Fix a reference sample from an approved batch, then compare every new lot with it. Track the key results from each certificate of analysis, retest some lots yourself, and run a short performance check on arrival.
Consistency matters more with specialty chemicals than with commodities. A small shift in active content or molecular weight can change how a product works on your line, even when every figure still sits inside the specification.
Build a reference
When a trial batch passes, keep a sealed sample of it as your reference. Store it as the data sheet says. Ask the supplier to keep one too.
Record the reference batch's full results, including figures the specification does not limit, such as colour, odour and viscosity at a set temperature. These often move first when something changes.
Replace the reference only on purpose. If you approve a new grade or the supplier makes an agreed change, test the new batch fully and record it as the new reference.
Track the numbers
- Put the key results from every certificate into one spreadsheet, batch by batch.
- Plot each figure over time, so you can see trends, not just pass or fail.
- Ask the supplier to give the actual measured value, not just "complies".
- Flag any batch that sits at the edge of the specification, even if it passes.
- Compare the supplier's figures with your own retest results to check their laboratory.
Test on arrival
Retest a sample of each lot, or every few lots once trust is built, for the two or three figures that matter most. Add a quick performance test that mirrors your process, run beside the reference sample.
Draw samples correctly. A sample from the top of one drum does not represent a lot of forty drums. See drawing a chemical sample for the method.
If a lot differs from the reference, test the supplier's retained sample too. This tells you whether the product changed at the plant or in transit.
Look for figures that never change. A certificate that shows exactly the same values for every batch is a warning sign. Real measurements vary a little, and identical numbers often mean the certificate was copied, not tested.
Why batches drift
The usual causes are a change of raw material source, a new operator, a scale-up to a bigger reactor, or blending old stock into a new batch. Write a change notification clause into your contract, so the supplier must tell you before any of these happen.
Ask the supplier to show its own process control records during an audit. A plant that tracks its own trends usually catches drift before you do. The general patterns are covered in why quality drops after the first container.
Next step
We hold reference samples, check certificates against them, and have lots tested before loading. Book a free sourcing review.