QASoft Application Examples
Measurement of Trace Gases in Air
Trace gases can be measured by infrared absorption over a long path through the atmosphere. Figure 1. shows three different ways by which this has been done. Method c.–transmitter-to-receiver at the earth’s surface--is usually carried out with a multiple-pass “White” cell, as shown in Figure 2. Measurements with the long path cell have been done in the atmosphere for more than 50 years, as listed after Figure 2. It is from this experience that the QASoft technique of measurement has been developed.
At the present time, the Fourier Transfer Infrared Spectrometer (FT-IR) is a widely used instrument for air studies. Infrared Analysis, Inc. has found that a multiple-pass cell with 50-meters of optical path mounted in an FT-IR spectrometer sample compartment is about the optimum instrumentation for the trace gas measurement. This is discussed in detail in the book PROCEDURES IN INFRARED ANALYSIS OF GASES.
We have also found that an automation of the traditional band-area method of doing quantitative analysis is a superior way to get the trace gas concentrations. This, too, is discussed in detail in the book. An air analysis system is shown in Figure 3. This system may be used in conjunction with any of the presently available high-quality commercial FT-IR spectrometers.
The analytical method developed by Infrared Analysis, Inc. is called RIAS, which stands for Region Integration and Subtraction. From the detailed discussion in the book, we select Figure 4 to illustrate RIAS. The main features of RIAS are that the computer draws a baseline under a selected spectral feature and then determines the area between the base line and the line of the spectrum. It does this for both the sample spectrum and the reference spectrum. Then it compares the areas, calculates the gas concentration, and subtracts the entire spectrum of the measured compound from the sample spectrum. It does this for one compound after another, as directed by the user. See detailed discussions in the book.
EXPERIENCE LEADING TO THE DEVELOPMENT OF QASOFT BY INFRARED ANALYSIS, INC. ( 50 Years of research and development)
SPECTROSCOPIC PROOF OF THE FORMATION OF OZONE IN THE SMOG. (1953)
DISCOVERY OF ORGANIC PEROXY NITRATES AS PRODUCTS OF SMOG REACTIONS. (1954)
ANALYSIS OF POLLUTED AIR BY IN-SITU LONG PATH INFRARED ABSORPTION (1956)
INVESTIGATION OF POLLUTION MEASUREMENT FROM SATELLITES. (1967)
TRANSITION FROM DISPERSIVE IR TO FOURIER TRANSFORM IR IN THE STUDY OF AIR POLLUTANTS AND OTHER TRACE GASES (1970)
DEMONSTRATION OF THE POWER OF THE SPECTRAL SUBTRACTION TECHNIQUE WHEN CARRIED OUT BY THE DIGITAL COMPUTER. (1973)
USE OF LONG PATH INFRARED IN THE DISCOVERY OF CARBONYL SULFIDE IN THE AIR. (1975)
SPECTROSCOPIC MEASUREMENT OF HALOGENATED POLLUTANTS IN THE ATMOSPHERE (1975)
MEASUREMENT OF CHLORINE NITRATE IN THE ATMOSPHERE (1978)
CREATED DATABASE OF QUANTITATIVE REFERENCE SPECTRA OF AIR CONTAMINANTS (1985)
DEVELOPED TECHNIQUE OF REGION INTEGRATION AND SUBTRACTION (RIAS) (1990 TO NOW)
SEE DISCUSSIONS IN OUR BOOK: PROCEDURES IN INFRARED ANALYSIS OF GASES
HOW RIAS WORKS
1. Integrate over a line or "spike", with zero on the left.
2. Integrate around a spectral feature, with zero on top.
3. Integrate near the shoulder of a band, with zero at the bottom.
4. Integrate on the sides of two overlapping bands.
5. Integrate in a region between water lines, with zero on the side.
Let us now consider an example of air analysis, using spectra that for demonstration purposes are included in the QASoft program.
To measure the trace gases in an air sample, one must prepare sample spectra, such as Samp1, a background spectrum named Back, a water subtraction spectrum named w, and a CO2 subtraction spectrum, named c. For demonstration, these spectra were made for a sample of air collected in Anaheim and are included in the QASoft folders WORK and SAMPLE SPECTRA. Detailed discussions are given in the book PROCEDURES IN INFRARED ANALYSIS OF GASES, as well as in the QASoft Application Article EXPLANATION OF THE QASOFT WORKBOOK PAGE.
The first sample spectrum in absorbance form, named Samp1a, is shown as the upper plot in Figure 4.
The same spectrum with water and CO2 lines removed is shown in the lower plot of Figure 4. These spectra are displayed in the QASoft workbook page. Figure 5 shows an expanded plot of the two spectra from Figure 4.
The calculation of concentrations may be carried out in either of two ways: (1) it may be done one compound at a time using the action button ANALYZE SC, or it may be done in an automatic sequence using either the action button ANALYZE SEQ or the blue AUTO SEQUENCE buttons. For details, see the book PROCEDURES IN INFRARED ANALYSIS OF GASES and the QASoft Application article EXPLANATION OF THE QASOFT WORKBOOK PAGE.
The folders named WORK and EXAMPLE SPECTRA actually contain nine spectra of the same air sample named Samp1, Samp2,.....Samp9. The purpose of having multiple spectra for the same sample is to see how the measured values scatter as a result of the different noise patterns in the spectra. This is discussed further in the TEXT button for Auto Sequence One.
We proceed to the measurements for the first three spectra, Samp1, Samp2 and Samp3, using AP1, AP2 and AP3 to prepare the spectra, and the 1st, 2nd and 3rd buttons of Auto Sequence One.
15 compounds were measured in a double run, with the results printed out in Figures 6, 7 and 8.
Here are some comments on the results. At 3.16 PPM, the methane concentration was well above its normal background concentration of about 1.8 PPM. This might be due to gas leaks or to a passing methane-powered vehicle, but we do not really know. At 2.86 PPM, carbon monoxide was next highest in concentration, after methane. This was most likely due to auto exhaust. At 0.32 PPM, the nitrous oxide concentration was near normal. At 0.29 PPM, the nitric oxide concentration was significantly high. This was no doubt due to vehicle exhaust. NO2 was near zero because the sample was gathered early in the day, before there was much chance for photo-oxidation of the NO. At 0.13 PPM, acetone was significant. The presence of acetone bands in the spectrum was verified by visual inspection of the sample spectra. The small negative values for ammonia and methanol are correct because visual inspection of the spectra showed these trace compounds to be present in the water subtraction spectrum. The subtraction put them into the sample spectrum as negatives. Note that for all compounds the noise in the spectra produced variations in the measured numbers that were only in the range of a few parts-per-billion.
Figures 6, 7 and 8
Print-outs of measurement
results for three spectra run on the same sample of air.
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