Simulation of ion motion in FAIMS through combined use of SIMION and modified SDS

A key application of field asymmetric waveform ion mobility spectrometry (FAIMS) has been in selectively transmitting trace analyte ions that are present in a complex ion mixture to a mass spectrometer (MS) for identification and quantification. The overall sensitivity of FAIMS-MS, however, still needs to be significantly improved through the optimization of ion transmission into FAIMS and at the FAIMS-MS interface. Processes that cause ion losses include diffusion, space charge, separation field in the FAIMS and fringe fields around the edges of the FAIMS electrodes. These were studied here by first developing an algorithm using SIMION as its core structure to compute ion trajectory at different ratios of electric field to buffer gas number density (E/N). The E/N was varied from a few Td to approximately 80 Td by using an asymmetric square waveform. The algorithm was then combined with statistical diffusion simulation (SDS) model, columbic repulsion, and a parabolic gas flow profile to realistically simulate current transmission and peak shape. The algorithm was validated using a FAIMS model identical to the Sionex Corporation SVAC model. Ions modeled included low mass ions with K(o) in the range of 2.17 (m = 55) to 1.39 cm(2) x V(-1) x s(-1) (m = 368). Good agreement was achieved between simulated and experimental CV (peak maxima) values, peak width (fwhm), and transmitted ion current I(output). The model was then used to study fringe fields in a simple arrangement where a 0.5 mm (w) gap was created between the FAIMS exit and a capillary inlet (i.d. = 0.5 mm). At an optimum CV (11.8 V), only approximately 17% (1.3 pA) of the total ion current that correlate to CV = 11.8 V, entered the capillary; bulk of the ion loss was caused by the fringe fields. Current transmission into the capillary was improved, however, by applying a 500 V DC bias across w (0.5 mm).