A slope-ratio method for quantitative open-path FTIR

Abstract

Two characteristics of ambient background sources must be addressed: their low intensity and their instability. Reference spectra—normally subtracted from sample spectra to remove the source characteristics, instrument response, and other spectral characteristics common to both the reference and sample spectra—are impossible to obtain when an ambient background source that may change rapidly is used. Data collection must be rapid because the source temperature may change, producing changes in the overall shape and intensity of the spectra. The low intensity of ambient sources makes noise a problem that is compounded by subtracting the reference spectrum from the analyte spectrum. We have developed a quantitative method for measuring volatile organic compounds (VOCs), the slope-ratio method (SRM), which is less susceptible to these problems.

The absorbance and the concentration for each standard taken at the time the analyte is measured and the absorbance for the analyte are substituted into the equation created from the linear equations of the standard and analyte. For ambient backgrounds, artificial reference lines are created by using points on either side of the peak of interest to create a straight line that can be subtracted from the peak. The log of the artificial reference spectrum subtracted from the raw sample spectrum produces absorbance intensities that still follow Beer's Law. Representative absorbance equations are generated for a range of intensities in the laboratory for both the standard and the analyte. The raw spectrum of the analyte measured under experimental conditions must be included with the series because the spectra for standards and the spectrum of the unknown must match within ±10.0%. This procedure has been successfully used for VOCs such as methanol, acetone, and t-butyl ethyl ether, and shows promise in measuring methane under laboratory conditions. © 2000 John Wiley & Sons, Inc. Field Analyt Chem Technol 4: 127–133, 2000