Quantitative Infrared Spectrophotometry: Empirical Extrapolation from Calibration Conditions

摘要

Quantitative infrared spectrophotometry measurements of infrared active compounds in the gaseous phase depend upon many variables, some of which are dependent on instrument capabilities, environmental conditions, and sample definitions. In quantitative measurements, calibration of an instrument in a set environment provides the necessary constants to determine the concentration of a particular compound in a sample gas. However the constants may change if the conditions of measurement vary. This study concerns estimating infrared constants when the following conditions change from calibration conditions using a particular infrared spectrophotometer (25-mm AgCl windows): pathlength, sample temperature, absorptivity level, and scanning speed. The intent of the study is to make users of process control infrared spectrophotometers aware that varying conditions affect results obtained from such instruments. Equations for the four specific condition changes explored are: Pathlength, K/sub c/ = K sub 0 (L sub 0 /L); Temperature, K/sub c/ = K sub 0 (T/T sub 0 ) exp 2 ; Absorptivity, K/sub c/ = (K sub 0 )exp(-(Log A) exp 2 /7); and Scanning Speed, K/sub c/ = K sub 0 (1.000 + 0.00032(S - S sub 0 )), where K is the infrared constant, L is pathlength, T is absolute temperature, A is the absorbance, and S is scanning speed. The subscript c and o refer to corrected and original (calibration) values, respectively. (ERA citation 07:063287)