Coupling of a High-Resolution Ambient Pressure Drift Tube Ion Mobility Spectrometer to a Commercial Time-of-flight Mass Spectrometer
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Ion mobility spectrometry provides information about molecular structures of ions. Hence, high resolving power allows separation of isomers which is of major interest in several applications. In this work, we couple our high-resolution ion mobility spectrometer (IMS) with a resolving power of Rp = 100 to a time-of-flight mass spectrometer (TOF-MS). Besides, the benefit of an increased resolving power such an IMS-MS also helps analyzing and understanding the ionization processes in IMS. Usually, the coupling between IMS and TOF-MS is realized by synchronizing data acquisition of the IMS and MS resulting in two-dimensional data containing ion mobility and mass spectra. However, due to peak widths of less than 100 μs in our high-resolution IMS, this technique is not practicable due to significant peak broadening during the ion transfer into the MS and an insufficient data acquisition rate of the MS. Thus, a novel but simple interface between the IMS and MS has been designed which minimizes ion losses, allows recording of ion mobility at full IMS resolving power, and enables a shuttered transmission of ions into the MS. The interface is realized by replacing the Faraday plate used in IMS by a Faraday grid that is shielded by two additional aperture grids. For demonstration, positive product ions of benzene, toluene, and m-xylene in air are investigated. The IMS is equipped with a radioactive 3H source. Besides the well-known product ions M+ and M·NO+, a dimer ion is also observed for benzene and toluene, consisting of two molecules and three further hydrogen atoms.
KeywordsHigh-resolution ion mobility spectrometer Gated IMS-MS interface BTX
Ion mobility spectrometry (IMS) is a well-known technique for fast online trace gas detection [1, 2]. Detection limits in the low ppbV (parts per billion) and even pptV (parts per trillion) range combined with fast response times make this technique very interesting for a wide range of applications [3, 4, 5]. IMS separates ions based on their mobility in an inert drift gas in the presence of an electric field. In the last years, we developed very compact high-resolution drift tube IMS with short drift lengths between 40 and 150 mm [6, 7] and different ion sources such as radioactive 3H  and non-radioactive electron sources , corona , X-ray , ultra violet (UV) , and electrospray ionization (ESI) sources  reaching a resolving power up to 250 . High-resolution IMS allow the investigation of ions with respect to size, shape, and charge gaining insights into molecular structures. In particular, with high IMS resolving power isomers, isobars and conformers can be separated which is of major interest in several applications, such as pharmaceutical, food, fragrance, and flavor research, where coupling IMS with mass spectrometry provides a new dimension in the analysis of complex mixtures. In this work, we couple our compact high-resolution ambient pressure IMS to a commercial time-of-flight mass spectrometer (Bruker micrOTOF II). Besides the benefit of an increased resolving power, our IMS also supports a wide range of interchangeable ion sources, which facilitates the investigation of a wide range of chemical compounds. Thus, this IMS-MS helps analyzing and understanding the ionization processes and eventually identifying the ion species formed in IMS at ambient pressure.
Over the last 40–50 years, numerous IMS-MS devices have been developed. Different combinations of IMS and MS instruments have been realized, including quadrupole mass spectrometers, ion trap mass spectrometers, Fourier transform mass spectrometers, magnetic sector mass spectrometers, and time-of-flight mass spectrometers as well as high-field asymmetric waveform ion mobility spectrometer (FAIMS), traveling wave IMS, trapped IMS, and drift tube IMS with different ion sources operated from low to ambient pressure. Since the original development, a number of commercially available instruments have been developed including those by TOFWERK (IMSTOF), Excellims (RA4100 HPIMS-MS), Agilent (Ion Mobility Q-TOF), Waters Corporation (Synapt-G2), and Bruker (timsTOF), democratizing the field. In particular, the timsTOF has shown high mobility resolving powers when applied to large, multiple-charged ions, reaching resolving powers of up to Rp = 250 . However, for smaller molecules, the drift tube technique reaches better performance. Thus, the aim of this work is to hyphenate our in-house developed drift tube IMS to the commercial Bruker micrOTOF II instrument to enable a high-resolution analysis of a wide range of chemical compounds, as our IMS is designed to operate with a variety of interchangeable ion sources. It is also important to note that our IMS operates at ambient pressure, while most IMS in IMS-MS operate at much low pressure which leads to an increasing effect of m/z on ion separation and less separation by collision cross section.
Considering drift tube IMS-TOF-MS couplings in the literature, it can be distinguished between nested and selective techniques . Clemmer and coworkers developed the first fully nested drift tube IMS-TOF-MS technique . After being separated in a drift tube, all ions are injected into the mass spectrometer. Since the time frame of an ion mobility spectrum is typically in the millisecond range and the flight time in the mass spectrometer is typically in the microseconds range, for every peak in the ion mobility spectrum, several mass spectra can be obtained. This results in a nested two-dimensional spectrum containing ion mobility spectra and mass spectra [16, 17]. The bin size in the drift time is the periodicity of the TOF pulser; within this bin size, no differences in drift times are distinguishable. Furthermore, the transfer of ions into the flight tube of the MS leads to peak broadening. Thus, compared to a continuous detector at the end of the IMS drift tube, there is a small loss in IMS resolving power. In common devices, the loss of resolving power is negligible, since the periodicity of the TOF-MS pulser is significantly lower than the IMS peak widths. However, in our compact high-resolution IMS, the peak width is less than 100 μs for a resolving power of Rp = 100  or even less than 30 μs for a resolving power of Rp = 250 . Thus, peak broadening in the transfer region of the MS becomes more significant, decreasing the effective IMS resolving power and the maximum repetition rate of, e.g., 20 kHz of the Bruker micrOTOF II is no longer sufficient to resolve the ion mobility spectrum.
Nonetheless, in addition to nested IMS-MS techniques, there are selective techniques for transmitting ions of only a selected mobility and then analyzing the resulting mass spectrum. This technique of coupling implies no requirements on the relative analysis times of the coupled devices and is therefore more appropriate for the coupling of our high-resolution IMS to the Bruker micrOTOF II. Early drift tube IMS-MS devices are based on this selective technique using a dual gate ion mobility setup to determine the mobility of ions by only gating out those ions that traversed the drift tube in a predetermined time, and then scanning the delay between the entrance and exit gates [18, 19]. More recently, Hill and coworkers developed high-resolution IMS-MS devices using a drift tube IMS with two Bradbury-Nielsen gates situated at the entrance and the exit of the drift tube [20, 21], which has been applied later in several devices [22, 23, 24]. This dual gate technique requires scanning the pulses at the second gate to obtain a full IMS spectrum. Thus, the bin size in the drift time is determined by the minimal gate opening time of the second gate. However, the minimal pulse width of Bradbury-Nielsen shutters of 50 μs  is insufficient to resolve the ion mobility spectrum of our compact high-resolution IMS. Hence, the full resolving power of our IMS is only maintained when the IMS spectrum is measured directly at the end of the drift tube, as shown in  or .
In this work, a simple IMS-MS interface is presented, minimizing ion losses, allowing the direct measurement of the ion mobility spectra, and enabling a shuttered transmission of ions into the MS to investigate single ion peaks and defined time domains of the ion mobility spectrum. Using this interface, it is possible to couple our high-resolution IMS to a Bruker micrOTOF II. Besides maintaining full IMS resolving power important for many applications, this IMS-MS allows investigating the ion chemistry employing different ion sources operated at ambient pressure. In this work, positive ions formed by a radioactive 3H source are investigated. As proof of concept study, measurements of benzene, toluene, and xylene are presented.
Time-of-flight Mass Spectrometer
The Bruker micrOTOF II is an atmospheric pressure ionization (API) TOF-MS with an ion flight tube containing an orthogonal acceleration stage, a reflector, and a microchannel plate detector. Ions from the API source pass through a glass inlet capillary before they enter the first vacuum stage. Using skimmers, RF hexapoles, and multi-lens systems, ions are transported through further vacuum stages to the finale pressure stage of the ion flight tube . The transfer parameters are summarized in the supplementary material A. By inversion of the applied voltages, the Bruker micrOTOF II is capable of measuring positive and negative ions. In general, the Bruker micrOTOF II is able to measure masses up to 20,000 m/z. Since this work focusses on the investigation of compounds with low m/z, the mass range is optimized for masses from 30 to 200 m/z.
For IMS-MS coupling, the API source is replaced by our high-resolution ambient pressure IMS. Thus, the IMS is directly coupled to the inlet capillary of the MS.
Ion Mobility Spectrometer
Operating parameters of the IMS
Drift gas flow
Sample gas flow
Length drift tube
For simple coupling our IMS to the Bruker micrOTOF II without further modifying the MS, only the Faraday detector of the IMS has to be modified. The novel interface is realized by replacing the Faraday plate of the IMS by a Faraday grid that is shielded by two further aperture grids, as shown in Figure 1b. By directly inserting the interface into the capillary shield of the MS, the IMS can be easily adapted to the MS inlet capillary. The optical transmittance of each grids is 80%. The potentials of the second aperture grid, the MS inlet capillary, and the MS capillary shield can be adjusted to control the ratio of ions that discharges at the Faraday grid to the ions transported by electrical fields towards the MS inlet capillary. The inlet gas flow of dry (< 1 ppmV H2O), purified air is set to 1350 mls/min and is split into 180 mls/min IMS drift gas and 1170 mls/min MS inlet flow. This high MS inlet flow combined with the small inner diameter of the inlet capillary (500 μm) results in very high flow velocities at the MS inlet capillary. Thus, near the capillary, convective ion transport dominates and leads to ion transmission into the MS.
However, with decreasing opening times of the ion gate, discrimination of larger ions with lower ion mobility will increase at the ion gate . To overcome this issue, a novel ion gating concept based on a similar three-grid ion gate and a specific waveform with three different states can be used .
In summary, this interface allows two operation modes. In the selective mode, the dual gate IMS can be used to transfer single ion peaks or defined time domains of the ion mobility spectrum into the MS to obtain the corresponding mass spectra, as known from . In the IMS mode, the IMS spectrum is measured directly using the Faraday grid. Thus, in contrast to prior dual gate work, in this work, the second gate is not scanned to generate an IMS spectrum, but rather the IMS spectrum is measured directly at the end of the drift tube to maintain full IMS resolving power.
Characterization of the IMS-MS System
For comparison, the commercial drift tube IMS-TOF-MS devices by Excellims (RA4100 HPIMS-MS) and Agilent (Ion Mobility Q-TOF) reach a resolving power of Rp = 60–100  and Rp = 60  respectively. The device of TOFWERK (IMSTOF) reaches a resolving power of Rp = 150  including a 205-mm drift tube working at ambient pressure. However, with post-processing, the resolving power exceeds Rp = 200 .
Considering ion transmission into the transfer stage of the MS, this simple interface should allow higher ion transmission rates in contrast to systems with a pinhole interface, due to the significantly higher ion transmittance of grids. Comparing this novel interface with a previous pinhole interface we used , an increase in ion transmission by factor 4 can be observed. However, there are limitations in sensitivity due to the duty cycle of the drift tube IMS. To improve signal intensity and duty cycle, ions could be pulsed into the drift region using a multiplexed gating scheme, as applied in the IMSTOF of TOFWERK .
Ion Cluster Dissociation and Ion Fragmentation Processes in the MS Transfer Stage
Due to the partial dissociation of the acetone dimer ion with 1.31 eV binding energy, reduced electrical field strengths up to 100 Td can be assumed in the transfer stage of the Bruker micrOTOF II. Thus, inside the interface, ion fragmentation processes are also possible as known from PTR-MS studies [35, 36].
The molecular mass of 1-octanol is 130 u. In the mass spectrum, the most abundant peak is 113 m/z, which results from the protonated molecule minus water (130 u − 18 u + 1 u = 113 u). Additionally, the protonated molecule plus water is present (130 u + 18 u + 1 u = 149 u). Furthermore, alkane fragments at m/z = 57 and m/z = 71 can be observed.
The ion mobility spectrum of dry (< 1 ppmV H2O), purified air shows one major peak (K0 = 2.05 cm2/Vs) attributed to the positive reactant ion peak (RIP+) accompanied by two smaller peaks (K0 = 2.16 cm2/Vs and K0 = 2.23 cm2/Vs). It is well known that the ion chemistry in atmospheric pressure chemical ionization sources is dominated by ion clustering processes, especially those that involve water molecules. In previous work, the three reactant ion peaks have been identified as NH4+(H2O)n, NO+(H2O)n, and H+(H2O)n [37, 38, 39]. However, in the corresponding mass spectra, apart from H+(H2O)n ions, no other ions are observed.
Kebarle et al. and many other research groups have investigated and quantitatively described the thermodynamic and kinetic properties of the thermalized proton-bound water cluster system [37, 40, 41, 42]. Based on this literature and kinetic simulations , proton-bound water clusters with n = 4 are expected as major species in a thermally equilibrated system at atmospheric pressure and < 1 ppmV humidity. However, in the corresponding mass spectrum, the highest abundance of water clusters is measured for the H+(H2O)3 ions (55 u). Additionally, H+(H2O)2 ions (37 u) are present in the mass spectrum. Thus, it becomes obvious that cluster dissociation processes occur inside the transfer stage and ions gain enough kinetic energy to break the binding between H+(H2O)3 and H2O (binding energy of 0.78 eV ), the binding between H+(H2O)2 and H2O (binding energy of 0.85 eV ), and to some extent, to break the binding between H3O+ and H2O (binding energy of 1.38 eV ). However, the single protonated water molecule H3O+ (19 u) cannot be detected due to the mass discrimination of the MS used in this work. Since the binding energy of NH4+ with H2O is about 0.75 eV  and the binding energy of NO+ with H2O is about 0.81 eV , the bare NH4+ and NO+ ions are expected to be present after the transfer stage. Again, due to the mass discrimination of small molecules, NH4+ and NO+ cannot be detected by the MS.
These examples show the occurrence of ion cluster dissociation and ion fragmentation processes inside the transfer stage of the Bruker micrOTOF II mass spectrometer. Thus, when analyzing the ion mobility spectra and the corresponding mass spectra, one has to be aware that the mass spectrum might not show the ions present in the IMS. However, IMS-MS data help to better understand the ion formation in ambient pressure IMS.
Furthermore, for high analyte concentrations (> 1 ppmV), a dimer peak is present in the ion mobility spectrum of benzene and toluene. In both cases, this peak consists of two molecules and three hydrogen atoms resulting in m/z = 159 for benzene ((2·78 u) + 3 u = 159 u) and m/z = 187 for toluene ((2·92 u) + 3 u = 187 u). However, such a dimer peak cannot be detected for m-xylene even at very high xylene concentrations. Moreover, this dimer seems to be less stable for toluene in comparison to benzene. In previous work, the benzene dimer is observed to consist of two benzene molecules [49, 50]. So far, to the best of our knowledge, the threefold hydrogenated dimer has not been reported for aromatic hydrocarbons. Thus, the formation process of this dimer ion is unknown and needs to be further investigated.
In this work, a simple IMS-MS interface to couple our compact high-resolution ambient pressure IMS (drift length of just 72 mm, resolving power of Rp = 100) to a Bruker micrOTOF II mass spectrometer is presented. The interface is realized by replacing the Faraday plate of the IMS by a Faraday grid that is shielded by two further aperture grids. Due to the high ion transmittance of the used grids in contrast to pinholes, ion losses are minimized. Furthermore, this interface allows recording the ion mobility spectrum at full resolving power and enables a gated transmission of ions into the MS to investigate single ion peaks or defined time domains of the ion mobility spectrum by simply varying the potential of the second aperture grid. Due to tailored electronics, peak widths in the low microsecond range can be gated. Considering the positive ions formed by a radioactive 3H source, the IMS-MS data show the expected ion cluster dissociation and ion fragmentation processes inside the transfer stage of the Bruker micrOTOF II mass spectrometer. Reduced electrical field strengths up to 100 Td can be assumed in the transfer stage. As proof-of-concept study, measurements of benzene, toluene, and xylene in dry (< 1 ppmV H2O), purified air are presented. Besides the well-known product ions M+ and M·NO+, a dimer ion can be observed for high benzene and toluene concentrations, consisting of two molecules and three hydrogen atoms. Future work aims for investigating the ionization processes at ambient pressure of different ion sources such as radioactive 3H and non-radioactive electron sources, corona, X-ray, ultra violet (UV), and electrospray ionization (ESI) sources.
This work was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – ZI 1288/8-1
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