[5kov]

Hello guys.
I need a little help on this one. Currently I'm conduction an assay determination of a pharmaceutical active ingredient. According to the European Pharmacopeia the substance I'm testing meets the assay requirements if the the sample contains not less than 99.0 and not more than the equivalent of 100.5 per cent of the active ingredient, calculated with reference to the dried substance. What does "calculated with reference to the dried substance" mean? Should I dry the sample first, or i should use undried sample, and recalculate the end result. I think that the right way to do it is with dried and cooled in an exicator sample.

[enahs]

I assume it means the pure substance is hygroscopic to some extent and that when making the standards they used the dry pure form (when massing out).

[Borek]

I was told by a friend of mine that standard procedure is to separately determine water in the sample.

[Arkcon]

There are procedures defined in the US Pharmacopeia (also in the EU Pharmacopeia, JP Pharmacopeia, and any others I've forgotten -- not trying to leave anyone out) that define the proper way to handle each pharmaceutical: you may use as-is, quantitate the moisture or dry.  The drying method may be gentle heating for a long time, vigorous heating for a short time, open to the incubator used or under vacuum.  You'll have to look up your specific pharmaceutical, in the correct regulatory book, to see what to do.

[elixir2011]

that means you take a representative sample of the ingredient you are testing. Find its loss on drying (LOD) value exactly following the process explained in the monograph of that material. The monograph will explain all the criterias ex. temperature, pressure, time etc.

to compute your assay value, use your LOD value in consideration.
For Example
Your LOD = 1.5052%
Your Sample weight = 100.4856 mg (if u are using micro balance)
Your true sample weight = 98.9731mg (use this value for your assay determination)

Hope that helps