Near-Infrared Spectra of Gases: Identification of Mixtures

Abstract

A 100-compound near-infrared (IR) spectral library of environmentally important gases was generated. All the library spectra were measured from 4000 to 9000 cm⁻¹ at a resolution of 2 cm⁻¹. Spectra of 17 test mixtures including 9 two-component mixtures, 5 three-component mixtures, and 3 four-component mixtures were measured. Two of the 9 two-component mixture spectra were measured at a resolution of 8 cm⁻¹ and two at 16 cm⁻¹; all the other mixtures were measured at 2 cm⁻¹ resolution. A mixture search routine was used for identifying the components in the mixtures. In a search using the near-IR region of 4000 to 8000 cm⁻¹, all components in the 9 two-component mixtures, all components in 5 three-component mixtures except for one component (o-xylene in a mixture of the three xylenes) and all components in the 3 four-component mixtures except for one component (a ketone) were correctly identified. The mixture search was repeated with the 4000 to 6200 cm⁻¹ region and all components in all the 17 mixtures were correctly identified. Thus, by the decision to limit the near-IR fingerprint region of 4000–5000 cm⁻¹ and the overtone region of the C-H stretching vibration at 5600–6200 cm⁻¹, the informational content was optimized. The results suggest that the 4000–6200 cm⁻¹ region could be used for monitoring gas mixtures.