How to Read a
Certificate of Analysis
(CoA)

What Is a CoA and Why It Matters

A Certificate of Analysis (CoA) is a quality assurance document issued by a manufacturer or testing laboratory that confirms a specific batch of material has been tested against defined acceptance criteria. For anyone purchasing chemicals — whether technical-grade solvents, agrochemical active ingredients, pharmaceutical excipients, or dye intermediates — the CoA is the primary documentary evidence that what you ordered is what was actually shipped.

In practice, most procurement teams treat the CoA as a box-ticking formality: they receive it, file it, and move on. This is a mistake that costs companies in two directions. First, an unchecked CoA from a supplier operating outside specification can result in downstream product failures, regulatory non-compliance, or batch rejections at the point of use. Second, a CoA that hasn't been interrogated thoroughly may be outright fraudulent — a recycled document from a different batch or, in extreme cases, a fabricated document that never corresponds to any analytical test at all.

Understanding how to read a CoA transforms it from a piece of paper into a genuine quality gate. This guide walks through every field you should expect, what the values mean, and the red flags that should trigger further investigation before you accept a shipment.

The Standard Fields Every CoA Should Contain

While there is no single universal template mandated for CoAs in most commodity chemical trade (unlike, say, a Certificate of Conformity in regulated industries), established industry practice — and the expectations of most quality systems under ISO 9001, ICH Q7, or FDA 21 CFR Part 211 — defines a clear minimum. The table below maps each standard field to what a diligent buyer should check, and the red flags worth escalating.

Purity and Assay Values Explained

The assay (also called purity) figure is the most important number on any CoA, and it is also the most frequently misread. Assay is expressed as a percentage and represents the proportion of the stated compound in the sample — but what that percentage is measured against matters enormously.

Assay on anhydrous vs. as-is basis

A 99.5% assay result can mean very different things depending on whether it was calculated on an as-is basis (including any residual water or solvent) or an anhydrous basis (corrected for moisture). For hygroscopic materials like certain amino acids, organic salts, and amine compounds, an as-is assay of 96% might correspond to an anhydrous purity of 99.2% once moisture is accounted for. Always confirm which basis applies.

The analytical method used to generate the assay value also determines how much weight you can place on the number. High-Performance Liquid Chromatography (HPLC) with a UV or refractive index detector is the gold standard for most organic compounds because it separates and quantifies each component. A simple titration, while reliable for straightforward acid-base chemistry, will not detect structurally similar impurities that could affect your process. Gas Chromatography (GC) is preferred for volatile solvents and lower-molecular-weight compounds.

Moisture Content, Ash Residue, and Heavy Metals

Three secondary parameters that buyers frequently overlook are moisture, residue on ignition (ash), and heavy metals. Each tells a different story about the material's handling history, source, and potential downstream risk.

Moisture content (Karl Fischer or loss on drying method) is critical for any material that reacts with water, is used in moisture-sensitive formulations, or where water activity affects microbial risk. A moisture value higher than specification can indicate improper storage, container breach, or deliberate adulteration with water to make up weight. Even for materials without a strict moisture specification, unexpectedly high moisture values across multiple batches from the same supplier should prompt an audit.

Residue on ignition (ROI), also called sulphated ash, measures inorganic impurities remaining after a sample is burned and treated with sulphuric acid. High ROI values can indicate contamination with inorganic salts, catalyst residues from synthesis, or adulteration with fillers. For fine chemicals and pharmaceutical intermediates, the typical specification is ≤0.1%.

Heavy metals are reported as a total (ppm) or by individual element (lead, mercury, arsenic, cadmium) using ICP-MS or atomic absorption spectroscopy. For agrochemical intermediates and food/pharma-contact materials, this is not optional — it is a regulatory and ethical imperative. The CoA should state the limit (typically ≤10 ppm total heavy metals for technical-grade material; ≤5 ppm or elemental limits for pharmaceutical grade) and the actual result with the test method cited.

Chromatographic Data: Reading HPLC, GC, and TLC Results

Chromatographic data on a CoA comes in two forms: a summary table (listing retention times, peak areas, and percentage composition) or an attached chromatogram. Both are valuable, but the chromatogram is the more trustworthy of the two because it cannot easily be fabricated without corresponding instrumentation.

When reviewing HPLC data, look for the following: the main peak should represent the stated compound and account for the assay percentage. Any peak with an area greater than 0.1% should be identified or at minimum flagged as an unknown impurity. The related substances profile (i.e., the impurity pattern) should remain broadly consistent between batches. A sudden appearance of a new peak or a sharp increase in an impurity level compared to historical data is a process change signal worth investigating.

For GC data, pay attention to the column type and temperature programme — different GC methods will resolve different compounds. A capillary column method at high resolution will typically detect residual solvents (ethanol, methanol, acetone, DMF, DMSO) that a packed-column method may miss. For pharmaceutical-grade materials, residual solvent limits are governed by ICH Q3C, which classifies solvents into Class 1 (avoid), Class 2 (limit), and Class 3 (low risk) categories with specific ppm limits for each.

Microbiological Parameters for Pharma and Food-Grade Materials

If you are purchasing excipients, active pharmaceutical ingredients, nutraceutical raw materials, or food-grade chemicals, the CoA should include a microbiological testing section. This is absent from technical-grade CoAs but mandatory for pharmaceutical and food supply chains.

Standard microbiological parameters include Total Aerobic Microbial Count (TAMC), Total Yeast and Mould Count (TYMC), and tests for specified pathogens including Escherichia coli, Salmonella, Staphylococcus aureus, and Pseudomonas aeruginosa. Limits are set by the relevant pharmacopoeia (USP <1111>, EP 5.1.4, or BP equivalent) or by your own specification, whichever is more stringent.

For injectable-grade or ophthalmic materials, the requirements extend to sterility testing and bacterial endotoxins (LAL test, with limits typically ≤0.5 EU/mg). These tests require separate documentation and, depending on the risk profile, may require testing from an independent ISO 17025-accredited laboratory rather than the manufacturer's in-house facility.

Batch Number, Expiry, and Shelf Life Verification

The batch or lot number is the critical link between the CoA document and the physical material you received. Before doing anything else with a new shipment, verify that the batch number on every drum, bag, or container label matches the batch number on the CoA. This sounds basic, but it is one of the most commonly found discrepancies in incoming goods inspections — especially when a distributor combines product from multiple manufacturer batches into a single shipment order.

Shelf life is not just an expiry date — it is a statement about the storage conditions under which that shelf life is valid. A material stated as "shelf life 24 months from manufacture date" stored at ambient temperature and humidity may have a very different useful life if stored in an unconditioned tropical warehouse. For moisture-sensitive, oxidation-prone, or photodegradable materials, the storage conditions on the CoA (e.g., "store below 25°C in a tightly sealed container, protected from light") define the reference conditions for the stated shelf life.

When purchasing material that will be held in inventory for an extended period before use, calculate the remaining shelf life at the point of use, not at the point of purchase. If the remaining shelf life at the anticipated time of use is less than your internal minimum holding period plus your process cycle time, the batch is not fit for purpose regardless of its current specification compliance.

Grade Comparison: CoA Fields by Standard

The depth and nature of CoA testing varies significantly depending on the intended grade. Technical-grade materials have the lightest documentation burden; pharmaceutical-grade the heaviest. The following table summarises which parameters you should expect at each level.

When to Reject a CoA

When you reject a CoA, do so in writing and retain the document. If you suspect fabrication or systematic fraud rather than an honest quality failure, escalate to your quality system's supplier management process. In regulated industries, this may trigger a formal Supplier Corrective Action Request (SCAR). For commodity trade, a single OOS result may warrant re-testing by a mutually agreed independent laboratory before final acceptance or rejection — a right that should ideally be built into your supply agreement.

Chelora Chemicals issues CoAs for every batch from our own accredited QC laboratory, with full chromatographic data available on request. Our CoAs follow the format standards expected by buyers operating under ISO 9001, ICH Q7, and US FDA cGMP frameworks. For any questions about interpreting a CoA you've received from us — or evaluating one from another supplier — our technical team is available for a direct consultation.