Part I: Exploring integration limits
Series by Ali and TBV.
Integration is the process by which we get the carbon isotope ratios out of the IRMS peaks. The sample gets burned up into carbon dioxide, ionized, and detectors for the m=44 and m=45 ions determine how many of carbon 12 molecules vs. the number of carbon 13 molecules there are.
Because they are of slightly different mass, the heavier 13 comes through a before the 12, by a little bit, so the resulting peak is not completely symmetrical, but carries a lopsidedness dependent on the ratio of the carbons. As a result, you need to be careful where you start and stop counting molecules for determining the ration.
This is explained for smart people in the Meier-Augenstein paper starting in GDC 1101 at GDC 1107. It has this picture showing the offset:
To explore integration, we've created a spreadsheet to make the following simplified examples that illustrate the issues. The spreadsheet is in the archive here, and you're invited to use it to play with values yourself, and to find any errors we've made in it.
Starting off with a well separated and perfectly formed peak, the following series of examples illustrates the sensitivity of the o/oo value to integration limits, background noise and interfering peaks. In part I, we're going to show the effects of changing left and right integration limits on the computed isotope (o/oo) ratio.
In showing this, we are not saying or suggesting anybody or anything is doing anything wrong of this sort in the tests. The automatic selection of limits has the same set of issues. We're just showing the kinds of effect changing the limits does have on computed results.
Next in Part II, background and background subtraction.