Anal. Chem. 2004, 76, 7366-7374
Understanding and Designing Field Asymmetric Waveform Ion Mobility Spectrometry Separations in Gas Mixtures Alexandre A. Shvartsburg, Keqi Tang, and Richard D. Smith
Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, MS K8-98, 3335 Q Avenue, Richland, Washington 99352
Field asymmetric waveform ion mobility spectrometry (FAIMS) has significant potential for post-ionization separations in conjunction with MS analyses. FAIMS fractionates ion mixtures by exploiting the fact that ion mobilities in gases depend on the electric field in a manner specific to each ion. Nearly all previous work has used pure gases, for which FAIMS fundamentals are understood reasonably well; however, unexpected phenomena observed in some gas mixtures (e.g., N2/CO2) but not in others (N2/O2) remain unexplained. Here, we introduce and experimentally test a universal model for FAIMS separations in mixtures, derived from formalisms that determine highfield mobilities in heteromolecular gases. Overall, the theoretical findings are consistent with data for N2/CO2 (although quantitative discrepancies remain), while results for N2/O2 fit Blanc’s law, in agreement with measurements. Calculations for He/N2 and He/CO2 are also consistent with observations and suggest why adding He to the working gas generally enhances FAIMS performance. As predicted, mixtures of gases with extremely disparate molecular masses and collision cross sections, such as He/SF6, exhibit spectacular non-Blanc effects, which greatly improve the resolution and peak capacity of technique. Understanding FAIMS operation in gas mixtures is expected to enable the rational design of media for both targeted and global analyses. Over the past decade, rapid development of approaches to the analysis of extremely complex samples, especially in proteomics1 and metabolomics,2 has highlighted the need for better separations methods. Most established approaches involve separations in condensed phases [e.g., SDS-PAGE,3 liquid chromatography (LC),4 strong-cation exchange (SCX),5 and capillary electrophoresis6]. Many of these methods provide high peak capacities (∼102 to 103) and near-complete sample recovery. Even higher peak capacities (in the 103-104 range) are provided by multidimensional separations, such as 2-D gel7 and SCX/reversed-phase LC (the (1) Aebersold, R.; Mann, M. Nature 2003, 422, 198. (2) Griffin, J. L. Curr. Opin. Chem. Biol. 2003, 7, 648. (3) Laemmli, U. K. Nature 1970, 227, 680. (4) Shen, Y.; Zhao, R.; Berger, S. J.; Anderson, G. A.; Rodriguez, N.; Smith, R. D. Anal. Chem. 2002, 74, 4235. (5) Washburn, M. P.; Wolters, D.; Yates, J. R. Nat. Biotechnol. 2001, 19, 242. (6) Shen, Y.; Xiang, F.; Veenstra, T. D.; Fung, E. N.; Smith, R. D. Anal. Chem. 1999, 71, 5348.
7366 Analytical Chemistry, Vol. 76, No. 24, December 15, 2004
MudPIT protocol8); however, all condensed-phase separations inherently take substantial time (typically minutes to hours) because of analyte diffusion rates, and multidimensional separations can last hours to tens of hours.8 Hence, the throughput is limited to a few samples per day, which effectively precludes many potential bioanalytical applications. In most proteomics and metabolomics applications, separations are coupled with MS analyses. Condensed-phase methods typically provide only a few fractions (i.e., distinct peaks) per minute. This rate is far short of possible MS acquisition speeds; for example, a typical time-of-flight MS collects ∼106 spectra/min. This disparity has created a strong interest in faster gas-phase separations based on ion mobility spectrometry (IMS).9-16 In conventional IMS, a static uniform electric field pushes ions through a nonreactive buffer gas at a constant velocity (v). In the zero-field limit, v is proportional to the field intensity (E), and the mobility (K) is defined as v/E. Converting K to the standard gas number density (N) yields the reduced mobility (K0). This quantity depends on the temperature (T), and the K(T) curve uniquely characterizes the ion/buffer gas combination. Typically IMS is implemented in drift tubes that support a fixed axial electric field.9-16 A packet encompassing ions with a range of K0 is partitioned while traversing a tube, and a mass spectrometer interfaced after the IMS can sequentially probe exiting components. Although any type of MS can be employed, the IMS/TOF hybrid that disperses analyte mixtures in both mobility and mass dimensions15,16 is especially promising as a high-throughput platform. A related but intrinsically different technique of field asymmetric waveform ion mobility spectrometry (FAIMS) has gained acceptance over the past few years.17-45 FAIMS analyses utilize the fact that ion mobilities depend on the electric field intensity: (7) Shevchenko, A.; Wilm, M.; Vorm, O.; Mann, M. Anal. Chem. 1996, 68, 850. (8) Wolters, D. A.; Washburn, M. P.; Yates, J. R. Anal. Chem. 2001, 73, 5683. (9) Hill, H. H., Jr.; Siems, W. F.; St. Louis, R. H.; McMinn, D. G. Anal. Chem. 1990, 62, A1201. (10) Eiceman, G. A. Crit. Rev. Anal. Chem. 1991, 22, 17. (11) von Helden, G.; Hsu, M. T.; Kemper, P. R.; Bowers, M. T. J. Chem. Phys. 1991, 95, 3835. (12) Dugourd, P.; Hudgins, R. R.; Clemmer, D. E.; Jarrold, M. F. Rev. Sci. Instrum. 1997, 68, 1122. (13) Fromherz, R.; Gantefo ¨r, G.; Shvartsburg, A. A. Phys. Rev. Lett. 2002, 89, 083001. (14) Bluhm, B. K.; Gillig, K. J.; Russell, D. H. Rev. Sci. Instrum. 2000, 71, 4078. (15) Srebalus Barnes, C. A.; Clemmer, D. E. Anal. Chem. 2001, 73, 424. (16) Hoaglund-Hyzer, C. S.; Lee, Y. J.; Counterman, A. E.; Clemmer, D. E. Anal. Chem. 2002, 74, 992. (17) Gorshkov, M. P. U.S.S.R. Inventor’s Certificate 966583, 1982. 10.1021/ac049299k CCC: $27.50
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K0 is really K0(0), that is, the E w 0 limit of K0(E) function that is a polynomial of even powers of E/N.
K0(E) ) K0(1 + a(E/N)2 + b(E/N)4 + c(E/N)6 + ...) (1)
At realistic field intensities (
