It is necessary for the user to prepare his own reference spectra for subtraction of water lines from the sample spectra. 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 its own 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.

Another problem is that most of the water lines will have a high degree of absorbance and therefore an absorbance versus concentration plot will be non-linear for most lines, and, furthermore, the extent of non-linearity will vary from line to line. This further complicates the water subtraction operation. It is therefore necessary that the spectrum used for water line subtraction should exhibit overall integrated absorbances close to those exhibited by the

sample spectrum. In other words, the reference gas sample should have about the same humidity as the gas sample being analyzed.

To prepare the water vapor subtraction spectra, a double tube is supplied with a dry side (the plastic tube) and a wet side (the tube with beads in it). Here is a procedure for preparing the water spectra.


1. Wet the beads with distilled water. Then make an estimate of the relative humidity in your sample, and try to match it by mixing wet and dry nitrogen in the cell or in the syringe. For example, if you want 50% humidity, you evacuate the cell and then put in half an atmosphere of dry nitrogen and half an atmosphere of wet nitrogen. Ou can also put a single-beam sample spectrum on the screen, superimpose the wet nitrogen spectrum, and use the wet-dry tube to adjust the humidity of the nitrogen sample until it matches that of the actual sample.


2. Record a low-noise single-beam spectrum of the humid nitrogen and save it in C:\QAS5\Work with the name W.


3. Evacuate the cell and re-fill it with one atmosphere of dry nitrogen.


4. Record a low-noise single-beam spectrum for use as background and save it in C:\QAS5\Work under the name Back.


5. Make sure the two single-beam spectra are in SPC format.


6. Next, click on the action button MAKE H2O, for preparation and saving of spectrum Wa.spc.

When working with samples of various degrees of relative humidity, the user can accumulate a family of water reference spectra. Eight of these may be saved in the database, Chapter W, under the names W1a, W2a.........W8a. Any one of these can be called up and re-named as the working spectrum, Wa. The existing Wa spectrum will be over-written, but it is not lost because it also exists as one of the numbered water spectra.

The WATER action button will use Wa in the removal of water lines from the sample spectrum. The measurement and removal of water can also be written into any measurement sequences that are created.

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