Practical implementation of absorption spectroscopy

This simple principle has been maintained con-cepmally in the set-up shown in the bottom part of 

If the amplitude of the amplifier output is normalized to unity, then this signal represents the sample absorption in per cent. If, furthermore, the amplifier has a logarithmic response (so-called balanced detectors are such devices), then the signal output is ln(/ //o), or log(/i//o), and thus is directly proportional to the absorbance A (see Equations (6.1) and (6.2)). Thus, an unknown absorber particle density N can easily be derived from the signal, provided the absorption cross-section a is known. Alternatively, one can calculate the absorption cross-section if the absorber particle density is predetermined. 

The method just described is becoming increasingly popular in the monitoring of environmental trace gases, including chemical combustion products, utilizing semiconductor diode lasers. Diode lasers are small and relatively inexpensive, and thus mobile systems for in-situ measurements can and have been devised. The method is now commonly known as tuneable diode laser absorption spectroscopy (TDLAS). For a few examples of its application see Chapter 28. 

The centre laser frequency j/l is modulated at the modulation frequency r mod which tunes the laser frequency periodically away from the centre 

As we just learned, referencing of the absorption signal to the input laser radiation eliminates a substantial part of (time-varying) noise contributions from the laser itself. What this approach may not be 

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