Development of A Cryogenic Drift Cell Spectrometer and Methods for Improving the Analytical Figures of Merit for Ion Mobility-Mass Spectrometry Analysis

Abstract

A cryogenic (325-80 K) ion mobility-mass spectrometer was designed and constructed in order to improve the analytical figures-of-merit for the chemical analysis of small mass analytes using ion mobility-mass spectrometry. The instrument incorporates an electron ionization source, a quadrupole mass spectrometer, a uniform field drift cell spectrometer encased in a cryogenic envelope, and an orthogonal geometry time-of-flight mass spectrometer. The analytical benefits of low temperature ion mobility are discussed in terms of enhanced separation ability, ion selectivity and sensitivity. The distinction between resolving power and resolution for ion mobility is also discussed. Detailed experimental designs and rationales are provided for each instrument component. Tuning and calibration data and methods are also provided for the technique. Proof-of-concept experiments for an array of analytes including rare gases (argon, krypton, xenon), hydrocarbons (acetone, ethylene glycol, methanol), and halides (carbon tetrachloride) are provided in order to demonstrate the advantages and limitations of the instrument for obtaining analytically useful information. Trendline partitioning of small analyte ions based on chemical composition is demonstrated as a novel chemical analysis method. The utility of mobility-mass analysis for mass selected ions is also demonstrated, particularly for probing the ion chemistry which occurs in the drift tube for small mass ions. As a final demonstration of the separation abilities of the instrument, the electronic states of chromium and titanium (ground and excited) are separated with low temperature. The transition metal electronic state separations demonstrated here are at the highest resolution ever obtained for ion mobility methods. The electronic conformational mass isomers of methanol (conventional and distonic) are also partially separated at low temperature. Various drift gases (helium, neon, and argon) are explored for the methanol system in order to probe stronger ion-neutral interaction potentials and effectuate higher resolution separations of the two isomeric ions. Finally, two versatile ion source designs and a method for axially focusing ions at low pressure (1-10 torr) using electrostatic fields is presented along with some preliminary work on the ion sources.