This paper reviews the theory and applications of photoacoustic (also called optoacoustic) methods belonging to the more general area of photothermal measurement techniques. The theory covers excitation of gaseous or condensed samples with modulated continuous light beams or pulsed light beams. The applications of photoacoustic methods include spectroscopy, monitoring deexcitation processes, probing physical properties of materials, and generating mechanical motions. Several other related photothermal methods, as well as particle-acoustics and wave-acoustics methods are also described. This review complements an earlier and narrower review [Rev. Mod. Phys. 53, 517 (1981)] that is mainly concerned with sensitive detection by pulsed photoacoustic spectroscopy in condensed matter.

Applications of photoacoustic sensing techniques

The first part of this two-part review focused on the fundamental and diagnostics aspects of laser-induced plasmas, only touching briefly upon concepts such as sensitivity and detection limits and largely omitting any discussion of the vast panorama of the practical applications of the technique. Clearly a true LIBS community has emerged, which promises to quicken the pace of LIBS developments, applications, and implementations. With this second part, a more applied flavor is taken, and its intended goal is summarizing the current state-of-the-art of analytical LIBS, providing a contemporary snapshot of LIBS applications, and highlighting new directions in laser-induced breakdown spectroscopy, such as novel approaches, instrumental developments, and advanced use of chemometric tools. More specifically, we discuss instrumental and analytical approaches (e.g., double- and multi-pulse LIBS to improve the sensitivity), calibration-free approaches, hyphenated approaches in which techniques such as Raman and fluorescence are coupled with LIBS to increase sensitivity and information power, resonantly enhanced LIBS approaches, signal processing and optimization (e.g., signal-to-noise analysis), and finally applications. An attempt is made to provide an updated view of the role played by LIBS in the various fields, with emphasis on applications considered to be unique. We finally try to assess where LIBS is going as an analytical field, where in our opinion it should go, and what should still be done for consolidating the technique as a mature method of chemical analysis.

https://journals.sagepub.com/doi/pdf/10.1366/11-06574

Laser-induced breakdown spectroscopy (LIBS), part II: review of instrumental and methodological approaches to material analysis and applications to different fields.

Hydrogen/deuterium exchange (HDX) mass spectrometry (MS) has become a key technique for monitoring structural and dynamic aspects of proteins in solution. This approach relies on the fact that exposure of a protein to D2O induces rapid amide H → D exchange in disordered regions that lack stable hydrogen-bonding. Tightly folded elements are much more protected from HDX, resulting in slow isotope exchange that is mediated by the structural dynamics (“breathing motions”) of the protein. MS-based peptide mapping is a well established technique for measuring the mass shifts of individual protein segments. This tutorial review briefly discusses basic fundamentals of HDX/MS, before highlighting a number of recent developments and applications. Gas phase fragmentation strategies represent a promising alternative to the traditional proteolysis-based approach, but experimentalists have to be aware of scrambling phenomena that can be encountered under certain conditions. Electron-based dissociation methods provide a solution to this problem. We also discuss recent advances that facilitate the applicability of HDX/MS to membrane proteins, and to the characterization of short-lived protein folding intermediates. It is hoped that this review will provide a starting point for novices, as well as a useful reference for practitioners, who require an overview of some recent trends in HDX/MS.

Hydrogen Exchange Mass Spectrometry for Studying Protein Structure and Dynamics

Statistics for analytical chemistry

Communication systems: An introduction to signals and noise in electrical communication

Protein amide hydrogen exchange (HDX) is a convoluted process, whose kinetics is determined by both dynamics of the protein and the intrinsic exchange rate of labile hydrogen atoms fully exposed to solvent. Both processes are influenced by a variety of intrinsic and extrinsic factors. A mathematical formalism initially developed to rationalize exchange kinetics of individual amide hydrogen atoms is now often used to interpret global exchange kinetics (e.g., as measured in HDX MS experiments). One particularly important advantage of HDX MS is direct visualization of various protein states by observing distinct protein ion populations with different levels of isotope labeling under conditions favoring correlated exchange (the so-called EX1 exchange mechanism). However, mildly denaturing conditions often lead to a situation where the overall HDX kinetics cannot be clearly classified as either EX1 or EX2. The goal of this work is to develop a framework for a generalized exchange model that takes into account multiple processes leading to amide hydrogen exchange, and does not require that the exchange proceed strictly via EX1 or EX2 kinetics. To achieve this goal, we use a probabilistic approach that assigns a transition probability and a residual protection to each equilibrium state of the protein. When applied to a small protein chymotrypsin inhibitor 2, the algorithm allows complex HDX patterns observed experimentally to be modeled with remarkably good fidelity. On the basis of the model we are now in a position to begin to extract quantitative dynamic information from convoluted exchange kinetics.

/pdf/mapping-protein-energy-landscapes-with-amide-hydrogen-4kklpdo0m9.pdf

Mapping protein energy landscapes with amide hydrogen exchange and mass spectrometry: I. A generalized model for a two-state protein and comparison with experiment.

The fluorescence and photoacoustic characteristics of a windowless flow cell intended for liquid chromatographic applications are compared with respective characteristics of a static cuvette cell. In addition, a photoionization mode of operation for the flow cell is exhibited which utilizes the ionization products of two-photon excitation of polynuclear aromatic hydrocarbons in n-alkanes to effect a sensitive detection of those PAHs.

Comparison of laser excited fluorescence and photoacoustic limits of detection for static and flow cells

Two-photon photoionization detection of polycyclic aromatic hydrocarbons and drugs in a windowless flow cell

A simple piezoelectric detection system suitable for performing highly sensitive liquid-phase photoacoustic spectroscopy is presented. Performance of the system with respect to linearity, solvent effects, and excitation pulse characteristics is experimentally evaluated and compared with theoretical figures of merit. Limits of detection for various porphyrins, laser dyes, and drugs are presented.

Edward Voigtman

Papers

Mapping protein energy landscapes with amide hydrogen exchange and mass spectrometry: I. A generalized model for a two-state protein and comparison with experiment.

Comparison of laser excited fluorescence and photoacoustic limits of detection for static and flow cells

Two-photon photoionization detection of polycyclic aromatic hydrocarbons and drugs in a windowless flow cell

Condensed phase photoacoustic spectroscopic detection of porphyrins and dyes

Statistical properties of limit of detection test statistics.