Loss on drying (revision 6)
Old revision·06:56, 6 Oct 2024·PancreasPru
| Loss on drying | |
|---|---|
| Abbreviation | LOD |
| Measures | Total volatile mass lost under defined conditions |
| Contrast with | Karl Fischer titration, which measures water specifically |
| Compendial chapter | USP <731> |
| Analytical method infobox · conventions | |
Loss on drying determines the mass a sample loses when heated under defined conditions of temperature, pressure and time. It is simple, requires no specialised reagent, and is widely reported on peptide certificates.[1]
It is not a water determination. The mass lost comprises water and any other volatile component — residual acetonitrile, acetic acid, ammonia from a salt-exchange step — so a loss-on-drying figure is an upper bound on water content rather than a measurement of it. Karl Fischer titration measures water specifically and will generally give a lower number on the same material.[2]
For peptides the two are frequently confused, and a certificate reporting "water: 6.2% (LOD)" is reporting something other than water. The distinction matters because water is subtracted in calculating peptide content, and using a loss-on-drying figure for that subtraction understates the peptide.[3]
Method
[edit]A weighed sample is dried to constant mass under stated conditions — an oven at a specified temperature, or a desiccator over a specified desiccant, or under vacuum — and the mass difference expressed as a percentage of the starting mass.[1]
The conditions are part of the result. Drying at 105 °C removes more than drying at 60 °C; drying under vacuum removes more than at atmospheric pressure. A figure quoted without its conditions is not comparable with another figure quoted without its conditions.