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.     

Decompositions of odd- and even-electron anions derived from deoxy-polyadenylates

Radical anions have been formed via electron transfer from multiply charged 5′-d(AAA)-3′ and 5′-d(AAAA)-3′ anions to CCl₃ ⁺. These ions have been isolated in a quadrupole ion trap operated with helium bath gas at a pressure of 1 mtorr and subjected to resonance excitation (i. e., conventional ion trap collisional activation). Collisional activation of the even-electron species of the same charge state formed directly via electrospray was also performed by using essentially identical conditions. The collisional activation data can be compared directly without ambiguity arising from differences in parent ion internal energies and/or dissociation time frames. Both the odd- and even-electron anions yield extensive sequence-informative fragmentation but show significant differences in the extent of nucleobase loss and in the relative contributions from the various sequence diagnostic dissociation channels. The results of this study indicate that radical anions derived from multiply deprotonated oligo-deoxynucleotides that survive the electron transfer process are stable with respect to fragmentation in the ion trap environment under normal storage conditions and that the unimolecular dissociation behavior of these ions differs from the even-electron anions of the same charge state. These findings suggest, therefore, that odd- and even-electron anions might be used to provide complementary sequence information in cases in which neither ion type provides the full sequence.     

Rapid analysis of polychlorinated biphenyls in the gas phase with resonance-enhanced two-photon ionization: optimal injection of ions into the ion-trap storage/time-of-flight mass spectrometer.

LI-IT-TOFMS (laser ionization/ion-trap storage/time-of-flight mass spectrometry) is expected to be a powerful tool for environmental monitoring. In the research reported here, real-time LI-IT-TOFMS measurements were carried out on gaseous 2-4 chlorinated PCBs in order to evaluate the applicability of an environmental monitoring method. With respect to ion-trap storage for PCBs, we found that the effect was due to the driving RF voltage on the ring electrode in the ion trap. For PCBs ions produced by laser irradiation, we observed that it was more efficient to reach the center of the ion trap by using a gated RF voltage rather than by using a continuous RF voltage. The ion trajectories in the ion trap were simulated by SIMION 7.0. We found that the voltage of the exit end cap electrode affected both the number of ions trapped and the orbit of ions inside the trap cell. Optimization of this parameter was performed using both simulated and experimental results. The achievable PCBs sensitivity for real-time (1 min) measurement using the LI-IT-TOFMS method was found to be in the pptV range (<0.01 mg/m3N) by means of a comparison with the conventional gas sampling/GS-MS method. A satisfactory proportional relationship was confirmed between the laser-based and conventional results.     

High sensitivity and broad dynamic range infrared multiphoton dissociation for a quadrupole ion trap

Dynamic control of bath gas pressure in a quadrupole ion trap (QIT) achieved high sensitivity and broad dynamic range infrared multiphoton dissociation (IRMPD). Conventional IRMPD is not sensitive because the bath gas pressure in the QIT needs to be kept at less than 1 mTorr for an effective dissociation, whereas the pressure should be about 20 mTorr for maximum trapping efficiency during ion accumulation. By switching the bath gas pressure between about 20 mTorr during the ion accumulation period and less than 0.6 mTorr during the IRMPD period, it was possible to achieve both maximum trapping efficiency and effective IRMPD. An optimized method for gas introduction enables the trapping efficiency to remain constant during the accumulation period, which permits a broad dynamic range measurement.     

Determination of trace anions in liquefied petroleum gas using liquid absorption and electrokinetic migration for enrichment followed by ion chromatography

A simple sample enrichment technique, electrokinetic migration enrichment in single phase using a designed device, coupled with ion chromatography is presented for the determination of four anions (H(2)PO(4)(-), Cl(-), NO(3)(-), and SO(4)(2-)) in liquefied petroleum gas by liquid adsorption. The electrokinetic migration enrichment is based on the phenomenon of ion electrokinetic migration to the opposite electrode. When the anions migrated to the anode in a smaller volume chamber under the electric field, the concentration was realized. The main parameters affecting enrichment efficiency of applied voltage and enrichment time were investigated. The ion chromatography condition for anions separation was also studied. Under the optimal electrokinetic migration enrichment and ion chromatography conditions, the four anions were detected simultaneously with good linear relationship (r(2) = 0.9908-0.9968) and high precisions (less than 5% of the relative standard deviations of peak areas). The limits of detection of anions (S/N of 3) were in the range of 8-600 μg L(-1). The enrichment factors of the four anions ranged from 3.1 to 5.8. The established method was successfully applied to the analysis of the trace anions in liquefied petroleum gas by liquid adsorption with satisfactory results. The advantages of this method are simple operation and low cost.     

Low-energy electron attachment to SF6. III. From thermal detachment to the electron affinity of SF6

The thermal attachment of electrons to SF(6) is measured in a flowing-afterglow Langmuir-probe apparatus monitoring electron concentrations versus axial position in the flow tube. Temperatures between 300 and 670 K and pressures of the bath gas He in the range of 0.3-9 Torr are employed. Monitoring the concentrations of SF(6)(-) and SF(5)(-), the latter of which does not detach electrons under the applied conditions, an onset of thermal detachment and dissociation of SF(6) at temperatures above about 530 K is observed. Analysis of the mechanism allows one to deduce thermal detachment rate coefficients. Thermal dissociation rate coefficients for the reaction SF(6)(-)-->SF(5)(-)+F can only be estimated by unimolecular rate theory based on the results from Part I and II of this series. Under the applied conditions they are found to be smaller than detachment rate coefficients. Combining thermal attachment and detachment rates in a third-law analysis, employing calculated vibrational frequencies of SF(6) and SF(6)(-), leads to the electron affinity (EA) of SF(6)(-). The new value of EA=1.20(+/-0.05) eV is significantly higher than previous recommendations which were based on less direct methods.     

Corona Discharge Ionization Source for a Planar High-Field Asymmetric Waveform Ion Mobility Spectrometer

A corona discharge (CD) ionization source was prepared for a planar high-field asymmetric waveform ion mobility spectrometer (FAIMS). The effects of discharge current and discharge distance on ionization efficiency were investigated; and the electric field dependence of the ion injection in the reaction region was studied. The results showed that the discharge current of CD source had good linearity with the intensity of reactant ion peak (RIP), and the RIP intensity increased to a stable level at the discharge distance of >5 mm. An injection electrode was introduced to improve the ionization efficiency. A square-wave voltage applied to the electrode was found to provide optimal performance of ion injection and utilization. The operating parameters of the CD-FAIMS were optimized to achieve trace level detection of dimethyl methylphosphonate (DMMP) sample. The detection limit for DMMP was 0.5 µg/m^3.     

Collision-induced dissociation of glycero phospholipids using electrospray ion-trap mass spectrometry.

Characterisation of phospholipids was achieved using collision-induced dissociation (CID) with an ion-trap mass spectrometer. The product ions were compared with those obtained with a triple quadrupole mass spectrometer. In the negative ion mode the product ions were mainly sn-1 and sn-2 lyso-phospholipids with neutral loss of ketene in combination with neutral loss of the polar head group. Less abundant product ions were sn-1 and sn-2 carboxylate anions. CID using a triple quadrupole mass spectrometer, however, gave primarily the sn-1 and sn-2 carboxylate anions together with lyso-phosphatidic acid with neutral loss of water. For the ion trap a charge-remote-type mechanism is proposed for formation of the lyso-phospholipid product ions by loss of alpha-hydrogen on the fatty acid moiety, electron rearrangement and neutral loss of ketene. A second mechanism involves nucleophilic attack of the phosphate oxygen on the sn-1 and sn-2 glycerol backbone to form carboxylate anions with neutral loss of cyclo lyso-phospholipids. CID (MS(3) and MS(4)) of the lyso-phospholipids using the ion-trap gave the same carboxylate anions as those obtained with a triple quadrupole instrument where multiple collisions in the collision cell are expected to occur. The data demonstrate that phospholipid species determination can be performed by using LC/MS(n) with an ion-trap mass spectrometer with detection of the lyso-phospholipid anions. The ion-trap showed no loss in sensitivity in full scan MS(n) compared to multiple reaction monitoring data acquisition. In combination with on-line liquid chromatography this feature makes the ion-trap useful in the scanning modes for rapid screening of low concentrations of phospholipid species in biological samples as recently described (Uran S, Larsen A, Jacobsen PB, Skotland T. J. Chromatogr. B 2001; 758: 265).     

Competition between resonance ejection and ion dissociation during resonant excitation in a quadrupole ion trap

The competition between ion dissociation and ion ejection during resonant excitation in a quadrupole ion trap is investigated. Ions of similar mass but with a range of critical energies for the onset of dissociation have been examined. The effects of the amplitude and duration of the resonant excitation, the well depth in which the ions are trapped, and the pressure and nature of the collision gas are explored. Once the onset of ion ejection is reached, the rate of ion ejection increases with increased amplitude of the resonant excitation signal. The rate of ejection decreases or stays constant as a function of the duration of the resonant excitation, depending upon the ion species being excited. Increasing the trapping well depth increases the relative amount of dissociation versus ejection as does increasing the pressure of the bath gas. Adding heavier bath gases lowers the onset of ion dissociation and raises the onset of ion ejection.     

Energy-resolved collisional activation of dimethyl phosphonate and dimethyl phosphite ions in a quadrupole ion trap and a triple quadrupole mass spectrometer

Collision-activated dissociation spectra of dimethyl phosphonate and dimethyl phosphite ions were measured as a function of the amplitude of a supplementary AC voltage applied across the end-caps of an ion-trap mass spectrometer. These spectra yield breakdown graphs which bear a close resemblance to those obtained by varying collision energy in a triple-quadrupole mass spectrometer operating under multiple-collision conditions. Variation in the time of excitation at the resonance frequency provides an alternative route to breakdown graphs. The results demonstrate that energy deposition occurs via multiple activating collisions in the ion trap. Maximum energy deposition observed is somewhat smaller under normal operating conditions in the ion trap than in the triple-quadrupole mass spectrometer.     

Electrospray ionization ion mobility spectrometry of carboxylate anions: ion mobilities and a mass-mobility correlation

A number of carboxylate anions spanning a mass range of 87-253 amu (pyruvate, oxalate, malonate, maleate, succinate, malate, tartarate, glutarate, adipate, phthalate, citrate, gluconate, 1,2,4-benzenetricarboxylate, and 1,2,4,5-benzenetetracarboxylate) were investigated using electrospray ionization ion mobility spectrometry. Measured ion mobilities demonstrated a high correlation between mass and mobility in both N2 and CO2 drift gases. Such a strong mass-mobility correlation among structurally dissimilar ions suggests that the carboxylate functional group that these ions have in common is the source of the correlation. Computational analysis was performed to determine the most stable conformation of the studied carboxylate anions in the gas phase under the current experimental conditions. This analysis indicated that the most stable conformations for multicarboxylate anions included intramolecular hydrogen-bonded ring structures formed between the carboxylate group and the neutral carboxyl group. The carboxylate anions that form ring confirmations generally show higher ion mobility values than those that form extended conformations. This is the first observation of intramolecular hydrogen-bonded ring conformation of carboxylate anions in the gas phase at atmospheric pressure.     

Triboelectric spray ionization

Triboelectric spray ionization (TESI) is a variation of electrospray ionization (ESI) using common instrumental components, including gas flow, solvent flow rate and heat, the only difference being the use of a high‐voltage power supply for ESI or a static charge for TESI. The ionization of solvent or analyte is due to the electrostatic potential difference formed between the spray electrode and counter electrode. The ion source contains a pneumatic spray operated over a range of flow rates (0.15–1.5 µl/min) and gas pressures (0–100). This new design contains a standalone spray assembly and an optional metal mesh in front of the spray. There are several parameters that affect the performance during ionization of molecules including the flow rate of solvent, gas pressure, temperature, solvent acidity, distance and potential difference between emitter and counter electrode. A variable electrostatic potential can be applied for higher ionization efficiency. The new ionization method was successfully applied to solutions of various proteins under different conditions. The same charge‐state distributions compared to other ESI techniques are observed for all the protein samples. The unique feature of TESI is very efficient spraying by using a natural electrostatic potential even at the potential that a human body can produce. This provides very gentle ionization efficiency of peptides and proteins in different solvents. Copyright © 2013 John Wiley & Sons, Ltd.     

An unexpected ion-molecule adduct in negative-ion collision-induced decomposition ion-trap mass spectra of halogenated benzoic acids

The ion observed at m/z 145 when product ion spectra of iodobenzoate anions are recorded using ion-trap mass spectrometers corresponds to the adduct ion [I(H(2)O)](-). The elements of water required for the formation of this adduct do not originate from the precursor ion but from traces of moisture present in the helium buffer gas. A collision-induced decomposition (CID) spectrum recorded from the [M-H](-) ion (m/z 251) derived from 3-iodo[2,4,5,6-(2)H(4)]benzoic acid also showed an ion at m/z 145. This observation confirmed that the m/z 145 is not a product ion resulting from a direct neutral loss from the carboxylate anion. (79)Bromobenzoate anions produce similar results showing an ion at m/z 97 for [(79)Br(H(2)O)](-). The ion-molecule reaction observed here is unique to ion-trap mass spectrometers since a corresponding ion was not observed under our experimental conditions in spectra recorded with in-space tandem mass spectrometers such as triple quadrupole or quadrupole time-of-flight instruments.     

Structural characterization and differentiation of modified isomeric tryptophans

Different mass spectrometry (MS) techniques have been applied to the study of modified tryptophan isomers obtained by photochemical reactions. The gas phase behavior of the molecular ions and the most abundant fragment ions produced under electron ionization has been selectively studied by MS/MS experiments. Both the fragmentation reactions occurring in the ion source, as well as those produced under collision-induced dissociation conditions have allowed to characterize and differentiate each isomer from the others. Investigation of a bisubstituted derivative has been useful in the rationalization of the gas phase behavior of this series of modified tryptophans. This study has allowed the evaluation of the role played by the substituents and their positions at the indolic ring on the gas phase decompositions that are distinctive and selective for each isomer. The occurrence of regiospecific reactions suggests that isomerization phenomena do not occur either in the molecular ions or in the main fragment ions in the gas phase.     

Negative electron transfer dissociation Fourier transform mass spectrometry of glycosaminoglycan carbohydrates

Electron transfer through gas phase ion-ion reactions has led to the widespread application of electron- based techniques once only capable in ion trapping mass spectrometers. Although any mass analyzer can in theory be coupled to an ion-ion reaction device (typically a 3-D ion trap), some systems of interest exceed the capabilities of most mass spectrometers. This case is particularly true in the structural characterization of glycosaminoglycan (GAG) oligosaccharides. To adequately characterize highly sulfated GAGs or oligosaccharides above the tetrasaccharide level, a high resolution mass analyzer is required. To extend previous efforts on an ion trap mass spectrometer, negative electron transfer dissociation coupled with a Fourier transform ion cyclotron resonance mass spectrometer has been applied to increasingly sulfated heparan sulfate and heparin tetrasaccharides as well as a dermatan sulfate octasaccharide. Results similar to those obtained by electron detachment dissociation are observed.     

Anion photoelectron imaging of deprotonated thymine and cytosine

We report the anion photoelectron spectra of deprotonated thymine and cytosine at 3.496 eV photodetachment energy using velocity-mapped imaging. The photoelectron spectra of both species exhibit bands resulting from detachment transitions between the anion ground state and the ground state of the neutral radical. Franck-Condon simulations identify the anion isomers that contribute to the observed photoelectron spectrum. For both thymine and cytosine, the photoelectron spectra are consistent with anions formed by removal of a proton from the N atom that normally attaches to the sugar in the nucleotide (N1). For deprotonated thymine, the photoelectron spectrum shows a band due to a ring breathing vibration excited during the photodetachment transition. The electron affinity for the dehydrogenated thymine radical is determined as 3.250 +/- 0.015 eV. For deprotonated cytosine, the photoelectron spectrum lacks any resolved structure and the electron affinity of the dehydrogenated cytosine radical is determined to be 3.037 +/- 0.015 eV. By combining the electron affinity with previously measured gas phase acidities of thymine and cytosine, we determine the bond dissociation energy for the N-H bond that is broken.     

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.     

Infrared multiple photon dissociation in the quadrupole ion trap via a multipass optical arrangement

The design of a novel multipass optical arrangement for use with infrared multiple photon dissociation (IRMPD) in the quadrupole ion trap is presented. This design circumvents previous problems of limited IR laser power, small IR absorption cross sections for many molecules, and the limited ion statistics of trapping and detection of ions for IRMPD in the quadrupole ion trap. In contrast to previous designs that utilized the quadrupole ion store, the quadrupole ion trap was operated in the mass selective instability mode with concurrent resonance ejection. The instrumental design consisted of a modified ring electrode with three spherical concave mirrors mounted on the inner surface of the ring. This modified design allowed for eight laser passes across the radial plane of the ring electrode. IRMPD of protonated bis(2-methoxyethyl)ether (diglyme) was used to characterize the performance of the multipass ring electrode. Two consecutive reactions for the IRMPD of protonated diglyme were observed with a lower energy channel predominant at less than 0.6 J (irradiation times from 1 to 30 ms) and a second channel predominant at energies greater than 0.6 J (irradiation times > 30 ms). Other studies presented include a discussion of the dissociation kinetics of protonated diglyme, the use of a pulsed valve for increased trapping efficiency of parent ion populations, and the effects of laser wavelength and of ion residence time in the radial plane of the ring electrode on photodissociation efficiency.     

Electrospray ionisation tandem mass spectrometry in the characterisation of isomeric benzofurocoumarins

A set of aminoalkoxy-substituted, differently annullated furocoumarins, differing in the position of the aminoalkoxy chain and in the unsaturation level of the fused ring, has been subjected to electron impact and electrospray ionisation (ESI) experiments. In order to achieve a distinct characterisation of isomeric compounds, which partially failed under electron impact conditions, collision-induced dissociation experiments were performed on protonated molecules. The breakdown curves obtained by varying the tickle voltage on an ion trap ESI instrument led to the desired characterisation.     

Numerical Modeling of an RF Argon–Silane Plasma with Dust Particle Nucleation and Growth

A one-dimensional numerical model and simulation results are presented for a capacitively-coupled radio frequency parallel-plate argon–silane dusty plasma. The model includes self-consistently coupled numerical modules, including a plasma fluid model, a sectional aerosol model, and a simple chemistry model to predict rates of particle nucleation and surface growth. Operating conditions considered include 13.56 MHz frequency, 100 mTorr pressure, a 4-cm electrode gap, gas flow through the top electrode with a 30:1 ratio of argon to silane, and applied radio frequency voltage amplitude of either 100 or 250 V. In the higher voltage case two lobes of relatively large particles are formed by ion drag, while fresh nucleation occurs in the void between these lobes. It is shown that the reason that fresh nucleation occurs in the void involves an interplay among several coupled phenomena, including nanoparticle transport, the plasma potential profile, and trapping of silicon hydride anions that drive nucleation in this system.