Investigation of the measurable range of sample amount and bias of the carbon and nitrogen stable isotope ratio analyzed by elemental analyzer-isotope ratio mass spectrometer system
Bulletin of the National Research Institute of Fisheries Science 16: 27-37
The elemental analyzer-Isotope ratio mass spectrometer (EA-IRMS) system at the National Research Institute of Fisheries Science was tested for the measurable range of sample amount, the dependence of the isotope ratio on sample amount, and bias of the isotope ratio which is introduced by helium (He) dilution of CO2 gas. The nitrogen and carbon isotope ratio of L-alanine prepared as the working standard was -2.4permill (s.d.=0.10permill) and -22.06permill (s.d.=0.011 permill), respectively. Signal height of the major isotope (28 for N2, 44 for CO2) was linearly regressed on sample nitrogen (or carbon) content as long as the signal height was lower than 10 V. Measured delta value was linearly regressed on the signal height (< 10 V for delta15N measurement, < 7 V for delta13C measurement), except for delta13C measurement at He dilution pressure of 6 psi, where delta value was constant. delta13C value at He dilution of 12 psi was highly variable, therefore applicable He dilution pressure was 10 psi or lower. The measurable amount of nitrogen content was 0.021 - 0.265 mg, and carbon content was 0.006 - 4.82 mg.