E/N effects on K0 values revealed by high precision measurements under low field conditions

Ion mobility spectrometry (IMS) is used to detect chemical warfare agents, explosives, and narcotics. While IMS has a low rate of false positives, their occurrence causes the loss of time and money as the alarm is verified. Because numerous variables affect the reduced mobility (K-0) of an ion, wide detection windows are required in order to ensure a low false negative response rate. Wide detection windows, however, reduce response selectivity, and interferents with similar K-0 values may be mistaken for targeted compounds and trigger a false positive alarm. Detection windows could be narrowed if reference K-0 values were accurately known for specific instrumental conditions. Unfortunately, there is a lack of confidence in the literature values due to discrepancies in the reported K-0 values and their lack of reported error. This creates the need for the accurate control and measurement of each variable affecting ion mobility, as well as for a central accurate IMS database for reference and calibration. A new ion mobility spectrometer has been built that reduces the error of measurements affecting K-0 by an order of magnitude less than +/- 0.2%. Precise measurements of +/- 0.002 cm(2) V-1 s(-1) or better have been produced and, as a result, an unexpected relationship between K-0 and the electric field to number density ratio (E/N) has been discovered in which the K-0 values of ions decreased as a function of E/N along a second degree polynomial trend line towards an apparent asymptote at approximately 4 Td. Published by AIP Publishing.