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Ion-trap chemical-ionization performance has been improved by application of a modified scan function for the rejection of the undesired electron-ionization-like (EI-like) ions formed at the beginning of the reaction ionization period. The net effect of this software modification to the automatic reaction control is to produce chemical ionization (CI) spectra that are no longer adulterated with concentration-dependent EI-like ions. Under such improved conditions, CI spectra from an ion trap can now be directly compared with CI spectra produced on conventional quadrupole and magnet-scanning instruments.

During regulatory analysis of pesticides and industrial chemicals, compounds are frequently encountered which cannot be readily identified by gas chromatography alone on two or more stationary phases. Such incurred unknown analytical responses are examined by gas chromatography mass spectrometry, particularly if they have been shown to contain P, S, N or halogen via specific gas chromatography detectors. Seven case histories are presented to illustrate the analytical approach adopted to structurally elucidate such compounds at the ppm level. These examples demonstrate the potential complementary combination provided by low resolution electron impact and chemical ionization mass spectrometry in structural determinations.