Sterically hindered phenols in negative ion mobility spectrometry–mass spectrometry

Negative corona discharge atmospheric pressure chemical ionization (APCI) was used to investigate phenols with varying numbers of tert‐butyl groups using ion mobility spectrometry–mass spectrometry (IMS‐MS). The main characteristic ion observed for all the phenolic compounds was the deprotonated molecule [M–H]⁻. 2‐tert‐Butylphenol showed one main mobility peak in the mass‐selected mobility spectrum of the [M–H]⁻ ion measured under nitrogen atmosphere. When air was used as a nebulizer gas an oxygen addition ion was seen in the mass spectrum and, interestingly, this new species [M–H+O]⁻ had a shorter drift time than the lighter [M–H]⁻ ion. Other phenolic compounds primarily produced two IMS peaks in the mass‐selected mobility spectra measured using the [M–H]⁻ ion. It was also observed that two isomeric compounds, 2,4‐di‐tert‐butylphenol and 2,6‐di‐tert‐butylphenol, could be separated with IMS. In addition, mobilities of various characteristic ions of 2,4,6‐trinitrotoluene were measured, since this compound was previously used as a mobility standard. The possibility of using phenolic compounds as mobility standards is also discussed. Copyright © 2009 John Wiley & Sons, Ltd.     

Evaluation of capillary liquid chromatography-electrospray ionization ion mobility spectrometry with mass spectrometry detection

Due to the proteomics revolution, multi-dimensional separation and detection instruments are required to evaluate many peptides and proteins in single samples. In this study, electrospray ionization (ESI) ion mobility spectrometry (IMS) was evaluated as an additional separation after HPLC separations. Common HPLC mobile phase compositions (solvents, acid modifiers, and buffers) were assessed for the effect on ESI-IMS response. Up to 5 mM sodium phosphate, a non-volatile buffer, was able to be electrosprayed into the IMS without degradation of the instrumental performance. Due to the rapid separation times of IMS, multiple IMS spectra were obtained within a single HPLC peak. A five-peptide mixture was separated in a capillary HPLC column under isocratic conditions within 3 min. Coelution of two peaks due to non-optimal HPLC conditions occurred and these two peaks could not be distinguished by HPLC with UV detection. In contrast, the single ion mobility chromatograms provided separation of each peptide as well as providing a second degree of analyte identification (HPLC retention time and IMS mobility). Furthermore, IMS-MS analysis of the five peptides and comparison with HPLC retention times showed that each peptide had a unique retention time-ion mobility-mass to charge value. This work showed that IMS could be employed for direct separation and detection of HPLC eluents and also could be combined with HPLC-MS for three unique dimensions of separation.     

Buffer gas additives (modifiers/shift reagents) in ion mobility spectrometry: Applications,predictions of mobility shifts,and influence of interaction energy and structure

Ion mobility spectrometry (IMS) is an analytical technique used for fast and sensitive detection of illegal substances in customs and airports, diagnosis of diseases through detection of metabolites in breath, fundamental studies in physics and chemistry, space exploration, and many more applications. Ion mobility spectrometry separates ions in the gas‐phase drifting under an electric field according to their size to charge ratio. Ion mobility spectrometry disadvantages are false positives that delay transportation, compromise patient's health and other negative issues when IMS is used for detection. To prevent false positives, IMS measures the ion mobilities in 2 different conditions, in pure buffer gas or when shift reagents (SRs) are introduced in this gas, providing 2 different characteristic properties of the ion and increasing the chances of right identification. Mobility shifts with the introduction of SRs in the buffer gas are due to clustering of analyte ions with SRs. Effective SRs are polar volatile compounds with free electron pairs with a tendency to form clusters with the analyte ion. Formation of clusters is favored by formation of stable analyte ion‐SR hydrogen bonds, high analytes' proton affinity, and low steric hindrance in the ion charge while stabilization of ion charge by resonance may disfavor it. Inductive effects and the number of adduction sites also affect cluster formation. The prediction of IMS separations of overlapping peaks is important because it simplifies a trial and error procedure. Doping experiments to simplify IMS spectra by changing the ion‐analyte reactions forming the so‐called alternative reactant ions are not considered in this review and techniques other than drift tube IMS are marginally covered.     

Pressure effects on resolution in ion mobility spectrometry

This paper explains the effect of pressure on separation factor, resolving power (defined based on a single peak), and resolution (defined based on two adjacent peaks) in ion mobility spectrometry. IMS spectra were recorded at various pressures ranging from 39 hPa (29 Torr) up to atmospheric pressure and various ion gates ranging from 50 to 225 μs. The results show that the IMS peaks shift perfectly linear with pressure so that separation factors remain unaffected by pressure. However, pressure has strong influence on resolving power and resolution. Reducing pressure at constant pulse width decreases the resolving power and resolution. On the other hand, the decrease in resolution can be compensated by shortening the ion pulse width since reducing pressure results in a higher ion current.     

Mobility spectrometry of amino acids and peptides with matrix assisted laser desorption and ionization in air at ambient pressure

Gas phase ions for valine, glutamate, phenylalanine, angiotensin, bradykinin, LH-RH, and bombesin were formed through matrix assisted laser desorption-ionization (MALDI) in air at ambient pressure and were characterized by ion mobility spectrometry (IMS). The IMS drift tube was operated at 100 °C with air as the drift gas and without an ion shutter. Responses were obtained using α-cyano-4-hydroxycinnamic acid as the matrix and a Nd-YAG laser at 355 nm with an unfocused beam at 6 mJ per pulse and 7 mm² cross section. Matrix and analyte were applied to a borosilicate glass target and microgram amounts of sample provided responses lasting 10 to 15 s with the laser operated at 11 Hz. Detection limits for the peptides were estimated to be 10 to 100 pmol per laser shot. The mobility spectra for individual amino acids and peptides exhibited multiple peaks with spectral distortions and raised baselines. These features and calculated values for reduced mobilities were consistent with the existence of clusters between analyte ions and matrix neutrals and the dissociation of these clusters in the drift region of the analyzer. Mobility spectra with distinctive peaks were not obtained for MALDI-IMS of peptides larger than 5700 amu, though ion formation was suggested from the depletion of matrix signal.     

Ion mobility-mass spectrometry

This review article compares and contrasts various types of ion mobility-mass spectrometers available today and describes their advantages for application to a wide range of analytes. Ion mobility spectrometry (IMS), when coupled with mass spectrometry, offers value-added data not possible from mass spectra alone. Separation of isomers, isobars, and conformers; reduction of chemical noise; and measurement of ion size are possible with the addition of ion mobility cells to mass spectrometers. In addition, structurally similar ions and ions of the same charge state can be separated into families of ions which appear along a unique mass-mobility correlation line. This review describes the four methods of ion mobility separation currently used with mass spectrometry. They are (1) drift-time ion mobility spectrometry (DTIMS), (2) aspiration ion mobility spectrometry (AIMS), (3) differential-mobility spectrometry (DMS) which is also called field-asymmetric waveform ion mobility spectrometry (FAIMS) and (4) traveling-wave ion mobility spectrometry (TWIMS). DTIMS provides the highest IMS resolving power and is the only IMS method which can directly measure collision cross-sections. AIMS is a low resolution mobility separation method but can monitor ions in a continuous manner. DMS and FAIMS offer continuous-ion monitoring capability as well as orthogonal ion mobility separation in which high-separation selectivity can be achieved. TWIMS is a novel method of IMS with a low resolving power but has good sensitivity and is well intergrated into a commercial mass spectrometer. One hundred and sixty references on ion mobility-mass spectrometry (IMMS) are provided.     

Alkali halides based on nano-alumina as positive and negative ion source for ion mobility and mass spectrometry

In this work, a new long-life alkali ion source is proposed that is based on alkali halide salts doped in nano-γ-alumina (Al₂O₃). Depending on the polarity, the ion source produces both alkali and halide ions. The source was characterized using different techniques such as scanning electron microscopy (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD), fourier transform infrared (FT-IR), and ion mobility spectrometry (IMS). SEM images confirm a strong interaction between the alkali halide (MX) and nano-γ-alumina. The average particle size of the doped nanoparticles was calculated to be 44 nm by TEM. Formation of new phases (KAlCl₂O and K₃AlF₆) was confirmed by XRD and that of Al–O–K group in the synthesized particles by FT-IR. Alkali and halide ion peaks were observed by IMS in the positive and negative modes, respectively. The lifetime of the ion source for different alkali halides was measured to range from 216 to 960 h. The total ion current emitted from the source was about 2 µA, while it was 12 nA at the collector plate of the IMS. Finally, application of the new source in ion mobility spectrometry was demonstrated by observing ion mobility spectra of compounds ionized via cation and anion attachment reaction.     

Development of imaging mass spectrometry (IMS) dataset extractor software,IMS convolution

Imaging mass spectrometry (IMS) is a powerful tool for detecting and visualizing biomolecules in tissue sections. The technology has been applied to several fields, and many researchers have started to apply it to pathological samples. However, it is very difficult for inexperienced users to extract meaningful signals from enormous IMS datasets, and the procedure is time-consuming. We have developed software, called IMS Convolution with regions of interest (ROI), to automatically extract meaningful signals from IMS datasets. The processing is based on the detection of common peaks within the ordered area in the IMS dataset. In this study, the IMS dataset from a mouse eyeball section was acquired by a mass microscope that we recently developed, and the peaks extracted by manual and automatic procedures were compared. The manual procedure extracted 16 peaks with higher intensity in mass spectra averaged in whole measurement points. On the other hand, the automatic procedure using IMS Convolution easily and equally extracted peaks without any effort. Moreover, the use of ROIs with IMS Convolution enabled us to extract the peak on each ROI area, and all of the 16 ion images on mouse eyeball tissue were from phosphatidylcholine species. Therefore, we believe that IMS Convolution with ROIs could automatically extract the meaningful peaks from large-volume IMS datasets for inexperienced users as well as for researchers who have performed the analysis.     

Determination of terpenes in humid ambient air using ultraviolet ion mobility spectrometry

Atmospheric humidity causes the major problem using ion mobility spectrometers (IMS) under ambient conditions. Significant changes of the spectra are decreasing sensitivity as well as selectivity. Therefore, the influence of humidity on the IMS signal was investigated in case of direct introduction of the analyte into the ionisation chamber and in case of pre-separation by help of a multi-capillary column (MCC). For direct analyte introduction, a significant decrease of the total number of ions in the range of 28-42% with increasing relative humidity was found. Simultaneously additional peaks in the spectra were formed, thus complicating the identification of the analytes. In case of pre-separation of the analyte, the spectra do not change with increasing relative humidity, due to the successive appearance of the analyte and the water molecules in the ionisation chamber. Detection limits were found in the range of 5 μg/m³ (about 1 ppbv) for selected terpenes and—with pre-separation—independent on relative humidity of the analyte. Without pre-separation, detection limits are in the same range for dry air as carrier gas but in the range of 200-600 μg/m³ when relative humidity reaches 100%. Thus, MCC-UV ion mobility spectrometry is optimally capable for the detection of trace substances in ambient air (e.g. indoor air quality control, process control, odour detection) without further elaborate treatment of the carrier gas containing the analyte and independent on relative humidity.     

Atmospheric-pressure ionization studies and field dependence of ion mobilities of isomeric hydrocarbons using a miniature differential mobility spectrometer

The ionization pathways and ion mobility were determined for sets of structural isomeric and stereoisomeric non-polar hydrocarbons (saturated and unsaturated cyclic hydrocarbons and aromatic hydrocarbons) using a novel miniature differential mobility spectrometer with atmospheric-pressure photoionization (APPI) to assess how structural and stereochemical differences influence ion formation and ion mobility. The analytical results obtained using the differential mobility spectrometry (DMS) were compared with the reduced mobility values measured using conventional time-of-flight ion mobility spectrometry (IMS) with the same ionization technique.The majority of differences in DMS ion mobility spectra observed among isomeric cyclic hydrocarbons can be explained by the formation of different product ions. Comparable differences in ion formation were also observed using conventional IMS and by investigations using the coupling of ion mobility spectrometry with mass spectrometry (APPI-IMS-MS) and APPI-MS. Using DMS, isomeric aromatic hydrocarbons can in the majority of cases be distinguished by the different behavior of product ions in the strong asymmetric radio frequency (rf) electric field of the drift channel. The different peak position of product ions depending on the electric field amplitude permits the differentiation between most of the investigated isomeric aromatics with a different constitution; this stands in contrast to conventional IMS in which comparable reduced mobility values were detected for the isomeric aromatic compounds.     

Shift reagents in ion mobility spectrometry: the effect of the number of interaction sites,size and interaction energies on the mobilities of valinol and ethanolamine

Overlapping peaks interfere in ion mobility spectrometry (IMS), but they are separated introducing mobility shift reagents (SR) in the buffer gas forming adducts with different collision cross‐sections (size). IMS separations using SR depend on the ion mobility shifts which are governed by adduct's size and interaction energies (stabilities). Mobility shifts of valinol and ethanolamine ions were measured by electrospray‐ionization ion mobility‐mass spectrometry (MS). Methyl‐chloro propionate (M) was used as SR; 2‐butanol (B) and nitrobenzene (N) were used for comparison. Density functional theory was used for calculations. B produced the smallest mobility shifts because of its small size. M and N have two strong interaction sites (oxygen atoms) and similar molecular mass, and they should produce similar shifts. For both ethanolamine and valinol ions, stabilities were larger for N adducts than those of M. With ethanolamine, M produced a 68% shift, large compared to that using N, 61%, because M has a third weak interaction site on the chlorine atom and, therefore, M has more interaction possibilities than N. This third site overrode the oxygen atoms' interaction energy that favored the adduction of ethanolamine with N over that with M. On the contrary, with valinol mobility shifts were larger with N than with M (21 vs 18%) because interaction energy favored even more adduction of valinol with N than with M; that is, the interaction energy difference between adducts of valinol with M and N was larger than that between those adducts with ethanolamine, and the third M interaction could not override this larger difference. Mobility shifts were explained based on the number of SR's interaction sites, size of ions and SR, and SR–ion interaction energies. This is the first time that the number of interaction sites is used to explain mobility shifts in SR‐assisted IMS. Copyright © 2016 John Wiley & Sons, Ltd.     

Ion characterisation by comparison of ion mobility spectrometry and mass spectrometry data

The major uncertainty related to ion mobility spectrometry is the lack of knowledge about the characteristics of the ions detected. When using a radioactive atmospheric pressure ionisation source (e.g. ⁶³Ni), from theory proton bound water clusters are expected as reactant ions. When analyte ions occur, proton transfer should lead to proton-bound monomer and dimer ions. To increase the knowledge about those ionisation processes in an ion mobility spectrometer (IMS), a ß-radiation ionisation source was coupled to a mass spectrometer (MS) and an identical one to an IMS. Exemplarily, acetone, limonene and 2- and 5-nonanone were introduced into both instruments in varying concentrations. By correlating the MS and IMS spectra, conclusions about the identities of the ions detected by IMS could be drawn. Proton-bound monomer, dimer and even trimer ions (MH⁺, 2MH⁺, 3MH⁺) could be observed in the MS spectra for acetone and 5-nonanone and could be assigned to the related signals detected by IMS. The oligomers could be expected from theory for increasing concentration. Limonene and 2-nonanone yielded in a variety of different ions and fragments indicating complex gas phase ion chemistry. Those findings on the obviously different behaviour of different analytes require further research focussed on the ion chemistry in IMS including the comparison of different ionisation sources.     

Temperature effects on resolution in ion mobility spectrometry

The separation efficiency of ion mobility spectrometry (IMS) may be measured in terms of either resolving power, based on a single-peak definition, or peak-to-peak resolution, based on the separation of pairs of adjacent peaks. Usually resolving power decreases with temperature. However, the experimental results show that the peak-to-peak resolution may be increased in some cases. Negative ion mobility spectra of halide ions are better resolved at elevated temperatures. In addition, the peaks corresponding to protonated monomer of amylacetate and the proton-bound dimer of ethylacetate are well separated at 100 °C while they fully overlap at 18 °C. This paper focuses on the effect of temperature on peak-to-peak resolution. It was also observed that in some cases peak-to-peak resolution decreases with temperature. Examples are the spectra of cyclohexanone and methyl-iso-butyl ketone (MIBK) as well as dimethyl methyl phosphonate (DMMP) and MIBK. The increase or decrease in resolution at elevated temperatures has been attributed to the changes in separation factor (α) which is governed by the different hydration and clustering tendency of ions.     

Improved design for high resolution electrospray ionization ion mobility spectrometry

An improved design for high resolution electrospray ionization ion mobility spectrometry (ESI-IMS) was developed by making some salient modifications to the IMS cell and its performance was investigated. To enhance desolvation of electrospray droplets at high sample flow rates in this new design, volume of the desolvation region was decreased by reducing its diameter and the entrance position of the desolvation gas was shifted to the end of the desolvation region (near the ion gate). In addition, the ESI source (both needle and counter electrode) was positioned outside of the heating oven of the IMS. This modification made it possible to use the instrument at higher temperatures, and preventing needle clogging in the electrospray process. The ion mobility spectra of different chemical compounds were obtained. The resolving power and resolution of the instrument were increased by about 15-30% relative to previous design. In this work, the baseline separation of the two adjacent ion peaks of morphine and those of codeine was achieved for the first time with resolutions of 1.5 and 1.3, respectively. These four ion peaks were well separated from each other using carbon dioxide (CO₂) rather than nitrogen as the drift gas. Finally, the analytical parameters obtained for ethion, metalaxyl, and tributylamine indicated the high performance of the instrument for quantitative analysis.     

Ion mobility spectrometer—field asymmetric ion mobility spectrometer-mass spectrometry

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 tyrosine-tryptophan-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 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.     

An ion mobility spectrometer sensor system for subsurface use

An ion mobility spectrometer (IMS) probe system for real-time, subsurface soil-gas sampling applications is presented. The system includes an IMS and supporting electronics encased in a 51 mm diameter stainless steel probe housing. The IMS was challenged in the laboratory with 2,6-di-tert-butylpyridine (DtBP) and tetrachloroethylene (PCE) in zero air yielding reduced ion mobility constants (K_o) values of 1.42 cm²/Vs (n = 3) and 1.79 ± 0.01 cm²/Vs (n = 3), respectively. A resolving power of 38 and 31 was obtained for DtBP and PCE, respectively. The system was deployed at a PCE-contaminated site to demonstrate its performance under field conditions. PCE was detected in the vapor samples as evidenced by peaks with a K_o value of 1.80 ± 0.01 cm²/Vs for two measurements that were taken 6 min apart. The presence of PCE at the contaminated site was confirmed by GC-MS analysis of a gas sample at an EPA-certified laboratory, suggesting that this IMS system can be used to detect PCE under field conditions.     

Influence of structural features of isomeric hydrocarbons on ion formation at atmospheric pressure

Ion mobility spectrometry (IMS) in combination with different techniques of atmospheric pressure ionization (⁶³Ni ionization, photoionization, Corona discharge ionization) was applied to determine the influence of structural features of aromatic and cyclic hydrocarbons on ion mobility spectra. For this purpose, different sets of isomeric hydrocarbons were investigated using the above-mentioned ionization techniques. We found different structural features of these isomeric non-polar compounds which cause distinct differences in ion mobility spectra. These differences result from the formation of different product ions or a different relative abundance of ions formed depending on the occurrence of certain structural features (position of the double bond, arrangement of double bonds within the carbon ring, configuration of aliphatic side chain in the space, position of aliphatic side chain on the carbon ring and the number of carbon atoms in the aliphatic side chain). The nature of product ions formed was determined using a coupling of IMS with mass spectrometry (MS).     

Optimum collision energies for proteomics: The impact of ion mobility separation

Ion mobility spectrometry (IMS) is a widespread separation technique used in various research fields. It can be coupled to liquid chromatography–mass spectrometry (LC–MS/MS) methods providing an additional separation dimension. During IMS, ions are subjected to multiple collisions with buffer gas, which may cause significant ion heating. The present project addresses this phenomenon from the bottom-up proteomics point of view. We performed LC–MS/MS measurements on a cyclic ion mobility mass spectrometer with varied collision energy (CE) settings both with and without IMS. We investigated the CE dependence of identification score, using Byonic search engine, for more than 1000 tryptic peptides from HeLa digest standard. We determined the optimal CE values—giving the highest identification score—for both setups (i.e., with and without IMS). Results show that lower CE is advantageous when IMS separation is applied, by 6.3 V on average. This value belongs to the one-cycle separation configuration, and multiple cycles may supposedly have even larger impact. The effect of IMS is also reflected in the trends of optimal CE values versus m/z functions. The parameters suggested by the manufacturer were found to be almost optimal for the setup without IMS; on the other hand, they are obviously too high with IMS. Practical consideration on setting up a mass spectrometric platform hyphenated to IMS is also presented. Furthermore, the two CID (collision induced dissociation) fragmentation cells of the instrument—located before and after the IMS cell—were also compared, and we found that CE adjustment is needed when the trap cell is used for activation instead of the transfer cell. Data have been deposited in the MassIVE repository (MSV000090944).     

Evaluation of micro-electrospray ionization with ion mobility spectrometry/mass spectrometry

In recent years, the resolving power of ion mobility instruments has been increased significantly, enabling ion mobility spectrometry (IMS) to be utilized as an analytical separation technique for complex mixtures. In theory, decreasing the drift tube temperature results in increased resolution due to decreased ion diffusion. However, the heat requirements for complete ion desolvation with electrospray ionization (ESI) have limited the reduction of temperatures in atmospheric pressure ion mobility instruments. Micro-electrospray conditions were investigated in this study to enable more efficient droplet formation and ionization with the objective of reducing drift tube temperatures and increasing IMS resolution. For small molecules (peptides), the drift tube temperature was reduced to ambient temperature with good resolution by employing reduced capillary diameters and flow rates. By employing micro-spray conditions, experimental resolution values approaching theoretically predicted resolution were achieved over a wide temperature range (30 to 250 °C). The historical heat requirements of atmospheric pressure IMS due to ESI desolvation were eliminated due to the use of micro-spray conditions and the high-resolution IMS spectra of GLY-HIS-LYS was obtained at ambient temperature. The desolvation of proteins (cytochrome c) was found to achieve optimal resolution at temperatures greater than 125 °C. This is significantly improved from earlier IMS studies that required drift tube temperatures of 250°C for protein desolvation.     

Monte Carlo simulation of ion transport in ion mobility spectrometry

A program for simulation of ion trajectories in ion mobility spectrometry (IMS) instruments has been developed and incorporated into SIMION 7.0 [Int. J. Mass Spectrom. 200 (2000) 3–25]. Simulations were based on elastic collisions between ions and gas particles and conducted for an IMS drift tube. The program was validated by comparing the reduced mobility of helium ions derived from the simulation with the experimental data for helium ions in neon drift gas in low electric fields. Typical IMS parameters, including pressure, temperature, and flow rate of the drift gas were taken into account in the simulations. The program demonstrates capabilities of generating IMS spectra and predicting ion transport efficiency and separating ions. For the IMS drift tube studied, a correlation between imperfection of the electric field distribution and low resolution has been observed.