Pressure Ion Source Research Articles

Effect of ion source pressure on ion formation of carbamates in particle-beam chemical-ionisation mass spectrometry

In this study the mass spectra of fourteen carbamate pesticides, obtained with desorption chemical ionisation (DCI) and flow injection (FIA)-particle beam (PB)-ammonia positive chemical ionisation (PCI)-mass spectrometry (MS), are compared. The mass spectra from the FIA-PB-PCI-MS experiments exhibit higher relative abundances for fragment ions. The influence of the ion source pressure and temperature on the ion abundances under ammonia positive chemical ionisation conditions was studied. The results indicate that thermal degradation of the carbamate pesticides takes place in the FIA-PB-MS system. In addition, the [M + NH 3 + H] + and [M + NH 3 + H - CH 3NCO] + ion intensities are strongly dependent on the ion source pressure, especially for carbofuran as an extreme. Both the ion source pressure and the temperature cause irreproducibility of the ammonia PCI mass spectra of carbamates under liquid chromatography PB-PCI-MS conditions and it is therefore of utmost importance to use well defined experimental conditions for the quantitative determination of carbamates.

Application of a new atmospheric pressure ionization source for double focusing sector instruments

In three application examples we demonstrate how the capabilities of a modern atmospheric pressure ionization (API) source, as a universal LC-MS coupling tool, are supplemented by the high mass resolution available on double focusing sector instruments. The examples include charge state determination in complex electrospray ionization-collisional induced dissociation (ESI-CID) spectra of multiply charged ions, mixture analysis of multicomponent samples without prior chromatographic separation, and very sensitive and highly specific detection of pesticides using atmospheric pressure chemical ionization (APCI).

High precision mass spectrometric analysis of isotopic abundance ratios in nitrous oxide by direct injection of N 2O

This paper presents the technical and theoretical background of a technique for directly determining the nitrogen and oxygen isotopic composition of N 2O by measuring simultaneously the m/ z 44, 45, and 46 beam intensities on a gas source ratio mass spectrometer. Mass interferences are caused by any CO 2 contaminant as CO 2 has the same molecular masses as N 2O. Any small amount of CO 2 remaining in the sample can be corrected for by measuring the ratio of m/ z 12 to 14 or m/ z 12 to 44 ratio in the sample. Under certain source conditions, NO 2 is produced in the ion source, causing a mass interference at m/ z 46. NO 2 production in the source can be corrected for by establishing an NO 2 slope correction factor through measuring the ratio of m/ z 46 to 44 in the sample at various ion source pressures. The production of NO 2 can be totally eliminated by increasing the draw-out voltage in the source to a value similar to that used for hydrogen isotope analysis. Experiments were carried out on both a Finnigan MAT251 and a Finnigan MAT252. Delta values for oxygen and nitrogen isotope abundances were obtained with an internal precision of better than 0.08 permil and 0.02 permil, respectively, on samples as small as 3 μmol on both instruments.

Concentration enrichment in the ion source of a pulsed electron beam high pressure mass spectrometer

The magnitude of concentration enrichment of methyl iodide in methane, helium, and argon buffer gases within the ion source of a pulsed e-beam high pressure mass spectrometer (PHPMS) is characterized here by a theoretical model and by kinetic measurements of the gas phase ion/molecule reaction, F − + CH 3I → CH 3F + I −, at 150°C. A relatively simple PHPMS ion source is used in which the magnitude of enrichment is related only to the ventilation of individual molecules out of the ion source by passage through its ion exit and electron entrance slits. It is shown that significant enrichment does occur under typical operating conditions of this source and that the magnitude of enrichment is strongly dependent on the choice of ion source pressure. These observations are explained in terms of the flow conditions thought to exist in the ion source. Recommendations for improving the accuracy of future kinetic and equilibrium measurements by the PHPMS are provided.

Atmospheric-pressure-ionization mass spectrometry: I. Instrumentation and ionization techniques

Mass spectrometer ion sources are normally located inside a high-vacuum envelope. Such low-pressure ion sources can make use of a range of different ionization methods and are in routine use in analytical mass spectrometers. An ion source operating at atmospheric pressure is better suited, and may be essential, for a growing number of applications. Mass spectrometric analysis of samples pyrolyzed under controlled conditions, the combination of liquid chromatography and capillary electrophoresis with mass spectrometry, and the determination of the molecular mass of proteins by electrospray ionization all benefit from, or require, an atmospheric pressure ionization source.

Quantification of fg-pg amounts by electron capture negative ion mass spectrometry ? Parameter optimisation and practical advices

Electron capture negative ion mass spectrometry (ECNI-MS) is a very suitable and popular technique for the identification and quantification of fg-pg amounts of compounds with a sufficiently high electron affinity. Many users of the ECNI mode have faced a lot of frustrating problems due to instrument contamination and wrong recommendations concerning instrument optimisation. This article summarises 14 years of experience with ECNI-MS using a large number of different instruments. Recommendations are given concerning optimisation procedures of important parameters such as ion source pressure and temperature as well as electron energy. An ion pressure optimisation method is proposed using a gas chromatograph. ECNI-MS is very sensitive against trace amounts of contaminants in the mass spectrometer and requires very clean components in the reagent gas line. Recommendations are given concerning suitable parts. Different other contamination sources are also discussed. The construction of a simple and clean gas inlet system is presented. Furthermore, contamination-free cleaning methods for the ion source are suggested. A test method based on the detection of hexachlorobenzene in the full scan mode (m/z 34–300) is proposed. It allows to evaluate both the background level in the mass spectrometer and the overall system performance. Clean instruments should show a signal-to-noise ratio of the total ion current GC signal of at least 20:1 without applying any mass signal area reject threshold. A linearity test procedure is also suggested. It shows that the linear range of a clean and optimised instrument is at least 3 orders of magnitude in the ECNI mode.

Collisional cooling effects on the charge reversal and neutralization—reionization mass spectra of the negative cluster ion of sulfur dioxide (SO 2) .−2

The charge reversal and neutralization—reionization mass spectra of the sulfur dioxide cluster anion (SO 2) 2 .− 2 are strongly affected by the ion-source pressure conditions and two possible origins for the collisional cooling effects are discussed.

Negative‐ and positive‐ion chemical ionization mass spectra of aromatic amines: Surface‐assisted reactions involving oxygen

AbstractThe O2–N2 and O2–Ar negative‐ion chemical ionization mass spectra of aromatic amines show a series of unusual ions dominated by an addition appearing at [M + 14]−˙. Other ions are observed at [M – 12]−˙, [M + 5]−˙, [M + 12]−˙, [M + 28]−˙ and [M + 30]−˙. Ion formation was studied using a quadrupole instrument equipped with a conventional chemical ionization source and a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer. These studies, which included the examination of ion chromatograms, measurement of positive‐ion chemical ionization mass spectra, variation of ion source temperature and pressure and experiments with 18O2, indicate that the [M + 14]−˙ ion is formed by the electron‐capture ionization of analytes altered by surfaceassisted reactions involving oxygen. This conversion is also observed under low‐pressure conditions following source pretreatment with O2. Experiments with [15N]aniline, [2,3,4,5,6‐2H5] aniline and [13C6]aniline show that the [M + 14]−˙ ion corresponds to [M + O − 2H]−˙, resulting from conversion of the amino group to a nitroso group. Additional ions in the spectra of aromatic amines also result from surface‐assisted oxidation reactions, including oxidation of the amino group to a nitro group, oxidation and cleavage of the aromatic ring and, at higher analyte concentrations, intermolecular oxidation reactions.

M + 11]+˙ and [M + 11]−˙ ions in the nitrogen positive and negative ion chemical ionization mass spectra of aromatic amines

AbstractThe N2 negative ion chemical ionization (NICI) mass spectra of aniline, aminonaphthalenes, aminobiphenyls and aminoanthracenes show an unexpected addition appearing at [M + 11]−˙. This addition is also observed in the N2 positive chemical ionization (PCI) mass spectra. An ion at [M – 15]− is found in the NICI spectra of aminoaromatics such as aniline, 1‐ and 2‐aminonaphthalene and 1‐ and 2‐aminoanthracene. Ion formation was studied using labeled reagents, variation of ion source pressure and temperature and examination of ion chromatograms. These experiments indicate that the [M + 11]−˙, [M – 15]−˙ and [M + 11]+˙ ions result from the ionization of analytes altered by surface‐assisted reactions. Experiments with 15N2, [15N] aniline, [2,3,4,5,6‐2H5] aniline and [13C6] aniline show that the [M + 11]−˙ ion corresponds to [M + N – 3H]−˙. The added nitrogen originates from the N2 buffer gas and the addition occurs with loss of one ring and two amino group hydrogens. Fragmentation patterns in the N2 PCI mass spectrum of aniline suggest that the neutral product of the surface‐assisted reaction is 1,4‐dicyanobuta‐1,3‐diene. Experiments with diamino‐substituted aromatics show analogous reactions resulting in the formation of [M – 4H]−˙ ions for aromatics with ortho‐amino groups. Experiments with methylsubstituted aminoaromatics indicate that unsubstituted sites ortho to the amino group facilitate nitrogen addition, and that methyl groups provide additional sites for nitrogen addition.

The determination of thermodynamics of oxygen complexes of anions of sulfur dioxide and fluoronitrobenzenes using atmospheric pressure ionization mass spectrometry

The thermodynamic properties of oxygen complexes of the negative ions of 1-fluoro-2,4-dinitrobenzene (FDNB), 1,5-difluoro-2,4-dinitrobenzene (DFDNB), and sulfur dioxide have been determined by measuring the temperature dependence of the pertinent ion ratios by using a mass spectrometer equipped with a63Ni atmospheric pressure ionization source. The values of -ΔH° and -ΔS°are SO2−(O2), 101 ± 2.1 kJ mole1 and 121 ± 3.8 J/K-mole; FDNB−(O2), 51.5 ±3.4 kJ mole−1 ± 9.2 J/K-mole; and DFDNB−(O2), 65.7 ± 0.4 kJ mole−2 and 110 ± 5.0 J/K-mole.

On the structure of water-alcohol and ammonia-alcohol protonated clusters

Collision-induced dissociation (CID) of protonated ammonia-alcohol and water-alcohol heteroclusters was studied using a triple quadrupole mass spectrometer with a corona discharge atmospheric pressure ionization source. CID results suggested that the ammonia-alcohol clusters had NH + 4 at the core of the cluster and that hydrogen-bonded alcohol molecules solvated this central ion. In contrast, CID results in water-alcohol clusters showed that water loss was strongly favored over alcohol loss and that there was a preference for the charge to reside on an alcohol molecule. The results also indicated that a loose chain of hydrogen-bonded molecules was formed in the water-alcohol clusters and that there appeared to be no rigid protonation site or a fixed central ion.

Identification of degradation products of some chemical warfare agents by capillary electrophoresis—ionspray mass spectrometry

Capillary zone electrophoresis—ionspray mass spectrometry (CZE—IS-MS) in the negative-ion mode was applied in the identification of five organophosphonic acids, which are the primary hydrolysis products of nerve agents. The spectra exhibit a very abundant (M  H) − ion with minimal fragmentation. Fragment ions were produced by raising the nozzle — skimmer voltage difference in the first vacuum stage between the atmospheric pressure ion source and the mass analyzer. CZE—IS-MS provides extremely good separation efficiency and very high sensitivity for the phosphoric acids. Sensitivity in the range 10–30 pg was achieved with standard solutions using selected ion monitoring. Linear regression analysis data showed correlation coefficients to be between 0.994 and 0.999, indicating good quantitative linearity of the method.

Why are electron capture negative ion mass spectra not reproducible? An ion source problem

Differences in the electron capture negative ion mass spectra of environmentally related organic compounds acquired on a VG 30-250 triple quadruple mass spectrometer and on an HP 5985B gas chromatography/mass spectrometry system were investigated with respect to the ion formation process. Neither ion source temperature nor pressure was responsible for the differences. The populations of thermal electrons in both ion sources were experimentally determined and found to be similar, suggesting that electron capturing reactions should proceed with comparable efficiencies in both ion sources. The ion extraction efficiencies of the two instruments were examined by monitoring the transmission profiles of low- and high-mass ions as a function of lens potentials. Results indicated that the HP 5985B extraction lens significantly suppressed low-mass ions. Further, theoretical evaluation of ion trajectories using SIMION suggested that on the HP 5985B, low-mass ions entered the mass analyzer as a defocused beam, but high-mass ions entered the analyzer as a well-collimated beam. On the VG 30-250, low- and high-mass ions were transmitted to the analyzer with equal efficiency by the ion extraction system.

Liquid chromatography chloride attachment negative chemical ionization mass spectrometry of diacylglycerol dinitrophenylurethanes

Chloride attachment and electron capture negative chemical ionization mass spectra were determined for the 3,5-dinitrophenyl urethanes of natural diacylglycerols following high-performance liquid chromatography on chiral-phase columns using dichloromethane as a source of chloride. For this purpose a chiral column containing the R(+) naphthylethylamine polymer was eluted with an isocratic mixture of either isooctane-f-butyl methyl ether-isopropanol-acetonitrile 80:10:5:5 (solvent A) containing 1% dichloromethane, or with solvent A (20%) and hexane-dichloromethane-ethanol 40:10:1 (solvent B, 80%). The dinitrophenylurethanes gave electron capture molecular and fragment ions in proportions which varied with the composition of the eluting solvent and the ion source pressure. The chloride attachment fragment ions corresponding to the intact diacylglycerols provided a sensitive method of identification and quantitation of the molecular species of the diacylglycerols under all conditions. The detection of the diacylglycerol dinitrophenylurethanes as the negative chloride attachment fragment ions increased the sensitivity and specificity of analysis of the solutes about 100-fold over positive chemical ionization mass spectrometric detection. The detection limit for the dinitrophenylurethanes of natural diacylglycerols was in the low-nanogram range, with the instrument response being linear over two orders of magnitude. For additional structural characterization the dinitrophenylurethanes were subjected to positive chemical ionization mass spectrometry, which resulted in prominent intensities for the [M - DNPU]+ and [RCO + 74]+ ions, but gave no molecular ions. Quantitative analysis of complex mixtures required calibration of the ionization response.

Ion-spray mass spectrometric analysis of glycosaminoglycan oligosaccharides

Oligosaccharides from hyaluronic acid and chondroitin 6-sulfate were prepared by digestion with testicular hyaluronidase and separated according to their degree of polymerization by gel-permeation chromatography. These materials were successively analyzed by negative-mode ion-spray mass spectrometry with an atmospheric-pressure ion source. An ion-spray interface was used to produce ions via the ion evaporation process, producing mass spectra containing a series of molecular species carrying multiple charges. Using two adjacent multiply charged molecular ions, the exact molecular weights up to the tetradecasaccharide were calculated with a precision of +/- 1 dalton. This type of mass spectrometry was also demonstrated to be feasible for the analysis of mixtures of oligosaccharides, including tetra-, hexa-, octa- and decasaccharides, from hyaluronic acid or chondroitin 6-sulfate without separation. Ion-spray mass spectrometry was thus shown to be applicable to the structural analysis of oligosaccharides from glycosaminoglycans.

Calibration of ion spray mass spectra using cluster ions

AbstractMass calibration for ion spray mass spectrometry can be achieved by using cluster ions formed by flow injection of solutions of alkali metal salts in aqueous acetonitrile into the liquid flowing to the ion spray needle. Source contamination is thereby reduced to a minimum. For quadrupole mass analyzers, sodium iodide provides an ideal compromise between undesirable spectral complexity and spacings between calibrant mass peaks sufficiently close that interpolation errors are negligible. When much closer spacings are required, protonated water clusters provide an excellent calibration up to about m/z 1000. If higher mass ranges are required with a large number of calibrant peaks, a solution of mixed alkali metal iodides does provide the expected spectra but intensities are poor at higher m/z values. For liquid chromatography with on‐line mass spectrometry (LC/MS) the mass calibration may be checked without changing the mobile phase by post‐column flow injection of a cesium carbonate solution, since the carbonate anion is wholly displaced by the anion of the mobile phase acid modifier, resulting in no mixed clusters. The metal salt calibrants have the additional advantage of being useful over a wide range of tuning parameters in the atmospheric pressure ionization source, covering those appropriate to both relative molecular mass determinations of large proteins and to LC/MS of small analyte species.

Collisional focusing effects in radio frequency quadrupoles

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

Ion-molecule equilibria, how and why

T his article describes how the occurrence of ion-molecule equilibria under thermal conditions was discovered when experiments at high, near-atmospheric, ion source pressures were undertaken for a totally different purpose. It also describes how the instrumentation for the determination of ion-molecule equilibria was developed, and how the field grew so as to provide a vast amount of data on ion-molecule energetics which find application in a number of fields. Results from ionic equilibria measurements in solution, compiled in the form of acid-base dissociation constants, stability constants for ion-l&and complexes, and electrochemical reduction potentials, represent the quantitative backbone of chemistry in solution. The measurement and recording of such data began in the early 1900s and at the present time this material is a major part of tist-year college chemistry. The same type of basic chemical reactions-proton transfer, ion-ligand association, and electron transfer -occur also in the gas phase. Therefore, valuable information was potentially available from the measurement of ion-molecule equilibria in the gas phase. The instrument with which to observe such reactions was obviously the mass spectrometer. However, when the research to be described started, there was among mass spectrometry researchers little, if any, awareness of this extraordinary opportunity. In the gas phase, at the low pressures of the then available mass spectrometers, ions become discharged on collision with the wall of the apparatus, and therefore the time required for the achievement of an ion-molecule reaction equilibrium is generally not available. Furthermore, the presence of space charge or imposed electric fields does not lead to thermal, MaxwellBoltzmann distribution ions. The thermal conditions required for meaningful equiliiria measurements are just not “natural” for this medium. Considering all this, one is not surprised that no mass spectrometry researcher had set about deliberately to develop apparatus for ion-molecule equilibria measurements. It is also not surprising that the discovery that this is possible was with apparatus that was radically diierent, but designed with other objectives in mind. The thermochemical data resulting from ion-

The design of an atmospheric pressure ionization/time-of-flight mass spectrometer using a beam deflection method

A novel time-of-flight (TOF) mass spectrometer configuration has been designed which can be interfaced to a continuous ion beam source produced by atmospheric pressure ionization. The TOF device uses a beam deflection method to sweep the ion beam past a slit placed near the ionization source in order to generate a start pulse for TOF detection. The beam sweep technique is modeled by computer simulation and optimized for the various experimental parameters. Nonvolatile samples are injected into the TOF device using liquid injection into a glow discharge atmospheric pressure ionization source in helium. A resolution of at least 519 at m/z 311 is obtained, which is limited by the experimental parameters available in our experiment. The mass resolution is computer modeled and it is shown that as the mass increases, the experimental constraints become less important, and the resolution will increase. It is predicted that using the correct experimental conditions and with the addition of an ion reflector that resolution of well over 1000 should be obtained.

Protonated ethanol and its neutral counterparts

Collisionally activated dissociation and neutralization-reionization experiments reveal that protonation of ethanol leads to two distinct isomers, the classical ion CH 3CH 2OH + 2 and the proton-bound complex C 2H 4…H +…OH 2. The neutral counterpart of the latter is unstable, whereas that of the former can be produced in a bound state if the CH 3CH 2OH + 2 precursor ion is formed under low ion source pressure conditions and, thus, with higher internal energies. This suggests that there are substantial differences in the geometries of CH 3CH 2OH + 2 and the hypervalent CH 3CH 2OH 2 ·. This provides only a partial explanation for unusual isotope effects; C 2H 5OD 2 ·, CH 3CD 2OD 2 ·, and CD 3CH 2OD 2 · are substantially more stable than C 2D 5OD 2 · and C 2H 5OH 2 ·.