Molecular Ion Species Research Articles

Gas-phase reactions during reactive laser ablation of Ti and Si in N2 and O2 rarefied atmospheres analysed using energy-dispersive mass spectrometry

We have utilised energy-dispersive mass spectrometry to investigate the energy and composition of ionic products formed during the pulsed laser ablation of Ti and Si targets. Specifically, we have observed the evolution of molecular ion species in backgrounds of O2 and N2 over a wide range of pressures (10−7–10−1 mbar). The yields of these species are significantly dependent upon the background gas pressure and, therefore, number density of the background species and the implied collisional frequency. We propose an interpretation of the relative yields of the species based upon their gas-phase binding energies and reaction cross-sections.

Application of electrospray mass spectrometry to the characterization of hydroxymethylphosphonium salts, -phosphines, and their oxide, sulfide and selenide derivatives

Air-stable hydroxymethylphosphonium salts, readily synthesized from the appropriate primary, secondary or tertiary phosphine, formaldehyde and HCl, provide an excellent means of characterizing phosphines by electrospray mass spectrometry. Results are discussed in terms of fragmentation and aggregation of the parent molecular ion species, where loss of a CH 2OH group is a common fragmentation route. For monophosphonium salts, parent ions M + were typically observed as the base peaks in positive-ion spectra, with the tendency to aggregate to form species of the type [M 2Cl] +, [M 3Cl 3] + (M=phosphonium ion) increasing with the number of CH 2OH groups. Related aggregates, e.g. [M 2Cl 3] −, were observed in the negative-uon spectra. In situ formation of silver(I) adducts of a number of phosphines and their sulphide derivatives provides an additional means of characterization, though this method was not applicable to phosphine selenides. For phosphine oxides, adducts are readily detected with alkali metal ions.

Analysis of Pulsed Excimer Laser Ablation of PZT, Pb And Ti in an Oxygen Ambient Using Energy Dispersive Mass Spectrometry

AbstractA number of factors have been identified as affecting the gas phase oxidation of cations during pulsed laser ablation of lead zirconate titanate (PZT), by using energy dispersive mass spectrometry (EDMS). These phenomena are known to be critical to the deposition of high quality thin films of PZT. However, to isolate the role of each cation species and the influence of its gas phase properties, we have also looked beyond the ablation of the PZT ceramic. In this paper, we detail our observations during pulsed laser ablation of electroceramic and metallic targets, specifically PZT, Pb and Ti. We have observed the evolution and transport of molecular species during gas phase expansion over a wide range of oxygen pressures (10−7 - 10−1 mbar) and laser fluences (0.3 -10 Jcm−2). The yields of the molecular ion species are strongly dependent upon the number density of the background gas. We propose an interpretation of the relative yields of the species based upon their respective gas phase binding energies and their ease of formation based upon the reaction thermodynamics and reaction cross-section of each species.

Thermospray-LC-MS analysis of various groups of polyphenols in tea. I. Catechins, flavonol O-glycosides and flavone C-glycosides.

This paper describes the application of thermospray-HPLC-MS (buffer ionization mode, single-stage MS, positive ion detection) to the analysis of flavanols (catechins), flavonol O-glycosides, flavone C-glycosides as well as caffeine, theobromine, theogallin and theanine from tea. All compounds are detected as pseudo-molecular ions [M+H]+. Other molecular ion species are adducts with sodium, potassium, ammonium and solvent clusters. The catechin gallates and the flavonol glycosides are fragmented. The fragmentation is temperature dependent. The glycoside bond is labile and consequently the flavonol glycosides have the protonated aglycone as this base peak. The ester bonds in the catechin gallates are more stable and the fragmentation is limited. The fragment pattern contributes to the structural information. LC-thermospray-MS is a good analytical tool for identifying the polyphenols mentioned above both in tea and other foodstuffs, especially in method development and structural elucidation.

Direct mass spectrometric analysis of ozonides: application to unsaturated glycerophosphocholine lipids.

The reaction of ozone with double bonds present in glycerophosphocholine lipids results in formation of ozonides that can be directly analyzed by mass spectrometry as either positive or negative molecular ion species generated by electrospray ionization. Polyunsaturated fatty acyl groups esterified to the phospholipid yielded a mixture of ozonide species with the maximum number of ozone molecules added equal to the total number of double bonds. Ozonide decomposition resulted in omega-aldehyde and omega-carboxylic acid products as revealed by ESI-MS. Collisional activation of the ozone adducts for mono- and polyunsaturated phospholipids gave rise to fragment ions indicative of the position of the double bonds in these molecules. The major decomposition pathway for either positive or negative ozonide ion species involved charge remote fragmentation of the ozonide initiated by homolytic cleavage of the peroxide bridge followed by rearrangement to form the omega-aldehyde and omega-carboxylate acyl species. The reaction of ozone with phospholipids containing polyunsaturated fatty acyl groups is a useful method to probe the position of double bonds by electrospray ionization mass spectrometry.

Tandem Mass Spectrometry of Polychlorodibenzo-p-Dioxin and Polychlorodibenzofuran in a Quadrupole Ion Trap. 1. Comparison of Single-Frequency, Secular Frequency Modulation, and Multifrequency Resonant Excitation Modes

An investigation has been carried out on three modes of resonant excitation in the development of an analytical protocol for the determination of polychlorodibenzo-p-dioxins (PCDDs) and polychlorodibenzofurans (PCDFs) by ion trap tandem mass spectrometry. The resonant excitation modes investigated are single-frequency irradiation (SFI), secular frequency modulation (SFM), and multifrequency irradiation (MFI) of isolated molecular ion species. Each excitation mode was examined for fragmentation channel selectivity so as to optimize instrument sensitivity. Collision-activated dissociation mass spectra obtained with each excitation mode are compared with those obtained by triple-stage quadrupole mass spectrometry (TSQMS). While the same reaction channels (corresponding to losses of Cl•, COCl•, and 2COCl• for PCDDs and Cl•, COCl•, and COCl2 and COCl3• for PCDFs) were observed for each excitation mode, the fragment ion relative abundances differed among the resonant excitation modes and showed that internal en...

The identification of in vitro metabolites of bupropion using ion trap mass spectrometry

AbstractWe have identified in vitro metabolites of bupropion (Wellbutrin®) from incubations with human liver S9 fraction and human liver microsomes based on molecular weight information from full scan experiments using a liquid chromatograph coupled to a quadrupole ion trap mass spectrometer capable of multi‐stage operation (LC/MSn). Preliminary experiments have shown that this instrument provides comparable sensitivity to conventional LC‐coupled triple quadrupole instruments for metabolic studies, while allowing detailed structural studies using MSn experiments and routine on‐line coupling with high performance liquid chromatography via an external atmospheric pressure chemical ionization (APCI) source. The LC/MS analysis of human S9 showed the presence of three isomeric monohydroxylated metabolites of bupropion. These were further characterized in a series of MS/MS experiments which gave characteristic spectra for the three isomers. A minor dihydroxylated species was also identified in the human S9 sample and further characterized in a series of MSn experiments. Detailed structural information was generated by the use of on‐line LC/MSn type experiments. We have followed the fragmentation pathways of several molecular ion species in a series of sequential LC/MSn experiments, extending as far as MS6 with scan cycle times of less than 1.5 s. Such experiments have provided insights into the structure of specific fragment ions. Additional metabolic products were identified in the rat liver microsomes incubation sample.

Reliable and sensitive method for determination of microcystins in complicated matrices by frit‐fast atom bombardment liquid chromatography/mass spectrometry

A reliable and sensitive method for determination of hepatotoxic microcystins in complicated matrices by frit-fast atom bombardment liquid chromatography/mass spectrometry (Frit-FAB LC/MS) is described. Immonium ions of constituent amino acids, which were obtained together with molecular ion species by FAB mass spectral analysis of standard microcystins RR, YR, LR, and [D-Asp3] and [Dha7]microcystins LR using flow injection system composed of Frit-FAB probe, showed potential for reliable identification of microcystins by Frit-FAB LC/MS. Frit-FAB LC/MS using a microbore column provided not only the baseline separation of standard microcystins RR, YR, and LR but 200-fold higher sensitivity than that using conventional column. Furthermore, when a selected ion monitoring (SIM) technique was used, the detection limits of microcystins RR, YR, and LR were 300, 350, and 400 pg, respectively, at a signal-to-noise ratio of 5:1, and calibration curves of each microcystin showed a linear relationship from 2 ng to 50 ng. Finally, identification and quantitative analyses of microcystins in water samples were carried out.

Using Electron Cyclotron Resonance Plasma for Depositing Epitaxial Titanium Nitride Thin Films

AbstractWe have employed a 2.45 GHz electron cyclotron resonance (ECR) plasma source to deposit single-crystal thin films of titanium nitride onto MgO substrates of (100) orientation. During deposition the ECR plasma beam delivering a mixture of excited species of molecular and atomic nitrogen ions, strikes a substrate while an electron beam deposits on the same substrate species of titanium. We have studied the formation of films at substrate temperatures of 200, 400, and 600°C, as well as at room temperature. X-Ray diffraction (XRD) revealed that a cubic Bl phase of titanium nitride forms predominantly at all the temperatures explored. Both channeling and Rutherford backscattering spectroscopy (RBS) showed epitaxial layers forming at the temperature as low as 400°C. The minimum relative backscattering yield, χmin decreased as the temperature increased, with the best result of 7.3% obtained for the film deposited at 600°C. Biasing the substrates with either negative or positive voltage at room temperature directly affects film crystallography.

Cooling time and pressure effects on competitive thermalization/activation processes by resonance excitation on ITMS

AbstractThe classical n‐butylbenzene model is used to estimate the qualitative variation of the precursor ion internal energy by the resonant excitation method in an ion‐trap mass spectrometer. The kinetic energy thermalization process is confirmed by insertion of a cooling time (⩽10ms under our conditions). An initial internal energy relaxation process due to collisions between the molecular ion and neutral species of n‐butylbenzene is proposed. The larger amount of internal energy deposited in ions as the helium pressure is increased can be explained in terms of the number of collisions between the molecular ion and helium atoms, the competition between ion dissociation and ion ejection, the rate constant of dissociation and the secular axial frequency resolution of the isolated ions. Increased analyte pressure does not accelerate the ion relaxation process, probably due to the space charge effects. In summary, variation of helium pressure and cooling time permit optimization of ion internal energy deposition leading to enhanced sensitivity in MS/MS experiments.

Peptide amino acid sequence analysis using matrix‐assisted laser desorption/ionization and fourier transform mass spectrometry

AbstractHigh‐performance matrix‐assisted laser desorption/ionization (MALDI) using 7 T Fourier transform mass spectrometry (FTMS) was investigated for peptide amino acid sequence analysis. Two synthetic peptides representative of the type which would be displayed by major histocompatibility complex molecules from tumor cells were investigated by MALDI/FTMS. Molecular ions of the two 9‐amino acids peptides were detected with resolving power of 8000–17 900 and mass measurement accuracy between 8 and 81 ppm for the all 12C isotope ions. An ultra‐high resolution spectrum (RP 300 000) for the molecular ion of one of the two peptides was obtained. Structurally useful sequence information was obtained by use of surface‐induced dissociation (SID) of the molecular ion species. Interestingly, SID of a sodium‐attached peptide molecular ion resulted in the production of numerous sodium‐attached sequence ions.

Structure Elucidation of Glykenin, Glycosidic Antibiotics from Basidiomycetes sp. VI. Structure Characterization of the GK Components Using Frit-FAB LC/MS.

The structures of the glycosidic antibiotics, the glykenin (GK) family, produced by Basidiomycetes sp. were characterized using frit-FAB LC/MS. The GK was separated into nine components (GK-I-VII and DG) with differences in the number and positions of the acetyl groups in the sugar moiety. Although each of these components is further composed of three isomers depending on the three aglycones, it was difficult to isolate individual components because of the presence of the labile acetyl groups in the sugar moiety. For this reason, frit-FAB LC/MS in the negative ion mode was used for the structure characterization of each component. Three peaks were found in the mass chromatogram of each GK component monitored for the molecular ion species. The fragment ions related to the aglycone and sugar moieties were observed in each mass spectrum. Their molecular weights, the locations of the acetyl groups and the presence of three aglycones were confirmed using these ions. Furthermore, a profile analysis of the GK mixture was possible using a gradient elution mode.

Analysis of hydrocarbon dendrimers by laser desorption time-of-flight and Fourier transform mass spectrometry

The first mass spectrometric analysis of a new class of hydrocarbon dendrimers that result from a convergent synthetic approach is reported. Molecular weights of a series of phenylacetylene dendrimers (715 to 14776 u MW) are characterized by ultraviolet matrix-assisted laser desorption (MALD) time-of-flight (TOF) mass spectrometry, direct and silver chemical ionization infrared laser desorption Fourier transform mass spectrometry (FTMS), and ultraviolet matrix-assisted laser desorption silver chemical ionization Fourier transform mass spectrometry. New matrices and techniques were developed to facilitate analysis of the dendrimers. Mass measurement accuracies between 10 and 25 ppm are obtained for molecular ion species of the five dendrimers analyzed. Laser desorption time-of-flight and FTMS techniques are shown to be complementary, with FTMS providing high mass resolution (27,000–67,000 resolving power) and accuracy for lower mass dendrimers (10–14 ppm) and MALD TOF yielding the highest resolution (1100 resolving power) and accuracy (25 ppm) for the largest dendrimer. These results are consistent with proposed empirical formulas.

Ion measurements during Pioneer Venus reentry: Implications for solar cycle variation of ion composition and dynamics

During the final, low solar activity phase of the Pioneer Venus mission, the Orbiter Ion Mass Spectrometer measurements found all ion species, in the midnight‐dusk sector, reduced in concentration relative to that observed at solar maximum. Molecular ion species comprised a greater part of the total ion concentration as O+ and H+ had the greatest depletions. The nightside ionospheric states were strikingly similar to the isolated solar maximum “disappearing” ionospheres. Both are very dynamic states characterized by a rapidly drifting plasma and 30–100 eV superthermal O+ ions.

Primary to quaternary protein structure determination with electrospray ionization and magnetic sector mass spectrometry

AbstractElectrospray ionization with a forward‐geometry magnetic sector mass spectrometer was used for collisionally activated dissociation studies of multiply charged polypeptides and for studying non‐covalently bound protein systems. The high‐resolution capabilities of a high‐performance instrument allow the resolution of isotopic contributions for product ions and molecular ion species. Determination of product ion charge states by this method reduces difficulties in the interpretation of product ion mass spectra from multiply charged precursors, which are generated either in the atmospheric pressure/vacuum electrospray interface or in the collision chamber of the mass spectrometer. Extended tandem mass spectrometric experiments have the potential for sequencing larger polypeptides. However, evidence for isomerization of gas‐phase product ions from substance P and substance P analogues was observed, complicating the interpretation of product ion spectra. Non‐covalent complexes can also be studied by electrospray ionization magnetic sector MS. The higher m/z range of such an instrument is a major advantage for studying weakly bound systems, such as heme–protein systems (myoglobin, hemoglobin) and protein aggregates (concanavalin A), because of their tendency to form complex ions with relatively low charge states.

Confirmation of Identities of Propylene and Ethylene Glycols in Anchovies by Tandem Mass Spectrometry

A gas chromatographic/tandem mass spectrometric (GC/MS/MS) method has been developed for confirming the identity of propylene and ethylene glycols added to bait fish for preservation. Bait fish are occasionally illegally diverted to human food use. The bait fish were extracted with methanol, the extract was centrifuged and filtered, and the filtrate was concentrated 10-fold and then analyzed by GC/MS/MS. The glycols were separated chromatographically without derivatization or preliminary cleanup. Isobutane positive ion chemical ionization was used to generate the protonated molecular ion species of each glycol. Product-ion MS/MS experiments were performed to obtain spectra to confirm the identities of propylene and ethylene glycols. The identities of these 2 compounds in anchovy extracts were successfully confirmed by this approach.

Mechanism of production of ions in electrospray mass spectrometry

AbstractA mechanism for the production of multiply charged molecular ion species in electrospray mass spectrometry (ES‐MS) is still required. A concise discussion of a recently published ionic solution equilibrium model offering a partial mechanism is presented. That publication proposed that the ion abundance–charge profile could be fitted by one or a series of superimposed Gaussian functions, in accord with a solution equilibrium model. It is shown that indeed a simulated mass spectrum based on a solution odel can compare well with the observed spectrum. However, some new and recently published experimental evidence is presented which shows that the ES mass spectra of many proteins give rise to multiply charged molecular ions which carry higher charges than those calculated by the model. Further, the ion abundance–charge profile is very sensitive to some experimental parameters, e.g. cone voltage; it does not necessarily reflect the solution or gaseous ion populations in the mass spectrometer source. Therefore, the concept that gaseous multi‐charged ions originate from equivalently charged solvated ions in electrically neutral solution must be treated with caution.

The effect of oxygen flooding on the secondary ion yields of matrix and impurity species in InP and InGaAs

In the SIMS analysis of InP/InGaAs layered systems the secondary ion signals of the matrix elements show intensity variations that do not correlate with the compositional variations in the samples. Under some analysis conditions anomalous interface spikes are observed. A proposed solution to the problem of matrix element composition determination by SIMS is the use of the CsM + molecular ion species. This method is shown to be an improvement over the detection of other species but is not a complete solution to this problem. The use of oxygen flooding is shown to be more effective in producing a depth profile that is representative of the compositional variation in the sample in the analysis of InP/InGaAs structures using a Cameca ims 3f. The mechanism by which the oxygen flood acts is discussed and investigated through the study of useful ion yields of ion implants of dopant species in these two matrices as a function of oxygen flood conditions.

Competitive ionization of tetraphenylporphyrin in a laser-generated metal ion plasma

The ionization of tetraphenylporphyrin (TPP) in a laser-desorbed metal ion plasma is examined by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Competitive reaction pathways observed to generate abundant molecular ion species include electron detachment, cation attachment, charge exchange, metallation, and transmetallation in the positive ion mode and electron capture, metallation, and transmetallation in the negative ion mode. In general, cation attachment reactions dominate positive ion spectra below the laser irradiance threshold for plasma ignition, although the metallation product from [TPP] +· reaction with the metal atom, M, is observed. Negative ion products are not observed in the FT-ICR spectrum when a plasma is not formed. Under plasma ignition conditions, positive ion spectra include [TPP] +· formed by charge exchange with M +, which is also present in the spectrum. Negative ion spectra are dominated by [TPP] −· formed by charge exchange with M +, which is also present in the spectrum. Negative ion spectra are dominated by [TPP] −·, which is formed by attachment to thermal electrons generated in the plasma. Metallation reactions involving TPP and the metal substrate are examined. Positive ion metallation products are observed both in the absence of a plasma through reaction of [TPP] +· with M and by a second pathway under plasma ignition conditions through reaction of TPP with M +. In negative ion mode, metallation is only observed under plasma ignition conditions through reaction, of [TPP] −· with M. Observation of metallated products is found to be consistent with formation of stable metal oxidation states in the metallated porphyrin.

Maximum-Likelihood Restoration Data Processing Techniques Applied to Matrix-Assisted Laser Desorption Mass Spectra

Maximum-likelihood restoration (MLR) data processing is applied to matrix-assisted laser desorption time-of-flight mass spectra of several peptide/protein samples. It is shown that this technique can enhance mass resolution for peptides (porcine insulin; MW = 5777.58 Da) sufficiently to assist in the identification of various matrix adduct ion species formed in the laser desorption process. For higher-molecular-weight proteins, mass resolution enhancement is such that peak overlap contributions from partially resolved matrix adduct ion species can be minimized. Mass accuracies are improved by approximately a factor of six for a sample of a bacterial protease (Subtilisin Carlsberg; Subtilopeptidase A) from Bacillus licheniformis (MW = 27,288.4 Da). The MLR technique also supports the contention that the mass resolution degradation observed with increasing analyte mass is at least partially due to ion signal tailing, presumably caused by secondary ion species being produced in the detection process. This factor is also postulated as being at least partially responsible for the lack of success at resolution enhancement for higher mass analytes (bovine serum albumin; MW = 66,430.3 Da) where the multiple matrix adduct ion species produced can no longer be resolved from the protonated molecular ion species, thus limiting experimental mass measurement accuracies.