Int. J. Ion Mobil. Spec. (2011) 14:15–22 DOI 10.1007/s12127-011-0058-9

ORIGINAL RESEARCH

Ion mobility spectrometer—field asymmetric ion mobility spectrometer-mass spectrometry Matthew J. Pollard & Christopher K. Hilton & Hongli Li & Kimberly Kaplan & Richard A. Yost & Herbert H. Hill Jr.

Received: 13 December 2010 / Revised: 30 January 2011 / Accepted: 1 February 2011 / Published online: 9 March 2011 # Springer-Verlag 2011

Abstract Since the development of electrospray ionization (ESI) for ion mobility spectrometry mass spectrometry (IMMS), IMMS have been extensively applied for characterization of gas-phase bio-molecules. Conventional ion mobility spectrometry (IMS), defined as drift tube IMS (DT-IMS), is typically a stacked ring design that utilizes a low electric field gradient. Field asymmetric ion mobility spectrometry (FAIMS) is a newer version of IMS, however, the geometry of the system is significantly different than DT-IMS and data are collected using a much higher electric field. Here we report construction of a novel ambient pressure dual gate DT-IMS coupled with a FAIMS system and then coupled to a quadrupole ion trap mass spectrometer (QITMS) to form a hybrid three-dimensional separation instrument, DT-IMS-FAIMS-QITMS. The DT-IMS was operated at ~3 Townsend (electric field/number density (E/N) or (Td)) and was coupled in series with a FAIMS, operated at ~80 Td. Ions were mobility-selected by the dual gate DT-IMS into the FAIMS and from the FAIMS the ions were detected by the QITMS for as either MS or MSn. The system was evaluated using cocaine as an analytical standard and tested for the application of separating three isomeric tri-peptides: tyrosine-glycine-tryptophan (YGW), tryptophan-glycine-tyrosine (WGY) and tyrosinetryptophan-glycine (YWG). All three tri-peptides were separated in the DT-IMS dimension and each had one mobility peak. The samples were partially separated in the

M. J. Pollard : H. Li : K. Kaplan : H. H. Hill Jr. (*) Department of Chemistry, Washington State University, Pullman, WA 99164, USA e-mail: [email protected] C. K. Hilton : R. A. Yost University of Florida, Gainesville, FL 32611, USA

FAIMS dimension but two conformation peaks were detected for the YWG sample while YGW and WGY produced only one peak. Ion validation was achieved for all three samples using QITMS. Keywords Ion mobility spectrometry . Field asymmetric ion mobility spectrometry . Peptide separation

Introduction Ion mobility spectrometry (IMS) is a well established technique for ion separation based on collision cross section, Ω, and is often used for the detection of explosives and chemical warfare agents [1, 2, 11, 12, 15, 19]. IMS is designed as either a standalone system employing an electrometer (Faraday cup or plate) detection system, or coupled to any one of a variety of mass spectrometers [20]. When IMS is coupled to a fast detector, e.g. faraday plate or a time of flight mass spectrometer, the IMS effluent stream is analyzed in near real-time, yielding data for the complete ion mobility spectrum. In traditional or drift tube-ion mobility spectrometry (DT-IMS), the introduced ions traverse an electric field gradient of a few hundred volts per centimeter. The ion’s trajectory is not simply a straight line down this low electric field gradient, but rather one which is dependent upon the number of collisions with the countercurrent neutral drift gas. As the ions interact or collide with the neutral gas molecules, their microscopic trajectories change, with the small collision cross section ions undergoing fewer collisions, resulting in a longer mean free path. This longer path results in the smallest cross section ions exiting the system first. Conversely, the largest collision cross section ions undergo more collisions, have a shorter mean free path, and exit the system last. The ions exiting the mobility device yield a time- or size-dependent ion mobility spectrum.

16

FAIMS (field asymmetric waveform ion mobility spectrometry) has been described [4, 13, 16, 17, 21, 22, 25, 27]. The asymmetric waveform is composed of a high voltage component, lasting for a short period of time and a low voltage component, of opposite polarity, lasting a longer period of time. At high electric fields (e.g. 10,000 V/cm), the ion drift velocity is no longer directly proportional to the applied field; rather, ion mobility becomes dependent on the applied electric field [11, 24]. This high-field behavior of ion mobility is compound-dependent. Three changes in ion mobility as a function of electric field have been described by Buryakov et al. [6]. With the increase of the electric field strength, the mobility of an ion can increase, decrease or initially increase and then decrease. The separation of ions in FAIMS is based on the difference between the mobility of an ion at high electric field and its mobility at low electric field; each ion species will exhibit discrete mobility characteristics. Without further assistance, the ion’s trajectory would have them collide with an electrode and never exit the system. A direct current compensation voltage (CV) is superimposed on an asymmetrical radio frequency electric field in order to keep the ions of interest centered between the electrodes. By slow scanning the CV, ions with a different high-field to lowfield mobility ratio establish a stable trajectory and pass through the device. Ions that have an unstable trajectory strike the electrodes and are lost. The resulting spectrum is plotted as a function of the compensation voltage. Mixtures of different molecules are often resolved with IMS, but mixtures of isomers or mixtures of different conformational states pose more of a separation difficulty due to their similar collision cross sections. This has been a longstanding goal in separation science especially applied towards biological applications. Both high-field and conventional low-field ion mobility spectrometry have applied to bio-molecules [3, 14, 31] including separation of isomeric mixtures such as carbohydrates, peptides [2, 7, 8, 10, 29, 32, 33] and characterization of conformations of proteins [5, 23, 26]. Furthermore, species having same mobility at low electric field cannot be separated while may have different mobility at high electric field thus can be differentiated. Since the high-field and low-field techniques separate ions differently, it would naturally be beneficial to couple the instruments [28, 30]. The residence time of the ions in low field is tens of milliseconds where in high field are hundred milliseconds. When coupling the low-field (faster) instrument (DT-IMS) to the output of the high-field (slower) instrument (FAIMS), multiple low-field IMS scans can be performed in the time it takes for the ions to traverse the high-field system. Due to the discrepancies with the residence times, coupling the instruments in the reverse order (low-field, then high-field) will cause loss of resolution from the low-field IMS; the

Int. J. Ion Mobil. Spec. (2011) 14:15–22

resolution of the high-field IMS is not affected. To couple the instrument in the reverse order (low-field, then highfield), a second Bradbury-Nielsen ion gate would permit a small temporal window of ions to exit the low-field system and enter the high-field system. By adjusting the delay time and width of the second gate, an unresolved peak of ions could be selectively gated out of the low-field system for scrutiny by high-field system. Ion identification post-IMS is typically with a mass spectrometer. A fast mass spectrometer, (e.g. time-of-flight) yields a comprehensive MS spectrum for each IMS scan. A quadrupole MS is typically too slow to give a complete mass scan when used as an IMS detector; therefore, it is often used in single ion monitoring mode (SIM), resulting in sensitive detection of a specific target ion. Recently, a dual gate DT-IMS was coupled to a quadrupole ion trap MS [9]. Quadrupole ion trap (QITMS) operates by accumulating ions up to 8 s, and for in situ fragmentation for multiple stages of mass spectrometry in tandem (MSn). The accumulation is beneficial for very low concentration samples. Unfortunately, the QITMS is a rather slow mass analyzer, again not permitting a real-time IMS spectrum. The purpose of this research was to test the hyphenated system dual gate DT-IMSFAIMS-QITMS for using cocaine as an analytical standard and obtaining multi-dimensional information: drift time, compensation voltage, m/z and intensity. The second part was to determine the instrumental separation capability of three isomeric trip-peptides: tyrosine-glycine-tryptophan (YGW), tryptophan-glycine-tyrosine (WGY) and tyrosinetryptophan-glycine (YWG).

Experimental design Chemicals All chemicals including cocaine, tyrosineglycine-tryptophan (YGW), tryptophan-glycine-tyrosine (WGY) and tyrosine-tryptophan-glycine (YWG) used in this study were purchased from Sigma Chemical (St. Louis, MO) and were used without further purification. The standards were prepared for a final concentration of 200 μM in a 49.5:49.5:1 mixture of methanol: water: acetic acid solution and were electrosprayed into the DT-IMS system at 2 μL/min. High performance liquid chromatography grade solvents were purchased from J.T. Baker (Phillipsburgh, NJ). The ion trap’s ultra high purity helium was used without further purification, and all low- and high-field IMS gases were purified and dried prior to use with inline scrubbers and filters. Instrumentation A schematic of the electrospray ionizationambient pressure dual gate DT-IMS-FAIMS-QITMS is displayed in Fig. 1. The instrument consisted of four primary units: (1): The ESI source was constructed at

Int. J. Ion Mobil. Spec. (2011) 14:15–22

17

Fig. 1 Schematics of the electrospray ionization, ambient pressure dual gate ion mobility-field asymmetric ion mobility-quadrupole ion trap mass spectrometer (DT-IMS-FAIMS-QITMS)

Washington state university using 360 μm o. d.×75 μm i. d. fused silica capillary (Polymicro Technologies, Phoenix, AZ). The sample transfer line was connected through a zero-dead volume stainless steel union (Valco Instruments Co. Inc., Huston, TX) where the ESI voltage was applied. The ESI capillary was oriented at an upward angle of 30° from the center of the DT-IMS target screen in order to enhance complete desolvation. (2): The dual gate DT-IMS includes detailed units of desolvation region, drift region and two ion gates. A series of conducting and ceramic drift rings with length of 2.6 cm were extended after the second gate for coupling to MS. (3): The exit of the dual gate DTIMS system was connected to the curtain plate inlet of the Fig. 2 Instrument image (top-view) of dual gate DT-IMS-FAIMS-QITMS

FAIMS cell. A potential between the second gate and the curtain plate of the FAIMS was maintained at approximately 1,000 V DC. The outer and inner electrodes of cylindrical FAIMS are also shown in Fig. 1. (4) The FAIMS cell is directly coupled to the heated transfer capillary of QITMS to transmit ions into the MS for detection. Figure 2 is the top-view image of the combined instrument positioned in a direction consistent with Fig. 1. Ambient pressure dual gate ion mobility spectrometer A custom built dual gate atmospheric pressure low-field DTIMS was designed by Clowers and Hill [9] and has been described previously. However, instead of a 17.6 cm length

18

for the drift region as described previously, 25.5 cm was used here and the desolvation region was 7.5 cm. Ions produced entered the desolvation region in the DT-IMS system, where they were heated to a nominal temperature of 225 °C. Nitrogen drift gas was introduced at the lowvoltage end of the drift region at flow rate of 1 L/min, and flowed through the drift tube counter to the direction of ion motion. A Bradbury-Nielson (BN) gate was used to pulse the ions into the drift region. The BN gate was made of two sets of wires (California Fine Wire Co., Grove Beach, CA) separated at equivalent space of 0.6 mm. The two ion gates used were identical and had separate but identical control hardware. Two operation modes were achieved in dual gate DT-IMS: single ion monitoring mode (SIM) which allows a specific drift time window of ions to transmit to further device and dual gate scanning mode (DGS) which allows the ion populations drift times to be determined through a successive series of stepped ion gate pulsing experiments. The operation principles of these two modes have been explained in details by Clowers and Hill [8, 9] and Zhu et al. [33] and are not repeated here. Once the mobility spectrum of an ion species was determined by the mobility DGS mode, only a narrow range of drift time window was selected in the SIM mode to introduce ions of interest into FAIMS cell finally QITMS continuously and repetitively for further analysis. In this study, the first ion gate was open for 300 μs, allowing ions to enter the drift region. The second ion gate was opened for 1 ms, allowing a selected pulse of ions to exit the DT-IMS system and enter the highfield ion mobility device. The voltages throughout the system were as follows: The ESI was operated at 15 kV, the high-voltage end of the drift tube was 12 kV and the low voltage end was 2 kV, resulting in a 3.0 Td electric field. FAIMS instrumentation The high-field IMS instrument was an Ionalytics FAIMS system operated at a nominal 4,000 V dispersion voltage over a 2 mm analytical gap producing a separation field of ~80 Td. The curtain plate voltage was maintained at 1,000 V DC. Outer electrode bias voltage was maintained at 35 V, and the compensation voltage was applied to the inner electrode. The carrier gas used was a 1:1 ratio of He: N2 at 2 L/min. The potential between the second gate of the DT-IMS and the curtain plate of the FAIMS was 1,000 V DC over a separation of 1.5 cm. Quadrupole ion trap The FAIMS system was coupled to a LCQ Deca quadrupole ion trap mass spectrometer (Thermo Electron, San Jose, CA). The ionization source of the LCQ Deca quadrupole was removed to enable the end of the FAIMS device to fit directly onto the MS heated capillary interface. The 2 mm outlet of the FAIMS device was mounted in close contact with the heated capillary inlet of the LCQ Deca using a PEEK mounting ring designed

Int. J. Ion Mobil. Spec. (2011) 14:15–22

specifically to optimize alignment. The PEEK mounting ring was attached to the heated capillary using a simple set screw and connected to the FAIMS device with a transfer bushing. This arrangement required that the heated capillary be maintained at 150 °C or less to avoid causing thermal breakdown of the PEEK. Additionally, the capillary was operated at a low potential (typically 15 V) to avoid developing a significant potential difference between itself and the outer electrode of the FAIMS device. Ion transfer between the FAIMS device and the LCQ Deca was driven by the gas conductance limit of the capillary inlet which was measured to be approximately 1.9 L/min. The ratio of gas flow through the FAIMS device to gas the flow through the capillary is nearly 1:1 ensuring the majority of the ions not filtered are passed to the mass spectrometer inlet. Software The two ion gates in DT-IMS were controlled by custom built high-voltage gate control hardware and LabView 6.1 software (National Instruments, Austin, TX) written in house. The graphical interface allowed the user to define parameters for SIM mode: scan range, first ion gate pulse width, second ion gate scan window, number of ion gate pulses per ion trap injection cycle and for DGS mode: scan range, first and second ion gate pulse widths, first ion gate delay, step resolution and ion gate pulses per ion trap injection cycle. The ion mobility timing control software governed solely the pulsing characteristics of the DT-IMS system and was synchronized with the ion trap injection cycle. DT-IMS, FAIMS and MS data were collected using the ion trap Xcalibur suite of programs (Thermo Electron, San Jose, CA). The Xcalibur program allowed the user to define the number of microscans or ion trap scan averages included for each data point. Modes of operation With this dual gate DT-IMS-FAIMSQITMS system, there are four different modes of operation. First: the DT-IMS system can be used without the gates activated (both gates “open”) and the FAIMS system can be turned into “pass through” mode by turning off the dispersion voltage and leaving the outer electrode bias voltage to 35 V. This mode provides no mobility separation and only mass spectrometer can be used, but does allow the ions ample time for desolvation and helps to maintain the cleanliness of the whole system. Second, the FAIMS system can be turned into “pass through” mode and used as an ion transmission device. The system can be considered as dual gate DT-IMS coupled to mass spectrometer directly and the two operation modes of dual gate DT-IMS are fully utilized in this mode. With this configuration, a low-field ion mobility spectrum can be collected by operating dual gate DT-IMS in DGS mode and a specific drift time window of ions can be accumulatively pulsed into

Int. J. Ion Mobil. Spec. (2011) 14:15–22

19

requires long data acquisition time while SIM mode is good to study ions of interest efficiently. The above four modes can be referred as: MS only, dual gate DT-IMS-QITMS, FAIMS-QITMS and dual gate DT-IMS-FAIMS-QITMS.

1.00

Normalized Signal

0.80 0.60 0.40

Results & discussion

0.20 0.00 -10.0 -11.0 -12.0 CV (Volts)

-13.0

-14.0

41.0 40.0 39.0 Drift Time (ms) 38.0 -15.0

Fig. 3 Drift time and compensation voltage scan of cocaine as [M+H]+ at m/z 304.1

mass spectrometer by operating DT-IMS in SIM mode. Third: DT-IMS can be turned into “pass through” mode with two ion gates open and FAIMS cell can be activated. DT-IMS is effectively decoupled from the whole system. Only high field asymmetric mobility separation is achieved, no low field mobility separation is allowed in this mode. Four: both low electric field (DT-IMS) and high electric field mobility (FAIMS) separation can be activated. In this mode, FAIMS cell is activated and dual gate DT-IMS can work in either DGS mode or SIM mode. Ions separated in the dual gate DT-IMS are transferred into FAIMS cell stepwise to perform high field mobility separation. However, DGS focuses on all the ions in the sample which

Evaluation of DT-IMS-FAIMS-QITMS with an analytical standard The multi-dimensional data of the instrument was demonstrated using cocaine as a standard. Cocaine is an addictive powerful drug and has high potential for abuse which will cause various adverse effects on brain and body, thus the capability to detect cocaine by different techniques is critical for pharmaceutical and human health. In this example, both the low electric field DT-IMS and high electric field FAIMS were scanned, yielding three orthogonal identifications of the analyte species: 1) low-field drift time (ms), 2) high-field compensation voltage (CV), and 3) mass spectrum (m/z). Figure 3 is a multi-dimensional plot showing the drift time and compensation voltage scan for cocaine ([M+H]+, m/z 304.1). All the mobility spectra displayed in this study were the average of four mobility scans, each scan took 2 min to collect the data. The drift time of cocaine was measured at 39 ms and compensation voltage was at −12 V. DT-IMS data were collected at 0.5 ms intervals for a narrow drift time

Fig. 4 Structures of YGW, WGY and YWG and the mass spectrum of a 1:1:1 mixture

20

Fig. 5 Ion mobility spectra of the three isomeric tri-peptides obtained individually: YGW, WGY and YWG

range from 38 ms to 41.5 ms and FAIMS data was also collected over a narrow scan range from 10 to 15 V. Since two ion gates were employed in DT-IMS system, the overall sensitivity of the system is low compared to one gate IMS system. This also accounts for the narrow drift time range, narrow FAIMS scan range and wide gate pulse widths used for acquiring data in all the experiments. Isomeric separation of three tri-peptides: YGW, WGY, and YWG In order to evaluate the isomeric separation capability of the dual gate DT-IMS-FAIMS-QITMS system, three tripeptides were selected: YGW, WGY, and YWG. Peptides are considered as very flexible systems and are able to form typical distinct conformer states [18]. MS alone cannot distinguish among peptides with the same amino acid compositions but different sequences because they have same molecular weight and typically give identical MS spectra. Figure 4 shows the structures of three tripeptides and the MS spectrum of a 1:1:1 mixture. The three yield the same mass spectrum, with a base peak [M+H]+ at m/z 425. The two nitrogens highlighted in red

Int. J. Ion Mobil. Spec. (2011) 14:15–22

indicate proposed sites of protonation. These protanation sites distribute differently in the space which would lead to different collision cross sections. Therefore, development of peptide separation techniques coupled with MS is desirable. Dual gate DT-IMS-QITMS and dual gate IMS-FAIMSQITMS analyses of the three individual tri-peptides were performed. By turning the FAIMS as a pass-through device, low electric field DT-IMS spectra were obtained for YGW, WGY and YWG, as shown in Fig. 5. The drift times of YGW, WGY and YWG are observed at 37.4 ms, 38.2 ms and 39.1 ms, respectively, with resolution between the closest pairs of YGW and WGY of 1.5. This demonstrates that the DT-IMS can resolve these three isomeric based on differences in the ions’ Ω. It was noticed that only a single mobility peak was observed for each tri-peptide, which indicates no conformational differentiation of these tripeptide ions is achieved by DT-IMS. In addition, the resolving power of ~75 is achieved of these three tripeptides even with wide gate pulse widths. When the low-field mobility peak was gated into the high-field FAIMS system, the compensation voltage (CV) was scanned from −20 to 0 V in 2 min. Each FAIMS spectrum displayed in Fig. 6 is the average of four repeated CV scans. These spectra were collected in step-wise fashion: a mobility-selected slice of ions exited the lowfield system (DT-IMS) into the scanning high-field device (FAIMS); the ions were finally accumulated and mass analyzed by the ion trap mass spectrometer. YGW and WGY have similar CV values at −10 V, while YWG is detected at a significantly different CV of −5.6 V and with a lower intensity peak at CV of −2.8 V. This corresponds to resolution of 1.0 between the closest pairs of WGY and YWG. While this resolution is not as high as that provided by DT-IMS, it nevertheless complements the separation provided by DT-IMS. The two peaks observed for YWG at CV values of −5.6 and −2.8 V likely represent two different conformers of the YWG ion. Variation in the conformation is a common phenomenon for peptides; it results from the

Fig. 6 FAIMS scans/CV spectra of the three isomeric tri-peptides obtained individually: YGW, WGY and YWG

Int. J. Ion Mobil. Spec. (2011) 14:15–22

interaction between backbone and side-chain of the peptides to achieve the most energetically preferred state [18]. Although the different conformational structures of YWG corresponding for the two FAIMS peaks are not confirmed, this illustrates the potential of FAIMS for conformation study for peptides. Overall, separation of YGW, WGY and YWG was achieved by DT-IMS with the FAIMS separating YWG from the other two peptides and providing valuable conformer information, and the MS verifying ion identification. This study is significant because it demonstrates the potential of combined DTIMS-FAIMS-QITMS for identifying peptides and for further other bio-molecules application.

Conclusion We have successfully interfaced three well-established analytical techniques into one new and novel instrument which has not been demonstrated previously. The benefits include increasing flexibility of analysis based on over determination, increased insight into ion mobility at both high and low fields, and pre-filtration for MS analysis. Operation of the DT-IMS subsystem using a dual gating technique creates a pulsed source of ions for separation by FAIMS followed by mass analysis in a quadrupole ion trap. This creates a threedimensional response surface consisting of low-field mobility, high-field differential mobility, and mass-to-charge, demonstrating the strengths of the high resolution DT-IMS system coupled to FAIMS and a mass spectrometer with MSn capabilities. Through the use of a custom-built two-gate DTIMS coupled to a quadrupole ion-trap, isomeric tri-peptides were separated and analyzed. DT-IMS has successfully been used to separate isomeric peptides which cannot be resolved with mass spectrometry alone. Detection of two conformational stages of YWG ion by FAIMS shows the advantage of high electric field ion mobility. Further studies need to be performed, pushing the limits to complex mixtures and molecular sizes. And the interfaces between each stage could be optimized to increase sensitivity of the whole system. Acknowledgements This work was supported by IMPLANT SCIENCES under grant #: RSAR0700019. Additional acknowledgement was made to Thermo Finnigan who provided the LCQ Deca quadrupole ion trap mass spectrometer that made this study possible.

References 1. Asbury GR, Klasmeier J, Hill HH Jr (2000) Analysis of explosives using electrospray ionization/ion mobility spectrometry (ESI/IMS). Talanta 50:1291–1298 2. Asbury GA, Hill HH Jr (2000) Using different drift gases to change separation factors (α) in ion mobility spectrometry. Anal Chem 72:580–584

21 3. Barnett DA, Ells B, Guevremont R, Purves RW (2002) Application of ESI-FAIMS-MS to the analysis of tryptic peptides. J Am Soc Mass Spectrom 13:1282–1291 4. Barnett DA, Ells B, Guevremont R (2000) Evaluation of carrier gases for use in high-field asymmetric waveform ion mobility spectrometry. J Am Soc Mass Spectrom 11:1125–1133 5. Borysik AJH, Ashcroft AE (2004) Separation of β2-microglobulin conformers by high-field asymmetric waveform ion mobility spectrometry (FAIMS) coupled to electrospray ionization mass spectrometry. Rapid Commun Mass Spectrom 18:2229–2234 6. Buryakov IA, Krylov EV, Nazarov EG, Rasulev UK (1993) Int J Mass Spectrom Ion Process 128:143–148 7. Clowers BH, Dwivedi P, Steiner WE, Hill HH Jr (2005) Separation of sodiated isobaric disaccharides and trisaccharides using electrospray ionization-atmospheric pressure ion mobility-time of flight mass spectrometry. J Am Soc Mass Spectrom 16:660–669 8. Clowers BH, Hill HH Jr (2006) Influence of cation adduction on the separation characteristics of flavonoid diglycoside isomers using dual gate-ion mobility-quadrupole ion trap mass spectrometry. J Mass Spectrom 41:339–351 9. Clowers BH, Hill HH Jr (2005) Mass analysis of mobilityselected ion populations using dual gate, ion mobility, quadrupole ion trap mass spectrometry. Anal Chem 77:5877–5885 10. Dwivedi P, Bendiak B, Clowers BH, Hill HH Jr (2007) Rapid resolution of carbohydrate isomers by electrospray ionization ambient pressure ion mobility spectrometry-time-of-flight mass spectrometry (ESI-APIMS-TOFMS). J Am Soc Mass Spectrom 18:1163–1175 11. Eiceman GA, Karpas Z (2005) Ion mobility spectrometry, 2nd edn. Taylor and Francis Group, LLC, Boca Raton 12. Eiceman GA (2002) Ion-mobility spectrometry as a fast monitor of chemical composition. Trends Anal Chem 21:259–275 13. Eiceman GA, Krylov EV, Tadjikov B, Ewing RG, Nazarov EG, Miller RA (2004) Differential mobility spectrometry of chlorocarbons with a micro-fabricated drift tube. Analyst 129:297–304 14. Eiceman GA, Young D, Smith G (2005) Mobility spectrometry of amino acids and peptides with matrix assisted laser desorption and ionization at ambient pressure. Microchimica Acta 81:108–116 15. Ewing RG, Atkinson DA, Eiceman GA, Ewing GJ (2001) A critical review of ion mobility spectrometry for the detection of explosives and explosive related compounds. Talanta 54:515–529 16. Guevremont R, Purves RW (1999) High field asymmetric waveform ion mobility spectrometry–mass spectrometry: an investigation of leucine enkephalin ions produced by electrospray ionization. J Am Soc Mass Spectrom 10:492–501 17. Guevremont R (2004) High-field asymmetric waveform ion mobility spectrometry: a new tool for mass spectrometry. J Chromatogr A 1058:3–19 18. Guisasola EEB, Masman MF, Enriz RD, Rodríguez AM (2010) Structure of isolated tyrosyl-glycyl-glycine tripeptide: a comparative conformational study with peptides containing an aromatic ring. Cent Eur J Chem 8(3):566–575 19. Kanu AB, Hill HH Jr (2007) Identity confirmation of drugs and explosives in ion mobility spectrometry using a secondary drift gas. Talanta 73:692–699 20. Kanu AB, Dwivedi P, Tam M, Matz L, Hill HH Jr (2008) Ion mobility-mass spectrometry. J Mass Spectrom 43:1–22 21. Krylov EV, Coy SL (2009) Temperature effects in differential mobility spectrometry. Int J Mass Spectrom 279:119–125 22. Lambertus GR, Fix CS, Reidy SM, Miller RA, Wheeler D, Nazarov E, Sacks R (2005) Silicon microfabricated column with microfabricated differential mobility spectrometer for GC analysis of volatile organic compounds. Anal Chem 77:7563–7571 23. Li J, Taraszka JA, Counterman AE, Clemmer DE (1999) Influence of solvent composition and capillary temperature on the conformations of electrosprayed ions: unfolding of compact ubiquitin

22

24. 25.

26.

27.

28.

Int. J. Ion Mobil. Spec. (2011) 14:15–22 conformers from pseudonative and denatured solutions. Int J Mass Spectrom 185(186/187):37–47 Mason EA, McDaniel EW (1988) Transport properties of ions in gases. Wiley, New York Miller RA, Eiceman GA, Nazarov EG, King AT (2000) A novel micromachined high-field asymmetric waveform-ion mobility spectrometer. Sens Actuators B 67:300–306 Purves RW, Barnett DA, Guevremont R (2000) Separation of protein conformers using electrospray-high field asymmetric waveform ion mobility spectrometry. Int J Mass Spectrom 197:163–177 Purves RW, Guevremont R (1999) Electrospray ionization highfield asymmetric waveform ion mobility spectrometry-mass spectrometry. Anal Chem 71:2346–2357 Shvartsburg AA, Tang K, Smith RD (2009) Two-dimensional ion mobility analyses of proteins and peptides, Mass Spectrometry of Proteins and Peptides 492, doi:10.1007/978-1-59745-493-3_26

29. Shvartsburg AA, Tang K, Smith RD (2010) Differential ion mobility separations of peptides with resolving power exceeding 50. Anal Chem 82(1):32–35 30. Tang K, Li FM, Shvartsburg AA, Strittmatter EF, Smith RD (2005) Two-dimensional gas-phase separations coupled to mass spectrometry for analysis of complex mixtures. Anal Chem 77 (19):6381–6388 31. Verbeck GF, Russell DH (2002) A fundamental introduction to ion mobility mass spectrometry applied to the analysis of biomolecules. J Biomol Tech 13:56–61 32. Wu C, Siems WF, Klasmeier J, Hill HH Jr (2000) Separation of isomeric peptides using electrospray ionization/high-resolution ion mobility spectrometry. Anal Chem 72:391–395 33. Zhu M, Bendiak B, Clowers BH, Hill HH Jr (2009) Ion mobilitymass spectrometry analysis of isomeric carbohydrate precursor ions. Anal Bioanal Chem 394:1853–1867