AnalyticalMethods

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Electrothermal m

Departamento de Quımica Inorganica, A

Facultad de Ciencias Exactas y Naturales

Universitaria Pabellon 2, AR1428EHA Bue

fcen.uba.ar; tudino@qi.fcen.uba.ar

† Electronic supplementary informa10.1039/c6ay03331h

Cite this: Anal. Methods, 2017, 9, 756

Received 11th December 2016Accepted 5th January 2017

DOI: 10.1039/c6ay03331h

www.rsc.org/methods

756 | Anal. Methods, 2017, 9, 756–760

etallic furnace atomic absorptionspectrometry†

E. Morzan, G. Carrone, M. Tudino* and R. Etchenique*

We present an atomization system for atomic absorption spectrometry comprised of a stainless steel furnace

heated by the Joule effect by means of its intrinsic resistance. This new kind of furnace does not require any

gases during operation. Samples are introduced with an independently controlled thermospray injector. The

device outperforms conventional FAAS (Flame Atomic Absorption Spectrometry) for many analytes,

providing a very safe, compact and inexpensive alternative for many analytical determinations. Full

characterization of the system is presented, and theoretical simulations are contrasted with experimental

data.

Introduction

Atomic Absorption Spectrometry (AAS) has been one of the mostwidespread techniques for the determination of trace elementssince Alan Walsh introduced Flame Atomic Absorption Spec-trometry (FAAS) in 1955.1

Since then, many changes of the original idea have beeninvestigated and several new versions of AAS have beenproposed with diverse success.2,3

The use of a ame in FAAS implies the acceptation of twodrawbacks: a complex gas management system able to ensurea safe operation under all conditions, and a short residencetime of samples in the optical path,4 given the fact that theame is an open environment.

Electrothermal atomizers,5–8 mainly graphite furnaces,9–12 ortungsten coils13–16 to a lower extent, became an excellent alter-native to overcome the problems depicted above. Nonetheless,for this alternative regardless the employment of standardizedtemperature platform furnaces, transverse heating or matrixmodication,17,18 a high purity argon gas ow for the protectionof the heated cell is mandatory.

Another strategy to conne samples to the optical path hasbeen proposed recently by Berndt19 and rened by severalgroups.20–25 This arrangement, known as thermospray amefurnace atomic absorption spectrometry (TS-FFAAS), allows theintroduction of the totality of a liquid sample into a tube (orfurnace) mounted over a combustion ame in the optical pathof an atomic absorption spectrometer. The sample is injectedvia a peristaltic pump and transported to the ame furnace (FF)

nalıtica y Quımica Fısica, INQUIMAE,

, Universidad de Buenos Aires, Ciudad

nos Aires, Argentina. E-mail: rober@qi.

tion (ESI) available. See DOI:

through a ceramic capillary directly heated by the ame,producing the so called “thermospray” (TS). The combination ofthe whole sample introduction and the connement of theatomized sample has proven to optimize sensitivity for volatileelements when compared to FAAS with no need for majorchanges in the basic instrument.

Inspired by FFAAS and ETAAS, we present the electrothermalmetal furnace atomic absorption spectrometry (EMFAAS)approach that takes advantage of both techniques. EMFAASemploys a stainless steel alloy tube as the furnace. This tubewith relatively high electric resistivity is very well suited to beheated by conducting a high current through its body.

Thus, the high temperature of the furnace is attained with noneed for burning or protective gases. The total liquid sampleintroduction is performed with the assistance of a peristalticpump and a ceramic thermospray injector which is heatedelectrically and independent of the furnace heating.

In this way, EMFAAS combines electrical heating of both theFF and the TS injector, keeping away from pressurized gases foreither combustion, protection or typical pneumatic nebulisationof liquid samples.

Additionally, EMFAAS offers the simplicity and robustness ofFAAS together with increased sensitivity for volatile elements asit will be shown here as a proof of principle.

ExperimentalMaterials and methods

All solutions were prepared with analytical grade chemicalreagents and double deionized water (DDW) obtained froma Milli-Q purication system (Millipore, Bedford, MA, USA). Allglassware was washed with EXTRAN (Merck) 1% v/v and kept in10% (v/v) HCl with further cleaning with DDW. Standard solu-tions of all the analytes were prepared by appropriate dilution of1000 g L�1 stock solutions (Merck Darmstadt, Germany). A

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digital camera Samsung NX 1100 was used to take all thetemperature images. The heating of the cell was performed bymeans of a commercial 700 W microwave oven transformer, inwhich the original secondary wiring was replaced by a two-turncoil of a 7 mm diameter copper multi-strand cable.

An atomic absorption spectrometer Shimadzu AA6800 (Shi-madzu, Kyoto, Japan), hollow cathode lamps (Hamamatsu,Japan) and a deuterium lamp for background correction wereemployed throughout the measurements. Other instrumentalconditions were those provided by the manufacturer.

The EMFAAS system was assembled with a peristaltic pumpwith eight channels and six rollers (IPC, Ismatec, Glattbrugg-Zurich, Switzerland), a six-port rotatory valve VICI (ValcoInstruments, Houston, TX, USA), 0.5 mm i.d. PTFE® tubing,a ceramic capillary (0.5 mm i.d., 6 cm length) and the metallicame furnace atomizers placed in the optical path of thespectrometer with the assistance of a homemade holder.

In a typical experimental procedure, 500 mL of standardsolution was introduced into a carrier stream (DIW) and injec-ted into the atomization cell at a ow rate of 1.1 mL min�1. Anichrome coil wound around the ceramic injection capillarywas used to heat the solution above the vaporization tempera-ture prior to entering the cell. The heating was controlled bychanging the applied voltage to the coil between 10 and 24 V toensure full vaporization of the sample at different ow rates.

Fig. 1 Scheme of the electrothermal atomization cell of the EMFAAS.See the text for details.

Temperature measurements

A commercial digital camera was used to obtain the images ofthe cell during atomization heating. To overcome the intrinsicnon-linearity of the pixel values in images compressed to JPGwith respect to the light intensity and to the temperature,a two-step calibration process was followed. Image analysis wasdone using public access ImageJ soware.26

First, images of the heated cell were taken with constantdiaphragm (f/8) and ISO 100 settings, varying the exposuretime from 1 to 1/30 seconds. As the photon ux is linearlydependent on the exposure time, a calibration of pixel value vs.red or green intensity can be easily obtained. A polynomialexpression is useful to perform this calibration. The results aregiven as ESI.†

As a second step, a relationship between the light intensity ofa given colour and the temperature must be found. Although inprinciple it is possible to use the theoretical dependence forblack body emission, better results can be obtained throughempirical calibration of either red or green intensity vs.temperature, or ratiometrically from the relationship betweenred and green values. In brief, the logarithm of the emissionintensity is rather linear with the temperature. The details aregiven as ESI.† Both red and green can be used to obtaintemperatures within 5% uncertainty. Ratiometric measure-ments yield similar results, but implies the use of perfectlysteady images because the amount of red and green emission isstrongly dependent on the temperature and thus severaldifferent exposures must be done for a given situation. For theimages presented in this work the green channel measurementwas preferred.

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Results and discussion

Fig. 1 shows the general diagram of the Electrothermal MetalFurnace Atomic Absorption Spectrometer (EMFAAS) depictingits main building blocks. The heart of the system consists ofa 10 mm inner diameter cylindrical tube (A) made of a 304Lstainless steel27 alloy which is supported by two highly conduc-tive copper pillars (B and C) connected to a controlled lowvoltage–high current source (D). A ow of water through a pair ofholes in each pillar ensures that they remain at near-ambienttemperature to prevent the heat for reaching the power supplycables (E). The pillars are separated using polycarbonate isola-tors from the aluminium base. In front of the cylinder, a ceramiccapillary (F) is introduced by about 1–2 mm into the atomizertube. This capillary is heated from the exterior by means ofa nichrome wire resistance coil (G) connected to a regulatedvoltage power supply (H). The sample (I) is fed into the capillaryby means of a peristaltic pump (J).

Stainless steel (304L) presents a high electric resistivity of0.72 � 10�6 Um at 20 �C and 1.16 � 10�6 Um at 660 �C around60 times higher than that of copper. Together with its highmechanical, thermal and chemical resistance, this alloyconstitutes a superb option for an electrothermal atomizationcell. To achieve this goal, a 700 W AC transformer capable ofgenerating around 1 volt at several hundred amperes was con-nected to the tube ends through the cooled copper pillars. Thecontrol of the overall power delivered to the cylindrical cell canbe performed using a linear inductive autotransformer (variac)or changing the duty cycle of the primary coil of the maintransformer (220 VCA) using a phase controlled TRIAC(dimmer). The latter allows the possibility of temperaturecontrol using a calibrated K-type thermocouple and a thermo-stat for controlling the primary current. The thermal inertia ofthe heated cell guarantees that the 50 Hz cycle variation of thetemperature at the cell walls is completely negligible. A similar

Anal. Methods, 2017, 9, 756–760 | 757

Fig. 2 (a) Furnace cell. (b) False colour thermogram of the cellshowing the temperature field. The temperature scale is depicted onthe right. (c) The colour-coded temperature field calculated witha finite difference software.

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phase controlled dimmer was used to control the injectorheating.

For an 5 cm long furnace with both ends attached to refrig-erated copper pillars through ns of about 2 cm long each, thetotal resistance was RL ¼ 2.87 mU. In a typical experimenta current i ¼ 320 A circulates through the furnace, raising itscentral temperature to about 1000 �C. The voltage drop at thetube is VL ¼ 0.92 V. This low voltage implies that no hazard dueto electric shocks is present in the equipment. At the involvedhigh currents other voltage drops are important, such as thecontact resistance between the pillars and the furnace (225 mVeach contact) and on the cable itself (160 mV). Therefore, about60% of the total power of 505 W is dissipated along the furnace,

Fig. 3 Signal change for several analytes as a function of the injectionflow rate. Green ¼ Cd (100 mg L�1). Grey ¼ Zn (500 mg L�1). Red ¼ Cu(500 mg L�1). Orange ¼ Au (100 mg L�1). T ¼ 910�.

758 | Anal. Methods, 2017, 9, 756–760

increasing its temperature. This gure could be easily enhancedby means of gold-plated contacts.

Fig. 2 shows the cell geometry and its inset shows the ob-tained temperature prole as a false color picture taken witha digital RGB camera on the external wall of the atomizationcell. The simulation performed with a nite difference algo-rithm (solved by a numerical procedure using FlexPde 5.0soware, see the ESI†) is shown with the same colour scaleshowing a good correspondence even for the simple modelused.

Fig. 4 (a) Raw RGB picture of the EMFAAS-cell at i ¼ 320 A during aninjection flow of 1.2 mL min�1. (b–e) False color temperature fields ofthe EMFAAS-cell at 0, 0.6, 1.0 and 1.2 mL min�1 injection flows.

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Table 1 Figures of merit obtained by EMFAAS for several analytesunder the conditions described in the text (for EMFAAS: injectionvolume ¼ 500 mL, flow rate ¼ 1.1 mL min�1, and T ¼ 910 �C). Limits ofdetection are compared with those obtained by FAAS

Element

LOD (mg L�1) LR (mg L�1)%RSD (n ¼ 5)

FAAS EMFAAS EMFAAS EMFAAS

Ag 30 15 50 1000 3.92Au 40 29 96 1000 4.17Cd 36 2 7 750 2.5Cu 12 10 33 1000 3.43K 26 24 79 2000 1.42Pb 226 84 277 3000 2.58Mn 90 87 287 3000 3.23Zn 23 3 10 500 3.45

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One of the typical characteristics of thermospray injection isthe temperature drop that occurs at the position of the injectordue to the vaporization heat of the sample. These temperaturevariations are responsible for the changes of sensitivity atdifferent injection ows. Fig. 3 shows the signals obtained forseveral analytes at different injection ow rates. The responsepresents maxima around 1.1 mL min�1, indicating that thecooling effect of the injection at higher ows can overcome theincrement of the analyte concentration in the atomization cell.

In order to determine the magnitude of the cooling,temperature images were taken at different injection ows. Fig. 4shows the results. The top image corresponds to the raw pictureof the cell at the maximum injection ow (1.2 mL min�1). Thefalse color pictures are the thermal images, from 0 (top) to1.2 mL min�1 (bottom), showing that this maximum owcorresponds to a drop of about 90 �C in the coldest region.

As shown in Table 1, the dynamic linear range is about twoorders of concentration in most cases, and the repeatabilityexpressed as RSD% is less than 5% (n ¼ 5). These gures ofmerit are comparable with those obtained by FAAS. Neverthe-less, the LOD is up to 20 times lower than that of traditionalFAAS for Cd. It is expected that even lower LODs can be obtainedby raising the tube temperature, and/or extending the celllength. High temperature alloys like Kanthal® or metal–ceramic composites can help in increasing the maximumoperation temperature.

Conclusions

We have devised a new kind of electrothermal atomic absorp-tion spectrometer which allows robust and sensitive measure-ment of several elements with improved gures of merit withrespect to FAAS with a complete lack of any of the problems andnuisances due to the use of gases. The atomization is performedthrough a Joule effect self heating cell tube that can resisttemperatures up to 1000 �C, enough to get good sensitivity forseveral analytes. The employment of cells of new metal–ceramiccomposite materials capable of working at 1600–1800 �C couldbroaden the analytical capabilities to a larger number ofelements. The injection of samples through a heated capillary

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(thermospray injection) makes this approach fully compatiblewith ow systems. The independently electrically heatedceramic thermospray injector introduces a new variable tooptimize the operation at different throughput rates. Thecompactness of EMFAAS, lacking any gas management whichensures safe operative conditions, makes this kind of atomicspectrometry ideal for portable and in-eld measurements.

Acknowledgements

This research was supported by the National Agency for Scienceand Technology Promotion, CONICET, and University of Bue-nos Aires. RE and MT are members of the CONICET.

Notes and references

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