ACQUISITION AND PROCESSING OF DATA FOR ISOTOPE-RATIO-MONITORING MASS-SPECTROMETRY

Methods are described for continuous monitoring of signals required fo r precise analyses of C-13, O-18, and N-15 in gas streams containing v arying quantities of CO2 and N2. The quantitative resolution (i.e. max imum performance in the absence of random errors) of these methods is adequate for determination of isotope ratios with an uncertainty of on e part in 10(5); the precision actually obtained is often better than one part in 10(4). This report describes data-processing operations in cluding definition of beginning and ending points of chromatographic p eaks and quantitation of background levels, allowance for effects of c hromatographic separation of isotopically substituted species, integra tion of signals related to specific masses, correction for effects of mass discrimination, recognition of drifts in mass spectrometer perfor mance, and calculation of isotopic delta values. Characteristics of a system allowing off-line revision of parameters used in data reduction are described and an algorithm for identification of background level s in complex chromatograms is outlined. Effects of imperfect chromatog raphic resolution are demonstrated and discussed and an approach to de convolution of signals from coeluting substances described.