# Assay over 100% (L-Cysteine Hydrochloride Monohydrate)

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?

• 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. – Colin McFaul Jun 6 '13 at 1:42
• 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%. – jcalfee314 Jun 6 '13 at 10:38
• 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 %." – jcalfee314 Jun 7 '13 at 20:07