Characterization of proton-bound acetate dimers in ion mobility spectrometry

Ionized acetates were used as model compounds to describe gas-phase behavior of oxygen containing compounds with respect to their formation of dimers in ion mobility spectrometry (IMS). The ions were created using corona discharge at atmospheric pressure and separated in a drift tube before analysis of the ions by mass spectrometry. At the ambient operational temperature and pressure used in our instrument, all acetates studied formed dimers. Using a homolog series of n-alkyl-acetates, we found that the collision cross section of a dimer was smaller than that of a monomer with the same reduced mass. Our experiments also showed that the reduced mobility of acetate dimers with different functional groups increased in the order n-alkyl 相似文献   

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.     

Development of an ion mobility spectrometer for use in an atmospheric pressure ionization ion mobility spectrometer/mass spectrometer instrument for fast screening analysis

An ion mobility spectrometer that can easily be installed as an intermediate component between a commercial triple-quadrupole mass spectrometer and its original atmospheric pressure ionization (API) sources was developed. The curtain gas from the mass spectrometer is also used as the ion mobility spectrometer drift gas. The design of the ion mobility spectrometer allows reasonably fast installation (about 1 h), and thus the ion mobility spectrometer can be considered as an accessory of the mass spectrometer. The ion mobility spectrometer module can also be used as an independently operated device when equipped with a Faraday cup detector. The drift tube of the ion mobility spectrometer module consists of inlet, desolvation, drift, and extraction regions. The desolvation, drift and extraction regions are separated by ion gates. The inlet region has the shape of a stainless steel cup equipped with a small orifice. Ion mobility spectrometer drift gas is introduced through a curtain gas line from an original flange of the mass spectrometer. After passing through the drift tube, the drift gas serves as a curtain gas for the ion-sampling orifice of the ion mobility spectrometer before entering the ion source. Counterflow of the drift gas improves evaporation of the solvent from the electrosprayed sample. Drift gas is pumped away from the ion source through the original exhaust orifice of the ion source. Initial characterization of the ion mobility spectrometer device includes determination of resolving power values for a selected set of test compounds, separation of a simple mixture, and comparison of the sensitivity of the electrospray ionization ion mobility spectrometry/mass spectrometry (ESI-IMS/MS) mode with that of the ESI-MS mode. A resolving power of 80 was measured for 2,6-di-tert-butylpyridine in a 333 V/cm drift field at room temperature and with a 0.2 ms ion gate opening time. The resolving power was shown to be dependent on drift gas flow rate for all studied ion gate opening times. Resolving power improved as the drift gas flow increased, e.g. at a 0.5 ms gate opening time, a resolving power of 31 was obtained with a 0.65 L/min flow rate and 47 with a 1.3 L/min flow rate for tetrabutylammonium iodide. The measured limits of detection with ESI-MS and with ESI-IMS/MS modes were similar, demonstrating that signal losses in the IMS device are minimal when it is operated in a continuous flow mode. Based on these preliminary results, the IMS/MS instrument is anticipated to have potential for fast screening analysis that can be applied, for example, in environmental and drug analysis.     

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.     

Separation of sodiated isobaric disaccharides and trisaccharides using electrospray ionization-atmospheric pressure ion mobility-time of flight mass spectrometry

A series of isobaric disaccharide-alditols, four derived from O-linked glycoproteins, and select trisaccharides were rapidly resolved using tandem high resolution atmospheric pressure ion-mobility time-of-flight mass spectrometry. Electrospray ionization was used to create the gas-phase sodium adducts of each carbohydrate. Using this technique it was possible to separate up to three isobaric disaccharide alditols and three trisaccharides in the gas phase. Reduced mobility values and experimentally determined ion-neutral cross sections are reported for each sodium-carbohydrate complex. These studies demonstrated that ion mobility separations at atmospheric pressure can provide a high-resolution dimension for analysis of carbohydrate ions that is complementary to traditional mass spectral (m/z) ion analysis. Combining these independent principles for separation of ions provides a powerful new bioanalytical tool for the identification of isomeric carbohydrates.     

Two-dimensional separations with electrospray ionization ambient pressure high-resolution ion mobility spectrometry/quadrupole mass spectrometry

The coupling of ion mobility spectrometry (IMS) instruments with mass spectrometers has been described since early in IMS development, most commonly with quadrupole mass analyzers. The recent development of IMS with time-of-flight (TOF) instruments has demonstrated that the time compatibility (IMS milliseconds and TOFMS microseconds) of the two techniques enables rapid two-dimensional separations to be performed, theoretically in the order of seconds for a complete analysis. This study presents a unique way to operate a traditional IMS/QMS system to attain separations similar to those achieved with IMS/TOF. For this new approach, the quadrupole was slowly scanned in the single-ion monitoring mode while IMS spectra were continually embedded in each m/z step. In this way, two-dimensional separations (IMS drift times and m/z) were obtained using the traditional IMS/QMS arrangement. An example of a five amino acid separation (quadrupole scan of 40 m/z values at a rate of approximately 7 steps/min) led to a complete two-dimensional analysis within 6 min, comparable to rapid chromatographic separations with mass spectrometry. Proposed approaches to reduce the analysis time are discussed and a reduction in the analysis time to less than 1 min is feasible when the IMS/QMS separation conditions are optimized.     

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).     

Ribonucleotide and ribonucleoside determination by ambient pressure ion mobility spectrometry

Detection limits and reduced mobilities for 12 ribonucleotides and 4 ribonucleosides were measured by ambient pressure electrospray ionization-ion mobility spectrometry (ESI-IMS). With the instrument used in this study it was possible to separate some of these compounds within mixtures. Detection limits reported for ribonucleotides and ribonucleosides ranged from 15 to 300 pmol and the reduced mobilities ranged from 41 to 56 suggesting that ambient pressure ESI-IMS may be used for their rapid and sensitive separation and detection. This report demonstrates that it was possible to use ion mobility spectrometry (IMS) to obtain a spectrum for the separation of nucleotides and nucleosides in less than 1 min. The application holds great promise for nucleotide analysis in the area of separating DNA fragments in genome sequencing and also for forensics DNA typing examinations used for the identification of blood stains in crime scenes and paternity testing.     

Profiling and imaging of tissues by imaging ion mobility-mass spectrometry

Molecular profiling and imaging mass spectrometry (IMS) of tissues can often result in complex spectra that are difficult to interpret without additional information about specific signals. This report describes increasing data dimensionality in IMS by combining two-dimensional separations at each spatial location on the basis of imaging ion mobility-mass spectrometry (IM-MS). Analyte ions are separated on the basis of both ion-neutral collision cross section and m/z, which provides rapid separation of isobaric, but structurally distinct ions. The advantages of imaging using ion mobility prior to MS analysis are demonstrated for profiling of human glioma and selective lipid imaging from rat brain.     

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.     

An Ion Mobility/Ion Trap/Photodissociation Instrument for Characterization of Ion Structure

A new instrument that combines ion mobility spectrometry (IMS) separations with tandem mass spectrometry (MS(n)) is described. Ion fragmentation is achieved with vacuum ultraviolet photodissociation (VUV PD) and/or collision-induced dissociation (CID). The instrument is comprised of an approximately 1 m long drift tube connected to a linear trap that has been interfaced to a pulsed F(2) laser (157 nm). Ion gates positioned in the front and the back of the primary drift region allow for mobility selection of specific ions prior to their storage in the ion trap, mass analysis, and fragmentation. The ion characterization advantages of the new instrument are demonstrated with the analysis of the isomeric trisaccharides, melezitose and raffinose. Mobility separation of precursor ions provides a means of separating the isomers and subsequent VUV PD generates unique fragments allowing them to be distinguished.     

In situ isobaric lipid mapping by MALDI–ion mobility separation–mass spectrometry imaging

The highly diverse chemical structures of lipids make their analysis directly from biological tissue sections extremely challenging. Here, we report the in situ mapping and identification of lipids in a freshwater crustacean Gammarus fossarum using matrix‐assisted laser desorption/ionization (MALDI) mass spectrometry imaging (MSI) in combination with an additional separation dimension using ion mobility spectrometry (IMS). The high‐resolution trapped ion mobility spectrometry (TIMS) allowed efficient separation of isobaric/isomeric lipids showing distinct spatial distributions. The structures of the lipids were further characterized by MS/MS analysis. It is demonstrated that MALDI MSI with mobility separation is a powerful tool for distinguishing and localizing isobaric/isomeric lipids.     

Metabolic profiling of Escherichia coli by ion mobility-mass spectrometry with MALDI ion source

Comprehensive metabolome analysis using mass spectrometry (MS) often results in a complex mass spectrum and difficult data analysis resulting from the signals of numerous small molecules in the metabolome. In addition, MS alone has difficulty measuring isobars and chiral, conformational and structural isomers. When a matrix-assisted laser desorption ionization (MALDI) source is added, the difficulty and complexity are further increased. Signal interference between analyte signals and matrix ion signals produced by MALDI in the low mass region (<1500 Da) cause detection and/or identification of metabolites difficult by MS alone. However, ion mobility spectrometry (IMS) coupled with MS (IM-MS) provides a rapid analytical tool for measuring subtle structural differences in chemicals. IMS separates gas-phase ions based on their size-to-charge ratio. This study, for the first time, reports the application of MALDI to the measurement of small molecules in a biological matrix by ion mobility-time of flight mass spectrometry (IM-TOFMS) and demonstrates the advantage of ion-signal dispersion in the second dimension. Qualitative comparisons between metabolic profiling of the Escherichia coli metabolome by MALDI-TOFMS, MALDI-IM-TOFMS and electrospray ionization (ESI)-IM-TOFMS are reported. Results demonstrate that mobility separation prior to mass analysis increases peak-capacity through added dimensionality in measurement. Mobility separation also allows detection of metabolites in the matrix-ion dominated low-mass range (m/z < 1500 Da) by separating matrix signals from non-matrix signals in mobility space.     

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).     

Coupling of capillary electrophoresis with electrospray ionization multiplexing ion mobility spectrometry

In this work, ion mobility spectrometry (IMS) function as a detector and another dimension of separation was coupled with CE to achieve two‐dimensional separation. To improve the performance of hyphenated CE‐IMS instrument, electrospray ionization correlation ion mobility spectrometry is evaluated and compared with traditional signal averaging data acquisition method using tetraalkylammonium bromide compounds. The effect of various parameters on the separation including sample introduction, sheath fluid of CE and drift gas, data acquisition method of IMS were investigated. The experimental result shows that the optimal conditions are as follows: hydrodynamic sample injection method, the electrophoresis voltage is 10 kilo volts, 5 mmol/L ammonium acetate buffer solution containing 80% acetonitrile as both the background electrolyte and the electrospray ionization sheath fluid, the ESI liquid flow rate is 4.5 μL/min, the drift voltage is 10.5 kilo volts, the drift gas temperature is 383 K and the drift gas flow rate is 300 mL/min. Under the above conditions, the mixture standards of seven tetraalkylammoniums can be completely separated within 10 min both by CE and IMS. The linear range was 5–250 μg/mL, with LOD of 0.152, 0.204, 0.277, 0.382, 0.466, 0.623 and 0.892 μg/mL, respectively. Compared with traditional capillary electrophoresis detection methods, the developed CE‐ESI‐IMS method not only provide two sets of qualitative parameters including electrophoresis migration time and ion drift time, ion mobility spectrometer can also provide an additional dimension of separation and could apply to the detection ultra‐violet transparent compounds or none fluorescent compounds.     

Letter: A simple ion source set-up for desorption/ionization on silicon with ion mobility spectrometry and ion mobility spectrometry-mass spectrometry

Using a simple ion source set-up, laser desorption/ionization on silicon (DIOS) was demonstrated with the use of a custom-made drift tube ion mobility spectrometer (IMS), mounted on a commercial triple quadrupole mass spectrometer, and with an IMS equipped with a Faraday plate detector. DIOS was tested by mobility measurement of tetrapropylammonium iodide, tetrabutylammonium iodide and tetrapentylammonium iodide, whilst 2,6-di-tert- butylpyridine was used as a standard. The reduced mobilities measured for the test halides are in concordance with previously obtained ion mobility spectrometry-mass spectrometry data.     

Separation of benzodiazepines by electrospray ionization ion mobility spectrometry-mass spectrometry

Benzodiazepines are a commonly abused class of drugs; requiring analytical techniques that can separate and detect the drugs in a rapid time period. In this paper, the two-dimensional separation of five benzodiazepines was shown by electrospray ionization (ESI) ion mobility spectrometry (IMS)-mass spectrometry (MS). In this study, both the two dimensions of separation (m/z and mobility) and the high resolution of our IMS instrument enabled confident identification of each of the five benzodiazepines studied. This was a significant improvement over previous IMS studies that could not separate many of the analytes due to low instrumental resolution. The benzodiazepines that contain a hydroxyl group in their molecular structure (lorazepam and oxazepam) were found to form both the protonated molecular ion and dehydration product as predominant ions. Experiments to isolate the parametric reasons for the dehydration ion formation showed that it was not the result of corona discharge processes or the potential applied to the needle. However, the potential difference between the needle and first drift ring did influence both the relative intensity ratios of the two ions and the ion sensitivity.     

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.     

The novel use of gas chromatography-ion mobility-time of flight mass spectrometry with secondary electrospray ionization for complex mixture analysis

Increasing the dimensionality of an analysis enables more detailed and comprehensive investigations of complex mixtures. One dimensional separation techniques like gas chromatography (GC) and ion mobility spectrometry (IMS) provide limited chemical information about complex mixtures. The combination of GC, ion mobility spectrometry, and time-of-flight mass spectrometry (GC-IM-TOFMS) provides three-dimensional separation of complex mixtures. In this work, a hybrid GC-IM-TOFMS with a secondary electrospray ionization (SESI) source provided four types of analytical information: GC retention time, ion mobility drift time, mass-to-charge ratios, and ion intensity. The use of secondary electrospray ionization enables efficient and soft ionization of gaseous sample vapors at atmospheric pressure. Several complex mixtures, including lavender and peppermint essential oils, were analyzed by GC-SESI-IM-TOFMS. The resulting 3D data from these mixtures, each containing greater than 50 components, were plotted as 3D projections. In particular, post-processed data plotted in three dimensions showed that many mass selected GC peaks were resolved into different ion mobility peaks. This technique shows clear promise for further in-depth analyses of complex chemical and biological mixtures.