Over in the thread about visual identification of peaks (Discussion with BustinBilly), there have been some comments that lead naturally into a discussion about the applicability of relative retention time across two instruments.
As we know, Paragraph 185 of the Majority Award declared that it wasn't possible, therefore Landis' complaints about the RTTs not being within spec of TD2003IDCR didn't apply.
In the discussion of visual identification, I tried to show that some of the math used in paragraph 185 was incorrect:
Indeed, the majority decision in their mathematical example seems to suggest that what TBV is doing can never result in a match.
They say the retention times of the IRMS GC take longer than the GC-MS machine by some constant amount of time. They use the example of a relative retention time ratio of 10 to 5, becoming 11 to 6, after you add a 1 minute constant.
Well if in fact the IRMS retention times are off by some ADDITIVE constant relative to the GCMS machine, then you can't correct for that by MULTIPLYING in some proportionality correction. By stretching the scale I assume TBV is doing some multiplying. At least that is how I understand the example and the math.
The panel got their "math" example wrong. If they'd figured it out correctly, the example they gave as "broken" worked just fine. You are supposed to subtract the fixed offset before you do the multiplcation, so (11-1) / (6-1) works out to 10/5 just like they said it didn't.
What is the authority for these two statements:
1. "You are supposed to subtract the fixed offset before you do the multiplcation"
2 "and then scaling (doing the proportional multiplication)." (I asked before, on what basis are you claiming you are entitled to do proportional scaling?
You can google "relative retention time chromatography", and the first link you find (http://www.iupac.org/publications/analytical_compendium/Cha09sec237.pdf) will take you to a pdf document entitled "188.8.131.52 Retention Parameters in Column Chromatography". It has a section (p.2) describing the Adjusted Retention Volume (Time). It then goes on to discuss Relative Retention (r) on p. 4. For those who are interested, IUPAC is International Union of Pure and Applied Chemistry. This is pretty standard stuff.Another reference I've found is a tutorial at the Shimadzu web site, unfortunately in Flash.
(also some interesting stuff at scientific.org)
Although many consider GC/MS to be the "gold standard" in scientific analysis, GC/MS does have some limitations. Because great faith is maintained in GC/MS analysis, erroneous results are not expected and hard to dispute. However, false positives and false negatives are possible.
I'll confess that I've never quite understood some things about RTT. It's been represented to me that it does solve the problem in question (identifying peaks) across different runs, which might include different machines. The Majority of the Panel asserts this is not the case.
Let's explore together the same way we did with "visual gestalt". I'll post some things as I understand them, then comments can react and tell me what I've gotten wrong. I'll collect the ones I think are most relevant here and we'll see where we end up.
Before going on, commenter "M" writes, quoting the iupac reference above:
On the other hand, the unadjusted relative retention (and "relative retention time") values are only reproducible within a single chromatographic system."
There's an implication that RRTs are therefore not valid, except that we are dealing with a single chromatographic system, despite various caveats:
The GC column is, of course, the same in both instrumentsSo, we'll work through it as best we can and see who is hot-dogging who.-Majority award paragraph 188
Let's illustrate some terms, shamelessly borrowed from the Shimadzu tutorial, also from an Agilent document on terms.
The "Retention Time" is from injection to the peak detection.
The unretained compound peak seems to be time at which a test substance that won't be delayed by the column is injected until when it gets out. It goes through the column as fast as anything possible can, and becomes a fixed reference for this run.
The "Adjusted Retention Time" is the retention time less the hold-up time.
The figure also introduces K symbols, which are Retention Factors. The retention factor for a peak is the adjusted retention time divided by the hold up time. As a special case, the retention factor of the unretained peak is defined to be zero, to avoid a divide-by-zero.
This is how we know in the GCMS that the labeled peaks are what they are -- there was reference run in the GCMS with standards that identified the substances in question, and the retention times there were matched within TD2003IDCR of the Landis target samples.
Figure 4 shows the kind of match done in Figure 3 with the LNDD calibration sample and the Landis F3. The target peaks line up well, within TD2003IDCR. We note at the left edge by the known standard that the circled peaks of the standard in the cal mix acetate is very different height than it is in the Landis sample. If we'd gone on peak heights alone rather than look at the times, we might have lined the Landis next peak to the right with the cal mix.
[ coming, cal mix in the IRMS and the Landis F3 ]