Optical Methods of Chemical Analysis

Several highly applicable methods of analysis rely on the interaction between light (UV-visible-IR) and matter. The wavelengths (energies) at which the interactions occur are characteristic for the individual substances (spectral fingerprint ), and this forms the basis of qualitative analysis using light. Further, the intensities that are measured in such interactions are closely related to the concentration of the particular substance. Therefore quantitative analysis, which is frequently very accurate, can be performed. 

The optical methods of analysis, as well as those based on X-ray transitions (Chap. 5) are of great interest for determinations of the presence and concentration of a very large number of substances. The presence of chemical elements can be determined and molecular identification can also be made. Molecular analysis can be performed by IR absorption spectroscopy and also by XPS (Chap.5) and NMR (Chap.7). We will briefly describe some applications of optical analysis methods. 

In the steel industry, as well as in the chemical and pharmaceutical industries there are clearly many fields of application for analysis techniques. 

The development of analytical methods has been in progress since the beginning of the 19th century. J.J. Berzelius (1779-1848) determined the constituents of about 2000 chemical compounds. The periodic table of the elements was put forward by D. Mendeleyev in 1872. At that time only 67 elements were known. By 1900 the number had increased to 83. 

Analysis by optical techniques is frequently performed by measuring absorption. It is important to be able to correctly relate the absorption to the concentration. The relation which is called the Beer-LamheH law wiU now be considered. 

Consider monochromatic light of intensity Pq impinging on a sample of thickness b as illustrated in Fig. 6.55. The sample can be a solution in a cuvette or atoms in a flame from a specially designed bmuier. An intensity P is transmitted through the sample. (We disregard possible effects from the sample confinement). We now consider the conditions over a small interval Ax in the sample. Before the considered space interval, the intensity has been reduced to P, and it will be further reduced by AP in the interval Ax. The fractional attenuation AP/P is proportional to the number of absorbers, An, in the small interval Ax 

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T. R. P. Gibb, Optical Methods of Chemical Analysis, McGraw-Hill, New York, 1942, p. 239. 

An enormous technology base already exists for familiar optical methods of chemical analysis. There is probably no class of chemical analyte that has not at some time been the subject of optical determination through absorption or fluorescence spectroscopy. Methods also exist for labeling a target analyte with an appropriate chromophore or a fluorophore, a common practice in immunoassays. 

Optical Methods of Chemical Analysis 153 Table 6.3. Recommended lines for atomic absorption spectrophotometry... 

In spectroscopic analysis, species are identified by the frequencies and stmctures of absorption, emission, or scatteting features, and quantified by the iatensities of these features. The many appHcations of optical methods to chemical analysis rely on just a few basic mechanisms of light—matter iateraction. 

Almost all methods of chemical analysis require a series of calibration standards containing different amounts of the analyte in order to convert instrument readings of, for example, optical density or emission intensity into absolute concentrations. These can be as simple as a series of solutions containing a single element at different concentrations, but, more usually, will be a set of multicomponent solutions or solids containing the elements to be measured at known concentrations. It is important to appreciate that the term standard is used for a number of materials fulfilling very different purposes, as explained below. 

Nonstoichiometry is a pervasive aspect of oxide chemistry, particularly where the cation can assume two or more valences or aliovalent cation substitutions are facile. These can be classified into three rather broad ranges. Class 1 includes systems where the nonstoichiometry approaches or exceeds that which caimot be detected by classical methods of chemical analysis (i.e., less than 1 part in 1000) but may manifest itself in dramatic changes in electrical or optical properties. Class n includes systems where the nonstoichiometry is of the order of several mole % and readily discernible by chemical analysis, density measurements, or X-ray diffraction measurements of unit cell constants. Class 111 are those systems with broad ranges of nonstoichiometry such as the alkah metal tungsten bronzes. 

The erythrocyte has proved to be a useful experimental object for permeability studies since it is relatively easy to separate glucose metabolism from membrane permeability and to measure the latter by both rapid optical methods and chemical analysis. From the work of Wilbrandt and colleagues (1947, 1956 Wilbrandt and Rosenberg, 1950 Wilbrandt, 1954),... 

Analysis of Surface Molecular Composition. Information about the molecular composition of the surface or interface may also be of interest. A variety of methods for elucidating the nature of the molecules that exist on a surface or within an interface exist. Techniques based on vibrational spectroscopy of molecules are the most common and include the electron-based method of high resolution electron energy loss spectroscopy (hreels), and the optical methods of ftir and Raman spectroscopy. These tools are tremendously powerful methods of analysis because not only does a molecule possess vibrational modes which are signatures of that molecule, but the energies of molecular vibrations are extremely sensitive to the chemical environment in which a molecule is found. Thus, these methods direcdy provide information about the chemistry of the surface or interface through the vibrations of molecules contained on the surface or within the interface. 

Clark H.A., Kopelman R., Tjalkens R., Philbert M.A., Optical Nanosensors for Chemical Analysis inside Single Living Cells. 1. Fabrication, Characterization, and Methods for Intracellular Delivery of PEBBLE Sensors, Anal. Chem. 1999 71 4831— 4836. 

Clark HA, Hoyer M, Philbert MA, Kopelman R (1999) Optical nanosensors for chemical analysis inside single living cells. 1. Fabrication, characterization, and methods for intracellular delivery of PEBBLE sensors. Anal Chem 71 4831 1836... 

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