Co-occurrence of boundary and resonance ejection in a multiplexed rectilinear ion trap mass spectrometer

A method is reported for evaluating ion trap mass analyzers by selection of operating conditions under which both boundary and resonance ejection peaks occur in a single mass scan. The choice of frequency and amplitude of the auxiliary waveform applied for resonance ejection can be such as to produce a resonance ejection mass spectrum with unit resolution or, under selected conditions, signals attributable to both boundary and resonance ejection in a single mass scan. The contrasting mass resolution associated with these two ejection processes is evident in these data. The co-occurrence of the two ejection phenomena is ascribed to the effects of higher-order fields; it is more marked in some rectilinear ion traps (RITs) than in other nominally identical devices, leading to the possibility of using it to compare individual mass analyzers in multiplexed instruments. The method is used to compare multiple ion traps driven by the same RF signal in a fully-multiplexed mass spectrometer, composed of parallel ion source/mass analyzer/detector channels each housed in one quadrant of a specialized vacuum chamber.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

A digital ion trap mass spectrometer coupled with atmospheric pressure ion sources

In a digital ion trap (DIT), the quadrupole trapping and excitation waveforms are generated by the rapid switching between discrete d.c. voltage levels. As the timing of the switch can be controlled precisely by digital circuitry, the approach provides an opportunity to generate mass spectra by means of a frequency scan in contrast to the conventional voltage scan, thus providing a wider mass range of analysis. An instrument has been constructed which employs a 'non-stretched' ion trap and the field fault around the aperture of the end-cap electrode can be corrected electronically using a field-adjusting electrode. The ion trap was coupled with electrospray ionization (ESI) and atmospheric pressure matrix-assisted laser desorption/ionization (AP-MALDI) sources to demonstrate the capability of the digital method. AP-MALDI mass spectra of singly charged ions with mass-to-charge ratios upto 17 000 Th were generated using a trapping voltage of only 1000 V. Forward and reverse mass scans at resolutions up to 19 000 and precursor ion isolation at resolutions up to 3500 with subsequent tandem mass spectrometric analysis were demonstrated. The method of generating the digital waveforms and period scan is described. Discussion of the issues of mass range, scan speed, ion trapping efficiency and collision-induced dissociation efficiency are also provided.     

Miniature toroidal radio frequency ion trap mass analyzer

A miniature ion trap mass analyzer is reported. The described analyzer is a 1/5-scale version of a previously reported toroidal radio frequency (rf) ion trap mass analyzer. The toroidal ion trap operates with maximum rf trapping voltages about 1 kVp-p or less; however despite the reduced dimensions, it retains roughly the same ion trapping capacity as conventional 3D quadrupole ion traps. The curved geometry provides for a compact mass analyzer. Unit-mass resolved mass spectra for n-butylbenzene, xenon, and naphthalene are reported and preliminary sensitivity data are shown for naphthalene. The expected linear mass scale with rf amplitude scan is obtained when scanned using a conventional mass-selective instability scan mode combined with resonance ejection.     

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.     

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.     

Thermospray liquid chromatography-mass spectrometry with a quadrupole ion trap mass spectrometer

A theromospray ion source using corona discharge ionization was interfaced to a quadrupole ion trap mass spectrometer via a multi-element lens system. Ions were injected into the trap periodically where they were stabilized by collisions with helium bath gas. Mass spectra were recorded on the trapped ions using the mass-selective instability scan mode. Data are shown for a peptide and a nucleoside and the effects of some experimental variables on the spectra are explored.     

Selective ionization for elemental analysis with an ion trap mass spectrometer

The ion storage capacity (<10⁶) of ion trap mass spectrometers (ITMS) can sometimes limit the ability to analyze trace components in complex mixtures. We demonstrate here that resonant laser ablation (RLA) can offer a degree of selectivity in the ionization process, thus allowing the preferential accumulation of analyte ions in the trap. Selectivities of 75 and 50, for chromium and iron, respectively, are reported here for RLA of stainless steel in an ITMS. We offer suggestions to improve both the selectivity and the ionization efficiency, relative to the results reported here.     

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.     

"Fast excitation" CID in a quadrupole ion trap mass spectrometer

Collision-induced dissociation (CID) in a quadrupole ion trap mass spectrometer is usually performed by applying a small amplitude excitation voltage at the same secular frequency as the ion of interest. Here we disclose studies examining the use of large amplitude voltage excitations (applied for short periods of time) to cause fragmentation of the ions of interest. This process has been examined using leucine enkephalin as the model compound and the motion of the ions within the ion trap simulated using ITSIM. The resulting fragmentation information obtained is identical with that observed by conventional resonance excitation CID. "Fast excitation" CID deposits (as determined by the intensity ratio of the a(4)/b(4) ion of leucine enkephalin) approximately the same amount of internal energy into an ion as conventional resonance excitation CID where the excitation signal is applied for much longer periods of time. The major difference between the two excitation techniques is the higher rate of excitation (gain in kinetic energy) between successive collisions with helium atoms with "fast excitation" CID as opposed to the conventional resonance excitation CID. With conventional resonance excitation CID ions fragment while the excitation voltage is still being applied whereas for "fast excitation" CID a higher proportion of the ions fragment in the ion cooling time following the excitation pulse. The fragmentation of the (M + 17H)(17+) of horse heart myoglobin is also shown to illustrate the application of "fast excitation" CID to proteins.