Explanation of the QASoft Workbook Page
After QASoft is installed in GRAMS AI, a click on the toolbar button QASoft will bring up the single workbook page of the QASoft program. This page controls all the actions of QASoft. The screen is printed below with a spectrum of air.
There are two classes of buttons on the screen:
(1) The red, white and blue action buttons that call up actions to be performed on the active trace.
(2) The yellow Text buttons that explain the functions of the adjacent action buttons.
Let us review the actions of the red, white and blue buttons. This review will include much of the explanatory text that is called up by clicking on the yellow buttons.
SET UP is used once during installation of the software to write the INI files into the home directory.
QASOFT INFO brings up the registration details on the licensee of this copy.
ALPHABET INDEX calls up a listing of all the compounds in the database of quantitative reference spectra. At present there are about 420 of these. Each compound name is followed by letters that indicate the chapter in which the spectrum appears and the position in that chapter. The molecular weight is also given for use in converting parts-per-million to micrograms per cubic meter.
H2O .5, H2O 1, CO2 .5, CO2 1 control the preparation of water and carbon dioxide reference spectra.
The buttons H2O .5 or H2O 1 will make absorbance spectra (at the indicated resolution) from the wetted nitrogen spectrum, w, and the background spectrum, back. The computer writes the integration parameters into the header and the spectrum is saved in C:\QAS5\1QASData under the name Wa. The operator should also save the spectrum in C:\QAS5\1QASData as a numbered spectrum, such as W1a. Eight numbered water spectra may be saved in the database, Chapter W,
under the names W1a, W2a......W8a. Any one of these can be called up and re-named as
the working spectrum, Wa. The existing Wa spectrum will be over-written, but it is not
lost because it also exists as one of the numbered spectra.
The action buttons CO2 .5 or CO2 1 will make an absorbance spectrum from C and Back, and save it in C:\QAS5\1QASData under the name Ca. Like in the case of the water spectra, the
CO2 spectrum should be saved in C:\QAS5\1QASData as a numbered spectrum, such as C1a.
Ca is automatically given integration parameters.
The PPM-Meters value for Ca can be determined by clicking the action button MEASURE CO2.
The user can accumulate eight CO2 subtraction spectra under the names C1a, C2a....C8a.
Any one of these can be called up from Chapter X and re-named Ca, the working spectrum.
The program removes water lines from the CO2 spectra.
DATABASE gives access to all the digitized reference spectra (in the present degree of resolution). The spectra are classified in chapters according to chemical type. To find out which chapter a spectrum is in, click on the action button ALPHABET INDEX.
The analytical technique used in QASOFT is called RIAS, for REGION INTEGRATION AND SUBTRACTION. RIAS is an automation of the spectroscopist's traditional method of
comparing band areas in sample and reference spectra. With a computer doing the comparisons rather than a human operator, the analyses have become faster and more accurate than before. In addition, the computer can immediately subtract the measured spectrum from the sample spectrum. The data system can then proceed to the measurement of other designated compounds. The subtraction "peels down" a complex spectrum, one compound at a time.
RIAS can succeed where other quantitative methods will fail because RIAS uses the characteristic structure of a spectrum to distinguish it from other spectra. This allows RIAS to take a complex spectrum apart one compound at a time, measuring as it goes. If spectral
overlaps cause errors, a second run of a RIAS sequence will make corrections.
Here are six important features of RIAS.
(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. As long as your reference
spectrum is right, you know that you got the right answer.
(4) In the presence of a major component, like water, RIAS can operate in the spaces between the lines for valid measurements of other compounds in trace amounts.
(5) With RIAS you can measure your chosen compounds even if the relevant spectral region has unknown contributors to the 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.
SELECTION OF INTEGRATION REGIONS AND ZERO POINTS
A click on the action button RIAS PARAM brings up the QASOFT REFERENCE FILE INFO screen that applies to a chosen spectrum. This screen calls for the designation of the integration region and the zeroing region. There is also an entry for the concentration-pathlength product, which may be scaled for one reason or another to make the print-out readings higher or lower
than the true concentration values.
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 little way into the band. If there are two broad bands of different compounds that partially overlap, 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 and zeroing 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 one compound creates an error in the measurement of another, that error will most likely be corrected when the measurement and subtraction sequence is run a second time. A third run-through usually shows no further correction. As the default choice, all sequences are run twice, with the results being added together
Proper selection of the zeroing points is important in 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 band
can be designated 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 the feature, both integrals will be negative. To reduce the effects of noise on the zero level the zero line may be chosen by averaging many points. Graphic examples of zeroing and integrating are given in the manual.
ONE SHOULD MAKE SURE THAT ALL INTEGRATIONS ARE PERFORMED ONLY ON
SPECTRAL FEATURES WITH LOW ABSORBANCE. HOW LOW IT SHOULD BE DEPENDS ON THE RESOLUTION AND THE TYPE OF SPECTRAL FEATURE. AT HALF WAVENUMBER, THE ABSORBANCES SHOULD NOT GO HIGHER THAN 0.1 ABSORBANCE UNITS.
IF HIGH ABSORBANCE FEATURES ARE INCLUDED IN AN INTEGRATION, THE
RESULTS WILL MOST LIKELY BE ERRONEOUS.
THIS LOW ABSORBANCE REQUIREMENT APPLIES NOT ONLY TO RIAS, BUT ALSO
TO ANY OTHER QUANTITATIVE ANALYSIS PROCEDURE. WHEN WORKING WITH HIGH ABSORBANCE SPECTRAL FEATURES, ERROR IS INTRODUCED BY LINE SATURATION, BY SCATTERED LIGHT, BY BASELINE IRREGULARITIES, BY NOISE, AND BY OTHER FACTORS.
LOW ABSORBANCE FEATURES ARE GENERALLY NOT DISTORTED BY THE ABOVE MENTIONED FACTORS. IF THE SPECTRAL RESOLUTION IS THE SAME IN SAMPLE AND REFERENCE SPECTRA, INTEGRATION OVER A LOW ABSORBANCE FEATURE WILL GIVE A CORRECT QUANTITATIVE ANALYSIS.
IF THE ABSORBANCE OF THE STRONG SPECTRAL FEATURES IS HIGH AND THEY
ARE THEREFORE DISTORTED, A VALID MEASUREMENT CAN STILL BE MADE BY
SHIFTING THE INTEGRATION TO A WEAK SPECTRAL FEATURE. THE MEASUREMENT RESULT WILL THEN BE CORRECT, BUT THE STRONG SPECTRAL FEATURES WILL END UP OVER-SUBTRACTED.
THE ABILITY TO CHOOSE EITHER STRONG OR WEAK SPECTRAL FEATURES
ALLOWS RIAS TO MAKE VALID MEASUREMENTS OVER CONCENTRATION RANGES AS WIDE AS SIX ORDERS OF MAGNITUDE.
ROUTINE FOR WRITING IN THE INTEGRATION PARAMETERS:
1. Put the reference spectrum on the screen.
2. Decide on the left (high wavenumber) and right (low wavenumber) sides of the integration region. Also decide on the left side and the extent of your zeroing region.
3. Click RIAS PARAM.
4. Choose ON SCREEN.
5. Under VIEW LIMITS, write in the left and right limits.
6. Under ANALYSIS BAND, click SAME AS VIEW.
7. For ZERO, write in the wavenumber of the left side of the region. Other zero points will be to
right of that.
8. For NUMBER OF POINTS write in a number large enough so that you can average out the noise without including water lines or other interferences, as discussed above.
9. For PPM-Meters, write in the number from the reference spectruim, such as 100.
Also consult the detailed examples in the manual.
MEAS CO2 .5, MEAS CO2 1
These buttons will measure the CO2 concentration in your air sample. They operate on the
spectra prepared from the AP buttons, but water lines must be removed before measuring
the CO2. The sequence of operations is:
(1) Click on an AP button.
(2) Click on REMOVE H2O.
(3) Click on MEAS CO2 .5 or MEAS CO2 1
To analyze for other compounds, click again on the AP button, then click REMOVE H2O and
REMOVE CO2; and then perform further operations on the residual spectrum.
BROWSE is an interactive search function. The browse function allows the user to compare an unknown spectrum--or a portion thereof--to all the spectra in the database. The comparisons may start at the beginning of the database, the end of the database or the beginning of any chapter.
You may browse while viewing only a portion of the spectrum.
The database spectra are moved across the unknown by means of the
page-up and page-down keys.
The ABSORBANCE function calculates and displays an absorbance spectrum from sample and background single-beam spectra that are chosen by the operator.
The single beam spectra are created by means of the spectrometer software. QASoft should be used to make the absorbance calculations. This will allow you to avoid erroneous high absorbance values that are created by some versions of spectrometer software.
The errors come about in regions of intense absorption in the single-beam spectra, where noise may cause some points to have negative values. Trying to calculate the logarithm of negative numbers causes the computer to put out meaningless false high absorbance readings.
The QASoft absorbance calculation corrects these false readings by eliminating the negative values before the absorbance calculation is made.
The ANALYZE SC function enables the measurement of the sample spectrum for all of 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 the 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.
See the examples in the manual.
The Analyze Sequence function allows the operator to designate a series of compounds for measurement. The computer couples the measurements together in an automatic sequence. The sequence then runs, with the succession of measurements and subtractions appearing on the screen. At the end of the sequence the measurement results are printed out. The sequence may be saved and called up again whenever needed. A sequence may be run twice in succession on a single sample spectrum. In most cases, the second run-through will produce all zeros as measurement values. Sometimes, however, when there are severe spectral overlaps, the first run-through will be sightly in error, and the second run will provide corrections. A third run will almost always produce no further corrections. The default choice is two runs, with the sum of the two measurements being printed out. The manual presents an example of ANALYZE SEQ that may be followed through on the computer.
To remove water interference it is necessary for the user to prepare his own water reference spectra. The reason for this is that a good subtraction requires an exact match of the line positions, line shapes and line widths between the sample spectrum and the reference spectrum. Since each spectrometer has its own "spectrum signature" with its own combination of line
positions, widths and shapes, the water and CO2 subtraction spectra must be made on the same instrument that is used to record the sample spectra.
Another problem is that most of the water lines will have a high degree of absorbance and therefore an absorbance versus concentration plot will be non-linear for most lines. Furthermore, the extent of non-linearity will vary from line to line. This further complicates the water subtraction operation. It is therefore necessary that the spectrum used for water line subtraction should exhibit overall integrated absorbances close to those exhibited by the sample spectrum. In other words, the reference gas sample should have about the same humidity as the gas sample being analyzed.
To assist in preparing the water vapor subtraction spectrum, a tubing assembly is supplied with a double segment that has a dry side--the plastic tube with nothing inside, and a wet side--the
plastic tube with wetted beads inside.
Here is a procedure for preparing the water spectra.
1. Put your single-beam sample spectrum on the screen and note the absorbance values in the vicinity of 1600 cm-1. Next put the single-beam spectrum of nitrogen on the screen and humidify it to the same extent as the sample by mixing wet and dry nitrogen.
2. When you have matched the water absorbance values of the wetted nitrogen with those of the sample, record a low-noise single-beam spectrum of the humid nitrogen and save it with the name W.
3. Evacuate the cell and re-fill it with one atmosphere of dry nitrogen.
4. Record a low-noise single-beam spectrum for background and save it under the name Back.
5. Move the two single-beam spectra to C:\QAS5\Work and import them into SPC format.
6. Make an absorbance spectrum, and save it to C:\QAS5\1QASData under the name Wa. It should also be saved as a numbered water spectrum, such as W1a.
When working with samples of various degrees of relative humidity, the user can accumulate a family of water reference spectra. Eight of these may be saved in the database, Chapter W, under
the names W1a, W2a.....W8a. Any of these can be called up and re-named as the working spectrum, Wa. The existing Wa spectrum will be over-written, but it will not be lost because is also exists as one of the numbered spectra.
Integration parameters are automatically written into the water spectra.
The action button REMOVE CO2 will remove carbon dioxide lines from the sample spectrum.
PREPARING THE FAMILY OF CARBON DIOXIDE SUBTRACTION SPECTRA
For reasons discussed in the text of the water button, it is necessary to prepare CO2 subtraction spectra on the same spectrometer that is used for recording the sample spectra.
A wide range of CO2 concentrations may be encountered in sample analysis. In polluted air, for example, the CO2 concentration will fall near 500 PPM. In combustion effluents, the CO2 concentration may be near to 100,000 PPM (10%). If the combustion effluent is diluted 10-fold, the CO2 concentration will still be 10,000 PPM. If the user knows approximately what his CO2 pathlength-concentration product is going to be, he can match it in metering out his CO2 gas.
A simple way to create the desired CO2 concentration is to calculate the needed volume of CO2 at one atmosphere pressure and introduce that volume of CO2 into the cell. For example, to create 1000 PPM of CO2 in nitrogen, with a cell of 14 liters, just meter out 14 cc. of CO2, and
allow the nitrogen to carry it into the cell.
QASoft allows the user to create eight CO2 subtraction spectra that are stored in the computer and then are used to automatically subtract the CO2 lines from the sample spectrum. The user calls for the subtraction spectrum that has the closest match to his sample spectrum and saves it as the working CO2 spectrum, named Ca.
The GAS ANALYSIS SYSTEM includes a CO2 syringe.
The following steps will produce a CO2 subtraction spectrum.
1. Fill the CO2 syringe with one atmosphere of CO2 from a tank, a fire extingisher, or gas from a piece of dry ice placed in a plastic bag.
2. Fill the long path cell with dry nitrogen. Then inject the CO2 into the cell.
3. Record a low-noise single-beam spectrum and save it in C:\QAS5\1QASoft5 with the name C.
4. The action button MAKE CO2 will combine the spectra C and Back into an absorbance spectrum named Ca, which will be saved in C:\QAS5\1QASData. Integration parameters are automatically written into Ca. Also water lines are automatically removed from Ca. The user should also save the absorbance spectrum in C;|QAS5\1QASData as a numbered spectrum,
such as C1a.
The user can accumulate eight CO2 subtraction spectra saved under the names C1a, C2a.....C8a. Any of these can be called up from Chapter X and re-named as the working spectrum, Ca. The existing Ca will be over-written, but it will not be lost because it still exists as one of the numbered spectra.
Clicking on RESOLUTION allows all program operations to be changed from the present degree of spectral resolution to the other degree of spectral resolution.
AUTO SEQUENCE ONE
The buttons 1st, 2nd, and 3rd activate an automatic analysis for up to 20 compounds.
The buttons operate on the active spectrum that is on the screen. For example they may act on the test spectra of air named Samp1a, Samp2a, and Samp3a. These are the spectra prepared by clicking on the white buttons AP1, AP2 and AP3. Sequences are prepared through the button ANALYZE SEQ. The analytical sequence is written out in the file Autseq1.seq. Measurement results may be printed out. Each of the three buttons runs the same analytical sequence, but each button prints the results differently. 1st prints the header information for the spectrum to which it is applied and then prints the compound names and the PPM concentrations. 2nd prints only the PPM concentrations for its spectrum in a column displaced from the column of 1st. 3rd likewise prints PPM concentrations in a column further displaced.
The purpose of all this is to allow the printing of three sets of results side-by-side on a single page. To do this, the print-out page from 1st is fed back into the printer and then the print-out from 2nd is entered on it. Then the print-out page from 2nd is fed back into the printer to receive the print-out from 3rd.
A principal application of this printing technique is to compare and average the analytical results
from three separate spectra that have been recorded for a single sample.
After print-out, the residual spectrum remains on the screen for visual inspection. If other
compounds are recognized, they may be measured one at a time by means of the ANALYZE SC button. Also, any other analytical sequence may be run on the residual spectrum.
AUTO SEQUENCE TWO
See the discussion for the AUTO SEQUENCE 1. The sequence called by the buttons under
AUTO SEQUENCE TWO is named Autseq2.seq.
AUTO SEQUENCE THREE
See the discussion for AUTO SEQUENCE ONE. The sequence called by the buttons
under AUTO SEQUENCE THREE is named Autseq3.seq.
THE AP BUTTONS
The buttons AP1, AP2.......AP9 will automatically prepare spectra for quantitative analysis. AP stands for Automatic Preparation. There can be nine single-beam sample spectra named Spec1.spc, Spec2.spc.....Spec9.spc, and they must reside in the folder C:\QAS5\Work. The corresponding single-beam spectra for background, water removal and CO2 removal--named
Back.spc, w.spc, and c.spc--must also be in that folder. These spectra can be made on any FT-IR spectrometer and imported into GRAMS.
A click on AP1, for example, does the following:
1. Calls the background spectrum, back.spc, and removes carbon monoxide lines that may be in it. The CO-free spectrum is re-named backz.spc.
2. Makes an absorbance spectrum from Samp1.spc and Backz.spc and then returns that absorbance spectrum to C:\QAS5\Work with the name Samp1a.spc.
2. Puts the sample spectrum, Samp1a.spc, into slot 1, ready for the quantitative analysis. In most cases, the removal buttons REMOVE H2O and REMOVE CO2 should then be clicked. Next, an analysis button such as Spec. 1 may be clicked.
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