Qualitative and Semiquantitative Arc-Spark Emission Spectrochemical Analysis

Qualitative and Semiquantitative Arc—Spark Emission Spectrochemical Analysis 

Emission spectroscopy provides an ideal method for qualitative analysis, since each atomic species has its own unique line spectrum. Spectral lines have two characteristics useful for qualitative analysis (1) their wavelengths and (2) their intensities. It is the pattern of wavelength distribution that is primarily used for qualitative analysis, although the relative intensity distribution also can be helpful to verify spectral lines to identify an element. About 70 elements are easily identified by spectral methods. Those that are more difficult to identify include the gases and a few nonmetals, primarily because sensitive lines lie in the short ultraviolet portion of the spectrum that is difficult to observe. 

For qualitative spectrochemical analysis it is desirable to identify from three to five or more spectral lines of the element. This is necessary since the spectrum of a complicated sample may contain many spectral lines and the possibility of line overlap or of misidentification of a spectral line exists. Photographic recording of spectra for qualitative analysis is essential since identification depends on identifying several lines for each element rather than just one spectral line. Photorecording also provides a permanent record of the sample that can be referred to later if necessary. 

Qualitative spectrochemical analysis requires only a very small sample. Frequently a complete qualitative analysis can be obtained from a 1-5 mg sample and a single exposure. All readily detectable elements can be observed from one spectrum of the sample. Qualitative analysis also is possible with samples difficult to handle by more traditional chemical methods for example, glasses, refractory materials, slags, minerals, etc., can be handled by reducing the sample to a fine powder. No chemical treatment or chemical separation is required. 

Spectroscopic detection limits differ for different elements many elements can be detected at very low concentration levels, some as low as 10 g. The spectroscopist should become familiar with detection limits of elements of most concern in his particular field and under his excitation conditions. The sensitivity of qualitative spectral analysis is dependent on the type and size of the sample, the excitation conditions, and the sensitivity of the photographic emulsion and the optical system used with the spectrograph. For best results excitation conditions should be maintained as uniform as possible. 

---