Improving FAIMS sensitivity using a planar geometry with slit interfaces

Abstract

Differential mobility spectrometry or field asymmetric waveform ion mobility spectrometry (FAIMS) is gaining broad acceptance for analyses of gas-phase ions, especially in conjunction with largely orthogonal separation methods such as mass spectrometry (MS) and/or conventional (drift tube) ion mobility spectrometry. In FAIMS, ions are filtered while passing through a gap between two electrodes that may have planar or curved (in particular, cylindrical) geometry. Despite substantial inherent advantages of the planar configuration and its near-universal adoption in current stand-alone FAIMS devices, commercial FAIMS/MS systems have employed curved FAIMS geometries that can be more effectively interfaced to MS. Here we report a new planar (p-) FAIMS design with slit-shaped entrance and exit apertures that substantially increase ion transmission in and out of the analyzer. The entrance slit interface effectively couples p-FAIMS to multi-emitter electrospray ionization (ESI) sources, improving greatly the ion current introduced to the device and allowing liquid flow rates up to ϳ50 L/min. The exit slit interface increases the transmission of ribbon-shaped ion beams output by the p-FAIMS to downstream stages such as a MS. Overall, the ion signal in ESI/FAIMS/MS analyses increases by over an order of magnitude without affecting FAIMS resolution. (J Am Soc Mass Spectrom 2009, 20, 1768 -1774) © 2009 American Society for Mass Spectrometry F ield asymmetric waveform ion mobility spectrometry (FAIMS) is increasingly recognized as a powerful separation technique that can process ions in gases in milliseconds at ambient conditions [1][2][3][4][5][6] and is readily combined with other common analytical tools, including liquid chromatography (LC) [7][8][9] and/or mass spectrometry (MS) [9 -13]. The operation of FAIMS has been understood fundamentally [2,5,14] and modeled mathematically and in simulations [10,[15][16][17]. Briefly, the mobility of an ion in a gas mainly depends on E/N, where E is the electric field intensity and N is the gas number density [18]:where K(0) is the mobility in zero-field limit and f(E/N) is a function of field strength. This allows sorting ions by the difference of their mobility at high and low E/N [that is, the form of f(E/N)], regardless of K(0). On the contrary, the conventional drift tube ion mobility spectrometry (DTIMS) separates ions by absolute mobility, and the field is commonly weak enough to equate K(E/N) to K(0). In FAIMS, ions experience alternating strong and weak fields of opposite polarity, generated by a periodic asymmetric waveform [1] applied in the gap between two electrodes in the direction perpendicular to a gas flow. Ions carried through FAIMS by the flow are displaced toward one of the electrodes in each waveform cycle and would eventually be lost. To prevent this for species with a specific mobility difference between high and low E/N and keep them stable in the gap, the net displacement induced by the waveform is offset by adding to it an appropriate dc potential (the com...

Ultrafast Differential Ion Mobility Spectrometry at Extreme Electric Fields Coupled to Mass Spectrometry

“…With E D > 0, a positive E C that rises at higher E D means a mobility decreasing with increasing field. This classifies the ion as “type C” in line with previous data at absolute E D / N < ~80 Td, where | E C / N | was under 0.06 Td 16,22. (Those data were for E D < 0, hence E C < 0).…”

Differential Ion Mobility Separations of Peptides with Resolving Power Exceeding 50

“…Bradykinin (Bk, 1060 Da) is a common model peptide in MS and FAIMS research 15,18,19. ESI typically generates protonated Bk with z = 2 (dominant) and 3.…”

Separation of Peptide Isomers with Variant Modified Sites by High-Resolution Differential Ion Mobility Spectrometry

“…Ion transmission efficiency through the present FAIMS device is low (~0.1 - 1%), but losses at the FAIMS/MS interface can be ameliorated using slit-aperture inlets that match the geometry of ion beams exiting a planar FAIMS gap 45,55. The current EC/TD methods for localization of phosphorylations involve signal reductions by a similar 10 2 - 10 3 times compared to the parent ion, but cannot address mixtures of more than two variants because of non-unique fragmentation.…”