I try to prepare spectinomycin sulfate 5120 ug/ml in a stock solution for 5 ml, but I'm stuck with the problem that I don't know how to calculate the potency if the antibiotic contains salt as in this case "sulfate". The formula provided by CLSI to calculate the potency is shown as: Potency = (Assay purity) * (Active fraction) * (1-Water Content).
Now the manufacturer gives me the certified purity: 84.6% (Mass Balance/as is basis, as spectinomycin sulfate), so everything can be filled in the formula except for active fraction because if antibiotic is free base or acid they can be represented by 1 (100%), but for the spectinomycin sulfate it contains sulfate as a salt.
I don't know what active fraction in this case should be. Please suggest me how to calculate it. The MW of spectinomycinsulfate is 502.5.
Thanks for your help.
Jay
I am not sure, I understand the problem correctly. You name a quite specific concentration of 5120 ug/ml of streptomycin sulfate. So you already know you need this specific concentration and that this is sufficient potent for you?
If this is the case I would do he calculation as following:
5120 ug/ml streptomycin sulfate in 5 ml would be 25.6 mg streptomycin sulfate. But since the purity is 84.6%, you need to use 30.47 mg (25.6 mg/0.846) to have a sufficient amount of pure compound.
If the concentration you need would be 5120 ug/ml of streptomycin you calculate it as following:
5120 ug/ml for 5 mL = 25.6 mg streptomycin. 25.6 mg/(molecular weight of streptomycin) = mol of streptomycin
this multiplied by molecular weight of streptomycin sulfate = mg of streptomycin sulfate as pure compound. This divided by 0.846 to take the impurities into consideration = mg you need to dissolve in 5 ml.
I didnt calculate it with numbers, because you can find several streptomycin sulfates (but the molecular weight you posted doesnt seem to be correct, since streptomycin itself has a molecular weight of 581.6 g/mol)
I hope I could help you with this answer
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