Photothermal Spectroscopy Methods

Stephen E. Bialkowski, Nelson G.C. Astrath, Mikhail A. Proskurnin

• Publisher: Wiley
• Year: 2019
• ISBN:
  9781119279099
• Edition: 2
• Topic: Physical Sciences
• Subtopic: Analytic Chemistry

Covers the advantages of using photothermal spectroscopy over conventional absorption spectroscopy, including facilitating extremely sensitive measurements and non-destructive analysis

This unique guide to the application and theory of photothermal spectroscopy has been newly revised and updated to include new methods and applications and expands on applications to chemical analysis and material science. The book covers the subject from the ground up, lists all practical considerations needed to obtain accurate results, and provides a working knowledge of the various methods in use.

Photothermal Spectroscopy Methods, Second Edition includes the latest methods of solid state and materials analysis, and describes new chemical analysis procedures and apparatuses in the analytical chemistry sections. It offers a detailed look at the optics, physical principles of heat transfer, and signal analysis. Information in the temperature change and optical elements in homogeneous samples and photothermal spectroscopy in homogeneous samples has been updated with a better description of diffraction effects and calculations. Chapters on analytical measurement and data processing and analytical applications are also updated and include new information on modern applications and photothermal microscopy. Finally, the Photothermal Spectroscopy of Heterogeneous Sample chapter has been expanded to incorporate new methods for materials analysis.

• New edition updates and expands on applications to chemical analysis and materials science, including new methods of solid state and materials analysis
• Includes new chemical analysis procedures and apparatuses
• Provides an unmatched resource that develops a consistent mathematical basis for signal description, consolidates previous theories, and provides invaluable insight into laser technology

Photothermal Spectroscopy Methods, Second Edition will appeal to researchers from both academia and industry (graduate students, postdocs, research scientists, and professors) in the general field of analytical chemistry, optics, and materials science, and researchers and engineers at scientific instrument developers in fields related to photonics and spectroscopy.

1
Introduction

1.1 Photothermal Spectroscopy

Photothermal spectroscopy is a group of high sensitivity methods used to measure optical absorption and thermal characteristics of a sample. The basis of photothermal spectroscopy is a photoinduced change in the thermal state of the sample. Light energy absorbed and not lost by subsequent emission results in sample heating. This heating results in a temperature change as well as changes in thermodynamic parameters of the sample that are related to temperature. Measurements of the temperature, pressure, or density changes that occur due to optical absorption are ultimately the basis for the photothermal spectroscopic methods.

Ingle and Crouch (1988) classify photothermal spectroscopy as one of several indirect methods for optical absorption analysis. Indirect methods do not measure the transmission of light used to excite the sample directly, but rather measure an effect that optical absorption has on the sample. The term indirect applies to the light measurement, not to the optical absorbance. Photothermal spectroscopy is, in a sense, a more direct measure of optical absorption than optical transmission‐based spectroscopies. Sample heating is a direct consequence of optical absorption, and so photothermal spectroscopy signals are directly dependent on light absorption. Scattering and reflection losses do not produce photothermal signals. Subsequently, photothermal spectroscopy more accurately measures optical absorption in scattering solutions, in solids, and at interfaces. This aspect makes it particularly attractive for application to surface and solid absorption studies and studies in scattering media.

The indirect nature of the measurement also results in photothermal spectroscopy being more sensitive than optical absorption measured by transmission methods. There are two reasons for this. First, photothermal effects can amplify the measured optical signal. This amplification is referred to as the enhancement factor (Dovichi and Harris 1979; Mori, Imashaka, and Ishibashi 1982) and is the ratio of the signal obtained using photothermal spectroscopy to that of conventional transmission spectroscopy. Enhancement factors depend on the thermal and optical properties of the sample, the power or energy of the light source used to excite the sample, and the optical geometry used to excite the sample. Since the optical excitation power or energy and geometry are variable, the enhancement can be made very large, even for samples with relatively poor thermal and optical properties. In fact, the problem with photothermal spectroscopy is not the absorption detection limit. The problem is the detection of analyte absorbance in the presence of a relatively large (10⁻⁵ cm⁻¹) absorbance of the solvent. The second reason photothermal spectroscopy is more sensitive than transmission is that the precision of the measurement is inherently better than that of the direct transmission method. The fundamental limitation of conventional absorption spectroscopy, namely, shot noise, may be partially circumvented (Bialkowski et al. 1992). Because of the increased fundamental signal‐to‐noise ratios, the problem of being able to detect the analyte in the presence of a relatively large background absorption should be able to be overcome with perseverance.

The high sensitivity of the photothermal spectroscopy methods has led to applications for analysis of low absorbance samples. Dovichi (1987) reviewed the literature regarding the use of photothermal spectroscopy for chemical analysis. The magnitude of the photothermal spectroscopy signal depends on the specific method used to detect the photothermal effect and on the type of sample being analyzed. There are many different reported detection limits, and it is difficult to specify an absolute lower limit of detection since the method may be used to measure the background absorption of the solvent itself. But it is safe to say that optical absorbances of less than 10⁻⁶ can be detected with optimized experimental designs. Subsequently, photothermal spectroscopy is often characterized as a trace analysis method. Concentration limit of detection measurements can be impressive. Electronic transitions of strongly absorbing chromophores have molar absorptivities exceeding 10⁴ M⁻¹ cm⁻¹. Using photothermal methods, concentrations lower than 10⁻¹⁰ M of these strongly absorbing chromophores may be measured in standard cuvettes. These limits of detection are slightly higher than those obtained using laser‐excited fluorescence spectroscopy and are two to three orders of magnitude better than that obtained using conventional transmission spectroscopy. The low molar absorption detection limits coupled with the fact that the volume being probed can be very small result in extremely small numbers of molecules being detected. The high absorbance sensitivity of these methods has opened up new areas of trace chemical analysis based on optical absorption spectroscopy.

Photothermal signals depend on the thermodynamic and ene...

Table of contents

• Cover
• Table of Contents
• About the Authors
• Preface
• Acknowledgments
• 1 Introduction
• 2 Absorption, Energy Transfer, and Excited State Relaxation
• 3 Hydrodynamic Relaxation: Heat Transfer and Acoustics
• 4 Temperature Change, Thermoelastic Deformation, and Optical Elements in Homogeneous Samples
• 5 Photothermal Spectroscopy in Homogeneous Samples
• 6 Analytical Measurement and Data Processing Considerations
• 7 Analytical Applications
• 8 Photothermal Spectroscopy of Heterogeneous Samples
• Index
• End User License Agreement