Atomic-absorption spectrochemical analysis for ultratrace elements in geological materials by hydride-forming techniques: Selenium

0039.9140/81/030169-04SO2.00/0

Tolanra Vol.28,pp.169to 172 Pergamon PressLtd 1981. Printedin GreatBritain

ATOMIC-ABSORPTION SPECTROCHEMICAL ANALYSIS FOR ULTRATRACE ELEMENTS IN GEOLOGICAL MATERIALS BY HYDRIDE-FORMING TECHNIQUES: SELENIUM G. PAOLO SIGHINOLFI

and

CARLO GORC~NI

Istituto di Mineralogia, Universita di Modena, Italy (Received 16 July 1980. Accepted

3 October

1980)

method based on hydride generation for the AAS determination of selenium at nanogram levels in geological materials is described. The sample is decomposed by aqua regin attack in a sealed

Summary-A

Teflon bomb. After treatment with hydrochloric acid, selenium is converted into hydrogen selenide by reaction with sodium borohydride and determined by AAS. Matrix interference effects have been investigated, but though they are rarely significant, the standard-additions method is recommended. The absolute sensitivity of the method is about 2.0 ng of Se (in 10 ml of solution). Detection limits of about 5-10 ng in a 1.0-g sample have been achieved with the use of “Suprapure” reagents. The selenium content of some USGS, CRPG and ANRT reference samples is reported.

A number of studies have shown that the metal-hydride vapour procedures are among the most promising advances in analysis for ultratrace elements by atomic-absorption spectrophotometry (AAS).‘-” These techniques may be applied in the determination of Hg, Se, Te, As, etc., which can be obtained in the gaseous phase as a hydride or metal (Hg) vapour by chemical reduction. The major advantages of these techniques are that they are relatively simple and rapid, and characterized by very high analytical sensitivity. It is known, however, that the hydride generation process is frequently affected by severe interelemental interference effects, so this technique can usually be applied successfully only to the determination of a few elements in relatively simple media such as natural waters or biological solutions. In the case of more complex chemical matrices, such as geological materials, only mercury is currently determined by the vapour-forming technique. We propose to test the analytical capabilities of hydride-forming techniques for other ultratrace elements in geological materials. Here a method for the determination of selenium is described and analytical data relative to some geological reference 3amples are reported.

GENERAL

ANALYTICAL

COMMENTS

The analytical procedure used is based on the generation of a volatile selenium compound (hydrogen selenide) from an ionic selenium solution by reduction with sodium borohydride as described in the literature. In the case of analysis of geological materials, some aspects (sample dissolution, optimization of analytical sensitivity and control of matrix interelemental effects) need investigation in detail. TM..28/3--c

Sample decomposition

The following factors were considered in choosing the sample dissolution procedure: (a) selenium is a highly volatile element which may easily be lost during sample attack, even at moderate temperature; (b) selenium has a marked calcophilic character which leads to its virtual absence in non-sulphide mineral phases;” (c) the severity of the known interelemental interferences suggests that the selenium should be separated from matrix elements. Considerations (b) and (c) led to the choice of a partial sample dissolution (by aqua regia) that is strongly selective for sulphide phases. Loss by kolatilization was prevented by the use of a sealed Teflon bomb similar to the one described by Bernas.12 Repeated attack on the same sample has shown (Fig. 1) that small volumes of aqua regia (2 ml for up to 1 g of silicate rock) are sufficient for the complete dissolution of the selenium contents of common rock samples. The use of small amounts of reagents derives from the need to minimize blank values and control the interference of nitrate. The attack on the selenium-bearing phases when a sealed Teflon bomb is used at a relatively high temperature (160”) is very rapid: all the selenium is dissolved in about 60 min (Fig. 1). The controlled aqua regia atmosphere during the attack and the subsequent addition of concentrated hydrochloric acid should ensure the complete conversion of selenium into selenite, as required for optimal hydride generation. Optimization of analytical sensitivity

Selenium is an ultratrace element in most geological materials, frequently occurring at below the 100 rig/g level.” For this reason the optimization of sensi169

G. PAOLOSIGHINOLFJ and CARLOGORGONI

170

H-l o-o-

/

L

0.010

Fig. 1. Selenium recovered after aqua regia attack in the Teflon bomb. W-l: USGS reference sample; M-l, M-3: basaltic rock; G: granitic rock; M-lb, M-3b: second attack cycle on the M-l and M-3 samples. tivity and efficiency is critical. Previous studies suggest that a number of parameters affect metal vapour or hydride generation and the efficiency of the absorption phenomena. It has been shown13 that selenium must be present as selenite to give optimal hydride generation. Hydrochloric acid is usually used as reducing agent.14 The effects of parameters such as acid concentration and cell temperature have already been investigated. l5 Here the effects of the pressure of the charring gas and of the volume of the reacting solution are studied. The absorption signal increases with decreasing argon or nitrogen pressure (Fig. 2). Thus a low (about 1.2 bar) gas pressure should be used. The absorption signal tends to increase with decreasing volume of reaction solution (Fig. 2), ,as is also the case for mercury.” At the same time, however, the reproducibility of the signal becomes poorer, so very small volumes cannot be used. A lo-ml volume seems to be the best compromise. Control

of interelemental

interference

effects

The determination of metals by hydride-forming techniques may be affected by serious matrix interference effects.16-‘9 It has been shown that the most severe interferences arise from any element or com-

pound affecting the reduction to the hydride form. A number of elements may interfere in selenium hydride generation, but the most serious interferences arise from nitrate and from transition metals (Ni, Cu, Co, etc.) and noble metals (Au, Pt, Pd, etc.), these last usually present as trace or ultratrace elements in geological materials. The inhibitory effect of nitrate can easily be avoided by evaporation of the reagent (aqua regia) after the attack in the Teflon bomb. However, a series of experiments on solutions of geological materials and on aqueous selenium solutions suggested that the formation of selenium hydride from about 4M chloride solution is unaffected by the presence of nitrate at tincentrations up to about 2M. For this reason, as the nitric acid concentration in the analytical solution is only about 0.3M, its removal seems unnecessary; moreover, omission of the evaporation step also avoids any possible loss of volatile selenium compounds during this stage. As regards the interference effects of metals, the compositional complexity of geological materials makes the investigation of the interference of single elements unrealistic. The sum of interferences by the silicate or non-silicate matrices may be estimated by analysing with the standard-additions method geological materials of widely varying composition (acid, mat%, ultramafic rocks, sulphides, etc.). Most of the standard-additions calibration plots obtained are practically parallel to the standard calibration plot (Fig. 3) and this indicates that in most cases interelemental interferences are absent or negligible. In other cases, such as for the nickel-rich (about 0.1%) P-l ultramafic rock, the standard-additions calibration plot shows that matrix interferences are present. The plot for aqueous selenium solutions containing gold confirms the interfering effect of the noble metals. This means that in the analysis of geological materials only the interferences by specific elements (Ni, Cu, Au, Pt, etc.), which are very occasionally present in discrete amounts in common rocks,

I 0

10

20 ml.vOl

*

Fig. 2. Effectsof the calibration volume (0: PAr = 1.2 bar) and of the gas pressure (Cl: calibration volume = 10 ml) on the absorbance reading. M-l, M-4: basaltic rock [ppM (parts per milliard = ng/ml].

Fig. 3. Study of matrix interferences on by standard-additions calibration plots. bration plot; M-l, M-2, M-3: basaltic rock: P: ultramafic rock (pyroxenite); .” blank.

Se determination S: standard calirock; G: granitic Su: pyrites; B:

AAS ultratrace determination

need be considered. The use of masking agents is in some cases effective in removing interference in hydride generation. ‘O For example, the use of tellurium(IV) has been found useful in the determination of selenium,‘l because the Te’- formed by borohydride reduction will form very stable tellurides with some interfering ions. ” Nevertheless, the addition of tellurium(IV) was found ineffective in removing or reducing matrix interference in analysis of the sample P-l (the interference being attributed to the presence of nickel) (Fig. 3). For these reasons we recommend the use of the standard-additions procedure for the analysis of geological materials of unknown composition, since it should eliminate the effect of strongly interfering elements such as Ni, Cu, noble metals etc.

171

of Se

stir the mixture to wet the sample. Place the Teflon vessel in the bomb and seal tightly. Heat the bomb at 160”for 90 min. After cooling, add 10 ml of concentrated hydrochloric acid, filter and transfer the filtrate to a 25-ml standard flask. Place three equal fractions (from 1 to 8 ml, depending on the Se content of the sample) of this solution in the reaction tubes and dilute them to the lo-ml mark with 2M hydrochloric acid. With a micropipette add known amounts of the 0.1~pg/ml Se solution to two aliquots and from the three absorption readings plot the standard-addition graph. Calculate the Se content in the sample from Se = -25 PI

~

“W

mig

where u is the volume (ml) of the fraction used for preparing each standard-additions solution, w is the weight (g) of the sample, and [Se] is the intercept on the x-axis of the plot of absorption signal against weight (ng) of selenium added to the standard-additions solution ([Se] should be zero or negative). RESULTS

atomic-absorption spectrophotometer, Model 56 recorder, electrodeless discharge selenium lamp, and MHS-10 Mercury/Hydride System were used, under the following conditions.

Wavelength 196.0 nm Spectral band-width 0.7 nm MHS-I 0 module

Cell heating Reductant Charring gas Calibration vol.

Air/CzH2 3% NaBH,, + 1% NaOH solution Ar (1.2 bar) 10 ml

Reagents and standards

Certified standard selenium solution (1000 pg/ml) was obtained commercially. A O.l-pg/ml Se solution was prepared daily by diluting the concentrated standard with demineralized water. “Suprapure” hydrochloric and nitric acids were used.

Sensitivity and reproducibility

The absolute sensitivity of the method depends on the degree of contamination from reagents and handling. With the use of “Suprapure” chemical reagents in the amounts recommended, the blank signal is very low (Fig. 3). Ten ml of 02-ng/ml Se solution (2 ng of Se absolute) gave an absorption reading about twice that of the blank. Thus, 2 ng of selenium represents the-detection limit. This corresponds to determination limits of around Z-10 rig/g in the sample when the standard-additions procedure is used. This is adequate for determination of selenium in most geological materials. Instrumental precision was checked by means of a series of replicate readings on selenium solutions at different concentrations. Very good reproducibility (2-3% coefficient of variation) was found over the whole concentration range 0.2-5.0 ng/ml. Geological

Procedure

Weigh 0.1-1.0 g of powdered sample into a 60-ml Teflon vessel (for design see Bernas12), add 2 ml of aqua regia and

reference

samples

Selenium has been determined in some USGS, CRPG and ANRT geological reference samples. The

Table 1. Selenium in geological reference samples (ng/g)

Sample W-l BCR-1 GSP-1 PCC-1 AGV-1 G-2 DTS-1 BR UB-N DT-N GH JG-1 Syenite-1 Sulphide M-l (alkalic basalt)

Se found 104,114 80,84 72,80 32,42 ill <16 <16 35 125,136 <15 <14 115 11,17 27,32 x lo3 178t

t Average of 10 determinations;

Literature data Ref. 23 Ref. 24 Ref. 25 Ref. 26 Ref. 27 Ref. 28 Ref. 29 130 100 <400 <180 tl40 <700 <3c0

110 103 59 22 8 5 4

110

124 124

102

coefficient of variation 9%.

100 118 88 31

79

172

G. PAOLOSIGHINOLFI and CARLOGORGONI

results are reported in Table 1. The limited availability of some samples, such as G-2, GH, AGV-1, which are very poor in Se, permits only estimates of the upper concentration limits. For most of the samples sufficiently high in selenium (e.g., W-l, BCR-1, GSP-1, etc.) there is relatively good agreement between our data and the literature data. Duplicate analysis on most of the reference samples and replicate determinations (10) on the non-reference sample M-l (basaltic rock) indicate an acceptable reproducibility of the bulk analysis. An error of less than 10% may be considered satisfactory. Data on the Sulphide reference material suggest that the method is particularly suitable for microanalysis (about 1 mg of sample) of natural selenium-rich materials such as sulphide ores and meteoritic materials. Acknowledgement-This work was carried out with financial support of the Consiglio Nazionale delle Ricerche, Roma. REFERENCES 1. W. R. Hatch and W. L. Ott, Anal. Chem., 1968, 40, 2085. 2. G. Lindstedt, Analyst,

7. D. D. Siemer, P. Koteel and V. Jariwala, ibid., 1976, 48, 836.

8. G. M. Hieftje and T. R. Copeland, ibid., 1978, 50, 300R. 9. D. D. Siemer and P. Koteel, ibid., 1977, 49, 1096. 10. G. P. Sighinolfi and C. Gorgoni, Geostandards Newsletter, in the press. 11. F. Leutwein, Handbook of Geochemistry, Chap. 34, _ Selenium, Springer, Berlin, i971. 12. B. Bernas. Anal. Chem.. 1968.40. 1682. 13: M. Landsford, E. M. ?&Phdrson and M. J. Fishman, At. Abs. Newsl., 1974, 13, 103. 14. K. C. Thompson and D. R. Thomerson, Analyst, 1974, 99, 595. 15. E. Jackwerth, P. G. Willmer, R. HGhn and H. Berndt, At. Abs. Newsl., 1979, 18, 66. 16. A. E. Smith, Analyst, 1975, 100, 300. 17. F. D. Pierce and H. R. Brown, Anal. Chem., 1976, 48, 693. 18. Idem, ibid., 1977, 49, 1417. 19. H. D. Fleming and R. G. Ide, Anal. Chim. Acta. 1976., 83, 67. 20. G. F. Kirkbright and M. Taddia, ibid., 1978, 100, 145. 21. Idem., At. Abs. Newsl., 1979, 19. 68. 22. L. G. Sill&n and A. E. Martell,’ Stability Constants, p. 151. Spec. Publ. No. 17, The Chemical Society, London, 1964. 23. F. J. Flanagan, Geochim. Cosmochim. Acta, 1973, 37, 1189. 24. A. 0. Brunfelt and E. Steinnes, ibid., 1967, 31, 283. 25. L. A. Haskin, R. 0. Allen, P. A. Helmke, T. P. Paster, M. R. Anderson, R. L. Korotev and K. A. Zfeifel, ibid., Suppl. I, 1970, 2, 1213. 26. E. P. Mignonsin and I. Roelandts, Chem. Geol., 1975, 16, 137. 27. R. A. Nadkarni and B. C. Haldar, Radioanal. Lett., 1971, 7, 303. 28. M. M. Schnepfe and F. J. Flanagan, Chem. Geol., 1973, 12, 77. 29. J. W. Morgan, R. Ganapathy, H. Higuchi and U. KrPhenbiihl, Geochim. Cosmochim. Acta, 1976,40, 861.