In reading a COA (Certificate of Analysis), I came across an Assay line:

The Assay is 100.5% where "Lower Limit" is 98.5 and "Upper Limit" is 101.0

This product is L-Cysteine Hydrochloride Monohydrate. How can the Assay be over 100%?

I have found this statement: "If the substance under test is purer than the standard, the assay comes more than 100 pct." If that is the case, then the question becomes, where can I find out about this standard?

  • $\begingroup$ Can you post an excerpt from the COA? It probably won't help me figure out what's going on, but it might help someone else. $\endgroup$ Jun 6, 2013 at 1:42
  • $\begingroup$ I created a free login at ajiaminoacids.com. The include the COA under Additional Info. I don't really have (and maybe should not) post the whole thing. Is there something specific I can paraphrase? I did include the entire Assay line so you can see how they anticipate a value higher than 100%. $\endgroup$
    – jcalfee314
    Jun 6, 2013 at 10:38
  • $\begingroup$ another comment from a supplier: "we measure the % of compound based on the ions present in the compound reference to a standard. In some cases there may be other ions present in compound also detected in that test. When this result is compared to a standard it shows a higher %." $\endgroup$
    – jcalfee314
    Jun 7, 2013 at 20:07

1 Answer 1


There is a simple reason to have the purity greater than 100% for this compound.

If the substance was exposed to a dry environment for several hours, a small amount of the water of hydration could be lost, causing the calculation to have a higher purity.

Also, I believe that the purity is calculated from the perchlorate titration of the substance in acetic acid (I will need to check the USP monograph). There are many sources for error in the calculations: the weight of the L-Cysteine Hydrochloride Monohydrate, the volume of the perchlorate solution, the error from the standardization of the perchlorate solution, etc.

Under the right conditions, you can easily get 101% for the purity.


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