From HPLC data, assay value is getting 99.5% and we can consider this value is “as is” value. Here water content is 2%. So anhydrous basis will be 101.5%.
Theoretically for this material, if I take weight 100 mg, we will get 98 mg API as 2% water is present. If I calculate theoretically, “as is” potency is 98% (100 mg – 2 mg= 98 mg) but practical value is 99.5%. On the other hand, if we consider assay on anhydrous basis is 100% instead of 101.5% then it will match theoretical value.
Is this regard, which value on “as is basis” is correct?
The correct “as is” assay is 99.5%, as it reflects the actual API content in the received sample without moisture correction. Theoretical 98% based on water content is not used for reporting. Anhydrous basis (101.5%) applies only if specified in the monograph.
How did you get 99.5% assay if you have 2% water? Your weight adjustment on your assay samples (I assume a minimum of triplicate) should have come back closer to 97.5-98.0%. Surely you're not reporting "chromatographic purity" as assay, correct? An actual assay uses reference materials, calibration curves and multiple replicates to determine the actual purity of the material, and water will most assuredly factor into this.
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