The practical answer
Use an as-received assay for an as-received mass calculation unless an approved procedure specifies a justified correction. Water determination and loss on drying are not automatically equivalent. Before converting a result, identify the excluded mass, measurement method and assumptions behind the reported basis.
Read the method as well as the number
Loss on drying measures mass driven off under specified conditions; that loss can include volatile material other than water. A water-determination method addresses water using a procedure appropriate to the material. USP describes these as distinct tests. [1] [2] A COA should therefore not relabel every drying loss as a water result.
The distinction matters when a magnesium assay is corrected to a dried or anhydrous basis. Ask whether the reported value was measured directly after preparation or calculated using another result. Record the method and correction formula so that purchasing and formulation compare the same quantity.
Use a worked reconciliation
For a hypothetical lot with 12.0% magnesium as received and 4.0% water, dividing 12.0% by 0.96 gives 12.5% on a water-free basis, if water is the only mass excluded and the correction is appropriate. Starting with 12.5% on that basis and multiplying by 0.96 returns 12.0% as received.
Do not correct the 12.0% again when calculating input from the powder as weighed. Double correction creates a formulation error. Similarly, the moisture specification maximum is not automatically the actual lot water content. Substituting a maximum for an actual result changes the meaning of the calculation.
| Reported term | Interpretation to confirm |
|---|---|
| As is or as received | Fraction in the material as tested, without a stated drying correction |
| Dry basis | Which drying procedure and excluded mass define the denominator? |
| Anhydrous basis | How was water determined and the correction performed? |
| Loss on drying | Mass lost under the stated test conditions |
| Water content | Water measured by the specified suitable method |
On smaller screens, scroll the table horizontally.
Resolve cross-laboratory differences
- Confirm that both laboratories tested material linked to the same lot and handled samples comparably.
- Compare method versions, test conditions and reporting bases before interpreting a numerical difference.
- Check whether sample exposure during storage or handling could have changed its condition.
- Ask for the original assay and correction inputs when a certificate presents only a calculated result.
- Have the responsible laboratory explain disagreements rather than averaging values that measure different quantities.
Turn the explanation into specification language
State the accepted assay basis beside the magnesium range, then identify the water or drying-loss procedure separately. If a correction is used, define when it applies. Keep actual lot data, typical values and acceptance limits in separate fields in purchasing documents and spreadsheets.
A drying or water result does not identify all the material’s chemical components and cannot establish a chelation percentage. The conversion examples here are mass-balance illustrations. They are not specifications for a MagneINNO grade, instructions to dry a commercial lot or evidence that a particular material should have a particular water level.
Sources & further reading
- USP General Chapter 731: Loss on drying, public preview
- USP General Chapter 921: Water determination, public preview