Laser Mass Spectrometric Analysis of Polycyclic Aromatic Hydrocarbons with Wide Wavelength Range Laser Multiphoton Ionization Spectroscopy

Characterization of Sulfur-Rich Microbial Organic Matter in Jurassic Carbonates Using Laser-Assisted Mass Spectrometry

Laser desorption-ionization mass spectrometry (MS) shows great potential for in situ molecular analysis of planetary surfaces and microanalysis of space-returned samples or (micro)fossils. Coupled with pyrolysis gas chromatography-mass spectrometry (Py-GC-MS) in ESA's ExoMars project, this technique could help assess further the origin of sulfur-bearing organic matter (OM) recently detected on Mars. To unravel this potential, we analyzed sulfurized microbial OM from ca. 150 million year-old carbonates with laser desorption-ionization mass spectrometry (single- and two-step: LDI-MS and L2MS), in comparison with time-of-flight secondary-ion mass spectrometry (ToF-SIMS), gas chromatography-mass spectrometry (GC-MS), and Py-GC-MS. We show that LDI-MS and L2MS readily detect sulfur-bearing moieties such as (alkyl)thiophenes and (alkyl)benzothiophenes. The mineral matrix, however, made the identification of sulfur-bearing molecules challenging in our L2MS experiment. The dominance of small aromatic hydrocarbons (≤14 carbons) in the LDI-MS and L2MS of the extracted soluble and insoluble OM and of the bulk rock is consistent with the low thermal maturity of the sediment and contrasts with the predominance of larger polycyclic aromatic structures commonly observed in meteorites with these techniques. We detected inorganic ions, in particular VO+, in demineralized OM that likely originate from geoporphyrins, which derive from chlorophylls during sediment diagenesis. Finally, insoluble OM yielded distinct compositions compared with extracted soluble OM, with a greater abundance of ions of mass-to-charge ratio (m/z) over 175 and additional N-moieties. This highlights the potential of laser-assisted MS to decipher the composition of macromolecular OM, in particular to investigate the preservation of biomacromolecules in microfossils. Studies comparing diverse biogenic and abiogenic OM are needed to further assess the use of this technique to search for biosignatures.

IR/UV Double Resonance Study of the 2-Phenylallyl Radical and its Pyrolysis Products

Isolated 2-phenylallyl radicals (2-PA), generated by pyrolysis from a nitrite precursor, have been investigated by IR/UV ion dip spectroscopy using free electron laser radiation. 2-PA is a resonance-stabilized radical that is considered to be involved in the formation of polycyclic aromatic hydrocarbons (PAH) in combustion, but also in interstellar space. The radical is identified based on its gas-phase IR spectrum. Furthermore, a number of bimolecular reaction products are identified, showing that the self-reaction as well as reactions with unimolecular decomposition products of 2-PA form several PAH efficiently. Possible mechanisms are discussed and the chemistry of 2-PA is compared with the one of the related 2-methylallyl and phenylpropargyl radicals.

The gas-phase infrared spectra of the 2-methylallyl radical and its high-temperature reaction products

The resonance-stabilized 2-methylallyl radical, 2-MA, is considered as a possible intermediate in the formation of polycyclic aromatic hydrocarbons (PAHs) in combustion processes. In this work, we report on its contribution to molecular growth in a high-temperature microreactor and provide mass-selective IR/UV ion dip spectra of the radical, as well as the various jet-cooled reaction products, employing free electron laser radiation in the mid-infrared region. Small (aromatic) hydrocarbons such as fulvene, benzene, styrene, or para-xylene, as well as polycyclic molecules, like (methylated) naphthalene, were identified with the aid of ab initio DFT computations. Several reaction products differ by one or more methyl groups, suggesting that molecular growth is dominated by (de)methylation in the reactor.

Using Resonance-Enhanced Multiphoton Ionization Time-of-Flight Mass Spectrometry to Evaluate the Movement of a Constituent in a Multiple Emulsion

Herein, we propose a method for evaluating the movement of a constituent in a multiple emulsion while maintaining its original dispersed condition. In this study, an oil-in-water-in-oil (O1/W/O2) emulsion was prepared using a two-step emulsification method with styrene as an analyte species in the inner phase (O1). The emulsion was measured using resonance-enhanced multiphoton ionization time-of-flight mass spectrometry without pretreatment such as centrifugation. From a series of obtained mass spectra, a time profile for the peak areas arising from styrene was constructed. When the emulsion was measured immediately following preparation, a time profile composed of a base, positive, and negative signals confirmed the presence of styrene in the O2, O1, and W phases, respectively. Moreover, while a small amount of styrene was present in the inner O1 phase, almost all of the styrene was found in the outer O2 phase. Furthermore, the results of the obtained time profile were converted into a box plot, and a method for the selection of the base, positive, and negative signals was tentatively determined. Then, the movement of styrene among the phases could be evaluated using the time courses of these signals; the time constant of the movement of styrene from an O1/W droplet to the O2 phase was calculated to be 0.8 h.

Vacuum Laser Photoionization inside the C-trap of an Orbitrap Mass Spectrometer: Resonance-Enhanced Multiphoton Ionization High-Resolution Mass Spectrometry

State-of-the-art mass spectrometry with ultraviolet (UV) photoionization is mostly limited to time-of-flight (ToF) mass spectrometers with 1000-10 000 m/Δm mass resolution. However, higher resolution and higher spectral dynamic range mass spectrometry may be indispensable in complex mixture characterization. Here, we present the concept, implementation, and initial evaluation of a compact ultrahigh-resolution mass spectrometer with gas-phase laser ionization. The concept is based on direct laser photoionization in the ion accumulation and ejection trap (C-trap) of an Orbitrap mass spectrometer. Resonance-enhanced multiphoton ionization (REMPI) using 266 nm UV pulses from a frequency-quadrupled Nd:YAG laser was applied for selective and efficient ionization of monocyclic and polycyclic aromatic hydrocarbons. The system is equipped with a gas inlet for volatile compounds and a heated gas chromatography coupling. The former can be employed for rapid system m/z-calibration and performance evaluation, whereas the latter enables analysis of semivolatile and higher-molecular-weight compounds. The capability to evaluate complex mixtures is demonstrated for selected petrochemical materials. In these experiments, several hundred to over a thousand compounds could be attributed with a root-mean-square mass error generally below 1 ppm and a mass resolution of over 140 000 at 200 m/z. Isobaric interferences could be resolved, and narrow mass splits, such as 3.4 mDa (SH₄/C₃), are determined. Single laser shots provided limits of detection in the 20-ppb range for p-xylene and 1,2,4-trimethylbenzene, similar to compact vacuum REMPI-ToF systems.

Using SPME-GC/REMPI-TOFMS to Measure the Volatile Odor-Active Compounds in Freshly Cooked Rice

The volatile odor-active compounds of cooked rice were evaluated using a method that combined solid-phase microextraction (SPME) with gas chromatography-resonance-enhanced multiphoton ionization time-of-flight mass spectrometry (GC/REMPI-TOFMS). An SPME fiber was held at the upper levels of the cooked rice and given an extraction time of 5 min. By using a nanosecond ultraviolet (266 nm) pulsed laser for ionization, two compounds, 4-vinylphenol and indole, which are considered to be important for the characteristic flavor of cooked rice, could be detected from all types of cultivars measured in the present study-nonglutinous rice, glutinous rice, and aromatic rice. In the case of fresh nonglutinous rice, the amounts of introduction for 4-vinylphenol and indole to GC were ca. 70 and 20 pg, respectively. While both peak areas decreased with increases in the time needed to maintain warmth, the decreasing behaviors differed slightly with a noteworthy rapid decrease for indole. For nonglutinous rice, the peak areas for 4-vinylphenol were almost the same, whether it was fresh (measured within 1 month from harvest) or aged (measured 6-12 months after harvest), but those of indole significantly decreased following storage. We also found differences among cultivars: the peak area for 4-vinylphenol in nonglutinous rice was somewhat strong; the peak area for indole was intensely strong in glutinous rice; however, the peak areas for both 4-vinylphenol and indole were weak in aromatic rice. Volatile odor-active compounds were detected in a sensitive and time-resolved manner; therefore, the proposed method could be useful for differentiating varieties of cooked rice from the viewpoints of cooking conditions, freshness, and cultivar types.

On the benefits of using multivariate analysis in mass spectrometric studies of combustion-generated aerosols

The intricate chemistry of the carbonaceous particle surface layer (which drives their reactivity, environmental and health impacts) results in complex mass spectra. In this respect, detailed molecular-level analysis of combustion emissions may be challenging even with high-resolution mass spectrometry. Building on a recently proposed comprehensive methodology (encompassing all stages from sampling to data reduction), we propose herein a comparative analysis of soot particles produced by three different sources: a miniCAST standard generator, a laboratory diffusion flame and a single cylinder internal combustion engine. The surface composition is probed by either laser or secondary ion mass spectrometry. Two examples of multivariate analysis, Principal component analysis and hierarchical clustering analysis proved their efficiency in both identifying general trends and evidencing subtle differences that otherwise would remain unnoticed in the plethora of data generated during mass spectrometric analyses. Chemical information extracted from these multivariate statistical procedures contributes to a better understanding of fundamental combustion processes and also opens to practical applications such as the tracing of engine emissions.

Using Resonance-Enhanced Multiphoton Ionization Time-of-Flight Mass Spectrometry to Quantitatively Analyze the Creaming of an Emulsion

In this study, we used a quantitative analytical method to indicate creaming behavior in an emulsion. An oil-in-water emulsion was directly measured by resonance-enhanced multiphoton ionization time-of-flight mass spectrometry, and the time profiles of the peak areas of an oil component, styrene, were obtained at heights of 1, 2, and 3 cm from the bottom of a sample that had a height of 4 cm. All time profiles roughly indicated that the signal intensity increased once, then decreased, and finally settled. Moreover, we proposed a fitting equation for the time profiles by subtracting two sigmoid functions, whereby the degree of the signal increases at the initial stage, the degree of the signal decreases after the increase, and the times for continuing the higher signal intensities were all longer as the monitoring positions were raised. This method would surely provide useful information about emulsions that undergo creaming behavior.

The Gas-Phase Infrared Spectra of Xylyl Radicals

The three isomers of the xylyl radical, C₈H₉, are possible intermediates in the formation of soot and polycyclic aromatic hydrocarbons (PAH). Their infrared spectra have been recorded by IR/UV ion dip spectroscopy using free electron laser radiation. The radicals were generated by flash pyrolysis from the corresponding nitrites and resonantly ionized via the D₃ ← D₀ transition around 310 nm. Mid-infrared spectra of the three xylyl isomers were recorded between 550 and 1700 cm^-1 and are in excellent agreement with computations, provided that overtones and combination bands are included in the simulation. The results show that the three xylyl isomers can be distinguished by their infrared spectra and that no isomerization occurs in the pyrolysis reactor. The IR spectra obtained at m/z = 208 indicate that dimerization of xylyl radicals leads to substituted stilbenes, which has not been observed for benzyl.

Low-temperature synthesis of polycyclic aromatic hydrocarbons in Titan's surface ices and on airless bodies

Titan's equatorial dunes represent the most monumental surface structures in our Solar System, but the chemical composition of their dark organics remains a fundamental, unsolved enigma, with solid acetylene detected near the dunes implicated as a key feedstock. Here, we reveal in laboratory simulation experiments that aromatics such as benzene, naphthalene, and phenanthrene-prospective building blocks of the organic dune material-can be efficiently synthesized via galactic cosmic ray exposure of low-temperature acetylene ices on Titan's surface, hence challenging conventional wisdom that aromatic hydrocarbons are formed solely in Titan's atmosphere. These processes are also of critical importance in unraveling the origin and chemical composition of the dark surfaces of airless bodies in the outer Solar System, where hydrocarbon precipitation from the atmosphere cannot occur. This finding notably advances our understanding of the distribution of carbon throughout our Solar System such as on Kuiper belt objects like Makemake.

Self-Reaction of ortho-Benzyne at High Temperatures Investigated by Infrared and Photoelectron Spectroscopy

ortho-Benzyne, a Kekulé-type biradical is considered to be a key intermediate in the formation of polycyclic aromatic hydrocarbons (PAH) and soot. In the present work we study the ortho-benzyne self-reactions in a hot microreactor and identify the high-temperature products by IR/UV spectroscopy and by photoion mass-selected threshold photoelectron spectroscopy (ms-TPES) in a free jet. Ms-TPES confirms formation of ortho-benzyne as generated from benzocyclobutenedione, as well as benzene, biphenylene, diacetylene, and acetylene, originating from the reaction o-C₆H₄ → HCC-CCH + C₂H₂, and CH₃. PAH molecules like naphthalene, 2-ethynylnaphthalene, fluorene, phenanthrene, and triphenylene are identified based on their IR/UV spectra. By comparison with recent computations their formation starting from o-benzyne can be readily understood and supports the importance of the biradical addition (1,4-cycloaddition followed by fragmentation) pathway to PAH molecules, recently proposed by Comandini et al.

Evaluating the Creaming of an Emulsion via Mass Spectrometry and UV-Vis Spectrophotometry

The creaming behavior of a turbid oil-in-water emulsion was observed via the processes of multiphoton ionization time-of-flight mass spectrometry (MPI-TOFMS) and ultraviolet-visible spectrophotometry (UV-vis), and the results were compared. The transmittance measurement by UV-vis showed that the turbidity of the toluene emulsion was decreased with time. However, non-negligible errors are common in the measurement of a sample with high turbidity. The online measurement by MPI-TOFMS detected many spikes in the time profile, which revealed the existence of toluene droplets in the emulsion. A smooth time profile suggested that the signal intensity had initially increased, and then decreased with time; the initial concentration of toluene was 3 g/L, which had decreased by half after 60 min. The signal behavior obtained using MPI-TOFMS differed only slightly from that obtained using UV-vis. Since a change in turbidity is not the same as a change in the local concentration of an oil component, MPI-TOFMS is useful for the analysis of a turbid emulsion and offers additional information concerning the creaming phenomenon of an emulsion.

Determination of Relative Ionization Cross Sections for Resonance Enhanced Multiphoton Ionization of Polycyclic Aromatic Hydrocarbons

Resonance enhanced multiphoton ionization (REMPI) is a powerful method for the sensitive determination of polycyclic aromatic hydrocarbons (PAHs) in gaseous mixtures via mass spectrometry (MS). In REMPI, ions are produced by the absorption of at least two photons including defined electronic intermediate states. As a result—unlike other laser-based ionization techniques—spectroscopic selectivity is involved into the ionization process. Nevertheless, these wavelength-dependent ionization rates impede the quantification using REMPI. For this purpose, relative photoionization cross sections (relPICS) give an easy-to-use approach to quantify REMPI-MS measurements. Hereby, the ionization behavior of a single compound was compared to that of a reference substance of a given concentration. In this study, relPICS of selected single-core aromatics and PAHs at wavelengths of 266 nm and 248 nm were determined using two different time-of-flight mass spectrometric systems (TOFMS). For PAHs, relPICS were obtained which showed a strong dependence on the applied laser intensity. In contrast, for single-core aromatics, constant values of relPICS were determined. Deviations of relPICS between both TOFMS systems were found for small aromatics (e.g., benzene), which can be assigned to the differences in UV generation in the particular system. However, the relPICS of this study were found to be in good agreement with previous results and can be used for system-independent quantification.

A comprehensive protocol for chemical analysis of flame combustion emissions by secondary ion mass spectrometry

RATIONALE

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is used to provide detailed information on the surface chemical composition of soot. An analytical protocol is proposed and tested on a laboratory flame, and the results are compared with our previous measurements provided by two-step laser mass spectrometry (L2MS).

METHODS

This work details: (1) the development of a dedicated apparatus to sample combustion products from atmospheric flames and deposit them on substrates suitable for TOF-SIMS analysis; (2) the choice of the deposition substrate and the material of the sampling line, and their effect on the mass spectra; (3) a method to separate the contributions of soot and condensable gas based on impact deposition; and finally (4) post-acquisition data processing.

RESULTS

Compounds produced during flame combustion are detected on the surface of different deposition substrates and attributed a molecular formula based on mass defect analysis. Silicon and titanium wafers perform similarly, while the surface roughness of glass microfiber filters results in a reduced mass resolution. The mass spectra obtained from the analysis of different locations of the deposits obtained by impaction show characteristic patterns that are attributed to soot/condensable gas.

CONCLUSIONS

A working method for the analysis of soot samples and the extraction of useful data from mass spectra is proposed. This protocol should help to avoid common experimental issues like sample contamination, while optimizing the setup performance by maximizing the achievable mass resolution.

Development of Multiphoton Ionization Time-of-Flight Mass Spectrometry for the Detection of Small Emulsion Droplets

A system for measuring small oil droplets in an oil-in-water (O/W) emulsion was developed using multiphoton ionization time-of-flight mass spectrometry. In the present study, a capillary column with an inner diameter of 15 µm was used for sample introduction. Moreover, a compact microscopic system was constructed for observing an emulsion flowing through a capillary column. As a result, the length for sample introduction was shortened, which is preferable for the direct evaluation of an emulsion. Using this system, the minimum diameter of a detectable toluene droplet in an O/W emulsion was decreased to 1.7 µm. The present system could be used to evaluate the local microenvironment and stability of an emulsion.

Online Monitoring of a Styrene Monomer and a Dimer in an Emulsion via Laser Ionization Time-of-Flight Mass Spectrometry

Laser ionization time-of-flight mass spectrometry was applied to the online monitoring of a styrene monomer and dimer in an emulsion. During the measurement of a styrene monomer oil-in-water emulsion for this study, a styrene dimer, 1,3-diphenylpropane, was dropped into the emulsion. As a result, signal spikes from both analytes occurred simultaneously, which suggested that either the dimer had moved to the monomer droplets or that the monomer and dimer droplets had aggregated. We concluded that this method could be useful for the direct monitoring of monomers and oligomers in the early stages of emulsion polymerization.

Multiphoton Ionization Time-of-Flight Mass Spectrometry for the Detection of Bioactive Lignan

Multiphoton ionization time-of-flight mass spectrometry (MPI-TOFMS) combined with a pulsed laser for sample vaporization was developed for the detection of a low-volatile compound in a solution. A solution containing Taiwanin A ((3E,4E)-3,4-bis(1,3-benzodioxol-5-ylmethylene)dihydro-2(3H)-furanone), which is a lignan that has an anticancer effect, was employed in the present study. Consequently, Taiwanin A could be detected by irradiating a laser pulse for vaporization to an inlet nozzle, rather than by heating. Therefore, the present method could be effective for detecting compounds with lower volatilities in a liquid sample.

Pyrolysis-gas chromatography-mass spectrometry with electron-ionization or resonance-enhanced-multi-photon-ionization for characterization of polycyclic aromatic hydrocarbons in the Baltic Sea

Polycyclic aromatic hydrocarbons (PAH), as a part of dissolved organic matter (DOM), are environmental pollutants of the marine compartment. This study investigates the origin of PAH, which is supposed to derive mainly from anthropogenic activities, and their alteration along the salinity gradient of the Baltic Sea. Pyrolysis in combination with gas chromatography and two mass selective detectors in one measurement cycle are utilized as a tool for an efficient trace analysis of such complex samples, by which it is possible to detect degradation products of high molecular structures. Along the north-south transect of the Baltic Sea a slightly rising trend for PAH is visible. Their concentration profiles correspond to the ship traffic as a known anthropogenic source, underlined by the value of special isomer ratios such as phenanthrene and anthracene (0.31-0.45) or pyrene and fluoranthene (0.44-0.53). The detection of naphthalene and the distribution of its alkylated representatives support this statement.

Progress toward the Quantitative Analysis of PAHs Adsorbed on Soot by Laser Desorption/Laser Ionization/Time-of-Flight Mass Spectrometry

Ex situ analyses of substances extracted from flames provide useful albeit mostly qualitative information on the formation process of soot and on the impact of exhausts on the environment. An experimental setup based on the coupling of laser desorption, laser ionization and time-of-flight mass spectrometry (LD/LI/ToF-MS) is presented in past works as an alternative means to more traditional techniques like gas chromatography (GC) to characterize the polycyclic aromatic hydrocarbons (PAHs) content of soot. In this paper, we go one step further in the understanding of the laser desorption/laser ionization dynamics and propose a combined experimental/simulation approach: we estimate the limit of detection of LD/LI/ToF-MS as low as [0.2, 2.8] fmol per laser pulse and we make quantitative predictions on the concentration of PAHs desorbed from soot. In particular, external calibration with model samples where PAHs are adsorbed on black carbon at known concentrations allows us to link the concentration of PAHs desorbed and detected by photoionization ToF-MS to the concentration of PAHs adsorbed on soot. The comparison of data obtained from the analysis of flame sampled soot with standard commercial GC-MS run in parallel validates the approach and defines limits and potentialities of both techniques.

Ionization of EPA contaminants in direct and dopant-assisted atmospheric pressure photoionization and atmospheric pressure laser ionization

Seventy-seven EPA priority environmental pollutants were analyzed using gas chromatography-mass spectrometry (GC-MS) equipped with an optimized atmospheric pressure photoionization (APPI) and an atmospheric pressure laser ionization (APLI) interface with and without dopants. The analyzed compounds included e.g., polycyclic aromatic hydrocarbons (PAHs), nitro compounds, halogenated compounds, aromatic compounds with phenolic, acidic, alcohol, and amino groups, phthalate and adipatic esters, and aliphatic ethers. Toluene, anisole, chlorobenzene, and acetone were tested as dopants. The widest range of analytes was ionized using direct APPI (66/77 compounds). The introduction of dopants decreased the amount of compounds ionized in APPI (e.g., 54/77 with toluene), but in many cases the ionization efficiency increased. While in direct APPI the formation of molecular ions via photoionization was the main ionization reaction, dopant-assisted (DA) APPI promoted ionization reactions, such as charge exchange and proton transfer. Direct APLI ionized a much smaller amount of compounds than APPI (41/77 compounds), showing selectivity towards compounds with low ionization energies (IEs) and long-lived resonantly excited intermediate states. DA-APLI, however, was able to ionize a higher amount of compounds (e.g. 51/77 with toluene), as the ionization took place entirely through dopant-assisted ion/molecule reactions similar to those in DA-APPI. Best ionization efficiency in APPI and APLI (both direct and DA) was obtained for PAHs and aromatics with O- and N-functionalities, whereas nitro compounds and aliphatic ethers were the most difficult to ionize. Halogenated aromatics and esters were (mainly) ionized in APPI, but not in APLI.

Application of pyrolysis-mass spectrometry and pyrolysis-gas chromatography-mass spectrometry with electron-ionization or resonance-enhanced-multi-photon ionization for characterization of crude oils

A novel analytical system for gas-chromatographic investigation of complex samples has been developed, that combines the advantages of several analytical principles to enhance the analytical information. Decomposition of high molecular weight structures is achieved by pyrolysis and a high separation capacity due to the chromatographic step provides both an universal as well as a selective and sensitive substance detection. The latter is achieved by simultaneously applying electron ionization quadrupole mass spectrometry (EI-QMS) for structural elucidation and [1+1]-resonance-enhanced-multi-photon ionization (REMPI) combined with time-of-flight mass spectrometry (ToFMS). The system has been evaluated and tested with polycyclic aromatic hydrocarbon (PAH) standards. It was applied to crude oil samples for the first time. In such highly complex samples several thousands of compounds are present and the identification especially of low concentrated chemical species such as PAH or their polycyclic aromatic sulfur containing heterocyclic (PASH) derivatives is often difficult. Detection of unalkylated and alkylated PAH together with PASH is considerably enhanced by REMPI-ToFMS, at times revealing aromatic structures which are not observable by EI-QMS due to their low abundance. On the other hand, the databased structure proposals of the EI-QMS analysis are needed to confirm structural information and isomers distinction. The technique allows a complex structure analysis as well as selective assessment of aromatic substances in one measurement. Information about the content of sulfur containing compounds plays a significant role for the increase of efficiency in the processing of petroleum.

The ionization mechanisms in direct and dopant-assisted atmospheric pressure photoionization and atmospheric pressure laser ionization

A novel, gas-tight API interface for gas chromatography-mass spectrometry was used to study the ionization mechanism in direct and dopant-assisted atmospheric pressure photoionization (APPI) and atmospheric pressure laser ionization (APLI). Eight analytes (ethylbenzene, bromobenzene, naphthalene, anthracene, benzaldehyde, pyridine, quinolone, and acridine) with varying ionization energies (IEs) and proton affinities (PAs), and four common APPI dopants (toluene, acetone, anisole, and chlorobenzene) were chosen. All the studied compounds were ionized by direct APPI, forming mainly molecular ions. Addition of dopants suppressed the signal of the analytes with IEs above the IE of the dopant. For compounds with suitable IEs or Pas, the dopants increased the ionization efficiency as the analytes could be ionized through dopant-mediated gas-phase reactions, such as charge exchange, proton transfer, and other rather unexpected reactions, such as formation of [M + 77](+) in the presence of chlorobenzene. Experiments with deuterated toluene as the dopant verified that in case of proton transfer, the proton originated from the dopant instead of proton-bound solvent clusters, as in conventional open or non-tight APPI sources. In direct APLI using a 266 nm laser, a narrower range of compounds was ionized than in direct APPI, because of exceedingly high IEs or unfavorable two-photon absorption cross-sections. Introduction of dopants in the APLI system changed the ionization mechanism to similar dopant-mediated gas-phase reactions with the dopant as in APPI, which produced mainly ions of the same form as in APPI, and ionized a wider range of analytes than direct APLI.

Ultraviolet laser desorption/ionization mass spectrometry of single-core and multi-core polyaromatic hydrocarbons under variable conditions of collisional cooling: insights into the generation of molecular ions, fragments and oligomers

The ultraviolet laser desorption/ionization of polyaromatic hydrocarbons (PAHs) has been investigated under different background pressures of an inert gas (up to 1.2 mbar of N2) in the ion source of a hybrid, orthogonal-extracting time-of-flight mass spectrometer (oTOF-MS). The study includes an ensemble of six model PAHs with isolated single polyaromatic cores and four ones with multiple cross-linked aromatic and polyaromatic cores. In combination with a weak ion extraction field, the variation of the buffer gas pressure allowed to control the degree of collisional cooling of the desorbed PAHs and, thus, to modulate their decomposition into fragments. The dominant fragmentation channels observed are related to dehydrogenation of the PAHs, in most cases through the cleavage of even numbers of C-H bonds. Breakage of C-C bonds leading to the fragmentation of rings, side chains and core linkages is also observed, in particular, at low buffer gas pressures. The precise patterns of the combined fragmentation processes vary significantly between the PAHs. The highest abundances of molecular PAH ions and cleanest mass spectra were consistently obtained at the highest buffer gas pressure of 1.2 mbar. The effective quenching of the fragmentation pathways at this elevated pressure improves the sensitivity and data interpretation for analytical applications, although the fragmentation of side chains and of bonds between (poly)aromatic cores is not completely suppressed in all cases. Moreover, these results suggest that the detected fragments are generated through thermal equilibrium processes rather than as a result of rapid photolysis. This assumption is further corroborated by a laser desorption/ionization post-source decay analysis using an axial time-of-flight MS. In line with these findings, covalent oligomers of the PAHs, which are presumably formed by association of two or more dehydrogenated fragments, are detected with higher abundances at the lower buffer gas pressures.

The characterization of selected drugs with infrared laser desorption/tunable synchrotron vacuum ultraviolet photoionization mass spectrometry

Some selected drugs including captopril, fudosteine and racecadotril have been analyzed by infrared (IR) laser desorption/tunable synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry (PIMS). The molecular ions of captopril and racecadotril are exclusively observed without any fragments at near threshold single-photon ionization (SPI). However, fudosteine easily forms fragments even at a photon energy near the ionization threshold, indicating the instability of its molecular ion. For these drugs, a number of fragments are yielded with the increase of photon energy. The structures of such fragments proposed by IR LD/VUV PIMS are supported by electron ionization time-of-flight mass spectrometry (EI-TOFMS) results. Fragmentation pathways are discussed in detail.

Analysis of organoselenium and organic acid metabolites by laser desorption single photon ionization mass spectrometry

A method for analyzing organoselenium and organic acid metabolites using laser desorption from graphite surfaces coupled to vacuum ultraviolet single photon ionization mass spectrometry (LD/SPI MS) is described. The 1-10-fmol sensitivity and linear dynamic range allows quantitative detection of selenomethionine, trimethylselenonium ion, methylselenogalactosamine, and 1beta-methylseleno-N-acetyl-D-galactosamine in complex biological samples such as human urine. In addition, common urinary metabolites such as tartronic, glutaric, orotic, uric, suberic, and hydroxyhippuric acids, are readily detected. Screening and quantitative detection of these organoselenium and organic acid metabolites is achieved within minutes. The results are also consistent with those obtained using high-performance liquid chromatography tandem mass spectrometry techniques. The study demonstrates the viability of matrix-free LD/SPI MS for molecular characterization and quantitative analysis of biological metabolites in the m/z 10-500 range that are present in complex biological fluids.

Thermal Desorption/Pyrolysis Coupled with Photoionization Time-of-Flight Mass Spectrometry for the Analysis of Molecular Organic Compounds and Oligomeric and Polymeric Fractions in Urban Particulate Matter

Atmospheric aerosols are subject to be responsible for human health effects. In this context, besides mass and number concentration of particles, their chemical composition has gained interest recently. However, knowledge about the organic content of particulate matter is still relatively scarce; i.e., only 10-40% of compounds present in the aerosol are as yet identified. By means of a newly developed measurement technique, thermal desorption/photoionization time-of-flight mass spectrometry (TOFMS), organic species evolved from urban aerosol samples collected at Augsburg, Germany, are analyzed. Thereby, compounds desorbed according to a temperature protocol following procedures for OC/EC analysis (120, 250, and 340 degrees C as desorbing temperatures) are ionized by soft, fragmentationless resonance multiphoton ionization (REMPI) and single photon ionization (SPI), respectively. With REMPI-TOFMS, a large variety of PAH is detectable. A comprehensive analysis is enabled by adding SPI-TOFMS, which gives access to aliphatic and carbonylic hydrocarbons as well as alkanoic acids and esters. Analysis of the data showed a high abundance of phenol and guiacol as well as retene, which are known markers for wood combustion. Similar patterns were found with ash from spruce wood combustion. An increase of volatile substances at 340 degrees C gave rise to the suggestion that these compounds are re-formed by pyrolytic decomposition reactions from oligomeric, polymeric, and polyfunctional oxygenated species. This was corroborated by the investigation of the behavior of cellulose acetate, which exhibited a similar pattern in its SPI-TOFMS spectrum at 340 degrees C as the aerosol. More thorough investigations of urban aerosol and source material with respect to problems such as the mass closure of carbonaceous material, indications for source apportionment, and allotment of organic species on a molecular level to fractions of organic and elemental carbon seem feasible with this measurement method.

Application of Infrared Laser Desorption Vacuum-UV Single-Photon Ionization Mass Spectrometry for Analysis of Organic Compounds from Particulate Matter Filter Samples

A new built instrument suitable for laser desorption-single photon ionization time-of-flight mass spectrometry (LD-SPI-TOFMS) with use of Vacuum-UV photons with a wavelength of 118 nm was used for the analysis of organic compounds. Fragmentation-free analysis of a variety of substances was achieved for desorption experiments with pure compounds desorbed from quartz glass filters applying low desorption energies. It was further found that the rate of fragmentation is strongly dependent on the desorption energy. Matrix effects were investigated by desorption experiments utilizing soot spiked with several organic compounds.The characteristics of the desorption process are assessed in more detail and the impact on the analysis of ambient particulate matter (PM) samples on filters are discussed. First results obtained from the application of the technique to the analysis of organic compounds from ambient PM are presented. Furthermore, possibilities of future developments of the method, in particular for analysis of ambient PM, are discussed.

Application of multi-way models to the time-resolved fluorescence of polycyclic aromatic hydrocarbons mixtures in water

The time-resolved laser-induced fluorescence of a series of polycyclic aromatic compounds (PAHs) and mixtures of these latter in aqueous solution was measured by means of an apparatus equipped with optical fibers, which allows their real time in situ monitoring. The potential of such spectroscopic technique, yielding 4-way fluorescence data arrays, together with the application of multi-way models to the matricized data, was tested for the resolution of complex aqueous mixtures containing low concentrations of PAHs, as typical fluorescent pollutants in aquatic systems. PARAllel FACtors analysis was employed for the qualitative resolution of PAHs mixtures and for calculating the fluorescence lifetimes of single PAHs; n-way partial least squares analysis was applied for evaluating the concentration of the single PAHs in the aqueous mixtures.

Selective Detection of Volatile Aromatic Compounds Using a Compact Laser Ionization Detector with Fast Gas Chromatography

We report results for a new gas chromatography detector that is comparatively sensitive and far more selective for aromatic compounds than the traditional photoionization detector. The detection means is multiphoton ionization at atmospheric pressure. The ionization source in these experiments is a diode-pumped passively Q-switched microchip laser operating at 266 nm. Experiments were conducted with the detector interfaced to a fast gas chromatograph. For <20 s elution time, limits of detection were <1 pg for toluene, ethylbenzene, xylenes, and isopropylbenzene; the limit of detection for benzene is approximately 10 pg. Detector response was linear over 5 orders of magnitude, including these low levels. Negligible signals were observed for nonaromatic ketones, aldehydes, ethers, and cycloalkanes at levels as high as 0.1 microg (10 mg/L concentration). Detector efficiency after fast GC separation was 0.002% when using a detector cell with a radius of 1.1 cm and a purge gas flow of 500 mL/min. The advantages of this detector are further illustrated by the fast GC analysis of fuel samples.

Direct determination of polycyclic aromatic hydrocarbons in solid matrices using laser desorption/laser photoionization ion trap mass spectrometry

The development and characterization of a new instrument for solid sampling which couples IR laser desorption followed by UV laser photo-ionization and analysis using an ion trap mass spectrometer has been investigated. For calibration, a new type of solid sample preparation involving activated charcoal as the solid substrate was used. This solid sample provided a steady signal for several thousand laser shots, which allowed optimization of the experimental procedure. It was found that both the IR and UV intensity and the delay between them play an important role in both the magnitude and type of signals observed. A method of gas phase accumulation with multiple laser shots was examined. Finally, this technique was demonstrated to be effective in providing direct qualitative information for N.I.S.T. SRM 1944 river sediment sample with no sample pre-treatment.

Mass-selected resonance-enhanced multiphoton ionisation spectra of laser-desorbed molecules for environmental analysis: 16 representative polycyclic aromatic compounds

Mass-selected gas-phase UV spectra of laser-desorbed molecules at room temperature have been measured via resonance-enhanced multiphoton ionisation (REMPI) and time-of-flight mass selection. The wavelength range of 260 to 320 nm is optimal for detection of polycyclic aromatic hydrocarbons (PAHs) using REMPI-mass spectrometry. A new laser desorption/laser ionisation source has been used which features a compact size and thermal equilibrium of the desorbed molecules. 16 PAHs have been investigated which have been selected by the US Environmental Protection Agency (EPA). These 16 EPA-PAHs are commonly used world-wide to characterise the PAH-load of environmental samples.

Scanning ultraviolet two-step laser mass spectroscopy of polycyclic aromatic hydrocarbon distributions on creosote-contaminated soil particles

The distribution of polycyclic aromatic hydrocarbons (PAHs) on creosote-contaminated soil has been examined with scanning ultraviolet two-step laser desorption/laser ionization mass spectroscopy (UV-L2MS). The instrument has been constructed in-house by modifying a reflectron time-of-flight mass spectrometer. Two-dimensional chemical maps were accurately generated from model patterned PAH distributions. From examination of three-dimensional substrates, the depth of field of the experiment allows surfaces with roughness of up to 120 microm to be treated as a two-dimensional system and still achieve an accurate representation of the surface deposits. Soil was obtained from a former wood treatment facility. Individual particles of 100-1000 microm were mounted on indexed sample plates and examined by reflectance infrared microscopy, optical microscopy, and imaging UV-L2MS. The most intense PAH signals were associated with regions on the particles where clay/organic carbon deposits were found.

Analysis of trans-resveratrol by laser desorption coupled with resonant ionization spectrometry. Application to trans-resveratrol content in vine leaves and grape skin

A method has been developed for nonvolatile compound analysis in fruits, namely, trans-resveratrol in grapes and vine leaves. This has been accomplished by the combination of laser desorption with resonance-enhanced multiphoton ionization coupled to time-of-flight mass spectrometry detection. After the optimization of the experimental conditions and the finding of the resonant wave-length of the substance, the full validation of the technique (i.e., linearity, repeatability, reproducibility, accuracy, detection limit, and quantification limit) was carried out with satisfactory results. Essential features of the method are as follows: (1) an enhanced desorption yield due to the mixing of the analyte with metal powder and (2) a high resolution and sensitivity and a low detection limit due to laser resonant ionization and mass spectrometric detection. Application to the analysis of trans-resveratrol in vine leaves and grape skin demonstrated the capabilities of the analytical method reaching detection limits of only few ppb.

Analysis of water contaminants and natural water samples using two-step laser mass spectrometry

The applicability of two-step laser mass spectrometry (L2MS) to the analysis of water contaminants and environmental water samples is demonstrated. First, the ionization characteristics of a selection of naphthyl and carbamate pesticides and of phenol were determined. The ion signal of all compounds increased with ionization laser pulse energy, within the investigated range (20-200 microJ). Ion yields relative to an internal standard, benz[alanthracene, reached 30% for naphthyl pesticides ionized at 225 nm and 2-8% at 266 nm. At 266 nm, similar relative ion yields were found for phenol. Carbamate pesticides showed lower relative ion yields at all wavelengths, by a factor of approximately 10-100, but higher relative ion yields, on the order of 1%, were obtained when using short (ps) laser pulses for ionization. These data allow one to estimate the detection limits of these analytes in a variety of matrixes once they are known for one of the compounds. Second, the quantitative analysis of carbaryl, phenol, and polycyclic aromatic hydrocarbons in rainwater is demonstrated. The aqueous samples were frozen to permit direct L2MS analysis of organic pollutants without tedious sample preparation. Detection limits were in the low-microgram per liter concentration range and recoveries of phenol from spiked rainwater samples were above 90%. The specific advantages are exemplified with the investigation of dynamic washout processes of atmospheric organic pollutants with a resolution of 0.01 mm of precipitation.

Laser mass spectrometry for environmental and industrial chemical trace analysis

Resonant laser mass spectrometry is a promising method for chemical trace analysis since it combines selectivity, sensitivity and rapidity of measurement. It is a two-dimensional technique incorporating medium- or high-resolution UV spectroscopy and time-of-flight mass spectrometry. No sample preparation and chemical clean-up is necessary to reach detection limits in the sub-ppb range even when highly complicated mixtures of chemical species are analyzed. After an introduction to the principles of resonant laser mass spectrometry, illustrative examples of applications are presented. Drawbacks, possibilities of overcoming them, some interesting features and future developments of resonant laser mass spectrometry are discussed.

Surface mass spectrometry of molecular species

This tutorial discusses the predominant methods available for surface mass spectrometry (MS) of molecular species: thermal desorption spectroscopy, laser desorption MS, secondary ion MS, post-ionization of desorbed neutrals and surface matrix-assisted laser desorption/ionization. Each of these has the capability to analyze molecular species that are chemisorbed, physisorbed, covalently bound to or the predominant component of a solid surface. These surface MS methods are briefly described, then their capabilities demonstrated using data predominantly from the authors' work. Comparisons are made with related methods in conventional MS. A very brief discussion is provided on the importance of complementing surface MS data with data from x-ray photoelectron spectroscopy and other surface analysis tools.