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SEQUENCE ANALYSIS


The Analyze Seq 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 flashing across 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. 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 slightly in error, and the second run-through will provide corrections. A third run-through will almost always produce zeros.



To illustrate Analyze Seq, we return to the spectrum of ambient air (dcaa.spc on disk). With this spectrum in the view, we click on Analyze Seq. A dialog box appears asking if we wish to create a new sequence or retrieve a stored sequence. If we clicked NEW we would be asked to select a compound, confirm the analysis parameters and proceed to the next compound. After the sequence was built, we would click Sequence Complete. The list of selected compounds then would appear. We would then name the sequence, save it, and proceed to run the analysis.


However, we are going to click RETRIEVE because for analysis of the air sample we have already prepared a 15 compound sequence that is stored in the 1QASoft5 folder under the name Air.seq. When we select that sequence, the list of compounds appears. We click Continue,and confirm the sample file and the pathlength. The sequence then runs and the measured concentrations are printed on the screen. We then repeat the sequence to get corrections that may result from spectral overlaps. The corrections are added and the screen then appears as shown here. The only significant correction was on nitric oxide, and a third run will add no further corrections. If we then click NO on the repeat sequence question, we will be given the opportunity to print out the final results on paper.


We now proceed to another example--the measurement of aromatic compounds in the EXXON 87 gasoline sample whose spectrum is in our database. The gasoline spectrum is brought up from the database. We then we click our action button Analyze Seq and the question of building a new sequence or retrieving a stored sequence comes up. We have an aromatic sequence stored (it came from the installation disk), so we click RETRIEVE. A screen then comes up asking us to select the sequence. We choose aromatic.seq, and the screen shows a six molecule sequence (the group of compounds known as BETEX).


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We could now modify the sequence by adding or removing compounds, but we wish to use it as it is, so we click on a - Continue. We are then asked if we wish to change the sample file.

We say NO, and we are asked to confirm the sample spectrum and the optical pathlength.

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Because our sample was 100 ppm-meters of gasoline vapor, and our reference spectra are each for 100 ppm-meters of compound, we change the pathlength from 144 meters to 1 meter. Then our calculated amounts of aromatic compounds (called parts-per-million) will be equal to per cent. Clicking on OK then causes the analysis to proceed, and the screen shows the results. We make a second run-through and see that it shows some small corrections (resulting from the overlapping of the spectra), so we run the sequence a third time. This produces essentially all zeros. The results may now be printed on paper.

 



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