Field Dependence of Mobilities for Gas-Phase-Protonated Monomers and Proton-Bound Dimers of Ketones by Planar Field Asymmetric Waveform Ion Mobility Spectrometer (PFAIMS) E. Krylov, E. G. Nazarov, R. A. Miller, ² B. Tadjikov, and G. A. Eiceman* Department of Chemistry and Biochemistry, New Mexico State UniVersity, Las Cruces, New Mexico 88003, and SIONEX Corporation, Wellesley Hills, Massachusetts 02481 ReceiVed: January 8, 2002; In Final Form: March 19, 2002 The dependence of the mobilities of gas-phase ions on electric fields from 0 to 90 Td at ambient pressure was determined for protonated monomers [(MH + (H 2 O) n ] and proton bound dimers [M 2 H + (H 2 O) n ] for a homologous series of normal ketones, from acetone to decanone (M ) C 3 H 6 O to C 10 H 20 O). This dependence was measured as the normalized function of mobility R(E/N) using a planar field asymmetric waveform ion mobility spectrometer (PFAIMS) and the ions were mass-identified using a PFAIMS drift tube coupled to a tandem mass spectrometer. Methods are described to obtain R(E/N) from the measurements of compensation voltage versus amplitude of an asymmetric waveform of any shape. Slopes of R for MH + versus E/N were monotonic from 0 to 90 Td for acetone, butanone, and pentanone. Plots for ketones from hexanone to octanone exhibited plateaus at high fields. Nonanone and decanone showed plots with an inversion of slope above 70 Td. Proton bound dimers for ketones with carbon numbers greater than five exhibited slopes for R versus E/N, which decreased continuously with increasing E/N. These findings are the first alpha values for ions from a homologous series under atmosphere pressure and are preliminary to explanations of R(E/N) with ion structure. Introduction The motion of gas-phase ions in electric fields at pressures greater than 1 Torr has been explored for over a century 1-3 and velocities of ion swarms in controlled atmospheres of drift tubes have been used since the 1960s to model interactions between ions and molecules. 4-7 In recent years, these principles have been developed in mobility spectrometers for the detection of chemical warfare agents and explosives. 8 In general, ion motion is measured as velocity (V d , cm/s) in uniform gradients of electric fields (E, V/cm) low enough so that velocity is proportional to the electric field through a constant, the coefficient of mobility (i.e., K )V d /E, cm 2 /Vs). One relationship between the mobility coefficient, ion structure, and the gas atmosphere is eq 1: 5 where the terms are e, elementary charge; N, drift gas density; μ, reduced ion/neutral mass; T eff , effective temperature of the ion; and Ω, collisional cross section. The mobility coefficient is characteristic of the structure of an ion and the ion-molecule interactions in a gas; also, K is influenced by collision frequency and energy obtained from the field by ions between collisions. The average energy acquired from the electric field is determined by E/N and is considered negligible when E/N is small since any energy gained by the ion from the field is dissipated at high pres- sures by collisions with the supporting gas. Under such conditions, K is a constant and independent of E/N. However, the mobility coefficient becomes dependent on electric field with increasing values of E/N as shown in eq 2: 5,7,9 where terms are K(0), the mobility coefficient under low field conditions and R 2 , R 4 , ..., R 2n , specific coefficients of even powers of the electric field; E/N is in units of Td (under normal conditions 1 Td corresponds to 268.67 V/cm since E ) N 0 10 -17 , where N 0 is Loschmidt’s constant). The mobility coefficient should be expressed as an even power series in E/N due to symmetry considerations (i.e., the absolute value for ion velocity is independent of electric field direction). When experimental conditions are constant, unique patterns for K versus E/N exist for different ions due to characteristic values of R 2n .A simplification of eq 2 has been described 10 where an alpha function is used for the electric field dependence of the coefficient of mobility per eq 3: where R(E/N) )R 2 (E/N) 2 +R 4 (E/N) 4 + .... This formula is a function showing the nonlinear electric field dependence of mobility for specific ions. Extensive experimental findings exist for the nonlinear dependence of the mobility coefficient on electric field though studies were restricted to simple ions or ions containing only a few atoms. Results available in the Atomic Data of Nuclear Data Tables show that K o is a nonlinear function with increasing E/N at subambient pressures. 11-14 Comparable findings at ambient pressure (high neutral gas density), where ion-neutral processes exert a dominant role in forming mobility spectra, are not generally available. Only a few studies for the measure- ment of K(E/N) at ambient pressure have been made owing to ² SIONEX Corporation. K ) 3e 16N  2π μkT eff 1 Ω 1.1 (T eff ) (1) K(E/N) ) K(0)[1 +R 2 (E/N) 2 +R 4 (E/N) 4 + ...] (2) K(E/N) ) K(0)[1 +R(E/N)] (3) 5437 J. Phys. Chem. A 2002, 106, 5437-5444 10.1021/jp020009i CCC: $22.00 © 2002 American Chemical Society Published on Web 05/14/2002