A method for the accurate determination of ultratrace
selenium species of relevance to
cancer research, such as
gamma-glutamyl-Se-methylselenocysteine (
gamma-glutamyl-SeMC), using species-specific double
isotope dilution analysis (IDA) with HPLC-ICP-MS is reported for the first time. The (77)Se-enriched
gamma-glutamyl-SeMC spike was produced in-house by collecting the fraction at the retention time of the
gamma-glutamyl-SeMC peak from a chromatographed aqueous extract of (77)Se-enriched yeast, pooling the collected fractions and freeze-drying the homogenate. The Se content of this spike was characterised using reverse
isotope dilution mass spectrometry (IDMS) and the isotopic composition of this spike was checked prior to quantification of the natural abundance
dipeptide species in garlic using speciated IDA. The extraction of the
gamma-glutamyl-SeMC species in water was performed in a sonication bath for 2 h after adding an appropriate quantity of (77)Se-enriched
gamma-glutamyl-SeMC to 50 mg of garlic to give optimal (78)Se/(77)Se and (82)Se/(77)Se ratios of 1.5 and 0.6, respectively. The effect of ultrasonic nebulisation, in comparison with the loading of the ICP with
carbon (through the addition of methane gas on-line), on the detection of Se associated with
gamma-glutamyl-SeMC using collision/reaction cell ICP-MS with
hydrogen as collision gas was investigated. Sensitivity enhancements of approximately fourfold and twofold were achieved using USN and methane mixed plasma, respectively, in comparison with conventional nebulisation and conventional Ar ICP-MS. However, an approximately twofold improvement in the detection limit was achieved using both approaches (42 ng kg(-1) for (78)Se using peak height measurements). The use of species-specific IDMS enabled quantification of the
dipeptide species at ng g(-1) levels (603 ng g(-1) Se) in the complex food matrix with a relative standard deviation (RSD, n = 4) of 4.5%, which was approximately half that obtained using standard addition as a confirmatory technique. Furthermore, good agreement was found between the
gamma-glutamyl-SeMC species concentrations obtained using both calibration methods.