Reagent Gas Pressure Research Articles

Isobutane Made Practical as a Reagent Gas for Chemical Ionization Mass Spectrometry.

As a reagent gas for positive- and negative-mode chemical ionization mass spectrometry (CI-MS), isobutane (i-C4H10) produces superior analyte signal abundance to methane. Isobutane has never been widely adopted for CI-MS because it fouls the ion source more rapidly and produces positive CI spectra that are more strongly dependent on reagent gas pressure compared with methane. Isobutane was diluted to various concentrations in argon for use as a reagent gas with an unmodified commercial gas chromatograph-mass spectrometer. Analyte spectra were directly compared using methane, isobutane, and isobutane/argon mixtures. A mixture of 10% i-C4H10 in argon produced twice the positive-mode analyte signal of methane, equal to pure isobutane, and reduced spectral dependence on reagent gas pressure. Electron capture negative chemical ionization using 1% i-C4H10 in argon tripled analyte signal compared with methane and was reproducible, unlike pure isobutane. The operative lifetime of the ion source using isobutane/argon mixtures was extended exponentially compared with pure isobutane, producing stable and reproducible CI signal throughout. By diluting the reagent gas in an inert buffer gas, isobutane CI-MS experiments were made as practical to use as methane CI-MS experiments but with superior analytical performance. Graphical Abstract ᅟ.

Determination of synthetic nitro-musks in cosmetics by gas chromatography coupled with negative chemical ionization-triple quadrupole mass spectrometry

A confirmatory method is presented for the determination of five nitro-musks (musk ambrette, musk xylene, musk moskene, musk tibeten and musk ketone) in different cosmetics by gas chromatography coupled with negative chemical ionization (NCI)-triple quadrupole mass spectrometry (GC-MS/MS). The samples were extracted under ultrasonication using a mixture of acetone and hexane. The extract was concentrated and then cleaned up by CNWBOND Si solid phase extraction cartridge. Five different instrument parameters such as the temperature programmed, ion source temperature, reagent gas pressure, collision energy, monitoring ion pairs were optimized for higher sensitivity. Then the analytes were qualitatively and quantitatively analyzed under the multiple reaction monitoring (MRM) mode after the chromatographic separation on an HP-5MS capillary column (30 m x 0.25 mm, 0.25 microm), and employing d15-musk xylene as internal standard. The mixed standards were spiked in the blank cosmetics samples (each nitro-musk was about 500 ng/kg), and the recoveries were in the range of 85.81%-103.77% with the relative standard deviations (RSDs) not more than 5.32%. The limits of quantification of the method were about 50.0-500 ng/kg. The method is accurate, rapid, sensitive and can be used in the inspection of the five nitro-musks in cosmetics.

Monomer Formation Model versus Chain Growth Model of the Fischer–Tropsch Reaction

One of the great challenges in molecular heterogeneous catalysis is to model selectivity of a heterogeneous catalytic reaction based on first principles. Molecular kinetics simulations of the Fischer–Tropsch reaction, which converts synthesis gas into linear hydrocarbons, demonstrate the need for microkinetics approaches that do not make a priori choices of rate controlling steps. A key question pertaining to this reaction, in which hydrocarbons are formed through consecutive insertion of adsorbed CHx monomers into adsorbed growing hydrocarbon chains, is whether the CO consumption rate depends on the rate of the CHx insertion polymerization process. Microkinetic theory of this heterogeneous catalytic reaction based on quantum-chemical data is used to deduce expressions for the CO consumption rate and chain growth parameter α in the two limiting cases where chain growth rate is fast compared to the formation of CHx (monomer formation limit) or where the reverse relation holds (chain growth limit). The conventional assumptions that CHx formation is rate controlling and that change in CO coverage due to reaction is negligible lead to substantial overestimation of the rate of CO consumption. It appears that intermediate reactivity of the catalytic reaction center, with neither too low nor too high activation energies for C–O bond cleavage, and low reagent gas pressure lead to such monomer formation limiting type behavior, whereas maximum rate of CO consumption is found when chain growth rate is limiting.

Determination of the volatiles from tobacco by capillary gas chromatography with atomic emission detection and mass spectrometry

A new gas chromatograph-atomic emission detector (GC-AED) coupled with Deans switching technique for analyzing volatiles from tobaccos were developed. The detector operating parameters (reagent gas pressure and make-up gas flow rate) were optimized. The detection limits for the elements carbon (193 nm), hydrogen (486 nm) and oxygen (171 nm) ranged 0.05-0.2, 0.05-0.3 and 1-11 ng, respectively, depending on the compound. The sensitivity and linearity for the elements carbon (193 nm), hydrogen (486 nm) and oxygen (171 nm) decreased in the order O>H>C. Calibration curves were obtained by plotting peak area versus concentration, and the correlation coefficients relating to linearity were at least 0.9359. Elemental response factors measured on these channels, relative to the carbon 193-nm channel, were hydrogen, 0.38-0.48 (mean %RSD=5.64), and oxygen, 0.085-0.128 (mean %RSD=14.9). The evaluation was also done for the new technique and for an established GC-MS technique for the same real samples. The results of GC-AED and GC-MS showed that there was a relatively good agreement between the two sets of data.

ChemInform Abstract: Optimization of Pyrolysis-Desorption Chemical Ionization Mass Spectrometry and Tandem Mass Spectrometry of Polysaccharides.

The operating conditions for pyrolysis–desorption ammonia chemical ionization mass spectrometry and tandem mass spectrometry have been optimized and the technique evaluated for the production and analysis of structurally-informative pyrolytic fragmentation ions corresponding to intact anhydrohexose oligosaccharides, using amylose as the model polysaccharide. Among the various parameters examined it was found that the nature of the solvent used to adhere the sample to the emitter coil and the configuration of the emitter and the rate at which it is heated all play important roles in determining the efficiency of the pyrolytic process and the production of high mass fragment ions. Adjustment of reagent gas pressure together with source temperature also influence the chemical integrity of high mass oligomeric pyrolysis products. Under optimal operating conditions using ammonia reagent gas, the analyses of cellulose, laminarin, agars, and chitin gave relatively abundant ions corresponding to ammonium (or prot...

Gas chromatography negative ion chemical ionization mass spectrometry: Application to the detection of alkyl nitrates and halocarbons in the atmosphere

Alkyl nitrates and very short-lived halocarbon species are important atmospheric trace gas species that are present in the low to sub parts per trillion concentration range. This presents an analytical challenge for their detection and quantification that requires instrumentation with high sensitivity and selectivity. In this paper, we present a new in situ gas chromatograph negative ion chemical ionization mass spectrometer (GC/NICI–MS) coupled to a non-cryogen sample pre-concentration system. This instrument, with detection limits of <0.01 ppt, is capable of detecting and quantifying a large suite of alkyl nitrate and halocarbon species with high sensitivity and precision. The effects of ion source temperature and reagent gas pressure on the ionization efficiency of the NICI mode are investigated and the results are used to optimize the sensitivity. The NICI mode is compared to the more frequently used electron impact (EI) ionization and the enhancements in sensitivity are presented for all the calibrated compounds.

Mass Spectral Analysis of Chloropicrin under Negative Ion Chemical Ionization Conditions

A chemical ionization (CI) method is developed for the first time to obtain molecular weight information for chloropicrin (CP), which is used as a chemical warfare agent and as an insecticide. The study includes a detailed investigation on the behavior of CP under electron impact (EI) and CI. Reagent gases of different nature, i.e., methane, isobutane, ammonia, hydrogen, and carbon dioxide, were used for CI analysis. Negative ion mode is found more sensitive than positive ion mode for the EI/CI mass spectrometric analysis of CP, but none of the methods provided molecular weight information, except negative ion CI using ammonia as the reagent gas (NICI (NH3)). The NICI (NH3) showed formation of the quasi-molecular ion, [M + H]-, in addition to other adduct ions. The [M + H]- abundance critically depends on the source temperature, reagent gas pressure, and concentration of the analyte, and it can be 13% under optimized conditions by which CP can be confirmed unambiguously. This method meets the criteria used in official proficiency tests conducted by OPCW for confirming the molecular weight of the unknowns.

Probing the Effects of Reagent Gas Pressure and Ion Source Temperature for Dimethyl Ether Chemical Ionization of Tricyclic Antidepressants in an External Source Ion Trap Mass Spectrometer

AbstractThis study examines the reagent gas pressure and ion source temperature dependence for dimethyl ether chemical ionization (DME CI) mass spectra recorded with an external source ion trap mass spectrometer (ITMS). Information for better controls of the reagent gas pressure in order to obtain fair CI spectra is provided. The origin of M+• ions observed in DME CIMS is discussed in detail. Furthermore, the ion source temperature effect on the DME CI is also investigated.

The utility of ion–molecule reactions in a quadrupole ion trap mass spectrometer for analyzing metal complex coordination structure

Gas-phase ion–molecule reactions of metal complex ions with acetonitrile in a quadrupole ion trap mass spectrometer are shown to have some potential for determining the number and types of functional groups bound to a metal. Metal complexes with varying coordination spheres show significant differences in their gas-phase reactivity with acetonitrile. The relative product ion intensities observed in the mass spectrum after a given reaction time provide the appropriate data to distinguish complexes with different coordination spheres. Experimental parameters suspected to have an effect on the reproducibility of these intensities are examined in order to understand better which factors need to be most closely controlled to maximize precision. Ni(II) and Cu(II) complex ions of the pentadentate ligands 1,9-bis(2-imidazolyl)-2,5,8-triazanonane (DIEN-(imi) 2) and 1,9-bis(2-tetrahydrofuranyl)-2,5,8-triazanonane (DIEN-(THF) 2) are reacted in the gas-phase at different temperatures, pressures of acetonitrile, and pressures of buffer gas (helium) in a modified quadrupole ion trap mass spectrometer. The effects of such experimental variations on the product ion intensities are measured and analyzed. Under conditions where the precision is maximized, the temperature and acetonitrile pressure seem to have the biggest impact on the reproducibility of the resulting product ion intensity with the acetonitrile level being slightly more important. The reproducibility of ion intensities for the reactions of the Ni(II) complexes are the lowest due to their higher reactivity, while the measurements for the Cu(II) complexes have a higher degree of precision. In general, with careful control of the temperature and reagent gas pressure, ion–molecule (I–M) reactions seem to be a promising method for providing a rapid and sensitive analysis of the functional groups bound to a metal in a given complex.

Electron CaptureNegative Ionization Mass Spectrometric Characteristics of Bioaccumulating Methyl SulfoneSubstituted Polychlorinated Biphenyls

Gas chromatography/electron-capture negative ionization mass spectrometry (GC/ECNI-MS) was used to separate and detect a mixture of 24, 3-and 4-methyl sulfone-substituted polychlorinated biphenyls (MeSO2-PCBs). The dominant fragment ions were [M-CH3]- and [M-Cl+H]-, and to a lesser extent [M-2Cl]-, [M-2Cl+2H]-, [M-CH3-Cl+H]- and [M-SO2CH3+H]-). The structures of the 3-and 4-MeSO2-PCBs varied in the degree (i.e. tetrachloro-to heptachloro-) and position of chlorine substitution. With the assistance of multivariate statistical analysis, the abundance of several isotopic clusters of fragment ions relative to that of the molecular ion (M-) was found to vary as a function of MeSO2-PCB structure. Compared with other congeners, the relative abundance of [M-CH3]- was significantly lower for MeSO2-PCBs containing the 4-MeSO2-2,5-dichloro moiety, and the hexachloro-and heptachloro-MeSO2-PCBs had higher relative abundances of [M-Cl+H]-. Subtle ECNI mass spectral fragmentation rules therefore exist for 3-and 4-MeSO2-PCBs with respect to congener structure; however, both ion source temperature and pressure must be controlled. Increasing the ion source temperature from 100 to 260°C resulted in an large increase in the abundance of [M-CH3]- relative to M- from <20% to >100%, for all the MeSO2-PCBs studied. For the set of ten 3-/4-MeSO2-PCB pairs, the [M-CH3]- relative abundance ratio was independent of ion source temperature, and was ∽4.5 and ∽1.5 for congeners possessing 2,5-dichloro and 2,5,6-trichloro substitution on the MeSO2-containing phenyl ring, respectively. The effect of an increase in ion source temperature, over the same temperature range, on the relative abundance of hydrogen inclusion-type fragment ions was much less dramatic. [M-CH3-Cl+H]- and [M-Cl+H]- abundance often increased, but never exceeded ∽40%. Decreasing the methane reagent gas pressure from 0.5 to 0.05 mbar did not significantly alter the ion source temperature effect on the relative abundance of the fragment ions. However, the ECNI response sensitivity decreased by an order of magnitude. © 1997 by John Wiley & Sons, Ltd.

Electron Capture/Negative Ionization Mass Spectrometric Characteristics of Bioaccumulating Methyl Sulfone-Substituted Polychlorinated Biphenyls

Gas chromatography/electron-capture negative ionization mass spectrometry (GC/ECNI-MS) was used to separate and detect a mixture of 24, land 4-methyl sulfone-substituted polychlorinated biphenyls (MeSO 2 PCBs). The dominant fragment ions were [M-CH 3 ] - and [M - Cl + H] - , and to a lesser extent [M - 2Cl] - , [M - 2Cl + 2H] , [M - CH 3 -Cl + H] - and [M -SO 2 CH 3 + H] - ). The structures of the 3-and 4-MeSO 2 -PCBs varied in the degree (i.e. tetrachloro-to heptachloro-) and position of chlorine substitution. With the assistance of multivariate statistical analysis, the abundance of several isotopic clusters of fragment ions relative to that of the molecular ion (M - ) was found to vary as a function of MeSO 2 -PCB structure. Compared with other congeners, the relative abundance of (M-CH 3 ] - was significantly lower for MeSO 2 -PCB 8 containing the 4-MeSO 2 -2,5-dichloro moiety, and the hexachloro-and heptachloro-MeSO2-PCBs had higher relative abundances of [M - Cl + H] - . Subtle ECNI mass spectral fragmentation rules therefore exist for land 4-MeSO2-PCBs with respect to congener structure; however, both ion source temperature and pressure must be controlled. Increasing the ion source temperature from 100 to 260°C resulted in an large increase in the abundance of [M - CH 3 ] - relative to M - from 100%, for all the MeSO 2 -PCBs studied. For the set of ten 3-/4-MeSO 2 -PCB pain, the [M - CH 3 ] - relative abundance ratio was independent of ion source temperature, and was ∼4.5 and ∼1.5 for congeners possessing 2,5-dichloro and 2,5,6-trichloro substitution on the MeSO 2 -containing phenyl ring, respectively. The effect of an increase in ion source temperature, over the same temperature range, on the relative abundance of hydrogen inclusion-type fragment ions was much less dramatic. [M-CH 3 -Cl+ H] - and [M - Cl + H] - abundance often increased, but never exceeded 40%. Decreasing the methane reagent gas pressure from 0.5 to 0.05 mbar did not significantly alter the ion source temperature effect on the relative abundance of the fragment ions. However, the ECNI response sensitivity decreased by an order of magnitude.

Real-time monitoring of the gas phase reactions of a single ion population using the remeasurement experiment in Fourier transform ion cyclotron resonance mass spectrometry.

A single population of multiply charged protein ions formed by electrospray ionization is held in a trapped ion cell and remeasured continuously by Fourier transform ion cyclotron resonance (FTICR) mass spectrometry while undergoing multiple reactions with diethylamine. In a first example, electrosprayed horse myoglobin with an average of 16 attached protons is reacted with base at a pressure of 1.5 x 10(-8) Torr for a period of 60 s. A total of 31 spectra acquired with a duty cycle of 2 s exhibit the charge state-dependent formation of up to three diethylamine adducts and removal of up to nine protons. A realtime measuring experiment is then conducted over a 1 h period to observe charge stripping of electrosprayed myoglobin ions, leaving as few as seven charges. Realtime monitoring is used to evaluate the effect of reagent gas pressure on adduct formation and is used in conjunction with high mass resolution FTICR detection to resolve the isotopic peaks within individual charge states of adduct spectra. The attractive features of real-time reaction monitoring include a dramatic reduction in experiment time and sample consumed while concurrently observing long-lived intermediates and reaction products as they form.

Optimization of pyrolysis–desorption chemical ionization mass spectrometry and tandem mass spectrometry of polysaccharides

The operating conditions for pyrolysis–desorption ammonia chemical ionization mass spectrometry and tandem mass spectrometry have been optimized and the technique evaluated for the production and analysis of structurally-informative pyrolytic fragmentation ions corresponding to intact anhydrohexose oligosaccharides, using amylose as the model polysaccharide. Among the various parameters examined it was found that the nature of the solvent used to adhere the sample to the emitter coil and the configuration of the emitter and the rate at which it is heated all play important roles in determining the efficiency of the pyrolytic process and the production of high mass fragment ions. Adjustment of reagent gas pressure together with source temperature also influence the chemical integrity of high mass oligomeric pyrolysis products. Under optimal operating conditions using ammonia reagent gas, the analyses of cellulose, laminarin, agars, and chitin gave relatively abundant ions corresponding to ammonium (or protonated) adducts of up to anhydrohexose tetrasaccharide. More importantly, the generation of such higher mass fragment ions provided a sustained ionic current of sufficient duration to perform tandem mass spectrometric analyses.

Gas Phase Reactions of Transition Metal Clusters

AbstractThe chemical reactions of isolated clusters of transition metal atoms with small molecules will be reviewed. Reactions of iron, cobalt, and nickel clusters with hydrogen, ammonia, water, and nitrogen will be considered. Experiments which determine the uptake of adsorbate molecules vs. reagent gas pressure provide information on kinetic and equilibrium processes, as well as a measure of the number of adsorbate binding sites on a cluster's surface. More detailed, quantitative determinations of reaction rate constants often show dramatic variation of reactivity with cluster size. Equilibrium studies provide measurements of cluster‐adsorbate binding energies. A knowledge of the number of adsorbate binding sites often provides information about cluster geometrical structure.

Ionization mechanisms in capillary supercritical fluid chromatography—chemical ionization mass spectrometry

Ionization mechanisms have been studied for supercritical fluid chromatography (SFC) with mass spectrometric (MS) detection. One of the problems associated with SFC-MS is the interference of mobile phase constituents in the ionization process, which complicates the interpretation of the resulting mass spectra. This interference can be reduced by adding a reagent gas to the ion source. It was found that the properties and the pressure of this reagent gas control the ionization process. In this study ammonia was used as a chemical ionization (CI) reagent gas. An increase in the reagent gas pressure generally resulted in higher abundances of the protonated molecular ion. The presence of an excess of reagent gas suppresses charge exchange processes between the mobile phase constituents and the solutes. Charge exchange causes a more pronounced fragmentation than proton transfer in CI processes. The spectra obtained by charge exchange ionization, with helium as the reagent gas at moderately high pressures, are comparable to electron ionization spectra from standard MS libraries.

Negative ion desorption chemical ionization mass spectrometry of flavonoid glycosides

Negative ion mass spectra of underivatized flavonoid O- and C-glycosides were recorded using desorption chemical ionization (DCI) at a low reagent gas pressure. Advantages and limitations of the method for the characterization of the mono-, di- and triglycosides are discussed. For all O-glycosides investigated, the aglycone moieties are found to be base anions. The molecular masses of the glycosides are indicated by the presence of [M]−. ions with fairly high abundances for monoglycosides, lower abundances for diglycosides and a very low intensity for triglycosides. In addition to the sugar moiety losses, anions which resulted from the stepwise elimination of water from the sugar moiety were also observed at m/z 290 or 306 in 1 6 linked bioses, m/z 127, 145 and 163 in deoxyhexose as well as m/z 161 and 143 in hexose. Those ions except ions due to the molecule and the retro-Diels–Alder cleavage of the aglycone moiety could not be found in electron ionization mass spectra. Consequently, information concerning the structure of the aglycone moiety could be obtained using the EI technique and of sugar moiety using the negative ion DCI technique; also the combined positive and negative ion was shown to be a good approach for the structural characterization of flavonoid glycosides.

Low‐resolution accurate mass measurement in self‐chemical ionization mass spectrometry

AbstractLow‐resolution (2000, 10% valley definition) accurate mass measurements using self‐chemical ionization (self‐CI) have been evaluated as an alternative to the conventional chemical ionization (CI) method. In conventional CI experiments a high pressure of reagent gas is required to induce ionization while in self‐CI no reagent gas is used and the self‐CI is produced presumably by molecular/fragment ion–molecule reactions. Nine compounds ranging in mass from 50–500 daltons were examined. Results obtained by the self‐CI method indicate that the elemental composition assignment can be obtained simultaneously for the protonated molecule and/or molecular ion. It is also shown that perfluorokerosene can be used routinely in self‐CI as an internal reference standard over a broad mass range (50–500 daltons). It is sometimes difficult to use a single reference standard in conventional low‐resolution CI accurate mass spectrometry over a similar mass range.

Application of isobutane and ammonia chemical ionization mass spectrometry for the analysis of volatile terpene alcohols and esters

AbstractThe ionization and fragmentation behaviour of mono‐and sesqui‐terpene alcohols and esters in CI mass spectrometry was investigated. The reagent gases used were isobutane and ammonia. Isobutane yields the species [MH]+ as the quasimolecular ions which, however, are extremely small for many of the investigated compounds. Ammonia gives intense [M + 18]+ quasimolecular adduct ions. The assignment of the molecular mass is more reliable in ammonia than in isobutane CI. Alcohols and esters form characteristic fragment ions. Alcohols show the mass difference of 18 mass units and esters display the difference of the respective acid in the patterns of the CI spectra. This fact enables one to distinguish the classes of the terpene alcohols and esters by means of CI. Particular compounds, as well as cis/trans and stereoisomers, can be differentiated by their ion patterns. CI spectra are strongly dependent on experimental conditions. High ion source temperature favours fragmentation diminishing the relative intensity of the quasimolecular ion. High ammonia reagent gas pressure reduces the relative intensities of fragment ions enhancing the relative abundancies of the quasimolecular adduct ions.Mass spectrometry Chemical ionization (CI) Isobutane reagent gas Ammonia reagent gas Monoterpenes Sesquiterpenes Alcohols Esters.

Structures of gas phase C5H8 radical cations: A collisional ionization study

AbstractCollisional ionization (charge stripping) and charge exchange ionization spectrometry were utilized to determine structures of fourteen cyclic and acyclic C5H8 radical cations, including ionized 1,2‐ 1,3‐, 1,4‐ and 2,3‐pentadienes (‐PD), isoprene, 1‐ and 3‐methylcyclobutenes (1‐ and 3‐MCB), 3‐methyl‐1,2‐butadiene (3‐M‐1,2‐BD), methenecyclobutane (MECB), cyclopentene, 3‐methyl‐1‐butyne (3‐MB), 1‐ and 2‐pentynes and vinylcyclopropane (VCP). The pressure of the charge exchange reagent gas in the ion source was adjusted to generate ions of different energy contents. The structures of the C5H8 ions are energy dependent, and their isomerization reactions can be monitored as a function of the amount of internal energy deposited by charge exchange. 1,3‐PD, isoprene and cyclopentene radical cations are identified as stable ion structures. 1‐MCB, 3‐M‐1,2‐BD and 3‐MB radical cations isomerize to isoprene ions, whereas ionized VCP, 3‐MCB, 1,2‐PD, 2,3‐PD, 1,4‐PD, 1‐pentyne and 2‐pentyne ultimately isomerize to the [1,3‐PD]+˙. Thermodynamic arguments are invoked to corroborate these isomerization reactions. The critical energies of the isomerizations are also estimated.

On the evaluation of ion source conditions for optimum sensitivity of chemical ionization

AbstractThe best operating conditions that yield maximum sensitivity at minimum source pressure have been evaluated for the chemical ionization source in a Kratos MS‐25 mass spectrometer. The best reagent gas pressure is shown to be different for each gas and is significantly lower than is usually reported in the literature.