use of supercritical fluids in inorganic analysis

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ISSN 00271314, Moscow University Chemistry Bulletin, 2010, Vol. 65, No. 1, pp. 1–18. © Allerton Press, Inc., 2010. Original Russian Text © E.M. Basova, V.M. Ivanov, O.A. Shpigun, 2010, published in Vestnik Moskovskogo Universiteta. Khimiya, 2010, No. 1, pp. 3–26. 1 INTRODUCTION The ability of supercritical fluids to dissolve sub stances was discovered as far back as 1879, when the solubility of cobalt and iron chlorides in supercritical ethanol was being studied [1]. However, practical application of this phenomenon was discovered only in 1960. The dissolving ability of supercritical fluids is used mainly in supercritical fluid extraction (SFE) and supercritical fluid chromatography (SFC). At present, liquid chromatography is widely used in inorganic analysis, particularly for the separation and determination of metal ions, making it possible to sep arate numerous inorganic compounds as ions or non volatile polar and nonpolar derivatives (metal chelates and metal organic substances) [2–7]. However, despite the high sensitivity and selectivity of high per formance liquid chromatography (HPLC) and mod ern spectroscopic, electrochemical, and nuclear physical methods of analysis, in order to decrease the detection limit and increase the reproducibility and validity of the analytical results at the sampling step, it is important to preconcentrate the microelements. Extraction, an effective and widespread concentration method, is applicable for discarding the matrix and for individual, group, or consecutive separation of micro elements [8]. The application of HPLC and liquid extraction requires the large expenditure of organic solvents, which, first, is a health hazard for laboratory staff, since they must work in an atmosphere of toxic, inflammable vapors, and, second, leads to ecological problems. For example, the expenditure of the mobile phase in HPLC can amount to several milliliters per minute, it is contaminated by toxic substances, the amount of waste is large, and storage and processing often cause problems. The desire to decrease the amount of organic solvents used in laboratories and to accelerate chromatographic separation and sampling has led experts to develop the alternative methods of SFC and SFE. The advantage of SFC is the high degree of separa tion at low temperature and short duration of analysis. In addition, the expenditure of solvent is substantially less than in HPLC and the waste is easily neutralized, since it is chemically simple and gaseous due to the slow expansion of the supercritical mobile phase out side the chromatographic system. The advantages of SFE are the possibility of analy sis without predecomposition of environmental objects and biological matrices, and the high rate of extraction and nontoxicity of the majority of super critical fluids. The devices used for analysis differ in their capabilities, available options, and low cost. For example, the cost of extractors varies between 10000 and 80 000 dollars, on average not exceeding 40000 dollars; and chromatographs, from 26000 to 64 000 dollars [9]. The theory of the SFC and SFE methods has been sufficiently developed and presented in monographs [10–16]. The latest achievements of the methods are generalized in [17–23]. There are special journals dedicated to the application of supercritical fluids Use of Supercritical Fluids in Inorganic Analysis E. M. Basova a , V. M. Ivanov b , and O. A. Shpigun b a International University of Nature, Society, and Man, Dubna, Russia b Division of Analytical Chemistry, Moscow State University, Moscow, Russia email: [email protected] Received May, 22, 2009 Abstract—The possibilities, advantages, shortcomings, and prospects of using supercritical fluids for separat ing and extracting metal complexes with organic reagents are considered. The theoretical bases of supercrit ical fluid chromatography and factors influencing the separation of metal complexes (nature of the organic reagent, solubility of reagents and complexes in a supercritical fluid, type of column, motionless phase, addi tion of a modifier into the mobile phase, and the test solvent) are discussed. The processes occurring in com plexes during chromatography are discussed. The bases of supercritical fluid extraction and factors influenc ing extraction of metals (nature and solubility in a supercritical fluid of an organic reagent and complexes; concentration and ways of introducing the reagent into the system; addition of the modifier, water, and sur factants; the collector; and the matrix) are considered. The possibilities of methods for determining metals in various objects are shown. Key words: supercritical fluid, fluid chromatography, supercritical fluid extraction. DOI: 10.3103/S0027131410010013

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Page 1: Use of supercritical fluids in inorganic analysis

ISSN 0027�1314, Moscow University Chemistry Bulletin, 2010, Vol. 65, No. 1, pp. 1–18. © Allerton Press, Inc., 2010.Original Russian Text © E.M. Basova, V.M. Ivanov, O.A. Shpigun, 2010, published in Vestnik Moskovskogo Universiteta. Khimiya, 2010, No. 1, pp. 3–26.

1

INTRODUCTION

The ability of supercritical fluids to dissolve sub�stances was discovered as far back as 1879, when thesolubility of cobalt and iron chlorides in supercriticalethanol was being studied [1]. However, practicalapplication of this phenomenon was discovered onlyin 1960. The dissolving ability of supercritical fluids isused mainly in supercritical fluid extraction (SFE) andsupercritical fluid chromatography (SFC).

At present, liquid chromatography is widely used ininorganic analysis, particularly for the separation anddetermination of metal ions, making it possible to sep�arate numerous inorganic compounds as ions or non�volatile polar and nonpolar derivatives (metal chelatesand metal organic substances) [2–7]. However,despite the high sensitivity and selectivity of high per�formance liquid chromatography (HPLC) and mod�ern spectroscopic, electrochemical, and nuclear�physical methods of analysis, in order to decrease thedetection limit and increase the reproducibility andvalidity of the analytical results at the sampling step, itis important to preconcentrate the microelements.Extraction, an effective and widespread concentrationmethod, is applicable for discarding the matrix and forindividual, group, or consecutive separation of micro�elements [8].

The application of HPLC and liquid extractionrequires the large expenditure of organic solvents,which, first, is a health hazard for laboratory staff,since they must work in an atmosphere of toxic,inflammable vapors, and, second, leads to ecological

problems. For example, the expenditure of the mobilephase in HPLC can amount to several milliliters perminute, it is contaminated by toxic substances, theamount of waste is large, and storage and processingoften cause problems. The desire to decrease theamount of organic solvents used in laboratories and toaccelerate chromatographic separation and samplinghas led experts to develop the alternative methods ofSFC and SFE.

The advantage of SFC is the high degree of separa�tion at low temperature and short duration of analysis.In addition, the expenditure of solvent is substantiallyless than in HPLC and the waste is easily neutralized,since it is chemically simple and gaseous due to theslow expansion of the supercritical mobile phase out�side the chromatographic system.

The advantages of SFE are the possibility of analy�sis without predecomposition of environmentalobjects and biological matrices, and the high rate ofextraction and nontoxicity of the majority of super�critical fluids. The devices used for analysis differ intheir capabilities, available options, and low cost. Forexample, the cost of extractors varies between 10000and 80000 dollars, on average not exceeding40000 dollars; and chromatographs, from 26000 to64000 dollars [9].

The theory of the SFC and SFE methods has beensufficiently developed and presented in monographs[10–16]. The latest achievements of the methods aregeneralized in [17–23]. There are special journalsdedicated to the application of supercritical fluids

Use of Supercritical Fluids in Inorganic AnalysisE. M. Basovaa, V. M. Ivanovb, and O. A. Shpigunb

a International University of Nature, Society, and Man, Dubna, Russiab Division of Analytical Chemistry, Moscow State University, Moscow, Russia

e�mail: [email protected] May, 22, 2009

Abstract—The possibilities, advantages, shortcomings, and prospects of using supercritical fluids for separat�ing and extracting metal complexes with organic reagents are considered. The theoretical bases of supercrit�ical fluid chromatography and factors influencing the separation of metal complexes (nature of the organicreagent, solubility of reagents and complexes in a supercritical fluid, type of column, motionless phase, addi�tion of a modifier into the mobile phase, and the test solvent) are discussed. The processes occurring in com�plexes during chromatography are discussed. The bases of supercritical fluid extraction and factors influenc�ing extraction of metals (nature and solubility in a supercritical fluid of an organic reagent and complexes;concentration and ways of introducing the reagent into the system; addition of the modifier, water, and sur�factants; the collector; and the matrix) are considered. The possibilities of methods for determining metals invarious objects are shown.

Key words: supercritical fluid, fluid chromatography, supercritical fluid extraction.

DOI: 10.3103/S0027131410010013

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(Journal of Supercritical Fluids and Sverkhkriticheskiyefluidi: teoriya i praktika (published in Russia since2006).

These methods are used mainly in the analysis offood, pharmaceuticals, and objects in the environ�ment (determination of pesticides, polycyclic aro�matic hydrocarbons, phenols, and other widespreadcontaminants). SFC and SFE are used rarely in inor�ganic analysis, although in [21], SFE of metal chelatesis presented as an interesting example.

The present review is dedicated to the possibilitiesof separation and extraction of metals as complex sub�stances with organic ligands using supercritical fluids.It should be noted that SFC was discovered on thebasis of HPGC in the separation of metal complexes,nickel porphyrines, with chlorofluoromethanes as themobile phases [24].

SUPERCRITICAL FLUIDSAND THEIR PROPERTIES

The supercritical state is the state in which phasesdo not change. At temperatures higher than critical,vapor and liquid have the same density and the fluidcannot be converted to the liquid state by increasingpressure. Under constant pressure (higher than thecritical value), continuous liquid supercriticalfluid transfer takes place with an increase in tempera�ture, and with an increase in pressure at constant tem�perature, the continuous gas supercritical fluidtransfer takes place.

The properties of some fluids applicable for SFCand SFE are given in Table 1. The most widely used iscarbon dioxide as the mobile phase for SFC or anextracting medium for SFE. This is due to the follow�ing: the critical temperature of carbon dioxide is low,while the critical pressure and critical density are fairlyhigh; it is nontoxic, nonflammable, and nonexplosive;it can easily be purified; it is cheap; and it makes it pos�sible to work with sensitive and universal flame ioniza�

tion detectors. In order to analyze thermally unstablecompounds, it is necessary to choose a supercriticalfluid with a low critical temperature (for example, it ispossible to work at temperatures lower than 40°C withcarbon dioxide, nitrogen (I) oxide, or trifluo�romethane).

Supercritical fluids can dissolve many substances,slowly volatile or high�molecular, since intermolecu�lar interactions are quite strong within them. In thefirst approximation, the value of critical pressure Pkcan be the degree of solubilizing power of the super�critical fluid. Usually, the Pk value is as high as thepolarity of the eluent.

Most supercritical fluids are nonpolar eluents orextragents and make it possible to analyze only nonpo�lar or weakly polar compounds. Supercritical ammo�nia has a high dissolving ability with respect to polarcompounds, the application of which is hindered dueto its aggressive chemical properties and, conse�quently, high requirements on the durability of equip�ment. The use of nitrogen (I) oxide in SFC is also lim�ited due to its high oxidizing ability with respect toorganic materials, including the sample, its solvent,and stationary phase. Supercritical xenon has gooddissolving ability; it is transparent in the IR range, butextremely expensive.

Other important properties of supercritical fluidsare the viscosity and diffusion coefficients of sorbates.Over the critical point for density and dissolving abil�ity, the compound approaches the liquid state; over thecritical point for viscosity, it approaches the gaseousstate and its diffusion coefficient has a medium valuebetween the given values for a gas and a liquid.

Thus, the properties of supercritical fluids make itpossible to use them as eluents in the SFC and SFE ofmetal complexes.

SUPERCRITICAL FLUID CHROMATOGRAPHY

SCF methods differ from typical methods in theiruse of temperature and pressure, the values of whichexceed the critical values for the mobile phase. Thereare no strict lines separating the high�temperature liq�uid chromatography (LC), enhanced�fluidity LC,SFC, and high�pressure gaseous chromatography(GC), although there are some practical differences.The only new requirement for intermediate methods isthe application of enough pressure to the end of thecolumn to prevent boiling and separation of the eluentduring phases. The end of the column in common LCmethods is under a pressure of 1 atm.

SFC is a method that does not compete with, butrather complements HPLC and GC. The diffusioncoefficients of sorbates in the liquid mobile phase areabout 105 times less with respect to diffusion coeffi�cients in gases, and in supercritical fluids, they areabout 100 times greater than in liquids. Consequently,the resolution of SFC must be intermediate between

Table 1. Physical parameters of several compounds used assupercritical fluids [10]

FluidCritical

temperature Tc, °C

Criticalpressure Pc,

atm

Criticaldensity ρc,

g/ml

CO2 31.3 72.9 0.47

N2O 36.5 72.5 0.45

NH3 132.5 112.5 0.24

n�butane 152.0 37.5 0.23

n�pentane 196.6 33.3 0.23

SF6 45.5 37.1 0.74

Xe 16.6 58.4 1.10

CCl2Fe2 111.8 40.7 0.56

CHF3 25.9 46.9 0.52

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USE OF SUPERCRITICAL FLUIDS IN INORGANIC ANALYSIS 3

GC and HPLC. In SFC, a higher degree of separationis achieved per unit time with respect to HPLC. Thefeatures of mass transfer, determined by the diffusioncoefficient values of sorbates, reduce the duration ofanalysis in the HPLC, SFC and GC row. The low vis�cosity of supercritical fluids [(0.2–1.0) × 10–3 P]makes it possible to decrease the pressure at the begin�ning of the column to a high extent (10–100 times) atthe given flow rate with respect to HPLC. The advan�tage of SFC is operation at substantially lower temper�atures with respect to GC, which makes it possible toeffectively separate complex mixtures of thermallyunstable compounds. This can be done by HPLC, butit is time�consuming to optimize the content of themobile phase.

COLUMNS AND STATIONARY PHASES

Separation by SFC can be carried out on packedand capillary columns. Usually, packed columns areused for HPLC, which are filled with modified andnonmodified silicagels, aluminum oxide, and polymersorbents. For SFC on packed columns, a large set ofstationary phases with variable selectivity are typical[20–23]. It was shown that the selectivity of polar sta�tionary phases strongly depends on temperature, andthis dependence is insignificant for nonpolar phases[22]. On polar packings, substantially larger shifts inselectivity were observed than on nonpolar ones, witha change not only in temperature, but also pressureand the modifier [21].

Capillary columns for SFC differ from capillarycolumns for GC: the internal diameter is usually lessand stationary phases must be chemically bonded withthe walls of the column or polymerized with the for�mation of cross bonds; before the stationary phase isestablished, the columns are deactivated in order toprevent retention, determined by the presence ofremaining silanol groups on the surface of quartz glass.Capillary columns for GC are not applicable in SFC,since the stationary phases cannot withstand the highdissolving ability of supercritical eluents. The thick�ness of the film of stationary phase in columns for SFCis usually 0.25 and 0.50 μm. The selection of stationaryphases differing in chemical properties is less in capil�lary SFC than in SFC with packed columns: usuallypolysiloxanes modified by n�octyl, methyl, phenyl, orcyanopropyl groups. The search is underway for poly�mers applicable for films of stationary phases usedthroughout specific separations [22].

The number of theoretical plates on packed col�umns for HPLC is 5000–10000; in capillary GC,there are 100000–200000 [10]. In routine determina�tions by SFC, common packed columns filled withparticles with size of 5 or 10 μm can provide the prep�aration of more than 20000 theoretical plates for sev�eral minutes. For packed capillary columns, the num�ber of theoretical plates is about three times greater,and under the given pressure, differential for capillary

columns with an internal diameter of 50 or 100 μm,this number is 100–500 times greater [10]. However, along time is required for determination on columnswith very high efficiency. Therefore, capillary SFCexceeds SFC on packed columns in the rate of analy�sis.

Lower values of viscosity of supercritical fluids withrespect to liquid mobile phases lead to lower values ofthe pressure differential in the column and, conse�quently, to the possibility of using longer columns thanin HPLC with a higher number of theoretical plates.However, in studying the effect of the pressure differ�ential in SFC in packed and common microcolumns,it was discovered that long columns with a higherapparent number of theoretical plates do not separatebetter [23]. A significant loss in apparent efficiencywas demonstrated, which increases in long columnswith an increase in the pressure differential [25]. Theauthors proposed the existence of a superposition ofthe temperature gradient, which to a certain extentcontradicts the longitudinal density gradient caused bythe pressure differential. The existence of a longitudi�nal temperature difference in columns having a longi�tudinal pressure differential has been proven [23]. InSFC with a contracting and solvating mobile phase,the space pressure gradient creates a correspondingspace gradient in the eluting power of the mobilephase, which leads to false direct calculations of thenumber of theoretical plates by the retention time andthe bandwidth at the end of the column.

The two types of columns have their own advan�tages and drawbacks. The selection between packed orcapillary column depends only on the actual analyticalproblem [10].

Mobile phase. The dissolving ability of eluents inSFC is enhanced by adding a modifier, an organic sol�vent, usually methanol, to the supercritical fluid.Modifiers are applied mainly for elution of polar sor�bates, and the introduction of polar modifiers causes asignificant decrease in retention. This effect is con�nected mainly with modification of the stationaryphase (molecules of a polar modifier solvate theremaining silanol groups in the case of grafted silicagels, making the surface of stationary phase morehomogeneous in terms of polarity), but not withenhancement of the solubilizing power of the mobilephase. At higher concentrations of modifier, the factordetermining the separation is the solubilizing power ofthe mobile phase. If the modifier is a weaker solventwith respect to sorbates in comparison to the maincomponent, the retention will increase with anincrease in the modifier concentration. When studyingthe effect of modifiers on retention, we succeeded inisolating effects related to a change in eluent densityfrom effects related to intermolecular interactions[25]. The effect of a cosolvent can be connected withthe formation of hydrogen bonds, complexes withcharge transfer, and dipole–dipole interactions [22]. Itwas shown that polar modifiers, which are capable of

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forming hydrogen bonds, can strongly influence theselectivity [22].

The effect of ionic additives, such as lithium,ammonium, tetramethylammonium, tetrabutylammonium acetates, and ammonium chloride, dis�solved in methanol, was studied in the retention ofsodium arylsulfonates in SFC on a nonmodified silicagel and the nitrile phase [26]. Different values of reten�tion time and different shapes of peaks correspond todifferent additives. Ammonium ions probably deacti�vate available silanol centers on two stationary phasesand also react with CN�groups of the grafted phase[27]. In another way, modifiers affect retention onporous graphite carbon [28]; regardless of the modi�fier’s origin, interactions of dissolved compounds withthe stationary phase were stronger than interactions ofdissolved compounds with the CO2–modifier mobilephase (methanol, ethanol, propanol, isopropyl alco�hol, tetrahydrofuran, acetonitrile, and hexane werestudied).

The work with modifiers is fraught with certain dif�ficulties. The eluent must be monophase, and the crit�ical parameters of binary mixtures depend nonlinearlyon the eluent composition. Therefore, in evaluatingthe physical state of the stationary phase there isuncertainty. In working with modifiers, it is necessaryto complicate the apparatus and work at higher tem�peratures. In addition, the introduction of modifiersleads to an increase in the background signal and noisewhen working with most detectors, as well as theimpossibility of applying a flame ionization detector.Modifiers are used more often with packed columnsrather than capillary ones.

Optimization of separation conditions. SFC is aflexible method. Whereas in GC the main ways of sep�aration are the change in (or programming of) temper�ature and the selection of the appropriate stationaryphase, in HPLC, they are selection of the stationaryphase and the variation of eluent composition, and inSFC, they are selection of the stationary phase, andvariation of content, temperature, pressure, and elu�ent density. The eluent content, temperature, pres�sure, and density can be varied separately or together.

The density of the mobile phase is the most impor�tant parameter; it characterizes solubility in the fol�lowing way [10]:

where Pc is the critical pressure, ρ is the density of thesolvent, ρliquid is the density in the liquid state. In orderto make the sorbate dissolve, the parameters of sorbateand solvent solubility should have almost equal values.At low density values, the parameters of mobile phasesolubility in SFC conditions are lower than for sor�bates; with an increase in density, they increase.Therefore, the strength of the mobile phase increaseswith an increase in density, and the capacity coeffi�cients decrease. Selectivity also decreases with anincrease in density. Due to the low rate of mass transfer

ρ 1.25Pc1/2

ρ/ρliquid( ),=

at high density diffusion, the coefficients of sorbatesand the coefficient of the mobile phase also decrease,which leads to diffusion of peaks.

Pressure influences the capacity coefficients simi�lar to density: retention decreases with an increase inpressure in a range higher than critical. With anincrease in operating pressure, the degree of separa�tion usually gets worse.

Pressure, density, and temperature are related toeach other nonlinearly. These parameters are includedin the equation of state. At constant density the depen�dence of capacity coefficient from the temperature canbe described by the Van Hoff thermodynamic equa�tion. However, under constant pressure, the retentionand degree of separation depend on temperature in acomplex way, which is determined by two factors, theeffect of which is contradictory. On the one hand, thedecrease in solubility of sorbate takes place due to thedecrease in eluent density with an increase in temper�ature and at constant pressure; on the other hand, theincrease in solubility takes place due to the increase invapors density with the increase in temperature. Thesecond effect dominates over the first one, if the tem�perature is elevated over definite level. If the tempera�ture of eluent exceeds critical, the capacity coeffi�cients and efficiency increase.

Most separation processes are carried out underprogramming the density of mobile phase. To decreasethe duration of separation, the pressure programmingcan be used. Since the degree of separation in bothcases decreases, in order to increase it, the tempera�ture should be varied. At constant density, duration ofthe determination reduces at higher temperatures; atconstant pressure, it reduces at temperatures a bit lessor slightly higher than the critical temperature of themobile phase. Maximal retention and separation areachieved at a temperature somewhat exceeding thecritical one. In SFC with density and temperature pro�gramming, the time required for equilibration of thecolumn is slightly less than in HPLC with gradientelution.

Models of retention. There is no clear understand�ing of retention in SFC. The results of investigation ofthe retention mechanism are generalized in [29],where different types of retention models are com�pared [29]. For example, in the separation of copoly�mers on capillary columns, it was shown that they sta�tionary phase influences the separation in signifi�cantly; with pressure programming, separation isgoverned by solubility in the mobile phase [22]. Mod�els have been developed that make it possible to pre�dict the degree of retention in capillary SFC in theabsence of programming or in conditions of tempera�ture (or pressure) programming [22]. With the exam�ple of ferrocene and its derivatives, it was shown thatretention in capillary SFC can be predicted by analyz�ing the solubility, sorbate molar volume parameters,and the dynamic characteristics of the chromatogra�phy column [29]. In SFC of alkyl aryl ketone

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homologs on packed columns filled with nitrile phase,it was shown that sorbates interact with silanol groups,which decrease substantially while adding methanol tothe mobile phase [22]. In a comparison of the reten�tion of cyanoalkanes, cyanoalkylbenzenes, andalkanes on alkyl and cyanoalkyl stationary phases,interactions with silanol groups were discovered thatcontribute to retention [21]. The retention of certainpolar sorbates is directly proportional to the area of thesurface of the diol stationary phase [22]. The numeri�cal ratios normally used in HPLC to describe reten�tion on porous graphite soot, were applied in SFC withCO2–methanol mobile phase [30]. The retention isdetermined by the polarizability and volume of mole�cule, while the basicity of the hydrogen bond is notincluded in the model of retention in any content ofmethanol studied.

Detectors. In SFC, detectors applicable for GCand HPLC were successfully used: spectrophotomet�ric ultraviolet, flame ionization, mass�spectrometric,IR�spectroscopic with Fourier transform, fluoromet�ric by light scattering during evaporation, etc.; themostly widely used are the first two. Recently, a newuniversal acoustic flame detector has been developed[31], the response of which is qualitatively comparableto the response of a flame ionization detector.

Supercritical Fluid Extraction

The development of SFE is directed mainlydecreasing the use of toxic flammable organic solvents.Actually, this method has many advantages withrespect to liquid extraction: high selectivity, decreasein extraction period, quantitative extraction, and lowcost of one determination. The system for SFE analy�sis consists of a high pressure pump, an extraction ves�sel, acounterpressure regulator, and a separation ves�sel. Usually, extraction from solid substances is carriedout. Extraction of liquid samples is carried out byadsorbing a liquid on a sorbent or by direct methods.Direct extraction of aqueous samples is used morerarely. The matrix of the sample strongly influences thedegree of extraction.

Supercritical carbon dioxide is used as the mainextragent. Freons are also finding application; forexample, the extraction of polycyclic aromatic hydro�carbons (PAHs) and polychlorinated biphenyls(PHBs) by Freon�22 is higher than by supercriticalCO2, SF6, and N2O due to its high dipole moment[22].

The strength of the supercritical fluid extragent canbe varied simply by changing temperature and/orpressure. The degree of extraction strongly depends onpressure and increases as pressure increases. Nonpo�larity of supercritical carbon dioxide limits its applica�tion. If it is necessary to extract polar compounds oranalytes that strongly interact with the matrix, twomethods are used.

The first is enhancement of the polarity of theextracting phase by the introduction of organic solventinto supercritical fluid. The origin of the solvent and itsvolume and contact period are essential. It was shownthat introduction of modifiers having acid–base prop�erties into supercritical CO2 enhances the extractionof PHB from river deposits, and modifiers capable ofdipole�induced dipole interactions and π–π interac�tions enhance the extraction of PAHs from solid parti�cles [21].

The second approach, allowing extraction of polarand even ionic compounds by nonpolar fluids of CO2type, is the preparation of derivatives in situ beforedynamic extraction. Different types of reactions areused: derivatization of active centers on the matrix sur�face, leading to release of analytes; transformations ofcarboxylic, hydroxyl, amino groups, and groups of sul�fonic acid into alkyl, acyl, or silyl derivatives; obtain�ing of ionic pairs (for example, to extract cationic SASfrom sewage sludge); and preparation of metal che�lates for the extraction of charged metal ions [23]. Thepreparation of analyte derivatives is required toenhance the solubility of analytes in the extractionfluid and reduce the interactions of analytes with thematrix of the sample.

Usually, extraction is carried out in two steps: first,in static conditions (no flux) and, then, in dynamic(collection of analytes into special receivers). Theduration of static and dynamic extraction is opti�mized. Analytes are collected in special collectors.The step of analyte collection is important throughoutthe overall extraction, since loss is possible that distortsthe real efficiency of extraction. The analyte collectioncan be done in two ways: into liquid solvent (catchingby solvent) and on the solid surface of a weighing bottleor test tube (by the change in solubility under decreas�ing pressure or temperature gradient), as well as on thesorbent surface.

The collection of analytes directly into an organicsolvent is the most convenient method for furtherdetermination, since the desorption step is notrequired. It can be collected into a pure solvent or mix�ture, since in mixed mediums the degree of extractioncan be higher. The efficiency of catching in an organicsolvent depends on the analyte partition coefficient,polarity, temperature and volume of the solvent,extraction pressure, fluid flux rate, and configurationof the device for collection [21]. It has been shown thatunder extraction by pure supercritical CO2, trappingwith a mixed solvent is more efficient, and when theextragent contains a modifier, higher degrees ofextraction are achieved upon collection into one sol�vent [21].

The methods of catching with sorbents have twoimportant advantages: limiters can be heated in orderto minimize possible cappings; volatile analytes arecaught more efficiently. The choice of sorbent dependson the origin of the analyte. Efficiency of catchingwith solid�phase traps, in addition to the origin of the

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sorbent, depends on the temperature of the trap, fluidflux rate, and concentration of the modifier. Cryogen�ically cooled traps have an advantage in analyzing vol�atile analytes. Since high concentrations of the modi�fier can lead to loss in sorption catching, two ways ofcatching have been proposed; for example, the mem�brane is placed before the flask with the solvent [23].

The important factor is the origin of the solvent,used for desorption of analytes from the sorbent layer.We carried out a comparative study of two ways ofcatching chlorinated substances after SFE: by 2,2,4�trimethylpentane and traps filled with silica gel, fluo�risyl, and octadecylsilicagel [23]. It is shown that fornonvolatile PHB, an insignificant difference in effi�ciency of the two ways of catching is observed, whilefor the more volatile chlorobenzenes, the use of solid�phase traps gives better results due to the collection ofanalytes from the collecting solvent.

In developing models of SFE, thermodynamic(solvatation, distribution, and diffusion) and kineticprocesses are taken into account. A model of dynamicSFE is proposed, based on the influence of solubilityon the kinetics of extraction and analyte diffusionfrom the matrix of the sample [22]. If the extraction islimited by the solubility of the analyte, it depends onthe flux rate; if extraction is limited by the desorptionof the analyte from the matrix and/or diffusionthrough the matrix, this dependence is absent [32]. Akinetical model of SFE is proposed, which accountsfor different processes reducing the rate of extraction:desorption kinetics, swelling, diffusion of the analyteinto the organic component of the sample, and parti�tion coefficients in the fluid�matrix system [22]. It isshown that the extraction is governed by model inwhich the step determining the rate is diffusion [22]. Amodel of dynamic SFE is worked out with the use ofdiffusion layer theory for predicting the time requiredfor extraction [21]. An SFE model is proposed thatallows for two steps of extraction: phase equilibriumand internal diffusion in particles [33]. What is new isa detailed description of different phase equilibria andflux characteristics. The number of parameters of themodel depends on the complexity of process: one tothree for mass transfer and one for flux. The possibilityof estimating the parameters of the model by extrac�tion curves is shown.

Extracts can be analyzed by different physico�chemical methods as in the mode with the separationof time (off�line), as in the mode of real time (on�line). The advantage of SFE is the possibility of directconnection with other analytical methods (GC,HPLC, SFE, FIA, IR spectroscopy with Fouriertransform, bioanalysis) [21–23]. For example, thecombination of commercial systems is described forSFE with supercritical CO2 modified by methanol or2�propanol, and HPLC with inverse phases in thereal�time conditions for determination of analytes inaqueous matrices [34]. A new automated analyticalsystem has been developed in which, in real�time con�

ditions SFE and separation by HPLC with UV detec�tion are carried out sequentially [35]. The system wasdeveloped during analysis of preparations of tradi�tional Chinese medicine.

The main field of application of SFE is the deter�mination of organic substances in food and objects ofthe environment [21–23, 36, 37]. The application ofsupercritical liquids as new reaction systems inorganic synthesis is promising [38]. SFE opens newopportunities in creating energy� and resource�con�servation technologies for processing and safe utiliza�tion of wastes and by�products of factories of the agri�cultural complex [39]. The method can be used todevelop chemical means for determining and extract�ing elements in environmental samples (water, soil,biota) [40].

EXTRACTION AND SEPARATION OF METALS

Nonpolar CO2�type supercritical fluids do notextract charged metal ions, and they cannot be sepa�rated by partition chromatography. Therefore, theyshould be transferred to neutral compounds and com�plex compounds of various types.

Choice of organic reagent. Neutral metal com�plexes must have high stability constants, dissolve wellin supercritical fluid, and have a quick kinetics of com�plex formation.

Solubility of complex�forming reagents. The maincriterion for the applicability of SFC and SFE is solu�bility of compounds in supercritical fluids. The data onsolubility of different complex�forming reagents insupercritical CO2 are generalized in [41]. The solubil�ity of free reagents in supercritical CO2 depends on theorigin of the reagent and varies substantially (Table 2).The solubility of reagents increases with an increase inpressure.

The length of an alkyl derivative chain at a nitrogenatom primarily influences the solubility of dithiocar�baminates (the solubility increases with increasinglength of the alkyl chain at the nitrogen atom). Fluori�nation enhances the solubility of dialkyldithiocarbam�inates: if at 50°C and 100 atm the solubility of sodiumdiethyldithiocarbaminate is 1.5 × 10–4 M, the solubil�ity of sodium bis(trifluoroethyl)dithiocarbaminate is4.7 × 10–4 [41]. The origin of a cation also influencesthe solubility of dithiocarbaminates, although not to alarge extent. Tetraalkylammonium salts have very highsolubility (Table 2).

β�diketones mix well with supercritical CO2 (Table 2).The solubility of fluorinated compounds is higher.With an increase in trioctylphosphineoxide alkyl radi�cal length, the solubility decreases, and the introduc�tion of a phenyl substituent also decreases the solubil�ity (Table 2).

As shown in Table 2, the solubility of bis(tria�zol)crown ethers is low. The introduction of tert�buthyl substituent into the benzene ring of dibenzo�

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MOSCOW UNIVERSITY CHEMISTRY BULLETIN Vol. 65 No. 1 2010

USE OF SUPERCRITICAL FLUIDS IN INORGANIC ANALYSIS 7

bis(triazol)crown�ether enhances the solubility by 40times [43]. While introducing methanol (5%) intosupercritical CO2 as a modifier, the solubility of thesecrown ethers increases by about one order [43].

The solubility of calixarenes in supercritical CO2 isincreased substantially when p�tert�butyl groups arereplaced by fluorinated lateral groups (reagents C1and C2, respectively, in Table 2). The introduction of asubstituent into the lower part (reagents C4 and C5)decreases the solubility of fluorinated reagents, espe�cially in the case of hydroxamic acid (C5). The solu�bility of hydroxamic acid decreases with an increase inhydrocarbon radical length, as well as with the intro�duction of a benzene ring into molecule (Table 2). Thesolubility of perfluorooctanehydroxamic acid is 20times higher than that of octanehydroxamic acid [45].

Thus, the solubility of the studied reagents insupercritical CO2 is quite high and allows them to beused for separation and extraction of metals. Regard�less of the type of organic reagent, the solubilityincreases with the introduction of fluorine atoms inthe molecule; consequently, it is favorable to use fluor�inated reagents. However, these reagents are notalways available and they are expensive, which limitstheir use. The reagents having a ring have the lowestsolubility and should not be used.

It is necessary to take into account the stability ofreagents in supercritical CO2. There are no data on thedecomposition of β�diketones, phosphorous�contain�ing organic compounds, crown ethers, or calixarenes.β�dithiocarbaminates are unstable in acidic condi�tions. It has been shown that water coming in contactwith supercritical CO2 has pH 2.80–2.95 in the rangeof pressures of 70–200 atm and temperatures of 25–70°C [46], which is determined by acid–base equilib�rium:

CO2 + H2O = H2CO3 H+ + HC .

Consequently, dithiocarbaminates should partiallydecompose in the extraction systems with supercriticalCO2 in the presence of water. However, this does notinterrupt metal extraction; it is enough to use a largeexcess of reagent.

The solubility of complexes is usually lower thanthe solubility of reagents. For example, the solubilityof iron(III)chelates with a stoichiometry of 1 : 3 withHHA, OHA, and BHA in supercritical CO2 is 0.6, 0.4,and 0.3 mM, respectively, that is, 8–15 times lowerthan the solubility of the reagents themselves (Table 2).

The solubility of metal dithiocarbaminates is low(10–5 to 10–6 M) [41] and increases by two to three

O3–

Table 2. Solubility (S, M) of various reagents in supercritical CO2

Reagent (designation) P, MPa (atm) T, °C S Literature

Sodium diethyldithiocarbaminate (DEDTC) 17.18 45 1.09* [42]

Tetrabutylammonium diethyldithiocarbaminate 17.18 45 2.91* [42]

Tetrabutylammonium dibutyldithiocarbaminate (DBDTC) 17.18 45 23.24* [42]

Ammonium pyrrolidine dithiocarbaminate (PDTC) 17.18 45 0.60* [42]

Acetylacetone (AA) (130) 600 4 × 10–4** [41]

Tenoiltrifluoroacetone (TTFA) (130) 600 2.3 × 10–2** [41]

Tributylphosphate (TBP) (120) 60 11% [41]

Tributylphosphineoxide (TBPO) (200) 60 0.85 [41]

Trioctylphosphineoxide (TOPO) (200) 45 6.6 × 10–2 [41]

Triphenylphosphineoxide (200) 45 7.7 × 10–3 [41]

Bis(2,2,4�trimethylphenyl)monothiophosphine acid (200) 60 40*** [41]

Bis(triazol)crown ethers (200) 60 (0.1–4.3) × 10–4 [43]

p�Tert�buthylcalix[4]arene (C1) (200) 60 0.62 × 10–3 [44]

p�Tert�buthylcalix[6]arene (C2) (200) 60 0.50 × 10–3 [44]

p�Heptadecafluorodecylthio�n�propylcalix[4]arene (C3) (200) 60 >0.12 [44]

p�Heptadecafluorodecylthio�n�propylcalix[4]arenetetra�ethylacetate (C4)

(200) 60 >0.094 [44]

p�Heptadecafluorodecylthio�n�propylcalix[4]arenetetrahy�droxamic acid (C5)

(200) 60 1.64 × 10–3 [44]

Heptanehydroxamic acid (HHA) (200) 60 9.4 × 10–3 [45]

Octanehydroxamic acid (OHA) (200) 60 3.8 × 10–3 [45]

Benzhydroxamic acid (BHA) (200) 60 2.3 × 10–3 [45]

Note: * µg/(min/ml); ** molar parts; *** g/L.

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8

MOSCOW UNIVERSITY CHEMISTRY BULLETIN Vol. 65 No. 1 2010

BASOVA et al.

orders when using sodium bis(trifluoroethyl)dithio�carbaminate (BTFEDTC) [47].

Hg(II), Zn(II), and Cu(II) dithiocarbaminateschange in the following order of decreasing solubility:M(BTFEDTC)2 > M(DHDTC)2 > M(DPDTC)2 >M(DBDTC)2 > M(DPRDTC)2 > M(DEDTC)2 >M(PDTC)2 [48], where DHDTC is dihexyldithiocar�baminate, DPDTC is dipentyldithiocarbaminate, andDPRDTC is dipropyldithiocarbaminate; i.e., the sol�ubility of chelates decreases with a decrease in hydro�carbon radical length at the nitrogen atom, like thesolubility of the reagents themselves (Table 2). Theintroduction of a ring in the molecule (PDTC) nega�tively influences the solubility. The use of reagents withlong�chain alkyl substituents is hindered by their com�mercial inavailability and the necessity of carrying outthe difficult synthesis. Optimal are the reagents withsubstituents from –C3H7 to –C5H11. It should benoted that metal dithiocarbaminates are substantiallymore stable in supercritical CO2 than initial reagents.

The solubility of metal β�diketonates, especiallyfluorinated ones, is high in supercritical CO2. Forexample, at 170°C and a CO2 density of 0.330 g/ml,the solubility of copper(II)acetylacetonate is 0.37 g/l;yttrium acetylacetonate, 0.082 g/ml; and yttriumhexafluoroacetylacetonate, 4.06 g/ml, which is50 times higher [49]. The solubility ofchrom(III)hexafluoroacetylacetonate at 60°C and200 atm is more than 40 times higher than that of itsnonfluorinated analog [41]. The solubility of lantaneand europium chelates with 2,2�dimethyl�6,6,6,6,8,8,8�heptafluoro�3,5�octanedion (FOD) isalso high: 5.5 × 10–2 and 7.9 × 10–2 M, respectively, at60°C and 150 atm [50]. The metal acetylacetonates at60°C and 200 atm change in the following order ofdecreasing solubility: Ga(AA)3 > In(AA)3 > Mn(AA)3> Zn(AA)2 > Co(AA)3 > Mn(AA)2 · H2O > Co(AA)2 ·H2O > Cu(AA)2 > LiAA [49]. It is seen that the solu�bility depends on the composition of complexes. Theincrease in the number of reagent molecules coordi�nated by metal ions leads to more efficient screeningof metal ion and an increase in solvatation of the form�ing complex in terms of nonpolar CO2 molecules. Itwas shown that metal chelates are more stable withnonfluorinated β�diketones and have a smaller ten�dency to decompose in the presence of water in SFE[41]. Copper(II) complex with the reagent Cyanex 302is also less soluble in comparison with reagent [41].

Study of structural influence of dithiocarbaminates[49] and β�diketones [51] on the solubility of Hg(II),

Chelate S, M (50°C, 100 atm)

Ni (DEDTC)2 8.5 × 10–7

Ni(BTFEDTC)2 7.2 × 10–4

Co (DEDTC)3 2.4 × 10–6

Co(BTFEDTC)3 8.0 × 10–4

Zn(II), Cu(I) and Cu(II), and Cr(III) chelates,respectively, revealed a correlation between the solu�bility of chelates and the Hildebrandt parameter ofsolubility δ of the free reagent: as the solubilityincreases, the lower the value of δ. By the way, in thecase of dithiocarbaminates, the dependences of loga�rithm for solubility from δ are nonlinear [49] and forβ�diketonates are satisfactory linear [51]. Thus,assuming the solubility parameter of the ligand, thesolubility of metal complexes in supercritical CO2 and,consequently, the possibility of their use in SFC andSFE can be predicted.

SFC of metal chelates did not receive a wide distri�bution [52]. The systems studied are presented on theTable 3. In all cases, except for [60], CO2 was used asthe supercritical fluid and solutions of ready chelatesin organic solvents were injected into the column,.

Optimal conditions of separation. During analysis,packed and capillary columns are used; in the firstcase, a spectrophotometric detector is used, typical forLC; in the second, a flame ionization detector, typicalfor GC. We focus on the influence of the origin of thestationary phase on the separation. In [57], six col�umns were studied; in [56], seven columns; and in[50], two columns. Optimization of separation condi�tions from the viewpoint of retention time, columnefficiency, shapes of peaks, and modifier use is deter�mined by the origin of interactions of chelates, sepa�rated by the stationary phase. The retention of chelatesoccurs due to the interaction of atoms of reagents withthe stationary phase. Therefore, the choice of the ori�gin of the stationary phase will be separate for eachreagent. For example, U(VI) complexes with 2,6�diacetylbis(benzoylhydrazone) (DABH) are stronglyretained on organic polymer stationary phases (PRP�1, Asahipac C18) even under elution by supercriticalCO2 containing 20% v/v of methanol (capacity coeffi�cient is 300 on PRP�1), probably by the π–π interac�tion between phenyl groups of the complex and aro�matic part of copolymer structure [56]. The reactionof this complex with remaining silanol groups deter�mines the strong retention and asymmetric peaks withtails on the Spherisorb ODS�2 column. The Capsell�Pack C18 stationary phase is optimal.

The efficiency of separation on packed columns islow: 1250–4600 theoretical plates at a column lengthof of 250 mm [56]. The efficiency of capillary columnsis several times higher [61]; also higher is the quality ofchromatograms in SFC on capillary columns [55, 61].

One more advantage of capillary columns is theabsence of a modifier (Table 4). In SFC on packed col�umns, in order to increase the eluting power, methanolis added to supercritical CO2. The retention decreaseswith an increase in temperature and pressure. Sincethe temperature of separation is not high (lower thanin separation of metal chelates by gas chromatogra�phy) [2], the separation and detection of iron(III) andarsenic(III)diisobutyldithiocarbaminate, which wasdestroyed in GC, became possible [61]. In several

Page 9: Use of supercritical fluids in inorganic analysis

MOSCOW UNIVERSITY CHEMISTRY BULLETIN Vol. 65 No. 1 2010

USE OF SUPERCRITICAL FLUIDS IN INORGANIC ANALYSIS 9

Tabl

e3.

Th

e se

para

tion

of m

etal

com

plex

es b

y S

FC

Rea

gen

tE

lem

ents

Typ

e of

colu

mn

Sta

tion

ary

phas

e(g

rain

ing,

μm

)M

odif

ier

(con

ten

t)T

, °C

PD

etec

tor

( λ,

nm

)L

iter

a�tu

re

8�H

ydro

xyqu

inol

ine

Pt(

II),

Pd(

II),

R

u(II

I),

Ir(I

II),

R

h(I

II)

Pac

ked

(250

× 4

.6 m

m)

Lic

hro

sorb

Si6

0 (7

.5)

Eth

anol

(15

mol

%),

met

han

ol (

15 m

ol %

)–

––

[53]

Pd,

Ru,

Ir,

Rh

Th

e sa

me

Th

e sa

me

(10)

Met

han

ol (

30 m

ol %

)75

100

bar

UV

(25

4)[5

4]

Die

thyl

dith

ioca

rbam

inat

eC

u(II

), Z

n, N

i,

Co(

II)

Th

e sa

me

Th

e sa

me

(7.5

)M

eth

anol

(10

mol

%)

––

–[5

3]

Cu,

Zn

, N

i, C

oT

he

sam

eT

he

sam

e (1

0)T

he

sam

e40

75 b

arU

V (

254)

[54]

Die

thyl

dith

ioca

rbam

inat

e,bi

s(tr

iflu

oroe

thyl

)dit

hio

carb

ami�

nat

e

As(

III)

, B

i(II

I),

Co(

III)

, Fe(

III)

, H

g(II

), N

i(II

),

Sb(

III)

, Z

n(I

I)

Cap

illa

ry(5

m ×

100

μm

)S

B�M

eth

yl�1

00�

supe

rbon

d–

100

Pro

gram

min

gfr

om 1

00 t

o20

0 at

m

FID

[55]

Dii

sobu

tyld

ith

ioca

rbam

inat

eC

d, P

b, Z

n,

Cu,

N

i, C

o(II

I),

Fe(

III)

, As(

III)

Cap

illa

ry(1

0 m

× 5

0 μ

m)

SB

�Ph

enyl

�5–

125

–F

ID[6

1]

SB

�Ph

enyl

�50

120

Oct

aneh

ydro

xam

ic a

cid,

ben

zhyd

roxa

mic

aci

dF

e(II

I)P

acke

dL

iCh

rosp

her

100

diol

(5)

Met

han

ol (

5 o

r 10

%)

60P

rogr

amm

ing

of

pre

ssu

re a

nd

ex

pen

dit

ure

SP

M[5

0]

Hyp

ersi

l OD

S (

5)

Ace

tyla

ceto

ne

V, M

n,

Fe,

Cr

Pac

ked

(250

× 4

.6 m

m)

Lic

hro

sorb

RP

�8

(7)

Met

han

ol (

10 m

ol %

)10

011

0 ba

rU

V (

254)

[54]

2,6�

Dia

cety

lpyr

idin

�bis

(ben

zoyl

�h

ydra

zon

e),

2,6�

diac

etyl

pyri

din

�bi

s(4�

tert

�but

ylbe

nzo

ylh

ydra

zon

e)

U(V

I),

Cu(

II)

Th

e sa

me

Cap

cell

�Pac

kC

18M

eth

ano

l (3

vol %

)45

265

bar

DM

D[5

6]

Ace

tyla

ceto

ne, h

exaf

luor

oace

tyl�

acet

one,

1�p

heny

l�1,

3�bu

tane

dion

eF

e(II

I), C

r(II

I),

Co(

II, I

II),

C

u(II

), N

i(II

)

Th

e sa

me

Ph

enyl

ph

ase

(5),

C

1�p

has

e,C

18�p

has

e

Met

han

ol (

20 v

ol %

)99

–10

038

00–

4750

psi

UV

(28

0)[5

7]

2,6�

Dim

eth

yl�6

,6,7

,7,8

,8,8

�hep

�ta

fluo

ro�3

,5�o

ctan

edio

ne,

1,

1,1,

6,6,

6�h

exaf

luor

open

tan

e�2,

4�di

one

lan

than

ides

, U

(VI)

––

––

––

[58]

Hex

aflu

oroa

cety

lace

ton

e, t

rifl

uo�

roac

etyl

acet

one,

2,2

,6,6

�tet

ram

e�th

yl�3

,5�h

epta

ned

ion

e

Cu(

II),

Mn

(II)

Cap

illa

ry(5

m ×

100

μm

)M

eth

ylsi

lico

n–

80P

rogr

amm

ing

FID

[59]

1�ph

enyl

�3�m

eth

yl�4

�ben

�zo

ylpy

razo

lon

e�5�

one

U(V

I)P

acke

d(2

50 ×

4.6

mm

)C

18M

eth

ano

l (7

vol %

)45

220

bar

SP

M

(280

)[6

0]

Not

e:S

PM

– s

pect

roph

otom

etri

c de

tect

or,

UV

– u

ltra

viol

et d

etec

tor,

DM

D –

dio

de�m

atri

x de

tect

or,

and

FID

– fl

ame

ion

izat

ion

det

ecto

r.

Page 10: Use of supercritical fluids in inorganic analysis

10

MOSCOW UNIVERSITY CHEMISTRY BULLETIN Vol. 65 No. 1 2010

BASOVA et al.

Tabl

e 4.

SF

E o

f met

al c

ompl

exes

Rea

gen

tE

lem

ents

Mod

ifie

rT

, °C

P,

atm

Met

hod

of

reag

ent

inje

ctio

nM

atri

xC

olle

ctor

Lit

era�

ture

Ace

tyla

ceto

ne,

tri

fluo

roac

etyl

acet

one,

h

exaf

luor

oace

tyla

ceto

ne,

ten

oylt

rifl

uoro

�ac

etyl

acet

one,

2,2

�dim

eth

yl�6

,6,7

,7,8

,8,8

�h

epta

fluo

ro�3

,5�o

ctan

edio

ne

(+T

BF

)

La(

III)

, Eu(

III)

, L

u(II

I)–

6015

0in

sit

uC

ellu

lose

�bas

ed fi

lter

p

aper

, sa

nd

, so

il,

wat

er

Ch

loro

form

[50]

Ten

oylt

rifl

uoro

acet

ylac

eton

eS

m,

Eu(

III)

, G

d, D

y, L

a,

Ce(

III)

, Y

b, L

u

TB

F(3

0 vo

l %)

6035

0on

�lin

e6

M H

NO

3 +

3 M

L

iNO

3

Eth

ano

l–w

ater

(50

: 50)

[63]

Ten

oylt

rifl

uoro

acet

ylac

eton

e (+

TB

F)

La,

Eu(

III)

, Lu,

U

(VI)

, Th

–60

150

Th

e sa

me

Fil

ter

pap

er–

[64]

Ace

tyla

ceto

ne,

hex

aflu

oroa

cety

lace

ton

eC

o(II

)–

6040

0in

sit

uC

ube

fro

m s

tain

less

st

eel

Met

han

ol

[65]

Tri

fluo

roac

etyl

acet

one

Al

–11

045

0T

he

sam

eA

mbe

rlit

e IR

A�4

00

imp

regn

ated

by

chro

maz

uri

le S

To

luen

e[6

8]

Tri

fluo

roac

etyl

acet

one

(+py

ridi

ne)

U(V

I)–

6030

0R

ead

y ad

du

ctP

ow

der

or

pap

er b

ull

ey

e, im

pre

gnat

ed

wit

h e

than

ol s

olu

tio

n

of

com

ple

x

Eth

ano

l[6

6]

Dip

ival

oylm

eth

ane,

tri

fluo

roac

etyl

ace�

ton

e, h

exaf

luor

oace

tyla

ceto

ne

(+T

BF

)U

(VI)

, Pd,

Np,

A

m–

6030

0T

he

sam

e–

–[6

7]

Flu

orin

ated

β�d

iket

ones

(+

TB

F)

La,

Eu(

III)

, Lu,

U

(VI)

–60

15*

–S

oli

d m

atri

ces

(so

il)

–[6

9]

Flu

orin

ated

β�d

iket

ones

(+

TB

F)

Act

inid

es–

––

in s

itu

So

ils,

dep

osi

ts,

filt

ers

Met

han

ol

[70]

2,2,

7�T

rim

eth

yl�3

,5�o

ctan

edio

ne

Fe(

III)

–60

20.8

*T

he

sam

eA

queo

us

solu

tio

nH

exan

e[7

1]2,

2�D

imet

hyl

�6,6

,7,7

,8,8

,8�h

epta

fluo

ro�

3,5�

octa

ned

ion

e (+

TB

F)

U(V

I)E

than

ol

(50

vol %

)70

5000

**T

he

sam

eS

oli

d m

atri

ces

(cao

lin

e, p

oly

eth

er,

glas

s w

oo

l)

Eth

ano

l[7

2]

2,2�

Dim

eth

yl�6

,6,7

,7,8

,8,8

�hep

tafl

uoro

�3,

5�oc

tan

edio

ne

(+T

BF

)L

a, E

u(II

I), L

u,

U(V

I)M

eth

ano

l(5

mo

l %)

6015

0T

he

sam

eC

ellu

lose

�bas

ed fi

lter

p

aper

Ch

loro

form

[73]

Ben

zoyl

acet

on

eC

u(II

)M

eth

ano

l(5

vo

l %)

6025

*R

ead

y ch

elat

eD

iato

mit

e gr

ou

nd

Eth

ano

l[8

7]

Ten

oyl

trif

luo

roac

etyl

acet

on

e (+

TB

F),

Hex

aflu

oro

acet

ylac

eto

ne

(+T

BF

)U

(VI)

, Th

–60

150

–W

ater

, so

il,

san

d–

[89]

2,2,

6,6�

Tetr

amet

hyl

�3,5

�hep

tan

edio

ne

Rh

(III

), P

d(II

)–

6015

*,

>40

*–

San

d,

hu

min

e ac

idC

artr

idge

s fo

r so

lid

�p

has

e ex

trac

tio

nD

ich

loro

met

han

e

[90]

Tet

rabu

tyla

mm

oniu

m d

ibut

yldi

thio

carb

a�m

inat

e, te

trab

utyl

amm

oniu

m d

ieth

yldi

thio

�ca

rbam

inat

e, t

etra

met

hyl

amm

on

ium

pyr

�ro

lid

ine

dit

hio

carb

amin

ate

Zn

, C

d, P

b–

5524

.05*

on�l

ine

Aqu

eou

s so

luti

on

sM

eth

ano

l[4

2]

Page 11: Use of supercritical fluids in inorganic analysis

MOSCOW UNIVERSITY CHEMISTRY BULLETIN Vol. 65 No. 1 2010

USE OF SUPERCRITICAL FLUIDS IN INORGANIC ANALYSIS 11

Tabl

e4.

(Con

td.)

Rea

gen

tE

lem

ents

Mod

ifie

rT

, °C

P,

atm

Met

hod

of

reag

ent

inje

ctio

nM

atri

xC

olle

ctor

Lit

era�

ture

Tet

rabu

tyl d

ibu

tyld

ith

ioca

rbam

inat

eC

d, C

u(II

), Z

n,

Mn

(II)

, P

b–

5024

.05*

(2

7.48

)T

he

sam

eT

he

sam

e–

[74]

Flu

ori

nat

ed d

ith

ioca

rbam

inat

eC

r(V

I)M

eth

ano

l (<

1%)

––

–S

ynth

etic

so

il m

atri

�ce

s–

[75]

Dit

hio

carb

amin

ates

Pb,

Hg(

II),

Cd

––

––

So

ils

–[7

6]

Bis

(tri

flu

oro

eth

yl)d

ith

ioca

rbam

inat

eC

r(V

I)M

eth

ano

l60

380*

**on

�lin

eB

row

nm

ille

rit,

ste

k�lo

vata

Met

han

ol

[77]

Die

thyl

dit

hio

carb

amin

ate

––

––

––

–[7

8]

Die

thyl

dit

hio

carb

amin

ate,

Bis

(tri

flu

oro

et�

hyl

)dit

hio

carb

amin

ate

Hg(

II)

Met

han

ol (

5%)

8040

0R

ead

y ch

elat

e ,

on�l

ine

Fil

ter

pap

er,

san

d,

slu

dge

, as

h,

soil

Tra

p w

ith

C18

so

r�be

nt

[79]

8�H

ydro

xyqu

ino

lin

eC

u(II

)T

he

sam

eR

ead

y ch

elat

eE

than

ol s

olut

ion

–[8

0]

8�H

ydro

xyqu

ino

lin

eC

u(II

), P

b, Z

n,

Cd,

Cr(

III)

, Ni,

C

o(II

)

Met

han

ol

5015

*–

––

[81]

8�H

ydro

xyqu

ino

lin

eC

dT

he

sam

e80

–12

017

.2*

on�l

ine

Fil

ter

pap

er[8

2]

p�H

epta

dec

aflu

oro

thio

dec

yl�n

�pro

pyl

�ca

lix[

4]ar

enet

etra

hyd

roxa

mic

aci

dF

e(II

I),

Pb,

C

u(II

), M

n(I

I)–

6035

0in

sit

uT

he

sam

eC

hlo

rofo

rm,

DM

SO

[44]

Bis

(tri

azo

l)cr

ow

n�e

ther

Hg(

II)

Met

han

ol (

5%)

6020

0on

�lin

eT

he

sam

eC

hlo

rofo

rm[6

5]

Bis

(tri

azo

l)cr

ow

n�e

ther

Hg(

II),

Th

e sa

me

6020

0in

sit

uF

ilte

r p

aper

, sa

nd

Th

e sa

me

[43]

Au(

III)

on�l

ine

Aqu

eou

s so

luti

on

Bis

(2,4

,4�t

rim

eth

ylp

enty

l)d

ith

iop

ho

s�p

hin

e ac

id (

Cya

nex

302

)C

u(II

), P

b, C

d,

Zn

–60

400

Th

e sa

me

So

il–

[83]

Bis

(2,4

,4�t

rim

eth

ylp

enty

l)d

ith

iop

ho

s�p

hin

e ac

id (

Cya

nex

302

)C

d, C

u, P

b, C

r,

As

Met

han

ol (

5%)

6020

0T

he

sam

eT

he

sam

e–

[88]

Su

lfu

r�co

nta

inin

g p

ho

sph

oro

us�

org

anic

re

agen

tsH

eavy

met

als,

u

ran

ium

––

–T

he

sam

eA

cid

ic s

olu

tio

ns

2�M

eth

yl�8

�qu

ino

lin

eP

dP

erfl

uo

rate

d

met

hyl

cycl

o�

hex

ane,

hep

tan

e,

cycl

oh

exan

e,

CC

l 4,

ben

zen

e

––

––

Ben

zoyl

acet

on

eC

u(II

)M

eth

ano

l(5

vo

l %)

6025

*R

ead

y ch

elat

eD

iato

mit

e gr

ou

nd

Eth

ano

l

Not

e:*

MP

a, *

* ps

i, *

** b

ar.

Page 12: Use of supercritical fluids in inorganic analysis

12

MOSCOW UNIVERSITY CHEMISTRY BULLETIN Vol. 65 No. 1 2010

BASOVA et al.

cases, the conditions of pressure programming areoptimal [50, 55, 59], or pressure and the expenditureof supercritical CO2 all at the same time [50].

Sequence of elution. 8�Hydroxyquinolinates ofplatinum metals were eluted from a Lichrosorb Si60column in the following order: Pd ~ Pt < Rh < Ru < Ir(15 mol. % of ethanol) [53] or Pt < Ru < Ir < Rh < Os(30 mol % of methanol) [54]. The duration of separa�tion was 20 min; peaks of ruthenium and iridium che�lates are not separated to baseline [54]. The sequenceof elution coincides with the sequence of elution innormal�phase HPLC [3]. Metal diethyldithiocarbam�inates were eluted in the sequence Cu+Zn < Ni < Co[54], which also coincides with the sequence of elutionof these complexes in normal�phase HPLC [4]. Theseparation of metal acetylacetonates on a LichrosorbRP�8 column was achieved within 5 min, although notto the baseline; chelates are eluted in the followingsequence: V < Mn < Fe < Cr [54]. The retention ofmetal acetylacetonates on the phenyl phase increasesin the following order: Fe(III) < Cr(III) < Co(III)[57]. These chelates mixtures were not separated byinverse�phase HPLC [4]. Co(III) retention of acety�lacetonate on the C1 column is worse than that ofCo(II) acetylacetonate [57]. The retention correlateswith the solubility of these complexes in supercriticalCO2: 0.62 mg/l for Co(AA)3 and 0.25 mg/l forCo(AA)2 · H2O [49]. The retention of chromium(III)complexes with different reagents increases in the fol�lowing order: tris(hexafluoroacetylacetonato)chro�mium < tris(acetylacetonato)chromium < tris(1�phe�nyl�1,3�butandionato)chromium [57]. And in thiscase, the decrease in retention takes place due toenhanced interaction with the mobile phase.

In the SFC method on the capillary column, metaldiisobutyldithiocarbaminates in the series Cd < Pb <As(III) are eluted within 15 min (stationary phase isSB�Phenyl�5), and in the series Zn < Cd < Cu(II) <Ni(II) < Pb < Co(II) < Fe(III) < As(III), within22 min (stationary phase is SB�Phenyl�50) [61]. Theorder of retention on two polyphenylmethylsiloxanephases, differing by the content of polyphenylsiloxane(5 and 50% respectively), coincides, but differs fromthe sequence of retention of bis(trifluoroethyl)dithio�carbaminates: Zn < Ni(II) < Co(III) < Fe(III) <Hg(II) < As(III) < Sb(III) < Bi(III) on the SB�Methyl�100 stationary phase (the duration of separa�tion is 22 min) [55]. It was shown that the As(III)complex with sodium bis(trifluoroethyl)dithiocar�baminate is retained far more weakly than the complexof nonfluorinated reagent [55], which can beexplained by enhanced complex interaction with themobile phase, since the solubility of fluorinated com�pounds in supercritical CO2 is higher than that of non�fluorinated ones.

With gas chromatography on a packed column (thestationary phase is 3% OV�25 on W�HP chromosorb),nine metal di(trifluoroethyl)dithiocarbaminates wereseparated in the following sequence: Zn < Cu < Ni <

Cd < Hg < Co < Fe < Pb < Bi [2], which differs fromthe exit sequence in [55]. Consequently, the reactionof chelates with the stationary phase determines theorder of retention and chelate elution.

Solubility of the sample. The chelate solutions wereprepared and injected into the column in chloroform[53, 54, 57], dichlormethane [55], methanol [50], andethylacetate [56]. The influence of sample solvent onthe shape of the peaks was studied [56]. Earlier [62] itwas shown that while injecting the sample into SFC ina solvent with a higher solvating ability, than supercrit�ical CO2, the diffusion and even splitting of peaks arepossible, because part of the analyte moves throughthe column with the solvent, which is retained weakly[62]. The introduction of a sample with evaporation ofsolvent (ethylacetate) made it possible to increase theefficiency of separation by 8.5 times [56].

Decomposition of chelates under chromatography.The thermodynamic stability and kinetic inertness ofmetal chelates are the main requirements for chelatesin HPLC [3–5, 7], and in gas chromatography, ther�mal stability is also essential. Since the separation iscarried out at relatively low temperature (by 100–120°C lower than by separation of the same metal che�lates by GC), the range of chelates separated by SFCbecomes broader. In separation on packed columns,the possible decomposition of kinetically labile com�plexes leads to the diffusion of peaks, appearance ofmultiple peaks, and memory effect of the column.Irreversible sorption of chelates or their decomposi�tion products substantially decreases the durability ofcolumns.

The possibility of chelate separation in SFC ismentioned in [55, 57, 59]. In a schematical investiga�tion of how dialkyldithiocarbamine acids influencethe chromatographical behavior of complexes in nor�mal�phase HPLC, it was shown that DEDTC is themost convenient reagent of this class for multielementseparation of metals [4]. However, in the separation ofZn, Pb, Ni(II), and As(III) diethyldithiocarbaminatesby SFC on a capillary column [55], the inapplicabilityof DEDTC was demonstrated, determined by the lowsolubility of chelates in supercritical CO2 and the pos�sibility of their decomposition, which leads to lowreproducibility of results, the appearance of widepeaks, chromatographic memory of the column, andits contamination. Metal bis(trifluoroethyl)dithiocar�baminate complexes are chromatographed in theshape of sharp, symmetric, reproducible peaks, whichconfirms the stability of these chelates during chroma�tography [55]. Contamination of the column in thiscase was not observed either.

In the separation of metal β�diketonates on packedcolumns [57], it was shown that, as in the case ofHPLC [4], kinetically inert complexes are reversiblyadsorbed (peaks have symmetrical shape) and kineti�cally labile are irreversibly adsorbed, which is reflectedin strongly diffused peaks, the retention times of whichvary by the amount of sample, and the appearance of

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MOSCOW UNIVERSITY CHEMISTRY BULLETIN Vol. 65 No. 1 2010

USE OF SUPERCRITICAL FLUIDS IN INORGANIC ANALYSIS 13

multiple peaks. Labile β�diketonates are formed byCo(II), Cu(II), Fe(III), Ni(II), Pb(II), and Zn(II).Actually, the Fe(III) and Co(II) acetylacetonate peaksare diffused, and for Ni(II) and Cu(II) acetylaceto�nates double peaks are obtained [57].

For SFC, also like for HPLC, the material of thecolumn and other parts of the chromatographic sys�tem have a large significance. Packed columns are usu�ally prepared from the stainless steel. With repeatedintroductions of octanehydroxamic acid samplewithin 2 h, the appearance of iron(III)chelate peakwas present [50]. The peak was detected at 420 nm,and the reagent did not absorb light. The height of thispeak firstly increased and then started to decrease,probably due to the decrease in the content of availableiron from material from parts of the chromatographicsystem. For the other studied reagent, this effect wasnot observed.

Study of adduct formation. Bivalent metals formcoordinating unsaturated compounds with β�diketo�nates. These complexes contain water molecules (forexample, Cu(TFAA)2 · H2O and Cu(HFAA)2 · H2O),they interact with the stationary phase more strongly,and they have tendency to decompose with loss of theligand and further irreversible desorption. Coordi�nated unsaturated chelates our able to form adductswith Lewis bases: tributylphosphate, tributylphosphi�neoxide, and trioctylphosphineoxide.

The formation of adducts of copper(II) and man�ganese(II) trifluoroacetylacetonates, hexafluoro�acetylacetonates, and 2,2,6,6�tetramethyl�3,5�hep�tanedionates with TBFO was studied [59]. TBFO wasadded to a chloroform solution of chelates andinjected into the column. The formation of adducts ofCu(TFAA)2 · 2TBFO, Cu(HFAA)2 · TBFO,Mn(HFAA)2 · TBFO composition in situ in the mobilephase was proved. Peaks of the adduct had a moresymmetric shape and larger height than the peaks ofthe initial chelates. The peaks of adducts are well sep�arated from the peak of excess TBFO. In this case, it isalso more preferable to use fluorinated reagents, sincethe introduction of CF3 electron acceptor substituentinto molecules leads to a decrease in electron densityaround the central ion and, as a consequence, to theformation of more stable adducts.

The formation of adducts of lanthanide complexeswith 2,2�dimethyl�6,6,7,7,8,8,8�heptafluoro�3,5�octanedione (FOD) and uranium(VI) with1,1,1,6,6,6�hexafluoropentane�2,4�dione (FPD) withTBF, TBFO, and TOFO containing two and one mol�ecule of neutral phosphorous�organic reagent, thatimproves the separation [58]. It was shown that form�ing adducts change in the following order of decreas�ing stability: Ln(FOD)3 · 2TOFO > Ln(FOD)3 ·2TBFO > Ln(FOD)3 · 2TBF. For adducts of differentlanthanides with TBFO, the stability of adductsincreased along with the atomic number of lanthanide.

The formation of adducts in situ with a neutraldonor ligand, which was introduced into the mobile

phase, was used in determining uranium with 1�phe�nyl�3�methyl�4�benzoylpyrazolin�5�one [60]. TBFO,diphenylsulfoxide, and phenantrene were studied asdonor reagents.

Analytical application. A method of determininguranium(VI) with a detection limit of 2 ng (the volumeof the sample in the column is 10 μl) was developed inwhich the complex is formed in situ in the mobilephase (Table 4) [60]. The graduated plot is linear in therange of 0.02–2.00 μg of uranium(VI). REM and tho�rium do not disturb the determination of uranium.

A method of uranium(VI) determination in theform of complex with 2,6�diacetylpyridin bis(benzoyl�hydrazone) obtained by liquid extraction has beenproposed [56]. The graduated plot is linear in therange of 52–323 ng (the volume of the ethylacetatesolution of the sample in the column is 10 μl). Themethod was applied to analyze a model solution with arelative error of 0.5%.

A method of simultaneous determination of Zn,Ni, Co, Fe, Hg, As(III), Bi(III), and the general con�tent of arsenic and bismuth in the form of chelateswith sodium bis(trifluoroethyl)dithiocarbaminateafter extraction preconcentration was developed (theconcentration coefficient is 200) [61]. The detectionlimit is ~1 mln–1 (the volume of the sample in the cap�illary column is 80 nl). The method was applied toanazlyze a water sample from the pores of river depos�its. The results agree satisfactorily with the data onneutron�activation analysis and atom�emission spec�trometry with an inductive�bonded plasma.

SFE of metal complexes. The studied systems arepresented in Table 4. The majority of publications arededicated to the use of β�diketones, which have thehighest solubility in supercritical CO2 (see Table 2).

Reagent introduction. Preliminarily synthesizedchelates are rarely extracted (Table 4); usually the pur�pose of such investigations is not the extraction ofmetal ions, but the study of the composition and sta�bility of complexes. A reagent is introduced into sys�tem for SFE in two ways: in a supercritical fluid presat�urated with a reagent in a special vessel (on�line, Table4) and by direct addition into the matrix containingthe sample (in situ, Table 4). Comparison of the effi�ciency of extraction of ready chelate and that obtainedfrom the components in the matrix in situ [79] showedthat the degree of extraction regardless of the matrixused is higher for ready chelate, which points to reac�tion of metal ions with the matrix. For example, forready chelate and from that obtained in the matrix, thedegree of extraction of mercury(II)bis(trifluoroet�hyl)dithiocarbaminate by supercritical CO2 at 80°Cand 400 atm was 81.4 and 90% from filter paper, and93.8 and 88.7% from soil, respectively. Usually theaddition of a reagent into matrix was used in extractionfrom solutions (Table 4).

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BASOVA et al.

In all cases, the static and dynamic extractions werecombined and the duration of each step was opti�mized.

Different organic solvents were used as collectors ofthe chelates extracted, mainly chloroform or metha�nol (Table 4). Only in two studies were chelatesextracted on the trap by a sorbent (chucks for solid�phase extraction) [79, 90]. The use of a chuck made itpossible to combine the TFE in real�time conditionswith separation and determination by HPLC [90].However, it was found that the degree of extraction ofpalladium and rhodium β�diketonates in collectioninto a dichloromethane solution and determinationunder the condition with time separation (off�line)was higher than on the sorbent. In sorption on chucks,part of the chelates were irreversibly sorbed and theother part passed through.

Influence of pressure and temperature on the effi�ciency of extraction. With an increase in pressure, thedensity of supercritical CO2 increases, which leads toan increase in reagent and chelate solubility, and con�sequently, the efficiency of extraction. For example,the degree of extraction of cadmium hydroxyquinoli�nate at a pressure of 6.9 MPa was 1.4%, and at17.2 MPa, it was 70.0% [82].

The influence of temperature is more complex. Onthe one hand, the volatility of compounds and thekinetics of extraction usually increase with an increasein temperature. On the other hand, an increase intemperature decreases the density of the fluid. There�fore, the pressure, temperature, and density are opti�mized.

A modifier enhances the solubility of complexes insupercritical CO2 and decreases the interaction ofthem with the matrix. Therefore, most separations arecarried out with the addition of a small amount ofmodifier, usually methanol (see Table 3).

The concentration of the reagent influences thedegree of extraction. In the absence of reagents thedegree of extraction is not significant, the introductionof reagents in most cases leads to quantitative extrac�tion of metals. It is necessary to introduce an excess of

reagent, in order to prevent the dissociation or decom�position of complexes. The excess must be more, thanin the case of liquid extraction with the same reagent,since the extraction is carried out at high temperaturesand more reactable environment (extraction cell ismade from stainless steel). For example, in extracting0.5 μmol of cobalt(II) in the form of chelate withhexafluoroacetylacetone (reagent is introduced intomatrix), a more than 400�fold excess of reagent isrequired [64]: when 100, 250, and 700 μmol of reagentwere introduced, the degree of extraction was 78.3,90.7, and 90.5% respectively.

The influence of water. In extraction from solidmatrices (filter paper, sand, soil), the degree of extrac�tion usually substantially increases small amounts(10–100 μl) of water are added. In these amounts,water is insoluble in supercritical CO2 and there aretwo phases in the system: the aqueous phase and thesupercritical fluid phase aqueous. In the aqueousphase, the dissociation of salt and the formation ofmetal ions takes place; the distribution of reagent intoaqueous phase and the formation of adducts withwater in the case of coordinated saturated chelates arepossible. All these factors enhance the complex forma�tion and increase the rate and degree of complex for�mation. Additives of water can also decrease theadsorption of analytes by the surface of matrix via theconcurrent interaction and blocking of the active cen�ters of the matrix, alleviating the migration of complexinto the fluid phase.

Additives of water can also have a negative influ�ence on extraction of complexes. The pH of water,being in equilibrium with supercritical CO2, is ~2.9[46]. The formation of chelates depends on the pH ofthe medium: 2–8 for dithiocatbaminate, and fromneutral to weakly acidic for β�diketonates. Conse�quently, SFE conditions are good for the formation ofthese chelates. In acidic conditions, partial decompo�sition of reagents is possible; therefore, it is necessaryto introduce them in large excess.

The origin of matrix strongly influences the degreeof extraction of metal ions. When metal ions areplaced on filter paper or sand, the degree of extractionis high (Table 5). However, if a metal salt solution isadded to a soil sample, the degree of extraction sub�stantially decreases (Table 5). Real samples from theenvironment (natural waters, soil, sludge) containnatural ligands (humine acids), which can stronglybond metal ions, hindering complex formation withthe added reagent. In actual samples, metal ions canbe present in the form of low�soluble compounds(oxides, sulfides). For analysts, the largest concern isthe analysis of environmental objects. Therefore, inthe extraction of metal ions SFE from the samples ofsoil (and probably natural waters) really only mobileforms can be determined.

Actually, in determining heavy metals (Hg, Pb) insoils, the samples are decomposed by concentrated

Table 5. The dependence of degree of extraction of mer�cury(II) (%) (reagent was introduced to matrix) by super�critical CO2 (80°C, 400 atm) from the origin of matrix [79]

Matrix SodiumDEDTC

LithiumBTFEDTC

Standard aqueous solution 100.0 100.0

Sand 86.5 90.0

Filter paper 83.1 78.1

Soil 51.0 88.1

Ash 32.2 102.0

Sludge 18.1 56.8

Note: BTFEDTC is bis(trifluoroethyl)dithiocarbaminate.

Page 15: Use of supercritical fluids in inorganic analysis

MOSCOW UNIVERSITY CHEMISTRY BULLETIN Vol. 65 No. 1 2010

USE OF SUPERCRITICAL FLUIDS IN INORGANIC ANALYSIS 15

acids [91], and for natural waters, photolysis is used[92].

The application of solid concentrate is consideredpromising as a matrix after the step of sorption precon�centration of metal ions. Only one publication existson this topic [68]. The selective sorbent for extractionof aluminum ions from aqueous solutions is AmberlyteIRA�400 impregnated by chromazurole S. As thereagent for extraction of aluminum, trifluoroacetylac�etone was chosen, which was added to the sorbent, andthe amount of reagent exceeded the content of alumi�num by 3000 times. Unfortunately, the degree ofextraction was only 48.57%, which confirms the highstability of aluminum complex with chromazurole S.This approach makes it possible to substantiallydecrease the detection limits by the preconcentrationstep. Special attention in this approach should be paidto the choice of reagents both for sorption and forSPE.

Influence of SAS. To increase the degree of extrac�tion, the introduction of nonionogenic SAS Triton X�100 into the system was proposed [80, 87]. SAS wasadded to the sample. In the case of a solid matrix (diat�omite soil), cyclohexane�1�buthanol microemulsionwas added, containing Triton X�100; the sample wasstirred with a magnetic mixer and then dried [87]. Thechelates react with inversed micelles and solubilize(come inside micelles) into SAS, which reduces theinteraction between chelate and matrix, facilitatingdesorption of the complex. Nonpolar groups ofmicelles can dissolve in nonpolar CO2, enhancingchelate extraction. The degree of extraction in thepresence of Triton S�100 SAS increased from 83.6 to96.6% [80] and from 72.69 to 90.52 [87].

Kinetics of extraction. Even metal ions that formsoluble complexes in supercritical CO2 are oftenextracted weakly from aqueous solutions, since themetal ions are hydrophilic and the chelate�formingreagent is hydrophobic. A new mixing device has beenproposed for the introduction of supercritical CO2into the aqueous phase, which increases the contactbetween gaseous and aqueous phases, enhances masstransfer, and increases the rate of chelate formation[71]. The degree of extraction of iron(III) complexwith 2,2,7�trimethyl�3,5�octanedione (reagent isweakly soluble in water) increases from 26 to 79%using this mixing device. The study was performed ofthe influence of the concentration of the reagent insupercritical CO2 on the kinetics of extraction of toxicheavy metals and uranium from acidic solutions con�taining phosphorous�organic reagents; a mathemati�cal model was proposed [84].

Synergetic extraction. In several cases improve�ment in the degree of extraction of lanthanide andactinide β�diketonates of SFE is achieved by syner�getic extraction upon introduction of a second reagentinto the system, neutral Lewis base TBF [50, 63, 65,67, 69, 72] or pyridine [66]. In the absence of a che�late�forming reagent, the degree of extraction of met�

als from the solid matrices in the form of MAn · mTBFcomplexes, where A is the anion of aqueous phase, islow. In studying extraction from 6M HNO3 containing3M LiNO3 by supercritical CO2 with the addition of30% TBF, the extraction of Sm(III), Eu(III), Gd(III),Dy(III) was achieved almost quantitatively (the degreeof extraction is 85.1–91.8%) [63]. It was shown that inthis case, light and heavy lanthanides are extracted inthe form of Ln(NO3)3 · 3TBF and Ln(NO3)3 · 2TBFcompounds, respectively.

The composition of adducts of metal chelatesformed with TBF also depends on the size of lan�thanide ion: throughout SPE, Ln(TTFA)3 · 3TBFcomplexes are formed for Sm(III), Lu(III), Eu(III),Gd(III), and Dy(III) and Ln(TTFA)3 · 2TBF forYb(III) and Lu(III) [63]. In the latter case, lanthanidecontraction and steric hindrances prevent the forma�tion of adducts with a stoichiometry of 1:3 for chelatesof heavy lanthanides.

The degree of extraction of lanthanides depends onthe origin of β�diketone and changes in the followingorder: TTFA~FOD > HFAA > TFAA > AA. The effi�ciency of extraction by fluorinated reagents is higherfor heavy lanthanides [50]. In studying the extractionof ready adducts, the partial destruction of an adductof uranium(VI)trifluoroacetylacetonate with pyridineunder extraction was mentioned [66].

Analytical application. The method for ura�nium(VI) determination in water samples in an aban�doned uranium mine in the northwestern region of theUnited States was developed using synergetic extrac�tion with the mixture (1 : 1) TTFA–TBF (reagentswere added into sample) [89]. The degree of uraniumextraction is 78–80%. The method was applied to ana�lyze samples of the upper layer of soil from northernIdaho; water from the mine was added. In this case,thehighest efficiency (the degree of extraction is 89–91%)was obtained under synergetic extraction by the TFA–TBF (1 : 1) mixture.

A method of spectrophotometric determination ofuranium was developed in the concentration range of40 mlrd–1 to 90 mln–1 after synergetic extraction bysupercritical CO2 containing 5% ethanol, 0.10M TBF,and 0.10M FOD [72].

A method was proposed for determining the atomabsorption of heavy metals in soils after SFE CO2 con�taining the dissolved phosphorous organic reagentCyanex 302 [83].

A method was developed for spectrophotometricdetermination of copper in the form of a chelate with8�hydroxyquinoline after SFE from ethanol solutionsin the presence of Triton X�100 SAS by supercriticalCO2 containing 5% methanol [80]. The graduated plotis linear in the range of 1.905–15.237 μg/ml of copper.

A method was developed for determination, byatomic absorption with an inductively bonded plasma,of chromium(VI) after extraction of bis(trifluoroet�hyl)dithiocarbaminate complex by supercritical CO2

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with a methanol additive [77]. The duration of extrac�tion is 17 min. The degree of extraction is 88.4%. Themethod was applied for analysis of a standard sampleof K2Cr2O7.

A method was developed for determining alumi�num in beverages (tea, beer, cola, orange juice) and tapwater [68]. The content of aluminum in objects is notlarge; therefore, first it was extracted and concentratedon Amberlit IRA�400 oil impregnated by chromazurolS. Aluminum was extracted from oil by SPE using tri�fluoroacetylacetone as the chelate�forming reagent viasupercritical CO2 containing methanol. The extractwas washed with NaOH solution in order to removeexcess chelate�forming reagent and was analyzed byRP HPLC (the mobile phase of methanol) with UVdetection at 296 nm. The duration of extraction is15 min, and the degree of extraction is 48.57%. Thedetection limit is about 0.1 mln–1 (the signal�to�noiseratio is 3). The longest step is concentration: sorptionequilibrium is achieved within 7 days.

The method was developed for determining ura�nium by square�wave inversion voltamperometry inextracts after SFE of cadmium hydroxyquinolinate viasupercritical CO2 modified by methanol, [82]. Theduration of extraction is 35 min. The graduated plot islinear in the range of 0.02–0.16 μg/g. The method isapplied for analysis of samples hair from smokers andnonsmokers (one cigarette contains 1–2 μg of cad�mium). In the hair of smokers, 0.06–0.42 μg of cad�mium was present; in that of nonsmokers, 0.02–0.16 μg.

In the above�mentioned methods, the extractionand determination steps are separated in time. Trials ofSFE and method of determination in real�time condi�tions combinations were conducted.

A method for determining rhodium(III) and palla�dium(II) in the form of chelates with 2,2,6,6�tetrame�thyl�3,5�heptanedione was developed, combiningextraction by SFE and separation by RP HPLC in thegradient mode of elution with spectrophotometricdetection at 325 and 254 nm, respectively [90]. Che�lates are separated during SFE: rhodium chelate isextracted at 60°C and a pressure of 15 MPa, and palla�dium, at a pressure above 40 MPa. To combine the twomethods, chelates were captured on a chuck for solid�phase extraction and then eluted by the mobile phaseinto the column of the chromatograph. The drawbackof method is nonquantitative trapping of chelates onthe chuck: part of the chelate passed through, and theother part sorbed irreversibly and was washed out bymobile phase for HPLC. This approach is consideredpromising; however, optimization of the reagent andthe nature of the sorbent for the solid phase extractionis required.

A method was developed for determining Cd, Cu,and Zn combining in real�time conditions the SFE ofdibuthyldithiocarbaminates by pure supercritical CO2and flame AAS determination [74]. The detectionlimits are 0.13, 2.83, and 0.46 pmol, respectively. The

method is applied on a standard sample of comparisonof freeze�dried liver (the degree of extraction of zinc is~86%).

The generalization of the material obtained by SFCand SFE of metal complexes allows us to see that thistopic has not lost its actuality. Numerous publicationsafter 2000 confirm this. Attention is paid to the factthat SFE is attracting more interest than SFC. Theachievements in SFC of metal chelates by selectivityand efficiency of separation give way to achievementsin HPLC. The ionic pairs were not studied amongmetal chelates. In nonpolar supercritical CO2 the highsolubility and the absence of ionic pairs’ dissociationshould be anticipated. The high solubility of studiedreagents from the class of dithiocarbaminates withquaternary ammonium counterions confirms this.The better results are achieved on capillary columns byselectivity and efficiency of separation. Probably, theyare more prospective for these classes of compounds.

SFE is attracting interest primarily due to its possi�bilities in analyzing samples from the environment,especially solid ones. In this context, two directionsare interesting: the use of additional interaction, solu�bilization of chelate by SAS, which leads to a substan�tial increase in the degree of extraction and the use ofa solid concentrate after sorption concentration as thematrix, which allows analysis of real diluted solutionsof samples. The success of the latter approach lies inproperly choosing the functional group of sorbent andchelating reagent in SFE. Since the classes of chelat�ing reagents applied in SFC and SFE are the same, thedevelopment of device with the combination of thesemethod in real�time conditions and the developmentof methods of multielement determination are prom�ising, including group concentration by SFE and sep�aration by SFE (or HPLC).

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