Surface-induced dissociation of molecular ions in a quadrupole ion trap mass spectrometer

A method is reported by which surface-induced dissociation is used to activate ions stored in a quadrupole ion trap mass spectrometer. The method employs a short (< 5 μs), fast-rising (< 20-ns rise time), high voltage direct current (dc) pulse, which is applied to the endcaps of a standard Paul-type quadrupole ion trap. This is in contrast to the application of an alternating current (ac) signal normally used to resonantly excite and dissociate ions in the trap. The effect of the dc pulse is to cause the ions rapidly to become unstable in the radial direction and subsequently to collide with the ring electrode. Sufficient internal energy is acquired in this collision to cause high energy fragmentations of relatively intractable molecular ions such as pyrene and benzene. The dissociations of limonene are used to demonstrate that high energy demand processes increase in relative importance in the dc pulse experiment compared with the usual resonance excitation method used to cause activation. The fragments are scanned out of the ion trap using the conventional mass-selective instability scan mode. Simulations of ion motion in the trap provide evidence that surface collisions occur at kinetic energies in the range of tens to several hundred electronvolts. The experiments also demonstrate that production of fragment ions is sensitive to the phase of the main radiofrequency drive voltage at the point when the dc is initiated.     

Analysis of high mass peptides using a novel matrix-assisted laser desorption/ionisation quadrupole ion trap time-of-flight mass spectrometer

A novel matrix-assisted laser desorption/ionisation quadrupole ion trap time-of-flight (MALDI QIT ToF) mass spectrometer has been used to analyse high mass peptide ions exceeding 2000 Da. Human adrenocorticotropic hormone (fragment 18-39) and oxidised bovine insulin chain B were utilised to evaluate the performance of the instrument both in MS and in MS/MS mode. Its ability to efficiently isolate ions and to fragment them using collisionally activated decomposition (CAD) has been demonstrated using mixtures diluted to the low-femtomole level on target. Additionally, multiple stage mass spectrometry (MS/MS/MS) provides a second-generation product ion spectrum in which new fragment ions are detected and new stretches of amino acids are identified.     

Fragmentation of N-linked glycans with a matrix-assisted laser desorption/ionization ion trap time-of-flight mass spectrometer

N-Linked glycans were ionized from several matrices with a Shimadzu-Biotech AXIMA-QIT matrix-assisted laser desorption/ionization quadrupole ion trap time-of-flight mass spectrometer. [M+Na]+ ions were produced from all matrices and were accompanied by varying amounts of in-source fragmentation products. The least fragmentation was produced by 2,5-dihydroxybenzoic acid and the most by alpha-cyano-4-hydroxycinnamic acid and 6-aza-2-thiothymine. Sialic acid loss was extensive but could be prevented by formation of methyl esters. Fragmentation produced typical low-energy-type spectra dominated by ions formed by glycosidic cleavages. MS(n) spectra (n = 3 and 4) were used to probe the pathways leading to the major diagnostic ions. Thus, for example, an ion that was formed by loss of the core GlcNAc residues and the 3-antenna was confirmed as being formed by a B/Y rather than a C/Z mechanism. The proposed structures of several cross-ring cleavage ions were confirmed and it was shown that MS3 spectra could be obtained from as little as 10 fmol of glycan.     

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.     

Orthogonal trap time-of-flight mass spectrometer using a collisional damping chamber

We report a new hybrid mass spectrometer, which is a combination of a quadrupole ion trap and an orthogonal time-of-flight (TOF) mass spectrometer. This new configuration consists of a collisional-damping chamber (CDC) inserted between an MSn-capable ion trap and a high-mass-accuracy orthogonal-TOF mass spectrometer. Because the CDC converted an ion packet into an energy-focused and quasi-continuous beam, a high mass resolution of over 10,000 and a high mass accuracy of better than 3 ppm were achieved. Moreover, the ratio of the maximum detectable m/z to the minimum detectable m/z, which is referred to here as the mass window, was improved to more than 10.     

A pulsed time-of-flight mass spectrometer for liquid secondary ion mass spectrometry

The design and performance of a new time-of-flight mass spectrometer is reported. The instrument combines the advantages of a pulsed drawout TOF analyzer with a liquid secondary ion source. Differences from commercially available pulsed TOF analyzers (Wiley/McLaren type) are discussed with regard to operation with ion desorption from a liquid matrix.     

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.     

Cluster ions derived from sodium and potassium tetrafluoroborate and their collision induced dissociation in an ion trap mass spectrometer

Electrospray ionization was used to produce distributions of gas-phase cluster ions from solutions of sodium and potassium tetrafluoroborate. The majority of the cluster species followed the trend (MBF(4))(n)M(+), where M=Na and K. The values of n, for both salts, ranged from 1-15. Collision induced dissociation (MS/MS and MS(n)) in an ion trap mass spectrometer was used to determine the dissociation pathways for the cluster ions. The (NaBF(4))(n)Na(+) cluster ions fragmented via two pathways: (a) the loss of one or multiple neutral BF(3) molecules and (b) the loss of one or more NaBF(4) units. Of the two, the product ions corresponding to the loss of BF(3) units were more prominent. Unlike the Na salt, the (KBF(4))(n)K(+) cluster ions decomposed primarily by the loss of one or multiple KBF(4) units. Similar differences in dissociation behavior were observed when the heated transfer capillary, normally used to desolvate ions, was used to investigate cluster ion stability to thermal degradation and dissociation. The dissociation profiles (decrease in ion abundance with increasing activation amplitude) for several (NaF)(n)Na(+) and (KF)(n)K(+) cluster ions were measured and compared to probe the influence of the relative stability of the alkali fluorides (NaF and KF) on the dissociation behavior exhibited by the tetrafluoroborate cluster distributions. We found that the (NaF)(n)Na(+) cluster ions required higher activation amplitudes to induce fragmentation than the corresponding (KF)(n)K(+) species, indicative of stronger ionic bonding and higher gas-phase stability for the former. This in turn indicates that the reaction pathway involving only the loss of one or multiple units of BF(3), favored for the (NaBF(4))(n)Na(+) cluster series, but not for the analogous (KBF(4))(n)K(+) series, may be due to the high gas-phase stability of NaF, and relatively lower stability of KF, towards dissociation.     

Internal pulsed valve sample introduction on a quadrupole ion trap mass spectrometer

A pulsed valve positioned just outside the ion trap electrodes (within the vacuum chamber) has been characterized. The observed gas pulse widths and the maximum ion intensities were found to decrease as the distance between the pulsed valve and the ion trap electrodes increased. An explanation is presented within. The pulsed valve was found to impart temporal separation in ion-molecule reactions by permitting the removal of interfering neutrals. Other factors that affect the degree of temporal separation also are presented.     

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.     

Structural characterization of polyethers using matrix-assisted laser desorption/ionization quadrupole ion trap time-of-flight mass spectrometry

Fragmentation of polyethers, such as poly(ethylene glycol) (PEG), poly(propylene glycol) and poly(tetramethylene glycol) was analyzed by matrix-assisted laser desorption/ionization tandem mass spectrometry (MALDI-MS/MS) using a quadrupole ion trap time-of-flight mass spectrometer (QIT-ToF). The Li adduct ion provided more abundant fragments than the Na and K adduct ions in the MS/MS spectra. A previous study had demonstrated four series fragments of hydroxyl-, vinyl- and formyl-terminated ions, as well as distonic cations, in high-energy fast atom bombardment MS/MS and MALDI collision-induced dissociation measurements of poly(ethylene glycol). In the present study, the low-energy MS/MS measurements using MALDI-QIT-ToF, showed hydroxyl-, vinyl- and formyl- terminated fragments with or without other fragment groups, but not distonic cations. The fragmentation depended on the types of polyethers examined. MS/MS measurements using MALDI-QIT-ToF are expected to allow structural characterization of unknown components of polyethers.     

Fragmentation of phosphopeptides in an ion trap mass spectrometer

A systematic study of the fragmentation pattern of phosphopeptides in an electrospray (ESI) ion trap mass spectrometer is presented. We show that phosphotyrosine- and phosphothreonine-containing peptides show complicated fragmentation patterns. These phosphopeptides were observed to lose the phosphate moiety in the form of H₃PO₄ and/or HPO₃, but were also detected with no loss of the phosphate group. The tendency to lose the phosphate moiety depends strongly on the charge state. Thus, the highest observed charge state tends to retain the phosphate moiety with extensive fragmentation along the peptide backbone. We also show that phosphoserine-containing peptides have relatively simple fragmentation patterns of losing H₃PO₄. This loss is independent of the charge state. We suggest strategies for the accurate identification of phosphorylation sites using the ion trap mass spectrometer.     

Origin of mass shifts in the quadrupole ion trap: dissociation of fragile ions observed with a hybrid ion trap/mass filter instrument

A novel hybrid tandem mass analyzer, coupling a quadrupole ion trap with a quadrupole mass filter, has been constructed to permit mass analysis of ions ejected from the ion trap. The initial application of this instrument is the investigation of the origin of mass shifts in the ion trap due to ion fragility. We hypothesize that fragile ions undergo mass shifts, characterized by peak fronting, due to early ejection from the quadrupole ion trap. As these ions come into resonance with the ejection frequency, they gain kinetic energy, collide with buffer gas molecules and thus can dissociate to produce fragment ions. These fragment ions will not be stable within the ion trap as they are situated past the stability boundary at q(z) = 0. 908. Consequently the fragment ions are ejected prematurely. This results in an apparent mass shift due to peak fronting. The experiments reported here clearly document the production of fragment ions as the origin of mass shifts during the resonant ejection of fragile ions. Copyright 2000 John Wiley & Sons, Ltd.     

Identification of137Cs in an individual microparticle by laser desorption/ionization ion trap mass spectrometer

The detection of radiocesium in microparticles was performed by using an ion trap mass spectrometer coupled with laser desorption and ionization. Pulsed laser desorbed particle and the resulted ions were analyzed by an ion trap mass analyzer. The presence of radiocesium, especially about¹³⁷Cs, in microparticles was verified by single as well as successive particle analysis. The detection limit was reached to ≈ag/particle level with a signal-to-background ratio of 4. The inhomogeneous distribution of particle size and the irregular shapes of particle limit the quantitative evaluation of¹³⁷Cs concentration in the microparticle. But this high sensitivity allows to monitor directly the radiocesium from small amounts of a microparticle sample.     

Anionic copper complex fragmentations from enkephalins under low-energy collision-induced dissociation in an ion trap mass spectrometer

Peptide metallation with Cu2+ was explored in the negative ESI mode using an ion trap mass spectrometer. Under these conditions, the [(M-3H) + CuII]- species formed were investigated under low-energy collision-induced dissociation conditions. MS2 experiments indicate a very different behavior of CuII metallated complexes compared with [M-H]- species. CuII induces an easy loss of CO2 and specific side-chain cleavages (by radical losses) at the C-terminal residue, as observed previously by prompt 'in source' dissociation experiments. The loss of CO2 yields an unstable carbylide that leads to further dissociations involving the migration of a proton or a hydrogen radical (through the reduction of CuII). Multistage MS3 experiments were carried out to rationalize this behavior. Fragmentation pathways are proposed in order to explain the product ions observed. The side-chain radical loss at the C-terminus was demonstrated to be a consecutive process. Finally, evidence is provided that the specific side-chain cleavages can be used for the differentiation of Leu/Ile and Gln/Lys residues when they are located at the C-terminus. The existence of a zwitterionic form in the case of the anionic YGGFK-CuII complex is proposed.     

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.     

Energy-dependent dissociation of benzylpyridinium ions in an ion-trap mass spectrometer

Benzylpyridinium ions, generated via electrospray ionization of dilute solutions of their salts in acetonitrile/water, are probed by collisional activation in an ion-trap mass spectrometer. From the breakdown diagrams obtained, phenomenological appearance energies of the fragment ions are derived. Comparison of the appearance energies with calculated reaction endothermicities shows a reasonably good correlation for this particular class of compounds. In addition, the data indirectly indicate that at threshold the dissociation of almost all of the benzylpyridinium ions under study leads to the corresponding benzylium ions, rather than the tropylium isomers. Substituent effects on the fragmentation for a series of benzylpyridinium ions demonstrate that neither mass effects nor differences in density of states seriously affect the energetics derived from the ion-trap experiments.     

MS/MS of ions in a low pressure linear ion trap using a pulsed gas

A pulsed valve was used to increase the pressure within the trapping region of a low-pressure linear ion trap by situating the pulsed valve close to the ion trapping region. The pressure was estimated to increase from a background pressure of 3.5e–5 Torr of nitrogen to 0.49 mTorr at the center of the trap. The increased pressure allowed excitation periods to be reduced from 100 to 25 ms without suffering losses in MS/MS efficiency during dipolar excitation. The reduction in excitation period translates into an increase in the overall duty cycle of the scan.     

Rapid protein identification using a microscale electrospray LC/MS system on an ion trap mass spectrometer

A methodology has been developed for the rapid identification of gel separated proteins. Following in gel protein digestion with trypsin, the resulting peptide mixture is analyzed by on-line liquid chromatography, electrospray mass spectrometry (LC/MS). The mass spectral data containing either accurate mass values or sequence specific fragment ion information is then matched to a database of known protein sequences. Key features of the LC/MS system are the use of a novel integrated, microscale LC column-electrospray interface and variable flow solvent delivery to optimize the efficiency of sample loading and gradient elution. With these enhancements, only 10 min is required to analyze each sample. The method is routine for sample amounts ranging from 50 to 500 fmol. The analysis parameters for the ion trap mass spectrometer have to be carefully adjusted in order to keep pace with the rapidly eluting LC peaks. Although designed for rapid LC separations, the integrated column-electrospray interface is also able to provide extended analyses of selected components using a technique known as “peak parking. ”