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Methodological questions in application of spectroscopic techniques

The technique of open-path monitoring with a Fourier transform spectrometer is being vigorously developed.Many issues for the routine acquisition of data have been addressed and are fairly well understood. The primary questions that remain unanswered at the present time are how to measure the quantities accuracy and precision. When the FTIR system makes measurements, it uses a long, open path, and for that configuration, even the concepts of the accuracy and precision are difficult to define. The classical definition of accuracy is in reality a measure of how well the instrument responds to a known stimulus. Precision is classically defined as the standard deviation of a set of responses of an instrument to the same stimulus. While these definitions maybe applicable to FTIR measurements, actually challenging the instruments with a known quantity of gas presents many problems. One suggestion has been to place a short cell filled with a high concentration of gas in the beam. Then, if the concentration in the cell is known, both the accuracy and the precision can be determined. The use of a short cell poses several experimental problems that need clarification. For example, during routine operation the FTIR measures the absorbance due to gases along the path. If a short cell containing a high concentration of gas is to be used to measure either precision or accuracy, then should it be in the beam but empty while the routine measurements are being made. What time is to be used or the accuracy and precision measurements. Since the gas in the cell is added to the gas along the path, how can the variability of the atmospheric component be accounted for unless the acquisition time is short compared to the variability of the data. The answer to the acquisition time question obviously depends on the end use of the monitoring program data. It has been suggested that a naturally occurring gas such as N2O may be used for the precision and accuracy measurements. That idea suggests the following question: If a measurement of N2O is used for the determination of accuracy, can the accuracy of a measurement of toluene be determined simultaneously. The answer to that question cannot be given as a simple yes or no. Another possibility may be to use the absorbance due to water vapor. Water vapor exhibits absorbance throughout the mid-IR region and is easily measured by independent techniques.