Peptide photodissociation at 157 nm in a linear ion trap mass spectrometer

The photodissociation by 157 nm light of singly- and doubly-charged peptide ions containing C- or N-terminal arginine residues was studied in a linear ion trap mass spectrometer. Singly-charged peptides yielded primarily x- and a-type ions, depending on the location of the arginine residue, along with some related side-chain fragments. These results are consistent with our previous work using a tandem time-of-flight (TOF) instrument with a vacuum matrix-assisted laser desorption/ionization (MALDI) source. Thus, the different internal energies of precursor ions in the two experiments seem to have little effect on their photofragmentation. For doubly-charged peptides, the dominant fragments observed in both photodissociation and collisionally induced dissociation (CID) experiments are b- and y-type ions. Preliminary experiments demonstrating fragmentation of multiply-charged ubiquitin ions by 157 nm photodissociation are also presented.     

Rapid hydrocarbon analysis using a miniature rectilinear ion trap mass spectrometer

A miniature ion trap mass analyzer was applied to the analysis of traces of hydrocarbons and simple heteroatomics in the vapor phase and in aqueous solution. Vapors of acetone, acetic acid, acetonitrile, benzene, butanethiol, carbon disulfide, hexane, dichloromethane, naphthalene, toluene and xylenes were detected and quantified using solid sorbent trapping and, in some cases, by passage through a membrane interface. Aqueous solutions of benzene, toluene, xylenes, hexane and a petroleum naphtha distillate were examined using the membrane interface. Sampling, detection and identification of all compounds was completed in times of less than one minute. The gas-phase samples of toluene and benzene were detected at 200 ppt (limit of detection, LOD) for toluene and 600 ppt for benzene. Identification of benzene and xylene in aqueous solutions was readily achieved with LODs of 200 and 400 ppb, respectively. Quantification over a linear dynamic range of two orders of magnitude for the aqueous samples and three orders of magnitude for the vapor-phase samples was demonstrated.     

A secondary ion microprobe ion trap mass spectrometer

An ion trap mass analyzer has been attached to an organic secondary ion microprobe. A pressure differential >100 can be maintained between the ion trap and microprobe. The well-focused secondary ion beam can transit a small (2 mm) diameter tube, but gas flow from ion trap to microprobe is impeded. This pressure differential allows the microprobe to retain imaging capability. Ion trap and microprobe data systems are integrated by taking advantage of the highly reproducible periodicity of the ion trap operating in resonant ejection mode and asynchronous signal and data acquisition afforded by commercially available interface cards. Secondary ion mass spectra and images obtained indicate an approximately 10-fold improvement in sensitivity, although preliminary evidence indicates low (<1%) trapping efficiency. Image data acquisition using the ion trap for mass analysis requires at least 10 times as much time compared to using a quadrupole mass filter because the mass-selected instability mode is used for mass analysis, i.e., mass resolution in the ion trap is not continuous as it is in the quadrupole.     

Use of a quadrupole linear ion trap mass spectrometer in metabolite identification and bioanalysis

A new type of quadrupole linear ion trap mass spectrometer, Q TRAP trade mark LC/MS/MS system (Q TRAP trade mark ), was evaluated for its performance in two studies: firstly, the in vitro metabolism of gemfibrozil in human liver microsomes, and, secondly, the quantification of propranolol in rat plasma. With the built-in information-dependent-acquisition (IDA) software, the instrument utilizes full scan MS in the ion trap mode and/or constant neutral loss scans as survey scans to trigger product ion scan (MS(2)) and MS(3) experiments to obtain structural information of drug metabolites 'on-the-fly'. Using this approach, five metabolites of gemfibrozil were detected in a single injection. This instrument combines some of the unique features of a triple quadrupole mass spectrometer, such as constant neutral loss scan, precursor ion scan and multiple reaction monitoring (MRM), together with the capability of a three-dimensional ion trap. Therefore, it becomes a powerful instrument for metabolite identification. The fast duty cycle in the ion trap mode allows the use of full product ion scan for quantification. For the quantification of propranolol, both MRM mode and full product ion scan in the ion trap mode were employed. Similar sensitivity, reproducibility and linearity values were established using these two approaches. The use of the product ion scan mode for quantification provided a convenient tool in selecting transitions for improving selectivity during the method development stage.     

A two-dimensional quadrupole ion trap mass spectrometer

The use of a linear or two-dimensional (2-D) quadrupole ion trap as a high performance mass spectrometer is demonstrated. Mass analysis is performed by ejecting ions out a slot in one of the rods using the mass selective instability mode of operation. Resonance ejection and excitation are utilized to enhance mass analysis and to allow isolation and activation of ions for MS(n) capability. Improved trapping efficiency and increased ion capacity are observed relative to a three-dimensional (3-D) ion trap with similar mass range. Mass resolution comparable to 3-D traps is readily achieved, including high resolution at slower scan rates, although adequate mechanical tolerance of the trap structure is a requirement. Additional advantages of 2-D over 3-D ion traps are also discussed and demonstrated.     

High resolution on a quadrupole ion trap mass spectrometer

By using a modified ion trap mass spectrometer, resolution in excess of 30,000 (FWHM) at m I z 502 is demonstrated. The method of increasing resolution in the ion trap mass spectrometer operated in the mass-selective instability mode depends on decreasing the rate of scanning the primary radio frequency amplitude as well as using resonance ejection at the appropriate frequency and amplitude. A theoretical basis for the method is introduced.     

Proton transfer reaction ion trap mass spectrometer

Proton transfer reaction mass spectrometry is a relatively new field that has attracted a great deal of interest in the last few years. This technique uses H(3)O(+) as a chemical ionization (CI) reagent to measure volatile organic compounds (VOCs) in the parts per billion by volume (ppbv) to parts per trillion by volume (pptv) range. Mass spectra acquired with a proton transfer reaction mass spectrometer (PTR-MS) are simple because proton transfer chemical ionization is "soft" and results in little or no fragmentation. Unfortunately, peak identification can still be difficult due to isobaric interferences. A possible solution to this problem is to couple the PTR drift tube to an ion trap mass spectrometer (ITMS). The use of an ITMS is appealing because of its ability to perform MS/MS and possibly distinguish between isomers and other isobars. Additionally, the ITMS duty cycle is much higher than that of a linear quadrupole so faster data acquisition rates are possible that will allow for detection of multiple compounds. Here we present the first results from a proton transfer reaction ion trap mass spectrometer (PTR-ITMS). The aim of this study was to investigate ion injection and storage efficiency of a simple prototype instrument in order to estimate possible detection limits of a second-generation instrument. Using this prototype a detection limit of 100 ppbv was demonstrated. Modifications are suggested that will enable further reduction in detection limits to the low-ppbv to high-pptv range. Furthermore, the applicability of MS/MS in differentiating between isobaric species was determined. MS/MS spectra of the isobaric compounds methyl vinyl ketone (MVK) and methacrolein (MACR) are presented and show fragments of different mass making differentiation possible, even when a mixture of both species is present in the same sample. However, MS/MS spectra of acetone and propanal produce fragments with the same molecular masses but with different intensity ratios. This allows quantitative distinction only if one species is predominant. Fragmentation mechanisms are proposed to explain the results.     

Dual linear ion trap/orthogonal acceleration time-of-flight mass spectrometer with improved precursor ion selectivity

A new hybrid mass spectrometer based on dual linear ion traps (LITs) and an orthogonal acceleration time-of-flight mass spectrometer (oaTOF), that can achieve MS(n) analysis and high-mass-accuracy detection with high sensitivity, has been developed. Dual-LIT was necessary because, in a single LIT plus oaTOF combination, the LIT pressure favorable for high precursor selectivity in MS(n) analysis (less than 1 mTorr) was far different from an optimum damping pressure (50-100 mTorr) for efficient connection to the TOF mass spectrometer. A dual-LIT solved this problem of inconsistency of the optimum pressures by using the first LIT for MS(n) analysis and the second LIT for collisional damping. This dual-LIT/TOF instrument achieved high-sensitivity MS(n) analysis with high precursor-ion selectivity.     

A linear ion trap mass spectrometer with versatile control and data acquisition for ion/ion reactions

A linear ion trap (LIT) with electrospray ionization (ESI) for top-down protein analysis has been constructed. An independent atmospheric sampling glow discharge ionization (ASGDI) source produces reagent ions for ion/ion reactions. The device is also meant to enable a wide variety of ion/ion reaction studies. To reduce the instrument's complexity and make it available for wide dissemination, only a few simple electronics components were custom built. The instrument functions as both a reaction vessel for gas-phase ion/ion reactions and a mass spectrometer using mass-selective axial ejection. Initial results demonstrate trapping efficiency of 70% to 90% and the ability to perform proton transfer reactions on intact protein ions, including dual polarity storage reactions, transmission mode reactions, and ion parking.     

A novel approach for identification and characterization of glycoproteins using a hybrid linear ion trap/FT-ICR mass spectrometer

Combining source collision-induced dissociation (CID) and tandem mass spectral acquisition in a pseudo-MS(3) experiment using a linear ion trap results in a highly selective and sensitive approach to identifying glycopeptide elution from a protein digest. The increased sensitivity is partially attributed to the nonselective nature of source CID, which allows simultaneous activation of all charge states and coeluting glycoforms generating greater ion abundance for the mass-to-charge (m/z) 204 and/or 366 oxonium ions. Unlike source CID alone, a pseudo-MS(3) approach adds selectivity while improving sensitivity by eliminating chemical noise during the tandem mass spectral acquisition of the oxonium ions in the linear ion trap. Performing the experiments in the hybrid linear ion trap/Fourier transform-ion cyclotron resonance (FT-ICR) enables subsequent high-resolution/high-mass accuracy full-scan mass spectra (MS) and parallel acquisition of MS/MS in the linear ion trap to be completed in 2 s directly following the pseudo-MS(3) scan to collate identification and characterization of glycopeptides in one experimental scan cycle. Analysis of bovine fetuin digest using the combined pseudo-MS(3), high-resolution MS, and data-dependent MS/MS events resulted in identification of four N-linked and two O-linked glycopeptides without enzymatic cleavage of the sugar moiety or release of the sialic acids before analysis. In addition, over 95% of the total protein sequence was identified in one analytical run.     

Duty cycle enhancement of an orthogonal acceleration TOF mass spectrometer using an axially-resonant excitation linear ion trap

We report a new technique to enhance detection duty cycle of an orthogonal-acceleration time-of-flight mass spectrometer (oaTOF) over a broad mass range. To this end, we used an axially-resonant-excitation linear ion trap, which ejects ions axially and mass selectively into a non-mass-selective linear ion trap in front of the TOF pusher. A delay between the ejection timing of the non-mass-selective LIT and the push timing of the oaTOF was swept mass-synchronously with the axially-resonant-excitation linear ion trap, so that ions are detected with duty cycle larger than 60% over a wide mass range from m/z 174.1 to 1922.0, which is 3 to 10 times better than conventional oaTOF.     

Solid phase micro-extraction in a miniature ion trap mass spectrometer

Fiber introduction mass spectrometry (FIMS), a variation of solid-phase microextraction (SPME) and membrane introduction mass spectrometry (MIMS), is employed with a miniature mass spectrometer. The inlet system, constructed of commercially available vacuum parts, allows the direct introduction of the SPME needle vacuum chamber into the mass spectrometer. Thermal desorption of the analyte from the poly(dimethylsiloxane) (PDMS) coated fiber was achieved with a built in nichrome heater, followed by electron ionization of the analytes internal to the cylindrical ion trap (CIT). The system has been tested with several volatile organic compounds (VOC) in air and to analyze the headspace over aqueous solutions, with limits of detection in the low ppb range. The signal rise (10-90%) and fall (90-10%) times for the system ranged from 0.1 to 1 s (rise) and 1.2 to 6 s (fall) using heated desorption. In addition, this method has been applied to quantitation of toluene in benzene, toluene, xylene (BTX) mixtures in water and gasoline. This simple and rapid analysis method, coupled to a portable mass spectrometer, has been shown to provide a robust, simple, rapid, reproducible, accurate and sensitive (low ppb range) fieldable approach to the effective in situ analysis of VOC in various matrices.     

Double resonance ejection in a micro ion trap mass spectrometer

Ion ejection from a cylindrical micro ion trap by resonance excitation of the secular motion is observed to be strongly dependent on the frequency of the secular motion at resonance. Both the intensity of the ion signal and the mass resolution of the resulting mass spectrum are increased when the ion secular frequency is approximately that of a nonlinear resonance of the trap. The resonances are attributed to electrical as well as geometrical considerations.     

Sodium ion attachment reactions in an ion trap mass spectrometer

The capabilities of ion traps to perform attachment reactions with alkali cations using classical scanning sequences have been exploited here with an ion trap mass spectrometer equipped with an external ion source to generate the reagent Na(+) ions. Kinetic studies have shown that, as expected, the attachment efficiency is very high, near-collision efficiency, and illustrate how the present method is particularly well suited for ion trap mass spectrometers. The large adaptability of the experimental conditions suggests that a wide range of organic molecules, characterized by a large alkali ion affinity, could be readily detected even at low levels. Applications of sodium ion attachment reactions are illustrated by the detection and characterization of explosives and some of their correlated pyrolytic degradation products. Detection -limits for phthalate compounds are shown to reach the low ng range of injected samples, without any noticeable difficulties in the full scan mode of acquiring mass spectra. Copyright 2000 John Wiley & Sons, Ltd.     

Development and characterization of an electrospray ionization ion trap/linear time-of-flight mass spectrometer

On-line analysis of compounds from solution has been greatly facilitated by the advent of electrospray ionization mass spectrometry (ESI-MS). Although quadrupole mass analyzers are most commonly used with ESI at present, time-of-flight (TOF) mass spectrometers offer several potential advantages including high data acquisition rates, which are desirable for fast separation techniques. One method of coupling ESI and TOF uses an ion trap for temporary storage and accumulation of the electrosprayed ions prior to TOF mass analysis. Previous studies have not fully addressed the effects of several key variables on the analytical capabilities of this type of instrument. In this study, the characterization of an ion trap/linear TOF instrument for ESI is described. The behavior of various analytes is divided into two separate groups; each one is found to have its own optimal set of operating conditions. The reasons for the observed differences between groups are explored. Issues relevant to mass resolution, sensitivity, mass range, mass-to-charge ratio discrimination, and mass measurement accuracy are addressed. Finally, it is suggested that the analytical capability of this type of instrument could be significantly improved by changing the ion optics from the existing focusing lenses to a rf-only quadrupole lens.