QUANTITATIVE GAS-ANALYSIS WITH FT-IR - METHOD FOR CO CALIBRATION USING PARTIAL LEAST-SQUARES WITH LINEARIZED DATA

Calibration spectra of CO in the 2.38-5100 ppm concentration range (22 spectra) have been measured with a spectral resolution of 4 cm(-1), i n the mid-IR (2186-2001 cm(-1)) region, with a Fourier transform infra red (FT-IR) instrument. The multivariate calibration method partial le ast-squares (PLS1) was used to model the CO calibration spectra in ord er to improve the sensitivity and to flag possible outliers in the pre diction step. The relation between the absorbance values and concentra tions was strongly nonlinear. This result was caused mainly by the low spectral resolution of the instrument. To improve the model predictio ns, we have linearized the data prior to making the model calculations . The linearization scheme presented here simplified the data pretreat ment, because the function needed to linearize the data might be appro ximated by co-absorbance peak areas representing the concentrations. T he integrated absorbance areas, rather than the concentration values, were used as input to the PLS algorithm. A fifth-order polynomial was used to calculate the concentrations from the predicted absorbance are as. The PLS algorithm used on the linearized data reduced the number o f factors in the calibration model. Our results reveal that the calibr ation model based on the linearized data had a high concentration pred iction accuracy throughout the entire concentration range.