Chemical Ionization Reagent Gas Research Articles

Hydrogen Plasma-Based Medium Pressure Chemical Ionization Source for GC-TOFMS.

The construction, critical evaluation, and performance assessment of a medium-pressure (2-13 mbar), high-temperature chemical ionization (CI) source for application in GC-MS is described. The ion source is coupled to a commercial time-of-flight (TOF) mass analyzer. Reagent ions are generated in a two staged process. The first stage uses a filament free, helical resonator plasma (HRP) driven ion source for H3+ generation. Reagent gases, for example, nitrogen, isobutane, and methane are added in a second stage to the H3+ stream, which leads to the formation of final protonation reagents. The GC effluent is added subsequently to the reagent ion gas stream. Designed for the hyphenation with gas chromatography, this GC-CI-TOFMS combination produces GC limited Gaussian peak shapes even for high boiling point compounds. Limits of detection for the compounds investigated are determined as 0.4-1.2 pg on column with nitrogen, 0.6-12.6 pg with isobutane, and 2 pg to >25 pg with methane as reagent gas, respectively. An EPA 8270 LCS mix containing 78 main EPA pollutants is used to evaluate the selectivity of the different reagent ions. Using nitrogen as reagent gas, 74 of 78 compounds are detected. In comparison, 41 of 78 compounds and 62 of 78 compounds are detected with isobutane or methane as CI reagent gas, respectively.

Qualitative Gas Chromatography–Mass Spectrometry Analyses Using Amines as Chemical Ionization Reagent Gases

Ammonia is a very useful chemical ionization (CI) reagent gas for the qualitative analyses of compounds by positive ion gas chromatography-mass spectrometry (GCMS). The gas is readily available, inexpensive, and leaves no carbon contamination in the MS source. Compounds of interest to our laboratory typically yield abundant protonated or ammoniated species, which are indicative of a compound's molecular weight. Nevertheless, some labile compounds fragment extensively by substitution and elimination reactions and yield no molecular weight information. In these cases, a CI reagent gas mixture of methylamine in methane prepared dynamically was found to be very useful in obtaining molecular weight data. Likewise, deuterated ammonia and deuterated methylamine are useful CI reagent gases for determining the exchangeable protons in organic compounds. Deuterated methylamine CI reagent gas is conveniently prepared by dynamically mixing small amounts of methylamine with excess deuterated ammonia.

Analysis of Hydrazine in Drinking Water by Isotope Dilution Gas Chromatography/Tandem Mass Spectrometry with Derivatization and Liquid−Liquid Extraction

A new isotope dilution gas chromatography/chemical ionization/tandem mass spectrometric method was developed for the analysis of carcinogenic hydrazine in drinking water. The sample preparation was performed by using the optimized derivatization and multiple liquid-liquid extraction techniques. Using the direct aqueous-phase derivatization with acetone, hydrazine and isotopically labeled hydrazine-(15)N2 used as the surrogate standard formed acetone azine and acetone azine-(15)N2, respectively. These derivatives were then extracted with dichloromethane. Prior to analysis using methanol as the chemical ionization reagent gas, the extract was dried with anhydrous sodium sulfate, concentrated through evaporation, and then fortified with isotopically labeled N-nitrosodimethylamine-d6 used as the internal standard to quantify the extracted acetone azine-(15)N2. The extracted acetone azine was quantified against the extracted acetone azine-(15)N2. The isotope dilution standard calibration curve resulted in a linear regression correlation coefficient (R) of 0.999. The obtained method detection limit was 0.70 ng/L for hydrazine in reagent water samples, fortified at a concentration of 1.0 ng/L. For reagent water samples fortified at a concentration of 20.0 ng/L, the mean recoveries were 102% with a relative standard deviation of 13.7% for hydrazine and 106% with a relative standard deviation of 12.5% for hydrazine-(15)N2. Hydrazine at 0.5-2.6 ng/L was detected in 7 out of 13 chloraminated drinking water samples but was not detected in the rest of the chloraminated drinking water samples and the studied chlorinated drinking water sample.

Simultaneous profiling of multiple neurochemical pathways from a single cerebrospinal fluid sample using GC/MS/MS with electron capture detection

Biogenic amines and amino acids are widely characterized in the pathways representing neurotransmission. Although several analytical methodologies have been used to detect specific target molecules in relevant fluids such as cerebrospinal fluid (CSF), multiple assays must be used to survey the primary pathways involved. This article describes the development of a GC/MS/MS method capable of analyzing up to 43 analytes (representing 20 amino acids and more than seven neurochemical pathways) from a single 50 microl CSF sample. In this procedure, a CSF sample is first treated with acetonitrile to precipitate proteins. The dried sample is then derivatized with a mixture of 2,2,3,3,3-pentafluoro-1-propanol and pentafluoropropionic acetic anhydride to replace all active hydrogen atoms with fluorine-containing groups. Due to the concentration difference between amino acids and neurotransmitters, these two compound classes are analyzed in separate injections of the same derivatized extract. The total run time for each injection is approximately 15-20 min. An essential feature of the method is the use of argon as a reagent gas for electron capture chemical ionization (ECCI), as the use of the more traditional gas (methane) lacked sufficient durability to be considered for use with the present instrumentation. This article describes the development of this method including a detailed investigation of the chemical ionization conditions used. The resultant conditions allow for the profiling of biogenic amines (e.g. serotonin, norepinephrine, and dopamine) in the low picogram per milliliter range.

Intramolecular fluorine migration via four-member cyclic transition states

Gaseous CF(3)(+) interchanges F(+) for O with simple carbonyl compounds. CF(3)(+) reacts with propionaldehyde in the gas phase to produce (CH(3))(2)CF(+) via two competing pathways. Starting with 1-(13)C-propionaldehyde, the major pathway (80%) produces (CH(3))(2)CF(+) with the carbon label in one of the methyl groups. The minor pathway (20%) produces (CH(3))(2)CF(+) with the carbon label in the central position. The relative proportions of these two pathways are measured by (19)F NMR analysis of the neutral CH(3)CF=CH(2) produced by deprotonation of (CH(3))(2)CF(+) at <10(-)(3) Torr in an electron bombardment flow (EBFlow) reactor. Formation of alkene in which carbon is directly bonded to fluorine means that (in the minor product, at least) an F(+) for O transposition occurs via adduct formation followed by 1,3-atom transfer and then isomerization of CH(3)CH(2)CHF(+) to the more stable (CH(3))(2)CF(+). Use of CF(4) as a chemical ionization (CI) reagent gas leads to CF(3)(+) adduct ions for a variety of ketones, in addition to isoelectronic transposition of F(+) for O. Metastable ion decompositions of the adduct ions yield the metathesis products. Decompositions of fluorocycloalkyl cations formed in this manner give evidence for the same kinds of rearrangements as take place in CH(3)CH(2)CHF(+). Density functional calculations confirm that F(+) for O metathesis takes place via addition of CF(3)(+) to the carbonyl oxygen followed by transposition via a four-member cyclic transition state. A computational survey of the effects of different substituents in a series of aldehydes and acyclic ketones reveals no systematic variation of the energy of the transition state as a function of thermochemistry, but the Hammond postulate does appear to be obeyed in terms of progress along the reaction coordinate. Bond lengths corresponding to the central barrier correlate with overall thermochemistry of the F(+) for O interchange, but in a sense opposite to what might have been expected: the transition state becomes more product-like as the metathesis becomes increasingly exothermic. This reversal of the naive interpretation of the Hammond postulate is accounted for by the relative positions of the potential energy wells that precede and follow the central barrier.

Mass spectrometry analysis of polychlorinated biphenyls: Chemical ionization and selected ion chemical ionization using methane as a reagent gas

In the present paper a quadrupole ion trap mass spectrometer, coupled with a gas chromatograph, was used to compare the electron impact ionization (EI) and chemical ionization (Cl) technique, in terms of their selectivity in polychlorinated biphenyls (PCBs) quantitative analysis. The experiments were carried out with a modified Varian SATURN III quadrupole ion-storage mass spectrometer equipped with Varian waveform generator, coupled with a gas chromatograph with DB-5 capillary column. The disadvantage of using EI in the analysis of PCBs congeners is the extensive fragmentation of the molecular ion. The main fragmentation pattern recorded in the EI mass spectra of PCBs was the loss of a chlorine atom from the molecular ion. Therefore the fragment-ion signal overlapped with the molecular-ion cluster of lower mass congener. The fragmentation reactions of PCBs are suppressed if methane is used as a reagent gas for chemical ionization, but fragment ions are also present in the spectrum as an obstruction for quantitative analysis. The most selective method for PCBs quantitative analysis appears to be Cl with mass-selected C2H5 + ions from methane, which results in a mass spectrum with a negligible amount of fragment ions.

Negative-ion chemical ionization of phenothiazines with oxygen

When O2 is used as a reagent gas for negative-ion chemical ionization (NCI) of phenothiazines and related compounds, the majority of the ions observed are formed by surface-catalyzed reactions prior to ionization. Both oxidation of the sulfur atom and formation of quinoid systems are observed. Alkyl derivatives also show degradation in addition to loss of the substituent. Deuterium labeling studies allowed several of the reaction paths to be elucidated. Copyright © 1999 John Wiley & Sons, Ltd.

Determination of the double bond position in functionalized monoenes by chemical ionization ion-trap mass spectrometry using acetonitrile as a reagent gas

Acetonitrile may be used as a chemical ionization reagent gas in ion-trap mass spectrometry for the location of double bonds in functionalized long-chain monoenes. [C3H4N]+, derived from acetonitrile, reacts with carbon–carbon double bonds, generating one or a pair of fragment ions characteristic of the position of the double bond in the chain. Tolerance of a range of functional groups is achieved by isolating [C3H4N]+ from the other acetonitrile ions before reaction with the alkene. Under these conditions, competing processes such as protonation are minimized, and a different mode of fragmentation from that previously observed with unfunctionalized alkenes11 is obtained. The technique is highly sensitive (<0.5 ng), allowing its use in the identification of insect pheromones. Copyright © 1999 John Wiley & Sons, Ltd.

Techniques for quantifying gaseous HOCl using atmospheric pressure ionization mass spectrometry

HOCl is speculated to be an important intermediate in mid-latitude marine boundary layer chemistry and at high latitudes in spring-time when surface-level O3 depletion occurs. However, techniques do not currently exist to measure HOCl in the troposphere. We demonstrate that atmospheric pressure ionization mass spectrometry (API-MS) is a highly sensitive and selective technique which has promise for HOCl measurements both in laboratory systems and in field studies. While HOCl can be measured as the (HOCl·O2)- adduct with air as the chemical ionization (CI) reagent gas, the intensity of this adduct is sensitive to the presence of acids and to the amount of water vapor, complicating its use for quantitative measurements. However, the addition of bromoform vapor to the corona discharge region forms bromide ions that attach to HOCl and allow its detection via the (HOCl·Br)- adduct. The ion–molecule chemistry associated with the use of air or bromoform as the CI reagent gas is discussed, with particular emphasis on the effects of relative humidity and gas phase acid concentrations. HOCl is quantified by measuring the amount of Cl2 formed by its heterogeneous reaction with condensed-phase HCl/H2O. Detection limits are ∽3 parts per billion (ppb) using air as the CI reagent gas and ∽0.9 ppb using bromoform. The atmospheric implications of this work are discussed.

Block microstructural characterization of copolymers formed from fluorinated and non-fluorinated alkyl polyisocyanates using desorption chemical ionization mass spectrometry

Homopolymers and copolymers of 1-isocyanato-4,4,5,5,6,6,7,7,8,8,9,9,9-tridecafluorononane (monomer F) and n-hexylisocyanate (monomer H) were examined by desorption chemical ionization mass spectrometry (DCI-MS) to obtain information on the monomer distribution in the copolymers. Tandem mass spectrometry (MS/MS) was used to characterize ions generated by DCI in the mass spectrometer ion source; ammonia and isobutane were selected as chemical ionization (CI) reagent gases. The major peaks in the ammonia DCI mass spectrum of poly(4,4,5,5,6,6,7,7,8,8,9,9,9-tridecafluorononyl)isocyanate (poly ( F)) are protonated and ammoniated trimers. This result suggests that on pyrolytic degradation poly ( F) forms cyclic trimers as do alkyl isocyanate polymers. The isobutane DCI mass spectra display the characteristic alkene elimination sequence characteristic of poly( n-hexyl)isocyanate and poly(2,6-dimethylheptyl)isocyanate but with additional extensive fragmentation. The major fragment ion is the protonated monomer. The monomer distributions in copolymers comprised of monomer F and monomer H were deduced from the abundances of various protonated and ammoniated trimers in the ammonia DCI mass spectra using Markovian statistics. Both soluble and insoluble copolymer samples were isolated and found to have non-random monomer distributions. The soluble fraction is dominated by monomer H blocks while the insoluble fraction also contains a majority of monomer H blocks but relatively more monomer F blocks. This forms an example in the polyisocyanates, which hitherto exhibited only random copolymerization, of a non-living method of polymerization yielding a block microstructure for a mixture of two monomers with virtually identical polymerizable functions. Mass spectrometry offers information on chain microstructure which would be unavailable by other means.

Methyl Iodide—An Interesting Reagent for Positive Ion Chemical Ionization

Methyl iodide (CH3I) has been used as a chemical ionization reagent gas and the ion/molecule reactions of formed reagents ions with a number of nucleophiles were studied in a chemical ionization source of a conventional mass spectrometer. Generalizing, charge exchange reactions dominate for compounds with lower ionization energy than CH3I, while the others preferred protonation and methylation reactions. For ketones both protonation and charge exchange reactions were thermodynamically possible and the prevailing reaction was dependent on the compound. Surprisingly, all the isomeric C5 ketones gave rise to clearly different spectra. Alcohols, having higher ionization energies than CH3I and a lower proton affinity than that of CH2I., also reacted interestingly. Ionization through hydroxide abstraction prevailed for C3–C6 alkanols. The formation of direct adduct ions was rare but, in contrast, substitution product ions [M − OH + CH3I]+ and [M − H2O + I]+ were observed for alkanols. © 1997 John Wiley & Sons, Ltd.

Methyl Iodide—An Interesting Reagent for Positive Ion Chemical Ionization

Methyl iodide (CH3I) has been used as a chemical ionization reagent gas and the ion/molecule reactions of formed reagents ions with a number of nucleophiles were studied in a chemical ionization source of a conventional mass spectrometer. Generalizing, charge exchange reactions dominate for compounds with lower ionization energy than CH3I, while the others preferred protonation and methylation reactions. For ketones both protonation and charge exchange reactions were thermodynamically possible and the prevailing reaction was dependent on the compound. Surprisingly, all the isomeric C5 ketones gave rise to clearly different spectra. Alcohols, having higher ionization energies than CH3I and a lower proton affinity than that of CH2I., also reacted interestingly. Ionization through hydroxide abstraction prevailed for C3–C6 alkanols. The formation of direct adduct ions was rare but, in contrast, substitution product ions [M − OH + CH3I]+ and [M − H2O + I]+ were observed for alkanols. © 1997 John Wiley & Sons, Ltd.

Mass spectrometric behaviour of 1‐imino‐substituted pyrazolo[1,2‐a]‐[1,2,4]triazoles under electron and chemical ionization

AbstractThe spectra of five pharmacologically interesting substituted pyrazolo[1,2‐a][1,2,4]triazole hydroiodides were measured under electron and chemical ionization. In the electron ionization spectra, in addition to the intense molecular ion peak of the free base (M+*), there was also a relatively intense molecular ion peak of the hydroiodide form, which is unusual since the hydroiodides are rarely so stable. The phenylimino and phenylamino substituents of the triazole ring affected the fragmentation behaviour of the compounds very much. The chemical ionization reagent gases used in this work were methane, isobutane, deuterated ammonia and acetone. In all the cases practically only [M+H]+ ions were observed, the only exception being acetone which also gave rise to intense [M+C2H3O]+ and [M+C3H7O]+ adduct ions. None of the reagent gases used was able to cause any fragmentation.

Reactions of Hydantoin and Succinimide Anticonvulsants with Dimethyl Ether Ions in a Quadrupole Ion Trap Mass Spectrometer

The analysis of hydantoin and succinimide anticonvulsants was carried out using techniques of chemical ionization and collision activated dissociation (CAD) in a quadrupole ion trap. Dimethyl ether was used as the chemical ionization reagent gas to generate [M + H] + and [M + 13] + products, with the objective of determining whether the protonation and methylene substitution reactions occurred at the same site and whether the products provided the same structural information. CAD, including MS 3 , SWIFT double resonance experiments and deuterium-labeling studies suggest that protonation occurs on a nitrogen atom or carbonyl oxygen while methylene substitution occurs predominantly on the phenyl substituents. Both [M + H] + and [M + 13] + ions dissociate by similar pathways in which typically the attached proton or methylene group remains a spectator.

Ion-Molecule Reactions of 2-Methoxyethanol with Ketones, Aldehydes, Alcohols and Phenols under Chemical Ionization Conditions

2-Methoxyethanol has been used as a chemical ionization (CI) reagent gas in a conventional ion source and it has been found to produce numerous reagent ions, including an ion at m/z 89, [CH 3 OCH 2 CH 2 OCH 2 ] + . These ions react with different classes of compounds, e.g. by formation of variety of adduct ions. Neutral ketones (RC(O)R') were observed to yield without exception [M - R + 44] + and/or [M - R'+ 44] + product ions. Similarly, for aldehydes the formation of the product ion [M - H + 44] + is common. Selectivity was noticed especially for the neutral alcohols studied, which resulted in the formation of [M + 27] + and [M + 89] + product ions, and also for the neutral phenols studied, which resulted in the formation of [M + 13] + , [M + 27] + and [M + 89] + product ions. There is a clear correlation between the structure of isomeric neutral alcohols studied and the abundances of the product ion [M + 89] + ; the more hindered the alcohol, the lower is the abundance of the [M + 89] + ion. The [M + 13] + product ion for phenols is a characteristic fragment of the [M + 89] + ion produced by elimination of neutral 2-methoxyethanol. The greatest abundance of the [M + 13] + ion for m-cresol and the use of labelled compounds and labelled reagent gases, CD 3 OCH 2 CH 2 OH and CH 3 OCH 2 CH 2 OD, suggest that the addition occurs mainly to the aromatic ring, ortho and para positions being more favoured. The collision-induced dissociation technique was used to characterize the structures of the product ions

Quantitative analysis of fluorimated ethylchloroformate derivatives of non-protein amino acids using positive and negative chemical ionization gas chromatography-mass spectometry

The GC-MS characterization of the ethylchloroformate derivatives of amino acids in an aqueous medium has been applied to non-protein amino acids. Derivatization of non-protein amino acids using ethylchloroformate, trifluoroethanol, and pyridine produced strong [M + 1] + and [M - 1] − ions in positive and negative chemical ionization (CI) modes, respectively. Twenty-one out of the twenty-three non-protein amino acids studied produced detectable ion chromatograms in both ionization modes when methane was used as the CI reagent gas. Mass spectra of these non-protein amino acid derivatives showed characteristic [M - 19] +, [M + 1] +, [M + 29] +, and [M + 41] + peaks in the positive chemical ionization mode, and [M - 1] −, and [M + 35] − peaks in the negative chemical ionization mode. The detection limits and the linear dynamic range of trifluorethanol ethylchloroformate derivatives of non-protein amino acids were studied using positive chemical ionization. The detection limits are mostly in the femtomole range.

Trace level analysis of volatile and semi-volatile organic compounds in water using a membrane/jet separator interfaced to an ion trap mass spectrometer

An ion trap mass spectrometer, equipped with a membrane/jet separator interface, is used for the direct detection of volatile and semi-volatile organic compounds in aqueous solutions. Aqueous sample is passed through a capillary membrane, the outside surface of which is continuously purged by helium. The permeate is pneumatically transported to the mass spectrometer via a jet separator which acts as an additional enrichment device. The performance and response characteristics of non-porous silicone and microporous polytetrafluoroethylene (PTFE) membranes are studied. The microporous membrane allows sufficient water to pass for it to be used as a reagent gas for chemical ionization. Both types of membranes provide detection limits in the parts per trillion (pptr) to parts per billion (ppb) range with a linear dynamic range of 3 orders of magnitude for some volatile organic compounds. Results show that there is no detectable matrix effect on response in the selected cases examined. The use of microporous membranes to analyze more polar compounds, such as 5-hydroxymethyl furfuraldehyde and lactic acid, is also demonstrated. The effects of other experimental parameters, such as membrane temperature and length, on sensitivity are also investigated.

Triton X‐100 as a reference compound for ammonia high‐resolution chemical ionization mass spectrometry and as a tuning and calibration compound for thermospray

AbstractThe non‐ionic surfactant Triton X‐100, 1‐(1,1,3,3,‐tetramethylbutyl)‐4‐[poly(1,2‐ethanediol)]benzene, has been evaluted as a reference material for positive‐ion high‐resolution ammonia chemical ionization (CI) mass spectrometry and as a tuning and calibration material for positive‐ion thermospray mass spectrometry (TSP). When ammonia is used as the CI reagent gas, Triton‐X‐100 produces evenly spaced ions that 44 u apart. Triton X‐100 cab be volatilized from a solids prove and is routinely used as the reference material for high‐resolution measurements. A reference table has been constructed from the theoretical values for the different masses. Triton X‐100‐Reduced and Triton X‐405‐Reduced were also evaluated as additional reference com‐pounds. As a calibration and tuning compound for TSP, Triton X‐100 affords an envelope of ions centred around m/z 600–700 with ions observed above m/z 1000. Triton X‐100‐Reduced affords a similar envelope although the ions observed are 6 u higher in mass.

Determination of an irreversible inhibitor of monoamine oxidase B (MDL 72974A) in human plasma and urine by gas chromatography—positive-ion chemical ionization mass spectrometry

A sensitive and specific assay has been developed for the quantitative measurement in human plasma and urine of the irreversible inhibitor of monoamine oxidase B [( E)-4-fluoro-β-fluoromethylenebenzene-butanamine HCl salt] (MDL 72974A) (I). This assay is based on gas chromatography—mass spectrometry with ammonia as the chemical ionization reagent gas. After addition of 1-fluoro-2-(4-chlorobenzene)-ethanamine HCl salt (MDL 71946A) as the internal standard, plasma (1 ml) and urine (100 μl) samples were extracted using an automated solid—liquid extraction procedure on CN columns. The eluent was dried with a stream of nitrogen, and the residue was derivatized with pentafluoropropionic anhydride. Selected-ion monitoring of the [MNH 4] + ions m/z 361 (I) and 351 (internal standard) was used for quantification. The method yielded a linear response over the concentration range 0.25–100 pmol/ml in plasma with a limit of quantitation of 0.25 pmol/ml. The within-day reproducibility at a concentration of 5 pmol/ml was 4.6% and at a concentration of 50 pmol/ml was 1.3%. The day-to-day reproducibility was 5.2 and 7.0% at concentrations of 10 and 30 pmol/ml, respectively. The method was applied to the quantification of 1 in plasma and urine after the administration of 12-mg doses of I to a healthy male volunteer.

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.