SINGLE COMPONENT ANALYSES
The Analyze SC function enables the measurement of the sample spectrum for all the compounds in the database, chosen one at a time. The operator selects the compound, and the computer measures it. The computer also subtracts the complete spectrum of the compound from the sample spectrum. Stored measurement parameters may be used, or new ones may be chosen at the time of analysis. During analysis the sample spectrum and the difference spectrum are displayed. Interactive correction of the subtraction factor is allowed. After displaying the results of the analysis, the computer asks the operator to select another compound for measurement. The program can proceed through as many compounds as are designated.
We illustrate the operation of ANALYZE SC by proceeding with measurement of compounds in a sample of polluted air captured in the District of Columbia on a week-day morning. The spectrum was recorded at half wavenumber resolution in a 144 meter long path cell. This spectrum is in the folder 1QASoft5 and is named dcaa.spc. The spectrum is called up. It is shown in the figure here. A click is then made on the ANALYZE SC button. Dialog boxes appear, asking us to confirm the sample file and the pathlength.
Clicking OK on pathlength gives us access to the complete database through the chapter index, and to user supplied spectra through chapter V. We will first remove the water lines from the air spectrum. To do this we use a water reference spectrum that was produced from a sample of wet nitrogen placed in the absorption cell This spectrum, named dcwna.spc is stored in the folder 1QASoft5, which is accessed by a click on chapter V-User’s Choice.
When the water spectrum is selected, its quantitative analysis parameters appear, as shown here. The panel says that view limits are set at 2200 to 1952, the analysis band is set at 2014
to 1942, and the zero line will be drawn at the level of the point at 1973. We will accept these parameters by clicking on GO. This causes the measurement and subtraction to take place, and a screen with the results appears--shown below. The sample spectrum is shown in red and the residual spectrum is shown in blue. Note the pattern of CO lines in the residual spectrum.
If one now presses ENTER on the keyboard, a new query appears: “Analyze for Another Compound?”. We say YES and then choose methane from chapter A. The methane analysis parameters appear. These say that we are going to display the region 3230 to 2837, integrate over the region 3020 to 3016, and take the zero level from the average level of 6 points beginning at 3025.
We will accept the methane quantitative analysis parameters by clicking on GO. This causes the measurement and subtraction to take place, and a screen showing the results appears. Like in the case of water subtraction, the screen shows the sample spectrum in red, which was the residual after water subtraction, and the new difference spectrum in blue. The methane concentration of 2.4 parts-per-million is printed.
From the residual spectrum we can now measure non-methane hydrocarbons. There are some subtraction artifacts and spectrum noise where the methane lines were removed, but these will not have a significant affect on the measurement of non-methane hydrocarbons because we will integrate over the whole C-H absorption band. Pressing ENTER on the keyboard brings back the question of analyzing for another compound. Clicking on YES brings back the database chapter index. We have a spectrum of EXXON gasoline vapor in chapter T, which we can use as surrogate for all non-methane hydrocarbons. When the gasoline spectrum is selected, its analysis parameters appear. When they are accepted by clicking GO, the measurement and subtraction take place, the concentration is printed and the operator is invited to continue with measurement of other compounds. These can include CO, nitrogen oxides, olefin hydrocarbons, methanol and Freon 12.