The absorbance function of QASoft calculates and displays an absorbance spectrum from sample and background spectra that are chosen by the operator. If the resultant absorbance spectrum contains the false high absorbance values that result from negative values in the single beam spectra, the program will eliminate those negative values and then re-calculate and display a proper absorbance spectrum. The figure at the left shows the Infrared Analysis, Inc. view with two single-beam spectra that are to be used in making an absorbance spectrum. The upper spectrum is the background spectrum, recorded through vacuum in a 144 meter multiple-pass cell (vacuum.spc in the installation disk). The lower spectrum is for room air in the 144 meter cell (sample.spc on the disk). Clicking on absorbance brings up dialog that asks the user to designate the two spectra for an absorbance calculation. For the spectra shown at the left above, the absorbance spectrum shown at the left below appears, marred by the false high absorbance values. These false values are eliminated, however, by clicking YES on the dialog box at the bottom of the spectrum. That box is then replaced by one that gives a choice of correcting the spectrum either by negative clipping and slight offset (Offset by noise level), or negative clipping and absorbance capping (Cap at set absorbance). The default value of absorbance capping is set at one absorbance unit. For gas analysis, readings higher than one absorbance unit are usually quite worthless, and it is just as well to dispose of them. However, the user is still given the options of declining to clip the absorbances or to cap them at a higher value. The dialog box for choice of absorbance correction method is shown here:
Clicking on Offset by noise level produces the spectrum shown below, left.
Clicking on Cap at set absorbance produces the spectrum shown below, right.