Mikrochim. Acta 109, 141-144 (1992) Mikrochimica Acta

�9 by Springer-Verlag 1992 Printed in Austria

The Use of Sodium Tetraethylborate for the Derivatization and Analysis of Selenium Containing Compounds

Steven Clark and Peter J. Craig*

Department of Chemistry, De Montfort University, The Gateway, P.O. Box 143, Leicester, LE1 9BH, U.K.

Abstract. Several selenium containing compounds have been derivatized to diethyl selenide using sodium tetraethylborate. The evolved diethyl selenide was detected by gas chromatography interfaced with atomic absorption or mass spectroscopic methods. Derivatization was carried out on the analyte in a sealed vial using an aqueous solution of sodium tetraethylborate (2%) added by means of a syringe.

Key words: sodium tetraethylborate, ethylation, analysis, selenium.

Numerous organoselenium compounds have been observed in organisms or in the natural environment, eg. dimethylselenide, ((CH3)2Se) dimethyl dis- elenide, ((CH 3)2 Se2), selenocysteine (HSeCH 2 CH(NH/)COOH), dimethylselenone ((CH3)aSeO2), selenomethionine (CH3SeCH2CHzCH(NH2)COOH) [1]. Numer- ous selenium compounds exist in organisms, but the first two organoselenium species above are the only ones reported to be detected in the atmosphere. There are no reports of monomethylselenium species detected in the atmosphere, or after derivatization, perhaps because of their instability (eg. CH3 Sell). If therefore mono- methylselenium compounds are present, perhaps co-ordinated to natural donor ligands in the aqueous or sediment environment, they may not be detectable by conventional hydride derivatization using sodium borohydride (NaBH4). Where volatile selenium compounds have been observed in the environment, eg. after incubation experiments with organic or inorganic selenium species, they have been observed in the head space of a biological reactor without derivatization [2]. In a different system, the trimethylselenonium ion ((CH3)3Se § has been observed in urine [3]. Volatile selenium compounds have also been observed in the exhalation of animals feed with inorganic selenium species ie. biomethylation (see for example Ref. [4]).

It is important to obtain derivatives in a volatile form, but without loss of essential chemical information, as this allows chromatographic separation prior to

* To whom correspondence should be addressed

142 S. Clark and P. J. Craig

detection. NaBH 4 is often used to prepare volatile hydrides from non volatile species present in the environment eg. butyl tin hydrides from butyl tin oxide or halide species present in the environment.

Clearly the existence of a derivatizing technique that would allow detection of any monomethyl or other monoalkyl selenium derivatives from biological or envi- ronmental sources would be of potential interest. As mentioned above, NaBH 4 might be unsuccessful in this context. The use of sodium tetraethylborate NaB(C2Hs) 4 for the production of volatile alkyl derivatives from several metallic elements has been reported recently. NaB(C2Hs)4 has been used to prepared volatile metallic ethyl derivatives from inorganic and methyl lead compounds [5], from methyl and inorganic mercury I-6, 7, 8], and from alkyl and aryl tin compounds [-9, 10, 11]. Preparation of the volatile ethyl derivatives, proceeding without scrambling or other dismutations of the alkyl groups already associated with the metal in question, has usually been followed by gas chromatographic (GC) separation fol- lowed by some form of specific detection eg. atomic absorption (AA) or mass spectroscopic (MS). We report here the use of NaB(CzHs)4 for the derivatization and analysis of various selenium compounds, the prepared ethyl-selenium being stable enough to undergo GC-AA separation and analysis.

Prior to this the determination of selenium compounds, eg. in environmental matrices is confined to generation of inorganic selenium dihydride (H2Se) fol- lowed by quartz furnace AA or inductively coupled plasma (ICP) techniques (eg. Ref. [12]).

Experimental

Apparatus

A Pye Unicam 104 GC (oven 130~ injector 180~ was interfaced with a Varian 1000 AA instrument fitted with a quartz furnace atomization cell. The GC was fitted with a 2 m x 4 mm i.d. packed column packed with 10~o OV10t on Chromosorb W-HP (80-100 mesh). Nitrogen was the carrier gas (75 cm 3 rain-l). The effluent from the GC was carried to the AA by a heated, unpacked, stainless steel

transfer line (0.75 m • 1.4 mm o.d. and 0.7 mm i.d.). The transfer line (heated to 180~ was insulated from a coiled nichrome heating wire by outer and inner Teflon sleeves. A wrapping of asbestos cord

was used to provide thermal insulation and this in turn was surrounded by an outer PTFE tape wrapping. Hydrogen (250 cm 3 rain 1) and air (25 cm 3 rain -1) were fed into the quartz furnace (at 950~ through junctions immediately outside the heated part. The AA was fitted with a hollow

cathode lamp (Juniper Ltd, UK) at 4.7 mA lamp current, 196.0 nm and a slit width of 1.0 nm. The 10 mV output from the AA was connected to a chart recorder. The interfaced apparatus has been described in detail elsewhere [13].

GC-MS work was achieved by use of a magnetic deflection VG Micromass 16-F instrument coupled via a transfer line (180~ to a Pye Unicam 204 GC using an ionization potential of 70 eV. A VG data handling system 2000 was used. The GC contained a 2 m by 4 mm i.d. column packed with 10~o OVI on Chromosorb as above with helium carrier gas.

Reagents

The following selenium compounds were purchased and used without further purification (there was evidence from our analytical work of the presence of some sulphur compounds but these did not inhibit analysis): sodium selenide (NazSe), selenium dioxide (SeOz), sodium selenite (Na2SeO3), sodium selenate (Na2 SeO4).

Derivatization and Analysis of Selenium Containing Compounds 143

Et2Se

B

A

50 100 150 138 (C2H~)zSe + (Peak A)

123 C2HsSeCH2 + 110 CzHsSeH + (Peak B)

95 CH3Se + 82 Sell +

29 CzH ~-

Isotopes of selenium 74 76 77 78 80 82

Abundances (%) 1.0 9.0 7.5 23.6 50.0 9.0

Fig. 1. Mass spectrum of diethylselenium from derivatization peak M/e

Derivatization

1 cm 3 of the aqueous analyte solution (20/~g cm 3) was placed in a 50 cm 3 vial which was sealed with

a PTFE-lined crimpon and sealed with a septum. 0.5 cm 3 of an aqueous solution of 2% NaB(C2Hs)4 was added through the septum using a syringe. Samples of headspace (0.5 cm 3) were taken via a

gastight-syringe for GC-AA and GC-MS analysis. For Na/Se, SeO 2, Na 2 SeO 3 and Na2 SeO4 derivati- zation took place at pH 1 in HC1 to produce in each case, the species diethyl selenide ((CzHs)zSe) which was detected in the GC-AA at a retention time of 1.4 min or from its mass spectrum. When HCI was not added, derivatization was not observed to be successful.

Results and Discussion

This preliminary work shows the feasibility of the use of NaB(C2 H5)4 in preparing selenium compounds for speciation analysis by GC AA or GC MS (Fig. 1). It was applied here only to standard inorganic substances and not to environmental or organic selenium species. However, the potential of the method is demonstrated as an alternative to the hydride method for inorganic selenium compounds. Small amounts of sulphur dioxide (SO2) and diethyl suphide ((CIH5)2S) were shown to be present by MS and this method has allowed selenium compounds to be detected in the presence of sulphur compounds, present presumably as sulphite. The presence of SO 2 suggests sulphur compounds are less easily ethylated than selenium com- pounds. The production of a single derNative ((C2H5) 2 Se) from the various oxida- tion states of selenium suggests a reduction to selenium(II) in the derivatization perhaps followed by ethyl carbanion attack to give (C2 H5)2 Se. Clearly this method

144 Derivatization and Analysis of Selenium Containing Compounds

cannot separate the inorganic derivatives of selenium as (C2Hs)2Se is produced in each case, but it is successful in the presence of sulphur compounds, which hydride generation using NaBH4 may not be. There are reports of sulphide being an impediment to hydride generation [141. Clearly, use of this method for some environmental samples might require a preconcentration step, but the ultimate concentration limits for analysis are not yet established. Additionally, the method has not yet been explored for organoselenium compounds of environmental interest.

A further aspect of this method is that selenate (SeO 2-) can be derivatized directly using NaB((CzHs)4) to ( C z H s ) z S e whereas NaBH4 does not reduce sele- nate to H 2 Se directly, sometimes necessitating a further reduction step [15].

Acknowledgement. We are pleased to acknowledge support from the Science and Engineering Research Council, U.K. for this work (grant to SC).

References

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Report 1037, Naval Ocean Systems Centre, San Diego CA, USA 92152-2000, p. 25. [,15] J. Piwonka, G. Kaiser, G. T61g, Fresenius' Z. Anal. Chem. 1985, 321,225.