Developments of a rugged method for selenium speciation

The need for selenium speciation in an unknown bacterial sample brought by a customer initiated the setup of a hyphenated method of IEC-ICP-MS. First a published method from Gilon et al. (J. Chromatogr. A, 1996, 750, 327) was modified to analyze selenurea, seleno ethionine, seleno methionine (SeM), Se(IV), seleno cystine (SeC) and Se(VI) by a hyphenated ICP-MS technique. LODs of 4–15 µg L−1 were achieved and a 1/10 diluted protease digestion (Pronase E) of the Se-containing bacterial sample was measured. However, the ruggedness of the method was insufficient. Separation was compromised even at a 1/10 dilution of the sample and identification of the species was poor. Another method was set up and developed to analyze anionic Se species. This method used a Dionex AS 11 column with NaOH and tetramethylammonium hydroxide as eluents. Now, LODs of ca. 0.1 µg Se L−1 were achieved and the method proved to be very rugged. No alteration of the separation was observed. During 30 consecutive runs, retention times and peak areas varied only slightly in spite of the high material load on the column (e.g., for the major compound the retention time was 6.26 min ± 6 s, and the peak area variation was ±0.7%). The protease digestion of the bacterial sample could be analyzed without dilution. The separation was not compromised, although the bacterial material load of the sample was 30 mg mL−1. Therefore low concentration Se species were not below the detection limit. Several species were identified, such as Se(IV), SeC and in small amounts SeM. However, some species remained unidentified due to unavailable standards for comparison. Mass balances were also performed. The protease digestion provided 25% Se of the total Se determined after pressure digestion (148 mg kg−1), and 98.7% recovery was obtained when the protease digestion was speciated.