Drug concentration in blood (calculation)

[amateur23]

Apologies for a stupid question. I need to determine the concentration of drug that is reconstituted for GC-MS. Here how the prep work goes:

50 uL of 10 ug/mL of a drug is used to spike 0.5 g of blood. So the concentration in blood is 1 ug/g. But then blood is diluted with water using 500 uL of water which means the concentration of drug is halved i.e. 0.5 ug/g. The drug is extracted and then 75 uL is used to reconstitute the analyte. I am having problem how can I go from ug/g to whatever ug or ng is in 75 uL. should I assume density of water and blood is the same? so is it 0.5 ug in 1 mL or 1000 uL ( 1 g of water = 1 mL ). Hence 0.5 ug *75/1000 = 0.0375 ug in 75 uL so the concentration is 0.0375 ug/0.075 mL = 0.5 ug/mL or 500 ng/mL

I know, I know - a stupid question but I think I am missing something here. Please, don't be too mean

[Peter Apps]

If this "The drug is extracted and then 75 uL is used to reconstitute the analyte." means that you end up with all the drug that was in the blood in 75 ul of solvent, then you already know the mass of drug, and you know the volume, so you can calculate the concentration.

Peter

[amateur23]

Thank you for your reply, Peter Apps!

That means it is simply 0.5 ug / 0.075 mL = 6.67 ug/mL

I guess I should assume that all drug ends up in 75 uL as % recovery or loss during the extraction is not known.

[Peter Apps]

I wouldn't assume anything - spike some solvent with the drug at the same concentration as you would get from a 100% extraction, and compare peak areas.

Peter

[amateur23]

You're right, it's best to actually determine that. Thank you again!