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PubMed2006

Ab initio study of the ionization of the DNA bases: ionization potentials and excited states of the cations.

The ionization of the four DNA bases is investigated by means of ab initio calculations. Accurate values of the gas-phase vertical and adiabatic ionization potentials (IP) are obtained at the MP2/6-31G(2d(0.8,alpha(d)),p) level of theory. The need of introducing extra polarization to the standard 6-31G(d,p) basis set is demonstrated by test calculations and an optimal value of alpha(d) = 0.1 is obtained. Ionization to electronically excited radical cations is also considered. The low-lying excited states of the cations are characterized for the first time. The topology of the corresponding potential energy surfaces is qualitatively described in terms of the stationary points (minima and saddle points) located on these surfaces. A conical intersection is characterized for the first time on the ground-state potential energy surface of all cations. It arises from the crossing of the adiabatic surfaces of the ground and first excited state at planar geometries. A nonplanar minimum is observed for the cytosine cation only. The geometry and electronic changes occurring along these surfaces are analyzed, leading to a comparison between the different nucleobase cations. The study of larger ionized systems related to DNA is rendered possible thanks to the optimized medium size basis set proposed in this work, as exemplified by the calculation of the IP of a stacked dimer of guanines.

Computer Simulation↗
PubMed2001

Electron impact ionization by drifting electrons in weakly ionized plasmas.

The expression for electron impact ionization rate by a Maxwellian electron population drifting with respect to a uniform neutral atom background is derived. Depending on electron temperature, drift speeds between one to five times electron thermal velocity produce increments in the ionization rate from two to seven orders of magnitude. Local ionization takes place over shorter distances than predicted for nondrifting electron populations and the results agree with previous experimental evidence on ionizing plasma double layers and electron attracting sheaths.

Journal Article↗
PubMed2005

Ionization processes in fluorescent lamps: evaluation of the Hg chemi-ionization rate coefficients.

Chemi-ionization by two mercury atoms to produce mercury atomic and molecular ions has been considered an important process in fluorescent lamps (FLs) for a quarter of a century. Despite the absence of reliable data, these processes have been included in a number of numerical models to help explain some of the experimental observations. These models have shown that the most important process is the Penning ionization of two Hg metastable atoms Hg(6(3)P2)+Hg(6(3)P2) --> Hg(+)+Hg(6(1)S0)+e. Although there is no experimental measurement of this cross section, modelers have typically used values implied from measurements of other chemi-ionization cross sections, or values obtained by fitting parameters to numerical models to obtain agreement with experiment. Recent theoretical investigations have indicated that the cross sections for the important processes may not be as large as previously thought. The aim of the present paper is to critically review the historical development of studies of chemi-ionization in fluorescent lamps and to present new experimental evidence which is consistent with the theoretical calculations and contradicts the conclusions from previously published experiments.

Journal Article↗
PubMed2006

Comparison study of atomic and molecular single ionization in the multiphoton ionization regime.

In this Letter, we report, for the first time in the multiphoton ionization regime, a comparison study of single-electron ionization of diatomic molecules versus rare gas atoms with virtually the same ionization potentials. In comparing N2+ to Ar+, a higher ion signal is seen in N2+ compared to Ar+ for linear polarization but the difference vanishes in circularly polarized light. In comparing O2+ to Xe+, we observe a suppression in O2+ compared to Xe+ for both linear and circular polarization but this suppression exhibits an intensity dependence; i.e., there is little suppression for O2+ at the lowest intensity range, but the suppression becomes increasingly stronger as the laser intensity increases. The multielectron screening model is used to discuss possible mechanisms of this intensity dependent suppression in O2+ in the multiphoton ionization regime.

Journal Article↗
PubMed2006

Strong-field molecular ionization: determination of ionization probabilities calibrated with field-free alignment.

We report an original optical method providing the probability of molecular ionization induced by femtosecond laser pulses. The approach consists of exploiting molecular alignment to extract reliable information about ionization. The cross defocusing technique implemented for this purpose reveals a sensitivity with respect to postpulse alignment, as well as to the free electron density induced by the ultrashort laser pulse. The analysis of the resulting signal thus gives access to absolute single-ionization probabilities calibrated through the degree of alignment, provided that free electrons are produced mainly by single ionization. The relevance of the method is assessed in N2.

Journal Article↗
PubMed1998

Gas chromatography-electron ionization and chemical ionization mass spectrometric analysis of urinary phenmetrazine after derivatization with 4-carbethoxyhexafluorobutyryl chloride--a new derivative.

Phenmetrazine is a central nervous system stimulant currently used as an anorectic agent. The drug is abused and is reported to cause death from overdose. We describe a new derivatization method for phenmetrazine using 4-carbethoxyhexafluorobutyryl chloride. Quantitation of urinary phenmetrazine can be easily achieved by using N-ethyl amphetamine as an internal standard. The electron ionization mass spectrum of 4-carbethoxyhexafluorobutyryl derivative of phenmetrazine showed a molecular ion at m/z 427 and a base peak at m/z 70. In the methane chemical ionization mass spectrum, the base peak was observed at m/z 428 (protonated molecular ion). In the electron ionization mass spectrum of 4-carbethoxyhexafluorobutyryl derivative of the internal standard, N-ethyl amphetamine we did not observe a molecular ion. However, in the chemical ionization mass spectrum, the protonated molecular ion at m/z 414 was the base peak. The retention time of derivatized phenmetrazine (8.4 min) was substantially longer than the retention time of the underivatized molecule. Moreover, underivatized phenmetrazine showed poor peak shape (substantial tailing) while derivatized phenmetrazine had excellent chromatographic properties. The within-run and between-run precisions of the assay were 2.6% and 3.1% respectively at a urinary phenmetrazine concentration of 10 micrograms/mL. The assay was linear for urinary phenmetrazine concentration of 1 to 100 micrograms/mL with a detection limit of 0.2 microgram/mL.

Amphetamines↗
PubMed2006

Derivatization by 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate for enhancing the ionization yield of small peptides and glycopeptides in matrix-assisted laser desorption/ionization and electrospray ionization mass spectrometry.

The characterization of glycosylation in proteins by mass spectrometry (MS) is often impeded by strong suppression of ionization of glycopeptides in the presence of non-glycosylated peptides. Glycopeptides with a large carbohydrate part and a short peptide backbone are particularly affected by this problem. To meet the goal of generating mass spectra exhibiting glycopeptide coverages as complete as possible, derivatization of glycopeptides offers a practical way to increase their ionization yield. This paper investigated derivatization with 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC) which is a rapid labeling technique commonly used for fluorescence detection in high-performance liquid chromatography (HPLC) and capillary electrophoresis (CE). As test samples we used peptides and glycopeptides obtained by enzymatic digestion of three different glycoproteins, i.e., human antithrombin, chicken ovalbumin, and bovine alpha1-acid-glycoprotein. It was found that AQC derivatization resulted in strongly increased signal intensities when analyzing small peptides and glycopeptides by matrix-assisted laser desorption/ionization (MALDI)-MS. For these compounds the limit of detection could be reduced to low fmol amounts. Without derivatization only glycopeptides containing large peptide backbones were detected by MALDI-MS. This effect was even significant when glycopeptides were pre-separated and enriched by means of lectin affinity chromatography before MALDI-MS analysis and when using electrospray ionization (ESI). This labeling method, applied in combination with MS detection for the first time, was found to be well suited for the enhancement of detection sensitivity for small glycopeptides in MALDI-MS analysis and thus for reducing the need for pre-separation steps.

Aminoquinolines↗
PubMed2004

Partial characterization of glycosphingolipids of Agelas sponges in their peracetylated form by liquid secondary ionization mass spectrometry and high-performance liquid chromatography combined with direct electrospray ionization mass spectrometric detection.

Electrospray ionization (ESI) and liquid secondary ionization (LSI) mass spectrometry were applied for characterization of glycosphingolipids (GSLs) isolated in their peracetylated form from four Agelas marine sponge species. Since peracetylated glycosphingolipids are not soluble in solvents traditionally used for ESI, lithium chloride was added to the samples in order to obtain lithium cationized molecules. Although the preferred fragmentation seems to be the sequential loss of acetic acid molecules, it was found that tandem mass spectra obtained from peracetylated diglycosyl ceramides might provide direct information about the structure of the long-chain base (formation of W''/Z0 fragments). The utility of ESI and LSI in the analysis of these compounds has also been compared. It was found that the tandem mass spectra obtained by LSI-MS/MS experiments could provide information about the chain-length (carbon atom number) variations within a certain ceramide mass. Thus, from one of our samples, 25 different ceramide compositions have been identified from 8 precursor (Z0) ions. Comparison of the two ionization modes (LSI and ESI) highlights the fact that molecular mass distributions obtained by LSI-MS, especially the presence of unsaturated species, have to be interpreted carefully. For the first time a direct high-performance liquid chromatography (HPLC)/ESI-MS method was used for characterization of complex mixtures of peracetylated GSLs. The results demonstrate that HPLC/ESI-MS is able to analyze mono- and diglycosylated GSLs, and other kinds of glycolipids that are actually present in the sample.

Acetylation↗
PubMed2005

Characterization of the improvised explosive urea nitrate using electrospray ionization and atmospheric pressure chemical ionization.

Mass spectra of urea nitrate were measured in electrospray ionization and in atmospheric pressure chemical ionization in the negative mode. In both ionization methods two characteristic adduct ions containing the intact molecule [urea nitrate+NO3]- and [urea nitrate+HNO3+NO3]- are shown. The structure of the two adduct ions was deduced using measurements of isotopically labeled urea nitrate. Collision-induced dissociation measurements of the adduct ions show typical losses enabling the identification of urea nitrate in trace amounts. Using these methods urea nitrate was identified in real cases.

Atmospheric Pressure↗
PubMed2002

Dual parallel electrospray ionization and atmospheric pressure chemical ionization mass spectrometry (MS), MS/MS and MS/MS/MS for the analysis of triacylglycerols and triacylglycerol oxidation products.

Two mass spectrometers, in parallel, were employed simultaneously for analysis of triacylglycerols in canola oil, for analysis of triolein oxidation products, and for analysis of triacylglycerol positional isomers separated using reversed-phase high-performance liquid chromatography. A triple quadrupole mass spectrometer was interfaced via an atmospheric pressure chemical ionization (APCI) interface to two reversed-phase liquid chromatographic columns in series. An ion trap mass spectrometer was coupled to the same two columns using an electrospray ionization (ESI) interface, with ammonium formate added as electrolyte. Electrospray ionization mass spectrometry (ESI-MS) under these conditions produced abundant ammonium adduct ions from triacylglycerols, which were then fragmented to produce MS/MS spectra and then fragmented further to produce MS/MS/MS spectra. ESI-MS/MS of the ammoniated adduct ions gave product ion mass spectra which were similar to mass spectra obtained by APCI-MS. ESI-MS/MS produced diacylglycerol fragment ions, and additional fragmentation (MS/MS/MS) produced [RCO](+) (acylium) ions, [RCOO+58](+) ions, and other related ions which allowed assignment of individual acyl chain identities. APCI-MS of triacylglycerol oxidation products produced spectra like those reported previously using APCI-MS. APCI-MS/MS produced ions related to individual fatty acid chains. ESI-MS of triacylglycerol oxidation products produced abundant ammonium adduct ions, even for those molecules which previously produced little or no intact molecular ions under APCI-MS conditions. Fragmentation (MS/MS) of the [M+NH(4)](+) ions produced results similar to those obtained by APCI-MS. Further fragmentation (MS/MS/MS) of the diacylglycerol fragments of oxidation products provided information on the oxidized individual fatty acyl chains. ESI-MS and APCI-MS were found to be complementary techniques, which together contributed to a better understanding of the identities of the products formed by oxidation of triacylglycerols.

Chromatography, High Pressure Liquid↗
PubMed1999

Analysis of cyclic and acyclic analogs of retinol, retinoic acid, and retinal by laser desorption ionization-, matrix-assisted laser desorption ionization-mass spectrometry, and UV/Vis spectroscopy.

Laser desorption ionization (LDI)- and matrix-assisted laser desorption ionization (MALDI)-mass spectrometry (LDI-MS, MALDI-MS) at 337-nm laser wavelength were used to analyze retinol (vitamin A), retinoic acid, and retinal and their analogs 3-hydroxyretinol, 3-hydroxyretinoic acid, 3-hydroxyretinal, 4-oxoretinol, 4-oxoretinoic acid, 4-oxoretinal, 3,4-didehydroretinol (vitamin A(2)), 3,4-didehydroretinoic acid, 3,4-didehydroretinal, acycloretinol, acycloretinoic acid, and acycloretinal. The compounds exhibit sufficient ionizability which allows to obtain mass spectra by LDI which are similar in quality to those obtained by MALDI. Mass spectra were recorded with a linear time-of-flight (TOF) instrument or a reflectron-type (RETOF) instrument in positive-ion mode. Under the conditions of LDI-MS the compounds form abundant radical molecular ions (M+*), whereas in the MALDI mass spectra abundant protonated molecular ions ([M + H]+) are observed. Characteristic fragment ions provide additional structural information. High-performance liquid chromatography (HPLC) coupled with UV/Vis photodiode detection was used to assist in retinoid characterization. Synthesis of 3-hydroxyretinal, 4-oxoretinal, and acycloretinal was performed by oxidative cleavage of the all-trans-carotenoids of zeaxanthin, canthaxanthin, and lycopene.

Molecular Structure↗
PubMed2001

Retention of ionizable compounds on high-performance liquid chromatography. VII. Characterization of the retention of ionic solutes in a C18 column by mass spectrometry with electrospray ionization.

The elution of ions from a C18 column with mobile phases containing methanol (60%, v/v) and aqueous buffers is studied by mass spectrometry. It is demonstrated that the anions are excluded from the stationary phase by the ionized silanols. However, the ionized silanols interact strongly with cations, which are retained in the column. These cations are later eluted from the column by ion exchange with the cations present in the pH buffered mobile phase. The size of the ions, the mobile phase cation concentration and the mobile phase pH are the main parameters that affect elution of the retained cations. It is also demonstrated that there are at least two different types of ionizable silanols, with different acidities, that contribute to the retention of cations. An estimate of the pKa values of these two groups of silanols in 60% methanol is given.

Buffers↗
PubMed2003

Factors influencing the analytical performance of an atmospheric sampling glow discharge ionization source as revealed via ionization dynamics modeling.

A kinetic model is developed for the dynamic events occurring within an atmospheric sampling glow discharge that affect its performance as an ion source for analytical mass spectrometry. The differential equations incorporate secondary electron generation and thermalization, reagent and analyte ion formation via electron capture and ion-molecule reactions, ion loss via recombination processes, diffusion, and ion-molecule reactions with matrix components, and the sampling and pumping parameters of the source. Because the ion source has a flow-through configuration, the number densities of selected species can be estimated by applying the steady-state assumption. However, understanding of its operation is aided by knowledge of the dynamic behavior, so numerical methods are applied to examine the time dependence of those species as well. As in other plasma ionization sources, the ionization efficiency is essentially determined by the ratio of the relevant ion formation and recombination rates. Although thermal electron and positive reagent ion number densities are comparable, the electron capture/ion-molecule reaction rate coefficient ratio is normally quite large and the ion-electron recombination rate coefficient is about an order of magnitude greater than that for ion-ion recombination. Consequently, the efficiency for negative analyte ion formation via electron capture is generally superior to that for positive analyte ion generation via ion-molecule reaction. However, the efficiency for positive analyte ion formation should be equal to or better than that for negative analyte ions when both ionization processes occur via ion-molecule reaction processes (with comparable rate coefficients), since the negative reagent ion density is considerably less than that for positive reagent ions. Furthermore, the particularly high number densities of thermal electrons and reagent ions leads to a large dynamic range of linear response for the source. Simulation results also suggest that analyte ion number densities might be enhanced by modification of the standard physical and operating parameters of the source.

Air Ionization↗
PubMed2002

Ionization and fragmentation of humic substances in electrospray ionization Fourier transform-ion cyclotron resonance mass spectrometry.

Electrospray ionization (ESI) was combined with ultra-high-resolution Fourier transform-ion cyclotron resonance mass spectrometry (FTICR MS) to characterize complex humic and fulvic acid mixtures. Lower than expected molecular weight distributions previously observed for humics when analyzed by ESI-MS have fueled speculation about a bias in favor of low molecular weight. Multiply charged ions, ionization suppression, and sample fragmentation have all been suggested as sources of this low molecular weight bias. In this work, resolution of the individual components of humic mixtures within a 1 mass-to-charge unit window was accomplished by FTICR MS at 9.4 T. At mass resolving powers between 60,000 (high mass) and 120,000 (low mass), it was possible to determine that virtually all ions present in spectra of Suwannee River fulvic and humic acid are singly charged, thus eliminating inadequate accounting for multiply charged ions as a primary source of any low molecular weight bias. The high-resolution mass spectra also revealed the presence of molecular families containing ions that differ from each other in degree of saturation, functional group substitution (primarily CH vs N and CH4 vs O), and number of CH2 groups. Ionization suppression and ion fragmentation were addressed for humic and fulvic acid mixtures and well-characterized poly(ethylene glycol) (PEG) mixtures with average molecular weights of 8000 and 10,000. Although these high molecular weight PEG mixtures fragment extensively under traditional positive-ion mode ESI conditions, similar fragmentation could not be confirmed for humic and fulvic acid mixtures.

Cyclotrons↗
PubMed1996

Proteolysis of human calcitonin in excised bovine nasal mucosa: elucidation of the metabolic pathway by liquid secondary ionization mass spectrometry (LSIMS) and matrix assisted laser desorption ionization mass spectrometry (MALDI).

PURPOSE: Two calcitonins, i.e. human calcitonin (hCT) and, for comparison, salmon calcitonin (sCT), were chosen as peptide models to investigate nasal mucosal metabolism. METHODS: The susceptibility of hCT and sCT to nasal mucosal enzymes was assessed by in-and-out reflection kinetics experiments in an in vitro model based on the use of freshly excised bovine nasal mucosa, with the mucosal surface of the mucosa facing the peptide solution. The kinetics of CT degradation in the bulk solution was monitored by HPLC. Peptide sequences of the main nasal metabolites of hCT were analyzed by using both liquid secondary ionization mass spectrometry (LSIMS), following HPLC fractionation of the metabolites, and matrix-assisted laser desorption ionization mass (MALDI) spectrometry. For sCT, the molecular weights of two major metabolites were determined by LC-MS with electrospray ionization. RESULTS: Both CTs were readily metabolized by nasal mucosal enzymes. In the concentration range studied metabolic rates were higher with hCT than with sCT. Presence of endopeptidase activities in the nasal mucosa was crucial, cleaving both calcitonins in the central domain of the molecules. CONCLUSIONS: Typically, initial metabolic cleavage of hCT in nasal mucosa is due to both chymotryptic- and tryptic-like endopeptidases. The subsequent metabolic break-down follows the sequential pattern of aminopeptidase activity. Tryptic endopeptidase activity is characteristic of nasal sCT cleavage.

Amino Acid Sequence↗
PubMed2002

Negative ion chemical ionization gas chromatography-mass spectrometry and atmospheric pressure chemical ionization liquid chromatography-mass spectrometry of low-dosed and/or polar drugs in plasma.

In clinical and forensic toxicology, doping control, and therapeutic drug monitoring, specific and sensitive detection and precise quantification of xenobiotics in biosamples are great challenges. Today, mass spectrometry techniques, coupled with gas chromatography or liquid chromatography, are the most powerful methods in analytic toxicology. The pros and cons of electron ionization (EI) and negative ion chemical ionization (NICI) gas chromatography-mass spectrometry (GC-MS) and of atmospheric pressure chemical ionization liquid chromatography-mass spectrometry (APCI-LC-MS) are described for determination of the low-dosed benzodiazepine flunitrazepam and its 7-amino and its nor-metabolite in plasma. In addition, application of NICI-GC-MS is described for sensitive chiral determination of amphetamine derivatives in plasma and application of APCI-LC-MS for screening, library-assisted identification, and validated quantification of oral antidiabetics and for validated quantification of the neuroleptic risperidone and its 9-hydroxy metabolite. These examples show that NICI-GC-MS and LC-MS are powerful tools for determination of low-dosed and/or rather polar drugs or poisons, thus becoming indispensable supplements to classic EI-GC-MS in clinical and forensic toxicology as well as in doping control.

Amphetamine↗
PubMed2003

Electron ionization and atmospheric pressure photochemical ionization in gas chromatography-mass spectrometry analysis of amino acids.

The mass spectra of tert-butyldimethylsilyl (TBDMS) derivatives of 17 amino acids were obtained using electron ionization (EI) and atmospheric pressure photochemical ionization (APPhCI) mass spectrometry. The APPhCI mass spectra for all of the derivatives except arginine were shown to consist of only molecular [M](+.) and quasimolecular [MH](+) ions whereas, in the case of EI, the compounds in question underwent a drastic fragmentation. The application of APPhCI to gas chromatography-mass spectrometry enables a reliable identification of the TBDMS derivatives of amino acids in a mixture, even if its components are only partially resolved, due to the unique molecular masses for each compound. Comparison of the respective positive-ion chemical ionization (PICI) mass spectra available in the literature with APPhCI spectra has shown that, in the case of PICI, unlike APPhCI, noticeable fragmentation occurs.

Amino Acids↗
PubMed1992

Fast atom bombardment, electron ionization and chemical ionization mass spectrometry of 1,2,3-benzenetricarboxylic acids, inhibitors of the mitochondrial tricarboxylate carrier.

It has been discovered that 1,2,3-benzenetricarboxylic acid and some 5-substituted derivatives are specific and competitive inhibitors of the mitochondrial tricarboxylate carrier. In order to characterize these acids a study was carried out using electron ionization (EI), chemical ionization (CI) and fast atom bombardment (FAB) mass spectrometry on the free acids, the potassium salt of 1,2,3-benzenetricarboxylic acid and the corresponding methyl esters. Unimolecular decomposition processes were also studied using B/E linked scan experiments in order to correlate peaks present in the FAB mass spectra of the above compounds. The FAB ionization method, as expected, was the most suitable for analysing the very polar and thermolabile acids and the only method which could be used for the analysis of the potassium salt. EI and CI turned out to be appropriate methods for analysing the less polar and thermostable trimethyl esters.

Benzene Derivatives↗