Potency, Free Base and Lot Purity - (Aug/19/2010 )

Hi Everybody,

we have a problem concerning solutions. on our chemicals are several informations, such as molecular weights and water content and lot purity. there are also informations like potency. whats that? we like to know now what the mass/amount of free base in our dilutions is. how should we calculate this? is there a kind of "official calculation"?

Tobramycin sulfate salt as an example:

Molecular Weight: 467.51 (free base basis)
Weight Ratio (base/salt): 0.656
Water content: 2%
Sulfate content: 2.5mol/mol
Purity: 98%
Potency: 712 ug/mg

To repeat: We don't understand whats included in the value of potency. Is it the amount of pure free base per milligramm? Or is it the amount of salt per milligram? Or something else?

Thanks in advance,
Martin

Look into this PDF
http://www.sinoapi.com/Pharmacopoeia/usp30/USP79645-27-5.pdf