RULES FOR QUANTITATIVE ANALYSIS OF GASES

 

(1) The absorption law is most likely to fail for small molecules that have individual spectral lines, because it is probable that the resolution bandwidth will be larger than the spectral linewidth.

 

(2) The absorption law is not likely to fail for heavy gaseous molecules that do not have fine structure in their infrared bands.

 

(3) As resolution is increased, the absorbance errors are decreased.

 

(4) The total sample pressure should be kept high by the addition of inert gas. The total pressure should be the same for both the sample spectrum and the reference spectrum.

 

(5) The sample spectrum and the reference spectrum must both be recorded with the same spectral resolution.

 

(6) When lines are not fully resolved, it is advantageous to record both the sample spectrum and the reference spectrum with small maximum absorbance. Only one reference spectrum is then required for quantification over a wide range of values.

 

(7) In measuring gases, the signal-to-noise ratio in the spectrum should be kept as high as possible, thus maximizing the range over which valid measurements can be made. In many cases, a signal-to-noise ratio of 104 will allow valid concentration measurements over a range of 103, using only one reference spectrum.


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