How do you subtract Blank areas using PDA?

[EmpowersBane]

Is it possible to subtract the areas due to your blank or diluent from your samples when using a PDA and if so can it be done before or after processing?

I have in my Method Set: Instrument Method and Two Derived Channels for the processing method, one at X wavelength the other Y. I labelled my Blank "BL" in the sample set and there seems to be a "Second Channel" option to subtract from samples with labels but I cant quite figure out how you do this.

Its probably something small can anyone tell me if this is possible? Also there is another gap on the MS screen for Blank Subtraction but I don't know what goes in there...help!

[lmh]

Normally you wouldn't subtract a blank. Your blank run shouldn't have a peak in the same place as the analyte in a sample run. If it does, you have a serious problem; many peaks in blank runs gradually change in area, so at the very least you'd have to establish that the variability in the peak in blank runs was very low compared to the expected size and variation in your analyte peak. It's a very high-risk strategy.

Subtracting a blank run from other runs is also somewhat risky. If you have any variation in retention-time or peak-shape, then subtraction will generate spurious negative and positive peaks. If you subtract a Gaussian peak from another equally-sized peak that's shifted slightly to the right, you'll get a sharp negative peak, followed by a flat bit, and then a sharp positive peak. Of course if the blank sample has a smaller peak, there will still be a proper peak to see in the subtracted chromatogram, but it will be preceded by a negative spike that will fool most integrators into starting the baseline far too low (over-estimates peak area), and there will be funny tailing effects.

Good luck!

[lmh]

Oh, I should add: if you want an accurate spectrum for a peak, then of course you need to subtract a spectral blank (spectrum of solvent) from the peak's spectrum; usually you choose bits of baseline just before and/or after the peak as representative of the absorbance spectrum of the solvent.

[EmpowersBane]

Ok thanks very much for reply