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SAMPLING CONSIDERATIONS


Infrared Analysis, Inc. seeks to make gas sampling as simple and inexpensive as possible. We wish for the analyst to do his quantitative analysis at normal conditions of temperature and pressure. We wish to minimize the use of pumps in sample gathering and to avoid the use of heated sample lines and heated absorption cells.


Simplification of sampling and simplification of the analytical method go hand in hand. Our quantitative analysis program, QASoft, uses reference spectra that were recorded and calibrated at normal temperature and pressure. Then when the sample can be studied under the same normal conditions, a correct quantitative analysis is obtained.


Flue gases, engine exhausts, incinerator effluents and other products of combustion are hot, wet, and laced with toxic gases. There is a widespread impression that to do a spectroscopic analysis of such gas mixtures, one needs to maintain the high sample temperature. There is concern that if the gas sample is cooled, the water will condense and alter the sample by dissolving some of the toxic pollutants. It is the experience of Infrared Analysis, Inc. that such concern can be avoided by proper sample handling. Working at room temperature is feasible in almost all cases.


Mistaken efforts to maintain a high temperature in the combustion gases cause difficulties in the handling of the sample. Heated sampling lines and a heated absorption cell are then needed.. Furthermore, these efforts complicate the spectroscopic analysis by maintaining a large amount of high temperature water vapor in the optical path. To allow a good infrared analysis, the heavy pattern of water vapor lines must be subtracted out of the spectrum. This must be done with a water reference spectrum made at the same temperature at which the sample spectrum was made and with nearly the same pathlength-concentration product as applied to the sample spectrum.. At high temperature, this matching is difficult to do. You must have equipment to control the temperature and to monitor the temperature during recording of both the sample spectrum and the reference spectrum.


Another difficulty arising from the high temperature is that you do not have available calibration and reference spectra made at the same temperature. Most available digitized reference spectra have been recorded only at room temperature.


How much easier things are if one is able to do the analyses at room temperature! The heated lines and heating jackets are gone! The temperature controller and temperature monitor are gone! The reference spectra for subtraction of water and CO2 interference are available! The calibration spectra for the quantitative analysis are available!


What are the obstacles to doing the analysis at room temperature? None! It is an easy thing to dilute the sample as it is being taken in. The dilution gas may be instrument air, ambient air, or tank nitrogen. As the sample is being taken in and diluted, it is cooled down. The dilution must be great enough that the water does not condense. A hot combustion gas may contain up to 10% water vapor. However, at room temperature, a gas mixture cannot hold more than 3% water vapor. The dilution factor in sampling must therefore be at least 3-to-1.


One may ask: isn't the 3-to-1 dilution going to reduce the measurement sensitivity by a factor of 3? The answer is no. The dilution will reduce all absorbances by a factor of 3, but this includes the interfering water lines. Measuring compounds like NO, NO2 and SO2, whose bands are badly mixed in with the water, will be made easier by the dilution. For these compounds, the dilution may even increase the measurement sensitivity.


Water and CO2 Line Subtraction


One of the principal difficulties in measuring trace gases in air or in combustion effluents is the interference by the spectral lines of water vapor. To enable a complete, fully sensitive analysis, water lines should be removed from the spectrum before the other lines are measured. The water lines are removed by means of a water subtraction spectrum. Each user needs to prepare his own water subtraction spectrum as well as his own CO2 subtraction spectrum. The reason for this is that a good subtraction requires the sample spectrum and the subtraction spectrum to have an exact match of line positions, line widths and line shapes. Since each spectrometer has its own “spectrum signature” with a unique combination of lines positions, widths and shapes, the water and CO2 subtraction spectra must be made on the same instrument as the sample spectra. Many of the water and CO2 lines will have a high degree of absorbance, and therefore absorption saturation effects will distort the lines. Absorbances will not be proportional to concentration. It is therefore necessary that the spectra used for water and CO2 subtraction should have nearly the same absorbance values as the values in the sample spectrum.


Sampling Components


Infrared Analysis, Inc. offers five components of hardware that permit quantitative, room temperature gas handling:


            (1) Gas sampling syringes.

            (2) The flowing gas dilution system.

            (3) Gas sampling bags.

            (4) Humidification Device (Wet-dry tube)

            (5) A CO2 dispensing tube.


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