Spectroscopic Analysis

Type of physical science: Chemistry

Field of study: Chemical methods

Spectroscopic analysis has become the "workhorse" of laboratory analytical methods and has also taken a firm hold in industrial plants, where it is used for quality and process control. Its extended use arises from the wide range of substances that it can measure, both qualitatively and quantitatively, as well as its nondestructive character.

Overview

Spectroscopic analysis is used by chemists in a variety of ways, and there are dozens of techniques. Here, attention is focused on spectroscopy utilizing light in its manifold forms.

Application of the proper type of analysis can produce data that tell what and/or how much of a chemical substance is present in a sample, or the knowledge can be interpreted to provide information about the structures of the chemicals present. These types of data are available on both the molecular and the elemental level. Some of the techniques are also capable of yielding information about the surface layer of a sample, whereas the more usual methods deal only with the bulk of the sample. The range of concentrations that are detectable begins at the percentage level and proceeds down to the parts-per-billion level. The range of materials that can be analyzed varies from the simplest elements to highly complex polymers and biomolecules. The information provided can be used to bolster theoretical reasoning or to monitor the quality of a material being mass-produced and to control the process. Given its wide range of applicability, it is easily realized that this technique must be one of the most important in chemistry.

In a very general way, the process of spectroscopic analysis can be viewed by considering, in order, the components of an instrument designed for such analysis. The variety of forms that spectroscopy takes requires that this description be in broad and general terms. The process begins with a source, sometimes the sample itself, that generates light in some part of the electromagnetic spectrum. This signal, if not from the source itself, then interacts with the sample and is modified by the sample. At this point, the analytical signal contains information about the sample's composition, concentration, and structure. The signal is divided into its component frequencies, and the components of the signal are directed to a detector to be measured. The resulting measurements, containing information about the sample, are processed into a convenient form for display and for analysis. The analysis step is at times done manually by the operator, at times automatically by computer, and at times by a combination of these methods.

There are four fundamental modes by which this interaction of light and sample are determined. Emission spectra are recorded when the sample is stimulated by the addition of energy from a secondary energy source. When the stimulation is energetic enough, the sample emits electromagnetic radiation in one or more of the spectral regions. The emitted light is then subjected to analysis. In absorption spectroscopy, light containing all the frequencies of analytical interest is focused on the sample. Some energy is absorbed from the beam into the sample, and the frequencies thus lost from the beam are the subject of the analysis. Fluorescence and phosphorescence spectroscopy are similar to each other. In both cases, the light is focused on the sample and, in part, absorbed by the sample. The sample, in this way, is raised to a higher energy level, from which it returns to its normal state by giving off light. When this emission occurs in a very short time, the process is fluorescence, and when there is a significant time delay, it is known as phosphorescence.

The qualitative and structural information that comes from spectroscopy comes in the form of the pattern of frequencies of light that are either absorbed or emitted. In the case of an elemental analysis, each element has its own characteristic set of frequencies that can be used to identify it. In molecular analysis, it is the ways that various elements are bonded to one another that result in the characteristic set of absorptions or emissions.

Quantitative analysis is possible because the intensity of the light emitted or absorbed is related to the number of atoms or molecules present in the beam being detected. In the simplest case, the intensity and the concentration are directly proportional. Often, interferences obscure this simple relationship, and the light intensity from the sampled beam is compared with that from several references of known concentrations of the material in question. This comparison can provide the relationship between the intensity and the concentration. In good circumstances, it is possible to measure concentrations as low as a few parts per billion with good accuracy and precision.

The exact equipment needed and the information available depend on the region of the electromagnetic spectrum that is being examined and the technique being used. Spectroscopy is performed using light from all ranges of the spectrum, from radio waves through microwaves, infrared radiation, visible radiation, ultraviolet radiation, and X rays to γ rays. Each of these spectral regions has at least one technique pertaining to it, and many of them have several.

Each of these methods is capable of identifying the components of a sample and of measuring their concentrations. At the low-energy, radio-wave end of the spectrum, the information is about the molecules present. At the high-energy, γ-ray end, the information is about the elements in the sample. Both types of information are available in the visible and ultraviolet region.

A quick survey of these spectral regions will give insight into the type of chemical information contained in each one. Nuclear magnetic resonance spectroscopy utilizes the radio-wave region of the spectrum to find out about the placement of certain active atoms in a molecule. Among the atoms that can be located in this manner are hydrogen, carbon, nitrogen, oxygen, phosphorus, and sulfur. The main use of this technique is with organic chemicals. The microwave region is studied in electron spin resonance spectroscopy, a technique that is particularly suited to studying atoms, ions, and molecules that have unpaired electrons in their structures. Use is made of this technique in the study of chemical, photochemical, and electrochemical reactions. The microwave region is used to obtain information about the way that molecules, or parts of molecules, rotate. This, in turn, can be interpreted so as to find the distance between bound atoms and the angles at which they are arranged relative to one another.

The main features of infrared spectra are caused by the ways that the atoms and groups of atoms in a molecule vibrate. The stretches and bends that occur have characteristic energies associated with them, and thus analysis of the spectra can determine what atoms are bonded together and the strength of the bonding. The visible and ultraviolet regions both lead to interpretations about the electronic states of the electrons in the chemical bonds within molecules. X-ray spectroscopy depends on the behavior of the electrons in the shells closest to the nucleus and yields information about these electrons. In γ-ray spectroscopy, the radiation involved is associated with changes within the nucleus itself and thus gives insight into the structure of the nucleus.

Applications

Spectroscopic measurements are made for one of three reasons, or a combination of these reasons. The method is capable of providing qualitative data that tell what atoms or molecules are present in a sample. It is also possible to use the technique in a quantitative way so that, when the components of the sample are known, their amounts can be measured. The third task to which the methods are applied is establishing fundamental information about the chemical structure of materials.

Some of these applications come about in the course of research studies in physical and biological science and engineering. Illustrative of the work done in these research areas are studies of internal-combustion engine exhausts using infrared analysis of the emissions. The engines were fueled with methyl alcohol, and the purpose of the work was to assess the environmental impact of converting to this alternative fuel. The method involved recording the infrared absorption spectra of samples of exhaust gas by passing the source beam through a cell slightly more than 4 meters in length. Comparing the experimental results with some known, standard gases showed that the analysis was capable of yielding detailed analysis of components present from the percent level to the parts-per-million level. Included in the gases found were carbon dioxide, methane, formaldehyde, nitrogen oxides, ethylene, acetylene, and methyl nitrite.

The last of these was of particular interest because other methods of analyzing exhausts had not noted this compound, and it is a substance that would be severely detrimental to air quality if released to the atmosphere in quantity.

Industrial applications of spectroscopy abound in the areas of quality or process control. In some of these cases, the analysis is automated to the point where continuous spectroscopic measurements are used to regulate the process to keep some preset values within bounds. The food industry is one that benefits from spectroscopic analysis in analyzing food constituents and controlling food quality. Since the molecules of interest are, by and large, organic molecules, the spectral region most used is the infrared region, which is sensitive to the vibrations that occur in such molecules. Infrared measurements have been made to determine the amounts of proteins, simple and complex lipids, carbohydrates, vitamins, and color and flavor compounds present in a wide variety of natural foods. In processed foods, spectral analysis has been used to determine permitted additives and residues from these permitted additives. In both cases, the method has also been directed at the detection of food contaminants or residues of such things as pesticides. The samples can be examined in the form of solutions, vapor, powders, thin films, or pressed pellets. In the realm of process monitoring, the carbon-dioxide level in carbonated beverages, the citric acid and sugar content of fruit juices, and the fat content in peanut butter and chocolate are regularly followed by infrared spectroscopy to allow the formulation processes to be controlled at acceptable levels.

Medically related spectroscopic measurements make the physician's task of diagnosis and treatment easier by allowing for simple monitoring of chemical levels within the body. It is quite usual for a patient to submit to blood testing to provide the physician with information to guide treatment. The usual procedure is called a "chem twenty" and is actually a series of between twenty and thirty chemical tests on the blood sample. These tests are performed automatically by a machine that breaks the original sample into small drops, each of which becomes a sample itself. The drops are transported through tubes to separate sections of the machine, where measured amounts of reagents are added and mixed. The purpose of these reagents is to develop a color in the sample characteristic of the blood component of interest. The drops are then transferred to optical sample cells, where the sample is measured by absorption spectroscopy to determine the quantity of a specified chemical. The region of the electromagnetic spectrum applied here is mainly the visible region, with a few tests also utilizing ultraviolet light.

The results on each series of drops are recorded and compiled into the complete record of that patient's blood test. Methods other than spectroscopic ones are used for some of the tests, but at least three-quarters of them are spectroscopic. This use of spectroscopy also points out the increasing role that computers play in automating both the data-producing and data-interpreting steps of spectroscopic methods.

The growth of concern about the environment has come hand in hand with the ability of spectroscopists to measure trace amounts of environmental contaminants. A spectroscopic technique that has found a major role in measuring samples to assess water is atomic absorption spectroscopy. The sample, in the form of a solution, is energized in order to vaporize, decompose, and atomize it. The energy to accomplish the atomization is provided either by a high-temperature flame or by a small furnace capable of being heated to more than 2,000 degrees Celsius in a matter of seconds. A light beam, selected to contain the frequencies known to be absorbed by the element in question, is passed through this atom vapor, and the amount of light absorbed is measured. Using preestablished calibration data, the concentration of the element can be related to the amount of light absorbed. The method is capable of measuring amounts of metal ions dissolved in drinking water at the level of less than 100 parts per billion.

These four areas of application barely scratch the surface of the multitude of uses to which the various forms of spectroscopy have been placed. They do, however, show the variety and indicate the range of importance of the methods.

Context

Although many of the types of spectroscopy currently in use had been known earlier, there has been an accelerating growth in these techniques, beginning with the United States' entry into World War II. As a part of the war effort, large amounts of both financial and human resources were brought into research in the fields of optics and electronics. The advances that came out of this research made it possible for manufacturers to design more powerful spectroscopic instruments. Radiation sources of higher power and greater stability, optics with less distortion, detectors with greater sensitivity, and electronic circuits capable of processing the detector's signals faster and more accurately quickly brought the instruments to the attention of research scientists. Using their imaginations, these scientists developed methods to utilize the added power of these instruments and to push the capabilities of the instruments to their limits.

This demand led instrument developers to produce better instruments and to broaden the range of the electromagnetic spectrum that was measurable. Thus began a cyclic development process that continues today.

Early instruments took large amounts of time and a high degree of operator skill to produce results that were neither highly accurate nor highly precise compared to those of modern instruments, which are fast and relatively simple to operate. The development of lasers has aided several spectroscopic methods. Some techniques that were theoretically known but impractical until the laser's development have become highly useful tools. Much of the improvement in spectroscopic methods is directly related to advances in microelectronics and computers.

In this later category, the role being played by Fourier transform analysis in many spectral techniques is significant. It was known that numbers could be transformed mathematically to turn data measured as light intensity versus time into data giving intensity versus frequency (a spectrum). That meant that data recorded in seconds were equivalent to a spectrum that might take many minutes or hours to record. The mathematics involved in the transform was very time-consuming, and the method was not practical. Computers opened up transform techniques as possibilities, and the method has been put to use in practically every type of spectroscopy. The method offers not only increased speed but also increased precision, by averaging hundreds of spectra. Transform techniques have also led to the use of spectrophotometric means as detectors for several types of chromatographic separations. As a detector, spectroscopy not only senses the presence of a chemical but can tell the identity and determine the amount of the substance by its spectrum.

The use of spectroscopic analysis is expanding and will play a still wider role in science, and scientists will continue to develop new techniques to meet new needs.

Principal terms

ELECTROMAGNETIC RADIATION: the whole range of light energies, from the lowest-frequency, longest-wavelength, lowest-energy form to the highest-frequency, shortest-wavelength, highest-energy form

PARTS PER BILLION: a measure of concentration of trace materials equal to 1 gram of the material in a total of 1 billion grams of the mixture

QUALITATIVE ANALYSIS: the process of finding out the identity of an unknown on either the molecular or elemental level

QUANTITATIVE ANALYSIS: the process of determining the amount of a compound or element present in a sample

SPECTROSCOPY: the process of measuring the interaction between the energies present in a beam of radiation and the matter in a sample

Bibliography

Ajzenberg-Selove, Fay, and Ernest K. Warburton. "Nuclear Spectroscopy." PHYSICS TODAY 36 (November, 1983): 26-32. Provides a well-written overview of spectroscopy in the γ and X-ray regions, with discussions of equipment, methods, and applications.

Denney, Ronald C., and Roy Sinclair. VISIBLE AND ULTRAVIOLET SPECTROSCOPY: ANALYTICAL CHEMISTRY BY OPEN LEARNING. Edited by David J. Mowthorpe. New York: John Wiley & Sons, 1987. The books in this series are written in a tutorial form, for self-learning at an introductory college level. This volume begins by considering the connection between chemical structure and color, and proceeds through the principles of both qualitative and quantitative analysis.

George, W. O., and P. S. McIntyre. INFRARED SPECTROSCOPY: ANALYTICAL CHEMISTRY BY OPEN LEARNING. Edited by David J. Mowthorpe. New York: John Wiley & Sons, 1987. The books in this series are written in a tutorial form, for self-learning at an introductory college level. After placing this spectral region in context, this volume introduces instrumentation, qualitative and quantitative methods, and interpretation of spectra in structural terms.

Griffiths, Peter R. "Fourier Transform Infrared Spectroscopy." SCIENCE 222 (October 21, 1983): 297-302. Outlines the Fourier transform method from both a theoretical and a practical point of view and summarizes its advantages. Also included are several applications from the infrared spectral region.

Metcalfe, E. D. ATOMIC ABSORPTION AND EMISSION SPECTROSCOPY: ANALYTICAL CHEMISTRY BY OPEN LEARNING. Edited by F. Elizabeth Prichard. New York: John Wiley & Sons, 1987. The books in this series are written in a tutorial form, for self-learning at an introductory college level. This book provides a description of the basic principles of these methods, surveys the instrumentation and practices used, and describes the relative merits of various techniques.

Rendell, David. FLUORESCENCE AND PHOSPHORESCENCE SPECTROSCOPY: ANALYTICAL CHEMISTRY BY OPEN LEARNING. Edited by David J. Mowthorpe. New York: John Wiley & Sons, 1987. The books in this series are written in a tutorial form, for self-learning at an introductory college level. Following a consideration of the basis for these effects, the book provides an overview of the instrumentation and applications of the two techniques.

Skoog, Douglas A. PRINCIPLES OF INSTRUMENTAL ANALYSIS. New York: Saunders College Publishing, 1985. The first half of this introductory-level textbook is concerned with electromagnetic radiation and its use in the various types of spectroscopy. The details may be too much for the general reader, but the general concepts and the examples of applications are very well presented.

Whiston, Clive. X-RAY METHODS: ANALYTICAL CHEMISTRY BY OPEN LEARNING. Edited by F. Elizabeth Prichard. New York: John Wiley & Sons, 1987. The books in this series are written in a tutorial form, for self-learning at an introductory college level. This text includes an introduction to the theory and use of both X-ray diffraction and X-ray fluorescence. The first of these is not a spectroscopic method, but knowledge of it provides background for the second.