Assay, Water Content and Residual Solvents

Start with the definition beside each result

The word “assay” needs a method-specific definition. A result may describe the named compound against a calibrated reference, or a different content measurement. For example, Bachem uses “assay” for nitrogen-derived peptide content in some documentation. That result includes peptidic impurities; it is not automatically a target-specific assay. [A2]

If the report simply says “assay: 99%”, ask which of these meanings applies. Keep the original wording in your record until it is clarified. Identity, purity and assay explains the different analytical questions; net peptide content covers total peptidic material.

Water and solvents need their own methods

Karl Fischer titration measures water. Loss on drying measures a loss of mass under specified conditions and should not be assumed to be a water-only result. ICH Q6A distinguishes the two and prefers water-specific detection where relevant. This is an analytical distinction, not a claim that a research product meets pharmaceutical requirements. [A3]

Headspace gas chromatography samples the gas above a material to analyse volatile components. Its response depends on how the target compounds partition between the sample and that gas phase. The method and sample conditions therefore matter. [A5]

Read a solvent result as a result for the listed analyte. “Not detected” needs a stated detection limit; “below the quantitation limit” is not a measured zero. A blank cell may mean no result was reported. [A4] Ask the issuer which interpretation applies instead of converting all three statements to zero. The testing-methods overview provides context for the available analytical approaches.

A change of basis changes the percentage

Consider an invented 100.0 mg powder sample. A defined target-specific assay reports 90.0% w/w on an as-received basis, and a water-specific test reports 5.0% w/w on that same basis. Assume, for this arithmetic exercise, that “dry basis” excludes only the measured water.

The 90.0% and 94.7% values describe the same invented amount using different denominators. The calculation has not made the sample purer. In a real report, use the laboratory's definition: a correction may exclude water, solvents, counter-ions or a specified combination. Do not apply a second correction to an already corrected result.

Neither result establishes the contents of an individual container unless the sampling and fill records support that connection. A bulk-sample percentage and a nominal catalogue mass are separate records.

Do not force unrelated numbers to add to 100

Before attempting a mass balance, confirm that every term uses the same basis, refers to the same material and counts a distinct component. Adding a dry-basis assay to as-received water content mixes denominators. Adding HPLC area percentage to a mass percentage mixes measurements.

Even on a shared basis, an unexplained remainder is only a remainder. It does not identify a particular impurity. Missing components, overlapping definitions and measurement uncertainty can all matter. Salt forms and counter-ions explains another contribution to the supplied mass.

For an invented solvent result of 500 µg/g, the mass fraction is 0.050% w/w: 500 divided by 1,000,000, then multiplied by 100. This conversion is valid only when the units are mass per mass. A bare “ppm” label needs its basis confirmed.

Record the unresolved question precisely

1. Copy the result, units, qualifier and method reference without rewriting them.
2. Record the stated mass basis and any water, solvent or salt correction.
3. Ask for missing analytes, reporting limits or calculation definitions.
4. Compare only compatible results with the applicable specification.

Use the product-specification page to frame that request. The purity-claim explanation addresses the separate public purity statement. If the records do not define a result, retain it as unresolved in your quality documentation; the number alone cannot settle its meaning.