Infrared and Raman Spectroscopy of Respiratory Gases

The sensitivity of the method is illustrated by Fig. 10.49, which depicts the time variations of the CO2, O2, and N2 concentrations in the exhaled air of a human patient. Note the variation of the concentrations with changing breathing periods. The technique can be used routinely in clinical practice for anesthetic control during operations and obviously also for alcohol tests of car drivers. Instead of Raman spectroscopy, infrared absorption spectroscopy can be used. 

Many bioactive molecules have absorption bands in the infrared, which are accessible by existing infrared lasers or OPOs (Fig. 10.50). When enhancement cavities and cavity ringdown or cavity leak out spectroscopy are used detection sensitivities down into the ppb or even ppt range can be achieved [1545], This is important for sensitive measurements of breathing gases, where the composition of the exhaled gas gives information about possible diseases. One famous example is the presence of the bacteria Helicobacter pylori in the stomach, which can cause gastritis or even stomach cancer. Their presence and concentration can be monitored by the spec- 

In Fig. 10.51 a possible experimental arrangement for breath analysis is shown [1547, 1548]. The breathing gases are dried, mixed with a buffer gas and injected into the multipath absorption cell. 

Such sensitive detection techniques also allow the quantitative measurement of hemoglobin in the blood of athletes, enabling checks to be performed for possible doping. 

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