Electrothermal Atomic Absorption Spectrometric Determination of Ultratrace Amounts of Tellurium Using a Palladium-coated L'vov Platform After Separation and Concentration by Hydride Generation and Liquid Anion Exchange

M A R C 0 G R O T T I A N D AMBROGIO MAZZUCOTELLI

Zstituto di Chimica Generale, Universita di Genova, Viale Benedetto X V , 3-1 61 32-Genova, Italy

An ultrasensitive method is described for the measurement of nanogram amounts of tellurium in the presence of interferents. Tellurium is separated from the matrix by hydride generation, concentrated on a liquid anion exchanger (liquid Amberlite LA-2) and atomized from a platform, previously treated with palladium. Analytical performances are reported and the possibility of application of the method to the analysis of complex samples (sea water, sediments) is proved.

Keywords: Tellurium; electrothermal atomic absorption spectrometry; hydride generation; liquid anion exchanger; L'vov platform; coating; preconcentration; hydrogen telluride trapping

Tellurium is widely employed in the electronic and metallurgi- cal industries. In geochemistry, tellurium is used as a 'path- finder' or indicator of a certain type of deposit. Tellurium compounds are toxic and affect various organs, as does sel- enium. Although tellurium has not caused any social problem, because of its low abundance in the environment, it is generally accepted as moderately toxic to plants and highly toxic to mammals, even at trace levels.',2 The Italian Law No. 915/82 has included tellurium in the number of the elements that must be determined to classify a waste as 'toxic and harmful'.

The determination of tellurium in real samples is complicated by its very low concentration and by matrix interference

A widely employed technique for the determination of this element is hydride generation atomic absorption spec- trometry (HG-AAS), because of its relatively high sensitivity, precision and reasonable selectivity. However, this method is not sufficiently sensitive to determine very low concentrations of tellurium accurately and usually preconcentration processes are needed.

The enrichment of hydrogen telluride in a cryogenic trap, was attempted by Kobayashi et al.' but without success, probably because of the instability of this hydride. In situ concentration of the volatile hydrides within the pre-heated graphite furnace and subsequent atomization was first pro- posed by Lee6 for the collection of bismuthine, and sub- sequently applied by Andreae7 and by Yoon et a1.* for the determination of tellurium in environmental samples. An improvement in sensitivity and precision has been reported by Doidge et aL9 and Zhang et al.," by employing a palladium- coated graphite furnace. Ni et al." also studied the trapping capability of a silver-coated graphite tube. Hydrogen telluride has also been collected by bubbling it through a potassium iodide-iodine solution," but this method is limited by inter- ferences, from copper, mercury, silver and selenium, that must be removed prior to HG.

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Finally, good results have been reported by Tsalev and Mandj~kov, '~ by trapping the hydrogen telluride in a cerium( 1V)-iodide absorbing solution.

In the present work a new method, based on the generation of hydrogen telluride, its trapping on a liquid anion exchanger (Amberlite LA-2) and analysis by electrothermal AAS (ETA AS), is presented. Analytical performances are reported and the possibility of application of the method to the analysis of complex matrices (sea-water, sediments) is proved.

EXPERIMENTAL

Apparatus

Hydride generation was accomplished in a Varian VGA-76; the black fluoroelastomer tubing emerging from the gas-liquid separator was dipped through a glass pipette into the trapping solution and held in a graduated test-tube. Helium was used as the inert gas.

A Varian SpectrAA 300 atomic absorption spectrometer, equipped with a Zeeman graphite tube atomizer was used. Pyrolitic graphite coated graphite tubes with forked pyrolytic platforms were used. The operating parameters were: lamp current, 10 mA; wavelength, 214.3 nm; bandpass, 0.2 nm; and measurement mode, integrated absorbance (QA).

Reagents

All the reagents were of reagent grade quality: Tellurium standard solution, 1000 ppm. A Spectrosol solution

from Merck. Working standards were prepared daily by serial dilutions with Milli-Q water.

Copper, selenium, mercury, silver, titanium and silicon, 1000 ppm and iron, calcium, magnesium, sodium, potassium and aluminium standard solutions, 10 OOO ppm. Spectrosol solutions from Merck. The required concentration of each element was obtained by diluting the respective standard solution with Milli-Q water.

Calcium carbonate, 99.995%. From Aldrich. NaBH, solution, 5% m/v. Prepared by dissolving 5.0 g of

NaBH, (analytical-reagent grade, Aldrich) and 2.0 g of NaOH pellets (Carlo Erba pure reagent) in 100 mi of Milli-Q water.

Liquid anion-exchange solution. Prepared by adding 10 ml of Amberlite LA-2 (Merck) to 5 ml of 6 ml 1-' HC1, stirring continuously and diluting with 10 ml of isobutyl methyl ketone (IBMK). Finally, the organic phase was treated with 2 moll-' NH,C1 and 2 moll-' NH, to pH 7.

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Palladium solution, 100 ppm. Obtained from a palladium standard solution, 1000 ppm (Aldrich).

Procedure

The sample, in 100 ml of 6 mol I-' HCl, was boiled for 45 min, and then delivered to the system for HG. The hydrogen telluride was collected into 1.8ml of the trapping solution (0.5 ml of aqueous phase and 1.3 ml of organic phase), held in a graduate test-tube. During the generation time, part of the trapping solution evaporated and the final volume of the organic phase was z 1.0 ml. The concentrate was centrifuged at 2000 rev min-l, for 10 min. Finally, 10 pl of the organic phase were injected onto the palladium-treated platform and atomized according to the temperature programme reported in Table 1.

Sample Preparation

A 0.2g amount of a fine powdered Reference Sediment Material, [MESS- 1, National Research Council Canada (NRCC)] were transferred into the Teflon vessel of the pressure 'bomb' (Perkin Elmer, Italy) and 1 ml of a 100ngml-' tellurium standard solution was added. Then, 4ml of aqua regia (hydrochloric acid-nitric acid, 3 + 1) were added and the 'bomb' was heated at 150°C for about 20 h. After cooling and centrifugation, the solution was transferred into a 100 ml graduated flask and diluted with 6mol1-1 HC1. 100ml of a Nearshore Seawater Reference Material (CASS-2, NRCC) were transferred into a 200ml graduated flask and 0.5 or 1 ml of a 100 ng ml -' tellurium standard solution was added.

RESULTS AND DISCUSSION

Reduction of Tew to TeN

Since only Te" forms the hydrogen telluride, it is necessary to reduce Tev' to Te", prior to HG. In agreement with several author^,'^*'^,'^ this was carried out by boiling aqueous solu- tions of tellurium in 6mol1-' HC1 for 45min. Also, the selectivity of HG for TeIV allows the speciation of TeIV and ~ ~ v J . 7 ~ 8

Table 1 Furnace temperature programme

Temperature/ "C

50 150 350

1200 1200 2600 2600

Ramp time/

5.0 50.0 30.0 10.0 0.0 0.7 0.0

S

Hold time/

0.0 0.0 0.0 2.0 2.0 2.0 2.0

S

Gas flow/ 1 min-'

3.0 3.0 3.0 3.0 0.0 0.0 3.0

Hydrogen Telluride generation

The effect of the concentration and the flow rate of HC1 and NaBH, on the evolution of the hydrogen telluride had been already in~es t iga t ed .~ ,~ , '~ Good results were obtained using the following conditions: 6 mol I-' HCl; 5% m/v NaBH,; and flow rate for HCl, NaBH, and sample of 1 , l and 4-5 ml min-', respectively.

Trapping on Amberlite

The collection of the hydrogen telluride was obtained by trapping it on the liquid anion exchanger, Amberlite (LA-2). Since the formation of the species Se2- during the stripping of gaseous selenium hydride has been propo~ed, '~ an analogous species can be considered to explain the fixing of tellurium (which belongs to the same Group 16) on the liquid exchanger:

TeH- +Te2- + H + (LA-2)Cl- +Te2- -+(LA-2)Te2- +C1-

As can be deduced from the first two equilibria, an acid environment does not facilitate the formation of Te2-. 'Therefore, the increase of the media pH, by treating the anion exchanger with a base, was expected to improve the trapping efficiency. By changing the pH value from 1 to 7, the efficiency of the process was increased from 40 to 90%. At higher pH values, the exchange capability of LA-2 decreases, probably owing to competition from OH- ions.

,Qtomization

'Tellurium was atomized from a platform, previously treated with palladium by pre-injecting a solution of the modifier, by carrying out the furnace temperature programme, until the pyrolysis temperature was reached. The use of a palladium- coated platform has the following advantages: the application of the L'vov p la t f~rm; '~ . '~ the covering of the graphite surface; and the employment of a chemical m~difier. '*~'~ The result is an improvement of the atomization of tellurium, as shown in 'Table 2, where the analytical performances obtained by employing different atomization systems (wall, platform, pal- ladium-treated platform) are reported, for the analysis of a 25 ppb Te standard solution. Finally, the higher pyrolysis temperature that can be used in the presence of palladium, permits the organic matrix (IBMK, LA-2) to be vaporized before the analyte is atomized.

Interferences

Hydride generation offers, besides the possibility of a precon- centration of the analyte, the achievement of a satisfying separation of the analyte from the matrix. Several elements were considered as possible interferents in the ETAAS determi- nation of tellurium in environmental samples (Table 3). No

Table 2 Integrated absorbance, repeatability and maximum pyrolysis temperature without loss of analyte, T,,,(max), for the atomization of a 25 ppb Te standard solution, from different atomization systems (n = 2)

Atomization system

Wall* Pyrolytic graphite coated graphite platform? Pd-treated platform$

Integrated Absorbance RSD (%) Tp&W/"C 0.079 & 0.004 5.5 0.1 63 +_0.008 4.8 0.1 65 & 0.007 4.3

700 700

1300

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Table 3 List of interferents typically occuring in three environmental matrices

Interferent

NaCl NaF Na,SO, NaHCO, KBr KCl CaCI, MgC12 SrCl, H3BO3

Sea-water/

27.65

g 1-1

0.019 2.660 0.133 0.020 0.466 0.533 1.560 0.008 0.020

Interferent

Si A1 Fe K Mg Ca Na Ti Mn

Sediment/ Calcium carbonate/ mg I-' Interferen t g 1-'

3 50 CaCO, 24.97 500 250 150 100 100 50 30 10

interferences were found in the determination of 2 ng ml-1 of tellurium.

It is well known that HG is susceptible to interferences from transition metals and hydride forming elements.20 In the deter- mination of tellurium by HG and trapping, Maher" found severe interferent effects from copper, mercury, silver and selenium. The interferences of the same elements, at the same analyte: concomitant ratio, were studied. No significant (< 10%) interferences were found.

Analytical Figures of Merit

The analytical figures of merit of the proposed method are reported in Table 4. The sensitivity of the method depends on the atomization efficiency and on the enrichment due to the preconcentration step. From the data in Table 2, we can deduce a characteristic mass of 16.7 pg for an integrated signal of 0.0044 s, in agreement with the instrument's performance.

Considering the whole process, the characteristic concen- tration and the detection limits are 25 and 100 pg ml-1 respect- ively. The relative standard deviation (RSD) of the method, evaluated by analysing nine samples on different days is 4.5%.

The efficiency of the process has been evaluated by compar- ing the absorbance of a 1 ng ml-' tellurium standard solution after concentration with the absorbance of a lOOngml-' tellurium standard solution directly introduced into the furnace.

The calibration graph shows a good linearity (r=0.9991), in the range 0-2ngml-', and can be adequately conveyed by the following equation:

QA = [Te]0.182+0.014

Higher analyte concentrations are assessable by working with lower samples volumes. Finally, in order to evaluate the analytical accuracy of the method and to prove the possibility of application of the method to the analysis of real samples, the method was applied in the analysis of seawater and sediment reference materials CASS-2 and MESS-1. Since these materials are not certified for tellurium, it was added before sample preparation. The analytical results are summarized

Table 4 Analytical figures of merit

Integrated absorbancels Blank 1 ng ml-' tellurium standard solution after

concentration 100 ng ml-' tellurium standard solution

directly introduced into the furnace Characteristic concentration/ng m1-I Detection limit/ng ml-' Trapping efficiency (YO) RSD (%) Correlation coefficient

0.016 0.006 0.189 t0.009

0.209 k 0.004

0.025 0.100

4.5 0.999 1

90

Table 5 Analytical results

Te added/ Sample Pg I-'

CASS-2 1 .o CASS-2 1 .o CASS-2 0.5 MESS-1 l.O* MESS-1 1.0* MESS-1 1.0*

Te found/

1 .oo 1.18 0.44 0.68 0.79 0.70

I % 1-l Recovery

100 118 88 68 79 70

(%I

* Concentration in solution (after solubilization).

in Table 5. The recovery is very high for the sea-water samples and lower but satisfactory, considering their very low concentration, for the sediment samples.

REFERENCES

1

2

3

4 5

6 7 8

9

10

11

12 13 14

15

16 17

18

19

20

Cooper, W. C., Tellurium, Van Nostrand Reinhold, New York, 1971, ch. 7. Gerhardsson, L., Glover, J. R., Nordberg, G. F., and Vouk, V., in Handbook on the Toxicology of Metals, eds. L. Friberg, Nordberg, G. F., and Vouk, V., Elsevier Science Publishers B.V., Amsterdam 2nd edn., 1986. Sedykh E. M., Belyaev, Yu. I., and Sorokina, E. V., J. Anal. Chem. (USSR), 1980, 35, 2162. Ni, Z.-m., and Shan, X.-q., Spectrochim. Acta, Part B, €987 42,937. Kobayashi, R., Imai, M., and Hashimoto, Y., Bunseki Kagaku, 1982, 31, 467. Lee, D. S., Anal. Chem., 1982, 54, 1682. Andreae, M. O., Anal. Chem., 1984,56,2064. Yoon, B. M., Shim, S. C., Pyun, H. C., and Lee, D. S., Anal. Sci., 1990, 6, 561. Doidge, P. S., Sturman, B. T., and Rettberg, T. M., J. Anal. At . Spectrom., 1989, 4, 251. Zhang, L., Ni, Z.-m., and Shan, X.-q., Spectrochim. Acta. Part B, 1989, 44, 751. Ni, Z.-m., He, B., and Han, H.-b., J. Anal. At . Spectrom., 1993, 8, 995. Maher, W. A., Analyst, 1983, 108, 305. Tsalev, D. L., and Mandjukov, P. B., Microchem. J., 1987, 35, 83. Yammarnoto, M., Yasuda, M., and Yamamoto, Y., Anal Chem., 1985,57, 1382. Agterdenbos, J., Bussink, R. W., and Bax, D., Anal. Chim. Acta, 1990 232, 405. L'vov, B. V., Spectrochim. Acta, Part B, 1978, 33, 153. Slavin, W., Manning, D. C., and Carnrick, G. R., At. Spectrosc., 1981, 2, 73. Tsalev, D. L., Slaveykova, V. I., and Mandjukov, P. B., Spectrochim. Acta Rev., 1990, 13, 225. Weibust, G., Langmyhr, F. J., and Thomassen, Y., Anal. Chim. Acta, 1981, 128, 23. Welz, B., and Melcher, M., Analyst, 1984, 109, 569.

Paper 4/03359K Received June 6, 1994

Accepted December 1 , 1994

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