The Present Status of Excitation in Spectrographic Analysis
Authors: Scribner Bourdon F. 
Published: July 2009

The light source in spectrographic analysis serves a two-fold function, first, as a means of vaporizing and dissociating the sample and, second, of exciting the atoms to radiate their characteristic spectra. Both factors are important to the development of the intensities of spectral lines; consequently a discussion of excitation in spectrographic analysis will necessarily involve the consideration of sample forms and physical states as well as the processes occurring in excitation itself. When it is considered that the material to be analyzed may be in the form of a gas, a liquid, or a solid, that it may be a conductor or nonconductor of electricity, and that the concentrations of atoms are to be measured over wide ranges, it is apparent that a variety of factors are involved in the general problem of excitation. Various light sources employing electrical discharges or gas flames have been devised to meet the needs of spectrographic analysis. However, it has become evident that the future development of spectrographic analysis will depend largely on improvements in present excitation sources or even in the application of radically different means of excitation. The sources in practical use are well described in standard texts on the subject or in the general literature. A recent review by Convey (1) provides a detailed treatment of the usual light sources and their applications in analysis. The following discussion outlines briefly certain of the problems that are involved in excitation and indicates lines of investigation that are being followed toward their solution. Quantitative spectrographic analysis is based on the observation that the intensities of spectral lines of elements excited in a light source vary in proportion to the concentrations of the elements present. For light sources in which excitation is mainly thermal, the intensity of a spectral line radiated by the excited atoms in the column of hot gas may be described (2) by the expression: I∼NPe−(ΔE∕KT) where: N=thenumberofatmosperunitvolume, P=thetransitionprobability, ΔE=thedifferenceinenergybetweeninitialandfinalenergylevelsfortheline, K=theBoltzmannconstant,and T=theabsolutetemperature. For a given line, P and ΔE are constants and the intensity is a function of the concentrations of the atoms and the effective temperature. Excitation in the sources employed for spectrographic analysis, namely flames, arcs, and sparks, has been shown to be mainly thermal. For a single gaseous element excited by a carefully controlled electrical discharge in a discharge tube protected from variable heat losses, the relative intensities of spectral lines of the one element may be maintained at a high order of constancy. Such tubes may in fact be employed as reliable reference sources for spectral lines of known relative intensities. In this case the factor N, which is proportional to the pressure of the gas, and the factor T, the temperature, may be maintained constant for long periods. In spectrographic analysis where mixtures of elements are excited and it is desired to measure the concentrations of a given element varying in different samples, the situation is more complicated. In the internal standard method (3) commonly employed in analysis, the intensity of a line of the element present in unknown concentration is measured relative to that of an invariant line of a reference element. Sawyer (2) has pointed out that, for successful analysis, this intensity ratio must be highly reproducible and that any variable that affects the ratio of N of the unknown element to N of the standard, or that affects T, is certain to affect adversely the ratio of intensities and so the accuracy of analysis. The temperature attained in the luminous source depends on the rate at which energy is supplied, the specific heat and conductivity of the gas column, and the rate at which energy is lost by radiation.