Quantitative open-path FTIR with the use of isotopic standards

Emphasis on air quality has increased the need for a rapid method of measuring air pollutants. Remote Fourier-transform infrared spectroscopy (FTIR) offers rapid, on-site analysis of large areas and requires no sample preparation. Simple quantitative methods that reflect conditions present at the time of the measurement are other advantages of remote FTIR. This is especially important when one is attempting to use ambient background sources where the intensity of the source alters absorption intensities and calibration curves. Quantitative remote FTIR is based on Beer's law and matching the analyte spectrum with reference spectra. The reference spectra are measured separately and do not account for variation in source intensity or temperature, or for unknown interferences. Substituting deuterium for one or more hydrogens in a volatile organic compound (VOC) shifts the vibrational frequency, allowing the deuterated compound to be measured simultaneously with the VOC. The reference spectra for the Beer's law plot can be measured at the same time as the analyte and a slope and y intercept conversion allows determination of the analyte concentration. We have used isotopic references for quantitative open-path FTIR measurements of acetone and methanol and have achieved better than 10% accuracy in the parts-per-million concentration range. © 1999 John Wiley & Sons, Inc. Field Analyt Chem Technol 3: 105–110, 1999