Characterization of protonated phospholipids as fragile ions in quadrupole ion trap mass spectrometry

Some ions exhibit "ion fragility" in quadrupole ion trap mass spectrometry (QIT-MS) during mass analysis with resonance ejection. In many cases, different ions generated from the same compound exhibit different degrees of ion fragility, with some ions (e.g., the [M+H](+) ion) stable and other ions (e.g., the [M+Na](+) ion) fragile. The ion fragility for quadrupole ion trap (QIT) mass spectrometry (MS) for protonated and sodiated ions of three phospholipids, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine, PC (16:0/16:0), 1,2-dipalmitoyl-sn-glycero-3-phophoethanolamine, PE (16:0/16:0), and N-palmitoyl-D-erythro-sphingosylphosphorylcholine, SM (d18:1/16:0), was determined using three previously developed experiments: 1) the peak width using a slow scan speed, 2) the width of the isolation window for efficient isolation, and 3) the energy required for collision-induced dissociation. In addition, ion fragility studies were designed and performed to explore a correlation between ion fragility in QIT mass analysis and ion fragility during transport between the ion source and the ion trap. These experiments were: 1) evaluating the amount of thermal-induced dissociation as a function of heated capillary temperature, and 2) determining the extent of fragmentation occurring with increasing tube lens voltage. All phospholipid species studied exhibited greater ion fragility as protonated species in ion trap mass analysis than as sodiated species. In addition, the protonated species of both SM (d18:0/16:0) and PC (16:0/16:0) exhibited greater tendencies to fragment at higher heated capillary temperatures and high tube lens voltages, whereas the PE (16:0/16:0) ions did not appear to exhibit fragility during ion transport.     

Selective chemical ionization of nitrogen and sulfur heterocycles in petroleum fractions by ion trap mass spectrometry

A procedure is reported for the selective ammonia chemical ionization of some nitrogen and sulfur heterocycles in petroleum fractions using ion trap mass spectrometry (ITMS). The ion trap scan routine is designed to optimize the population of ammonium reagent ions and eject from the trap (by radio frequency/direct current isolation) electron ionization products formed during reagent ion formation prior to ionization of the sample. The ITMS procedure is compared with standard ion trap detector and conventional quadrupole ammonia chemical ionization for the determination of nitrogen and sulfur heterocycles in gas oil and kerosine samples. Greatly enhanced selectivity is shown for the ITMS procedure by suppression of competing charge-exchange processes.     

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.     

Screening of methylenedioxyamphetamine‐ and piperazine‐derived designer drugs in urine by LC–MS/MS using neutral loss and precursor ion scan

This study describes a method for the screening of methylenedioxyamphetamine‐ and piperazine‐derived compounds in urine by liquid chromatography‐tandem mass spectrometry. These substances, characterized by possessing common moieties, are screened using precursor ion and neutral loss scan mode and then quantified in multiple reaction monitoring acquisition mode. Based on the product‐ion spectra of different known molecules, chosen as ‘model’, characteristic neutral losses and product ions were selected: piperazines were detected in precursor ion scan of m/z 44 and neutral loss of 43 and 86 while amphetamines in precursor ion scan of m/z 133, 135 and 163. The applicability of the screening approach was studied in blank urine spiked with selected analytes and processed by solid‐phase extraction. Linearity, matrix effect, precision, accuracy, limits of detection and limits of quantification were evaluated both for the screening and the quantification methods. The ability of the screening method to provide semi‐quantitative data was demonstrated. This method appears to be a useful tool for the identification of designer drugs derived from piperazines or methylenedioxyamphetamines and can be potentially applied to other drug classes. Copyright © 2013 John Wiley & Sons, Ltd.     

Asymmetric rectilinear ion trap with unidirectional ion ejection capability

In this paper, the shapes of the electrodes are modified based on a rectilinear ion trap to achieve unidirectional ejection of ions. The designed asymmetric rectilinear ion trap (ARIT) analyzer adds convex and concave circular structures with a height of 0.5 mm on the two X‐electrodes, so that the electric field center of the ion trap is inclined to the concave side. The electric field lines of the convex side are compressed to the concave side. Both simulations and experimental results show that ions are more likely to emit from the slit on the concave side plate when performing ion resonance ejection. The mass spectrum signal intensity can reach more than twice that of the original rectilinear trap when using only one detector. Calculations of the electric field components in the trap show that the even‐order higher field proportion in the ion trap has not been significantly affected. Combined with the experimental test results, the study further confirmed that the developed ARIT has no significant loss in mass resolution, tandem mass spectrometry capability, and quantitative analysis capability. The proposed asymmetric structure modification scheme can achieve single‐side ejection without significantly affecting other performances of the analyzer, which provides a new idea for the structural optimization of the subsequent ion trap analyzers.     

Analytical atomic spectroscopy using ion trap devices

Investigations using ion trap devices for analytical atomic spectroscopy purposes have focused on the use of an inductively coupled plasma (ICP) ion source with ion trap mass spectrometric (ITMS) detection. Initial studies were conducted with an instrument assembled by simply appending an ion trap as the detector to a fairly conventional ICP/MS instrument, i.e. leaving an intermediate linear quadrupole between the plasma source and the ion trap. The principal advantages found with this system include the destruction of nearly all problematic and typical ICP/MS polyatomic ions (e.g., ArH(+), ArO(+), ArCl(+), Ar(2)(+), etc) and a dramatic reduction of the primary plasma source ion, Ar(+). These results prompted the development of a second-generation plasma source ion trap instrument in which direct coupling of the ICP and ion trap has been effected (i.e. no intermediate linear quadrupole); the same performance benefits have been largely preserved. Initial operation of this instrument is described, characterized, and compared to the originally described ICP/ITMS and conventional ICP/MS systems. In addition, experiments aimed at improving ICP/ITMS sensitivity and selectivity using broadband resonance excitation techniques are described. Finally, the potential for laser optical detection of trapped ions for analytical purposes is speculated upon.     

CID of singly charged antioxidants applied in lubricants by means of a 3D ion trap and a linear ion trap-Orbitrap mass spectrometer

The aim of this study was to investigate the fragmentation behavior induced by low‐energy collision‐induced dissociation (LE‐CID) of four selected antioxidants applied in lubricants, by two different types of ion trap mass spectrometers: a three‐dimensional ion trap (3D‐IT) and a linear IT (LIT) Orbitrap MS. Two sterically hindered phenols and two aromatic amines were selected as model compounds representing different antioxidant classes and were characterized by positive‐ion electrospray ionization (ESI) and LE‐CID. Various types of molecular ions (e.g. [M]^+?, [M + H]⁺, [M + NH₄]⁺ or [M + Na]⁺) were used as precursor ions generating a significant number of structurally relevant product ions. Furthermore, the phenolic compounds were analyzed by negative‐ion ESI. For both IT types applied for fragmentation, the antioxidants exhibited the same unusual LE‐CID behavior: (1) they formed stable radical product ions and (2) C? C bond cleavages of aliphatic substituents were observed and their respective cleavage sites depended on the precursor ion selected. This fragmentation provided information on the type of structural isomer usually not obtainable for branched aliphatic substituents utilizing LE‐CID. Comparing the two instruments, the main benefit of applying the LIT‐Orbitrap was direct access to elemental composition of product ions enabling unambiguous interpretation of fragmentation trees not obtainable by the 3D‐IT device (e.g. loss of isobaric neutrals). It should be emphasized that the types of product ions formed do not depend on the type of IT analyzer applied. For characterizing degradation products of antioxidants, the LIT‐Orbitrap hybrid system, allowing the determination of accurate m/z values for product ions, is the method of choice. Copyright © 2011 John Wiley & Sons, Ltd.     

Analytical atomic spectroscopy using ion trap devices

Investigations using ion trap devices for analytical atomic spectroscopy purposes have focused on the use of an inductively coupled plasma (ICP) ion source with ion trap mass spectrometric (ITMS) detection. Initial studies were conducted with an instrument assembled by simply appending an ion trap as the detector to a fairly conventional ICP/MS instrument, i.e. leaving an intermediate linear quadrupole between the plasma source and the ion trap. The principal advantages found with this system include the destruction of nearly all problematic and typical ICP/MS polyatomic ions (e.g., ArH⁺, ArO⁺, ArCl⁺, Ar₂⁺, etc) and a dramatic reduction of the primary plasma source ion, Ar⁺. These results prompted the development of a second-generation plasma source ion trap instrument in which direct coupling of the ICP and ion trap has been effected (i.e. no intermediate linear quadrupole); the same performance benefits have been largely preserved. Initial operation of this instrument is described, characterized, and compared to the originally described ICP/ITMS and conventional ICP/MS systems. In addition, experiments aimed at improving ICP/ITMS sensitivity and selectivity using broadband resonance excitation techniques are described. Finally, the potential for laser optical detection of trapped ions for analytical purposes is speculated upon.     

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

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

Signal losses in pesticide analysis using the broadband isolation waveform in a commercial quadrupole ion trap to accomplish precursor ion isolation in tandem mass spectrometry

Signal losses due to precursor ion isolation in a quadrupole-ion-trap mass spectrometer were studied using selected pesticides as model compounds. These signal losses originate from isolations of ion populations employing the broadband isolation (bbiso) waveform used in the Varian quadrupole ion-trap precursor ion isolation protocol. Signal losses were found to be ‘precursor ion structure’ dependent upon isolation using the bbiso. The effect of the bbiso waveform on the ionic structure and nature of substituents on the precursor ion was investigated. Isolation of old electron radical molecular ions of the type [M^+?] showed remarkable signal losses compared with isolation of fragment ions derived from the same compounds. The impact of the bbiso waveform on the response of the instrument using mass spectrometry/mass spectrometry and the bbiso waveform was also examined. The response of the instrument as related to the calculated Instrument Detection Limits was observed to parallel ion population losses.     

Gas chromatography/ion trap tandem mass spectrometry for the analysis of halobenzenes in soils by solid-phase microextraction

Headspace solid-phase microextraction combined with gas chromatography/ion trap tandem mass spectrometry (HS-SPME/GC/ITMS/MS) was used for the analysis of 12 halobenzenes from soil samples. For MS/MS optimisation, the experiments were performed by precursor ion selection and software controlled operations. Collision-induced dissociation (CID) can be achieved by two different approaches, resonant and non-resonant excitation modes. Different results were obtained using the two approaches, and the resonant excitation mode was chosen as the best for all halobenzenes. Parameters such as the CID excitation amplitude, excitation RF storage level and CID bandwidth frequency were optimised to maximise the formation of halobenzene product ions. A 100-microm polydimethylsiloxane fibre was used for the isolation and preconcentration of the analytes. The HS-SPME/GC/ITMS/MS method was applied to the analysis of halobenzenes in an agricultural soil sample. The halobenzenes were quantified by standard addition, which led to good reproducibility (RSD between 4.7 and 9.2%) and detection limits in the low pg/g range. The method was validated by comparing the results with those obtained in a European inter-laboratory exercise.     

Laser desorption lamp ionization source for ion trap mass spectrometry

A two‐step laser desorption lamp ionization source coupled to an ion trap mass spectrometer (LDLI‐ITMS) has been constructed and characterized. The pulsed infrared (IR) output of an Nd:YAG laser (1064 nm) is directed to a target inside a chamber evacuated to ~15 Pa causing desorption of molecules from the target's surface. The desorbed molecules are ionized by a vacuum ultraviolet (VUV) lamp (filled with xenon, major wavelength at 148 nm). The resulting ions are stored and detected in a three‐dimensional quadrupole ion trap modified from a Finnigan Mat LCQ mass spectrometer operated at a pressure of ≥ 0.004 Pa. The limit of detection for desorbed coronene molecules is 1.5 pmol, which is about two orders of magnitude more sensitive than laser desorption laser ionization mass spectrometry using a fluorine excimer laser (157 nm) as the ionization source. The mass spectrum of four standard aromatic compounds (pyrene, coronene, rubrene and 1,4,8,11,15,18,22,25‐octabutoxy‐29H,31H‐phthalocyanine (OPC)) shows that parent ions dominate. By increasing the infrared laser power, this instrument is capable of detecting inorganic compounds. Copyright © 2015 John Wiley & Sons, Ltd.     

Infrared ion spectroscopy inside a mass‐selective cryogenic 2D linear ion trap

We demonstrate operation of the first cryogenic 2D linear ion trap (LIT) with mass‐selective capabilities. This trap presents a number of advantages for infrared ion “action” spectroscopy studies, particularly those employing the “tagging/messenger” spectroscopy approach. The high trapping efficiencies, trapping capacities, and low detection limits make 2D LITs a highly suitable choice for low‐concentration analytes from scarce biological samples. In our trap, ions can be cooled down to cryogenic temperatures to achieve higher‐resolution infrared spectra, and individual ions can be mass selected prior to irradiation for a background‐free photodissociation scheme. Conveniently, multiple tagged analyte ions can be mass isolated and efficiently irradiated in the same experiment, allowing their infrared spectra to be recorded in parallel. This multiplexed approach is critical in terms of increasing the duty cycle of infrared ion spectroscopy, which is currently a key weakness of the technique. The compact design of this instrument, coupled with powerful mass selection capabilities, set the stage for making cryogenic infrared ion spectroscopy viable as a bioanalytical tool in small molecule identification.     

Probing the mechanisms of the self ion-molecule reactions of dopamine in an ion trap mass spectrometer

Our previous work was the first to report [M+CH](+) and [M+C(2)H(3)](+) ions in the self ion-molecule reactions (SIMR) of two aza-crown ethers in an ion trap mass spectrometer (ITMS). In this study, the CH and C(2)H(3) addition ions were also found in the SIMR of dopamine. The SIMR of dopamine lead to the formation of the protonated molecules ([M+H](+)), of adduct ions ([M+F](+), where F represents fragment ions), and of [M+CH](+), [M+C(2)H(3)](+) and [2M+H](+) ions. Based on the combination of the results of isolation experiments and semi-empirical calculations, the reactive site for the formation of the [M+H](+) and [M+CH](+) ions of dopamine is proposed to be the amino group.     

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.     

Identification and quantification of atractylenolide I and atractylenolide III in Rhizoma Atractylodes Macrocephala by liquid chromatography–ion trap mass spectrometry

Rhizoma Atractylodes Macrocephala (RAM) is an important traditional Chinese medicinal herb that is used for treatment of dyspepsia and anorexia. The active ingredients, atractylenolide I (AO‐I) and atractylenolide III (AO‐III), were identified by direct‐injection ion trap‐mass spectrometry (IT‐MS) for collecting MSⁿ spectra. The major fragment ions of AO‐I and AO‐III were confirmed by MSⁿ both in negative ion mode and in positive ion mode. The possible main cleavage pathway of fragment ions was studied. The determinations of AO‐I and AO‐III were accomplished by liquid chromatography (LC) with UV and MS. The analytes provided good signals corresponding to the protonated molecular ions [M + H]⁺ and product ions. The precursor ions and product ions for quantification of AO‐III and AO‐I were m/z 249 → 231 and m/z 233 → 215, respectively, using selected ion monitoring by LC‐IT‐MS. Two methods were evaluated for a number of validation characteristics (repeatability, limit of detection, calibration range, and recovery). MS provides a high selectivity and sensitivity for determination of AO‐III and AO‐I in positive mode. After optimization of the methods, separation, identification and quantification of the two components in RAM were comprehensively tested by HPLC with UV and MS. Copyright © 2012 John Wiley & Sons, Ltd.     

Fragmentation of phosphopeptides by atmospheric pressure MALDI and ESI/ion trap mass spectrometry

An investigation of phosphate loss from phosphopeptide ions was conducted, using both atmospheric pressure matrix-assisted laser desorption/ionization (AP MALDI) and electrospray ionization (ESI) coupled to an ion trap mass spectrometer (ITMS). These experiments were carried out on a number of phosphorylated peptides in order to investigate gas phase dephosphorylation patterns associated with phosphoserine, phosphothreonine, and phosphotyrosine residues. In particular, we explored the fragmentation patterns of phosphotyrosine containing peptides, which experience a loss of 98 Da under collision induced dissociation (CID) conditions in the ITMS. The loss of 98 Da is unexpected for phosphotyrosine, given the structure of its side chain. The fragmentation of phosphoserine and phosphothreonine containing peptides was also investigated. While phosphoserine and phosphothreonine residues undergo a loss of 98 Da under CID conditions regardless of peptide amino acid composition, phosphate loss from phosphotyrosine residues seems to be dependent on the presence of arginine or lysine residues in the peptide sequence.     

A fragmentation study of isoflavones in negative electrospray ionization by MSn ion trap mass spectrometry and triple quadrupole mass spectrometry

This study has elucidated the fragmentation pathway for deprotonated isoflavones in electrospray ionization using MS(n) ion trap mass spectrometry and triple quadrupole mass spectrometry. Genistein-d(4) and daidzein-d(3) were used as references for the clarification of fragment structures. To confirm the relationship between precursor and product ions, some fragments were traced from MS(2) to MS(5). The previous literature for the structurally related flavones and flavanones located the loss of ketene (C(2)H(2)O) to ring C, whereas the present fragmentation study for isoflavones has shown that the loss of ketene occurs at ring A. In the further fragmentation of the [M-H-CH(3)](-*) radical anion of methoxylated isoflavones, loss of a hydrogen atom was commonly found. [M-H-CH(3)-CO-B-ring](-) is a characteristic fragment ion of glycitein and can be used to differentiate glycitein from its isomers. Neutral losses of CO and CO(2) were prominent in the fragmentation of deprotonated anions in ion trap mass spectrometry, whereas recyclization cleavage accounted for a very small proportion. In comparison with triple quadrupole mass spectrometry, ion trap MS(n) mass spectrometry has the advantage of better elucidation of the relationship between precursor and product ions.     

Metastable atom‐activated dissociation mass spectrometry: leucine/isoleucine differentiation and ring cleavage of proline residues

Extensive backbone fragmentation resulting in a‐, b‐, c‐, x‐, y‐ and z‐type ions is observed of singly and doubly charged peptide ions through their interaction with a high kinetic energy beam of argon or helium metastable atoms in a modified quadrupole ion trap mass spectrometer. The ability to determine phosphorylation‐sites confirms the observation with previous reports and we report the new ability to distinguish between leucine and isoleucine residues and the ability to cleave two covalent bonds of the proline ring resulting in a‐, b‐, x‐, y‐, z‐ and w‐type ions. The fragmentation spectra indicate that fragmentation occurs through nonergodic radical ion chemistry akin to electron capture dissociation (ECD), electron transfer dissociation (ETD) and electron ionization dissociation mechanisms. However, metastable atom‐activated dissociation mass spectrometry demonstrates three apparent benefits to ECD and ETD: (1) the ability to fragment singly charged precursor ions, (2) the ability to fragment negatively charged ions and (3) the ability to cleave the proline ring that requires the cleavage of two covalent bonds. Helium metastable atoms generated more fragment ions than argon metastable atoms for both substance P and bradykinin regardless of the precursor ion charge state. Reaction times less than 250 ms and efficiencies approaching 5% are compatible with on‐line fragmentation, as would be desirable for bottom‐up proteomics applications. Copyright © 2009 John Wiley & Sons, Ltd.     

Optimization and application of high-resolution gas chromatography with ion trap tandem mass spectrometry to the determination of polychlorinated biphenyls in atmospheric aerosols

Optimization of the Finnigan GCQ ion trap mass spectrometry (ITMS) system and a clean-up procedure were carried out in order to apply high-resolution gas chromatography-tandem mass spectrometry for the analysis of polychlorinated biphenyls (PCBs) in aerosols. Six ITMS operating parameters, including isolation time, excitation voltage, excitation time, "q" value, ion source temperature and electron energy were adjusted in order to optimize the instrument analytical performance. The adjustment of all parameters substantially increased the sensitivity of ITMS in the MS-MS mode. Changes in isolation time did not particularly affect ITMS sensitivity while ion source temperature had the strongest influence. After optimization, a limit of detection of 600 fg/microl with S/N varying from 8 up to 91 was achieved. The application of the optimized ITMS parameters conjointly with the developed clean-up procedure resulted in method detection limits of 10-20 fg/m3 for the determination of PCBs, in the particulate and gas phase of the atmospheric aerosol of background areas in the Eastern Mediterranean and Sweden.