Space-charge effects with mass-selective axial ejection from a linear quadrupole ion trap

Methods to reduce mass shifts caused by space charge with mass‐selective axial ejection from a linear quadrupole ion trap are investigated. For axial ejection, dipole excitation is applied to excite ions at q ≈ 0.85. The trapping radiofrequency (rf) voltage is scanned to bring ions of different m/z values into resonance for excitation. In the fringing field at the quadrupole exit, excited ions gain axial kinetic energy, overcoming the trapping potential, and are ejected from the trap. Space charge causes the frequencies of ion oscillation to decrease. Thus, greater rf voltages are required to bring ions into resonance for excitation and ejection, and the ions shift to higher apparent masses in a mass spectrum. At the same time, the peaks broaden, lowering resolution. The effects of injection q value, ejection q value, excitation amplitude, quadrupole dc voltages applied to the electrodes, applying an rf voltage to the exit lens, and scan speed, on mass shifts have been studied experimentally. Most experiments were done with only ions of protonated reserpine (m/z 609.3 and its isotopic peaks) in the trap. Some experiments were done with ions of protonated reserpine and ions of m/z 622 in the trap. In general, the mass shifts are reduced with higher ejection q values, higher excitation amplitudes, with quadrupole dc applied, and at higher scan speeds. The application of quadrupole dc appears to increase the ion cloud temperature, which lowers mass shifts. Thus, a proper choice of operating conditions can reduce, but not eliminate, mass shifts caused by space charge. Copyright © 2011 John Wiley & Sons, Ltd.     

A segmented ring,cylindrical ion trap source for time-of-flight mass spectrometry

An ion trap source has been designed for use with time-of-flight (TOF) mass analysis. Two thin diaphragms make up a segmented ring electrode; the end cap electrodes are planar wire mesh. The potential field produced by the rf voltage applied between the ring and end cap electrodes resembles that of the cylindrical ion trap. The trapped ion population for ions created by electron impact exhibits linear growth against a first-order loss that has a time constant of about 50 µs; no ion loss occurs when the electron beam is off. The observed value of q _z at low-mass cutoff for rf ion storage is ?0.84. Pulsed extraction of all ions is accomplished by switching the trap electrodes from rf to voltages required to provide a linear dc extraction field. The TOF flight path includes a wide energy range reflectron. Better than unit mass resolution is achieved through m/z 500 without collisional ion cooling. With an extraction rate of 1 kHz and a recording rate of 4 spectra per second, a linear working curve is obtained between 36 pg and 18 ng of chlorobenzene delivered chromatographically. The system has demonstrated the potential to achieve a very high sample utilization efficiency at high spectral generation rates.     

Use of region II of the a/q stability diagram for fast scanning of a linear quadrupole mass spectrometer

For evaluation as a detector for high-speed gas chromatography, a linear quadrupole mass spectrometer was operated in rf only mode by using Region II (a = 0; 7.514 < q < 7.580) of the Mathieu a/q stability diagram. The available power supply and the diameter of the quadrupole rods of the mass spectrometer placed an upper mass limit of ～ m/z 93. Scan rates of 1000 scans/s were obtained with mass spectral peaks resolved over an 80-u range The m/z 91 and 92 ions produced from the electron ionization of toluene are resolved with an R _1/2 of 135. A potential difference between the source and the quadruple mass filter of up to 1000 V was used to accelerate ions into the quadrupole. Broadening of mass-to-charge ratio peaks results from the time constant of the signal amplification rather than the small number of rf cycles the ions experience. The expected loss of sensitivity relative to Region I is observed, and the problem of mass aliasing is discussed.     

Ion internal temperature and ion trap collisional activation: protonated leucine enkephalin

Protonated leucine enkephalin has been used as a prototypical high-mass ion to yield a quantitative estimate of the relationship between the amplitude of the resonance excitation voltage used in an ion trap collisional activation experiment, and the internal temperature to which an ion can be elevated over the bath gas temperature. The approach involves the measurement of the ion dissociation rate as a function of resonance excitation voltage, and the correlation of dissociation rate with ion internal temperature. The relatively high ion trap dissociation rates observed under typical resonance excitation conditions preclude the direct application of the Arrhenius equation to derive internal temperatures. An empirical determination of the relationship between ion internal temperature and dissociation rate over the rate range of interest here was made via the systematic variation of bath gas temperature. The data suggest a very nearly linear relationship between ion internal temperature and resonance excitation voltage, at least under conditions in which ion ejection is minimal. It is shown that protonated leucine enkephalin ions can be elevated by about 357 K over the bath gas temperature using a monopolar resonance excitation voltage of 540 mV p − p(q_z = 0.163) without significant ion ejection. It is also demonstrated that ion internal temperature can be readily increased by increasing the bath gas temperature, by accelerating the ions in the presence of a room temperature bath gas (i.e. conventional ion trap collisional activation), or by a combination of the two approaches.     

Fluorescence Imaging for Visualization of the Ion Cloud in a Quadrupole Ion Trap Mass Spectrometer

Laser-induced fluorescence is used to visualize populations of gaseous ions stored in a quadrupole ion trap (QIT) mass spectrometer. Presented images include the first fluorescence image of molecular ions collected under conditions typically used in mass spectrometry experiments. Under these “normal” mass spectrometry conditions, the radial (r) and axial (z) full-width at half maxima (FWHM) of the detected ion cloud are 615 and 214 μm, respectively, corresponding to ~6 % of r ₀ and ~3 % of z ₀ for the QIT used. The effects on the shape and size of the ion cloud caused by varying the pressure of helium bath gas, the number of trapped ions, and the Mathieu parameter q _z are visualized and discussed. When a “tickle voltage” is applied to the exit end-cap electrode, as is done in collisionally activated dissociation, a significant elongation in the axial, but not the radial, dimension of the ion cloud is apparent. Finally, using spectroscopically distinguishable fluorophores of two different m/z values, images are presented that illustrate stratification of the ion cloud; ions of lower m/z (higher q _z ) are located in the center of the trapping region, effectively excluding higher m/z (lower q _z ) ions, which form a surrounding layer. Fluorescence images such as those presented here provide a useful reference for better understanding the collective behavior of ions in radio frequency (rf) trapping devices and how phenomena such as collisions and space-charge affect ion distribution.

Equilibrium space charge distribution in a quadrupole ion trap

A simple model provides a basis for evaluating the ion spatial distribution in a uadrupole (Paul) ion trap and its effect on the total potential (trap potential plus space charge 3 acting on ions in the trap. By combining the pseudopotential approximation introduced by Dehmelt 25 years ago with the assumption of thermal equilibrium (leading to a Boltzmann ion energy distribution), the resulting ion spatial distribution (for ions of a single mass-to-charge ratio) depends only on total number of ions, trap pseudopotential, and temperature. (The equilibrium assumption is justified by the high helium bath gas pressure at which analytical quadrupole ion traps are typically operated.) The electric potential generated by the ion space charge may be generated from Poisson’s equation subject to a Boltzmann ion energy distribution. However, because the ion distribution depends in turn on the total potential, solving for the potential and the ion distribution is no longer a simple boundary condition differential equation problem; an iterative procedure is required to obtain a self-consistent result. For the particularly convenient operating condition, (a _z = -8qU/m? ₀ ² Ω², and q _z =-4qV m? ₀ ² Ω², where U and V are direct current and radiofrequency (frequency, ω) voltages applied to the trap, m/q is ion mass-to-charge ratio, and ?₀ is the radius of the ring electrode at the z=0 midplane], both the pseudopotential and the ion distribution become spherically symmetric. The resulting one-dimensional problem may be solved by a simple optimization procedure. The present model accounts qualitatively for the dependence of total potential and ion distribution on number of ions (higher ion density or lower temperature flattens the total potential and widens the spatial distribution of ions) and pseudopotential (higher pseudopotential increases ion density near the center of the trap without widening the ion spatial distribution).     

Diffusion coefficients of ions through ion excange membrane in Donnan dialysis using ions of different valence

Donnan dialysis with an ion exchange membrane was investigated for ions of different valence. The effective diffusion coefficients (D_e) of various kinds of ions in the membrane were obtained by fitting of the equation derived from the Nernst–Planck equation to three or more sets of experimental data for Donnan dialysis. It became apparent that the value of D_e/D_s of monovalent ions (e.g., K⁺ or Na⁺ ions) at z_A=1 and z_B=2 (feed ions are monovalent ones and driving ions are bivalent ones) remained constant at ca. 1/210 and that of bivalent ions (e.g., Ca²⁺, Cu²⁺, or Mg²⁺ ions) remained constant at ca. 1/526 where D_s denotes the diffusion coefficient of ions at infinite dilution in water calculated from the Nernst–Einstein equation, and z_A and z_B represent the valences of the feed and driving ions, respectively. D_e/D_s of monovalent ions (e.g., H⁺, K⁺, or Na⁺ ions) at z_A=2 and z_B=1 (feed ions are bivalent ones and driving ions are monovalent ones) was constant at ca. 1/23.3 and that of bivalent ions remained constant at ca. 1/58.4. It was proved that D_e/D using D_e at z_A=1 and z_B=2 was constant at 1/3.0 and that at z_A=2 and z_B=1 remained constant at 3.0 where D represents the diffusion coefficient of ions in the membrane at z_A=z_B (the valences of both feed and driving ions are equal). Therefore, it was found that a large flux of ions could be obtained using the monovalent driving ions in Donnan dialysis. On the other hand, the small flux can be obtained using bi- or higher-valent driving ions.     

Multiparticle simulation of ion motion in the ion trap mass spectrometer: Resonant and direct current pulse excitation

A PC-based program that simulates the behavior of a collection of ions is extended to include the effects of collisions with the buffer gas and enhanced visualization methods. The simulations are based on the quadrupole field associated with the actual ion trap electrode structure. Ionization is simulated in such a way as to distribute ionization events randomly over rf phase angles and yield a realistic collection of stored ions. The effects of buffer gas collisions on ion motion during both mass-selective instability and resonance ejection scans are found to include the expected dampening of spatial excursions as well as limitation of the kinetic energy of trapped ions. In both experiments, ion ejection occurs over a number of secular cycles in the vicinity of the theoretical instability point. Activation via a resonant ac signal or a short dc pulse is shown to result in phase-locking of the ions as well as the expected increase in the size of the excursions in the z direction and in ion kinetic energy. Collisions cause dephasing and loss of kinetic energy. Radial dc activation is compared with activation in the axial direction. Experimental data for dc pulse activation of the n-butylbenzene molecular ion are analyzed in phase space and the onset of surface-induced dissociation is correlated with changes in the experimental m/z 91 to m/z 92 fragment ion ratio. Poincaré sections are shown for resonantly excited ions and their value in demonstrating improvement of the resolution of these experiments over conventional mass-selective instability scans is shown.     

Single series peptide fragment ion spectra generated by two-stage collision-induced dissociation in a triple quadrupole

By using nanoelectrospray ionization and a triple quadrupole analyzer, simplified fragment ion spectra of peptides have been recorded by combining skimmer collision-induced dissociation with precursor ion scanning or neutral loss scanning. These pseudo-MS³ scan modes are characterized by two-stage collision-induced dissociation and have been termed sCID/precursor and sCID/neutral loss scan, respectively. By these scan modes, peptide fragment ion spectra can be generated that predominantly show signals of a single fragment ion series, such as the B or Y″ series. Skimmer collision-induced dissociation combined with scanning for neutral loss of 28 generates spectra showing B ions, whereas combination with precursor ion scanning for the Y″₁ ion results in spectra showing Y″ ions for tryptic peptides (Y″₁=m/z 147 for C-terminal lysine, Y″₁=m/z 175 for C-terminal arginine). Sequence information including the direction of the sequence is easily extracted from the simplified fragment ion spectra generated by two-stage collision-induced dissociation, because the scan mode defines the type of fragments observed. The analytical results reported are similar to those that have been achieved in MS³ experiments using a hybrid BEQQ or a pentaquadrupole mass spectrometer (Schey, K. L.; Schwartz, J. C.; Cooks, R. G. Rapid Commun. Mass Spectrom. 1989, 3, 305–309). The pseudo-MS³ technique used in this study has some limitations with respect to sample purity, because there is no step of mass selection before the first stage of collisional activation; however, it has the advantage that a standard triple quadrupole instrumentation can be used.     

Analysis of long-chain fatty acyl coenzyme a thioesters by negative ion fast-atom bombardment mass spectrometry and tandem mass spectrometry

Long-chain acyl Coenzyme A (CoA) is essentially composed of three major chemical groups, fatty acyl-, phosphopantetheino-, and 3′, 5′,-adenosine diphospho-moieties. The negative ion fast-atom bombardment mass spectrometry spectra of long-chain acyl CoA thioesters were characterized by the formation of abundant [M ? H]^? and two distinct classes of fragment ions, one class which retained the acyl group and another class which is related to CoA that contains the phosphopantethene and adenine. The ions which retained the acyl group in the spectrum of palmitoyl CoA appeared at m/z 675, 657, 595, and 577 and were found to decompose by loss of alkylketene observed at m/z 357 and 339. Those ions which retained the adenine group were observed at m/z 426 and 408. In contrast to these ions observed following fast-atom bombardment ionization, tandem mass spectrometry of the [M ? H]^?, from palmitoyl CoA (m/z 1004), yielded the adenine-containing ions as major products and the acyl-containing ions were of low abundance or not detected. These results suggested that the formation of many characteristic ions observed in direct FAB analysis occurred during the desorption process. The unique relationship between ions which involved the transition from acyl-containing ions to only CoA-containing ions by the loss of alkylketene allowed the development of tandem mass spectrometry protocols for the analysis of acyl CoA mixtures. Precursor scans of either m/z 357 or 339 yielded the identification of each species in a complex mixture. Identification of specific species was obtained with a neutral loss scan of the mass for a specific alkylketene.     

Controlled Ion Ejection from an External Trap for Extended m/z Range in FT-ICR Mass Spectrometry

An auxiliary rf waveform of the same amplitude and phase applied to all the rods of an ion accumulation multipole creates an m/z-dependent axial pseudo potential. Controlled decrease of the auxiliary rf amplitude releases ions from the accumulation multipole sequentially from high to low m/z. The slope of the auxiliary rf voltage ramp is adjusted so that ions of different m/z reach the center of the ICR cell at the same time point, which mitigates the typical time dispersion observed in external source FT-ICR and extends the observable mass range for a single data acquisition by 2- to 3-fold. For complex mixture analysis, twice the number of elemental compositions are assigned when the auxiliary rf ejection is applied compared with the standard gated trapping.

Quantification of Competing H3PO4 Versus HPO3 + H2O Neutral Losses from Regioselective 18O-Labeled Phosphopeptides

Abundant neutral losses of 98 Da are often observed upon ion trap CID-MS/MS of protonated phosphopeptide ions. Two competing fragmentation pathways are involved in this process, namely, the direct loss of H₃PO₄ from the phosphorylated residue and the combined losses of HPO₃ and H₂O from the phosphorylation site and from an additional site within the peptide, respectively. These competing pathways produce product ions with different structures but the same m/z values, potentially limiting the utility of CID-MS³ for phosphorylation site localization. To quantify the relative contributions of these pathways and to determine the conditions under which each pathway predominates, we have examined the ion trap CID-MS/MS fragmentation of a series of regioselective ¹⁸O-phosphate ester labeled phosphopeptides prepared using novel solution-phase amino acid synthesis and solid-phase peptide synthesis methodologies. By comparing the intensity of the –100 Da (–H₃PO₃ ¹⁸O) versus –98 Da (–[HPO₃ + H₂O]) neutral loss product ions formed upon MS/MS, quantification of the two pathways was achieved. Factors that affect the extent of formation of the competing neutral losses were investigated, with the combined loss pathway predominantly occurring under conditions of limited proton mobility, and with increased combined losses observed for phosphothreonine compared with phosphoserine-containing peptides. The combined loss pathway was found to be less dominant under ion activation conditions associated with HCD-MS/MS. Finally, the contribution of carboxylic acid functional groups and backbone amide bonds to the water loss in the combined loss fragmentation pathway was determined via methyl esterification and by examination of a phosphopeptide lacking side-chain hydroxyl groups.

Structural characterization and identification of oleanane-type triterpene saponins in Glycyrrhiza uralensis Fischer by rapid-resolution liquid chromatography coupled with time-of-flight mass spectrometry

Oleanane‐type triterpene saponins (OTS) are major active ingredients in Glycyrrhiza uralensis. In this work, a rapid‐resolution liquid chromatography with time‐of‐flight mass spectrometry (RRLC/TOF‐MS) method has been developed to characterize and identify OTS from G. uralensis. The major diagnostic ions and fragmentation pathways from thirteen OTS have been characterized for the first time. At a low fragmentor voltage of 120 V in positive ion mode, the precursor ion [M + H]⁺ or/and [M + Na]⁺ was obtained for accurate determination of molecular formula. When the fragmentor voltage was increased to 425 V, abundant characteristic fragment ions were observed for structural characterization. Neutral losses of sugar moieties, such as glucuronic acid (GlcA, 176 Da), glucose (Glc, 162 Da) and rhamnose (Rha, 146 Da), were commonly observed in the MS spectra for prediction of the sugar number and sequences. Other typical losses included AcOH (60 Da), CH₂O (30 Da), 2 × H₂O (2 × 18 Da) and HCOOH (46 Da) from [Aglycone + H–H₂O]⁺ (named [B]⁺), corresponding to the presence of a C₂₂‐acetyl group, C₂₄‐hydroxyl group, C₂₂‐hydroxyl group or C₃₀‐carboxyl group on the aglycone moiety, respectively. In particular, characteristic ring cleavages of the aglycone moieties on A‐ and B‐rings were observed. Based on the fragmentation patterns of reference compounds, nineteen OTS have been identified in an extract of G. uralensis, thirteen of which were unambiguously identified and the other six were tentatively assigned. Copyright © 2010 John Wiley & Sons, Ltd.     

Fragmentation of the metastable molecular ion of methyl lactate: The formation of oxygen-protonated methanol [CH3OH2]+ involving double hydrogen atom transfers

The spontaneous unimolecular dissociation reaction of methyl lactate (1) ionized by electron impact was investigated by a combination of mass-analyzed ion kinetic energy spectrometry and deuterium labeling. The metastable ions 1^+· decompose in a variety of ways: four fragment peaks are observed at m/z 89, 76, 61, and 45, which correspond to the losses of ?H₃, CO, CH₃?O, and ?OOCH₃, respectively. Double hydrogen atom transfer occurs in the third reaction. The source-generated m/z 61 ions decompose into oxygen-protonated methanols at m/z 33 ([CH₃OH ₂ ⁺ ]) by the loss of CO with double hydrogen atom migration. Both hydroxyl and methyne hydrogen atoms in 1 ^+· are present in the resultant protonated methanols.     

Mass and charge assignment for electrospray ions by cation adduction

The assignment of the mass (m) value from the m/z value for ions with a multiple number of charges (z) in electrospray mass spectra usually utilizes multiple peaks of the same m but different z values, or unit-mass—separated isotopic peaks of the same z value from high resolution spectra. The latter approach is also feasible with much less resolving power using adduct ions of much higher mass separation. The application of this to mixture spectra containing many masses, such as spectra from tandem mass spectrometry (MS/MS) ion dissociation, does not appear to have been pointed out previously. Thus, replacing two protons by one Cu²⁺ ion increases the mass by 61.5 Da, with this shift providing a mass scale for assignment of m and z from this pair of m/z values. The more common Na⁺ adduct peaks provide a 22.0 Da separation, of utility for 1000 resolving power only below approximately 10 kDa. Further, collisional dissociation lowers the degree of Cu²⁺ adduction in the resulting sequence-specific fragment ions much less than that of the corresponding Na⁺ adducts, making the Cu²⁺ adducts far more useful for m and z determination in MS/MS studies.     

Collisional focusing effects in radio frequency quadrupoles

The transmission of ions through a conventional two-dimensional radiofrequency-only (rf) quadrupole has been studied for comparatively high operating pressures between 5 × 10^?4 and 1 × 10^?2 torr. Measurements of signals from mass-resolved analyte ions and total ion currents show that, provided the initial injection ion energy is low (1–30 eV), the ion transmission observed through a small aperture at the exit of the rf quadrupole first increases as the gas pressure increases, reaching a maximum at ? 8 × 10^?3 torr before decreasing at higher pressures. This is in direct contrast to the expectations of classical scattering. This “collisional focusing” appears to be analogous to effects seen in three-dimensional ion traps. The collisional focusing increases with the mass of the ion (not mass-to-charge ratio) for masses up to at least 16,950 u. The collisional focusing of the ions is found to be accompanied by significant losses of axial kinetic energy. A Monte Carlo simulation of the energy loss process is reported that can provide agreement with the observed losses for reasonable collision cross-sections. The results suggest that operation of rf quadrupoles at relatively high pressure may find practical application in sampling ions from high (e.g., atmospheric) pressure ion sources.     

Loss of Internal Backbone Carbonyls: Additional Evidence for Sequence-Scrambling in Collision-Induced Dissociation of y-Type Ions

It is shown that y-type ions, after losing C-terminal H₂O or NH₃, can lose an internal backbone carbonyl (CO) from different peptide positions and yield structurally different product fragment ions upon collision-induced dissociation (CID). Such CO losses from internal peptide backbones of y-fragment ions are not unique to a single peptide and were observed in four of five model peptides studied herein. Experimental details on examples of CO losses from y-type fragment ions for an isotopically labeled AAAAHAA-NH₂ heptapeptide and des-acetylated-α-melanocyte-stimulating hormone (dα-MSH) (SYSMEHFRWGKPV-NH₂) are reported. Results from isotope labeling, tandem mass spectrometry (MSⁿ), and ion mobility-mass spectrometry (IM-MS) confirm that CO losses from different amino acids of m/z-isolated y-type ions yield structurally different ions. It is shown that losses of internal backbone carbonyls (as CID products of m/z-isolated y-type ions) are among intermediate steps towards formation of rearranged or permutated product fragment ions. Possible mechanisms for generation of the observed sequence-scrambled a-“like” ions, as intermediates in sequence-scrambling pathways of y-type ions, are proposed and discussed. ?      

Determination of the fragmentation mechanisms of organophosphorus ions by H2O and D2O atmospheric-pressure ionization tandem mass spectrometry

Dimethylmethyl phosphonate (DMMP), dimethyl phosphite (DMPI), trimethyl phosphite (TMPI) and trimethyl phosphate (TMP) were investigated using H₂O and D₂O atmospheric-pressure ionization (API) tandem mass Spectrometry. All daughter ions could be explained by losses of one or a successive number of stable molecules as opposed to losses of radicals such as the hydride, methyl and methoxy species. Losses of neutral methanol and dimethyl ether and of protonated methanol and formaldehyde ions from all four organophosphorus pseudo-molecular ions were observed. The DMMP and DMPI MH⁺ pseudomolecular ions produced the losses of neutral C₂H₆ and water, respectively. Formaldehyde loss was not observed for the MH⁺ ions, but it was well represented in the decomposition pathways of daughter ions. The D₂O reagent gas highlighted the role of the ionizing proton/ deuteron in the various daughter ions, including m/z 95, 79, 65, 49, 33, 31 and 47. The last ion was found to be isobaric in that m/z 47 and 48 both appeared with similar abundances in the D₂O-API daughter ion mass spectra of TMPI and TMP.     

EPR spectroscopic studies in (30 − x) Li2O-xK2O-10CdO-59B2O3-1MnO2 multi-component glass system

EPR studies were carried out in (30 - x) Li₂O-xK₂O-10CdO-59B₂O₃-1MnO₂ multi-component glass system to understand the effect of the variation in the alkali ratios on the EPR parameters. The observed EPR spectra of Mn²⁺ ion exhibits resonances at g = 2.0, 3.3 and 4.3. The resonance at g = 2.0 is due to Mn²⁺ ions in an environment close to the octahedral symmetry, where as the resonances at g = 3.3 & 4.3 are due to the rhombic surroundings of Mn²⁺ ions. Hyperfine splitting constant values at g = 2.0 and number of paramagnetic centers & paramagnetic susceptibility at different observed resonances were evaluated. These parameters show non linear variation with progressive substitution of Li⁺ ion with K⁺ ions may be due to the changes in cation field strengths and local structural variation due to the variation in mixed alkali ion ratios.