Ion-trap detector electron impact mass spectrometry : practical considerations

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 + 11 +, 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.