Ion‐trap detector electron impact mass spectrometry: Practical considerations

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Abstract
An investigation of the quality of mass spectra obtainable by the Finnigan ion‐trap detector (ITD) and on how this quality may depend on sample size is described. For compounds which are not very thermolabile or too prone to fragmentation, near‐normal spectra can be expected if the sample sizes do not exceed amounts of the order of 100 ng. Analyte loads which generate acceptable total ion current chromatograms with narrow‐bore capillary columns (about 500 ng) could still produce ITD mass spectra having enhanced quasi‐molecular ions, [M + 1]+, abnormal distributions of peak intensities and unusual fragments. Even under optimum conditions, certain analytes, e.g. acyclic alkanes, n‐alkanols and pyridines, will manifest spectral characteristics which are strictly typical of the ITD. In favourable cases, the formation of quasi‐molecular ions can be exploited for relative molecular mass determination. Even under conditions of self‐chemical ionization, the ITD spectra of most analytes generally retain enough electron impact mass spectral character to be readily recognizable by inspection or by library algorithms.