Hydrogen/deuterium exchange on aromatic rings during atmospheric pressure chemical ionization mass spectrometry

It has been demonstrated that substituted indoles fully labelled with deuterium on the aromatic ring can undergo substantial exchange back to partial and even fully protonated forms during atmospheric pressure chemical ionisation (APCI) liquid chromatography/mass spectrometry (LC/MS). The degree of this exchange was strongly dependent on the absolute quantity of analyte, the APCI desolvation temperature, the nature of the mobile phase, the mobile phase flow rate and the instrument used. Hydrogen/deuterium (H/D) exchange on several other aromatic ring systems during APCI LC/MS was either undetectable (nitrobenzene, aniline) or extremely small (acetanilide) compared to the effect observed for substituted indoles. This observation has major implications for quantitative assays using deuterium‐labelled internal standards and for the detection of deuterium‐labelled products from isotopically labelled feeding experiments where there is a risk of back exchange to the protonated form during the analysis. Copyright © 2010 John Wiley & Sons, Ltd.     

Determination of polycyclic aromatic hydrocarbons in fractions in asphalt mixtures using liquid chromatography coupled to mass spectrometry with atmospheric pressure chemical ionization

An analytical method using liquid chromatography coupled to mass spectrometry with atmospheric pressure chemical ionization for the determination of polycyclic aromatic hydrocarbons in asphalt fractions has been developed. The 14 compounds determined, characterized by having two or more condensed aromatic rings, are expected to be present in asphalt and are considered carcinogenic and mutagenic. The parameters of the atmospheric pressure chemical ionization interface were optimized to obtain the highest possible sensitivity for all of the compounds. The limits of detection ranged from 0.5 to 346.5 μg/L and the limits of quantification ranged from 1.7 to 1550 μg/L. The method was validated against a diesel particulate extract standard reference material (NIST SRM 1975), and the obtained concentrations agreed with the certified values. The method was applied to asphalt samples after its fractionation according to ASTM D4124 and the method of Green. The concentrations of the seven polycyclic aromatic hydrocarbons quantified in the sample ranged from 0.86 mg/kg for benzo[ghi]perylene to 98.32 mg/kg for fluorene.     

Analysis of high-molecular-mass polycyclic aromatic hydrocarbons in environmental samples using liquid chromatography-atmospheric pressure chemical ionization mass spectrometry

Polycyclic aromatic hydrocarbons (PAHs) with molecular masses higher than 300 u were analysed using LC-atmospheric pressure chemical ionization (APCI) MS in extracts of environmental samples from Hamilton, Canada including zebra mussels from Hamilton Harbour, air particulate and coal tar. The LC-APCI-MS profiles of three molecular mass classes of PAHs (326 u, 350 u and 374 u) were compared to identify potential sources of PAH contamination in Hamilton Harbour. The Hamilton air particulate profile was also compared with an urban air reference standard (NIST SRM 1649) from Washington, DC, USA. Profiles of all extracts were similar and suggested an environmental predominance of PAHs within the three isomeric molecular mass classes studied. However, PAHs of molecular mass 326 u and 350 u were detected in extracts of coal tar and zebra mussels from Hamilton Harbour but were not detected in Hamilton air. These results indicated that some high-molecular-mass PAHs may be characteristic of contamination by coal tar.     

Analysis of polycyclic aromatic hydrocarbons by liquid chromatography/tandem mass spectrometry using atmospheric pressure chemical ionization or electrospray ionization with tropylium post-column derivatization

Polycyclic aromatic hydrocarbons (PAHs) with four to six rings are potent carcinogens. This study analyzed ten of the sixteen US EPA priority PAHs using reversed-phase liquid chromatography/tandem mass spectrometry (LC/MS/MS) in selected reaction monitoring mode with two ionization sources: positive atmospheric pressure chemical ionization (APCI+) or positive elecrtrospray ionization (ESI+) with tropylium post-column derivatization. Several factors were investigated, including mobile phases, stationary phases of columns and chromatographic temperature, to determine how optimal separation and sensitivity might be achieved. Methanol used as an organic mobile phase provided better sensitivities for most PAHs than acetonitrile, although some PAHs co-eluted. Acidic buffers did not increase analyte signals. Use of Restek Pinnacle II PAH columns (250 x 4.6 mm or 250 x 2.1 mm, 5 microm) with water/acetonitrile gradient at 27 degrees C made possible a good separation of the ten analytes. [M]+. were the best precursor ions in both APCI and ESI, although fluoranthene could not be detected in ESI mode when tropylium post-column derivatization was performed. [M-28]+ and [M-52]+ were the major product ions of PAHs after collision-induced dissociation, a result of neutral losses of C(2)H(4) and (C(2)H(2))(2), respectively. Chromatographic separation for PAH isomers was crucial because the mass spectra were so similar that even MS/MS could not distinguish them from each other. The recoveries of sample preparations of PAHs spiked onto air-sampling filters ranged between 77.5 and 106% with relative standard deviations between 1.1 and 15.9%. This method was validated by analyzing NIST SRM 1649a (urban dust), producing results comparable with the certified PAH concentrations. The detection limits using APCI and ESI interfaces, defined as three times the noise levels, ranged between 0.23 and 0.83 ng and between 0.16 and 0.84 ng of on-column injection, respectively.     

Targeted lipidomics using electron capture atmospheric pressure chemical ionization mass spectrometry

There is an increasing need to be able to conduct quantitative lipidomics analyses as a complement to proteomics studies. The highest specificity for proteomics analysis can be obtained using methodology based on electrospray ionization (ESI) or atmospheric pressure chemical ionization (APCI) coupled with liquid chromatography/tandem mass spectrometry (LC/MS/MS). For lipidomics analysis it is often necessary to be able to separate enantiomers and regioisomers. This can be very challenging when using methodology based on conventional reversed-phase chromatography. Normal-phase chromatography using chiral columns can provide dramatic improvements in the resolution of enantiomers and regioisomers. However, conventional ESI- and APCI-MS/MS has limited sensitivity, which makes it difficult to conduct studies in cell culture systems where only trace amounts of non-esterified bioactive lipids are present. The use of electron capture APCI-MS/MS overcomes this problem. Enantiomers and regioisomers of diverse bioactive lipids can be quantified using stable isotope dilution methodology coupled with normal-phase chiral chromatography and electron capture APCI-MS/MS. This methodology has allowed a lipidomics profile from rat epithelial cells maintained in culture to be delineated and allowed the effect of a non-selective lipoxygenase inhibitor to be assessed.     

In-source fragmentation of partially oxidized mono- and polycyclic aromatic hydrocarbons in atmospheric pressure chemical ionization mass spectrometry coupled to liquid chromatography

Partially oxidized derivatives of polycyclic aromatic hydrocarbons (PAHs) are known to be important environmental pollutants. For the identification of these substances in complex mixtures, e.g. atmospheric aerosol samples, liquid chromatography/mass spectrometry with atmospheric pressure chemical ionization (LC/APCI-MS) has been found to be a suitable analytical technique. In this study 31 derivatives of mono- and polycyclic aromatic hydrocarbons with up to five condensed aromatic rings carrying different functional groups (carboxyl, dicarboxylic anhydride, lactone, hydroxyl, and carbonyl) were characterized by LC/APCI-MS. Each substance was measured in positive and negative ion detection mode at four different fragmentor voltages (90 to 190 V). For the first time, the results show that characteristic and well-interpretable fragmentation patterns can be obtained for these classes of compounds by in-source collision-induced dissociation in a single quadrupole LC/APCI-MS system. For each class of compounds typical spectral features and optimum measurement conditions are reported, and fragmentation pathways are proposed. The study demonstrates the applicability of LC/APCI-MS for the determination of most of the investigated compounds at trace levels, and it provides a database for the identification of unknown partially oxidized aromatic hydrocarbons. Copyright 1999 John Wiley & Sons, Ltd.     

Super‐atmospheric pressure chemical ionization mass spectrometry

Super‐atmospheric pressure chemical ionization (APCI) mass spectrometry was performed using a commercial mass spectrometer by pressurizing the ion source with compressed air up to 7 atm. Similar to typical APCI source, reactant ions in the experiment were generated with corona discharge using a needle electrode. Although a higher needle potential was necessary to initiate the corona discharge, discharge current and detected ion signal were stable at all tested pressures. A Roots booster pump with variable pumping speed was installed between the evacuation port of the mass spectrometer and the original rough pumps to maintain a same pressure in the first pumping stage of the mass spectrometer regardless of ion source pressure. Measurement of gaseous methamphetamine and research department explosive showed an increase in ion intensity with the ion source pressure until an optimum pressure at around 4–5 atm. Beyond 5 atm, the ion intensity decreased with further increase of pressure, likely due to greater ion losses inside the ion transport capillary. For benzene, it was found that besides molecular ion and protonated species, ion due to [M + 2H]⁺ which was not so common in APCI, was also observed with high ion abundance under super‐atmospheric pressure condition. Copyright © 2013 John Wiley & Sons, Ltd.     

The analysis of salen complexes by fast atom bombardment mass spectrometry and atmospheric pressure chemical ionization mass spectrometry

Fast atom bombardment (FAB) mass spectrometry provides useful structural information about salen complexes and salen-based oxo transfer catalysts that are not appreciably soluble in organic solvents. It was discovered that initial dissolution of these complexes in trifluoroacetic acid was crucial for producing good FAB mass spectra. Trifluoroacetic acid helps dissolve the salen-based catalysts, concentrates the analyte molecules at the matrix surface, and most importantly, suppresses the reduction process, which is a well-known phenomenon when protic matrices are used. The best FAB matrices for these catalysts were found to be thioglycerol and “magic bullet.” However, dechlorination occurred under the acid conditions for complexes containing iron chloride and manganese chloride. Demetalation also occurred for nickel-containing oxo transfer salen-based complexes. When the salen-based complexes are soluble in LC solvents, they can be analyzed easily by atmospheric pressure chemical ionization (APCI) mass spectrometry without the employment of relatively nonvolatile matrices. In addition, APCI/MS provides much more sensitive detection for manganese-salen complexes when compared with FAB results. No dechlorination or demetalation were observed when a negative ion mode APCI was employed. To our knowledge, this is the first time that an intact molecule of this type of complex has been observed by mass spectrometry.     

Atmospheric pressure chemical ionization of fluorinated phenols in atmospheric pressure chemical ionization mass spectrometry, tandem mass spectrometry, and ion mobility spectrometry

Atmospheric pressure chemical ionization (APCI)-mass spectrometry (MS) for fluorinated phenols (C6H5-xFxOH Where x = 0-5) in nitrogen with Cl- as the reagent ion yielded product ions of M Cl- through ion associations or (M-H)- through proton abstractions. Proton abstraction was controllable by potentials on the orifice and first lens, suggesting that some proton abstraction occurs through collision induced dissociation (CID) in the interface region. This was proven using CID of adduct ions (M Cl-) with Q2 studies where adduct ions were dissociated to Cl- or proton abstracted to (M-H)-. The extent of proton abstraction depended upon ion energy and structure in order of calculated acidities: pentafluorophenol > tetrafluorophenol > trifluorophenol > difluorophenol. Little or no proton abstraction occurred for fluorophenol, phenol, or benzyl alcohol analogs. Ion mobility spectrometry was used to determine if proton abstraction reactions passed through an adduct intermediate with thermalized ions and mobility spectra for all chemicals were obtained from 25 to 200 degrees C. Proton abstraction from M Cl- was not observed at any temperature for phenol, monofluorophenol, or difluorophenol. Mobility spectra for trifluorophenol revealed the kinetic transformations to (M-H)- either from M Cl- or from M2 Cl- directly. Proton abstraction was the predominant reaction for tetra- and penta-fluorophenols. Consequently, the evidence suggests that proton abstraction occurs from an adduct ion where the reaction barrier is reduced with increasing acidity of the O-H bond in C6H5-xFxOH.     

Chloride-enhanced atmospheric pressure chemical ionization mass spectrometry of polychlorinated n-alkanes

The use of high-performance liquid chromatography combined with chloride-enhanced atmospheric pressure chemical ionization for the determination of polychlorinated n-alkanes (PCAs, also called chlorinated paraffins or CPs) is described as an alternative to gas chromatographic methods. Atmospheric pressure chemical ionization in the negative ion mode formed exclusively [M+Cl](-) adduct ions and suppressed fragmentation when a chlorinated solvent was added. Limits of detection were 1-2 ng/microL for technical PCA mixtures. Response factors for single short-chain PCA homologues with different degrees of chlorination varied by not more than a factor of 6.5. The developed method was applied for the determination of the composition of technical PCA mixtures as well as for the analysis of PCAs in household commodities. Medium-chain PCAs were found in paint samples at concentrations of 8.2-11.5% (w/w), compared with 7.4-11.5% obtained by gas chromatography combined with electron ionization tandem mass spectrometry.     

Gas chromatography/mass spectrometry of polychlorinated biphenyls using atmospheric pressure chemical ionization and atmospheric pressure photoionization microchips

Gas chromatography (GC) and ion trap mass spectrometry (MS) were combined with microchip atmospheric pressure chemical ionization (microAPCI) and microchip atmospheric pressure photoionization (microAPPI) sources. Selected polychlorinated biphenyls (PCBs, IUPAC Nos. 28, 52, 101, 118, 138, 153 and 180) were analyzed by GC/microAPCI-MS and GC/microAPPI-MS to demonstrate the applicability of the miniaturized ion sources in negative ion mode analysis. The microAPCI and microAPPI methods were evaluated in respect of detection limit, linearity and repeatability. The detection limits for the PCB congeners were somewhat lower with microAPCI than with microAPPI, whereas microAPPI showed slightly wider linear range and better repeatability. With both methods, the best results were obtained for highly chlorinated or non-ortho-chlorinated PCBs, which possess the highest electron affinities. Finally, the suitability of the GC/microAPPI-MS method for the analysis of PCBs in environmental samples was demonstrated by analyzing soil extracts, and by comparing the results with those obtained by gas chromatography with electron capture detection (GC/ECD).     

Quantitative aspects of and ionization mechanisms in positive-ion atmospheric pressure chemical ionization mass spectrometry

The behavior in atmospheric pressure chemical ionization of selected model polycyclic aromatic compounds, pyrene, dibenzothiophene, carbazole, and fluorenone, was studied in the solvents acetonitrile, methanol, and toluene. Relative ionization efficiency and sensitivity were highest in toluene and lowest in methanol, a mixture of molecular ions and protonated molecules was observed in most instances, and interferences between analytes were detected at higher concentrations. Such interferences were assumed to be caused by a competition among analyte molecules for a limited number of reagent ions in the plasma. The presence of both molecular ions and protonated analyte molecules can be attributed to charge-transfer from solvent radical cations and proton transfer from protonated solvent molecules, respectively. The order of ionization efficiency could be explained by incorporating the effect of solvation in the ionization reactions. Thermodynamic data, both experimental and calculated theoretically, are presented to support the proposed ionization mechanisms. The analytical implications of the results are that using acetonitrile (compared with methanol) as solvent will provide better sensitivity with fewer interferences (at low concentrations), except for analytes having high gas-phase basicities.     

Determination of oxygen and nitrogen derivatives of polycyclic aromatic hydrocarbons in fractions of asphalt mixtures using liquid chromatography coupled to mass spectrometry with atmospheric pressure chemical ionization

Liquid chromatography coupled to mass spectrometry with atmospheric pressure chemical ionization was used for the determination of polycyclic aromatic hydrocarbon derivatives, the oxygenated polycyclic aromatic hydrocarbons and nitrated polycyclic aromatic hydrocarbons, formed in asphalt fractions. Two different methods have been developed for the determination of five oxygenated and seven nitrated polycyclic aromatic hydrocarbons that are characterized by having two or more condensed aromatic rings and present mutagenic and carcinogenic properties. The parameters of the atmospheric pressure chemical ionization interface were optimized to obtain the highest possible sensitivity for all compounds. The detection limits of the methods ranged from 0.1 to 57.3 μg/L for nitrated and from 0.1 to 6.6 μg/L for oxygenated derivatives. The limits of quantification were in the range of 4.6–191 μg/L for nitrated and 0.3–8.9 μg/L for oxygenated derivatives. The methods were validated against a diesel particulate extract standard reference material (National Institute of Standards and Technology SRM 1975), and the obtained concentrations (two nitrated derivatives) agreed with the certified values. The methods were applied in the analysis of asphalt samples after their fractionation into asphaltenes and maltenes, according to American Society for Testing and Material D4124, where the maltenic fraction was further separated into its basic, acidic, and neutral parts following the method of Green. Only two nitrated derivatives were found in the asphalt sample, quinoline and 2‐nitrofluorene, with concentrations of 9.26 and 2146 mg/kg, respectively, whereas no oxygenated derivatives were detected.     

Determination of heterocyclic aromatic amines in meat extracts by liquid chromatography-ion-trap atmospheric pressure chemical ionization mass spectrometry

When protein-rich foods are processed under normal cooking conditions, heterocyclic aromatic amines (HAAs) can be generated at a few parts per billion level. In this work, we have analyzed the HAAs present in a lyophilized meat extract by means of a simplified solid-phase extraction procedure. All the analytes were collected in a single extract with recoveries in the range of 45.6-75.2%, so the analysis time has been greatly reduced. Problems derived from the less exhaustive purification of the extract have been solved by using MS(ion trap) detection. The RSD for quantification ranged from 2.1% to 5.1% for run-to-run precision and from 5.2% to 11% for day-to-day precision. The limits of detection for standard solutions ranged from 20 to 150 pg injected. For the meat extract analyzed limits of detection from 0.9 to 11.2 ng g(-1) were obtained. Results of the quantification are in agreement with those obtained using different clean-up procedures.     

Negative ion atmospheric pressure ionization mass spectrometry: Ionization of aromatic hydrocarbons

A corona discharge atmospheric pressure ionization source generates the reagent ions, OH^? and O^? ions in addition to better known O₂^? ions, when ambient air is used as the carrier. All three ions are gas-phase bases that could form negative ions from organics via proton abstraction. Ionization of simple aromatic hydrocarbons by O₂^? is thermodynamically not feasible. Simple aromatic hydrocarbons are ionized only by O^? and/or OH^? to form [M ? H]^? ions. However, [M ? H]^? ions do not appear in the mass spectrum as they undergo stabilization via clustering with predominantly oxygen atoms.     

Quantitative determination of polar and ionic compounds in petroleum fractions by atmospheric pressure chemical ionization and electrospray ionization mass spectrometry

The capabilities of atmospheric pressure chemical ionization (APCI) and electrospray ionization (ESI) methods for quantitative analysis of polar and ionic compounds in petroleum fractions have been examined. The requirements of the analysis for sensitivity, linear dynamic range, and structural characterization have been discussed. ESI was found to be approximately two orders of magnitude more sensitive than APCI and is most suitable for the detection of analytes in weak concentrations. Equivalent relative linear dynamic ranges were observed by the two methods (at least three orders of magnitude). For the relatively high analyte concentrations examined here (e.g., 1-100 ppm or higher), the absolute area counts increased linearly with the analyte amount only in APCI, making this method more attractive for quantitative liquid chromatography/mass spectrometry (LC/MS) applications. Nevertheless, a wider range of ionic compounds can be detected by ESI than by APCI.     

Optimization of the atmospheric pressure chemical ionization liquid chromatography mass spectrometry interface

Use of optimized instrument parameters that result from statistical experimentation revealed that the sensitivity of atmospheric pressure chemical ionization (APCI) liquid chromatography-mass spectrometry (LC/MS) is greater than the sensitivity of an optimized Thermabeam? LC/MS interface by about 3 orders of magnitude, when tested on aromatic compounds. APCI is one of the few LC/MS techniques in which the chromatogram is directly comparable with liquid chromatographs that use ultraviolet detection. The optimum instrument parameters for a Finnigan SSQ-7000 APCI LC/MS interface were found at low flow rates (e. g., 0. 1 mL/min), relatively low capillary heat (e. g., 225 °C), and high sheath-gas pressure (e. g., 60 lb/in²). The optimization was achieved by monitoring the responses of sensitivity, fragmentation, and cluster ion formation. The fine tuning for high sensitivity calls for a high percentage of water in the mobile phase. In contrast, a high percentage of organic content in the mobile phase is required to obtain abundant protonated molecular ions with respect to fragmentation and clustering. This is an important consideration for analyses of unknowns.     

Rapid detection of drugs in biofluids using atmospheric pressure chemi/chemical ionization mass spectrometry

We have demonstrated that, with simple pH adjustment, volatile drugs such as methamphetamine, amphetamine, 3,4‐methylenedioxymethamphetamine (MDMA), ketamine, and valproic acid could be analyzed rapidly from raw biofluid samples (e.g. urine and serum) without dilution, or extraction, using atmospheric pressure ionization. The ion source was a variant type of atmospheric pressure chemical ionization (APCI) that used a dielectric barrier discharge (DBD) to generate the metastable helium gas and reagent ions. The sample solution was loaded in a disposable glass pipette, and the volatile compounds were purged by nitrogen gas to be reacted with the metastable helium gas. The electrodes of the DBD were arranged in such a way that the generated glow discharge was confined within the discharge tube and was not exposed to the analytes. A needle held at 100–500 V was placed between the ion‐sampling orifice and the discharge tube to guide the analyte ions into the mass spectrometer. After pH adjustment of the biofluid sample, the amphiphilic drugs were in the form of a water‐insoluble oil, which could be concentrated on the liquid surface. By gentle heating of the sample to increase the evaporation rate, rapid and sensitive detection of these drugs in raw urine and serum samples could be achieved in less than 2 min for each sample. Copyright © 2009 John Wiley & Sons, Ltd.     

High-throughput assay of rotenone in olive oil using atmospheric pressure chemical ionization tandem mass spectrometry

A high-throughput method is presented for the determination of rotenone in contaminated matrices. The procedure involves atmospheric pressure chemical ionization tandem mass spectrometry with a triple-quadrupole instrument and the use as internal standard of a cycloadduct derivative of rotenone itself. The method was applied to the evaluation of the decay of rotenone in olives and olive oil at various times after exposure to the contaminant.