01Start with the quantity the method actually measures

USP's public preview of General Chapter 731 defines loss on drying in terms of volatile matter driven off under the specified conditions. This is broader than water determination. The definition makes the conditions part of the result.USP — General Chapter 731: Loss on Drying (opens in a new tab)

Time, temperature, pressure and the endpoint criterion therefore belong with the number. Two results labelled loss on drying may not be directly comparable if they were generated under materially different conditions.

For a peptide powder, the analytical question might be whether a controlled preparation meets a defined mass-loss specification. That can be a legitimate question without claiming that the test independently identifies every volatile constituent.

02Subtract the container before calculating the percentage

Consider an original weighing example. An empty container weighs 15.4200 g. Container plus sample before drying weighs 15.5200 g, and container plus sample afterwards weighs 15.5160 g. The original sample mass is 0.1000 g, and the loss is 0.0040 g.

The calculation is 100 × 0.0040/0.1000 = 4.00%. Dividing by the container-plus-sample mass would be incorrect because most of that mass is the container. Dividing by the final sample mass would use a different reporting basis.

The displayed precision is illustrative. A real result needs suitable weighing performance and a justified endpoint. Four decimal places on a balance do not establish the uncertainty of the entire drying measurement.

03Do not identify the lost material from its weight alone

Mettler Toledo distinguishes total gravimetric loss from water-specific chemical measurement. Its analytical guidance notes that the loss can involve water, other volatile substances or decomposition products, with further analysis needed to distinguish them.Mettler Toledo — Moisture and water measurement methods (opens in a new tab)

In a simplified original comparison, one 100 mg sample loses 3 mg water and 1 mg of another volatile constituent. Another loses 1 mg water and 3 mg of that constituent. Both lose 4% by mass, even though their water contents differ.

This example assumes complete loss of those stated constituents and no other changes. Its purpose is to show that one total can arise from different compositions, not to predict a real drying process.

The same logic applies to the residue: 96 mg remaining after drying does not establish 96 mg peptide. Non-volatile counterions and other material can remain. A peptide-specific amount measurement asks a separate question.

04Investigate disagreement without forcing the numbers to match

If loss on drying and Karl Fischer results differ, first compare their measurands, sample bases and preparation conditions. They are not required to agree when one measures a broader mass loss than the other.

For example, a hypothetical 4% drying loss and 3% water result could justify investigating non-water volatiles. The difference alone does not identify the missing 1%, because recovery, preparation and uncertainty may also contribute.

An appropriate conclusion states the drying result under its actual method and uses separate measurements to investigate composition. Do not subtract two uncertain results and label the remainder a named solvent without selective evidence.

The cited USP page is a public chapter preview used for its definition. It is not a complete method or evidence that a particular peptide has been tested against an applicable current monograph.