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.     

Use of a five-channel multiplexed electrospray quadrupole time-of-flight hybrid mass spectrometer for metabolite identification

Metabolism data provided with reduced cycle time has become of increasing importance for the early evaluation of DMPK properties of drugs in discovery. In this regard, quadrupole time-of-flight hybrid mass spectrometers (Q-TOF) can provide very reliable metabolite identification via accurate mass measurement of ions and the consequent access to the elemental composition of the metabolite. However, due to their cost, they are often used for drug metabolism studies on later stage drug candidates or to address challenging metabolism questions. A new prototype, consisting of a five-channel multiplexed electrospray ionization (ESI) source on a Q-TOF with one channel used for lock-mass compound infusion, was evaluated for metabolite identification. The goal was to increase the sample throughput of a single ESI-MS system by a factor of 4, while maintaining efficient metabolite separation in high-performance liquid chromatography (HPLC) as well as adequate sensitivity and mass accuracy, and ultimately improve the speed and quality of metabolism studies supporting drug discovery. The analytical performance of the system was assessed by evaluating the sensitivity and mass accuracy (using real in vitro and in vivo samples), inter-channel differences in retention times, MS/UV response, and cross-talk among channels. The sensitivity using the multiplexed ESI source was on average 2-fold lower than with single ESI, correlating well with previous literature data. The mass accuracy was comparable to that obtained using single ESI in both MS and MS/MS modes, making the metabolite identification process using the multiplexed ESI source as reliable as with single ESI. Compound-dependent differences in ionization efficiencies were observed among channels, and were minimized by analyzing related samples on the same channel. Finally, the level of cross-talk among channels was acceptable (around 0.3%) and comparable to levels previously published for quantitative applications using multiplexed ESI. The paper also focuses on the advantages and disadvantages of this new approach compared to other approaches in the literature in the field of metabolite identification.     

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.     

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.     

Triple quadrupole linear ion trap mass spectrometer for the analysis of small molecules and macromolecules

Recently, linear ion traps (LITs) have been combined with quadrupole (Q), time-of-flight (TOF) and Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry (MS). LITs can be used either as ion accumulation devices or as commercially available, stand-alone mass spectrometers with MSn capabilities. The combination of triple quadrupole MS with LIT technology in the form of an instrument of configuration QqLIT, using axial ejection, is particularly interesting, because this instrument retains the classical triple quadrupole scan functions such as selected reaction monitoring (SRM), product ion (PI), neutral loss (NL) and precursor ion (PC) while also providing access to sensitive ion trap experiments. For small molecules, quantitative and qualitative analysis can be performed using the same instrument. In addition, for peptide analysis, the enhanced multiply charged (EMC) scan allows an increase in selectivity, while the time-delayed fragmentation (TDF) scan provides additional structural information. Various methods of operating the hybrid instrument are described for the case of the commercial Q TRAP (AB/MDS Sciex) and applications to drug metabolism analysis, quantitative confirmatory analysis, peptides analysis and automated nanoelectrospray (ESI-chip-MS) analysis are discussed.     

"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.     

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.     

Plasticizer contamination from vacuum system O-rings in a quadrupole ion trap mass spectrometer

The outgassing of plasticizers from Buna-N and Viton o-rings under vacuum lead to undesired ion-molecule chemistry in an Electrospray Quadrupole Ion Trap Mass Spectrometer. In experiments with the helium bath gas pressure >1.2 mTorr, or whenever analyte ions were stored for >100 ms, extensive loss of analyte ions by proton transfer or adduction with o-ring plasticizers bis(2-ethylhexyl) phthalate and bis(2-ethylhexyl) adipate occurred. A temporary solution to this contamination problem was found to be overnight refluxing in hexane of all the o-rings in the vacuum system. This procedure alleviated this plasticizer contamination for approximately 100 hours of operation. These results, and those that lead to identification of the contamination as plasticizers outgassing from o-rings are described.     

Coupling a particle beam interface directly to a quadrupole ion trap mass spectrometer

A particle beam interface has been coupled to a quadrupole ion trap mass spectrometer. The system allows the collection of electron ionization mass spectra from analyte in solution. The interface incorporates a pneumatic nebulizer, a heated desolvation chamber, and a three-stage separator region. Additional helium, for improved performance, is added through stage 3. The particles formed in the interface are separated from solvent molecules and are transferred directly to the ion trap where they are expected to collide with the hot hyperbolic surface of the end cap. The end cap serves both as a heated target used to vaporize the particles and as an ion-trapping electrode. Mass analysis is achieved with the mass-selective instability scan supplemented with resonance ejection. Electron ionization spectra from 100 ng of caffeine [molecular weight (MW) = 1941; 1-naphthalenol methylcarbamate (carbaryl) (MW = 2011, 17α-hydroxyprogesterone (MW = 330), and reserpine (MW = 608) are shown using sampling by a segmented flow analysis. Some charge exchange is evident with methanol as well as self-chemical ionization at higher analyte levels. The interface shows a nonlinear caffeine calibration curve for analyte amounts below 30 ng and a more linear response at higher amounts. Caffeine was detected at 25 pmol (5 ng), with a signal-to-noise ratio of 50, 20-μL loop, full scan.     

Investigations into the use of a reverse scan in a quadrupole ion trap mass spectrometer

The forward scan (i.e. an increasing RF voltage ramp for the mass-selective instability scan) is commonly used as an analytical scan for ion detection with quadrupole ion trap instruments. A number of phenomena have been observed while using this scan technique. These include space charge effects resulting in the delayed ejection of ions from the ion trap, and the fragmentation of fragile ions producing very broad peaks. Here the use of a reverse scan (i.e. a decreasing RF voltage ramp) is examined to determine the effect of the above phenomena in this acquisition method. With regard to space charge effects, the apparent reduction of the carbon isotope spacing below one Thomson (for singly charged ions) that is observed with the forward scan is now replaced by an apparent increase in this spacing. The reverse scan, which optimizes at lower axial modulation ejection voltages than the forward scan, allows for the intact ejection of fragile ions under its typical operating conditions whereas the forward scan results in fragmentation. Reducing the axial modulation voltage for the ejection of ions in the forward scan results in less dissociation of the fragile ions during ion ejection, but with the observation of ghost peaks due to incomplete ejection of all of the ions at the resonance ejection condition. While performing the reverse scan experiment, the formation of product ions from dissociation of the MH(+) ion has also been observed.     

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.     

Coupling of ion-molecule reactions with liquid chromatography on a quadrupole ion trap mass spectrometer

We report for the first time a coupling of gas-phase ion-molecule reactions with chromatographic separations on a quadrupole ion trap mass spectrometer. The interface was accomplished by using a pulsed valve for the introduction of a volatile neutral into the ion trap. The pulsed valve controller is synchronized with the mass spectrometer software. The setup requires some minor modifications to the vacuum system of the commercial quadrupole ion trap but most of the modifications are external to the mass spectrometer. Two applications of this interface are described: differentiation between two phosphoglucose positional isomers and detection of a phosphopeptide in a peptide mixture. Both applications are using the reactivity of trimethoxyborate towards a phosphate moiety in the negative ion mode. The detection of phosphopeptides hinges on our findings that non-phosphorylated peptide anions do not react with trimethoxyborate. This LC/MS detection can be easily visualized in terms of selected reaction monitoring.     

Ion motion in the rectangular wave quadrupole field and digital operation mode of a quadrupole ion trap mass spectrometer

A quadrupolar electric field driven by a rectangular wave voltage can be used for mass-selective storage and analysis. The ion motion in such an electric field is derived, and the stability of ions is presented in the a-q diagram that is commonly used for sinusoidal wave quadrupole mass spectrometry in association with the solution of the Mathieu equation. The pseudo-potential well is discussed in an approximation that leads to the relation of secular frequency to operating parameters. A scheme for a digital ion trap mass spectrometer is described, based on this theory. An ion optics simulation was performed to check the theory of resonant ejection, and to prove the feasibility of the mass scan method for a practical ion trap of such geometry.     

Sensitivity and amplitude calibration of a Fourier transform 3D quadrupole ion trap mass spectrometer

With a three-dimensional (3D) quadrupole ion trap running in a Fourier transform operating mode, the detected signal is an image of the collective motion of the confined ions. Consequently, it is assumed that the image signal is the sum of the axial trajectories of the simultaneously confined ions. The resulting frequency spectrum after Fourier transformation comprises frequency peaks at the axial secular frequencies of the confined species according to their mass/charge ratio. With a singly confined species, the maximal amplitude of the image signal is proportional to the amplitude of the secular axial frequency peak. The matrix method is employed to express the axial trajectory sampled at the confinement field period. In that case, the expression of the image signal, as well as its maximal amplitude, is calculated as a function of the trap operating conditions and initial axial positions and axial velocities of the ions. The initial position and velocity distributions are connected to the injection mode. With the steady ion flow injection mode (SIFIM) and an initial phase of the confinement field equal to kπ, the maximal amplitude of the image signal is proportional to either the sum of the initial axial positions or the number of confined ions and the mean value of the initial axial positions. By simulation, amplitude fluctuation of the frequency peak is then calculated for a number of ions ranging between some tens to some thousands of ions injected by SIFIM. The peak amplitude fluctuations induced by the fluctuations of the number of ions are seven times greater than those induced by the fluctuations of the distribution of the initial axial positions.     

Quantitative and qualitative profiling of endocannabinoids in human plasma using a triple quadrupole linear ion trap mass spectrometer with liquid chromatography

Owing to the large implication of endocannabinoids (ECs) in many physiological and pathophysiological processes, a rapid liquid chromatography/electrospray ionisation triple quadrupole linear ion trap mass spectrometric assay (LC/ESI‐QqQ_LIT) was developed for the detection and characterization of anandamide (AEA), 2‐arachidonoyl glycerol (2‐AG), virodhamine (VA), noladin ether (2‐AGE), and N‐arachidonoyl dopamine (NADA) in human plasma. The ECs were extracted from 500 µL of plasma by liquid–liquid extraction (LLE) and separated by using an XTerra C18 MS column (50 × 3.0 mm i.d., 3.5 µm) with gradient elution. The mobile phase was composed of a mixture of acetonitrile, water, and formic acid (0.1%). For confirmatory analysis, an information‐dependent acquisition (IDA) experiment was performed with selected reaction monitoring (SRM) as survey scan and enhanced product ion (EPI) as dependent scan. The assay was found to be linear in the concentration range of 0.1–5 ng/mL for AEA, 0.3–5 ng/mL for VA, 2‐AGE, and NADA and 1–20 ng/mL for 2‐AG using a 0.5 mL aliquot of plasma. Repeatability and intermediate precision were found less than 15% over the tested concentration ranges. The developed method thus provided the rapid, highly sensitive and highly selective requirement for assess quantitation, and identification of ECs in plasma. Copyright © 2009 John Wiley & Sons, Ltd.