Sulphated ash includes the volatile metals that may be lost during the normal ashing process. Hence it is used to look for inorganic compounds that are not directly detected by other methods, especially toxic heavy metals.
Ordinary ashing procedure may not be quantitative when such metals are presence, hence the addition of conc sulphuric acid before/during charring to form the non-volatile metal sulphates.
It was a simple, standard method for providing an indication of the metals that may be present, from an era when alternative instrumental techniques were not readily available.
It's usually done as part of the initial release specification for most drug candidates, especially if there are likely to be processing impurities ( metallic catalysts ( eg Ni ) or dissolved metal from vessels ).
Once it's shown there aren't such impurities present, it's usually dropped from the specification. Also these days, there are other techniques, such as ICP-MS, ICP-OES etc, that use much smaller quantities of sample.
Edit - I should add that if the sulphated ash value is significant, then it may be that your standard is in the form of a metal salt, and you should confirm that, and allow for that information in any calculations.
Generally, unless otherwise specified, whether you include the ash result in the calculations depends on it's significance. If it's low, it's usually ignored, if significant then it's included - once you understand why the sample has a high sulphated ash value.
Bruce Hamilton
