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

Development of a multipurpose ion source for LC-MS and GC-API MS

Over the past decade, multimode ion sources operating at atmospheric pressure (i.e., more than one ionization method is operative in the ion source enclosure) have received considerable interest. Simultaneous operation of different ionization methods targeting different compound classes within one analysis run has several advantages, including enhanced sample throughput and thus significant laboratory cost reductions. Potential drawbacks are enhanced ion suppression and other undesirable effects of the simultaneous operation of ionization methods. In this contribution we present an alternative approach-the development and characterization of a widely applicable, multipurpose ion source operating at atmospheric pressure. The optimized source geometry allows rapid changing from LC-API methods (ESI, APCI, APLI) to GC-API methods (APCI, APLI, DA-APLI) along with the appropriate coupling of chromatographic equipment required. In addition, true multimode operation of the source is demonstrated for LC-ESI/APLI and LC-APCI/APLI.

Combining chip-ESI with APLI (cESILI) as a multimode source for analysis of complex mixtures with ultrahigh-resolution mass spectrometry

Recently we have established atmospheric-pressure laser ionisation (APLI) as a method for coupling time-of-flight mass spectrometric detectors (TOF MS) with chromatographic systems (HPLC and GC) to allow two-photon ionisation of non-polar aromatic compounds. Here we demonstrate that APLI can be combined with chip-electrospray ionisation (cESI) coupled to Fourier-transform-ion cyclotron resonance mass spectrometry (FT-ICR MS) for ultrahigh-resolution analysis of complex samples. With the laser turned off, the analytes are ionised only by ESI, whereas when the laser is switched on non-polar aromatic substances also are ionised. In combination with the extremely high mass resolution of an FT-ICR MS, simultaneous qualitative analysis of polar and non-polar analytes is possible in both positive and negative modes, as is exemplified with a crude oil sample. Nevertheless, ion suppression was observed (up to ca. 70% for D(10)-pyrene) and thus sample preparation with chromatographic or electrophoretic pre-separation is necessary for quantitative analysis of targets. In addition, for the first time, the dopant-assisted APLI method in combination with cESI (DA-cESILI) was used for determination of 1-nitrocoronene.

The distribution of ion acceptance in atmospheric pressure ion sources: spatially resolved APLI measurements

It is demonstrated that spatially resolved mass selected analysis using atmospheric pressure laser ionization mass spectrometry (APLI MS) represents a new powerful tool for mechanistic studies of ion-molecule chemistry occurring within atmospheric pressure (AP) ion sources as well as for evaluation and optimization of ion source performance. A focused low-energy UV laser beam is positioned computer controlled orthogonally on a two-dimensional grid in the ion source enclosure. Resonance enhanced multiphoton ionization (REMPI) of selected analytes occurs only within the confined volume of the laser beam. Depending on the experimental conditions and the reactivity of the primary photo-generated ions, specific signal patterns become visible after data treatment, as visualized in, e.g., contour or pseudo-color plots. The resulting spatial dependence of sensitivity is defined in this context as the distribution of ion acceptance (DIA) of the source/analyzer combination. This approach provides a much more detailed analysis of the diverse processes occurring in AP ion sources compared with conventional bulk signal response measurements.

Ultrasensitive Determination of Polycyclic Aromatic Compounds with Atmospheric-Pressure Laser Ionization as an Interface for GC/MS

Recently we introduced atmospheric pressure laser ionization (APLI) as a complementary ionization method for coupling LC-MS systems (HPLC and CEC), allowing ionization of nonpolar aromatic compounds via near-resonant two-photon excitation. In this paper, we demonstrate that APLI with the same source enclosure as for LC coupling is also suited for hyphenation of GC with atmospheric-pressure ionization mass spectrometry. This technique permits the qualitative and quantitative determination of aromatic compounds in an ultralow concentration range, as we show here with polycyclic aromatic hydrocarbons (PAHs), alkylated PAHs, and hetero-PAHs as examples. The outstanding sensitivity is demonstrated for chrysene, with a detection limit of 22 amol. Polar functional groups reduce the sensitivity, but after methylation or silylation, the analytes can also be determined very sensitively in complex matrixes, as is shown with 1-hydroxypyrene in urine.

APPI-MS: effects of mobile phases and VUV lamps on the detection of PAH compounds

The technique of atmospheric pressure photoionization (APPI) has several advantages over electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI), including efficient ionization of nonpolar or low charge affinity compounds, reduced susceptibility to ion suppression, high sensitivity, and large linear dynamic range. These benefits are greatest at low flow rates (i.e., Collapse

Key Words

• atmospheric pressure photoionization
• appi
• mass spectrometry
• polyaromatic hydrocarbons
• pah
• krypton
• argon
• vuv
• solvent effects
• photoabsorption cross-section

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MESH Headings

• Atmospheric Pressure
• Benz(a)Anthracenes/analysis
• Benzo(a)pyrene/analysis
• Chromatography, High Pressure Liquid
• Mass Spectrometry/instrumentation
• Mass Spectrometry/methods
• Noble Gases/chemistry
• Photochemistry/instrumentation
• Photochemistry/methods
• Pyrenes/analysis
• Solvents/chemistry

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Grants

• R43 GM063430 NIGMS NIH HHS
• R44 GM063430 NIGMS NIH HHS
• R44 GM063430-03S1 NIGMS NIH HHS
• GM063430 NIGMS NIH HHS

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Affiliation(s)

Luke Chandler Short Syagen Technologies, Inc., Tustin, California 92780, USA
Sheng-Suan Cai
Jack A Syage

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Electrospray ionization/atmospheric pressure photoionization multimode source for low-flow liquid chromatography/mass spectrometric analysis

Analysis of several polar and non-polar compounds is performed with a newly developed dual electrospray ionization/atmospheric pressure photoionization (ESI/APPI) or ESPI source. Several variables are considered in the source, such as ESI probe heater temperature, solvent flow, dopant effects, repeller plate voltage, source geometry and photon energy (Kr vs. Ar lamp). Direct photoionization resulting in a molecular radical cation [M](*+) dominates at high temperatures (>400 degrees C) and low flow rates (<200 microL/min). Indirect photo-induced chemical ionization (PCI) involving solvent molecules becomes important at lower temperatures and higher solvent flow rates. Indirect PCI is enhanced using an Ar lamp, which yields comparable [M+H](+) signal but poorer [M](*+) signal than the Kr lamp at lower temperatures and higher flow rates. This is in support of our recent finding that the Ar lamp results in a solvent-dependent enhancement of analyte molecules via PCI. Analysis of 12 compounds in methanol under low-flow conditions (10 microL/min) demonstrates that the dual ESPI source performs favorably for most compounds versus the standard ESCI source, and significantly better than ESCI for the analysis of unstable drugs, like flurbiprofen. Several factors contributing to the benefits of the ESPI source are the shared optimal geometry for ESI and APPI sources and soft ionization of APPI versus APCI.

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

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

Single photon ionization time-of-flight mass spectrometry with a pulsed electron beam pumped excimer VUV lamp for on-line gas analysis: setup and first results on cigarette smoke and human breath

Single-photon ionization (SPI) using vacuum ultraviolet (VUV) light produced by an electron beam pumped rare gas excimer source has been coupled to a compact and mobile time-of-flight mass spectrometer (TOFMS). The novel device enables real-time on-line monitoring of organic trace substances in complex gaseous matrixes down to the ppb range. The pulsed VUV radiation of the light source is employed for SPI in the ion source of the TOFMS. Ion extraction is also carried out in a pulsed mode with a short time delay with respect to ionization. The experimental setup of the interface VUV light source/time-of-flight mass spectrometer is described, and the novel SPI-TOFMS system is characterized by means of standard calibration gases. Limits of detection down to 50 ppb for aliphatic and aromatic hydrocarbons were achieved. First on-line applications comprised real-time measurements of aromatic and aliphatic trace compounds in mainstream cigarette smoke, which represents a highly dynamic fluctuating gaseous matrix. Time resolution was sufficient to monitor the smoking process on a puff-by-puff resolved basis. Furthermore, human breath analysis has been carried out to detect differences in the breath of a smoker and a nonsmoker, respectively. Several well-known biomarkers for smoke could be identified in the smoker's breath. The possibility for even shorter measurement times while maintaining the achieved sensitivity makes this new device a promising tool for on-line analysis of organic trace compounds in process gases or biological systems.

A silica-based monolithic column in capillary HPLC and CEC coupled with ESI-MS or electrospray-atmospheric-pressure laser ionization-MS

We describe the successful coupling of CEC and capillary HPLC with the recently developed atmospheric-pressure laser ionization (APLI) method. APLI is suitable for selectively and sensitively ionizing nonpolar aromatic compounds at ambient pressure for subsequent mass-selective detection. The polycyclic aromatic hydrocarbons used as analytes are first separated either by CEC on a silica-based monolithic column or by capillary HPLC. The eluent, along with a sheath flow, is volatilized by microelectrospray and then selectively ionized by excimer laser (KrF*) radiation via two-photon excitation. A QTOF-MS is used as mass-selective detector. This interface combination makes soft ionization of thermally labile nonpolar aromatic analytes possible.

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

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