Calculating change (%) of concentration from peak area

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A solution of 50 mL contains compound A with unknown concentration. The peak area from GC-MS for this compound is A1.
The same solution is treated to remove compound A. After treatment the solution volume is reduced to 30 mL and compound A also reduced to some extenet. The peak area from this sample is A2.

In this case how to calculate the removal % of compound A using peak area information?

Generally, we can calculate the removal % from peak area using the formula (A1-A2)/A1 *100.

How do we calculate the removal percentage when volume is changing? Do we need to consider the change in volume when the removal % is calculated from peak area?

It seems to me that by extracting your analyte from the 50 mL solution, you are essentially diluting it. Peak area is proportional to concentration of analyte in the solvent. Your expression only works if the volumes remain the same.

Since you are concentrating the analyte again after your first extraction, you are making the "effective amount of analyte" removed larger than what was actually extracted. Thus, you are able to weight the loss more heavily. In my mind, the correct way to calculate the % difference is:

(A1 - A2*5/3)/A1 * 100
The correct answer depends critically on details that are not given in the question. The simplest scenario to remove some A and to leave 30 ml of solution is to tip 20 ml down the sink. The concentration of A in the remaining solution, and so the peak size will remain the same. Or you could treat to remove A and recover 30 ml of solution. The peak area from the recovered solution will bear the same ratio to solution concentration as the peak area from the original solution. This will also apply if you treat and recover any volume larger than 30 ml and then discard the excess, but not if you recover less than 30 ml and then dilute to 30 ml.

Peter Apps
3 posts Page 1 of 1

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