Predicting Total Electron-Ionization Cross Sections and GC-MS Calibration Factors Using Machine Learning

Concentrations in GC-MS using electron-ionization mass spectrometry can be determined without pure calibration standards through prediction of relative total-ionization cross sections. An atom- and group-based artificial neural network (FF-NN-AG) model is created to generate EI cross sections and calibrations for organic compounds. This model is easy to implement and is more accurate than the widely used atom-additivity-based correlation of Fitch and Sauter (Anal. Chem. 1983). Ninety-two new measurements of experimental EI cross sections (70-75 eV) are joined with different interlaboratory datasets, creating a 396-compound cross-section database, the largest to date. The FF-NN-AG model uses 16 atom-type descriptors, 79 structural-group descriptors, and one hidden layer of 10 nodes, trained 500 times. In each cycle, 96% of the compounds in this database are freshly chosen at random, and then the model is tested with the remaining 4%. The resulting r² is 0.992 versus 0.904 for the Fitch and Sauter correlation, root mean square deviation is 2.8 versus 9.2, and maximum relative error is 0.30 versus 0.73. As an example of the model's use, a list of cross sections is generated for various sugars and anhydrosugars.

Tropylium, chlorine isotopic abundances, monomeric metaphosphate anion, and conestoga wagon theory

As I look back over a career in mass spectrometry, three high points stand out especially prominently. These are associated with (1) the tropylium model for the CuH 7 (+) ion in the mass spectra of toluene and other alkylbenzenes, (2) revision of the previously accepted value for the natural abundance of the chlorine isotopes, and (3) the first direct observation of the monomeric metaphosphate anion, which had been for a quarter of a century an elusive, suspected reaction intermediate. Studies of organic ions in the rarefied atmosphere of the mass spectrometer, where only unimolecular processes are allowed, have deepened my appreciation of the role and ubiquity of bimolecular processes in more conventional chemical contexts. Consideration of the two categories of molecular behavior has prompted me to seek and find, for a selected system in the mass spectrometer, parallels both in condensed-phase chemistry and, by an anthropomorphic extension, in human behavior.