Peak integration

Discussions about GC and other "gas phase" separation techniques.

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A number of methods that I am constrained to use show a front edge leading peak. Of course on the chromatogram it is not obvious due to scaling. My question is: does that front edge leading peak really effect the integration area vs. a symmetrical Gaussian peak? I can only speculate at this point that response and area would not be much different vs. a nice symmetrical peak shape. Perhaps there would be a sensitivity drop with the shark fin peak. Is there a study comparing or explaining any real difference in calculation? My assumption is that having a higher flow than recommended makes for a shorter method - so maybe I shouldn't get too annoyed with the ugly chromatography.

Thanks for any input.
None of the integration algorithms that I know of tied to any specific peak shape. The start of the peak is usually identified when the slope of the signal exceeds a certain trigger value, and the end of the peak is identified when returns near enough to zero the values in between are summed to give the area (yes, that's a gross oversimplification, but the point is that the details of peak shape don't matter).
What CAN matter with a distorted peak is a non-linear relationship between response and analyte concentration, but as long as the entire peak stays within the linear range of the detector, the shark-fin should not have a significant impact on integration.
-- Tom Jupille
LC Resources / Separation Science Associates
tjupille@lcresources.com
+ 1 (925) 297-5374
tom jupille wrote:
None of the integration algorithms that I know of tied to any specific peak shape. The start of the peak is usually identified when the slope of the signal exceeds a certain trigger value, and the end of the peak is identified when returns near enough to zero the values in between are summed to give the area (yes, that's a gross oversimplification, but the point is that the details of peak shape don't matter).
What CAN matter with a distorted peak is a non-linear relationship between response and analyte concentration, but as long as the entire peak stays within the linear range of the detector, the shark-fin should not have a significant impact on integration.


Thank you very much for putting me at ease on this! If I had the time I guess I could have run an area% and internal standard method then varied the column flow. I may have lost small peaks due to flattening putting the area/peak parameters below the integration parameters.

Thanks again!
In my opinion requiring limits on Peak Gaussian Factor, or Tailing Factor, ect. are holdovers from 30 years ago when peak integration software was not as robust as it is now. Current integration software is able to handle just about any peak shape if you get the settings adjusted.

Another thing that is somewhat of a holdover is requiring tight average response factor limits for calibration curves or 0.995 or higher r squared values for calibrations lines and frowning on the use of a quadratic fit. The EPA with the newest Drinking Water methods have began to accept calibration fits that were once questionable including curve weighting, with the requirement of tighter requant values. The new requirement makes more sense where after calibration you must recalculate all the points on the calibration and the point at or below the Minimum Reporting level must be within +/-50% and all other points within +/- normal calibration check limits (anywhere from 30% to 10% depending on the method) with the caveat that nothing is reported outside the upper and lower bounds of the calibration standards. This I believe gives good quality data. Before we just had the requirement that you do a linear calibration with a 0.995 or better fit and call it good, but I have seen a calibration of 0.5ppb to 500ppb give a curve fit of 0.998 but when recalculating the low standard the calculated value was 2.0ppb or 400% deviation. This may work well when you have mostly samples with detects near the mid point of the calibration but if you have most samples that are non-detect, just how sure can you be it is not really there?

As long as you get a good calibration and recalculation is accurate and reproducible, then peak shapes should not matter unless it leads to co-elutions, but that should be evident as a problem in the recalculation test.
The past is there to guide us into the future, not to dwell in.
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