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Photoionization and Photo–Induced Processes inMass Spectrome

Fundamentals and Applications

Edited by

Ralf Zimmermann
Luke Hanley






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The goal of this book is to explain the fundamentals of photoionization (PI) and the associated applications of PI that are playing an increasingly important role in mass spectrometry (MS). The target audience of this book includes practicing scientists, including PhD and MSc students, whose primary interest is in the application of PI to elemental or molecular analysis by mass spectrometry. An overly simplistic analysis provides several motivations for the use of PI in mass spectrometry:

  1. 1. Enhance ion yields for specific important compounds of substance classes where other ionization strategies have proven insufficient,
  2. 2. Selectively ionize individual molecular structures or classes of molecules from mixtures,
  3. 3. Form ions from neutrals with reduced or controlled degree of fragmentation, and
  4. 4. Induce desorption or ablation of solids for sampling.

Photoionization comes in different versions and technological realizations, making it somewhat more complicated and thus less popular than standard ionization methods in mass spectrometry such as electrospray ionization and electron ionization. Nevertheless, one goal of this book is to demonstrate how these and other motivations drive the use of PI in MS‐based analyses. The editors and authors hope that the readers will use this work as a series of building blocks for future advances in this promising area.

Actually, PI was central to the early development of mass spectrometry and single photon ionization (SPI) remains perhaps the best method for introducing a well‐defined amount of internal energy into a molecular ion. Early work in SPI used vacuum ultraviolet (VUV) gas discharge lamps. However, the advent of lasers and the associated nonlinear optical methods made multiphoton ionization (MPI) possible, dramatically expanding PI as a fundamental strategy to probe molecular structure. The continuing improvement of laser technology has also created ongoing opportunities for MS‐based applications of PI. For example, MPI via the excimer laser‐pumped dye laser was deployed in many early fundamental studies, but was costly, difficult to use, and relatively unreliable. However, the development of smaller and rugged, field‐deployable laser sources, such as compact excimer lasers, solid‐state lasers as Nd:YAG lasers with integrated harmonic generation, and tunable solid‐state lasers such as sealed, Nd:YAG‐pumped optical parametric oscillators (OPOs) or Ti:sapphire cavities rendered MPI sufficiently robust for more widespread MS applications.

This interplay between fundamental methods and instrumental considerations has driven the development of analytical applications of PI, so attention is paid here to both considerations. Photodissociation is only discussed here when it occurs in conjunction with PI, such as via the formation of fragment ions from neutral precursors during MPI or SPI. Thus, the burgeoning use of photodissociation of the precursor or molecular ions to form fragment ions for structural elucidation of the former is not discussed here. The fundamentals and mechanisms of low pressure, gas‐phase molecular PI for mass spectrometry is covered in Chapters 1, 2, 5, and 7, while molecular atmospheric pressure PI is a topic in Chapter 8. Applications and experimental methods of low‐pressure, gas‐phase photoionization mass spectrometry (PIMS) are presented in Chapters 3, 4, and 7. Elemental analysis by PI is covered exclusively in Chapter 5. Processes and applications that include a laser desorption (LD) or direct laser desorption/ionization (LDI) of analytes from the condensed phase are covered in Chapters 9 and 10, while Chapter 11 focuses on the direct analysis of individual aerosol particles using LDI, LD, and PI.

In the following, the content of the book is briefly highlighted chapterwise.

The first chapter entitled FUNDAMENTALS and MECHANISMS of VACUUM PHOTOIONIZATION, is serving as an introduction to mechanistic issues common to PI of molecules, atoms, and clusters under vacuum. It provides a fundamental description of light and the interaction of light and matter in the form of photoabsorption on a quantum mechanical level. This includes an excursion in perturbation theory and the dipole approximation and leads to an elucidation of the photon absorption selection rules. Finally, the SPI process, as an absorption in a continuum, nonbound state, is discussed and the most important parameters for SPI, the SPI cross sections and ionization energies, are deducted.

Fundamental aspects of MPI of molecules in the absence of gas‐phase collisions are covered in the following chapter, entitled FUNDAMENTALS and MECHANISMS of RESONANCE‐ENHANCED MULTIPHOTON IONIZATION (REMPI) in VACUUM and APPLICATION of REMPI for MOLECULAR SPECTROSCOPY (Chapter 2). Here, the theoretical consideration of photoionization processes is extended to MPI processes of molecular species and MPI‐based spectroscopy, while multiphoton ionization of atoms is covered in Chapter 6. Different MPI and REMPI processes are presented and a rate equation approach is used to deduct the influence of molecular physical properties on ionization efficiency. Special REMPI schemes are discussed, which can bypass unfavorable photophysical properties. In the following, MPI‐induced fragmentation and dissociation processes are discussed. The application of REMPI for molecular spectroscopy with and without supersonic jet cooling is demonstrated in an exemplary manner via a detailed discussion of the REMPI wavelength spectrum of supersonic jet‐cooled biphenylene and some other interesting aromatic molecules. REMPI wavelength spectroscopy, in particular with supersonic jet cooling, reveals the selectivity of the REMPI process that connects high‐resolution ultraviolet (UV) spectroscopy to mass spectrometry. The selective ionization of isomeric and isobaric compounds, however, can be extended to the differentiation of isotopomers and – by using a special REMPI technique with circular polarized laser light – even of enantiomers. Finally, in this chapter, REMPI‐based photoelectron spectroscopic (PS) techniques such as zero kinetic energy photoelectron spectroscopic (ZEKE‐PS) are introduced. The explanation of the fundamental aspects of SPI and REMPI leads to the analytical application of these two PI approaches.

In the beginning of Chapter 3, which is entitled “ANALYTICAL APPLICATIONS of SINGLE‐PHOTON IONIZATION MASS SPECTROMETRY,” a brief introduction into common VUV–light sources, is given. These can be distinguished as incoherent light sources (“lamp”‐based technologies) or coherent light sources (lasers). Also, a brief introduction to the generation of synchrotron light in the VUV range is elaborated (for applications of synchrotron‐based SPI, see Chapter 5). The general setup of SPI mass spectrometry systems with laser‐ and lamp‐based sources is introduced. Depending on the used VUV wavelengths, SPI can be a very soft ionization method, enabling the detection of the molecular mass fingerprint of complex mixtures. This renders SPI‐MS to an ideal approach for direct real time monitoring of complex gas and vapor mixtures. Different applications for online monitoring of combustion and pyrolysis processes are introduced, including online monitoring of gas phases from industrial processes, such as the coffee‐roasting and/or the biomass pyrolysis process. Hyphenated instrumental analytical concepts, e.g. gas chromatography (GC) or thermal analysis (TA) coupled to SPI mass spectrometry, are presented and typical results are shown.

Chapter 4, “ANALYTICAL APPLICATION of RESONANCE‐ENHANCED MULTIPHOTON IONIZATION MASS SPECTROMETRY (REMPI‐MS), gives, in a similar way as Chapter 3 for SPI, an overview of analytical applications of REMPI mass spectrometry. The preferential detection of aromatic analytes by the REMPI process with common laser is emphasized in Chapter 3. An overview on typical fixed‐frequency laser lines (excimer lasers or frequency‐multiplied, solid‐state lasers) is provided along with typical wavelength ranges of tunable laser systems. In the following, several exemplary analytical concepts and applications using tunable and fixed‐frequency lasers are presented. The tunable laser sources can be used to focus on specific analytes, which work particularly well with supersonic jet expansion inlet systems. For many applications, however, fixed‐frequency wavelength laser and effusive, heated gas inlets can be applied for a useful and often very sensitive “overview” MS‐profiling of many aromatic compounds. Exemplary process monitoring applications comprise detection of combustion by‐products in flue gases of incineration plants or flavor and roast degree, indicating compounds in coffee‐roasting off‐gas. Hyphenated instruments connecting gas chromatography or a thermo‐optical carbon analyzer (for aerosol loaded filters) to REMPI‐MS devices are discussed. A direct inlet of liquid samples into REMPI mass spectrometry is possible via a membrane inlet or a direct liquid introduction into the ion source. Note that SPI and REMPI processes can be performed in the same MS system. Finally, REMPI under atmospheric conditions is discussed (atmospheric pressure laser ionizationAPLI; see also Chapter 8 for SPI‐based atmospheric pressure‐based ionization).

The application of synchrotron VUV light for SPI‐MS‐based investigation of combustion and pyrolysis processes is laid out in Chapter 5, “PROBING CHEMISTRY AT VACUUM ULTRAVIOLET SYNCHROTRON LIGHT SOURCES.” By direct molecular beam sampling from flames as well as from oxidation or pyrolysis reactors, molecules from the reaction zone can be directly introduced in the mass spectrometer without wall contact. This enables the analysis of stable molecules and intermediates from the reaction zones. The flow reactors and model flame apparatus is often compact enough to be brought to synchrotron light source facilities and can be used for kinetic investigations. Unlike classical VUV sources, the synchrotron light from appropriate beamlines (i.e. equipped with VUV monochromators) exhibit a wide tunability, high energy resolution, and a relatively high photon flux. The identification of individual compounds from isobaric mixture components is possible by photoionization efficiency (PIE) spectroscopy, i.e. the recording of the ion yield as a function of the VUV wavelength. Chapter 5 also presents, an overview on different synchrotron‐based PIMS apparatus and analytical results for several reactors and applications.

REMPI of atoms is commonly referred to as “resonance ionization mass spectrometry (RIMS). The RIMS technology and applications are described in the Chapter 6, RESONANCE IONIZATION MASS SPECTROMETRY (RIMS): FUNDAMENTALS AND APPLICATIONS INCLUDING SECONDARY NEUTRAL MASS SPECTROMETRY. Unlike in molecules, atomic transitions are very narrow and thus very high ionization efficiencies can be achieved. Laser spectroscopic principles, atomic selection rules, and the principle setup of RIMS instruments are discussed. In many cases, two or more tunable lasers are required for high‐resolution RIMS. This high effort, however, in many cases allows even an isotope‐selective ionization and the suppression of nearly all interferences. Applications are focusing on the analyses of ultratraces of radioactive elements in the environment (e.g. plutonium fall out) or the characterization of very precious samples such as stardust grains.

MPI is often performed with nanosecond laser pulses, but shortening the pulses into the subpicosecond regime can fundamentally change the ionization process, as discussed in Chapter 7, “ULTRASHORT PULSE PHOTOIONIZATION for FEMTOSECOND LASER MASS SPECTROMETRY”. The mechanisms of ultrashort pulse ionization, in particular nonresonant multiphoton ionization (NRMPI) and strong field ionization (SFI), are elucidated. One advantage of ultrashort pulse ionization is to overcome problems in REMPI ionization caused by the short lifetime of intermediated states (see also Chapter 2). Thus, for example, molecular ions of explosives, such as trinitrotoluene (TNT), which exhibit an unstable S1 state due to fast predissociation (NO‐cleavage from nitro groups), can be obtained by a fs‐laser ionization process. In addition to gas‐phase ionization, ultrashort pulse ionization is also beneficial for postionization of neutrals to enhance ion signal in secondary ion mass spectrometry (SIMS) or laser desorption (LD)‐based mass spectrometry.

Mass spectrometry of samples at ambient pressure has become a major application, which is served by atmospheric pressure photoionization (APPI) using VUV for induction of the ionization process. Chapter 8, “PHOTOIONIZATION at ELEVATED or ATMOSPHERIC PRESSURE: APPLICATION of APPI and LPPI,” is devoted to such ionization mechanisms at elevated pressures and their experimental implementation. Furthermore, some characteristic applications of APPI are shown. The higher pressures during APPI allow abundant gas‐phase ion‐molecule and molecule‐molecule collisions that finally lead to energy and/or charge transfer in the APPI process. Therefore, very high sensitivities can be observed in APPI. However, although such secondary chemical ionization processes often are dominating the ionization process, APPI is commonly seen as a photoionization technology, although the term “photoinduced chemical ionization” may better describe the APPI process in many cases. In addition to SPI, the REMPI process can also be used to initiate ionization under atmospheric conditions (atmospheric pressure laser ionization, APLI, see Chapter 4).

Laser irradiation of condensed phase matter can also lead to PI as well as other energy transfer events that in turn lead to desorption and/or ablation that is widely used for sampling. Although this book is focusing on photoionization processes in vacuum or in the gas phase, laser desorption‐based processes and applications in mass spectrometry are not be excluded. Chapter 9, “FUNDAMENTALS of LASER DESORPTION/IONIZATION,” introduces the mechanisms of laser desorption/ionization (LDI) and matrix‐assisted laser desorption/ionization (MALDI), while Chapter 10, “APPLICATIONS OF LASER DESORPTION IONIZATION AND LASER DESORPTION/ABLATION WITH POSTIONIZATION,” introduces the corresponding applications. The MALDI approach for ionization of biopolymers (e.g. proteins or lipids), polymers, and other larger molecules is broadly applied, and numerous reviews and book chapters and whole books have already covered this topic. However, for completeness, the knowledge on the MALDI process is summarized in Chapter 9, although more focus is put on less common strategies such as fs‐laser ablation. In the laser desorption applications discussed in Chapter 10, in addition to the molecular detection also, MALDI imaging applications are considered and a focus is put on the approaches using postionization of laser desorbed neutrals.

Finally, Chapter 11, “LASER IONIZATION in SINGLE‐PARTICLE MASS SPECTROMETRY,” elaborates on an online laser mass spectrometric single (aerosol)‐particle measurement technique, which is based either on the LDI or on a laser desorption – laser postionization approach. After an introduction to the historical development of single‐particle laser ionization MS systems, the LDI process is discussed in context of the particle analysis background. This includes the ionization mechanism, laser sources, and quantification aspects. After the discussion of approaches based on the laser postionization of molecules desorbed from individual particles, instrumental realizations and applications are presented and discussed.

Finally, we hope that the readers will enjoy and learn from our exposition of photoionization fundamentals as well as the versatility of the photoionization mass spectrometry methodology for cutting edge applications.

Ralf Zimmermann

University of Illinois, Chicago, USA

Luke Hanley

University of Rostock, Germany