Chemical vapour generation of inorganic anionic species by aqueous phase alkylation

Complete text of publication follows. Derivatization methods based on the formation of volatile species represent an important analytical tool allowing, among the others, separation and preconcentration of the analytical species. In this contribution it is reported, for the first time, a derivatization method for the conversion of some common inorganic anionic species into volatile derivatives of well defined chemical composition. The method consists of aqueous phase alkylation by the powerful alkylating agents as trialkyloxonium tetrafluoroborates (R3O+BF4, R=Me, Et) and headspace analysis (HS) of evolved derivatives by GC-MS. Among the tested inorganic substrates, the halogen ions give the corresponding RX (X=I, Br, Cl), cyanide gives RCN, thiocyanate gives RSCN, and sulphide gives R2S. Most of the tested oxoanions (sulphate, sulphite, phosphate, borate, iodate, periodate, arsenite, arseniate, methylarsonate, dimethylarsinate, selenite, selenate) did not give any detectable derivatives. This was addressed to the fast decomposition of the corresponding alkyl esters in aqueous solution. The only exceptions are nitrates and nitrites, which were converted in to R-O-NO2 and R-O-NO, respectively. The reaction, which is largely controlled by the SN1 ionization reaction step, R3O+ R+ + R2O, is competitive with the hydrolysis of reagent, which is relatively slow for Et3O+BF4 (half-life about 7 min at 25 deg C) (G.J. King et al., Inorg. Chem. 25 (1986) 1078.). Detection limits (3s) were estimated even if a systematic approach for the optimization of HS/GC-MS method has not been performed (4mL reaction vial, 2mL sample, 0.1 mL of 1M R3O+, T=21 deg C, 0.5 mL of HS gases injected into GC-MS): 1.2 ng/mL for iodide, 2.6 ng/mL for bromide, 26 ng/mL for chloride, 36 ng/mL for nitrate, and they are in the range of 100-300 ng/mL for the other species nitrite, nitrate, thiocyanate, cyanide and sulphide. Application to the determination of bromide in ground water certified reference material, BCR-612, gave results in excellent agreement with certified values. A simple procedure for the determination and speciation of iodide and iodate by HS/GC-MS was developed. Application of the method to determination of iodine in table salt gave a good agreement with the results obtained by ion chromatography.