amateur23 wrote:
One more thing:
I have found a procedure of extraction of a drug from a biological matrix where it says:
Cal 2: 50 (ng/mL)-> 100 uL of 1 ug/mL standard
Cal 3: 100 -> 200 uL of 1 ug/mL standard
Volume of sample: 2 mL
That means that when I spike my solutions with 1 ug/mL STD, the concentration of drug is not 1 ug/mL (in the matrix). It is (1*100)/2000 = 50 ng/mL
Is this a correct approach to prepare drug standards in biological matrices?
1ug/ml is equal to 1ng/ul. If you spike 100ul into your 500ul matrix you now have 100ng in 600ul total volume which would be equal to 166.6ng/1ml.
If you make your spiking solution 10x more concentrated, then you have 10ng/ul and you can spike 10ul into the 500ul of matrix which gives you 100ng/510ul which would be 196ng/1ml.
If you make your spiking solution 100x more concentrated, then you have 100ng/ul and you can spike 1ul into the 500ul of matrix which gives you 100ng/501ul which would be 199.6ng/1ml.
When you inject onto the instrument, it does not read total mass per total amount of matrix, it reads mass per volume injected. The only way to make it work without having to calculate exact concentrations for your standards in you matrix would be to inject the full amount of matrix plus standard in to the instrument in a single injection. You also have to take into account what effects on instrument response are caused by injecting matrix alone, or matrix plus methanol from the standard. By adding less of a more concentrated spiking solution you introduce less effect of the mixed matrix.