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CARBON DIOXIDE INTERFERENCE REMOVAL


In many samples, CO2 is a major infrared absorber. Usually water is first and CO2 is second. This is the case with polluted air, engine exhausts and combustion effluents. After water lines have been subtracted from the spectrum of such a sample, CO2 lines will remain as the dominant feature. These lines can interfere with the measurement of many important trace gases, including nitrous oxide, carbon monoxide, olefinic hydrocarbons, acetylene, hydrogen cyanide, benzene and other aromatic hydrocarbons.


The CO2 lines can be subtracted away with correctly prepared CO2 reference spectra. As in the case of water line subtraction, the shapes, widths and positions of the CO2 lines must be the same in the reference spectra as in the sample spectra. The lines must also match in their distortions that result from line saturation. Since any two different spectrometers will be found to create slightly different lines shapes, widths and positions, the same spectrometer must be used in preparation of both the reference spectra and the sample spectra. In addition, the line distortions due to line saturation must be matched up. This requires that the pathlength-concentration product be similar for the reference spectra and sample spectra.


A wide range of CO2 concentrations may be encountered. In polluted air, for example, the CO2 concentration will fall near 500 PPM. In combustion effluents, the CO2 concentration may be near to 100,000 PPM (10%). If the combustion effluent is diluted 10-fold, the CO2 concentration will still be 10,000 PPM. If the user knows approximately what his CO2 pathlength-concentration product is going to be, he can match it in metering out the CO2 gas for the reference spectrum..


A simple way to create the desired CO2 concentration is to calculate the needed volume of CO2 at one atmosphere pressure and introduce that volume of CO2 into the cell. For example, to create 1000 PPM of CO2 in nitrogen in a cell of 14 liters, just meter out 14 cc. of CO2, and allow the nitrogen to carry it into the cell.


PREPARING THE FAMILY OF CARBON DIOXIDE SUBTRACTION SPECTRA


The Gas Analysis System includes a small syringe to be used in metering out a desired volume of CO2 and mixing it with nitrogen.

 

1. Fill the CO2 syringe with one atmosphere of CO2 from a tank, a fire extingisher, or a piece of dry ice placed in a plastic bag.


2. Mix the CO2 with nitrogen, either in the Mega Syringe or directly in the cell.

 

3. With the CO2-nitrogen mixture in the cell, record a low-noise single-beam spectrum and save it under the name C.spc. The previously-recorded background spectrum, back.spc is available for making a carbon dioxide absorbance spectrum.

 

4. Move the single-beam spectra to C:\QAS5\Work and import them into SPC format.

 

6. Click on the action button MAKE CO2 in order to prepare the CO2 absorbance spectrum. This will be saved in the database under the name Ca.spc. It should also be saved as a numbered CO2 spectrum, such as C1a.spc. The action of the AP buttons will also put the integration parameters into the header of spectrum Ca.spc.


The user can accumulate eight CO2 subtraction spectra saved under the names C1a, C2a.....C8a. Any one of these can be called up from Chapter X and re-named as the working spectrum, Ca. The existing Ca will be over-written, but it will not be lost because it still exists as one of the numbered spectra.


The REMOVE H,C,S action button will use Ca in the removal of CO2 lines from the sample spectrum. This button also removes water lines and the bands of any silicone vapor that may have been released by the valves or o-rings.


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