I've encountered in my recent analytical calculations for analytes in samples.
During my analysis, after processing a 750 mL sample and reconstituting it in 185 µL of solvent, I obtained a peak area of 4302.24 from the GC-MS/MS. The concentration in the 185 µL of solvent was calculated to be approximately 1.351 ng/mL, which is comfortably above the Limit of Detection (LOD) ie 0.03 ng/mL, and thus makes sense.
However, the confusion arises when I perform the back calculation to determine the concentration in the original 750 mL sample. This calculation results in a concentration of 0.33336 ng/L, which is puzzlingly lower than the LOD of 0.13495 ng/mL. This discrepancy is quite baffling, as the concentration in the reconstituted solvent indicates a detectable level, but the back calculation suggests otherwise.
Thank you for your time and support in helping me resolve this issue.
I'm not sure that you are carrying your units through the calculations consistently. If I take the concentration that you listed in the reconstituted solution of 1.351 ng/ml and multiple that by the final volume FV of 0.185 ml (185 ul) then I get a mass of 0.249935 ng. That means the original solvent solution had a concentration of 0.249935 ng divided by 0.750 L (750 ml) = 0.33325 ng/L. If your LOD is 0.03 ng/ml, convert this to ng/L by multiplying by 0.185 ml and then dividing that product by 0.750 L. That calculation results in an LOD of 0.0074 ng/L.
I was not sure of your results or calculations, but they seem OK.
If the 185 µL of solution results from an extraction of the 750 mL sample, the concentration of the analyte is multiplied by 750/0.185 = 4054 by the extraction process. If the extracted sample concentration is found to be 1.351 ng/mL, the initial 750 mL sample concentration was 1.351/4054 = 0.000333 ng/mL, quite close to your result. I don't see a problem at this step, but I don't understand the origin of the new LOD of 0.13495 ng/mL. There is no reason to have two LOD values for a given method. The only answer I can give is that the extraction process is indeed applied to get a much higher analyte concentration, well above the LOD. A direct analysis of the initial 750 mL sample is impossible because its concentration is lower than the LOD.
I don't think that the "calculated LOD" by Charles is correct. A method should have a unique LOD. 0.0074 ng/L is extraordinary low.....
Hello,
I'm not sure understanding correctly the values (as I use " ," before decimal and so I assume that 1.351ng/mL means 1351 ng/ml or 1,351µg/mL.
If that is correct, here are my calculation:
In your sample of 0,185 mL (185µL) you have:
(1351 ng*0,185mL)/1mL quantity of your compound (which is 249,93ng)
For me, this quantity is as well in your 750mL sample.
Hence, the concentration would be:
1000mL*[(1351ng*0,185mL)/1mL]/750mL=333ng
So, the concentration is 333ng/L
I think this should be in accordance with your expectation (there is just 1000 factor missing in your calculation).
Do not hesitate to put unity measures in all your calculation.
If my initial hypothesis is not correct, forget my answer!
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