Improved method for estimating molecular weights of volatile organic compounds from low resolution mass spectra

Abstract

Scott, D.R., 1991. Improved method for estimating molecular weights of volatile organic compounds from low resolution mass spectra. Chemometrics and Intelligent Laboratory Systems, 12: 189–200. An improved method of estimating the molecular weights of volatile organic compounds from their mass spectra has been developed and implemented with an expert system. The method is based on the strong correlation of MAXMASS, the highest mass with an intensity of 5% of the base peak in a mass spectrum, with the true molecular weight. Linear corrections to MAXMASS, which yield molecular weight estimates, were derived empirically using the expert system. The system is based on a sequential modular design with a primary classifier linked to molecular weight estimators for the resulting six classes. These are non-halobenzenes; chlorobenzenes; bromo- and bromochloroalkanes/alkenes; mono- and dichloroalkanes/alkenes; tri-, tetra- and pentachloroalkanes/alkenes; and ‘others’. In previous work only one iteration of correction terms was applied to MAXMASS. In the present system two iterations are used. The rules were derived from National Institute of Standards and Technology (NIST) reference spectra of 75 target toxic compounds from the first five classes and 32 from the ‘others’ class. Performance tests were made with the old and the improved expert systems and the Self Training Interpretive and Retrieval System (STIRS) on NIST reference spectra of the 107 training compounds; on a set of 32 test NIST reference spectra of other compounds found in air samples; on a set of 100 randomly selected NIST reference spectra; and on 27 spectra taken from actual field gas chromatography/mass spectrometry samples. These tests show that the domain of this system greatly exceeds the original one. The average absolute value deviations from the true values for the old and new expert system and STIRS were, respectively, 3.1, 1.0 and 11.6 mass units for training spectra and 13.5, 13.4 and 7.1 for test spectra. For field gas chromatographic/mass spectrometric spectra these were 6.0, 4.3 and 10.8 and for random spectra 15.5, 16.0 and 7.5. The expert system is very fast and will run on personal computers.