Conservation\nof the Charge in Signal from Drift Tube\nIon Mobility Spectrometers

Quantitative information obtained from drift tube detectors\nused\nin ion mobility spectrometry is contained in the area of peaks forming\nthe drift time spectrum. The area of all peaks corresponds to the\ntotal charge of ions entering the drift section of the spectrometer.\nIt was found that this charge is not conserved when the ion composition\nchanges. This work is devoted to studying the causes of this phenomenon.\nExperimental research consisted of recording drift time spectra for\n2-pentanone and <i>n</i>-heptanone, at various analyte concentrations\nand different opening times of the shutter grid. Measurements of the\ntotal ion current were also performed in static mode with an open\ngrid. The research results indicated that the reasons for the lack\nof ion charge conservation in the drift time spectrum are ion recombination,\nmutual repulsion, and mobility-dependent transmission of ions through\nthe shutter grid. The explanation of the relationships obtained experimentally\nwas based on a simple theoretical model, which considered the phenomenon\nof ion transport along the reaction section and the penetration of\nions through the shutter. The developed model provides a good description\nof the measurement results and allows the estimation of ion currents\nand ion concentrations in the reaction section upstream of the grid.\nThis information is important for proper quantitative analysis as\nwell as when the ion mobility spectrometer is used in quantitative\nstudies of chemical ionization processes.