Some Special Spectrochemical Problems

Several methods for sampling small areas are available. The methods frequently are used to sample metal surfaces for nonuniformity but are not limited to this. One technique is to use a spark discharge from the metal to a sharply pointed counter electrode. A nonconducting substance with a small hole through it is used to shield the metal around the area to be sampled. A mica sheet with a small hole can be used as a shield. Plastic electrician s tape also can be used, as can Teflon. 

Another method is to use a small wire in a capillary tube as the counter electrode with a spark discharge. The system does not possess high sensitivity but can reveal local differences in compositions that may be significant in quality control. 

A third technique is to use a laser beam to vaporize a very small sample area. A laser can be used to direct very high energy to a very small area. A microscope can be used to assist in aligning the laser beam. The sample is vaporized by the laser beam and excited between auxiliary electrodes to provide the energy to record a spectrum of the sampled area. Multiple exposures frequently are used to increase the intensity of the spectrum. The technique has been applied to metals and alloys and also to biological systems. 

Ahrens, L. H., and Taylor, S. R., Spectrochemical Analysis, 2nd ed., Addison-Wesley, Reading, Massachusetts (1961). 

Editor, Analytical Emission Spectroscopy, Vol. I, Part II, Marcel Dekker, New York (1972). 

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SOME SPECIAL PROBLEMS AND TECHNIQUES OF SPECTROCHEMICAL QUALITATIVE ANALYSIS... 

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