[Fun8420!]

Hello, we have a lab discussion about how to properly calculate a specific concentration, basically getting two different results. If i filter 5000 ml of a solution ("solution S") containing one specific compound ("compound A") onto one filter. Then I extract compound A from this one filter and re-dissolve the dry extract into 0.1 ml solvent. The final HPLC analysis including a prepared standard row now tells me I have 12.9 nM compound A in my 0.1 ml extract. How much is the concentration of "compound A" in the 5000 ml original solution S and how much compound A would i find in general per liter of solution S? The results we get would either be 0.000258 nM or 2.58 nM. Can anybody help with the correct results and calculation?

[DrCMS]

If compound A is dissolved to make solution S then you should not removed anything when filtering that solution.

If you do not have a solution but rather a suspension of A; then yes A can be filtered out. 
However if A has some solubility in S then only some of compound A will be filtered out.

With only the information you have given here there is no way anyone can answer your question.

[Hunter2]

Beside the specific problems with HPLC, the calculation is a simple dilution

The correct answer is 0.000258 nM.
First nM is Nanomole per liter

You got an result of 12.9 nM means 12.9 * 10^(-9) mol/l = 12.9 * 10^(-9) mmol/ml.
The volume of the sample is 0.1 ml
So your extract is then 12.9 * 10^(-9) mmol/ml* 0.1 ml = 12.9 *10^(-10) mmol.
This extract was obtained from 5000 ml means the concentration was 12.9 *10^(-10) mmol / 5000 ml = 2,58 *10^(-13) mmol/ml = 2,58 *10^(-13) mol/l
Convert in nM means divided by 10(-9) gives you 0.000258 nM