Practical Matters and Applications

There is more than one spectral line in the line spectrum of an element and therefore more than one line to choose from when setting the monochromator. For each element, there is one line that gives the optimum absorptivity for that element, and this line is therefore the most sensitive and useful. This line 

FIGURE 9.11 An illustration of a double-beam optical path. See text for explanation. 

The burner head can be adjusted vertically, horizontally (toward and away from the operator), and rotationally. Initially, it should be adjusted vertically so that the light beam passes approximately 1 cm above the center slot and horizontally so that the light beam passes directly through the center of the flame, end to end. All three positions can be optimized by monitoring the absorbance of an analyte standard while making the adjustments. A maximum absorbance would indicate the optimum position. 

The sources of acetylene, nitrous oxide, and sometimes air are usually steel cylinders of the compressed gases purchased from specialty gas or welders gas suppliers. Thus, several compressed gas cylinders are usually found next to atomic absorption instrumentation and the analyst becomes involved in replacing empty cylinders with full ones periodically. Safety issues relating to storage, transportation, and use of these cylinders will be addressed in Section 9.3.7. The acetylene required for atomic absorption is a purer grade of acetylene than that which welders use. 

There is an optimum fuel and oxidant flow rate to the flame, or, more precisely, an optimum fuel-oxidant flow rate ratio. If the flame is oxidant-rich, it is too cool. If it is fuel-rich, it is too hot. Again, monitoring the absorbance of an analyte standard while varying the flow rates helps to find the optimum ratio. Instrument manufacturers literature will also provide assistance. Safety issues relating to the proper flow rate of these gases will be addressed in Section 9.3.7. 

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