Investigation of the rapid scan on an electrospray ion trap mass spectrometer

A rapid scan on a quadrupole ion trap mass spectrometer can improve the signal intensity by over 200 times when scanning at 12 times the normal scan rate. This intensity increase is due to a 5.5-fold increase in mass peak height due to a reduction in the mass peak width over time and a 40-fold increase in signal from the increased number of ions that can be trapped without the deleterious effects of space charge. Detection limits can be further improved by signal averaging more scans in the same period that is required for the normal scan, and the greatest advantage occurs when scanning over the full mass range. The rapid scan impacts the mass accuracy and the resolution is reduced by 6 times. The molecular weight determination of 40 fmol/microL apomyoglobin was determined in 3 s using a rapid scan, but this was not possible when using the normal scan rate. Quantitation results showed that the relative standard deviations for the total ion current peak areas of 500 fmol of angiotensin I were improved by a factor of 2.6 when the rapid scan was used.     

Development of an ion mobility quadrupole time of flight mass spectrometer

We describe here a new ion mobility capable mass spectrometer which comprises a drift cell for mobility separation and a quadrapole time of flight mass spectrometer for mass analysis--the MoQTOF. A commercial QToF instrument (Micromass UK Ltd., Manchester, UK) has been modified by the inclusion of an additional chamber containing a drift cell and ancillary ion optics. The drift cell is 5.1 cm long made from a copper block and is mounted from a top hat flange in a chamber situated post source optics and prior to the quadapole analyzer. Details of this instrument are provided along with information about how it can be used to acquire mobilities of ions along with their mass to charge ratios. The MoQTOF is used to examine conformations of a series of antimicrobial peptides based on a beta-defensin template. In vivo, these cationic cystine-rich amphiphilic peptides are conformationally restrained by three or more disulfide bridges, although recent findings by several groups have cast doubt on the importance of canonical disulfide pairing to antimicrobial activities. By synthesizing a panel of variants to Defb14 (the murine orthologue of HBD3), we exploit ion mobility to distinguish conformational differences which arise due to disulfide formation and to the hydrophobicity of the peptide sequence. Our gas-phase results are interpreted in terms of the antimicrobial and chemotacic properties of beta-defensins, and this mass spectrometry based approach to discern structure may have a role in future design of novel antibiotics.     

Dual electrospray ion source for electron-transfer dissociation on a hybrid linear ion trap-orbitrap mass spectrometer

A dual electrospray ionization source (ESI) has been modified to simultaneously produce cations and anions, one from each emitter, for performing rapid electron-transfer dissociation (ETD) ion/ion reactions on a hybrid linear ion trap-orbitrap mass spectrometer. Unlike the pulsed dual ESI sources that were used to generate ETD reagent ions, this source separates the emitters in space, rather than time, by physically switching which one is in front of the atmospheric inlet. The new arrangement allows for substantially enhanced spray stability and decreased switching times (相似文献   

Space charge effects on resolution in a miniature ion mobility spectrometer

Miniaturization of ion mobility spectrometry (IMS) is expected to have many advantages, as well as difficulties, in the separation of chemical species at atmospheric pressure. We report the results of studies of a miniature ion mobility spectrometer that has a drift channel 1.7 mm in diameter, the smallest cross section reported to date. The miniature cell contains a homogeneous drift field and is operated at atmospheric pressure. The miniature IMS has been characterized by measuring both negative and positive ion spectra using a frequency-quadrupled Nd: YAG laser on samples of NO, O2, and methyl iodide; a useful resolution (> 10) was achieved with an operating voltage of 500 V. Peak broadening due to Coulomb repulsion was determined to have a major effect on the resolution of the miniature device.     

Ion mobility spectrometer with ion source based on laser-irradiated porous silicon

A new ion source for ion mobility spectrometry based on UV laser action upon the surface of porous silicon, is described for the first time. It is found that the yield of TNT negative ions from this source is higher by an order of magnitude than in the case of multistep laser ionization of TNT-containing air mixture. It is established that factors important for the formation of negative ions in the case of continuous supply of analyt to the ionization region are (i) electron emission from the irradiated surface and (ii) subsequent ionmolecular reactions in the gas phase.     

Prediction of gas-phase reduced ion mobility constants (K0)

A method of predicting reduced ion mobility values, K0, for use in ion mobility spectrometry is described. While the method is very similar to a previously reported method based on a neural network, the method described in this paper uses a purely statistical regression approach. Furthermore, it has been applied to a wider class of compounds, including chemical agents. Various molecular parameters were evaluated in the predictive model to determine the qualitative dynamics that have the greatest effect on K0. An R2 value of 80.1% was obtained when calculated K0 values were plotted against measured K0 values for 162 compounds for which experimental K0 values were available. However, when chloroacetophenone and 3-xylyl bromide (3-methylbenzyl bromide) were removed from the set due to their large residual values, the predictability increased to an R2 value of 87.4%. This compares well with the value of 88.7%, which was obtained in a regression step of a previous neural network study for a less diverse set of 168 compounds.     

A quadrupole ion trap mass spectrometer with three independent ion sources for the study of gas-phase ion/ion reactions

An instrument for the study of gas-phase ion/ion reactions in which three independent sources of ions, namely, two electrospray ionization sources and one atmospheric sampling glow discharge ionization source, are interfaced to a quadrupole ion trap mass analyzer is described. This instrument expands the scope of gas-phase ion/ion reaction studies by allowing for manipulation of the charge states of multiply charged reactant and product ions. Examples are provided involving the formation of protein-protein complexes in the gas phase. Complexes with charge states that cannot be formed from reactant ion charge states present in the normal electrospray charge state distributions can be formed in the new apparatus. Strategies that rely on both reactant ion charge state manipulation and product ion charge state manipulation are demonstrated. In addition, simplification of product ion spectra generated from dissociation of complexes formed via ion/ion reactions can be effected by using the discharge source to reduce the charge state of the product ions to primarily 1+.     

Rapid separation and quantitative analysis of peptides using a new nanoelectrospray- differential mobility spectrometer-mass spectrometer system

Differential mobility spectrometry (DMS) (see Buryakov, I. A.; Krylov, E. V.; Nazarov, E. G.; Rasulev, U. Kh. Int. J. Mass Spectrom. Ion Processes 1993, 128, 143-148), also commonly referred to as high-field asymmetric waveform ion mobility spectrometry (FAIMS) (see Purves, R. W.; Guevremont, R.; Day, S.; Pipich, C. W.; Matyjaszcyk, M. S. Rev. Sci. Instrum. 1998, 69, 4094-4105), is a rapidly advancing technology for gas-phase ion separation. The interfacing of DMS with mass spectrometry (MS) offers potential advantages over the use of mass spectrometry alone. Such advantages include improvements to mass spectral signal-to-noise, orthogonal/complementary ion separation to mass spectrometry, enhanced ion and complexation structural analysis, and the potential for rapid analyte quantitation. In this report, we investigate the use of our nanoESI-DMS-MS system to demonstrate differential mobility separation of peptides. The formation of higher order peptide aggregate ions (ion complexes) via electrospray ionization and the negative impact this has on DMS peptide separation are examined. The successful use of differential mobility drift gas modifiers (dopants) to reduce aggregate ion size and improve DMS peptide ion separation is presented. Following optimization of DMS peptide separation conditions, we examined next the feasibility of a new analytical platform which uses direct sample infusion with nanoESI-DMS-MS for ultrarapid analyte quantitation. Quantitation of a selected peptide from a semicomplex peptide mixture is presented. Initial feasibility results with this new approach demonstrate good accuracy and reproducibility, as well as an absolute mass sensitivity of 6.8 amol and a minimum dynamic range of 2500 for the peptide of interest. This report offers a first look at utilizing nanoESI-DMS-MS to create an ultrarapid (under 5 s) quantitative analysis platform and its potential in the high-throughput arena. Each ion separation technique, DMS and MS, offers orthogonal ion separation to one another, enhancing the overall specificity for this quantitative approach.     

Interfacing an ion mobility spectrometry based explosive trace detector to a triple quadrupole mass spectrometer

Hardware from a commercial-off-the-shelf (COTS) ion mobility spectrometry (IMS) based explosive trace detector (ETD) has been interfaced to an AB/SCIEX API 2000 triple quadrupole mass spectrometer. To interface the COTS IMS based ETD to the API 2000, the faraday plate of the IMS instrument and the curtain plate of the mass spectrometer were removed from their respective systems and replaced by a custom faraday plate, which was fabricated with a hole for passing the ion beam to the mass spectrometer, and a custom interface flange, which was designed to attach the IMS instrument onto the mass spectrometer. Additionally, the mass spectrometer was modified to increase the electric field strength and decrease the pressure in the differentially pumped interface, causing a decrease in the effect of collisional focusing and permitting a mobility spectrum to be measured using the mass spectrometer. The utility of the COTS-ETD/API 2000 configuration for the characterization of the gas phase ion chemistry of COTS-ETD equipment was established by obtaining mass and tandem mass spectra in the continuous ion flow and selected mobility monitoring operating modes and by obtaining mass-selected ion mobility spectra for the explosive standard 2,4,6 trinitrotoluene (TNT). This analysis confirmed that the product ion for TNT is [TNT - H](-), the predominant collision-induced dissociation pathway for [TNT- H](-) is the loss of NO and NO(2), and the reduced mobility value for [TNT - H](-) is 1.54 cm(2)V(-1) s(-1). Moreover, this analysis was attained for sample amounts of 1 ng and with a resolving power of 37. The objective of the research is to advance the operational effectiveness of COTS IMS based ETD equipment by developing a platform that can facilitate the understanding of the ion chemistry intrinsic to the equipment.     

Separation of isomeric peptides using electrospray ionization/high-resolution ion mobility spectrometry

In this paper, the first examples of baseline separation of isomeric macromolecules by electrospray ionization/ion mobility spectrometry (ESI/IMS) at atmospheric pressure are presented. The behavior of a number of different isomeric peptides in the IMS was investigated using nitrogen as a drift gas. The IMS was coupled to a quadrupole mass spectrometer, which was used for identification and selective detection of the electrosprayed ions. The mobility data were used to determine their average collision cross sections. The gas-phase ions of isomeric peptides were found to have different collision cross sections. In all cases, doubly charged ions exhibited significantly (8-20%) larger collision cross sections than the respective singly charged species. The analysis of mixtures of the isomeric peptides clearly demonstrated the capability of IMS to separate gas-phase peptide ions due to small differences in their conformational structures, which cannot be determined by mass spectrometry. An actual resolving power of 80 was achieved for two doubly charged reversed sequenced pentapeptides. Baseline separation was provided for ions differing by only 2.5% in their measured collision cross sections; partial separation was shown for isomeric ions exhibiting differences as small as 1.1%.     

Characterization of an improved electrodynamic ion funnel interface for electrospray ionization mass spectrometry.

An improved electrodynamic ion funnel for ion focusing at high pressure (> 1 Torr) has been developed for a triple quadrupole mass spectrometer and its performance compared with that of an earlier prototype previously reported. The ion funnel consists of a series of ring electrodes of progressively smaller internal diameters to which rf and dc electric potentials are co-applied. The new design utilizes ring electrodes possessing larger internal diameters that taper down to a relatively larger exit aperture. In the 1-10 Torr pressure range, the new design provides significant improvement in low m/z ion transmission. Additionally, the overall ion transmission range is improved by linked scanning of the ion funnel's rf voltage concomitantly with the scanning of the quadrupole mass analyzer. Transmission of a noncovalent complex through the interface demonstrated that excessive ion heating was not problematic. Computer simulations of ion transport support the ion funnel design and help explain the relative performance of both designs. Both ion simulations and experimental results are in accord and indicate close to 100% ion transmission efficiency for electrosprayed biopolymer ions through the interface and into the mass analyzer.     

A strategy for the determination of enzyme kinetics using electrospray ionization with an ion trap mass spectrometer.

A simple and rapid means of enzyme kinetic analysis was achieved using electrospray ionization mass spectrometry and a one-point normalization factor. The model system used, glutathione S-transferase from porcine liver, is a two-substrate enzyme catalyzing the conjugation of glutathione with a variety of compounds containing an electrophilic center. An internal standard that is structurally similar to the product was added to the reaction quench solution, and a single-point normalization factor was used to determine the product concentration without the need of a calibration curve. Kinetic parameters, such as Km, Vmax and Ki (for thyroxine), obtained by electrospray mass spectrometry agreed with those obtained from traditional UV-vis spectroscopy, and competitive vs noncompetitive inhibition reactions could be delineated via mass spectrometry. These results suggest that our method can be applied to enzymatic processes in which spectrophotometric or spectrofluorometric assays are not feasible or when the relevant substrates do not incorporate chromophores or fluorophores. This new method is competitive with traditional UV assays in that it is facile and it involves very little analysis time.     

Necessary conditions for peak resolution in an ion mobility increment spectrometer with a flat drift chamber

New analytical characteristics are proposed to describe the performance of an ion mobility increment spectrometer (IMIS). The dispersion describes the ability of such a spectrometer to separate ions with close values of the variable component of mobility. The necessary resolution limit with respect to this component characterizes the ability to separately detect different ion species. In accordance with the principle of ion separation in this instrument, it is suggested to redefine the resolving power of IMIS as the inverse of the necessary resolution limit with respect to the variable component of mobility.     

Characterization of benzodiazepine drugs by ion mobility spectrometry

Chemical ionization ion mobility spectrometry (CI-IMS) was used to characterize a number of benzodiazepines. In almost every example studied, the positive ion mobility spectrum consisted of a single ion peak corresponding to [M]+ or [MH]+. With some compounds, e.g., oxazepam, lorazepam, and chlordiazepoxide, fragment ions were noted that serve as good markers for the identification of these chemicals. Reduced mobility constants (K0) for the most significant peaks were calculated, and all ions produced were mass-analyzed by injection into a quadrupole mass spectrometer. The results of this study point to the potential of IMS as a qualitative tool for the rapid detection (analysis time less than 10 s) and reliable identification of benzodiazepines. Preliminary results on the application of digital signal processing and a second-derivative algorithm to partially overlapping IMS peaks are presented, and potential improvements are discussed.     

Electrochemically induced pH changes resulting in protein unfolding in the ion source of an electrospray mass spectrometer.

The operation of an electrospray ion source in the positive ion mode involves charge-balancing oxidation reactions at the liquid/metal interface of the sprayer capillary. One of these reactions is the electrolytic oxidation of water. The protons generated in this process acidify the analyte solution within the electrospray capillary. This work explores the effects of this acidification on the electrospray ionization (ESI) mass spectrum of the protein cytochrome c (cyt c). In aqueous solution containing 40% propanol, cyt c unfolds around pH 5.6. Mass spectra recorded under these conditions, using a simple ESI series circuit, display a bimodal charge-state distribution that reflects an equilibrium mixture of folded and unfolded protein in solution. These spectra are not strongly affected by electrochemical acidification. An "external loop" is added to the ESI circuit when the metal needle of the sample injection syringe is connected to ground. The resulting circuit represents two coupled electrolytic cells that share the ESI capillary as a common anode. Under these conditions, the rate of charge-balancing oxidation reactions is dramatically increased because the ion source has to supply electrons for both, the external circuit and the ESI circuit. The analytical implications of this effect are briefly discussed. Mass spectra of cyt c recorded with the syringe needle grounded are shifted to higher charge states, indicating that electrochemical acidification has caused the protein to unfold in the ion source. The acidification can be suppressed by increasing the flow rate and lowering the electrolyte concentration of the solution and by using an electrolyte that acts as redox buffer. The observed acidification is similar for sprayer capillaries made of platinum and stainless steel. Removal of the protective oxide layer on the stainless steel surface results in effective redox buffering for a few minutes.     

Measurement of ion swarm distribution functions in miniature low-temperature co-fired ceramic ion mobility spectrometer drift tubes

Measurements of the performance of a miniature, portable 12-mm-diameter, 57-mm-length low-temperature cofired ceramic (LTCC) ion mobility spectrometer drift tube were undertaken to verify models of ion transport and determine the physical shape of the ion "swarms" in the LTCC tube. Simplified two-dimensional Gaussian models of ion swarm shape were fit to measured data to extract geometrical shape parameters. Results indicate that tube-transfer function effects that produce asymmetric ion swarms are minimized in the tube reducing temporal dispersion. Data are presented that illustrate the swarm shape as a function of gate time, electric field magnitude, and total charge in the ion swarm. Characterization and understanding of the ion transport mechanisms and effects that limit the resolution and other performance parameters of miniature IMS drift tubes is essential to the development of practical, robust, portable systems for "first responder" and homeland security missions.     

Solvation energy and gas-phase stability influences on alkali metal cluster ion formation in electrospray ionization mass spectrometry

The ability to observe abundant gas-phase metal cluster ions in electrospray ionization mass spectrometry (ESI-MS) is highly dependent on experimental conditions. Alkali halides (MX) and other alkali metal salts were used to investigate the formation of cluster ions in ESI-MS. All compounds were found to give cluster ions of the form (M(n)(+1)X(n))(+) and (M(n)X(n+1))(-), with only two alkali salts yielding doubly charged cluster ions. In homologous alkali halide series, the relative abundances of cluster ions increased with increasing size of either the cation (positive ion mode) or the anion (negative ion mode). Calculations using an electrostatic model show that the gas-phase stability of cluster ions is greater for smaller cations or anions when a fixed counterion is employed. This stability calculation goes in a direction just opposite to the trend in cluster ion abundances observed in ESI-MS. Studies of equimolar mixtures consisting of two alkali halides reveal two distinct trends. When the equimolar mixture was composed of differing ions that participate in the droplet charge excess with the same counterion, the less solvated ions were found to form more abundant cluster ions. When the ions participating in the charge excess were fixed, the preferred counterion in observed clusters was the one that is more solvated in solution and forms more stable clusters in the gas phase. These observations can be rationalized by an extended form of the charged residue model where the weakly solvated ions that are part of the charge excess are preferentially enriched in offspring droplets during uneven fission. By contrast, transfer of a particular counterion located in the bulk of the droplets to the offspring droplets is not disfavored when this counterion is strongly solvated.     

Separation of variant methylated histone tails by differential ion mobility

Differential ion mobility spectrometry (field asymmetric waveform ion mobility spectrometry (FAIMS)) is emerging as a broadly useful tool for separation of isomeric modified peptides with post-translational modifications (PTMs) attached to alternative residues. Such separations were anticipated to become more challenging for smaller PTMs and longer peptides. Here, we show that FAIMS can fully resolve localization variants involving a PTM as minuscule as methylation, even for larger peptides in the middle-down range.     

Improvement in analysis of O2 in N2 by using Ar drift gas in an ion mobility spectrometer

Ion mobility spectrometry (IMS) is capable of providing real-time analysis of several impurities in inert gases. Because of its poor peak resolution, the IMS is susceptible to peak interferences. Previous study has shown that the detection limit and accuracy of O2 measurement in N2 suffers from peak interference. In many applications, the interference can be avoided by selecting a proper drift gas. For N2 analysis, we have demonstrated that the peak interference of O2 can be eliminated by using Ar as a drift gas. The use of Ar changes the chemical reactions that occur inside the drift region and results in different product ions. The peaks of new product ions are well-separated and do not coincide with the peak representing O2.