Understanding Purity and Assay Numbers on a CoA

You open a certificate of analysis from your citric acid supplier and see "Assay: 99.5-100.5%" sitting next to "Purity: 99.8%" on the same page, and it is tempting to treat those two numbers as the same thing wearing different labels. They are related, but they are not identical, and confusing them can quietly throw off your formula math without you noticing until something looks off in the finished product.

Assay is a measured test result, purity is often the specification

Assay is the actual result of a specific analytical test run on that particular lot, typically titration, HPLC, or a similar quantitative method depending on the substance. It tells you: this lot, tested this way, on this date, came back at this percentage of active substance.

Purity, as it appears on a CoA, is often the specification range the supplier commits to for the material generally, the acceptable band a lot has to fall within to pass. Sometimes a CoA reports both because the assay result and the purity spec answer slightly different questions: what did we measure versus what did we promise.

The distinction matters because assay results real lots to test against a purity spec. A lot with an assay of 99.5% against a purity spec of 98.0% to 102.0% passes comfortably. The same 99.5% assay against a tighter spec of 99.0% to 101.0% is still fine, but with much less room. If you are formulating close to a regulatory limit, knowing which number is the live test result and which is the acceptance range tells you how much variability to expect lot to lot.

Why this matters for your finished product concentration

If your CoA says your preservative's active content is 99% rather than the 100% you assumed, and you are dosing that preservative right at a restricted maximum, that 1% gap is not nothing. Multiply it across a raw material that makes up a meaningful share of your formula and the error compounds. This is the same logic that applies to supplier blends generally: you cannot just read a use percentage off a label, you have to know the actual concentration of the substance you care about inside whatever you are adding.

Reading a CoA like a formulator, not just filing it

A few fields worth actually reading rather than skimming past:

• Assay/Purity: the active content, as covered above
• Appearance: physical description, useful for catching a wrong shipment before you even open the drum
• Loss on drying / moisture content: matters for powders, since water content affects your real solids percentage
• Heavy metals, microbial limits: safety-relevant specs, especially for natural or botanical raw materials
• Batch/lot number: ties the CoA to the specific material you have in hand, not a generic spec sheet

CoA versus SDS, a common mix-up

Worth separating clearly: a certificate of analysis is not a safety data sheet. An SDS has 16 standard sections, with section 3 listing composition, and it exists for handling and hazard communication. A CoA exists to confirm a specific lot meets a specific quality specification. You need both documents for a serious raw material, and they serve different purposes, so don't treat one as a substitute for the other when a supplier only sends you one and you need the other for your records.

What to do when the numbers do not match your expectation

If an assay result comes in outside what you expected, do not just average it away or assume it is close enough. Recalculate your formula's actual active percentage using the real assay value, not the nominal 100% many people default to when a specific number is not top of mind. For anything sitting near a Hotlist restriction, that recalculation is not optional, it is the difference between a compliant filing and one built on an assumption.

Cosmetic Comply's ingredient screening works from the concentrations you actually enter, so feeding it the real assay-adjusted percentage rather than a rounded assumption gives you a screening result that reflects your actual formula, not an idealized version of it.