Determination of the H3 factor in hydrogen isotope ratio monitoring mass spectrometry

The H-3 factor, K, is a parameter required in high-precision, mass spectrometric analyses of hydrogen isotopic abundances. When H-2 is used as the sample gas, R* = R - Ki(2), where R* is the true HD/H-2 ratio, R is the observed (mass 3)/(mass 2) ion-current ratio, and i(2) is the ion current at mass 2, Four different methods for the determination of K were defined and tested under conditions characteristic of isotope ratio monitoring systems, Three of these were peak-based. The fourth employed steady flows of H-2 from a conventional inlet system, Results obtained using the latter method were more precise (standard deviation of K = 0.1 versus similar to0.6 ppm mV(-1) for the peak-based methods). However, use of the resulting values of K for correction of isotope ratio monitoring GC/MS results led to systematic errors as large as 9 parts per thousand, whereas use of the peak-based values led to no systematic errors. Values of K were only weakly dependent on the pressure of He, declining similar to5% for each 10-fold increase in P-He. Small variations in partial pressures of H2O and CH4, potential contaminants under isotope ratio monitoring conditions, had no significant effect on values of K.