Quantitative analysis of unlabelled and polydeuterated compounds by gas chromatography-mass spectrometry

Summary A system for computer evaluation of gas chromatographic-mass spectrometric analyses of unlabelled and polydeuterated compounds is described. The system has been in routine use for two years and has been found to fulfil most of the needs of the laboratory in connection with studies of steroid and bile acid metabolism. Complete or partial spectra are taken by repetitive magnetic scanning and are recorded on magnetic tape. Methods for qualitative evaluation have been described previously 6, 9]. Quantitative determinations of unlabelled compounds are based on areas of gas chromatographic peaks in selected specific fragment ion current chromatograms generated by the computer. Percentage distribution of molecular or fragment ion species containing 0–19 deuterium atoms is calculated from averaged spectra by comparison with spectra of the unlabelled reference compound analyzed on the same day. When small amounts of material are available and when ions occur in low abundance, repetitive accelerating voltage scanning over a short mass range is used. A slow scan rate, 1–5 s, and a high data sampling rate, 10 kHz, permit a 100–300 fold increase of amplification. After bunching of intensity readings, partial mass spectra are constructed which are treated in the same way as complete spectra.