"Ion Mobility-Mass Spectrometry" In

electric field. The mobility of ions is based on their size, Ion Mobility-Mass shape, and charge, thus IMS provides insights into struc- Spectrometry ture. In addition to being used for structural information, IMS can also be used as a separation device for complex mixtures. When coupled with mass spectrometry (MS), Wentao Jiang and Rena˜ A.S. Robinson IMS–MS offers a powerful hybrid analytical technique University of Pittsburgh, Pittsburgh, PA, USA that has many biological, pharmaceutical, structural, environmental, and other applications. This article provides an overview of IMS–MS, which focuses on principles 1 Introduction 1 of drift-time ion mobility spectrometry (DTIMS), high- 2 Principles of Ion Mobility Spectrometry 2 field-asymmetric ion mobility spectrometry (FAIMS), 2.1 Drift-Time Ion Mobility Spectrometry 2 and traveling wave ion mobility spectrometry (TWIMS) 2.2 High-Field-Asymmetric Waveform Ion methods. Several IMS–MS instruments are discussed and Mobility Spectrometry 4 examples of the current applications of the technology are 2.3 Traveling Wave Ion Mobility provided. Spectrometry 4 3 Ion Mobility Spectrometry–Mass Spectrometry Instrumentation 4 1 INTRODUCTION 3.1 Sources 5 3.2 Hybrid Instruments 6 IMS is a powerful analytical technique that has become 4 Multidimensional Ion Mobility more widespread in the last 40–50 years. IMS has several Spectrometry–Mass Spectrometry 10 capabilities as a stand-alone instrument and has been used (1–3) 4.1 Liquid Chromatography-Ion Mobility to monitor the detection of atmospheric compounds, (4–6) (7,8) Spectrometry-Mass Spectrometry 10 explosives, chemical warfare agents (CWAs), and (3) 4.2 Capillary Electrophoresis-Ion Mobility petrochemical reagents. In recent years, IMS tech- Spectrometry-Mass Spectrometry 11 nology has been used to detect explosives and narcotics in airport scanner devices. While it has been extremely 4.3 Ion Mobility Spectrometry-Ion Mobility effective in field applications as a stand-alone or portable Spectrometry-Mass Spectrometry 11 device, the coupling of IMS with MS extends the capabil- 4.4 Ion Mobility Spectrometry-Ion Mobility ities and applications of the technique tremendously. Spctrometry-Ion Mobility Spectrometry- IMS–MS is extremely useful for obtaining structural Mass Spectrometry 13 information on small polyatomic ions(9,10) to macromolec- 5 Applications of Ion Mobility ular ions, such as proteins(11–13) and even viruses.(12) Spectrometry–Mass Spectrometry 13 IMS–MS instruments can be operated in modes which 5.1 Proteomics 14 take advantage of IMS as a separation device allowing 5.2 Probing Structural Information 14 complex mixtures to be investigated and low-abundance 5.3 Lipidomics 14 species to be detected owing to the removal of chemical 5.4 Metabolomics 14 noise. Furthermore, IMS–MS provides fast measure- ments which allow it to be compatible with other front-end 5.5 Chiral Species 15 analytical separations, such as liquid chromatography and 5.6 Chemical Warfare Agents 15 capillary electrophoresis. 5.7 Pharmaceuticals 15 Owing to the growing interest in IMS–MS, this 5.8 Environmental 15 article seeks to provide a general overview of IMS–MS 6 Conclusions and Future Outlook 16 technology. There have been several notable advances in Acknowledgments 16 IMS–MS instrumentation which have led to a plethora of interesting applications. The reader will be introduced Abbreviations and Acronyms 16 to the basic principles surrounding three of the most Further Reading 16 commonly employed types of IMS separations. Current References 16 applications of IMS–MS have only been made possible due to the many advances that have taken place in instrumentation development and technology. Thus, an Ion mobility spectrometry (IMS) separates ions based on overview of several IMS–MS instrumentation setups is their mobility in an inert buffer gas in the presence of an also provided. Finally, examples of several applications

Encyclopedia of Analytical Chemistry, Online © 2006–2013 John Wiley & Sons, Ltd. This article is © 2013 John Wiley & Sons, Ltd. This article was published in the Encyclopedia of Analytical Chemistry in 2013 by John Wiley & Sons, Ltd. DOI: 10.1002/9780470027318.a9292 2 MASS SPECTROMETRY that stem from IMS–MS are discussed and this article In the above expression, ze refers to the charge on concludes with a future outlook on potential applications the ion, kB is Boltzmann’s constant, mI and mB are the and advancements of IMS–MS. masses of the ion and buffer gas, respectively, and N is the number density of the buffer gas.(14) Because IMS can be coupled with MS the mass and charge of an ion can 2 PRINCIPLES OF ION MOBILITY be readily deduced. By operating at specific fields (i.e. SPECTROMETRY low or high) or with different pressure regimes, different IMS methods can be developed. Here we discuss three The basic principle of ion mobility separation can common approaches: DTIMS, FAIMS, and TWIMS. be simply described as a gas-phase electrophoresis technique, whereby gaseous ions are separated according to their size, shape, and charge in the presence of a 2.1 Drift-Time Ion Mobility Spectrometry weak electric field. The drift tube is filled with an inert buffer gas (i.e. argon, helium, and nitrogen) at either low DTIMS–MS is the most widespread developed and vacuum pressures or at atmospheric pressure conditions. employed approach. DTIMS is the only IMS method Ions move according to diffusion processes through the which provides a direct measure of collision cross-section drift tube as the energies of the ions are similar to the based on an ion’s mobility. Figure 1(a) shows a simple thermal energy of the buffer gas. Various ions will have drift tube instrument that is filled with inert buffer gas different mobilities in a given drift tube device which in a counter direction of the ion motion. The weak elec- allows the separation of mixtures of ions and structural tric field applied to the drift tube is generated using information to be obtained. The simplest configuration a series of resistors and a DC potential. The electric of a drift tube is one in which a series of stacked ring field applied is generally around 2.5–20 V cm−1(15,16) in electrodes have a static direct current (DC) field applied reduced-pressure IMS (i.e. the drift pressure ranges from across the electrodes and the tube is filled with an inert 1 to 15 mbar). Higher voltages are applied across the drift buffer gas. As ions move under the influence of this tube when higher pressures, such as atmospheric pres- ν weakly applied electric field, they have a velocity, D, sures, are used.(17,18) Regardless of the pressure regime which is governed by the electric field, E, and mobility of used, it is important that the voltages applied do not cause the ion, K, in a specific buffer gas. the potential breakdown of the buffer gas. Traditionally used buffer gases are helium, nitrogen, and argon, or ν = KE (1) D mixtures thereof.(19–24) K is measured experimentally based on the time it takes DTIMS does not work with continuous injection of an ion to traverse the drift tube of length, L. ions, therefore packets of ions are introduced into the drift tube using an ion gate(17,25) or ion funnel.(15,26) Ion L packets can range in width from 100 to 200 μs. Because of K = ( ) 2 the use of ion packets, the overall sensitivity of the method tDE is reduced(27) such that only 0.1–1% of ions generated are Comparisons of reduced ion mobilities, K0, across sent to the IMS. After the ions are injected into the drift laboratories can be obtained by normalizing for buffer tube, the species begin to separate based on their mobility gas pressure, P , and temperature, T , as follows: through the buffer gas. For example, doubly-charged species experience the force of the electric field twice as L 273 P = much as singly-charged ions, therefore for ions of the same K0 (3) EtD T 760 shape the doubly-charged ion will have a higher mobility through the tube and thus a shorter drift time. Also, ions It is often useful to deduce information about the which have more elongated conformations will undergo structure (i.e. the size and shape) of specific ions based more collisions with buffer gas atoms and thus take a on a mobility experiment. This is possible using an experimentally derived collision cross-section, , for an longer time to drift through the tube than more compact ion, which represents the average area of the molecule structures. These concepts are illustrated in Figure 1(a). that interacts with the buffer gas over a range of three- Typically, ions travel through the drift tube on the (28) dimensional orientations. order of milliseconds which makes for a relatively fast separation. As can be inferred from the mobility (18π)1/2 ze 1 1 1/2 t E 760 T 1 equations described, the length of the drift tube can = + D 1/2 influence the transient time and mobility of an ion. The 16 (kBT) mI mB L P 273 N (4) drift resolving power (t/t) at full-width half maximum

Electric field (a)

Drift gas Ion

source Intensity

Drift time Drift tube (b) ~V(t)

Carrier CV1 gas

CV2

Ion Intensity CV3 source

CV CV2 CV (c)

Drift gas Ion source Intensity

t2 Drift time Time tn Traveling wave potential Figure 1 Illustration of drift tube separation principles. (a) Principle of DTIMS. Packets of ions are injected into a drift tube filled with an inert buffer gas. Under the influence of a weak electric field, ions are separated by charge, size, and shape. (b) Principles of FAIMS separation. An asymmetric waveform is applied to two cylindrical plates such that ions experience alternating high and low electric fields. Ions traverse the region between the plates moving in a perpendicular direction to the buffer gas and with the influence of a DC potential, termed the compensation voltage (CV). Only selected ions at a given CV will make it through the drift region. (c) Principles of TWIMS separation. An alternating phase radio-frequency (RF) potential is applied to a series of stacked ring ion guides (SRIGs). Ions are pushed through the drift region with a traveling potential wave and become mobility separated as higher mobility ions are able to ‘roll-over’ the traveling waves generated and exit the SRIG region. is approximated as follows: length for an in-house-built IMS drift tube is ∼1m. (30) (31) Clemmer et al. and Bowers et al. have shown t LEze 1/2 that increasing the tube length to 2 m or greater can ≈ (5) significantly improve the resolving power. A circular t 16kBT ln 2 drift tube design, which has effectively infinite length, This theoretical resolving power approximation(29) can extend the drift resolving powers of small peptides shows that increasing the length of the drift tube or to >300.(32) At higher electric fields, the buffer gas applied electric fields, or decreasing the buffer gas starts to break down, therefore higher electric fields temperature can increase the resolving power. A typical are employed only with higher pressure drift tubes

Encyclopedia of Analytical Chemistry, Online © 2006–2013 John Wiley & Sons, Ltd. This article is © 2013 John Wiley & Sons, Ltd. This article was published in the Encyclopedia of Analytical Chemistry in 2013 by John Wiley & Sons, Ltd. DOI: 10.1002/9780470027318.a9292 4 MASS SPECTROMETRY as mentioned earlier.(33,34) Cryogenically cooled drift 2.3 Traveling Wave Ion Mobility Spectrometry tubes with subambient temperatures have been recently demonstrated.(35) The ability to work with higher TWIMS has its origins in the commercialization of resolution drift instruments allows greater separation IMS–MS technology by Waters Corporation (see Section ( , ) power for complex mixtures or isomeric and isobaric 3.2.1). 42 43 The drift tube instrument in this case consists species with closely related mobilities and can give better of a series of three stacked ring ion guides (SRIG) in insight into structural transitions in the gas phase. which an radio frequency (RF) voltage is applied across It is worth briefly mentioning, as are further discussed consecutive electrodes and used to stop the radial spread subsequently, that the IMS and MS separations using this of ions. Superimposed on top of the radio frequency (RF) approach are not completely orthogonal. Because the fields is a DC voltage which is used to move ions down the mass and shape of ions strongly influence ion mobility, axial direction of the tube. As illustrated in Figure 1(c), there is a mass correlation observed in a two-dimensional the DC is pulsed so that in time ions begin to ride along the (2D) IMS–MS experiment. wells created by the potential field. Ions continue to move forward and are separated as higher mobility ions are also able to ride over the wave. The highest mobility ions ‘roll- 2.2 High-Field-Asymmetric Waveform Ion Mobility over’ the waves less times and have a faster transit through Spectrometry the SRIG.(43,44) The wave amplitude, velocity and buffer gas pressure can be altered in order to optimize ion At extremely high electric fields (i.e. >104 Vcm−1), the transmission through the SRIG.(43) For specific applica- velocity of ions does not follow the relationship shown tions, the SRIG can be used as a transmission device, in Equation (1).(36,37) The mobility of the ions is depen- storage device, or collision cell.(43) Conformational infor- dent on the strength of the electric field which changes mation can be obtained from TWIMS by using careful throughout the course of the experiment. As shown in calibrations to well-studied systems.(45) Figure 1(b), there are two closely spaced (∼2 mm) cylindrical or planar plates in which an asymmetric field is applied.(38) As ions move toward each of the plates, they experience different field strengths and form an oscil- 3 ION MOBILITY SPECTROMETRY–MASS lating motion between the plates. For example, the field SPECTROMETRY INSTRUMENTATION on one plate is twice the electric field that ions experience on the second plate. In contrast to DTIMS, the buffer gas is flowing in the direction of the ion motion which causes IMS–MS began with the work performed by McDaniel the path of the ions to be perpendicular to that of the et al.(46) in the late 1950s and 1960s when he developed electric field. In order for ions to traverse down the course an IMS–MS instrument to study ion molecule reactions of the electrodes and avoid hitting the electrode walls, a of noble gases and pure hydrogen. His instrument compensation voltage (CV) is applied to the plates. Under design was a low-pressure drift device that was coupled a given CV, only a specific ion is able to exit the end of to a magnetic sector mass spectrometer. Kebarle the drift field. Therefore, a scan of increasing CVs allows and Hogg(47) also created an early IMS–MS device a range of ions with different mobilities to be measured. for measuring ethylene gaseous ions. Over the last FAIMS acts similar to a quadrupole mass analyzer in this 40–50 years, there have been numerous developments way which is in contrast to DTIMS whereby all ions are in IMS–MS devices. The basic components found in transmitted simultaneously. Ion abundances are reported any IMS–MS instrument include the source, drift tube, as a function of CVs as opposed to drift times. FAIMS mass analyzer, focusing elements, and ion detector. also allows a continuous beam of ions to be introduced Technological advances in each of these components into the drift tube as opposed to a small packet of ions in have greatly added to the overall improvement of DTIMS.(7,38) IMS–MS instruments. Different combinations of IMS The exact mechanisms of FAIMS separation are not and MS instruments have been realized, including clearly understood and thus it becomes very difficult to IMS-time-of-flight mass spectrometers (TOF-MS), IMS- deduce structural information from this IMS method. quadrupole mass spectrometers (qMS), IMS-ion trap However, there are examples whereby FAIMS has mass spectrometers (IT-MS), IMS-Fourier transform been useful for obtaining conformations of well-studied mass spectrometers (FTMS), and IMS-magnetic sector systems.(39–41) In addition, because the field strength mass spectrometers. We provide a brief overview of employed is in the high-field limit the nature of the bath ionization sources and discuss the features of commonly gas can greatly influence the ion energy and mobility.(38) used IMS–MS instruments in the following sections.

3.1 Sources MALDI is a soft ionization method and unlike ESI, it is a pulsed source and primarily generates singly-charged 3.1.1 Electrospray Ionization ions.(52,53) The sample preparation procedure is simple, (54) Electrospray ionization (ESI) is one of the most popular making it a good choice for imaging MS applications. ion sources used in IMS–MS and MS since its discovery In addition, MALDI has better tolerance to salts and and application in biological molecules from the work detergents within samples. Analytes of interest are of Nobel laureate John Fenn et al.(48) ESI is a soft and mixed with selected small organic matrix compounds continuous ionization method that is nondestructive to (e.g. 3,5-dimethoxy-4-hydroxycinnamic acid, α-cyano-4- analytes and generates multiply-charged ions.(48) The hydroxycinnamic acid, and 2,5-dihydroxybenzoic acid). association of multiple charges on analytes extends the After solvent evaporation, the matrix and analyte cocrys- effective mass range of species that can be detected, tallize on the surface of a 96-well metal plate. The matrices thereby making molecules such as DNA and viruses have been selectively chosen so that they can absorb the (49–51) accessible. In addition, other types of nonvolatile radiation emitted by the laser (e.g. Nd:YAG or CO2). and thermally labile compounds, such as polymers and Spots are bombarded with laser pulses either under ( , ) small polar molecules, are accessible with ESI. vacuum source conditions, as in traditional MALDI, 52 53 The ESI source consists of a glass, metal, or fused- or at atmospheric conditions for atmosphere pres- silica sample capillary which contains liquid solutions of sure matrix-assisted laser desorption/ionization (AP- ( , ) the analyte of interest ﬂowing at rates of 0.1–1 mL min−1 MALDI). 55 56 As shown in Figure 2(b), charged analyte or higher. There is a potential drop (∼several kilovolts) and matrix ions are irradiated from the surface of the that is created between the capillary and source of the spot and are accelerated toward the IMS source. Because instrument (in this case the drift tube, see Figure 2a). MALDI is a pulsed ionization technique, it works well The distance between the capillary tip and the instrument with DTIMS as each laser shot generates a packet of ions source is 0.3–2 cm creating a ‘Taylor Cone’ which contains which can be injected into the drift tube without the use charged droplets of the analyte. As these droplets migrate of an ion gate. Several groups have employed MALDI as toward the entrance of the instrument, the droplets shrink an ion source before IMS–MS.(21,31,57–59) in size as the solvent evaporates. At a given point, the Coulombic repulsion in the droplet exceeds the surface 3.1.3 Laserspray Ionization tension and the droplets disperse into smaller droplets. This process continues until protonated analyte ions are Laserspray ionization (LSI) is a newly developed ioniza- left in the gas phase. Nebulizer and drying gases are often tion technique that resembles AP-MALDI, however used in order to assist with desolvation and aid in the results in multiply-charged ions similar to those produced transfer of ions to the source. For IMS–MS instruments, in ESI.(60,61) The setup is slightly different from that of such as those encountered in DTIMS and TWIMS, an ion AP-MALDI, as shown in Figure 2(c). The major differ- gate is located following the ESI source. ences are that a transparent slide is used to house the analyte/matrix crystals, no voltage is applied to the plate 3.1.2 Matrix-Assisted Laser Desorption/Ionization to accelerate ions into the source, and the laser beam is transmitted through the bottom portion of the glass slide Matrix-assisted laser desorption/ionization (MALDI) is as opposed to directly ablating the surface of the plate.(62) another attractive source for the study of large molecules. The distance of the sample plate to the inlet (∼1 mm) is

Matrix ESI MALDI LSI molecules Matrix molecules am e r b ase L Capillary M + M + + M M + + + Laser beam + + ++ M M M + + M M M + + + M + + + M + + + Metal plate + + + + + + + + + + + Taylor cone + + + + + +

V −

+ Metal plate Analyte Analyte molecules molecules + V − (a) (b) (c) Figure 2 Schematic diagram showing the mechanisms of (a) ESI, (b) MALDI, and (c) LSI.

Encyclopedia of Analytical Chemistry, Online © 2006–2013 John Wiley & Sons, Ltd. This article is © 2013 John Wiley & Sons, Ltd. This article was published in the Encyclopedia of Analytical Chemistry in 2013 by John Wiley & Sons, Ltd. DOI: 10.1002/9780470027318.a9292 6 MASS SPECTROMETRY important for minimizing sample loss. Matrices used by gate with a grid is used to pulse ion packets into the drift LSI are similar to those used with MALDI. tube. Figure 3(a) shows an example of a typical pulsing LSI features high sensitivity, easy sample prepa- diagram that may be used in an IMS-TOF-MS setup (see ration, simple laser focusing, and simple source Figure 3b). A packet of ions (100-μs wide) is injected instrumentation.(61) The combination of LSI with into the drift tube and mobility separated for a defined IMS–MS provides a solvent-free ionization and analysis period (e.g. ∼50 ms). The mobility period is selected to platform. Recently, Inutan and Trimpin(63) validated this correspond with the drift time of the lowest mobility coupling with a commercial Waters Synapt G2 IMS–MS species of interest. During mobility separation, TOF instrument on peptides and proteins ranging in molecular spectra are collected in a 50-μs window corresponding weight (MW) from 5.7 to 17 kDa. to the desired m/z range which generally spans up to 2000 m/z; although with the TOF analyzer, theoretically, there is no upper limit on the m/z to be measured. 3.1.4 Others Hundreds to thousands of TOF spectra are nested within While we highlight three ionization sources which are each IMS measurement. used for a range of higher MW compounds, there Data generated from this experiment can be displayed are other ion sources which have been applied in ina2Dplotofflighttime(orm/z) as a function of drift IMS–MS. Early studies of small gas-phase ions and time, similar to that shown in Figure 4(a), for a mixture molecules also applied corona ion discharge(19,64) and of tryptic peptides. The axes can also be switched for radioactive ion sources.(65) Laser desorption ionization these plots. It can be observed from the plot that there (77,78) is useful for the ionization of solid samples and has is a mobility–mass correlation which demonstrates been demonstrated on carbon clusters, silicon clusters, that the two techniques are not completely orthogonal. and fullerenes.(66–68) Other examples of sources include Specific trend lines appear in the spectrum, which corre- ultraviolet (UV) photoionization,(69) secondary ESI,(70) late with different charge-state families, allowing for desorption electrospray ionization (DESI),(71) and direct separation of multiply-charged ions generated during ESI analysis in real time.(72) For more detailed information on or LSI. In cases where the TOF resolution is limited, these sources compatible with IMS, we refer readers to a recent trend lines would allow charge-states to be assigned. review on the subject.(73) Mobility–mass behavior is not limited to peptides but also occurs for proteins and can be used to distinguish different classes of compounds such as proteins, lipids, 3.2 Hybrid Instruments DNA, and glycans.(11,58) 3.2.1 Ion Mobility Spectrometry–Time-Of-Flight-Mass A few points should be noted regarding the IMS- Spectrometry TOF-MS designs. Because the drift gas pressure (e.g. 0.5–15Torr)(15,16,26,31) is generally much higher than the Since the principles of TOF-MS have been proposed in the low vacuum necessary for TOF detection (i.e. 10−6 Torr), 1940s,(74,75) DTIMS-TOF-MS has attracted considerable a differential pumping region is necessary to couple the attention and undergone continuous development and devices. This can be done through designing an inter- improvement. McAfee et al.(19) constructed the first IMS- mediate vacuum stage consisting of multipoles or other TOF-MS in 1967 to study the mobilities and reactions focusing lenses. Due to the differences in pressure that can of small ions in the presence of argon gas. IMS is an occur between the ESI source and drift tube, consider- excellent match for TOF-MS analyzers owing to the able ion loss can occur because of diffusional losses.(46) A timescales of each technique. The scan time of TOF is major contribution to improve ion transmission efficiency the order of microseconds, which is much faster than the in DTIMS-TOF-MS is the ion funnel designed and intro- IMS separation that occurs on the order of milliseconds. duced by Smith et al.(26,80) Two ion funnels are present in Thus a ‘nested’ IMS-TOF measurement is obtained(25) the instrument shown in Figure 3(b): an hourglass-shaped and hundreds to thousands of MS spectra are acquired ion funnel after the source capillary and a second funnel for a single IMS pulse (Figure 3a). located after the drift region. The design is based on a The basic components of an IMS-TOF-MS instrument stacked ring RF ion guide, which is composed of a set include an ion source, a drift region, a TOF analyzer, of ring electrodes with opposite RF phases applied to focusing elements, and a detector. The layout of the every other electrode (Figure 3b inset). The ring elec- instrument can be similar to that shown in Figure 3(b), trodes have orifices with gradual decreasing diameters which is a design by Baker et al.,(76) in which the TOF that focus the ions into a collimated beam at pressures analyzer is orthogonal to the drift tube. Ions generated in up to 30 Torr.(76) A DC gradient is also applied along the ion source are injected into the drift region in packets. the funnel to push the ions in the z-direction. The hour- If a continuous ion source is used, such as ESI, an ion glass funnel is similar except for its geometry that allows

Mobility separation

100 µs

Ion injection IMS scan cycle 50 ms 2 µs TOF detection

(a) 50-µs TOF scan

Figure 3 (a) Example of a pulsing diagram for a DTIMS-TOF-MS experiment. The upper trace shows that an ion packet (100 μs width) is injected into the drift tube and mobility separated over the course of 50 ms. After a variable delay following ion injection, TOF scans of the order of 50 μs for a typical size m/z range, are acquired. Hundreds to thousands of TOF spectra are acquired within a single mobility experiment. (b) A schematic diagram of ESI-DTIMS-TOF-MS reprinted from Ref. 76. (Reproduced with permission from Ref. 76. Copyright 2007, Springer.) The inset shows a zoom-in of a typical ion funnel geometry. ions to be trapped for ion injection to the drift tube. Ion dissociation,(84) and other methods.(85–87) Due to the funnel functions include accumulating and focusing ions, timescale of the mobility separations and the short time increasing ion transmission efficiency, and collisionally of ion activation, the drift times of fragment ions are activating ions.(16,26) very similar to that of the precursor ions from which they Ions can also be collisionally activated in the IMS- arose. Figure 4(b) gives an example of fragmentation TOF-MS design at regions located after the drift tube spectra for a tryptic peptide that was isolated using a before entering the TOF analyzer.(42,81–87) Activation quadrupole mass filter and fragmented in a collision is possible with skimmer cones,(81) octopole collision cell. The b- and y-fragment ions detected for this cells,(82) Triwave cells,(42) split-fields,(83) surface-induced peptide all have similar drift times. One advantage of

y 46 46 12 (a)[M + 2H]2+ [M + H]+ (b)

44 1250 44 y11 1250 TLSDYNIQK+ y 42 42 10

40 + 40 ESTLHLVLR y9 1000 TITLEVEPSDTIENVK2+ 1000 TITLEVEPSDTIENVK2+ 38 + 38 EGIPPDQQR 2+ y8 MH–H2O 2+ 36 + 36

s) b or y s) MQIFVK 7 14 μ μ + 750 b –H O or 750 7 2 z QLEDGR z 34 34 + / / LEVEPSD m m AKIQDK+ 32 + 32 2+ LIFAGK Flight time ( Flight time ( ESTLHLVLR 30 30 TLTGK+ 500 28 2+ 500 28 EGIPPDQQR IQDK+ 26 26

24 24

22 22 LR+

20 250 20 250 Intensity 12345678 Intensity 12345678 Drift time (ms) Drift time (ms) Figure 4 (a) Nested drift (ﬂight) time distribution for an electrosprayed mixture of tryptic peptides of ubiquitin. The octopole collision cell was evacuated during the collection of these data, and the quadrupole was set to transmit all ions. The solid lines provide visual guides corresponding to the [M + H]+ and [M + 2H]2+ charge-state families. (b) Nested drift (ﬂight) time distribution for ubiquitin tryptic mixture. The quadrupole was used to select the [TITLEVEPDTIENVK + 2H]2+ ion (m/z = 849) and 3.8 × 10−4 Torr of argon was added to the octopole collision cell. Fragmentation of the [TITLEVEPSDTIENVK + 2H]2+ ion as well as the [TLTGK + H]+ ion is apparent. (Reproduced with permission from Ref. 79. Copyright 2000, American Chemical Society.)

Intelli Start. Analyte spray Lockmass spray

. . STEP WAVE TRI WAVE QUANTOF

Ion mobility High-field Ion detection Stepwave ion guide Quadrupole speration pusher Ion Trap Transfer mirror system

Helium cell

13452

Rotary pump Air-cooled turbomolecular pumps 6

Dual-stage reflectron

Figure 5 Schematic of Waters Synapt G2-STM HDMS instrument. It features a Stepwave ion guide, a quadrupole, a Triwave TWIMS, and a QuanTOF MS. (Image used with permission from Waters Corp.)

IMS-TOF-MS fragmentation is that without quadrupole FAIMS(41,88,89) and TWIMS(42) devices have also been selection, all ions are simultaneously fragmented. This coupled to TOF-MS analyzers. Waters Corporation ‘parallel fragmentation’(79,81) can increase the throughput commercialized the TWIMS-TOF-MS design in 2006(42) of Tandem mass spectrometry (MS/MS) experiments. in which the ﬁrst instrument design was called a Synapt. In

Quadrupole mass spectrometer

Chiral ion mobility spectrometer

Electrospray ionization source Desolvation region Drift region

LC pump Chiral modifier infusion Drift gas heater

Diffusion pump #1 Diffusion pump #2

Drift gas Figure 6 A simpliﬁed drawing of DTIMS–qMS instrument. A heated desolvation region was designed to help in desolvating the solvent. (Reproduced with permission from Ref. 91. Copyright 2006, American Chemical Society.)

2010, the Synapt was upgraded to the Synapt G2-S HDMS qMS) located in separate chambers with small orifices to (Figure 5) which features an improved performance sustain the pressure differences. A DTIMS–qMS instru- TOF analyzer, offline transfer lens design, and updated ment with improved resolution has been reported(90) and software. The major components are an ion source (ESI or is shown in Figure 6. The drift tube in this instrument MALDI), ion guides, a quadrupole, the TriWave which is operated under ambient pressure conditions and the includes the TWIMS, and a reflectron TOF-MS. The buffer gas can be manipulated with the addition of modi- ion beam generated is focused toward a ‘StepWave’ ion fiers that help achieve chiral selectivity.(91) The Excellims guide, in which neutral species are removed with an Corporation offers a commercial atmospheric pressure offline lens design. After selection or all-ion transmission DTIMS coupled to a qMS.(92) The drift tube in this config- in the quadrupole, ions are accumulated in a trap and uration is 10.85 cm. AB Sciex released an IMS–qMS ejected into the TWIMS for mobility separation, and instrument with different IMS electrode geometries(93) transferred to an orthogonal dual-reflection TOF-MS and a triple quadrupole/quadruple ion-trap MS. analyzer. Although the drift resolution of TWIMS is Wyttenbach et al.(15) implemented a funnel between lower than in-house-built DTIMS, the TWIMS offers the ESI source and reduced-pressure drift tube to focus high transmission efficiency and sensitivity.(42) the ion beam and prevent high-energy ion-neutral collisions. More recently, a DTIMS has been coupled to a (94) 3.2.2 Ion Mobility Spectrometry–Quadrupole Mass triple qMS for explosive detection. The triple qMS Spectrometry offers additional scanning modes and greater sensitivity than a single quadrupole analyzer. Cylindrical electrode Quadrupole mass analyzers can filter ions with specific FAIMS designs, which offer higher sensitivity than flat m/z ratios or allow all m/z ions to pass through the plate electrodes, have also been coupled to qMS.(95) detector. Although quadrupoles have slower scan speeds, Because FAIMS can act as a filter to eliminate chem- lower resolutions, and mass accuracies than TOFs, they ical contaminants, the signal-to-noise (S/N) ratio can be are widely used mass analyzers because of their simplicity, improved.(96,97) inexpensive cost, and utility for targeted analyses. The time required for a qMS scan is on the scale of 100 ms, 3.2.3 Ion Mobility Spectrometry–Ion Trap Mass which is far slower than TOF-MS and is similar to Spectrometry the timescales of DTIMS separations. Thus, it is more practical to use single-ion monitoring instead of full m/z IMS-IT-MS with quadrupole ion traps (QITs, also known scans in an IMS–qMS setup. as Paul ion trap) and linear ion traps (LITs) have been The hybrid IMS–qMS instrument setup was introduced reported.(18,98,99) The coupling of DTIMS-IT-MS requires in the 1970s(64) for monitoring ion–molecule reactions an ion gate at the IT-MS entrance to select a defined drift- of oxygen and nitrogen species. The instrument mainly time window. Clowers and Hill(18) have implemented a consists of three parts (the ion source, a DTIMS, and a dual-gate design. ESI-generated ions are pulsed into the

Flared inlet Hexapole 1 7-T magnet Focus Drift gas inlet capillary Transfer optics screen inlet Hexapole 2 Ion gate 1 Ion gate 2 Skimmers 1 and 2 Quadrupole

Nano-ESI Ion mobility spectrometer Ion transfer optics and Q-interface ICR cell Figure 7 Schematic of ESI-DTIMS-FTICR MS instrument which consists of a drift tube, three multipole ion guides, and an ICR cell with 7-T magnetic field. (Reproduced with permission from Ref. 102. Copyright 2007, John Wiley & Sons, Ltd.) drift region by the first ion gate, separated in the drift containing ion transfer optics, and an ICR cell. The region, selected by the second ion gate, focused through interface between the IMS and ICR cell serves as an the ion guides, and mass scanned by the QIT. Recently, intermediate pressure region before the high vacuum Zucker et al.(99) interfaced a DTIMS to a Thermo LTQ required in the ICR cell (i.e. 10−10 Torr). Although Velos instrument which features a dual LIT design. the duty cycle (more than 99% of the ion signal is Unique to this design is the introduction of a UV laser at wasted) has a long way to be improved for this setup, the back end of the second LIT for UV photodissociation there are sophisticated dynamic multiplexing approaches experiments. In this instrument, both UV photodisso- to combat these issues as has been recently presented ciation and collision-induced dissociation (CID) can be on Orbitrap FTMS instruments.(103–105) FAIMS-FTICR- employed to fragment specific m/z ions that are isolated MS(106) analysis on polymers has shown that this setup in the LIT. offers the advantages of higher sensitivity, extended FAIMS coupled to a modified LTQ has been dynamic range, high mass resolution and accuracy, and reported(98) and commercialized by Thermo Fisher.(100) lower limits of detection due to the elimination of Thermo Fisher provides the FAIMS interface in its chemical noise with FAIMS. triple quadrupole instrument (i.e. TSQ Quantum) and with LTQ and Orbitrap analyzers. The FAIMS is in a heated cube electrode design, in which selected ions can 4 MULTIDIMENSIONAL ION MOBILITY travel through a gap between two electrodes and enter SPECTROMETRY–MASS the transfer capillary of the MS. This technology offers SPECTROMETRY the advantage of reduced chemical noise from ESI or atmospheric pressure chemical ionization (APCI) sources For the analysis of simple systems, DTIMS, TWIMS, or and complex samples before MS analysis. FAIMS may provide enough increased peak capacity and separation space to identify all components in combination with MS. However, for many applications 3.2.4 Ion Mobility Spectrometry–Fourier Transform (as discussed subsequently) of biological nature, the Mass Spectrometry complexity of the system still requires increased dimen- The timescales of FT ion cyclotron resonance (ICR) sionality in order to detect more components, detect MS do not make it the most attractive choice of mass low-abundance species, resolve isomeric and isobaric analyzer for IMS coupling as scan times can be on the compounds, and extend dynamic range. To this end, order of seconds. However, because of the inherent high IMS–MS can be connected with other front-end separa- mass resolving powers attainable with FTICR-MS,(101) tions. a number of IMS applications can benefit from this hybrid setup. Coupling IMS to FTMS is similar to IMS- 4.1 Liquid Chromatography-Ion Mobility IT MS in which ion gates before and after the drift Spectrometry-Mass Spectrometry tube are required. Figure 7 shows an IMS-FTICR MS Liquid Chromatography (LC)-IMS-MS has been previ- constructed by Tang et al.(102) which is composed of ously reported(107–113) and is one of the most straight- a nano-ESI ion source, a DTIMS, an interface region forward front-end separations to couple with IMS–MS.

37 36 +4 Ubiquitin

34 1840

Compact 32 33 +5 30 1440 s) Elongated µ z Partially folded 28 / m +6

Flight time ( 26 Compact 1040 29 s)

µ +7 24

22 +13 – +10 +9 +8 +7 +6 Charge state fragment lanes Flight time ( 20 640 25 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Drift time (ms)

5+ 3+ 3+ y / y / b 5+ +8 – +13 59 35 36 [M + 5H] 5+ y60 5+ 2+ y63 / y25 5+ y 5+/ b 4+ y 21 58 47 61 y 5+ [M + 6H]6+ 4+ + 62 4+ 5+ 5+ y44 / b11 y58 / b72 y64 4+ 4+ 5+ y40 / b41 b65 y 4+/ y 3+/ b 2+ 4+ 7+ b 6+ 59 44 30 y37 / y66 63 Ubiquitin 17 1.5 2.0 2.5 3.0 3.5 4.0 4.5 640 1120 1600 2080 (a)Drift time (ms) (b) m/z Figure 8 Illustration of multidimensional IMS separation spectra. (a) The nested drift (flight) time distributions for ubiquitin 4+ 13+ tR = 38 min. ions with charge-states [M + 4H] to [M + 13H] are observed. In (b), the upper figure shows nested drift(flight) time distributions of the fragmentation patterns for ubiquitin, and the lower figure is an MS/MS spectrum integrated at tD = 2.96 ms corresponding to the fragmentation patterns observed for the [M + 7H]7+ charge-state of ubiquitin. (Reproduced with permission from Ref. 111. Copyright 2004, Springer.)

This is primarily because of the timescale of the sepa- approach to characterize the overall shapes of proteins ration and the generation of ions using sources such and collisionally activate precursor ions on the timescale as ESI. In comparison to IMS, liquid LC separations of an LC separation (Figure 8). occur on the order of hours with typical peak widths ranging from 15 to 45 s. This relatively long separa- 4.2 Capillary Electrophoresis-Ion Mobility tion allows the IMS–MS measurement to be introduced Spectrometry-Mass Spectrometry at no additional cost in experiment time. Essentially, a three-dimensional ‘nested’ experiment is performed Capillary electrophoresis (CE) was coupled to FAIMS- particularly if the IMS-TOF-MS platform is used. This MS for the study of complex lipopolysaccharides.(118) multidimensional separation method can dramatically They used a microspray interface to connect the CE increase the analytical peak capacity and reduce the eluent to the front end of a FAIMS-triple qMS. Due the chemical noise to benefit the analysis of complex samples, limited number of reports using CE with IMS–MS, there such as proteomes(114) or metabolomes.(110) is opportunity to make this coupling more desirable. The inclusion of MS/MS is often necessary to confidently identify biomolecules.(111,115,116) LC-IMS- 4.3 Ion Mobility Spectrometry-Ion Mobility MS/MS has been demonstrated in several applications Spectrometry-Mass Spectrometry of peptides and proteins(115,117) and generates a four- to five-dimensional data set which is gigabits to terabits A more straightforward approach to increase the dimen- in size. Sowell et al.(111) used nanoflow LC-DTIMS- sionality and peak capacity of IMS–MS technology is MS/MS to separate mixtures of intact small MW proteins, to exploit the mobility separation space. During the demonstrating the effectiveness of this multidimensional past decade, ion mobility spectrometry-ion mobility

Flow valve Gas inflow ESI emitter Manometer 1 Manometer 2 Regular Curtain gas ion funnel

Q00 Q0 Q1 Q2

Inner electrode

Carrier gas Hourglass Outer electrode ion funnel FAIMS IMS drift tube Quadrupole TOF MS stages Edwards EIM18 Alcatel 2033 mechanical pump mechanical pump

Drift time (D2, ms) 0 5 10 15 20

y 2+ 3 [M + 2H] y CID 4 y 2+ 4.78 ms 2+ 7 b [y –H O] 5 6 2 b b 2+ 4 8 y 2+ 4 y 2+,y 3 y 2 TOF MS y y 2+,F b 5 2 6 R = 3500–5000 y c 6 R

C 4.78 ms

F3 IA3 F1/IA1/G1 G2 F2 IA2

ESI b

[M + 3H]3+ C E [M + 2H]2+ B D [M + H]+ A

D1 R(D2) = 40–80 Intensity

a 22 25 Source Collision dynodel/ chamber R(D1 + D2) = 80–160 MCP detector 0 5 10 15 20 25 (a) (b) Drift time (total, ms) Figure 10 (a) Schematic of DTIMS-DTIMS-TOF-MS instrument. (Reprinted with permission from Analytical Chemistry.) (b) Multidimensional DTIMS-MS spectra. DTIMS-DTIMS drift-time distributions of electrosprayed bradykinin ions shown on two timescales: the total drift time (bottom) and the drift time observed after mobility selection (tD2), that is shifted by the selection time (as indicated by the dashed vertical line). Part (a) shows the distribution for all ions and includes an inset to show the low-intensity feature that is often associated with [Mn + nH]n+ ions. Part (b) shows the feature-labeled C after it has been selected at G2. Part (c) shows the distribution of fragment ions that are observed when C is activated at IA2 using 126 V. (Reproduced with permission from Ref. 16. Copyright 2006, American Chemical Society.) spectrometry-mass spectrometry (IMS-IMS-MS) and the separation principles of FAIMS and IMS are different, FAIMS-IMS-MS instruments have been reported. Tang thus this coupling is completely orthogonal. et al.(119) reported a FAIMS-IMS-TOF MS instrument Koeniger et al.(16) also constructed a two-stage IMS- which is shown in Figure 9. Ions produced from the ESI IMS-MS instrument using the DTIMS technology emitter are ﬁltered by the FAIMS, focused, and pulsed (Figure 10). Ion funnels connect the two drift tube into the DTIMS. The advantage of this approach is that sections which are ∼1 m in length and activation grids

Lipidomics

Environmental Chiral species analysis COOH COOH

H C R R C H

NH NH

Applications Acetyl-CoA

Oxaloacetate Citrate of IMS-MS NADH + H+

Chemical Malate Isocitrate warfare Metabolomics NADH + H+ CO Sccinate 2 α-Ketoglutarate NADH + H+ FADH2 Succinate CO Succinyl-CoA 2 GTP H H O H O N C C N C OH H R1 H R2

Proteomics Pharmaceuticals

Figure 11 Overview of example research applications accessible with IMS–MS.

Encyclopedia of Analytical Chemistry, Online © 2006–2013 John Wiley & Sons, Ltd. This article is © 2013 John Wiley & Sons, Ltd. This article was published in the Encyclopedia of Analytical Chemistry in 2013 by John Wiley & Sons, Ltd. DOI: 10.1002/9780470027318.a9292 14 MASS SPECTROMETRY from using the combined IMS–MS approach. There are capsids which are 3–4 MDa in size,(149) and GroEL chap- other reviews that may be helpful for obtaining an insight erone systems in Escherichia coli,(156) to name a few. into additional applications of IMS–MS.(3,126) Much of the understanding of small inorganic systems to large biological systems with IMS–MS is made possible 5.1 Proteomics through computer modeling algorithms which give insight into shapes and sizes of molecules measured with experi- The complexity of mixtures encountered in proteomics mental collision cross-sections.(12,13) is enormous, such that multidimensional separations before MS are necessary to obtain depth in proteome 5.3 Lipidomics coverage. It is clear that IMS has the capability to detect low-abundance species which would otherwise Lipidomics research, which studies the pathways and go undetected in an MS spectrum (see Figure 4a). As networks of cellular lipids, is currently a rapidly growing mentioned earlier, the incorporation of IMS–MS into a area in IMS–MS.(157) IMS–MS offers high-throughput chromatographic separation occurs at no experimental analysis, class identification, and structural information cost, therefore thousands of additional species can be of complex lipid samples, such as cellular membranes detected and/or identified with high throughput. In addi- and phospholipids. One of the earliest studies of lipids tion, the drift times of peptides can be used along with with IMS–MS reported on the ability of MALDI-IMS- the chromatographic retention time to ‘map’ peptides TOF MS to distinguish sphingomyelin from peptides and in multidimensional space.(127) Peptide mobilities have oligonucleotides ionized from the same mixture.(58) The specific signatures which can be correlated with chemical spingomyelin ions had lower mobilities than the peptide characteristics such as hydrophobicity.(128) Several groups and oligonucleotides and thus had a distinctive trend line have used IMS–MS for bottom-up proteomics analysis of in the 2D spectrum. Since these studies, other reports biological samples such as human plasma,(104,129) mouse investigating various classes of lipids were published.(157) plasma,(112) Drosophila,(114,127,130) combinatorial peptide Generally, the separation of lipids in IMS–MS is due to libraries,(131–133) and other standard tryptic peptide differences in the acyl chain length, degree of unsatura- mixtures.(78,104,119,134) Sowell et al.(111) reported on the tion, nature of the polar head group, and cationization use of LC-IMS-MS for top-down proteomics analysis of of individual species.(157) Different classes of phospho- intact proteins; however, this is an area that could be lipids in complex tissue extracts and slices have been further explored. Biomarkers of disease, such as cancer, studied with IMS–MS.(89,158) The analysis of unsaturated are also being explored using IMS–MS with MALDI phosphatidylcholines (PCs) with TWIMS-TOF MS iden- imaging.(135,136) tified a high mobility–mass correlation for saturated PC cations which was less significant for unsaturated PC cations.(159) Trimpin et al.(160) demonstrated the strengths 5.2 Probing Structural Information of multidimensional DTIMS-MS for reducing complexity Early applications of IMS–MS probed structures of small of lipid samples, detecting low-abundance lipids, and MW species, such as polyatomic ions and small inorganic elucidating structure in a global lipidomics analysis of clusters,(137–139) fullerenes,(140) and atomic cations.(141) phospholipids. With the advent of ESI and MALDI, gas-phase conformations of higher MW species, such as proteins,(142–145) 5.4 Metabolomics DNA,(50,146,147) RNA,(148) and even viruses,(149–151) have been accessible with IMS–MS. Characterization of Metabolomics studies focus primarily on the analysis of gas-phase structures has relevance for understanding metabolites for the purpose of early disease detection biological pathways in diseases, such as Alzheimer’s and prevention.(161) Chemical properties of metabo- disease.(152–155) Since the commercialization of TWIMS, lites, such as polarity, solubility, and volatility, can vary a number of groups have used IMS–MS to study biophys- tremendously among different classes. IMS–MS has the ical parameters of proteins and protein complexes.(12,13) capability of analyzing complex and diverse samples with These complexes have higher order tertiary and quater- high accuracy, high peak capacity, and fast speed and nary structures and dynamic binding events which are has been applied in target metabolomics analysis. For only partially understood with traditional biological and example, opiates and primary metabolites were analyzed other analytical techniques, such as nuclear magnetic by DTIMS-TOF MS such that metabolic isomers were resonance and X-ray crystallography. IMS–MS appli- separated based on mobility.(162) False-positive detec- cations of protein complexes have led to a better tion of methamphetamine in hair by stand-alone MS understanding of oligomeric tryptophan RNA binding was improved with DTIMS–qMS analyses.(163) Further- attenuation protein complexes,(148) Hepatitis B virus more, the usage of IMS–MS has extended to metabolite

Encyclopedia of Analytical Chemistry, Online © 2006–2013 John Wiley & Sons, Ltd. This article is © 2013 John Wiley & Sons, Ltd. This article was published in the Encyclopedia of Analytical Chemistry in 2013 by John Wiley & Sons, Ltd. DOI: 10.1002/9780470027318.a9292 ION MOBILITY-MASS SPECTROMETRY 15 profiling. With IMS–MS, more than 200 intracellular E. Hill et al.(17,87,173–176) used DTIMS-qMS to test degra- coli metabolites containing lipids, inorganic ions, volatile dation products of common nerve agents and sulfur alcohols and ketones, amino and nonamino organic mustard gas in liquid samples. Different experimental acids, hydrophilic carbohydrates, etc. were tentatively conditions were investigated to optimize the separation assigned.(164) Quantitative IMS–MS studies have been efficiency and sensitivity of CWAs’ detection including used to monitor metabolic changes in rat lymph fluid the ionization method (ESI, SESI, CI and Ni(63) radioac- caused by dietary stresses.(165) Keytothisworkwastheuse tive ionization), ionization mode (positive/negative), drift of positive- and negative-mode ionization for the detec- voltage, temperature, and matrices.(17,87,173–176) ACPI tion of amino acids, sugars, triglycerides and diglycerides FAIMS-MS has been applied to detect nanogram per in the presence of fatty acids and sugars, respectively. milliliter levels of CWAs in food and water samples.(177) Pharmaceutical metabolite profiling of leflunomide and DESI with TWIMS-MS has also been applied in the detec- acetaminophen has been reported(166) and suggests that tion of common organophosphorus CWAs.(178) While the IMS–MS is suitable for routine metabolomics analyses. number of reports in this area is limited, it is clear that the use of ambient ionization methods is critical for extending 5.5 Chiral Species CWA analysis to portable IMS–MS devices in the future. In some instances, typical IMS–MS separations are able to provide enough resolution so that isomeric and isobaric 5.7 Pharmaceuticals species are resolved.(167–169) However, in other cases there IMS–MS is growing in use in pharmaceutical detection are generally two IMS–MS strategies which are used to for its fast speed, low detection limit, and high obtain enantiomeric separation.(91) The first is to add a accuracy. However, it is unlikely that IMS–MS will selected chiral reference compound to the solution that be used for online quality control in pharmaceutical contains analyte enantiomers. Diastereomeric complexes production processes owing to the relatively high costs are formed by the chiral reference compound and analyte of MS, however, it is a powerful technique for in- enantiomers, which can be physically separated in the lab characterization and direct formulation analysis of drift tube. For example, separation of D- and L-lactic pharmaceuticals.(9) acids was possible with FAIMS-MS by spiking the Improvements in the accuracy of amine drug detec- analyte mixture solution with excess L-tryptophan.(88) tion are possible with IMS–MS over LC-MS techniques Also, six pairs of amino acid and terbutaline enantiomers because of the ability of IMS–MS to remove metabo- were characterized with IMS–MS through conversion lite interferences.(116) The successful quantification of of the enantiomers to metal-bound trimeric complexes amphetamine, methamphetamine, and methylenedioxy II + (117,170) of the form [M (L-Ref)2(D/L-A)-H] . ESI is the derivatives in human urine matrix demonstrated the preferred ionization method with this chiral approach so power of IMS–MS in the separation of rather complex that the interactions between the enantiomers and chiral pharmaceutical samples.(179) A strategy which can facil- reference are not disrupted. itate the detection of active pharmaceutical ingredients The second strategy of chiral separations, as mentioned in complex matrices has also been reported by using earlier, is to dope the drift gas with a chiral modifier. polyethylene-based ‘shift reagents’ in IMS–MS analysis. For example, (S)- and (R)-atenolol enantiomers were These shift reagents can form noncovalent complexes separated with DTIMS-MS after the N2 drift gas was with the active pharmaceutical ingredients which move doped with (S)-(+)-2-butanol.(91) There are other reports them to different regions of the 2D mobility–mass of IMS–MS being employed for chiral separation of space.(180) IMS–MS has been applied for the anal- small molecules including amino acids, pharmaceuticals, ysis of benzodiazepine drugs(181,182) and a range of and sugar enantiomers.(171) The reader is referred to a over-the-counter and prescription tablets and cream recent review of IMS–MS for chiral analyses.(172) formulations.(183)

5.6 Chemical Warfare Agents 5.8 Environmental IMS is considered the most efficient technique for the There are a very limited number of reports of IMS–MS detection of CWA for its high speed, simplicity, low cost, use for environmental analysis. For example, haloacetic and portability.(4,7) However, as a stand-alone technique acids were quantified from complex solutions with it suffers from low resolving power, limited identification FAIMS-MS to obtain nanogram per milliliter detec- ability, existence of false-positive results, and sample tion limits for samples without preconcentration or matrix effects.(8) Most of these disadvantages can be derivatization.(184) A class of important triazine pollu- overcomed when IMS is combined with MS. tants were analyzed with FAIMS-MS.(185) Clearly, this

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