It is a familiar story in process development. A small sample of an intermediate or excipient is tested, the results are excellent and a bulk order follows. Then the bulk material filters slowly, dissolves differently or shows a new peak in the chromatogram. Nobody did anything wrong on paper, because the bulk lot met the specification. The specification simply did not cover what changed.
Why bulk lots can differ from samples
The sample came from a different lot
Samples are usually drawn from whatever stock is available. The bulk order may be made or sourced weeks later, under different conditions or by a different route.
Scale changes the process
Reactions, crystallisations and drying behave differently at kilogram scale than at gram scale. Slower cooling can change crystal size, longer drying can change water content, and different equipment can introduce different trace metals.
Different raw material sources
A producer may switch starting material suppliers between lots. The product still meets specification, but its trace impurity profile shifts.
The specification is narrower than reality
Most specifications cover identity, assay, a few named impurities and water. Particle size, bulk density, polymorphic form, colour and minor unnamed impurities are often unspecified, yet they can dominate downstream behaviour.
Properties that often drift
- Impurity profile: new or larger unspecified peaks, including isomers and residual starting materials.
- Particle size and morphology: affects dissolution, filtration, blending and flow.
- Polymorphic form: affects solubility, stability and sometimes regulatory status.
- Water and residual solvents: affect assay, stability and reactions.
- Colour and appearance: often harmless, but can signal trace oxidation or metal contamination.
- Trace metals: matter for catalytic steps and for elemental impurity limits in pharmaceuticals.
How to protect yourself
1. Ask for a sample from the bulk lot
The simplest step is the most effective. Ask whether the sample can be drawn from the lot that will be supplied, or request a pre-shipment sample from the bulk lot before dispatch. Approve the lot, not just the product.
2. Reserve the lot
For materials with long qualification cycles, ask the supplier to hold enough of the approved lot for your expected demand. This is common for reference standards, excipients and critical reagents.
3. Extend the specification where it matters
If your process depends on particle size, add a particle size limit. If a particular isomer causes trouble, name it with a limit. Agree these additions before ordering, because the supplier may need to test or select lots specifically.
4. Compare certificates across lots
Ask for the certificate of the bulk lot before dispatch and compare it line by line with the sample certificate. Differences within specification can still be large enough to matter.
5. Keep retain samples
Keep a portion of the approved sample and of each bulk lot. If a problem appears later, side-by-side testing is far more informative than comparing certificates.
6. Run a use test
For critical materials, a small-scale run of your actual process with the bulk lot, before committing the whole delivery, often reveals issues no certificate would show.
Be realistic about timing as well. Approving a pre-shipment sample takes days, and testing a full use run can take weeks. Build that time into the purchase plan so that lot approval does not become the reason a project slips.
Change notification
For repeat supply, agree how the supplier will inform you of changes in route, site or key raw materials. A formal change control agreement is typical in regulated supply chains. For research supply, even an informal commitment to notify you of process changes is valuable.
What to write in the order
- Reference the approved sample lot number.
- Request that the supply lot is identical, or that a pre-shipment sample is sent for approval.
- Attach any additional specifications agreed beyond the standard certificate.
- Ask for the certificate of the supply lot before dispatch.
- State the number of lots acceptable within one delivery, ideally one.
A short case to illustrate
A formulation team approves a 100 g sample of an excipient grade inorganic salt. The bulk order of 200 kg arrives with a certificate that meets every listed test. During blending, however, the powder segregates and content uniformity fails.
Investigation shows that the sample was a finely milled lot, while the bulk material came from a coarser production run. Particle size was not in the specification, so neither the supplier nor the buyer had checked it. The fix was simple once identified: add a particle size distribution limit, ask for it on every certificate and approve future lots against that limit.
The lesson is that the properties your process depends on should be written down, even if no pharmacopoeial monograph requires them. A specification only protects what it lists.
Building a consistency file
For each critical material, keep a simple file that grows over time: the approved sample lot, each supply lot, their certificates, your own test results and any process observations. After four or five lots you will see the real range of variation, which helps set realistic specifications and spot an unusual lot before it causes trouble.
When variation is acceptable
Not every material needs this level of control. Common solvents and bulk reagents from a reputable producer are usually consistent enough that standard certificates suffice. Focus the effort on materials where a small change has a large effect: catalysts, chiral reagents, key intermediates, excipients and reference standards.
Zemixal supplies samples and bulk quantities across its Pharmaceutical Intermediates, Specialty Chemicals and other divisions, with a certificate of analysis for every lot. When you request a sample with a view to bulk supply, say so in your RFQ, so lot selection and reservation can be discussed from the start.