Predicting optimal resolving power for ambient pressure ion mobility spectrometry

Although diffusion theory predicts that IMS resolving power increases with the square root of the voltage applied across the drift tube, in practice, there exists an optimum voltage above which resolving power decreases. This optimum voltage was determined to be both compound and initial ion pulse width dependent. A "conditional" resolving power equation is introduced that can be used to quickly approximate realistic resolving powers for specific instrumental operating parameters and compounds. Using four common environmental contaminants (trichloroethylene, tetrachloroethylene, methyl tert-butyl ether, methyl isobutyl ketone), diffusion-limited (theoretical), R d, conditional, R c, and actual (or measured), R m, IMS resolving powers were determined and compared for a small IMS instrument designed for subsurface measurements. Detection limits determined at the optimal resolving power for the environmental contaminants ranged from 18 parts per trillion volume-to-volume (ppt v) to 80 parts per billion volume-to-volume (ppb v). The maximal measured resolving power for our small, ambient-pressure stand-alone IMS ranged from 42 to 54, yielding an IMS resolving power efficiency, defined as R m/ R c x 100%, of 56-74% of the maximal conditional resolving power possible.