Determination of polycyclic aromatic hydrocarbons in sediment by liquid chromatography-atmospheric pressure photoionization-mass spectrometry

Application of Dopant-Assisted Atmospheric Pressure Photoionisation HPLC-MS Method for the Sensitive Determination of Polycyclic Aromatic Hydrocarbons in Dark Chocolate

Multiple food research studies have shown that the polycyclic aromatic hydrocarbon (PAHs) are frequently found in processed cocoa products and chocolate. In a present study a method based on dopant-assisted atmospheric pressure photoionisation (DA-APPI) combined with a liquid chromatography/high-resolution mass spectrometer (HPLC-HRMS) for high-sensitivity analysis of four EU marker PAHs in dark chocolate samples was developed and fully validated according to the performance criteria set in EU guidelines. PAHs detection was achieved by HRMS in positive electrospray ionization mode with toluene used as a dopant to enhance the ionization efficiency of non-polar PAHs. The on-column instrument detection limits ranged from 0.8 to 1.2 pg for all four marker compounds. The method detection limits ranged from 0.016 to 0.024 μg kg-1 expressed on fat basis. The elaborated method was successfully applied to the analysis of four EU marker PAHs in dark chocolate samples. The presence of benzo[a]anthracene, benzo[b]fluoranthene and chrysene revealed at detectable levels in 100% of the samples, while benzo[a]pyrene was revealed in 77% of the samples, with its content ranging from 0.08 to 2.90 μg kg-1 fat.

Electrospray ionization mass spectrometric detection of low polar compounds by adding NaAuCl4

Liquid chromatography electrospray ionization mass spectrometry (LC/ESI/MS) has been widely used for various analyses. However, it is difficult to use LC/ESI/MS for the analysis of low polar compounds, such as polycyclic aromatic hydrocarbons. It is well known that AuCl₄^- ion decomposes to AuCl₃ by heating, and AuCl₃ is a strong π-electrophilic Lewis acid. Low polar compounds (pyrene, benzo[a]pyrene, perylene, benzo[ghi]perylene, dibenzothiophene and p-dimethoxybenzene) were detected by ESI/MS in the positive ion mode by adding NaAuCl₄ . The low polar compound interacts with AuCl₃ formed at the ESI interface, and undergoes electron transfer to AuCl₃ . The radical cation of the low polar compound was then detected by MS. In addition, the LC/ESI/MS determination of polycyclic aromatic hydrocarbons by the post-column addition of NaAuCl₄ was studied. © 2016 The Authors Journal of Mass Spectrometry Published by John Wiley & Sons Ltd.

VUV photo-processing of PAH cations: quantitative study on the ionization versus fragmentation processes

Interstellar polycyclic aromatic hydrocarbons (PAHs) are strongly affected by the absorption of vacuum ultraviolet (VUV) photons in the interstellar medium (ISM), yet the branching ratio between ionization and fragmentation is poorly studied. This is crucial for the stability and charge state of PAHs in the ISM in different environments, affecting in turn the chemistry, the energy balance, and the contribution of PAHs to the extinction and emission curves. We studied the interaction of PAH cations with VUV photons in the 7 - 20 eV range from the synchrotron SOLEIL beamline, DESIRS. We recorded by action spectroscopy the relative intensities of photo-fragmentation and photo-ionization for a set of eight PAH cations ranging in size from 14 to 24 carbon atoms, with different structures. At photon energies below ~13.6 eV fragmentation dominates for the smaller species, while for larger species ionization is immediately competitive after the second ionization potential (IP). At higher photon energies, all species behave similarly, the ionization yield gradually increases, leveling off between 0.8 and 0.9 at ~18 eV. Among isomers, PAH structure appears to mainly affect the fragmentation cross section, but not the ionization cross section. We also measured the second IP for all species and the third IP for two of them, all are in good agreement with theoretical ones confirming that PAH cations can be further ionized in the diffuse ISM. Determining actual PAH dication abundances in the ISM will require detailed modeling. Our measured photo-ionization yields for several PAH cations provide a necessary ingredient for such models.

Fast analysis of 29 polycyclic aromatic hydrocarbons (PAHs) and nitro-PAHs with ultra-high performance liquid chromatography-atmospheric pressure photoionization-tandem mass spectrometry

Polycyclic aromatic hydrocarbons (PAHs) and nitro-PAHs are ubiquitous in the environment. Some of them are probable carcinogens and some are source markers. This work presents an ultra-high performance liquid chromatography-atmospheric pressure photoionization-tandem mass spectrometry (UHPLC-APPI-MS/MS) method for simultaneous analysis of 20 PAHs and nine nitro-PAHs. These compounds are separated in 15 minutes in the positive mode and 11 minutes in the negative mode, one half of GC/MS analysis time. Two pairs of precursor/product ions are offered, which is essential for confirmation. This method separates and quantifies benzo[a]pyrene (the most toxic PAHs) and non-priority benzo[e]pyrene (isomers, little toxicity) to avoid overestimation of toxin levels, demonstrating its importance for health-related researches. With 0.5% 2,4-difluoroanisole in chlorobenzene as the dopant, limits of detection of PAHs except acenaphthylene and those of nitro-PAHs except 2-nitrofluoranthene are below 10 pg and 3 pg, respectively, mostly lower than or comparable to those reported using LC-related systems. The responses were linear over two orders of magnitude with fairly good accuracy and precision. Certified reference materials and real aerosol samples were analyzed to demonstrate its applicability. This fast, sensitive, and reliable method is the first UHPLC-APPI-MS/MS method capable of simultaneously analyzing 29 environmentally and toxicologically important PAHs and nitro-PAHs.

Letter: Fast detection of polycyclic aromatic hydrocarbons in complex mixtures of organic compounds based on gas chromatography-mass spectrometry with atmospheric pressure photoionization

The research is devoted to the investigation of the selectivity detection of 16 polycyclic aromatic hydrocarbons (PAHs) using gas chromatography-mass spectrometry (GC-MS), electron ionization (EI) GC-MS with atmospheric pressure photoionization (APPhI) and GC-MS atmospheric pressure photochemical ionization (APPhCI) when vapors of different substances (benzene, toluene, and naphthalene) were used as gas reagents. Capillary columns of different lengths were used for the separation of the components of the mixture of 44 semivolatile organic compounds. It was shown that the most-selective detection of 16 PAHs in a 44- component mixture was possible when GC-MS APPhCI was used. Only 16 PAHs were registered on the respective mass chromatograms and a fast detection of them was possible. The respective APPhI and APPhCI mass spectra consisted of only peak of the respective molecular ion, M⁺⁺ - the radical cation. Detection limits were 3 pg μL⁻¹ to 15 pg μL.

A critique on the structural analysis of lignins and application of novel tandem mass spectrometric strategies to determine lignin sequencing

This review is devoted to the application of MS using soft ionization methods with a special emphasis on electrospray ionization, atmospheric pressure photoionization and matrix-assisted laser desorption/ionization MS and tandem MS (MS/MS) for the elucidation of the chemical structure of native and modified lignins. We describe and critically evaluate how these soft ionization methods have contributed to the present-day knowledge of the structure of lignins. Herein, we will introduce new nomenclature concerning the chemical state of lignins, namely, virgin released lignins (VRLs) and processed modified lignins (PML). VRLs are obtained by liberation of lignins through degradation of vegetable matter by either chemical hydrolysis and/or enzymatic hydrolysis. PMLs are produced by subjecting the VRL to a series of further chemical transformations and purifications that are likely to alter their original chemical structures. We are proposing that native lignin polymers, present in the lignocellulosic biomass, are not made of macromolecules linked to cellulose fibres as has been frequently reported. Instead, we propose that the lignins are composed of vast series of linear related oligomers, having different lengths that are covalently linked in a criss-cross pattern to cellulose and hemicellulose fibres forming the network of vegetal matter. Consequently, structural elucidation of VRLs, which presumably have not been purified and processed by any other type of additional chemical treatment and purification, may reflect the structure of the native lignin. In this review, we present an introduction to a MS/MS top-down concept of lignin sequencing and how this technique may be used to address the challenge of characterizing the structure of VRLs. Finally, we offer the case that although lignins have been reported to have very high or high molecular weights, they might not exist on the basis that such polymers have never been identified by the mild ionizing techniques used in modern MS.

Performance of dielectric barrier discharge ionization mass spectrometry for pesticide testing: a comparison with atmospheric pressure chemical ionization and electrospray ionization

RATIONALE

The present study reports on the evaluation of dielectric barrier discharge microplasma ionization (DBDI) for liquid chromatography/high resolution mass spectrometry (LC/HRMS) analyses of pesticide residues in fruit and vegetables. Ionization, fragmentation, analytical performance and matrix effects displayed by LC/DBDI-MS were critically evaluated and compared with both atmospheric pressure chemical ionization (APCI) and electrospray (ESI), using a set of over 40 representative multiclass pesticides.

METHODS

Sample preparation was accomplished using standard QuEChERS procedure and the identification and quantitation of the pesticides tested accomplished by means of LC/MS with a hybrid linear quadrupole ion trap (LIT)-Fourier transform ion cyclotron resonance (FTICR) mass spectrometer operated in full-scan positive ion mode using DBDI, APCI and ESI sources.

RESULTS

The developed LC/DBDI-MS method allowed the screening of 43 pesticides in three different vegetable matrices: apple, orange and tomato. Minor matrix effects (i.e. signal suppression or enhancement ≤20%) were observed in most of the studied compounds: 95%, 70% and 81% of the studied compounds showed minor matrix effects in extracts of apple, orange and tomato, respectively. The results of the analysis of spiked orange extracts showed that the sensitivity obtained with LC/DBDI-MS is appropriate for multi-residue analysis of pesticide residues in fruit and vegetable samples. The limits of quantitation (LOQs) obtained for most of the studied pesticides were in compliance with the European Regulation 396/2005 (and subsequent updates) on food commodities (default maximum residue level of 10 µg kg(-1)).

CONCLUSIONS

Comparative studies with commercial sources demonstrate the suitability of DBDI as an ionization technique for residue analysis, because of the combination of the following two advantages: (1) the use of DBDI provides minimized matrix effects compared with APCI, and (2) improved the detection - in terms of sensitivity - of selected compounds that are not easily ionized by ESI, such as parathion.

Levels of PAHs in the waters, sediments, and shrimps of Estero de Urias, an estuary in Mexico, and their toxicological effects

PAHs were measured in water, sediment, and shrimps of Estero de Urias, an estuary in Sinaloa, Mexico, during the rainy and dry seasons, and analyzed for eleven PAHs routinely detected in samples. Phenanthrene was the most dominant congener in the water, sediment, and shrimp samples comprising about 38, 24, and 25%, respectively, of the eleven PAHs detected, followed by pyrene and naphthalene in water and sediment samples, and pyrene and fluorine in the shrimp samples. Total PAH concentrations ranged from 9 to 347 ng/L in water, 27 to 418 ng/g in sediments, and 36 to 498 ng/g in shrimps. The sources of contamination are closely related to human activities such as domestic and industrial discharge, automobile exhausts, and street runoff. High concentrations were also measured during the rainy season and during the first quarter of the year. Toxicity tests were also carried out, exposing fish embryos and juvenile shrimps to some of these PAHs. Fish embryos exposed to PAHs showed exogastrulation, while juvenile shrimps showed significantly lower growth rates than controls. DNA and protein alterations were also observed. These toxicity tests indicate that PAH concentrations measured could be dangerous to some aquatic organisms, particularly during early stages of development.

Detection and monitoring of PAH and oxy-PAHs by high resolution mass spectrometry: comparison of ESI, APCI and APPI source detection

The objective of this work was to compare direct infusion in a Q-TOF mass spectrometer through three different atmospheric pressure sources, electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), and atmospheric pressure photoionization (APPI) coupled to a high resolution Q-TOF mass spectrometer. A complex mixture of PAH and oxy-PAHs, obtained after the air oxidation of fluoranthene on mineral substrates, was used to compare the different ionization abilities of these sources. Here, we propose analytical methods for the use of all sources. Final goal was to provide background to the choice of the most appropriate source in order to analyze complex organic mixtures as those encountered in polluted soils, water, sediments, as well as in petroleum.

Analysis of carcinogenic polycyclic aromatic hydrocarbons in complex environmental mixtures by LC-APPI-MS/MS

Here we developed a highly sensitive, fast and reliable liquid chromatography tandem mass spectrometry (LC-MS/MS) method for the detection and analysis of 16 different polycyclic aromatic hydrocarbons (PAHs) and nitro-PAHs that have been identified as carcinogens and classified according to their biological potency. Comparison to standard analysis procedures based on gas chromatography-mass spectrometry (GC-MS) instrumentation demonstrated an improved easiness of sample preparation and sensitivity of detection achieved with the new LC-MS/MS method employing an atmospheric pressure photoionization (APPI) source attached to an API 4000 mass spectrometer (LC-APPI-MS/MS). The favorable outcome could be confirmed by analyzing complex mixtures such as certain Standard Reference Materials (SRMs) obtained from the National Institute of Standards & Technology (NIST), i.e., SRM 1975 and SRM 2975, and several diesel exhaust soots provided by the German automobile industry. Certified concentrations of individual analytes provided by NIST not only could be confirmed, but additional extremely potent carcinogens such as several isomeric hexacyclic dibenzopyrenes (DBPs), 5-methylchrysene (5-MC), and others have been detected in these crude samples in a concentration range down to below 1 ng g(-1) raw material.

Development and optimisation of a dopant assisted liquid chromatographic-atmospheric pressure photo ionisation-tandem mass spectrometric method for the determination of 15+1 EU priority PAHs in edible oils

European food legislation defines a set of 16 polycyclic aromatic hydrocarbons (PAHs) as of high concern for human health. The EU set contains structurally very similar PAHs with ring numbers between 4 and 6, and so raises some separation aspects and problems, which were not experienced with traditionally analysed PAHs. Many of the currently applied gas chromatographic mass spectrometric (GC-MS) methods suffer from separation problems, while high performance liquid chromatography with fluorescence detection (HPLC-FLD) is neither capable of detecting the whole set of EU priority PAHs nor does it (compared to GC-MS) allow structural identification. In addition HPLC-FLD shows limitations with difficult matrices due to interferences. The aim of this paper is to fill this gap by describing a liquid chromatographic dopant assisted atmospheric pressure photo ionisation tandem mass spectrometric (LC-DA-APPI-MS/MS) method for the determination of 15+1 EU priority PAHs in edible oil, which complies with the requirements set by European food legislation. Measurements were performed in positive ion mode. Anisole at a flow rate of 30 μl/min was used as dopant. Sample preparation was performed offline by donor-acceptor complex chromatography (DACC). Compared to HPLC-FLD methods the presented method enables the determination of all 15+1 EU priority PAHs at the low μg/kg concentration range including less fluorescence active compounds like benzo[j]fluoranthene and indeno[1,2,3-cd]pyrene. By analysing four reference materials it could be demonstrated that this method provides accurate results and is sufficiently sensitive for food control purposes. Statistically significant differences between the reference values and the measured analyte contents were not found. The method performs well also for very complex samples. Repeatability relative standard deviations (RSDr) of the determination of the target PAHs in olive oil were for most analytes below 5%. The limit of detection (LOD) of the method met the requirement set by EU legislation (0.3 μg/kg).

HPLC/APCI-FTICR-MS as a tool for identification of partial polar mutagenic compounds in effect-directed analysis

Identification of unknown compounds remains one of the biggest challenges for the assignment of adverse effects of sediment contamination and other complex environmental mixtures to responsible toxicants by effect-directed analysis (EDA). The identification depends on information gained from biotesting, chromatographic separation, and mass spectrometric detection. Thus, a methodology is provided for non-target identification of partial polar mutagenic polyaromatic compounds in sediment extracts by using polymeric reversed-phase HPLC column, high-resolution mass spectrometry and PubChem database. After visualization and processing the chromatogram constituents by using deconvolution software, the unambiguous elemental compositions generated were used as input in PubChem database to find a possible identity for the suspected species. The retrieved structures from the database search were refined by characterized chromatographic and mass spectrometric classifiers based on 55 model compounds comprising eight different classes representing mutagenic substructures. The applicability of the method was demonstrated by positive and tentative identification of constituents of mutagenic sediment fractions similar to selected model compounds.

Design and evaluation of a dopant-delivery system for an orthogonal atmospheric-pressure photoionization source and its performance in the analysis of polycyclic aromatic hydrocarbons

Atmospheric-pressure photoionization (APPI) mass spectrometry benefits from the addition of an ionization-enhancing dopant such as benzene. A passive dopant-delivery system has therefore been designed for use with the orthogonal APPI source within a commercial liquid chromatographic instrument with mass spectrometric detector. By providing the dopant in the gas phase, the newly designed equipment avoids mixing problems and other difficulties associated with liquid dopant addition. The system is a simple and durable design that can reliably deliver virtually any dopant with sufficient vapor pressure in the temperature range of 20 to 120 degrees C. At the optimum dopant flow rate (10% of the mobile phase flow rate) for high-performance liquid chromatography with narrow-bore (2.1 mm) columns, the system allows for uninterrupted routine analysis for up to two weeks. The performance of the device has been evaluated with benzene as dopant and with a test mixture consisting of four polycyclic aromatic hydrocarbons (PAH): naphthalene, 9H-fluorene, anthracene, and phenanthrene. All four PAH can be detected with an excellent signal-to-noise ratio in the scanning mode and a limit of detection down to 0.42 ng on column (51 pg in single-ion monitoring mode). The concentration calibration curves are linear over a range of three orders of magnitude, with correlation coefficients greater than 0.99. The utilization of benzene as dopant not only increases the sensitivity significantly - 20-fold, compared with dopant-free operation - but the low m/z values of the background ions observed also allow for the effective quantitative and qualitative analysis of PAH.

Quantification of acetylcholine, an essential neurotransmitter, in brain microdialysis samples by liquid chromatography mass spectrometry

Chemical neurotransmission has been the subject of intensive investigations in recent years. Acetylcholine is an essential neurotransmitter in the central nervous system as it has an effect on alertness, memory and learning. Enzymatic hydrolysis of acetylcholine in the synaptic cleft is fast and quickly metabolizes to choline and acetate by acetylcholinesterase. Hence the concentration in the extracellular fluid of the brain is low (0.1-6 nm). Techniques such as microdialysis are routinely employed to measure acetylcholine levels in living brain systems and the microdialysis sample volumes are usually less than 50 microL. In order to develop medicine for the diseases associated with cognitive dysfunction like mild cognitive impairment, Alzheimer's disease, schizophrenia and Parkinson's disease, or to study the mechanism of the illness, it is important to measure the concentration of acetylcholine in the extracellular fluid of the brain. Recently considerable attention has been focused on the development of chromatographic-mass spectrometric techniques to provide more sensitive and accurate quantification of acetylcholine collected from in-vivo brain microdialysis experiments. This review will provide a brief overview of acetylcholine biosynthesis, microdialysis technique and liquid chromatography mass spectrometry, which is being used to quantitate extracellular levels of acetylcholine.

Dopant-assisted atmospheric pressure photoionization mass spectrometry of polyisobutylene derivatives initiated by mono- and bifunctional initiators

Nine polyisobutylene (PIB) derivatives with different end groups (chlorine, vinyl, isobutenyl, 2,2-diphenylvinyl, and carboxyl) and molecular weights (1000 to 4500 g/mol), initiated by monofunctional and aromatic bifunctional initiators were studied by atmospheric pressure photoionization mass spectrometry (APPI-MS) in both negative and positive ion modes. Consistent with previous findings, negative ion APPI-MS revealed end-group identities through the formation of PIB adducts with chloride ions formed in situ from a chlorinated solvent (e.g., CCl4) in the presence of a dopant (toluene). In positive ion mode, considerable fragmentation of these PIB derivatives was observed, rendering end-group determinations very difficult. The M(n) values obtained by APPI(-)-MS were considerably lower than those determined by SEC for PIB derivatives with M(n) higher than 2000 g/mol. PIBs containing carboxyl termini can undergo collision-induced dissociation, yielding structurally important product ions. The resulting APPI-MS/MS intensities were found to reflect the "arm-length" distribution for PIBs with bifunctional aromatic moieties. In positive ion mode, [M + COCl]+ adducts were observed for PIBs with an aromatic initiator moiety. The origin of the COCl+ species is also discussed.

Ionization mechanism of negative ion-direct analysis in real time: a comparative study with negative ion-atmospheric pressure photoionization

The ionization mechanism of negative ion-direct analysis in real time (NI-DART) has been investigated using over 42 compounds, including fullerenes, perfluorocarbons (PFC), organic explosives, phenols, pentafluorobenzyl (PFB) derivatized phenols, anilines, and carboxylic acids, which were previously studied by negative ion-atmospheric pressure photoionization (NI-APPI). NI-DART generated ionization products similar to NI-APPI, which led to four ionization mechanisms, including electron capture (EC), dissociative EC, proton transfer, and anion attachment. These four ionization mechanisms make both NI-DART and NI-APPI capable of ionizing a wider range of compounds than negative ion-atmospheric pressure chemical ionization (APCI) or negative ion-electrospray ionization (ESI). As the operation of NI-DART is much easier than that of NI-APPI and the gas-phase ion chemistry of NI-DART is more easily manipulated than that of NI-APPI, NI-DART can be therefore used to study in detail the ionization mechanism of LC/NI-APPI-MS, which would be a powerful methodology for the quantification of low-polarity compounds. Herein, one such application has been further demonstrated in the detection and identification of background ions from LC solvents and APPI dopants, including water, acetonitrile, chloroform, methylene chloride, methanol, 2-propanol, hexanes, heptane, cyclohexane, acetone, tetrahydrofuran (THF), 1,4-dioxane, toluene, and anisole. Possible reaction pathways leading to the formation of these background ions were further inferred. One of the conclusions from these experiments is that THF and 1,4-dioxane are inappropriate to be used as solvents and/or dopants for LC/NI-APPI-MS due to their high reactivity with source basic ions, leading to many reactant ions in the background.

Atmospheric pressure photoionization mass spectrometry of polyisobutylene derivatives

Atmospheric pressure photoionization mass spectrometric (APPI-MS) study on three types of polyisobutylene derivatives is reported. Two of the polyisobutylenes investigated were polyisobutylene with dihydroxy and diolefinic end-groups derived from aromatic moieties [dicumyl chloride, 1,4-bis(2-chloro-2-propyl)benzene], and the third contained no aromatic moieties with a monohydroxy end-group. All three polyisobutylene derivatives (PIBs) had an average molecular weight (M(n)) of approximately 2000 g/mol, with a polydispersity lower than 1.2. In the positive ion APPI mode, protonated PIB molecules were formed, but the molecular weights obtained were considerably lower than those expected, indicating fragmentation of the PIB chains. In the negative APPI mode, using solvents such as tetrahydrofuran and toluene as dopants, no signal was obtained. However, in chlorinated solvents, such as CCl(4), CHCl(3), and CH(2)Cl(2), in the presence of toluene dopant, PIB adducts with chloride ions were formed with relatively high signal intensity. In the case of CH(2)Cl(2), no dopant (toluene) was necessary to generate chlorinated adduct ions, albeit increasing the toluene concentration in the flow increased the PIB signal intensity. The effect of the toluene concentration on PIB signal intensity was studied and models that include (1) photoionization of toluene, (2) formation of chloride ions from the chlorinated solvents by dissociative electron capture, (3) formation of chlorinated adduct ions and charge recombination reactions between the toluene radical cation, (4) chloride ions, and (5) chlorinated adduct ions are proposed based on the experimental results.

Determination of 10 carcinogenic polycyclic aromatic hydrocarbons in mainstream cigarette smoke

Polycyclic aromatic hydrocarbons (PAHs) are one class of chemical compounds that (1) are present at low to trace levels in unburned cigarette filler, and (2) are predominantly generated during combustion. According to a recent report of the International Agency for Research on Cancer, 10 carcinogenic PAHs together with 53 other known carcinogens are present in cigarette smoke. Accurate quantification of these chemicals helps assess public health risk to both smokers and nonsmokers exposed to second-hand smoke. We have developed and validated a specific and sensitive method for measuring these 10 carcinogenic PAHs in the particulate phase of mainstream tobacco smoke. Cigarette smoke particulate, produced using standard machine smoking protocols, was collected on glass fiber Cambridge filter pads. The particulate matter was solvent extracted, purified by solid-phase extraction, and analyzed by liquid chromatography/atmospheric pressure photoionization tandem mass spectrometry using isotopically labeled analogues as internal standards. Our method's limits of detection ranged from 11 to 166 pg and achieved sufficient reproducibility and accuracy to provide useful information on a range of cigarettes having dramatically different machine-smoked tar and nicotine deliveries. The identity of each PAH analyte was established from chromatographic retention time, analyte-specific fragmentation patterns, and relative peak area ratios of the product/precursor ion pairs. This new method provides higher sensitivity, specificity, and throughput than did earlier methods. We found relatively consistent PAH levels among a selection of domestic full-flavor cigarettes. The PAH levels in smoke from highly ventilated light and ultralight cigarettes were low when smoked using ISO (International Organization for Standardization) conditions. However, if highly ventilated cigarettes were smoked under more intense conditions (e.g., larger or more frequent puffs, vents blocked), their PAH levels equaled or exceeded their full-flavor counterparts under ISO conditions.

Electron capture atmospheric pressure photoionization mass spectrometry: analysis of fullerenes, perfluorinated compounds, and pentafluorobenzyl derivatives

An electron capture (EC) ionization mechanism has been found to be highly efficient in negative-ion atmospheric pressure photoionization (APPI) for the analysis of compounds with positive electron affinity (EA). Using negative-ion APPI, we first report the sensitive detection of natural electrophores with limited polarity, such as fullerenes and perfluorinated compounds, by mass spectrometry (MS). Using direct infusion on a quadrupole time-of-flight (QTOF) mass spectrometer, the limits of detection (LODs) for C(60) and perfluoromethylcyclohexane were determined to be 0.15 pg (0.2 fmol) and 1 femtoliter (fL) ( approximately 1.5 pg or 4.3 fmol), respectively. As the EA of the analyte increases, the detection sensitivity is enhanced. Making use of the accurate mass measurement capability of the QTOF mass spectrometer, we were able to investigate the elemental composition of the ions in each spectrum and attribute the observed high sensitivity to an EC-initiated ionization process. The proposed EC ionization mechanism is further supported by the observation of a dissociative EC reaction of pentafluorobenzyl (PFB)-derivatized phenols. The analysis of phenols by EC-APPI of their PFB derivatives resulted in very high sensitivity, with the lowest reported LOD of approximately 0.17 pg (0.5 fmol) being for 2,4-dinitrophenol. For future LC/EC-APPI-MS applications, the effect of additives and solvents on sensitivity was also tested and reported.

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.

Liquid chromatography/atmospheric pressure ionization mass spectrometry with post-column liquid mixing for the efficient determination of partially oxidized polycyclic aromatic hydrocarbons

The analytical hyphenation of micro-flow high-performance liquid chromatography (LC), with post-column liquid mixing and mass spectrometric detection (MS) was established to detect partially oxidized polycyclic aromatic hydrocarbons (oxy-PAHs) for low quantity samples. 100pmol injections of 30 reference standards could be detected in good sensitivity using either atmospheric pressure chemical ionization (APCI) and/or atmospheric pressure photoionization (APPI). The connected mass spectrometer was a single quadrupol analyzer realizing simultaneous registration of positive and negative ions in scan range width of 200 - 300Da. The ionization efficiency was compared using three ionization sources (incl. electrospray ionization (ESI)) for several oxy-PAHs. According to the mass spectra, the analytes behave differently in ionization properties. Ionization mechanism (e.g. deprotonated ions and electron captured ions) could be discussed with new inside views. Finally, the hyphenated system was applied to an exemplary aerosol extract and thus highlighting the expedient utilization of this downscaled method for real samples.

Optimization of the dopant for the trace determination of polycyclic aromatic hydrocarbons by liquid chromatography/dopant-assisted atmospheric-pressure photoionization/mass spectrometry

The composition of the dopant for the analysis of polycyclic aromatic hydrocarbons (PAHs) by liquid chromatography/dopant-assisted atmospheric-pressure photoionization/mass spectrometry under reversed-phase conditions was optimized to enhance the ionization efficiency for PAHs. The most suitable dopant was a toluene/anisole mixture (99.5:0.5, v/v) and it could improve limit of detections (LODs) to 0.79-168 ng mL(-1) (signal-to-noise (S/N)=3) for 16 common PAHs. The LODs are 3.8-40 times lower than those obtained with toluene alone and are comparable to those obtained using gas chromatography/mass spectrometry.

Capillary Electrochromatography Coupled to Atmospheric Pressure Photoionization Mass Spectrometry for Methylated Benzo[a]pyrene Isomers

Benzo[a]pyrene, one of the most carcinogenic PAHs, has 12 monomethylated positional isomers (MBAPs). A strong correlation between the carcinogenicity of these isomers and methyl substitution has been reported. In this study, on-line coupling of capillary electrochromatography (CEC) and atmospheric pressure photoionization mass spectrometry (APPI-MS) provides a unique solution to highly selective separation and sensitive detection of MBAP isomers. The studies indicated that APPI provides significantly better sensitivity compared to electrospray ionization and atmospheric pressure chemical ionization modes of MS. A systematic investigation of APPI-MS detection parameters and CEC separation is established. First, several sheath liquid parameters (including type and concentration of volatile buffers, type and content of organic modifiers, use of dopants and inorganic/organic additives, and sheath liquid flow rate) and APPI-MS spray chamber parameters (capillary voltage, vaporizer temperature, nebulizer pressure) were found to have effects on detection sensitivity as well as the profile of mass spectrum. For example, when ammonium acetate was replaced with acetic acid in the sheath liquid, the MS signal was enhanced as much as 90% and the formation of ammonia adduct was effectively suppressed. Next, the separation of MBAP isomers was conducted on internal tapered columns packed with polymeric C18 stationary phase. With the use of a mobile phase consisting of slightly higher acetonitrile content (90%,v/v) and a small amount of tropylium ion, the analysis times were significantly shortened by 20 min without compromising the resolutions between the isomers. Finally, quantitative aspects of the CEC-APPI-MS method were demonstrated using 7-MBAP as the internal standard. The calibration curves of three of the most carcinogenic isomers, namely, 1-MBAP, 3-MBAP, and 11-MBAP, showed good linearity in the range of 2.5-50 microg/mL with a limit of detection at 400 ng/mL.

Atmospheric pressure photoionization-mass spectrometry and atmospheric pressure chemical ionization-mass spectrometry of neurotransmitters

A group of five neurotransmitters with different properties was analyzed using atmospheric pressure photoionization (APPI) and atmospheric pressure chemical ionization-mass spectrometry (APCI-MS). The sensitivity of the techniques for the analytes was tested in six solvents and in positive and negative ion modes. APPI was found to be superior in sensitivity for all the compounds in both positive and negative ion modes. In positive ion mode, water/methanol/formic acid was found to be the best solvent, whereas in negative ion mode, water/methanol/ammonium hydroxide performed best. Detection limits using APPI were between 2.5-250 fmol, depending on the compound. The sensitivity was best for the neurosteroids dehydroepiandrosterone and beta-estradiol, and acetylcholine (LOD 2.5-10 fmol).

Study of a bisquaternary ammonium salt by atmospheric pressure photoionization mass spectrometry

A comprehensive atmospheric pressure photoionization (APPI) mass spectrometry investigation of hexamethonium bromide is reported. This bisquaternary ammonium salt is a model system for the investigation of multiply charged species and elucidation of ion formation processes. It has been used to elucidate the physicochemical phenomenon occurring when photoionization is carried out at atmospheric pressure. First, the in-source fragmentations were studied for aqueous solutions of the salt with the photoionization lamp switched off, i.e. under thermospray conditions. It is shown that, in this mode of operation, fragmentations are minor and may be classified into two classes, namely dequaternization and charge separation, arising from the two precursors, M2+ and [M+Br]+. Second, the fragmentation patterns have been monitored in dopant- assisted APPI for different dopants (toluene, toluene-d8, anisole and hexafluorobenzene) at various amounts. At low dopant flow rates, the [M+Br]+ and M2+ ions are still observed. As the flow rate is increased, these precursor ions lose intensity and are finally suppressed for all three dopants. Comparison of toluene and toluene-d8 reveals that H atoms may be transferred from the dopant to the molecular ions, very likely mediated by the solvent. The role of the solvent (water) was also investigated by using heavy water. Apart from the thermospray fragmentations, which are also observed in APPI, several fragmentation pathways appear to be specific to the photoionization process. Photoionization efficiencies are measured by determination of the relative photoionization cross sections with respect to toluene. It is found that, when the ionization efficiencies are taken into account, the depletion of the precursors as a function of the dopant flow rates is the same for all three dopant molecules. This result shows that the precursor ions are depleted by reactions with the photoelectrons released from the dopant. Three additional mechanisms are proposed to account for this effect: electron transfer or H atom transfer from negatively charged water nanodroplets and H atom transfer from the dopant.

Use of flow injection atmospheric pressure photoionization quadrupole time-of-flight mass spectrometry for fast olive oil fingerprinting

The recently introduced technique of an atmospheric pressure photoionization (APPI) source coupled to quadrupole time-of-flight mass spectrometry (QqTOFMS) has been applied to fast olive oil fingerprinting on the basis of the accurate mass measurements obtained with this instrumentation. The key compounds can be characterized as [M+H]+ (produced by proton transfer) or as [M]+* (by charge transfer) ions in the mass spectra. [M+H]+ ions, however, show higher abundance, especially for triacylglycerols. Other ions present in APPI-MS are the acylium ion [RiCO]+ and [RiCO-H2O]+. This latter ion is absent in the electrospray ionization (ESI)-MS spectra, and this represents valuable complementary information. Several critical parameters in the APPI source were optimized such as LC eluent composition, ion spray voltage and, especially, declustering potential. APPI-QqTOFMS allows easy discrimination among different edible oils: olive, extra virgin olive, olive-pomace, hazelnut, sunflower, corn and several mixed oils, with high throughput (approximately 1 min per sample). Cluster analysis was applied to obtain the best experimental conditions for oil discrimination on the basis of declustering potential. Principal components analyses of these APPI-MS spectra show that the approach can be used for studies of olive oil adulteration with other oils, even in the case of hazelnut oil that exhibits a high chemical similarity with olive oil.

From fundamentals to applications: recent developments in atmospheric pressure photoionization mass spectrometry

Only five years after the first publication on atmospheric pressure photoionization (APPI), this technique has evolved rapidly as a very useful complement to established ionization techniques for liquid chromatography/mass spectrometry (LC/MS). This is reflected in a rapidly increasing number of publications in this field. On the one hand, thorough studies into the photoionization mechanism have provided deep insights into the roles and influences of the solvent, the dopant and other additives. On the other hand, a large number of new and attractive applications have recently been introduced. New instrumental developments have resulted in combined APPI/ESI (PAESI) and APPI/APCI sources and a microfabricated APPI source. In this review, the most important developments within the field are summarized, focusing in particular on the applications of the technique.

Atmospheric-pressure laser ionization: a novel ionization method for liquid chromatography/mass spectrometry

We report on the development of a new laser-ionization (LI) source operating at atmospheric pressure (AP) for liquid chromatography/mass spectrometry (LC/MS) applications. APLI is introduced as a powerful addition to existing AP ionization techniques, in particular atmospheric-pressure chemical ionization (APCI), electrospray ionization (ESI), and atmospheric pressure photoionization (APPI). Replacing the one-step VUV approach in APPI with step-wise two-photon ionization strongly enhances the selectivity of the ionization process. Furthermore, the photon flux during an ionization event is drastically increased over that of APPI, leading to very low detection limits. In addition, the APLI mechanism generally operates primarily directly on the analyte. This allows for very efficient ionization even of non-polar compounds such as polycyclic aromatic hydrocarbons (PAHs). The APLI source was characterized with a MicroMass Q-Tof Ultima II analyzer. Both the effluent of an HPLC column containing a number of PAHs (benzo[a]pyrene, fluoranthene, anthracene, fluorene) and samples from direct syringe injection were analyzed with respect to selectivity and sensitivity of the overall system. The liquid phase was vaporized by a conventional APCI inlet (AP probe) with the corona needle removed. Ionization was performed through selective resonance-enhanced multi-photon ionization schemes using a high-repetition-rate fixed-frequency excimer laser operating at 248 nm. Detection limits well within the low-fmol regime are readily obtained for various aromatic hydrocarbons that exhibit long-lived electronic states at the energy level of the first photon. Only molecular ions are generated at the low laser fluxes employed ( approximately 1 MW/cm(2)). The design and performance of the laser-ionization source are presented along with results of the analysis of aromatic hydrocarbons.

Determination of aldehydes and ketones using derivatization with 2,4-dinitrophenylhydrazine and liquid chromatography–atmospheric pressure photoionization-mass spectrometry

Atmospheric pressure photoionization-mass spectrometry (APPI-MS) is used for the analysis of aldehydes and ketones after derivatization with 2,4-dinitrophenylhydrazine (DNPH) and liquid chromatographic separation. In the negative ion mode, the [M - H]- pseudomolecular ions are most abundant for the carbonyls. Compared with the established atmospheric pressure chemical ionization (APCI)-MS, limits of detection are typically lower using similar conditions. Automobile exhaust and cigarette exhaust samples were analyzed with APPI-MS and APCI-MS in combination with an ion trap mass analyzer. Due to improved limits of detection, more of the less abundant long-chain carbonyls are detected with APPI-MS in real samples. While 2,4-dinitrophenylazide, a known reaction product of DNPH with nitrogen dioxide, is detected in APCI-MS due to dissociative electron capture, it is not observed at all in APPI-MS.