Diffuse Reflectance Spectroscopy Environmental Problem Solving

R.W. Frei

• Publisher: CRC Press
• Year: 2019
• Print ISBN: 9781315892313
• eBook ISBN: 9781351088312
• Topic: Physical Sciences
• Subtopic: Biology

The main subject division of this book include the theory of Diffuse reflectance spectroscopy; measurement and standardization of diffuse reflectance; instrumentation; application to color measurement and physical, inorganic, and organic chemistry; and applications in chromatographic analysis.While the use of reflectance spectroscopy dates from the 1920s, it has only been in the last decade that its analytical potential has been developed. Interestingly, much of the early research involved industrial uses where measurement of color was required. The development and acceptance of thin-layer chromatography has opened up new areas of analysis for the application of this technique.It is not the purpose of this book to delve deeply into the theoretical aspects of reflectance spectroscopy, as this book has already been done in several previous books. Insofar as it is possible, this book is an up-to-date guide to instruments and techniques intended primarily for the chemical analyst, though it is hoped that it may contain information of interest to other scientists. The potential for the application of this technique is great and the authors feel confident that the coming decade will see many interesting developments in this type of spectroscopy study, particularly in the field of analysis.

Chapter 1

INTRODUCTION

Transmission spectroscopy is the technique that takes advantage of the transparent nature of samples to measure their spectral properties. With this method it is possible to measure the specific absorption of substances dissolved in a suitable solvent by transmitting monochromatic light through the resulting solution. This approach becomes difficult when applied to turbid or colloidal systems where substantial amounts of energy are lost due to light-scattering processes. Transmission measurements become completely unsuitable or even impossible to determine when absorption spectra of substances embedded or adsorbed on solid surfaces are to be measured. It is for problems of this nature that a solution can be provided by the technique of reflectance spectroscopy. Instead of measuring the light flux that is transmitted through a medium as a corresponding percent transmittance on the instrument meter, one measures the amount of light reflected from the sample surface with the aid of a suitably modified transmission spectrophotometer. The data are reported as percent reflectance (%R) read on the transmittance scale of commercial instruments and correspond to R = I/Io (where Io is the intensity of incident radiation and I is the intensity of radiation reflected from the medium). Compared to transmission spectroscopy, reflectance spectroscopy is still very much in its infancy. The technique is, however, not as new as one might expect, particularly if one is speaking about diffuse reflectance spectroscopy, which deals with the measurement of light reflected in a diffuse manner (in contrast to specular or direct reflectance). The paper, paint, dye, textile, printing, and ceramics industries, for example, have long made use of diffuse reflectance methods for measurement of color in routine quality-control processes.

Initial attempts to use diffuse reflectance spectroscopy as a potentially quantitative technique date back to about 1920 and resulted in the design by Taylor¹ and Benford² of the first relatively sophisticated filter-type reflectometers. A few years later Hardy³ developed a recording spectrophotometer-type reflectometer. These instruments had one common design feature, viz., they all used an integrating sphere (or “Ulbrichtkugel”⁴), which permitted the collection of diffusely reflected light and, in addition, either the exclusion of the generally undesirable specular component or alternatively, its inclusion to give total reflectance.

It was not until 1961⁵ that specular reflectance techniques, also known as “Internal Reflectance” or “Attenuated Total Reflectance” (ATR), emerged as an analytical tool of great potential. Specular reflectance methods will not be discussed in this book and the various terms used, such as reflectance spectroscopy, spectral reflectance, or reflectance techniques will refer only to diffuse reflectance spectroscopy unless otherwise specified.

As indicated earlier, a number of industries were among the first users of diffuse reflectance spectroscopy, and the first book on the subject of color measurements appeared in 1936.⁶ The most authoritative text has been written by Judd and Wyszecki,⁷ it contains many references to this technique as used in the industries already mentioned. Recently, Billmeyer and Saltzman⁸ published an excellent treatise on the principles of color measurement. This text gives a good introduction to the field, as well as an up-to-date treatment of the recent developments in color-measurement theory, instrumentation, and techniques. In analytical chemistry, however, there has been until recently little interest in the application of diffuse reflectance spectroscopy to the solution of qualitative or quantitative analytical problems. The situation now is rapidly changing due to the ready availability of diffuse reflectance attachments for practically all commercial spectrophotometers. This change, in turn, has been reflected in a rapid growth of the literature in the field. The most prominent contribution to the popularity of diffuse reflectance techniques in the field of chemistry, particularly for investigations of adsorption phenomena and the study of kinetics and equilibria on surfaces, must be credited to Kortüm. His work recently has culminated in a book⁹ that is also available in an English translation. The first comprehensive treatment on the subject of reflectance spectroscopy was written by Wendlandt and Hecht in 1966.¹⁰ Both these texts include a discussion of specular reflectance techniques. Also of interest are the published proceedings of the 1967 American Chemical Society Symposium on Reflectance Spectroscopy.¹¹

Reflectance spectroscopy has received a continuing increase in attention during the past decade. It is predicted that this trend will continue during the 1970’s and, in particular, that diffuse reflectance spectroscopy will become an increasingly useful tool in the field of chromatography.¹²

REFERENCES

1. Taylor, A. H., J. Opt. Soc. Am., 4, 9 (1919).

2. Benford, F., Gen. Elec. Rev., 23, 72 (1920).

3. Hardy, A. C., J. Opt. Soc. Am., 18, 96 (1929).

4. Ulbricht, T., Elektrotech. Z., 21, 595 (1900).

5. Fahrenfort, J., Spectrochim. Acta, 17, 698 (1961).

6. Hardy, A. C., Handbook of Colorimetry, M.I.T. Press, Cambridge, Mass., 1936.

7. Judd, D. B. and Wyszecki, G., Color in Business, Science and Industry, 2nd ed., John Wiley & Sons, New York, 1963.

8. Billmeyer, F. W., Jr. and Saltzman, M., Principles of Color Technology, John Wiley & Sons, New York, 1966.

9. Kortiím, G.,Reflexionsspektroskopie, Springer-Verlag, Berlin, 1969.

10. Wendlandt, W. W. and Hecht, H. G., Reflectance Spectroscopy, John Wiley & Sons, New York, 1966.

11. Wendlandt, W. W., Ed., Modern Aspects of Reflectance Spectroscopy, Plenum Press, New York, 1968.

12. Frei, R. W., in Progress in Thin-Layer Chromatography and Related Methods, Vol. II, Niederwieser, A. and Pataki, G., Eds., Ann Arbor Science Publishers, Ann Arbor, Mich., 1971.

Chapter 2
THEORY

1. The Kubelka-Munk Theory

The most generally accepted theory concerning diffuse reflectance and the transparency of light-scattering and light-absorbing layers has been developed by Kubelka and Munk.^1,2 It may be said that this theory has acquired an importance in the field of reflectance spectroscopy comparable to the importance of the Bouger-Beer law in transmittance spectroscopy. Development of the theory for an infinitely thick opaque layer yields the Kubelka-Munk equation, which may be

where R’_∞ is the absolute reflectance of the layer, k is its molar absorption coefficient, and s is the scattering coefficient. A derivation of the equation is presented in the Appendix.

Instead of determining R’_∞, however, it is customary to work with the more convenient relative diffuse reflectance, R_∞, which is measured against a standard such as MgO or BaSO₄. In these cases it is assumed that the k values for the standards are zero and that their absolute reflectance is one. However, since the absolute reflectance of the standards exhibiting the highest R’_∞ values never exceeds 0.98 to 0.99, one is actually dealing in such instances with the relationship

and it is essential to specify the standard used. If this expression is introduced into the initial equation it will assume the form

which indicates that a linear relationship should be observed between F(R_∞) and the absorption coefficient k, provided s remains constant. S is rendered independent of wavelength by using scattering particles whose size is large in relation to the wavelength used.

When the reflectance of a sample diluted with a non- or low-absorbing powder is measured against the pure powder, the absorption coefficient k may be replaced by the product 2.30∊C, where e is the extinction coefficient and C is the molar concentration.³ The Kubelka-Munk Equation 2.03 can then be written in the form

where k’ is a constant equal to s/303∊. Since F(R_∞) is proportional to the molar concentration under constant experimental conditions, the Kubelka-Munk relationship is analogous to the Beer-Lambert law of absorption spectrophotometry. At high enough dilutions, the regular reflection from the sample approximates that from the standard and is thus canceled out in any comparison measurement.

A straight-line relationship between F(R_∞) and C is only observed, however, when dealing with weakly absorbing substances and only when the grain size of the powders used is relatively small (ideally around 1 μ in diameter).^4-6 Furthermore, any significant departure from the state of infinite thickness of the adsorbent layer assumed in the derivation of the Kubelka-Munk equation results in background interference, which in turn is responsible for nonideal diffuse reflectance. When either adsorbents with a large grain size or large concentrations of the absorbing species are used, plots of F(R_∞) vs. concentration deviate from straight lines in that there is a decrease in slope at higher concentrations.

In his explanation of this phenomenon, Kortum^4-7 postulates that the reflected radiation is the result of both regular and diffuse reflectance. The first can be described as a mirror reflection, whereas the second occurs when impinging radiation is partly absorbed and partly scattered by a system so that it is reflected in a diffuse manner, i.e., with no defined angle of emergence. Regular reflectance for cases involving normal incidence is described by the Fresnel equation

where k is the absorption coefficient and n is the refractive index. Diffuse reflectance is described by the K...

Table of contents

• Cover Page
• Title Page
• Copyright Page
• Preface
• Table of Contents
• Chapter 1: Introduction
• Chapter 2: Theory
• Chapter 3: Measurement and Standardization of Diffuse Reflectance
• Chapter 4: Instrumentation
• Chpter 5: Application to Color Measurements and Color Comparison
• Chapter 6: Applications To Problems In Physical, Inorganic, and Organic Chemistry
• Chapter 7: Applications In Chromatography
• Appendix 1
• Appendix 2