Raman Scattering (contd...)

Typical Raman spectra, in the form of frequency shifts, are presented in Figure 7.13 for dry air (inhaled) as well as the air exhaled as human breath. Dry air is mainly nitrogen and oxygen, with traces of moisture (H₂O), indistinguishable levels of CO₂, and other gases that the measurement has not been able to resolve. The intensity peaks depend on pressure, temperature and species concentration. Since the nitrogen content in air is an invariant and so is pressure, the nitrogen peak can be used to determine air temperature. With this information, oxygen levels in air being inhaled by an individual can be determined.

Note that Raman spectroscopy has determined concentration of all species in a single step; it points towards the power of the technique in engineering applications.

The second spectrum in Figure 7.13 is that of air exhaled. As expected it is rich in moisture and CO₂, while the oxygen content has decreased. Nitrogen data can once again be used to fix temperature while other peaks will provide information on the respective concentrations.

Multiple peaks (such as CO₂ , Figure 7.12) are not uncommon. In fact, the identification of an intensity peak to a species is enabled by the vast amount of spectroscopic data available in the open literature.

Figure 7.13 Raman spectra and species detection in air inhaled by an individual and air exhaled. Figure adapted from Mayinger (1994).