WATER AND CO2 INTERFERENCE REMOVAL
The QASoft control screen has action buttons for removal of water and CO2 lines from an air spectrum.
It is necessary for the user to prepare the reference spectra that are called by these action buttons. The reason for this is that a good subtraction requires an exact match of the line positions,
line shapes and line widths between the sample spectrum and the reference spectra. Since each spectrometer has its own "spectrum signature" with a unique combination of line positions, widths and shapes, the water and CO2 subtraction spectra must be made on the same instrument that is used to record the sample spectra.
Incidentally, it is alright that the quantitative reference spectra in the database have been made on a different instrument because they are used only at low degrees of absorbance and they do not
disturb the whole spectrum the way a bad water spectrum would.
Since most of the water lines will have a high degree of absorbance, they are found to be "non-linear" in going from one water concentration to another. It is therefore necessary that the water spectrum used for water line subtraction should have absorbance values close to those in the sample spectrum. To cover a wide range of humidities in air, one needs a family of water reference spectra that correspond to various degrees of humidity. The closer
the humidity match between sample and reference, the better the water subtraction.
To assist in preparing the spectra for water and CO2 line removal, Infrared Analysis, Inc. supplies a WET-DRY TUBE and a CO2 TUBE. The procedure for using these tubes to make the needed water and CO2 reference spectra is given in the Manual for Getting Started.