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

Hyphenation of Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) with separation methods: The art of compromises and the possible - A review

This review provides an overview of the online hyphenation of Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS) with separation methods to date. The online coupling between separation techniques (gas and liquid chromatography, capillary electrophoresis) and FT-ICR MS essentially raises questions of compromise and is not look as straightforward as hyphenation with other analyzers (QTOF-MS for instance). FT-ICR MS requires time to reach its highest resolving power and accuracy in mass measurement capabilities whereas chromatographic and electrophoretic peaks are transient. In many applications, the strengths and the weaknesses of each technique are balanced by their hyphenation. Untargeted "Omics" (e.g. proteomics, metabolomics, petroleomics, …) is one of the main areas of application for FT-ICR MS hyphenated to online separation techniques because of the complexity of the sample. FT-ICR MS achieves the required high mass measurement accuracy to determine accurate molecular formulae and resolution for isobar distinction. Meanwhile separation techniques highlight isomers and reduce the ion suppression effects extending the dynamic range. Even if the implementation of FT-ICR MS hyphenated with online separation methods is a little trickier (the art of compromise), this review shows that it provides unparalleled results to the scientific community (the art of the possible), along with raising the issue of its future in the field with the relentless technological progress.

Charging Effects in Inlet Capillaries

Glass or metal inlet capillaries are commonly used for flow restriction in atmospheric pressure ionization mass spectrometers. They exhibit a high ion transmission rate and stability at most operating conditions. However, transferring unipolar currents of ions through inlet capillaries can lead to sudden signal dropouts or drifts of the signal intensity, particularly when materials of different conductivity are in contact with the capillary duct. Molecular layers of water and other gases such as liquid chromatography solvents always form on the surfaces of inlet capillaries at atmospheric pressure ionization conditions. These surface layers play a major role in ion transmission and the occurrence of charging effects, as ions adsorb on the capillary walls as well, charging the walls to electric potentials of up to kilovolts and eventually leading to a hindrance of ion transport into or through the capillary duct. In this work, surface charging effects are reported in dependence on the capillary material, i.e., borosilicate glass, (reduced) lead silicate, quartz, and metal. Low electrical conductance materials show a more pronounced long-term signal drift (e.g., quartz), while higher electrical conductance materials lead to stable long-term behavior.

LASER IONIZATION MASS SPECTROMETRY AT 55: QUO VADIS?

Laser ionization mass spectrometry (LIMS) was one of the first practical methods developed for in situ analysis of the surfaces of solid samples. This review will encompass several aspects related to this analytical method. First, we will discuss the process of laser ionization, the influence of the laser type on its performance, and imaging capabilities of this method. In the second chapter, we will follow the historic development of LIMS instrumentation. After a brief overview of the first-generation instruments developed in 1960-1990 years, we will discuss in detail more recent designs, which appeared during the last 2-3 decades. In the last part of our review, we will cover the recent applications of LIMS for surface analysis. These applications include various types of analyses of solid inorganic, organic, and heterogeneous samples, often in combination with depth profiling and imaging capability.

Development of a fast-switching dual (ESI/APCI) ionization source for liquid chromatography/mass spectrometry

RATIONALE

High-throughput liquid chromatography/mass spectrometry (LC/MS) is an increasing topic in analytical chemistry. Especially the idle time of a mass spectrometer should be reduced for an efficient and cost-saving use. Therefore, a fast-switching dual ion source was developed, which uses the most important ionization techniques at atmospheric pressure, electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI), with one or more LC systems.

METHODS

The performance of the developed ion source is shown by infusion experiments and chromatographic analyses of different standard substances. A high-throughput method is demonstrated by coupling two UHPLC systems to the dual ion source with a triple quadrupole mass spectrometer.

RESULTS

No decrease in the ion abundance and a stable performance of the mass spectrometer are presented while using the dual ion source. Instrumental limits of detection are 30 ng L^-1 for testosterone using ESI and 1 μg L^-1 for vitamin D₃ using APCI. A fast switching between two UHPLC systems and the dual ion source leads to a high sample throughput of 50 samples in 75 min with relative standard deviations for testosterone and vitamin D₃ of 1.5% and 3.8%, respectively.

CONCLUSIONS

This work presents the development of a dual ESI and APCI ion source operating simultaneously or in switched mode. The results show sensitive and reliable performance as well as the hyphenation to one or more HPLC systems.

Petroleomics: Tools, Challenges, and Developments

The detailed molecular characterization of petroleum-related samples by mass spectrometry, often referred to as petroleomics, continues to present significant analytical challenges. As a result, petroleomics continues to be a driving force for the development of new ultrahigh resolution instrumentation, experimental methods, and data analysis procedures. Recent advances in ionization, resolving power, mass accuracy, and the use of separation methods, have allowed for record levels of compositional detail to be obtained for petroleum-related samples. To address the growing size and complexity of the data generated, vital software tools for data processing, analysis, and visualization continue to be developed. The insights gained impact upon the fields of energy and environmental science and the petrochemical industry, among others. In addition to advancing the understanding of one of nature's most complex mixtures, advances in petroleomics methodologies are being adapted for the study of other sample types, resulting in direct benefits to other fields.

Systematic Ion Source Parameter Assessment by Automated Determination of the Distribution of Ion Acceptance (DIA) Using APLI

The determination of the spatially resolved ion signal with atmospheric pressure laser ionization (APLI), which was introduced as distribution of ion acceptance (DIA), serves as a valuable tool for the understanding of complex and highly dynamical conditions in modern atmospheric pressure (AP) ion sources. DIA provides information about fluid dynamics, ion transport, and ion transformation processes in such sources and is an ideal basis for the validation of numerical models of the dynamics in the ion source enclosure.We present a fully automated setup for DIA measurements, which enabled us to acquire a comprehensive dataset of over 700 individual DIA measurements in a commercial AP ion source (Bruker Multi Purpose Ion Source, MPIS). Ion source parameters as voltages, gas heater temperatures and gas flows, were varied, and the effect of those parameters on the DIA of a chemically inert analyte, pyrene, was systematically investigated. It is shown that the response of the DIA is nonlinear and that gas dynamics largely dominates the ion transport in the ion source. Particularly, the position of the heated nebulizer, which is used to introduce one of two gas flows and the analyte into the ion source chamber, had a profound effect on the DIA. This suggests that the gas dynamics in the source switches between different states. The now available comprehensive DIA dataset reveals such critical effects and will guide further numerical modeling efforts to understand the details of the dynamics of ions in the source chamber. Graphical Abstract.

Photoionization and photofragmentation in mass spectrometry with visible and UV lasers

Ever since the introduction of laser technology to the field of mass spectrometry, several disciplines evolved providing solutions to challenging scientific and analytical tasks in research and industry. Among these are techniques involving multiphoton ionization such as Resonance-Enhanced Multiphoton Ionization (REMPI, R2PI) and Mass-Analyzed Threshold Ionization (MATI) spectroscopy, a variant of Zero Kinetic Energy (ZEKE) spectroscopy, that possess the ability to selectively ionize certain preselected compounds out of complex mixtures, for example, environmental matrices, with a high level of efficiency. Another key feature of multiphoton ionization techniques is the ability to control the degree of fragmentation, whereas soft ionization is most highly appreciated in most applications. In cases where rich fragmentation patterns are desired for diagnostic purposes, Photodissociation mass spectrometry (PD-MS) is applied successfully. PD-MS allows for the cleavage of selected chemical bonds. With the introduction of chromophoric labels in PD-MS, it became possible to target certain molecules or groups within a molecule. In this review article, an overview of the basic principles and experimental requirements of REMPI and MATI spectroscopy and PD mass spectrometry are given. By means of selected examples, the latest developments and application possibilities in this field over the past decade with special focus on the German research landscape are pointed out. © 2018 Wiley Periodicals, Inc. Mass Spec Rev 38: 202-217, 2019.

Gas Flow Dynamics in Inlet Capillaries: Evidence for non Laminar Conditions

In this work, the characteristics of gas flow in inlet capillaries are examined. Such inlet capillaries are widely used as a first flow restriction stage in commercial atmospheric pressure ionization mass spectrometers. Contrary to the common assumption, we consider the gas flow in typical glass inlet capillaries with 0.5 to 0.6 mm inner diameters and lengths about 20 cm as transitional or turbulent. The measured volume flow of the choked turbulent gas stream in such capillaries is 0.8 L·min(-1) to 1.6 L·min(-1) under typical operation conditions, which is in good agreement to theoretically calculated values. Likewise, the change of the volume flow in dependence of the pressure difference along the capillary agrees well with a theoretical model for turbulent conditions as well as with exemplary measurements of the static pressure inside the capillary channel. However, the results for the volume flow of heated glass and metal inlet capillaries are neither in agreement with turbulent nor with laminar models. The velocity profile of the neutral gas in a quartz capillary with an inner diameter similar to commercial inlet capillaries was experimentally determined with spatially resolved ion transfer time measurements. The determined gas velocity profiles do not contradict the turbulent character of the flow. Finally, inducing disturbances of the gas flow by placing obstacles in the capillary channel is found to not change the flow characteristics significantly. In combination the findings suggest that laminar conditions inside inlet capillaries are not a valid primary explanation for the observed high ion transparency of inlet capillaries under common operation conditions. Graphical Abstract ᅟ.

A comparative study of APLI and APCI in IMS at atmospheric pressure to reveal and explain peak broadening effects by the use of APLI

The details of the ionization mechanism in atmospheric pressure are still not completely known. In order to obtain further insight into the occurring processes in atmospheric pressure laser ionization (APLI) a comparative study of atmospheric pressure chemical ionization (APCI) and APLI is presented in this paper. This study is carried out using similar experimental condition at atmospheric pressure employing a commercial ion mobility spectrometer (IMS). Two different peak broadening mechanisms can then be assigned, one related to a range of different species generated and detected, and furthermore for the first time a power broadening effect on the signals can be identified.

1- and 2-photon ionization for online FAIMS-FTMS coupling allows new insights into the constitution of crude oils

Photoionization techniques (APPI and APLI) are important for the mass spectrometric analysis of crude oils, given the mainly unpolar character of the sample. Ultrahigh resolving Fourier Transform mass spectrometry (FTMS) allows to distinguish between most isobaric compounds as well as to unambiguously determine the elemental compositions of the detected ions. Nevertheless, the complexity of crude oil makes its thorough analysis a difficult task. Besides discriminating effects that can be avoided and depth of information that can be gained by simplification of the sample prior to the MS analysis the presence of numerous isomeric compounds limits the amount of information that can be gained by mass spectrometry alone. Ion mobility spectrometry (IMS) has been shown to be a valuable tool for isomer separation and has also been employed for the analysis of crude oils using IMS-TOF MS. The application of an online FAIMS-FTMS coupling after photoionization for the analysis of crude oils is shown. With this setup the complementarity of data obtained from both APPI and APLI ionization is demonstrated. Online separation and individual detection of different hydrocarbon isomers is achieved.

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.

Mutagenicity, dioxin-like activity and bioaccumulation of alkylated picene and chrysene derivatives in a German lignite

In a former study, a German lignite extract exhibited toxicity to Danio rerio and Caenorhabditis elegans and was shown to have mutagenic and dioxin-like activity. Besides the comparatively low content of known toxic polycyclic aromatic hydrocarbons (PAH), highly intensive peaks of m/z 274 and m/z 324 were observed during the chromatographic analysis. These compounds are assumed to be alkylated chrysenes and picenes (3,3,7-trimethyl-1,2,3,4-tetrahydrochrysene, 1,2-(1'-isopropylpropano)-7-methylchrysene and an isomer of the latter, 1,2,9-trimethyl-1,2,3,4-tetrahydropicene and 2,2,9-trimethyl-1,2,3,4-tetrahydropicene). These compounds are intermediates in the diagenetic formation of chrysene and picene from triterpenoids. Due to their general high abundance in lignites and the toxicity observed for the lignite extract, the mechanism-specific toxicity and bioavailability of these compounds were investigated in the present study using the approach of effect-directed analysis. After the separation of the compounds from other PAH, their mutagenic activity (Ames Fluctuation test) and dioxin-like activity (EROD activity) were studied. Both, mutation induction factor (up to 2.9±2.7) and dioxin-like activity (Bio-TEQ of 224±75 pg/g; represents the amount (pg) 2,3,7,8-tetrachlorodibenzo-p-dioxin per g coal that would provoke the same toxic effect) were rather low. Bioavailability estimated by the bioaccumulation test with Lumbriculus variegatus was also very limited. Based on the obtained results, the environmental risk of the highly abundant alkylated chrysenes and picenes in lignites is concluded to be low.

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.

Are clusters important in understanding the mechanisms in atmospheric pressure ionization? Part 1: Reagent ion generation and chemical control of ion populations

It is well documented since the early days of the development of atmospheric pressure ionization methods, which operate in the gas phase, that cluster ions are ubiquitous. This holds true for atmospheric pressure chemical ionization, as well as for more recent techniques, such as atmospheric pressure photoionization, direct analysis in real time, and many more. In fact, it is well established that cluster ions are the primary carriers of the net charge generated. Nevertheless, cluster ion chemistry has only been sporadically included in the numerous proposed ionization mechanisms leading to charged target analytes, which are often protonated molecules. This paper series, consisting of two parts, attempts to highlight the role of cluster ion chemistry with regard to the generation of analyte ions. In addition, the impact of the changing reaction matrix and the non-thermal collisions of ions en route from the atmospheric pressure ion source to the high vacuum analyzer region are discussed. This work addresses such issues as extent of protonation versus deuteration, the extent of analyte fragmentation, as well as highly variable ionization efficiencies, among others. In Part 1, the nature of the reagent ion generation is examined, as well as the extent of thermodynamic versus kinetic control of the resulting ion population entering the analyzer region.

Online normal-phase high-performance liquid chromatography/Fourier transform ion cyclotron resonance mass spectrometry: effects of different ionization methods on the characterization of highly complex crude oil mixtures

RATIONALE

Characterization of crude oil represents a challenge for researchers due to its complexity. While Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) is the method of choice for such complex matrices the high number of ions present limits the efficiency of the analysis due to charge competition and space charge effects. One way to solve this problem is the direct coupling of FT-ICR MS with high-performance liquid chromatography (HPLC).

METHODS

Normal-phase liquid chromatography was applied on a deasphalted crude oil sample by using a polar aminocyano-bonded stationary phase with n-hexane and isopropyl alcohol as a mobile phase. The HPLC system was coupled online to a 12 T ultrahigh-resolution FT-ICR mass spectrometer. Ion chromatograms were obtained with electrospray ionization (ESI), atmospheric pressure photoionization (APPI), atmospheric pressure chemical ionization (APCI), and atmospheric pressure laser ionization (APLI).

RESULTS

The chromatographic separation yielded a group separation into two peaks according to the polarity of the components. Each ionization technique was able to uniquely assign components differing in polarity and aromaticity. Additionally, an increase in aromaticity in the course of retention time for nonpolar species in the first peak was observed. Monitoring the ratio between protonated and radical mono-nitrogen species was achieved.

CONCLUSIONS

For the analysis of a crude oil sample, online coupling of a normal-phase HPLC system to an FT-ICR mass spectrometer was achieved. The results of the different ionization techniques were compared with each other, which enables a detailed characterization of the complex sample. Copyright © 2014 John Wiley & Sons, Ltd.

Resonance-enhanced multiphoton ionization mass spectrometry (REMPI-MS): applications for process analysis

Process analysis is an emerging discipline in analytical sciences that poses special requirements on analytical techniques, especially when conducted in an online manner. Mass spectrometric methods seem exceedingly suitable for this task, particularly if a soft ionization method is applied. Resonance-enhanced multiphoton ionization (REMPI) in combination with time-of-flight mass spectrometry (TOFMS) provides a selective and sensitive means for monitoring (poly)aromatic compounds in process flows. The properties of REMPI and various variations of the ionization process are presented. The potential of REMPI for process analysis is highlighted with several examples, and drawbacks of the method are also noted. Applications of REMPI-TOFMS for the detection and monitoring of aromatic species in a large variety of combustion processes comprising flames, vehicle exhaust, and incinerators are discussed. New trends in technical development and combination with other analytical methods are brought forward.

Atmospheric pressure ion source development: experimental validation of simulated ion trajectories within complex flow and electrical fields

Three-dimensionally (3D) resolved ion trajectory calculations within the complex viscous flow field of an atmospheric pressure ion source are presented. The model calculations are validated with spatially resolved measurements of the relative sensitivity distribution within the source enclosure, referred to as the distribution of ion acceptance (DIA) of the mass analyzer. In previous work, we have shown that the DIA shapes as well as the maximum signal strengths strongly depend on ion source operational parameters such as gas flows and temperatures, as well as electrical field gradients established by various source electrode potentials (e.g., capillary inlet port potential and spray shield potential). In all cases studied, distinct, reproducible, and, to some extent, surprising DIA patterns were observed. We have thus attempted to model selected experimental operational source modes (called operational points) using a validated computational flow dynamics derived 3D-velocity field as an input parameter set for SIMION/SDS, along with a suite of custom software for data analysis and parameter set processing. Despite the complexity of the system, the modeling results reproduce the experimentally derived DIA unexpectedly well. It is concluded that SIMION/SDS in combination with accurate computational fluid dynamics (CFD) input data and adequate analysis software is capable of successfully modeling operational points of an atmospheric pressure ion (API) source. This approach should be very useful in the computer-aided design of future API sources.

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.

The role of ion-bound cluster formation in negative ion mass spectrometry

RATIONALE

The ionization mechanisms operative in negative ion atmospheric pressure mass spectrometry are far from being properly understood. In an excess of oxygen superoxide (O(2)(-)) is generally the primary charge-carrying species that is generated. However, subsequent reactions leading to the finally detected ion signals remain obscure.

METHODS

Since adiabatic expansion induced cluster growth and collision-induced dissociation (CID) processes rendered a representative sampling of ion distributions present in the source difficult, a custom-built thermally sampling time-of-flight mass spectrometer was used for the investigations. Using atmospheric pressure laser ionization of toluene as the reagent gas, high yields of thermal electrons were observed, but only negligible amounts of by-products. Ab initio calculations for individual ion/molecule reaction pathways were performed.

RESULTS

Electron capture by molecular oxygen resulted in the formation of subsequent superoxide water clusters as well as distinct analyte-adduct ions. By adjusting the extent of CID within the ion optical stages of the mass spectrometer, the cluster distribution changes to smaller cluster sizes and the analyte signals strongly shifted towards M(-) or [M-H](-). The observed superoxide water cluster distribution was close to thermal. The theoretical results confirmed the experimental findings.

CONCLUSIONS

In negative atmospheric pressure mass spectrometry the water concentration in the ion source (determining the ionization efficiency) and the CID energy provided through electrical fields (determining the ion distribution) are primary, critical parameters for the observed overall ionization mechanism and efficiency.

Impact of different ionization methods on the molecular assignments of asphaltenes by FT-ICR mass spectrometry

Over the years, ultrahigh resolution mass spectrometry has successfully illustrated the extreme complexity of crude oil and related solubility or polarity based fractions on a molecular level. However, the applied ionization technique greatly influences the outcome and may provide misleading information. In this work, we investigate the atmospheric pressure laser ionization (APLI) technique coupled with Fourier transform ion cyclotron resonance mass spectrometer to analyze the asphaltene fraction of a crude oil. These results were compared to data obtained by using other existing atmospheric pressure ionization methods. Furthermore elemental analysis and solid state NMR were used to obtain the bulk characteristics of the asphaltene sample. The results of the different ionization techniques were compared with the bulk properties in order to describe the potential discrimination effects of the ionization techniques that were observed. The results showed that APLI expands the range of the assigned molecules, while retaining information already observed with the generally used ion sources.

A novel APPI-MS setup for in situ degradation product studies of atmospherically relevant compounds: capillary atmospheric pressure photo ionization (cAPPI)

We report on the development of a novel atmospheric pressure photoionization setup and its applicability for in situ degradation product studies of atmospherically relevant compounds. A custom miniature spark discharge lamp was embedded into an ion transfer capillary, which separates the atmospheric pressure from the low pressure region in the first differential pumping stage of a conventional atmospheric pressure ionization mass spectrometer. The lamp operates with a continuous argon flow and produces intense light emissions in the VUV. The custom lamp is operated windowless and efficiently illuminates the sample flow through the transfer capillary on an area smaller than 1 mm(2). Limits of detection in the lower ppbV range, a temporal resolution of milliseconds in the positive as well as the quasi simultaneously operating negative ion mode, and a significant reduction of ion transformation processes render this system applicable to real time studies of rapidly changing chemical systems. The method termed capillary atmospheric pressure photo ionization (cAPPI) is characterized with respect to the lamp emission properties as a function of the operating conditions, temporal response, and its applicability for in situ degradation product studies of atmospherically relevant compounds, respectively.

Photoemission ambient pressure ionization (PAPI) with an ultraviolet light emitting diode and detection of organic compounds

The development of compact, rugged and low-power ion sources is critical for the further advancement of handheld mass analyzers. Further, there is a need to replace the common (63)Ni source used at atmospheric pressure with a non-radioactive substitute. We present here a description of a light emitting diode (LED) photoemission ionization source for use in mass spectrometry for the detection of volatile organic compounds. This technique relies upon the generation of photoelectrons from a low-work function metal via low-energy ultraviolet (UV) light (280 or 240 nm) generated by a single LED in air at atmospheric pressure. These low-energy photoelectrons result in either direct electron capture by the analyte or chemical ionization. Currently, only negative ions are demonstrated due to operation at atmospheric pressure. Ion generation occurs without use of high electric fields such as those found in corona discharge or electrospray ionization. This source is effective for measuring organic vapors from gases, liquids and surface residues via atmospheric pressure chemical ionization, initiated by photoemission off a conductive surface. Several classes of organic vapors are analyzed and found to be effectively detected, including compounds that ionize via electron attachment, dissociative electron capture, proton abstraction, adduct formation and replacement ionization.

Atmospheric pressure laser ionization (APLI) coupled with Fourier transform ion cyclotron resonance mass spectrometry applied to petroleum samples analysis: comparison with electrospray ionization and atmospheric pressure photoionization methods

The analysis of crude oil samples remains a tough challenge due to the complexity of the matrix and the broad range of physical and chemical properties of the various individual compounds present. In this work, atmospheric pressure laser ionization (APLI) is utilized as a complementary tool to other ionization techniques for crude oil analysis. Mass spectra obtained with electrospray ionization (ESI) and atmospheric pressure photoionization (APPI) are compared. APLI is primarily sensitive towards non-polar aromatic hydrocarbons, which are generally present in high amounts especially in heavy crude oil samples. The ionization mechanisms of APLI vs. APPI are further investigated. The results indicate the advantages of APLI over established methods like ESI and APPI. The application of APLI in combination with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) is thus demonstrated to be a powerful tool for the analysis of aromatic species in complex crude oil fractions.

Evaluation of the performance of small diode pumped UV solid state (DPSS) Nd:YAG lasers as new radiation sources for atmospheric pressure laser ionization mass spectrometry (APLI-MS)

The performance of a KrF* bench top excimer laser and a compact diode pumped UV solid state (DPSS) Nd:YAG laser as photo-ionizing source in LC-APLI MS is compared. The commonly applied bench-top excimer laser, operating at 248 nm, provides power densities of the order of low MW/cm(2) on an illuminated area of 0.5 cm(2) (8 mJ/pulse, 5 ns pulse duration, beam waist area 0.5 cm(2), 3 MW/cm(2)). The DPSS laser, operating at 266 nm, provides higher power densities, however, on a two orders of magnitude smaller illuminated area (60 μJ/pulse, 1 ns pulse duration, beam waist area 2 × 10(-3) cm(2), 30 MW/cm(2)). In a common LC-APLI MS setup with direct infusion of a 10 nM pyrene solution, the DPSS laser yields a significantly smaller ion signal (0.9%) and signal to noise ratio (1.4%) compared with the excimer laser. With respect to the determined low detection limits (LODs) for PAHs of 0.1 fmol using an excimer laser, LODs in DPSS laser LC-APLI MS in the low pmol regime are expected. The advantages of the DPSS laser with respect to applicability (size, cost, simplicity) may render this light source the preferred one for APLI applications not focusing on ultimately high sensitivities. Furthermore, the impact of adjustable ion source parameters on the performance of both laser systems is discussed in terms of the spatial sensitivity distribution described by the distribution of ion acceptance (DIA) measurements. Perspectives concerning the impact on future APLI-MS applications are given.

Improved abundance sensitivity of molecular ions in positive-ion APCI MS analysis of petroleum in toluene

Positive-ion atmospheric pressure chemical ionization (APCI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analyses of petroleum sample were performed with higher sensitivity by switching the solvent composition from toluene and methanol or acetonitrile to a one-component system consisting only of toluene. In solvent blends, molecular ions were more abundant than were protonated ions with increasing percentages of toluene. In 100% toluene, the double-bond equivalence (DBE) distributions of molecular ions obtained by APCI MS for each compound class were very similar to those obtained in dopant assisted atmospheric pressure photo ionization (APPI) MS analyses. Therefore, it was concluded that charge-transfer reaction, which is important in toluene-doped APPI processes, also plays a major role in positive-ion APCI. In the DBE distributions of S(1), S(2), and SO heteroatom classes, a larger enhancement in the relative abundance of molecular ions at fairly specific DBE values was observed as the solvent was progressively switched to toluene. This enhanced abundance of molecular ions was likely dependent on molecular structure.

Atmospheric pressure ion sources

This review of atmospheric pressure ion sources discusses major developments that have occurred since 1991. Advances in the instrumentation and understanding of the key physical principles are the primary focus. Developments with electrospray and atmospheric pressure chemical ionization and variations encompassing adaptations for surface analysis, ambient air analysis, high throughput, and modification of the ionization mechanism are covered. An important and limiting consequence of atmospheric pressure chemical ionization, chemical noise, is discussed as is techniques being employed to ameliorate the problem. Ion transfer and transport from atmospheric pressure into deep vacuum is an area undergoing constant improvement and refinement so is given considerable consideration in this review.

Evidence of neutral radical induced analyte ion transformations in APPI and near-VUV APLI

We report on the reactions of neutral radical species [OH, Cl, O(3P), H], generated in a typical atmospheric pressure ionization (API) source upon irradiation of the sample gases with either 193 nm laser radiation or 124 nm VUV light, the latter commonly used in atmospheric pressure photoionization (APPI). The present investigations focus on the polycyclic aromatic hydrocarbon pyrene as representative of the aromatic compound class. Experimental results are supported by computational methods: simple kinetic models are used to estimate the temporal evolution of the concentrations of reactants, intermediates, and final products, whereas density functional theory (DFT) energy calculations are carried out to further elucidate the proposed reaction pathways. The neutral radicals are generated upon photolysis of background water and oxygen always present in appreciable mixing ratios in typical API sources. Substantial amounts of oxygenated analyte product ions are observed using both techniques. In contrast, upon atmospheric pressure laser ionization (APLI) with 248 nm radiation, oxygenated products are virtually absent. In addition, kinetic data evaluation yielded a bimolecular rate constant of k = (1.9 +/- 0.9) x 10(-9) cm3 molecule(-1) s(-1) for the reaction of the pyrene radical cation with OH radicals.

Critical factors in chemical characterization for the evaluation of decontamination in solids using advanced oxidation

Advanced oxidation technologies (AOT) have been applied to the treatment of numerous organic pollutants embedded in solid matrices (e.g., soil, sediments, sludge, etc.). Given potentially strong matrix-analyte interactions in solids, chemical characterization of both the target contaminants and their oxidation products is critical for the evaluation of any decontamination method. The success of AOT applications has been evaluated either directly (based on the removal of original contaminants, extent of mineralization, and/or formation of by-products), or indirectly, e.g., based on toxicity or chemical oxygen demand. Since indirect methods do not provide comprehensive understanding of the pollutants' fate, direct analytical approaches are covered in this review while focusing on sample preparation and detailed chromatographic characterization, assessing the strengths and weaknesses of these methods. The significance of sample preparation, in particular extraction, is discussed with respect to the nature of matrix-analyte interactions, as those may also affect the selection of the remediation method. The ultimate goal of this review is the presentation of methods employed to achieve mass balance closure, which is essential to ensure the full understanding of degradation pathways.

Light and molecular ions: the emergence of vacuum UV single-photon ionization in MS

Thanks to recent technological advances and single-photon ionization's (SPI's) ability to detect all organics, the technique could become the long-sought universal soft ionization method. (To listen to a podcast about this feature, please go to the Analytical Chemistry Web site at pubs.acs.org/journal/ancham.).

Atmospheric pressure free liquid infrared MALDI mass spectrometry: toward a combined ESI/ MALDI-liquid chromatography interface

A new atmospheric pressure (AP)-MALDI-type interface has been developed based on a free liquid (FL) microbeam/microdroplets and a mid-infrared optical parametric oscillator (mid-IR OPO). The device is integrated into a standard on-line nanoESI interface. The generation of molecular ions in the gas phase is believed to be the result of a fast (explosive) laser-induced evaporative dispersion(not desorption) of the microbeam into statistically charged nanodroplets. Only the lowest charge states appear insignificant abundance in this type of experiment. Mass spectra of some common peptides have been acquired in positive ion mode, and the limit-of-detection of this first prototype (liquid microbeam setup) was evaluated to be 17 fmol per second. To improve the duty cycle and to reduce the sample consumption, a droplet-on-demand system was implemented (generating 100 pL droplets).With this setup, about 20 attomole of bradykinin were sufficient to achieve a signal-to-noise ratio better than five.This setup can be operated at flow rates down to 100 nL/min and represents a liquid MALDI alternative to the nanoESI. Our particular interest was the application of the developed ion source for on-line coupling of liquid chromatography with mass spectrometry. The flow rates(>100 microL/min), required for stable operation of the ion source in continuous liquid microbeam mode, matches perfectly the flow rate range of micro HPLC. Therefore, online LC/MS experiments have been realized, employing a microbore C18 reversed-phase column to separate an artificial peptide mixture and tryptic peptides of bovine serum albumin (performing a peptide mass fingerprint). In the latter case, sequence coverage of more than 90%has been achieved.

Mechanism of [m+h]+ formation in atmospheric pressure photoionization mass spectrometry: identification of propionitrile in acetonitrile with high mass accuracy measurement and tandem mass spectrometry and evidence for its involvement in the protonation phenomenon

The role of propionitrile in the production of [M+H]+ under atmospheric pressure photoionization (APPI) was investigated. In dopant-assisted APPI using acetone and anisole, protonated acetone and anisole radical cations were the most prominent ions observed. In dopant-free or direct APPI in acetonitrile, however, a major ion in acetonitrile was detected and identified as propionitrile, using high accuracy mass measurement and collision induced dissociation studies. Vaporizing ca. 10(-5) M althiazide and bendroflumethazide under direct APPI in acetonitrile produced their corresponding protonated species [M+H]+. In addition to protonated acetonitrile, its dimers, and acetonitrile/water clusters, protonated propionitrile, propionitrile dimer, and propionitrile/water clusters were also observed. The role of propionitrile, an impurity in acetonitrile and/or a possible product of ion-molecule reaction, in the production of [M+H]+ of althiazide and bendroflumethazide was further investigated in the absence of dopant using propionitrile-d5. The formation of [M+D]+ species was observed, suggesting a possible role of propionitrile in the protonation process. Additionally, an increase in the [M+H]+ signal of althiazide and bendroflumethazide was observed as a function of propionitrile concentration in acetonitrile. Theoretical data from the literature supported the assumption that one possible mechanism, among others, for the formation of [M+H]+ could be attributed to photo-initiated isomerization of propionitrile. The most stable isomers of propionitrile, based on their calculated ionization energy (IE) and relative energy (DeltaE), were assumed to undergo proton transfer to the analytes, and mechanisms were proposed.