An assay percentage needs a reporting basis. As-received, anhydrous and dried-basis results can use different sample masses in the denominator. Comparing the percentages directly can therefore suggest a change in composition even when the calculation starts with the same measured target amount.
Read the basis alongside the assay name
USP’s submission guidance distinguishes dried-basis corrections using loss on drying from anhydrous-basis corrections using water content. The applicable definition and analytical procedure still determine how a particular material’s result is calculated.USP — General Information for All Submissions (opens in a new tab)
| Basis | Denominator in this article’s example |
|---|---|
| As received | Original sampled material mass |
| Anhydrous | Original mass minus measured water |
| Dried | Original mass minus measured drying loss |
Anhydrous basis is a reporting convention here. It does not establish that the supplied material is a crystalline anhydrate or that somebody physically dried the portion used for the assay.
Likewise, a dried-basis calculation must identify the drying-loss measurement used. It cannot be reconstructed reliably from a generic claim that a powder looks dry.
Recalculate a single hypothetical sample
Consider an original example with 100 mg of as-received material, 90 mg of the defined target, 5 mg of water and a measured drying loss of 7 mg. Assume the drying loss includes that water plus 2 mg of other lost material, with no target lost or transformed.
The as-received assay is 90 ÷ 100 × 100 = 90.0%. Excluding water gives 90 ÷ 95 × 100 = approximately 94.74%. Excluding the full drying loss gives 90 ÷ 93 × 100 = approximately 96.77%.
Subtracting five percentage points from 90%, or adding five to it, would not perform the water-basis conversion. The operation divides by the retained mass fraction; it is not an addition or subtraction of assay percentages.
Check whether the simplifying assumptions hold
The example deliberately separates water from total drying loss. Treating both as 5 mg would erase the distinction that produces the two corrected values. A report that gives only one measurement cannot automatically supply the other.
The conversion also assumes that the measured excluded fraction represents the material used for the assay. If separate portions differ in moisture, or acquire different exposure histories, combining their measurements can introduce an additional mismatch.
If drying removes or transforms the target, keeping the numerator unchanged is no longer a description of the physically dried material. A calculation based on an independently measured loss and a direct assay after drying are then different evidence.
Do not use these equations to convert chromatographic area percentage into mass assay. The example starts with a target mass and a material mass. Peak areas do not supply those two quantities merely because they are reported as percentages.
Put two certificates on an interpretable basis
When comparing certificates, record the assay’s chemical definition as well as its moisture basis. A peptide-equivalent result and a salt-form result may differ in the numerator; correcting water alone does not reconcile them.
For a reverse check in the example, multiplying 94.74% by the retained fraction 0.95 gives approximately 90.0% as received. Minor final-digit differences can arise from rounding, so retain the unrounded inputs when they are available.
Ask for the measured water or drying loss, its unit, the calculation equation and the relationship between test portions. If the basis is undocumented, explain that the comparison remains unresolved rather than choosing the higher percentage.
Sources and further detail
- USP — General Information for All Submissions (opens in a new tab)
The indexed official guidance section on assay-basis terminology was read: correction for loss on drying versus water content. Full PDF retrieval was unavailable. The 100 mg example and its assumptions are original.
Sources checked 19 September 2026. Worked examples are illustrative unless a supplied report is explicitly identified. This article has not undergone independent scientific peer review.