Using Matrix Isolation-FTIR Trace Atmospheric Gas Analysis

Abstract

Abstract : The matrix isolation technique combined with FTIR spectroscopy has been used for atmospheric trace gas analysis. The main objective was to develop this technique into a simple, reliable field method using the simplest possible apparatus to give reliable results. As a preliminary to the research, calibration curves of the intensity of IR absorptions of acetone, acetaldehyde, acetic acid, formic acid, N2O, SO2, and ethylene were constructed for a range of concentrations of these reagents in CO2. All spectra were recorded on solid films at 77 K. Linear responses were observed for absorbance versus concentration, and the technique showed the best sensitivity for N2O, which was detected at a minimum atmospheric concentration of ca. 32.5 ppb. Acetone, acetaldehyde, acetic acid, and ethylene were detected at a minimum atmospheric concentration of ca. 3.3 ppm, while formic acid was detected at a minimum atmospheric concentration of ca 2.3 ppm. SO2 was detected at a minimum atmospheric concentration of ca. 325 ppb. A thorough spectroscopic study was done on some carbonyl compounds in CO₂ matrices, including the effect of CO₂ matrix on the IR spectra, as in most cases spectra have never been previously recorded for these species in CO₂ matrices. Another study of some isotopic species of these compounds in CO₂ matrices was then done to see if there was any interaction between the compounds and CO₂ matrix, which could lead to slightly different spectra from those of the gas phase or even from the spectra of the same compounds isolated in different matrices, which have been reported in other studies A new method for infrared analysis of multicomponent gas mixtures is described, involving a condensed phase sampling technique using commercially available cryogenic equipment. Molecules are characterized by a small number of well-defined infrared absorptions. As a method for pre-concentrating the atmospheric carbonyl compounds, various adsorbent materials were studied, and activated carbon was chosen as it gave the best recoveries (85–91%) and selectivity. The MI-FTIR technique, combined with the adsorbent tubes, was used in field work for sample collection and analysis. The method allows analyses to be executed rapidly and is capable of producing reliable quantitative results. The results are encouraging, indicating that this sampling method provides a practical basis for chemical analysis of these compounds.