Inductively Coupled Plasma Mass Spectrometer, elan DRC (ICP-MS)

Inductively coupled plasma dynamic reaction cell mass spectrometry (ICP-DRC-MS) was characterised for the detection of the six naturally occurring selenium isotopes. The potentially interfering argon dimers at the selenium masses m/z 74, 76, 78 and 80 were reduced in intensity by approximately five orders of magnitude by using methane as reactive cell gas in the DRC. By using 3% v/v methanol in water for carbon-enhanced ionisation of selenium, the sensitivity of 80Se was 104 counts s−1 per ng ml−1 of selenium, and the estimated limit of detection was 6 pg ml−1. The precision of the isotope ratios was close to the theoretical values for selenium concentrations at 1 and 10 ng ml−1. The accuracy of the isotope ratios, however, was improved by correcting the count rate of all selenium isotopes equivalent to the formation of SeH at 9.6 ± 0.5% one mass unit above the selenium isotopes. A linear relationship (r < 0.98) was found between the error of the corrected isotope ratios and the difference in mass from the 80Se reference isotope. This indicated that the error was caused by mass bias. The slope of the curve at −3.0% error per mass unit can be used for correction of the measured ratios. Deuterated methane used as the DRC gas showed that hydrogen transfer from methane was not involved in the formation of SeH as SeD was absent in the mass spectrum. The almost interference-free detection of selenium by ICP-DRC-MS made the detection of the 80Se isotope possible for detection of selenoamino acids separated by cation exchange HPLC. The limit of detection of the HPLC-ICP-DRC-MS method was in the range 3–5 pg as selenium.