Microchim Acta 153, 45–49 (2006)

DOI 10.1007/s00604-005-0431-7

Original Paper

Determination of Arsenic in Dolomites with a SimpleField Spectrometric Device

Katarzyna Stec1, Andrzej Bobrowski2;�, Kurt Kalcher3, Helmut Moderegger3,

and Walter Goessler3

1 Institute of Refractory Materials, Toszecka 99, PL-44-100 Gliwice, Poland2 Department of Building Materials Technology, Faculty of Materials Science and Ceramics,

AGH – University of Science and Technology, Al. Mickiewicza 30, PL-30-059 Krakow, Poland3 Institute of Chemistry, Analytical Chemistry, Karl-Franzens University, Universit€aatsplatz 1, A-8010 Graz, Austria

Received June 13, 2005; accepted September 13, 2005; published online November 7, 2005

# Springer-Verlag 2005

Abstract. A simple and an inexpensive procedure for

the determination of total arsenic in dolomites is pro-

posed. The method applies the spectrometric field de-

vice Supralab SD which was initially designed for the

determination of arsenic in drinking water. The method

relies on the formation of the volatile AsH3 and its

reaction with mercuric bromide, immobilized in a

membrane, to yield a yellowish-brown reaction prod-

uct, which is spectrophotometrically detected. The

dolomite samples were rapidly dissolved in hot hydro-

chloric acid in an open vessel and then were analyzed

with the portable instrument. The 3 s detection limit of

the developed method was 0.1mg L�1 as in the solution

containing dissolved dolomites. The time of As deter-

mination in the solution did not exceed 5 min.

To validate the results obtained with the field de-

vice, hydrogen generation inductively coupled plasma

optical emission spectrometry (HG-ICP-OES) and

inductively coupled plasma mass spectrometry (ICP-

MS) were used as reference methods using micro-

wave-assisted mineralization. Excellent agreement

between the methods was obtained.

Key words: Arsenic; dolomite samples; hydride generation; spec-

trophotometry; ICP-OES; ICP-MS; field device; SupraLab.

It has been found long ago that mineral salts, contain-

ing proper concentrations of Ca and Mg, consumed

with the human diet are very important agents playing

an essential role in the regulation of many biological

processes and affecting human health. Dolomite used

as an additive to food and to some medical formula-

tions is the main source for these compounds. How-

ever, there exist restrictions concerning the admissible

levels of the concentration of toxic elements present

in dolomite applied in pharmaceutical, food and fer-

tilizers industries. One of minor and trace compo-

nents, which may be very toxic, is arsenic. The human

organism can be seriously poisoned after consumption

of 100 mg dose of inorganic arsenic [1]. On the other

side arsenic in very low concentration is considered as

a microelement which can stimulate the human body

with respect to its basic metabolism. The Polish stan-

dard allows up to 0.0005% maximum concentration of

this element in the mineral dolomite when added to

food and pharmaceuticals [2]. For the above reasons

the precise analytical control of the arsenic concentra-

tion in dolomites which are used in the mentioned

industrial productions as well as in the production of� Author for correspondence. E-mail: gcbobrow@cyf-kr.edu.pl

high purity optical glass is very important. To deter-

mine low concentrations of arsenic in minerals mainly

the spectrometric methods are used, such as electro-

thermal atomic absorption spectrometry (ET-AAS),

inductively coupled plasma optical emission spectrom-

etry (ICP-OES) and inductively coupled plasma mass

spectrometry (ICP-MS). However, determination of

arsenic traces in dolomites by means of ET-AAS or

of ICP-OES may be troublesome on the account of

matrix effects, caused by the presence of high concen-

tration of Ca and Mg in the tested dolomite samples,

or by the presence of chloride. The application of

hydride generation (HG) pre-separation combined

with spectrometric techniques has proved to be a pow-

erful analytical tool for the determination of arsenic

and other elements able to form volatile hydrides,

which allows to avoid matrix effects, because Ca

and Mg do not form volatile hydride compounds.

The field device, the SupraLab SD, has been re-

cently designed to determine the concentration of ar-

senic and selenium in water samples [3, 4]. The

principal design of this analytical method is based

on the conversion of inorganic arsenic (arsenite and

arsenate) to arsine by reduction using tetrahydrobo-

rate in hydrochloric acid and its ensuing release from

the sample solution in a gas stream, which undergoes

a chemical reaction within the detector cell (Gutzeit

reaction) [5]. The yellowish-brown reaction product,

formed from arsine and mercury bromide and prob-

ably consisting of mixtures of As(HgBr)nH3-n, is spec-

trophotometrically monitored.

The main goal of the paper is to apply the above

field spectrophotometric device coupled with hydride

generation [3, 4] to the rapid determination of the ar-

senic content in various high purity dolomite samples

and to validate the results with ICP-MS and ICP-OES.

Experimental

Methods and Instrumentation

HG-Spectrophotometric Method with Supralab SD Device

All spectrophotometric measurements were carried out using the

Supralab SD (HELMOTEC. Kalsdorf, Austria [3] www.supra-lab.

com). An Erlenmayer flask (100 cm3) is connected with the instru-

ment and used for the generation of the hydride. The sodium tetra-

hydroborate reagent is added in form of tablets to the sample

solution (tablet HyGen), which facilitates easy and simple operation

in the field. The core of the device is a filter, impregnated with

mercuric bromide. The impregnated paper, the LED (light emitting

diode) and the photodiode are arranged linearly in a measuring cell,

which is mounted with its inlet directly onto the Erlenmayer flask,

where hydride generation is performed. The liberated arsine enters

the cell and penetrates the impregnated filter reacting with the mer-

curic bromide and forming a yellowish-brown reaction product,

which is monitored with an LED and a photodiode in transmittance

mode. The apparatus was connected with a personal computer via

an RS232 interface in case that monitoring of data is needed.

HG-ICP-OES and ICP-MS

HG-ICP-OES determinations were done with an ICP optical emis-

sion spectrometer (JOBIN YVON JY 36, www.jyemission.com) in

combination with a hydride cassette [6], the corresponding mass

spectrometric determination with an ICP-MS (Hewlett Packard

4500, www.chem.agilent.com), employing standard procedures.

For the reference determinations with atomic spectrometry the

dolomites were mineralized with a microwave digestion system

(MLS-1200 MEGA, Milestone, www.spectro-lab.com.pl).

Reagents and Materials

Nitric acid, hydrochloric acid, sodium hydroxide and standard ar-

senic solution (1 g dm�3), all of analytical reagent grade, were pur-

chased from Merck.

Sodium tetrahydroborate tablets (HyGen) and the impregnated

membranes (InMem, ReMem) were obtained from Helmotec.

Deionised water was purified with a cartridge system from

NANOpure BARNSTEAD.

Dolomite samples

The dolomites were of Polish origin. In lithological respect they

were homogeneous, white or yellow without any visible deposits or

contaminations. The samples were washed and ground prior to

analysis. The finely-ground dolomite samples with a grain size less

than 63mm were investigated. The composition of the dolomite

samples comprising main and trace elements was given in [7].

Digestion of the Dolomite Samples. Three different procedures of

the dolomite decomposition were elaborated. The digestion proce-

dures were selected depending on the analytical techniques used for

As determination. In the elaborated spectrophotometric method a

simple procedure was used for the digestion of a small amount

(0.01 g) of dolomite in hydrochloric acid, which made quick sample

decomposition in field conditions possible. The HCl present in the

analysed solution provides the most suitable condition for the

generation of arsine. In case of ICP AES and ICP MS procedures

0.5 g of sample was digested because the analyte was then used not

only for the determination of As but also for the quantification of

main and trace elements present in the dolomite [7]. For the multi-

elements determination of the trace elements (including As) by

means of the ICP MS technique the dolomite samples were digested

in nitric acid, which, does not, as is the case with other mineral

acids, interfere in the determination of other elements. In the ICP

AES method the mixture of the acids (HCl and HNO3) was chosen

so as to facilitate and reduce the time needed for the digestion

process. The obtained solutions were strongly acidified no matter

which digestion procedure was used.

For determination with Supralab SD a simple open acid digestion

procedure was applied. Aliquots of 0.01 g to 0.1 g, were weighed

into 50 cm3 beakers and moistened with a few cm3 of deionised

water. Hydrochloric acid (conc., 2 cm3) was added and the mixture

was quickly heated for 2 minutes to dissolve the samples. Then, a

further aliquot of 1 cm3 HCl was introduced. In case of the five

investigated dolomite samples containing less than 2.5% of SiO2,

46 K. Stec et al.

after digestion no precipitate was observed in the solutions. Only for

the ‘‘Niwnice’’ sample, containing ca. 18% SiO2, the digestion pro-

cess was not complete. After cooling, solutions were diluted with

deionised water to 50 cm3 in a volumetric flask. Solutions were

determined directly with the Supralab SD analyser.

For HG-ICP-OES purpose the samples were decomposed in the

microwave multi-digestion system. Subsamples of 0.5 g plus 5 cm3

HCl and 1 cm3 HNO3 were added to the digestion vessels, which

were placed in the carousel of MW oven and heated accordingly.

After digestion, solutions were transferred to the 100 cm3 volumetric

flask and diluted with deionised water. The total digestion time was

3 hrs. The line at 193,759 nm was used for the measurement.

For ICP-MS determination of As, the dolomite samples were also

digested in the microwave multi-digestion system in nitrogen atmo-

sphere. 0.5 g of subsamples and 5 cm3 HNO3 were placed in vessels

of MW system and decomposed. Total digestion time was equal to

3 hrs. Before ICP MS determination the solution were 1000 times

diluted with deionised water. Mass 75 was monitored with the

instrument.

Analytical Procedure for the As Determination with Supralab SD.

The impregnated filter paper (InMem) is inserted into the detector

cell between LED and photodiode. In order to absorb hydrogen

sulphide and selenide from the sample solution a second membrane

impregnated with lead acetate (ReMem) is placed in front of the

reactive membrane in the gas stream. A defined amount of the

solution of dissolved dolomite (50 cm3) is transferred to the reaction

vessel (Erlenmayer flask equipped with a Teflon-coated stirring bar).

A tablet of sodium borohydride (HyGen) is added to the sample

solution. Then the connector of the cell was set immediately onto

the flask. At the same time recording of the response curve was

started. The sample output curve was registered on a personal com-

puter with the corresponding software. The reaction (generation of

arsenic hydride) was in all cases finished after three minutes.

In order to check sorption of the arsine, lead impregnated filters

(commercially available ReMem) and cottons were tested for their

effect on the hydride. It was found that practically no notable

amount of the generated arsine is retained in both cases. Optical

inspection of the filters (InMem) by eye after reaction did not show

any inhomogeneities of the colour due to non-uniform impregnation.

In order to avoid analytical errors and artefacts it is necessary that

the measuring cell should be mounted immediately on the flask after

adding the sodium borohydride tablet. Delay may cause loss of ar-

sine gas and may produce erroneous results. Also, the dissolution of

the samples must be performed carefully in order to avoid losses by

spraying.

The evaluation of the concentration of arsenic in the sample

solution was done by the internal calibration curve of the instrument

supplied by the producer.

Results and Discussion

Optimization of Spectrophotometric Method

Employing Supralab DF Instrumentation

For the spectrophotometric method for the determina-

tion of arsenic in dolomite the sample solution should

not contain significantly more than 10mg dm�3 As

in order to be in the optimum working range of the

instrument.

For the determination of As in dolomite samples

the standard configuration of the instrument was

employed. The light source was the integrated photo

diode with an emission maximum of 450 nm. As reac-

tion matrix the commercially available membranes

impregnated with mercuric bromide were used

(Gutzeit reaction with arsine); after the use usually a

yellowish-reddish colour could be observed.

The instrument displays the concentration of

arsenic in the water sample already, but nevertheless

a calibration curve was established with standard solu-

tions of arsenic acidified in the same way as the dolo-

mite samples (Fig. 1). The signal was evaluated as the

difference of the initial intensity of light and the low-

est value of the measured intensity (given as bits) after

the reaction, obtained in all cases in less than three

minutes. The calibration curve corresponded in fact

Fig. 1. Calibration curve as obtained with the portable device

SupraLab; difference (i0� i) of the signal (i) and the signal ob-

tained before reaction (i0, 1000 bits), given in bits

Fig. 2. Dependence of the signal on the concentration of arsenic

with the signal calculated as absorption

Determination of As in Dolomites with a Simple Field SD 47

very well to the displayed values of arsenic concen-

trations as evaluated by the instrument’s internal cali-

bration curve, not deviating more than two percent

(mean of three determinations) over the whole range.

The signal shown in Fig. 1 corresponds to the inten-

sity decrease of light due to the formation a coloured

reaction product (Gutzeit reaction). The initial inten-

sity is set automatically to 1000 (bits); this transfor-

mation of the shown signals into absorption units is

possible by forming the corresponding logarithmic

ratio (Fig. 2). Such calibration graphs relating absor-

bance to As concentration show linearity up to around

10mg dm�3 As in the solution. The detection limit (3 s)

was found 0.07mg dm�3 arsenic in the investigated

solution, corresponding well with 0.1 ppb indicated

by the producer. The limit of quantification (10 s)

was determined as 0.23mg dm�3. The reproducibility

of the determination with the same dolomite sample

solution was typically 1% RSD, the repeatability with

the same sample, but dissolved individually ranged

from 1 to 10% RSD (3 determinations).

The analysis time was around six minutes including

dissolution and measurement. The total time for the

whole procedure, including weighing, was less than

ten minutes.

Severe interferences may arise from antimony if it

is present in the sample in a tenfold excess or more

(mass-to-mass ratio). Sulphide, selenite, tellurite do

not interfere even in a hundred fold excess because

they are absorbed from the gas stream by lead acetate-

impregnated membrane.

The comparison of the results of the As determina-

tion in various dolomite samples obtained by the three

methods is presented in Table 1. The results for samples

1 to 4 agree very well with all three methods. Sample 5

deviates 5 percents from the ICP-MS result which

represents still an acceptable result. Higher deviations

were observed with the sample 6 between plasma

spectroscopic and photometric determinations (19%),

but in this case obviously the mere dissolution of the

dolomite in hydrochloric acid seems to be inferior to

the micro-wave assisted digestion over three hours.

Arsenic seems to be included in some silicate residue,

and is therefore not accessible for the determination.

Nevertheless, for a quick field method with a portable

instrument, also this result is still acceptable.

Conclusions

A rapid and simple method for the determination of

arsenic in dolomite samples was developed, using a

portable spectrometric device. The results usually agree

very well with atomic optical and mass spectrometric

methods using microwave assisted digestion, supposed

that arsenic is not retained in silicate residues.

All the investigated methods possess the sufficient

sensitivity and similar precision, however, the cost of

the applied instrumentation is considerable different.

The method of the As determination in dolomite

samples based on coupling the hydride generation

technique with spectrophotometric detection in the

field device ensures to obtain a low LOD (0.1mg dm�3

As in the solution, [3]), wide linearity range, lack of

interferences from the Ca and Mg matrix elements,

good precision and accuracy. The analytical procedure

enables to perform field analysis after the simple and

quick digestion of the dolomites samples in hydro-

chloric acid what can also be easy executed in field

conditions.

When comparing the ability of the three investi-

gated methods of the As determination in dolomites

it can concluded that the spectrophotometric method

with Supralab field device could be a method of

choice due to its sufficient sensitivity, simplicity, high

speed of the analysis and low cost of apparatus. It may

be concluded that new method using the device Supra-

lab SD has some unique advantages in comparison

with other methods and is well suitable for trace deter-

minations of arsenic in dolomites used for pharmaceu-

tical purpose.

Table 1. Arsenic concentration (mean � s) in dolomite samples determined by means of the three spectrometric methods1

Sample

number

Source2 Supralab FD

[mg=kg]

ICP-AES

[mg=kg]

ICP-MS

[mg=kg]

1 Oldrzychowice 0.59 � 0.02 0.59 � 0.02 0.58 � 0.02

2 Niwice 30.26 � 0.25 30.12 � 0.27 29.90 � 0.37

3 Walisz�oow 10.3 � 0.6 10.0 � 0.5 10.0 � 0.2

4 Redziny I 0.51 � 0.03 0.51 � 0.02 0.55 � 0.03

5 Pogorzyce I 8.09 � 0.08 8.09 � 0.08 8.50 � 0.43

6 Pogorzyce II 5.72 � 0.52 5.7 � 0.5 6.80 � 0.14

1 The mean of 9 determinations (3 measurements of each of the 3 parallelly digested dolomite sample).2 Source indicates the sampling place in Poland.

48 K. Stec et al.

Acknowledgement. Financial support from the AGH-University of

Science and Technology (Project no. 11.11.160.698) and from the

CEEPUS project Pl-110 is kindly acknowledged.

References

[1] Bowen H J M (1966) Trace elements in biochemistry.

Academic Press, London-New York

[2] Polish standard ZN-86=MO-126, IMO Gliwice

[3] SupraLab SD Instruction Manual, HELMOTEC 2002;

http:==www.supra-lab.com

[4] Omanovic E, Moderreger H, Kalcher K (2002) Anal Lett 35:

943

[5] Buckett J, Duffield W D, Milton R F (1955) Analyst 80:

141

[6] Stec K, Bobrowski A (1999) Ceramics – building materials (in

Polish) 3: 81

[7] Stec K (2003) PhD Thesis, AGH-University of Science and

Technology, Krak�oow

Determination of As in Dolomites with a Simple Field SD 49