REGION INTEGRATION AND SUBTRACTION (RIAS)
Concentration information is extracted from the spectrum by the technique of Region Integration and Subtraction (RIAS), developed by Infrared Analysis, Inc. RIAS is an automation of the traditional method of making quantitative measurements in which areas under spectral features are calculated and compared. Band areas are compared not by the former method of hand and eye, but by the computer. The program not only does this better than a human being, but it also immediately removes the spectrum of the measured compound from the sample spectrum. The data system will continue on through as many compounds as are designated in the program. With RIAS one can measure any compound that has an available digitized reference spectrum.
RIAS can succeed where other quantitative analysis methods will fail. The reason for this is that RIAS can take advantage of fine structure in the spectra. RIAS works best when measuring narrow spectral features--spikes (Q-branches), shoulders of bands, small bundles of lines, or even individual lines. The use of narrow spectral regions is not especially detrimental to the sensitivity of the measurements because the spectral information is concentrated in spectrum details. The use of detailed features also promotes discrimination between compounds with overlapping spectra. In atmospheric analysis, for example, the use of spectral details allows pollutant measurement while working between water lines. This avoids noise and interference from subtraction artifacts.
In many samples there is interference by the absorption spectrum of water vapor. The problem is especially severe in the analysis of the ambient air. Even though the water lines are subtracted away, some residuals will remain and only noise will be present where the water absorption was intense. Since RIAS can operate in narrow spectral regions, water interference can be avoided for nearly all compounds. The AP buttons and their associated procedure of making water and CO2
subtraction spectra will yield an excellent removal of water and CO2 interference.
Listed below are nine advantages that RIAS has over the multivariate quantitative analysis methods, such as the methods of classical least squares (CLS) and partial least squares (PLS).
(1) In RIAS you can measure one compound at a time in the presence of many other compounds, even if there are things in the spectrum that you do not understand and cannot account for.
(2) In RIAS you can measure a selected few compounds in the presence of others.
(3) In RIAS you see what you are doing. You have visual verification of the success of your measurement. If you oversubtract or undersubtract, you can see it. With a correct reference spectrum, you know that you got the right answer.
(4) In the presence of a major component, like water, RIAS allows you to go between the lines for valid measurements of other compounds.
(5) With RIAS you can measure your chosen compounds even if the relevant spectral region has unknown contributors to the total absorption.
(6) With RIAS, baseline irregularities have little effect on the measurements; degradation of the optics has little effect on the measurements; the presence of light-scattering particles has little effect on the measurements.
(7) RIAS does not require the preparation of standards; the standards are already in the QASoft database.
(8) With RIAS, there is no plotting of calibration curves and no creation of mathematical models.
(9) When using RIAS there is no need to perform matrix algebra or factor analysis.
FOR DETAILS ON THE SELECTION OF INTEGRATION REGIONS, SEE THE TOPIC INTEGRATION REGION AND ZERO LEVEL SELECTION.
DETAILED EXAMPLES OF THE OPERATION OF RIAS ARE GIVEN IN THE TOPIC: COMPUTER OPERATIONS INVOLVED IN THE AUTOMATIC SEQUENTIAL ANALYSIS OF 13 COMPOUNDS IN AIR.