E/N effects on K 0 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 cm2 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.