Analysis with Direct Read-Out

For routine analytical control the direct reading spectrometers described in Chapter 4 are ideal. These are special-purpose instruments that can be adjusted for specific purposes. Once adjusted they can provide a rapid, precise method for the analysis of the chosen elements in a single matrix. The instruments use photomultiplier tubes, one for each element to be determined, mounted along the focal plane of the spectrometer, immediately back of a carefully adjusted exit slit. The number of elements that can be determined simultaneously can be as many as 30-35. One channel usually is used 

Direct reading spectrometers can provide analytical data on routine samples in 2-3 min with an average error of 1-2 %. In the metallurgical industries the direct reader easily provides valuable data while a metal is in the molten state so composition can be adjusted as desired. 

Direct reading has been applied successfully to carbon, sulfur, and phosphorus analyses in the iron and steel industries since concentrations of these elements are critical in the final product. Since the most sensitive lines of these elements lie in the vacuum ultraviolet, a vacuum spectrometer is required. These usually are separate units but can be constructed in combination with an air-path unit so a single sample can provide data on these three elements as well as those usually determined in the air-path spectrometer. 

Since direct readers are not versatile or easily readjusted for different samples, most laboratories using them also have available another instrument using photographic recording. The photographic units usually are used for qualitative scanning, nonroutine quantitative analysis, and method development. 

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