INTEGRATION REGION AND ZERO LEVEL SELECTION
When using QASoft, each compound to be measured needs to have an integration region written into the header information of the reference spectrum. The left and right sides of the integration band are specified, along with a zeroing region. The computer draws a straight zero line across the integration region, measures the y-values with reference to the zero line, and then adds them up. It does this for both sample and reference spectra. The comparison of the two integrals then gives the concentration of the gas, along with the subtraction factor that will properly remove the spectrum of the compound from the sample spectrum. The computer then carries out the subtraction. Although the integration operates only in a narrow spectral region, the subtraction removes the entire spectrum of the compound.
It is best to integrate over a distinct spectral feature that is separate from the features of other compounds. Integration across a single spectral line may suffice. One can also choose a bundle of spectral lines or a Q-branch, which usually appears as a "spike". A shoulder of a band may be chosen, with the integration starting at a minimum on the side of the shoulder and going a short distance into the band. If there are two broad bands of different compounds that overlap partially, one compound may be measured by integration on the left side of the composite band, and the other by integration on the right side. In the study of air, one should seek integration regions that do not include water lines. For measurement of a broad band, one may choose to integrate over a segment of the band that is between two water lines while designating a zero level on the side of the band, also between water lines.
If a compound creates an error in the measurement of another compound, the error will cause either an under-subtraction or an over-subtraction of the spectrum of the compound being measured. That error will be corrected when the measurement sequence is run a second time. A third run-through usually shows no further correction. As the default choice, all sequences are run two times, with the results being added together.
Proper selection of the zeroing point is important to obtaining a correct concentration value. Points on either side of a spectral feature may be chosen as the zero level. Even the top of a spectral feature may be chosen as zero. If the zero is at the bottom of a feature, the integrals for sample and reference spectra are positive. If the zero is at the top of a feature, both integrals are negative. To reduce the effects of noise on the results, the zero line position may be chosen by averaging many points. A set of zero points runs from the designated point in the direction of lower wavenumbers. Following are some examples of zeroing and integrating.
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.
The setting of the integration regions and zero regions is done by action button RIAS Param . A sample spectrum from which a compound is to be measured may first be put into the Infrared Analysis, Inc. main view. Then a click on RIAS Param produces a
dialog box that asks you to designate the reference spectrum in which the limits are to be set. If we designate methanol, for example, its QASoft Reference File Info screen will appear, as shown here.
To see a graphical presentation of limits and zero points, go to Zero Point Information and click Set by Graph. A screen appears showing both spectra. If a sample spectrum was not put in only the reference spectrum will show. The integration region will be marked by green vertical lines. The points for zero level are marked by black squares. An example is presented here for the case of methanol measurement. The red line in the figure is a portion of the spectrum of air captured in the District of Columbia, while the blue line is the database methanol spectrum. With this type of figure on the monitor, one may practise changing the integration limits and zero points.
Under View Limits, click Full Scale. The view limits will change to the full spectrum. Under View Limits, click Set by Graph. A dialog box will appear telling you to use the right mouse button to set the view limits and the left button to accept. Click OK and a cross appears. Position the cross and click the right button to set the left view limit; then move the cross and click the right button to set the right limit. Put the cross between the limit lines and click the left button to accept the limits. The header information screen will reappear with the new limits.
Move to Analysis Band and click Same as View. Analysis limits are taken from view limits.
Under Analysis Band click Set by Graph. The spectrum will appear with the cross. Again using the cross and the right mouse button, set the integration limits around a feature in the spectrum (left first, then right). Click the left button between the limit lines. The header information screen will appear with the new limits. Write in the desired zero point (left side of zero region) and the number of zero points to average. Click Save As, followed by Yes