title: Isotope dilution quantification of ultratrace gamma-glutamyl-Se-methylselenocysteine species using HPLC with enhanced ICP-MS detection by ultrasonic nebulisation or carbon-loaded plasma.
abstract: 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.
authors.affiliation: LGC Limited, Queens Road, Teddington, Middlesex, TW11 OLY, UK. Heidi.Goenaga-Infante@lgc.co.uk
authors.forename: Heidi
authors.initials: H
authors.lastname: Goenaga Infante
authors.forename: María del Carmen
authors.initials: Mdel C
authors.lastname: Ovejero Bendito
authors.forename: Carmen
authors.initials: C
authors.lastname: Cámara
authors.forename: Linda
authors.initials: L
authors.lastname: Evans
authors.forename: Ruth
authors.initials: R
authors.lastname: Hearn
authors.forename: Sven
authors.initials: S
authors.lastname: Moesgaard
mesh_terms: D002244:Carbon / Q000737:chemistry*; D002851:Chromatography, High Pressure Liquid / Q000379:methods; D003545:Cysteine / Q000031:analogs & derivatives* / Q000032:analysis; D005737:Garlic / Q000737:chemistry*; D007554:Isotopes; D013058:Mass Spectrometry / Q000379:methods*; D016566:Organoselenium Compounds / Q000032:analysis*; D012643:Selenium / Q000032:analysis / Q000737:chemistry; D012680:Sensitivity and Specificity; D013997:Time Factors
chemical_list: D007554:Isotopes; D016566:Organoselenium Compounds; C424943:glutamyl-Se-methylselenocysteine; D002244:Carbon; D012643:Selenium; D003545:Cysteine