Application of isotope dilution to ion trap gas chromatography mass spectrometry

Isotope dilution has been applied to an approach recently developed to quan tify inorganic mercury compounds by ion trap gas chromatography/mass spectr ometry (GC/MS). In this approach, a chemical alkylation reaction is used to convert an inorganic mercury salt (Hg(NO3)(2)) to an organometallic compou nd (methylmercury iodide), before sampling with a solid-phase microextracti on fiber. The adsorbed analytes are subsequently desorbed and analyzed by G C/MS, Isotope dilution adds only one additional step: an enriched isotopic spike is added to the sample before chemical alkylation. Before applying is otope dilution, however, it was necessary to show that the transient nature of the gas chromatographic profile and the pulsed nature of the ion trap m ass spectrometer did not severely limit the precision with which an isotope ratio could be measured. Neither had a significant influence, as demonstra ted by the precision, which ranged from similar to 0.2 to similar to 2.5% R SD, depending upon the relative abundance of the two isotopes and the absol ute concentration of mercury in the sample. Calibration curve and standard addition approaches were the benchmarks to which isotope dilution was compa red for quantification. When isotope dilution was employed, accuracy was im proved by a factor of 30 over the calibration curve approach and a factor o f 14 over the standard addition approach.