Effect of Moisture on the Field Dependence of Mobility for Gas-Phase Ions of Organophosphorus Compounds at Atmospheric Pressure with Field Asymmetric Ion Mobility Spectrometry

Abstract

The electric field dependence of the mobilities of gas-phase protonated monomers [(MH+(H2O)n] and proton-bound dimers [M2H+(H2O)n] of organophosphorus compounds was determined at E/N values between 0 and 140 Td at ambient pressure in air with moisture between 0.1 and 15 000 ppm. Field dependence was described as alpha (E/N) and was obtained from the measurements of compensation voltage versus field amplitude in a planar high-field asymmetric waveform ion mobility spectrometer. The alpha function for protonated monomers to 140 Td was constant from 0.1 to 10 ppm moisture in air with onset of effect at approximately 50 ppm. The value of alpha increased 2-fold from 100 to 1000 ppm at all E/N values. At moisture values between 1000 and 10 000 ppm, a 2-fold or more increase in alpha (E/N) was observed. In a model proposed here, field dependence for mobility through changes in collision cross sections is governed by the degree of solvation of the protonated molecule by neutral molecules. The process of ion declustering at high E/N values was consistent with the kinetics of ion-neutral collisional periods, and the duty cycle of the waveform applied to the drift tube. Water was the principal neutral above 50 ppm moisture in air, and nitrogen was proposed as the principal neutral below 50 ppm.

Feasibility of Higher-Order Differential Ion Mobility Separations Using New Asymmetric Waveforms

“…55 The resulting values (at T = 20 °C, in 10 −14 Td −6 ) are 1.29 -4.76 with 〈| c|〉 = 2.86, which allows projecting typical separation parameters for 4 th -order HODIMS. In this scenario, a useful operation could be achieved at E D /N ≈ 110 -125 Td, which is just above the highest E D /N ~110 Td employed in FAIMS 33,54 and well below the breakdown threshold for any gap width.…”

“…5a), a field already employed in some FAIMS studies. 33,54 Recalling that FAIMS becomes useful at E D / N ~ 40 -50 Td, one may estimate the fields needed for similar HODIMS performance as S 100 -115 Td for an "average" amino acid and ~ 90 -105 Td for H + lysine (Fig. 5a).…”

“…There also is a significant orthogonality between FAIMS and IMS dimensions, 63,64 which enables 2-D separations by FAIMS/IMS coupling. 64 However, FAIMS is still substantially correlated to MS. For example, in FAIMS in N 2 or air buffer, small ions with masses up to several hundred Da (including monatomics, 30 all amino acid ions, 31 and other simple organic ions 33,54 ) are "A-type" 49 (i.e., have a positive a), while large ions (including all peptides 24,53,63,64 ) are "C-type" 48 (i.e., have a negative a). The inverse correlation between a and m is also found within many homologous series, e.g., for the previously introduced organophosphorus compounds, 54 ketones, 33 and aromatic amines, 55 halogenate anions, 65 and amino acids (below).…”

Optimum waveforms for differential ion mobility spectrometry (FAIMS)

“…For species with a 1 Ͼ 0, typically a 2 Ͻ 0, and a increases up to a maximum at certain E/N and decreases at greater E/N. This behavior (called "type B") [5] is ubiquitous for both atomic and polyatomic cations and anions with m Ͻ ϳ400 Da in N 2 or air at room temperature, including 13 of 17 protonated and 15 of 17 deprotonated amino acids [54], protonated benzene, and all seven amines studied [55], eight protonated ketones up to decanone and five of their proton-bound dimers [56], all 10 protonated organophosphorus compounds investigated and seven of their dimers [57], and I Ϫ and anions of five common explosives and their degradants: 1,3-dinitrobenzene, 1,3,5-trinitrobenzene, p-mononitrotoluene, 2,4-dinitrotoluene, and 2,4,6-trinitrotoluene (TNT) [28]. The magnitude of a 2 /a 1 for those 72 species spans Ͼ3 orders of magnitude from Ͻ10 Ϫ6 to Ͼ10 Ϫ3 Td Ϫ2 ( Figure 5), but most values are about 10 Ϫ5 to 10 Ϫ4 Td Ϫ2 regardless of the ion mass.…”

“…[54], protonated benzene and amines (crosshair) [55], protonated ketone monomers (filled circle) and dimers (open circle) [56], protonated monomers (filled square), and dimers (open square) of organophosphorus compounds [57], and deprotonated or radical anions of explosives (open diamond) [28]. Horizontal lines mark values providing a R ϭ Ϫ1 at the stated E/N.…”