Selective Chemical Ionization Research Articles

Pesticide Residue Fast Screening Using Thermal Desorption Multi-Scheme Chemical Ionization Mass Spectrometry (TD-MION MS) with Selective Chemical Ionization.

In this work, the detection characteristics of a large group of common pesticides were investigated using a multi-scheme chemical ionization inlet (MION) with a thermal desorption unit (Karsa Ltd.) connected to an Orbitrap (Velos Pro, Thermo Fisher Scientific) mass spectrometer. Standard pesticide mixtures, fruit extracts, untreated fruit juice, and whole fruit samples were inspected. The pesticide mixtures contained 1 ng of each individual target. Altogether, 115 pesticides were detected, with a set of different reagents (i.e., dibromomethane, acetonylacetone, and water) in different polarity modes. The measurement methodology presented was developed to minimize the common bottlenecks originating from sample pretreatments and nonetheless was able to retrieve 92% of the most common pesticides regularly analyzed with standardized UHPLC-MSMS (ultra-high-performance liquid chromatography with tandem mass spectrometry) procedures. The fraction of detected targets of two standard pesticide mixtures generally quantified by GC-MSMS (gas chromatography with tandem mass spectrometry) methodology was much less, equaling 45 and 34%. The pineapple swabbing experiment led to the detection of fludioxonil and diazinon below their respective maximum residue levels (MRLs), whereas measurements of untreated pineapple juice and other fruit extracts led to retrieval of dimethomorph, dinotefuran, imazalil, azoxystrobin, thiabendazole, fludioxonil, and diazinon, also below their MRL. The potential for mutual detection was investigated by mixing two standard solutions and by spiking an extract of fruit with a pesticide's solution, and subsequently, individual compounds were simultaneously detected. For a selected subgroup of compounds, the bromide (Br-) chemical ionization characteristics were further inspected using quantum chemical computations to illustrate the structural features leading to their sensitive detection. Importantly, pesticides could be detected in actual extract and fruit samples, which demonstrates the potential of our fast screening method.

Pesticide Residue Pre-screening Using Thermal Desorption Multi-Scheme Chemical Ionization Mass Spectrometry (TD MION MS) with Selective Chemical Ionization

Pesticide Residue Pre-screening Using Thermal Desorption Multi-Scheme Chemical Ionization Mass Spectrometry (TD MION MS) with Selective Chemical Ionization

The new approach to determination of the combined isotopic composition of methane for its genetic characterization

A new approach to mass-spectrometric determination of the combined isotopic composition of methane has been put forward. The approach is based on the preliminary on-line conversion of methane to methanol in corona discharge and selective chemical ionization. The approach provides means to lower requirements for equipment, consumables, and operation conditions and offers a challenge to develop a new field method.

Water Radical Cations in the Gas Phase: Methods and Mechanisms of Formation, Structure and Chemical Properties.

Water radical cations, (H2O)n+•, are of great research interest in both fundamental and applied sciences. Fundamental studies of water radical reactions are important to better understand the mechanisms of natural processes, such as proton transfer in aqueous solutions, the formation of hydrogen bonds and DNA damage, as well as for the discovery of new gas-phase reactions and products. In applied science, the interest in water radicals is prompted by their potential in radiobiology and as a source of primary ions for selective and sensitive chemical ionization. However, in contrast to protonated water clusters, (H2O)nH+, which are relatively easy to generate and isolate in experiments, the generation and isolation of radical water clusters, (H2O)n+•, is tremendously difficult due to their ultra-high reactivity. This review focuses on the current knowledge and unknowns regarding (H2O)n+• species, including the methods and mechanisms of their formation, structure and chemical properties.

Real-time measurements of gas-phase organic acids using SF<sub>6</sub><sup>−</sup> chemical ionization mass spectrometry

Abstract. The sources and atmospheric chemistry of gas-phase organic acids are currently poorly understood, due in part to the limited range of measurement techniques available. In this work, we evaluated the use of SF6- as a sensitive and selective chemical ionization reagent ion for real-time measurements of gas-phase organic acids. Field measurements are made using chemical ionization mass spectrometry (CIMS) at a rural site in Yorkville, Georgia, from September to October 2016 to investigate the capability of this measurement technique. Our measurements demonstrate that SF6- can be used to measure a range of organic acids in the atmosphere. One-hour averaged ambient concentrations of organic acids ranged from a few parts per trillion by volume (ppt) to several parts per billion by volume (ppb). All the organic acids displayed similar strong diurnal behaviors, reaching maximum concentrations between 17:00 and 19:00 EDT. The organic acid concentrations are dependent on ambient temperature, with higher organic acid concentrations being measured during warmer periods.

A multi-residue method for pesticides analysis in green coffee beans using gas chromatography–negative chemical ionization mass spectrometry in selective ion monitoring mode

In this study, a new gas chromatography–mass spectrometry (GC–MS) method, using the very selective negative chemical ionization (NCI) mode, was developed and applied in combination with a modified acetonitrile-based extraction method (QuEChERS) for the analysis of a large number of pesticide residues (51 pesticides, including isomers and degradation products) in green coffee beans. A previously developed integrated sample homogenization and extraction method for both pesticides and mycotoxins analysis was used. An homogeneous slurry of green milled coffee beans and water (ratio 1:4, w/w) was prepared and extracted with acetonitrile/acetic acid (1%), followed by magnesium sulfate addition for phase separation. Aliquots from this extract could be used directly for LC–MS/MS analysis of mycotoxins and LC-amenable pesticides. For GC–MS analysis, a further clean-up was necessary. C18- and PSA-bonded silica were tested as dispersive solid-phase extraction (d-SPE) sorbents, separate and as a mixture, and the best results were obtained using C18-bonded silica. For the optimal sensitivity and selectivity, GC–MS detection in the NCI-selected ion monitoring (SIM) mode had to be used to allow the fast analysis of the difficult coffee bean matrix. The validation was performed by analyzing recovery samples at three different spike concentrations, 10, 20 and 50μgkg−1, with 6 replicates (n=6) at each concentration. Linearity (r2) of calibration curves, estimated instrument and method limits of detection and limits of quantification (LODi, LODm, LOQi and LOQm, respectively), accuracy (as recovery %), precision (as RSD%) and matrix effects (%) were determined for each individual pesticide. From the 51 analytes (42 parent pesticides, 4 isomers and 5 degradation products) determined by GC–MS (NCI-SIM), approximately 76% showed average recoveries between 70–120% and 75% and RSD≤20% at the lowest spike concentration of 10μgkg−1, the target method LOQ. For the spike concentrations of 20 and 50μgkg−1, the recoveries and RSDs were even better. The validated LOQm was 10, 20 and 50μgkg−1 for respectively 33, 3 and 6 of the analytes studied. For five compounds, the European Union method performance requirements for the validation of a quantitative method (average recoveries between 70-120% and repeatability RSD≤20%) were not achieved and 4 problematic pesticides (captan, captafol, folpet and dicofol) could not be detected as their parent compound, but only via their degradation products. Although the matrix effect (matrix-enhanced detector response) was high for all pesticides studied, the matrix interference was minimal, due to the high selectivity obtained with the GC–NCI-MS detection. Matrix-matched calibration for applying the method in routine analysis is recommended for reliable quantitative results.

Liquid chromatography–atmospheric pressure chemical ionization tandem mass spectrometry for the simultaneous determination of dimethoxyaschantin, dimethylliroresinol, dimethylpinoresinol, epimagnolin A, fargesin and magnolin in rat plasma

A rapid, selective, and sensitive liquid chromatography-atmospheric pressure chemical ionization (APCI) tandem mass spectrometry method was developed for the simultaneous determination of dimethoxyaschantin, dimethylliroresinol, dimethylpinoresinol, epimagnolin A, fargesin and magnolin, the pharmacologically active ingredients of Magnolia fargesii in rat plasma. These tetrahydrofurofuranoid lignans were extracted from rat plasma using tert-butyl methyl ether at pH 7.4. The analytes were separated on a Pinnacle DB biphenyl column with 65% methanol in 10 mm ammonium formate (pH 3.0) and detected by APCI tandem mass spectrometry in the selective reaction monitoring mode. The calibration curves were linear (r(2) ≥ 0.996) over the concentration range of 20.0-1000 ng/mL for six tetrahydrofurofuranoid lignans. The lower limit of quantification for these lignans was 20.0 ng/mL with 50 µL of plasma sample. The intra- and inter-assay coefficient of variation and relative error for the six tetrahydrofurofuranoid lignans at four quality control concentrations were 0.2-9.9% and -8.5-8.2%, respectively. There was no matrix effect for the six tetrahydrofurofuranoid lignans and tolterodine (internal standard). The pharmacokinetics of dimethylliroresinol, dimethylpinoresinol, epimagnolin A, fargesin and magnolin were evaluated after oral administration of a purified extract isolated from dried flower buds of Magnolia fargesii at doses of 5.5, 11.0 and 22.0 mg/kg in male rats.

Use of the dried blood spot sampling process coupled with fast gas chromatography and negative-ion chemical ionization tandem mass spectrometry: application to fluoxetine, norfluoxetine, reboxetine, and paroxetine analysis

The objective of this work was to combine the advantages of the dried blood spot (DBS) sampling process with the highly sensitive and selective negative-ion chemical ionization tandem mass spectrometry (NICI-MS-MS) to analyze for recent antidepressants including fluoxetine, norfluoxetine, reboxetine, and paroxetine from micro whole blood samples (i.e., 10 microL). Before analysis, DBS samples were punched out, and antidepressants were simultaneously extracted and derivatized in a single step by use of pentafluoropropionic acid anhydride and 0.02% triethylamine in butyl chloride for 30 min at 60 degrees C under ultrasonication. Derivatives were then separated on a gas chromatograph coupled with a triple-quadrupole mass spectrometer operating in negative selected reaction monitoring mode for a total run time of 5 min. To establish the validity of the method, trueness, precision, and selectivity were determined on the basis of the guidelines of the "Société Française des Sciences et des Techniques Pharmaceutiques" (SFSTP). The assay was found to be linear in the concentration ranges 1 to 500 ng mL(-1) for fluoxetine and norfluoxetine and 20 to 500 ng mL(-1) for reboxetine and paroxetine. Despite the small sampling volume, the limit of detection was estimated at 20 pg mL(-1) for all the analytes. The stability of DBS was also evaluated at -20 degrees C, 4 degrees C, 25 degrees C, and 40 degrees C for up to 30 days. Furthermore, the method was successfully applied to a pharmacokinetic investigation performed on a healthy volunteer after oral administration of a single 40-mg dose of fluoxetine. Thus, this validated DBS method combines an extractive-derivative single step with a fast and sensitive GC-NICI-MS-MS technique. Using microliter blood samples, this procedure offers a patient-friendly tool in many biomedical fields such as checking treatment adherence, therapeutic drug monitoring, toxicological analyses, or pharmacokinetic studies.

A sensitive liquid chromatography–tandem mass spectrometry method for the quantification of mometasone furoate in human plasma

A robust, rapid, selective and sensitive liquid chromatography-negative atmospheric pressure chemical ionization (LC–(APCI −)–MS–MS) method has been developed for the quantification of mometasone furoate (MF) in human plasma utilizing a solid-phase extraction clean-up step and 13C-fluticasone propionate as internal standard. The intra- and inter-day coefficients of variation were ≤15% and the lower limit of quantification (LLOQ) was 15 pg/ml. This method is ideally suited for pharmacokinetic investigations of low MF levels following inhalation of MF.

Enhanced signal generation for use in the analysis of synthetic pyrethroids using chemical ionization tandem quadrupole ion trap mass spectrometry

Synthetic pyrethroids fragment extensively under electron ionization (EI) conditions to give low mass ions, most of them with the same m/z ratios. This fragmentation is primarily due to the labile ester linkage found in these compounds. In this research we established the best gas chromatography (GC) conditions in the EI mode that served as a benchmark in the development of a chemical ionization (CI) protocol for ten selected synthetic pyrethroids. Based on proton affinity data, several reagent gases were evaluated in the positive CI ionization mode. Methanol was found to produce higher average ion counts relative to the other gases evaluated, which led to the development of an optimized method consisting of selective ejection chemical ionization (SECI) and MS/MS. Standard stainless steel ion trap electrodes produced significant degradation of chromatographic performance on late eluting compounds, which was attributed to electrode surface chemistry. A dramatic improvement in signal-to-noise (S/N) ratios was observed when the chromatographically inert Silcosteel coated electrodes were used. The resulting method, that has significant S/N ratio improvements resulting from a combination of septum programmable injections (SPI), optimized CI and inert Silcosteel-coated electrodes, was used to determine instrument detection limits.

High-pressure ion source combined with an in-axis ion trap mass spectrometer. 2. Application of selective low-pressure negative ion chemical ionization

Negative ion chemical ionization was carried out using a quadrupole ion trap mass spectrometer with selected reactant negative ions, primarily injected from a homemade dual EI/CI external ion source. Hence, selective ion/molecule reactions were provided according to the reaction time, which induce a greater control over bimolecular ionization mechanisms than in conventional a high-pressure ion source combined with beam instruments, where several competitive ionization processes take place mainly due to source conditions (e.g., temperature, pressure, and repeller). By selecting the reactant ions, ion/molecule reactions were specifically produced (i.e., charge exchange, proton transfer, nucleophilic substitution, and/or alpha-beta elimination) with several organic target compounds. Gas-phase reactivity of phosphorus- and nitrogen-containing compounds (such as phosphonates as representative for chemical warfare agents and phosphorothionates, phosphorodithionates, and triazines for pesticides) as well as dinitro aromatic compounds (for pesticides) has been explored, in the present work, to ensure further unambiguous detection.

Mass-Spectrometric Study on Ion-Molecule Reactions of CH5+, C2H5+, and C3H5+ with Benzoic Acid and Benzoates

Abstract The gas-phase ion-molecule reactions of CH5+, C2H5+, and C3H5+ with PhCOOX = M (X = H, CH3, C2H5, and Ph) have been studied under a reactant-ion selective chemical ionization mode of an ion-trap type of GC/MS. The major product channel is non-dissociative proton transfer leading to (M+H)+ in most cases. Dissociative proton transfer leading to (M+H-CO2)+ and/or (M+H-XOH)+ competes with the above channels. In the C3H5+/PhCOOX (X = H, CH3, and C2H5) reactions, initial (M+C3H5)+ adduct ions and/or their decomposition products are found. The reaction mechanism is discussed based on the reaction-time dependence of product-ion distributions and semi-empirical calculations of the energies of intermediates and products.

Mass-Spectrometric Study on Ion-Molecule Reactions of CH5+, C2H5+, and C3H5+ with Monosubstituted Benzenes Carrying a Hydroxy Group

Abstract The gas-phase ion-molecule reactions of CH5+, C2H5+, and C3H5+ with four monosubstituted benzenes carrying a hydroxy group in the substituent [PhX : X = OH, CH2OH, CH2CH2OH, and CH(OH)CH3] have been studied under a reactant-ion selective chemical ionization mode of an ion-trap type of GC/MS. The major product channel for PhOH is non-dissociative proton transfer leading to (M+H)+ ions, while that for the other three reagents is dissociative proton transfer leading to (M + H-H2O)+ ions in most cases. Small amounts of dissociative alkylation channels are found in some reactions with C2H5+ and C3H5+. The reaction mechanism is discussed based on the dependence of product-ion distributions on the reaction time and semi-empirical calculations of the energies of intermediates and products.

Analysis of pristanic acid β-oxidation intermediates in plasma from healthy controls and patients affected with peroxisomal disorders by stable isotope dilution gas chromatography mass spectrometry

In this paper we report the development of highly sensitive, selective, and accurate stable isotope dilution gas chromatography negative chemical ionization mass spectrometry (GC-NCI-MS) methods for quantification of peroxisomal β-oxidation intermediates of pristanic acid in human plasma: 2,3-pristenic acid, 3-hydroxypristanic acid, and 3-ketopristanic acid. The carboxylic groups of the intermediates were converted into pentafluorobenzyl esters, whereas hydroxyl groups were acetylated and ketogroups were methoximized. Hereafter, the samples were subjected to clean-up by high performance liquid chromatography. Analyses were performed by selected monitoring of the carboxylate anions of the derivatives. Control values of all three metabolites were established (2,3-pristenic acid: 2–48 nm, 3-hydroxypristanic acid: 0.02–0.81 nm, 3-ketopristanic acid: 0.07–1.45 nm). A correlation between the concentrations of pristanic acid and its intermediates in plasma was found. The diagnostic value of the methods is illustrated by measurements of the intermediates in plasma from patients with peroxisomal disorders. It is shown that in generalized peroxisomal disorders, the absolute concentrations of 2,3-pristenic acid, 3-hydroxypristanic acid, and 3-ketopristanic acid were comparable to those in the controls, whereas relative to the pristanic acid concentrations these intermediates were significantly decreased. In bifunctional protein deficiency, elevated levels of 2,3-pristenic acid and 3-hydroxypristanic acid were found. 3-Ketopristanic acid, although within the normal range, was relatively low when compared to the high pristanic acid levels in these patients.—Verhoeven, N. M., D. S. M. Schor, E. A. Struys, E. E. W. Jansen, H. J. ten Brink, R. J. A. Wanders, and C. Jakobs. Analysis of pristanic acid β-oxidation intermediates in plasma from healthy controls and patients affected with peroxisomal disorders by stable isotope dilution gas chromatography mass spectrometry. J. Lipid Res. 1999. 40: 260–266.

A Fast-Response Method for Determination of Atmospheric Isoprene Using Quadrupole Ion Trap Mass Spectrometry

A fast-response detection system for measurement of atmospheric isoprene (2-methyl-1,3-butadiene) is described which involves a tubular silicone membrane sampling system interfaced to a quadrupole ion trap. This method can be adapted to determination of a variety of atmospherically important alkenes that have short atmospheric lifetimes and, thus, for which high-frequency measurements are desirable. In this paper, we describe the development of a novel multimembrane air-sampling system in conjunction with selective chemical ionization and tandem mass spectrometry for the rapid quantitative determination of isoprene and other alkenes. In this method, we use vinyl methyl ether as an alkene-selective CI reagent, with which isoprene undergoes a [2 + 4] cycloaddition reaction, followed by loss of methanol, to generate an adduct ion at m/z 94 [M + 58 − 32]•+. This adduct ion undergoes loss of a methyl radical upon collision-induced dissociation. Quantitation is based on measurement of this fragment ion (m/z = 7...

Monitoring Indoor Ambient Atmospheres for Volatile Organic Compounds Using an Ion Mobility Analyzer Array with Selective Chemical Ionization

Abstract Three ion mobility analyzers with different reagent gases for chemical ionization were interfaced to a single computer and operated with a software package created for simultaneous and continuous monitoring of 1 to 8 analyzers. Mobility spectra were collected and digitized on separate input channels with 200 ms sampling intervals for individual analyzers. Volatile organic compounds (VOCs) in indoor ambient atmospheres were monitored in two scenarios: laboratory air during designed releases of several VOCs and ambient air in a central chemistry storeroom used for undergraduate laboratories. The ion mobility analyzer with reagent ions of H(H2O)+ n (n = 2 – 4) exhibited response toward alcohols, aldehydes, and other VOCs with low proton affinities. However, analyzers with reagent ions of elevated proton affinity including acetone and dimethylsulfoxide showed selective response toward only VOCs with proton affinities greater than that of the reagent ion. Thus, two tiers in selectivity of array respon...

Selective chemical ionization of nitrogen and sulfur heterocycles in petroleum fractions by ion trap mass spectrometry

A procedure is reported for the selective ammonia chemical ionization of some nitrogen and sulfur heterocycles in petroleum fractions using ion trap mass spectrometry (ITMS). The ion trap scan routine is designed to optimize the population of ammonium reagent ions and eject from the trap (by radio frequency/direct current isolation) electron ionization products formed during reagent ion formation prior to ionization of the sample. The ITMS procedure is compared with standard ion trap detector and conventional quadrupole ammonia chemical ionization for the determination of nitrogen and sulfur heterocycles in gas oil and kerosine samples. Greatly enhanced selectivity is shown for the ITMS procedure by suppression of competing charge-exchange processes.

Selective adduct formation by dimethyl ether chemical ionization in a quadrupole ion trap mass spectrometer and a conventional ion source

Dimethyl ether is evaluated as a selective chemical ionization agent for mono- and bisubstituted oxyaromatic compounds. The relative abundance and variety of adducts formed are compared

Identification and determination of propafenone and its principal metabolites in human urine using capillary gas chromatography/mass spectrometry

The capillary gas chromatography/mass spectrometry of trimethylsilyl-trifluoroacetyl, trifluoroacetyl and pentafluoropropionyl (PFP) derivatives of the antiarrhythmic agent propafenone (Rytmonorm), as well as its main metabolites N-despropyl-propafenone and 5-hydroxy-propafenone, have been investigated. Both electron impact and positive isobutane chemical ionization mass spectrometry using the Ion Trap Detector have been evaluated. The presence of propafenone and its co-extracted metabolites in human urine at time intervals after the oral administration of 150 mg Rytmonorm to healthy volunteers was established, and the urinary excretion of propafenone and 5-hydroxy-propafenone was calculated using selective chemical ionization mass spectrometric detection. Only a few per cent of the dose was excreted unchanged in the urine. Large intersubject variabilities had been observed also. The large dynamic range of the Ion Trap Detector and the high correlation coefficients (0.92-0.99) of the calibration curves were striking.

Benzene as a selective chemical ionization reagent gas

AbstractDilute mixtures of C6H6 or C6D6 in He provide abundant [C6H6]+· or [C6D6]+· ions and small amounts of [C6H7]+ or [C6D7]+ ions as chemical ionization (CI) reagent ions. The C6H6 or C6D6 CI spectra of alkylbenzenes and alkylanilines contain predominantly M+· ions from reactions of [C6H6]+· or [C6D6]+· and small amounts of MH+ or MD+ ions from reactions of [C6H7]+ or [C6D7]+. Benzene CI spectra of aliphatic amines contain M+·, fragment ions and sample‐size‐dependent MH+ ions from sample ion‐sample molecules reactions. The C6D6 CI spectra of substituted pyridines contain M+· and MD+ ions in different ratios depending on the substituent (which alters the ionization energy of the substituted pyridine), as well as sample‐size‐dependent MH+ ions from sample ion‐sample molecule reactions. Two mechanisms are observed for the formation of MD+ ions: proton transfer from [C6D6]+· or charge transfer from [C6D6]+· to give M+·, followed by deuteron transfer from C6D6 to M+·. The mechanisms of reactions were established by ion cyclotron resonance (ICR) experiments. Proton transfer from [C6H6]+· or [C6D6]+· is rapid only for compounds for which proton transfer is exothermic and charge transfer is endothermic. For compounds for which both charge transfer and proton transfer are exothermic, charge transfer is the almost exclusive reaction.