How to verify purity in a batch of agrochemical technical material

A supplier’s certificate of analysis is a starting point—not proof that every batch meets your purity and safety requirements. Verifying purity in agrochemical technical material requires representative sampling, suitable analytical methods, and assessment of both active ingredient content and relevant impurities against approved specifications.

For quality control and safety teams, the difficult question is rarely whether a laboratory can produce a purity percentage. It is whether that result represents the delivered batch, measures the right chemical entity, and supports a defensible release decision. A high assay can coexist with an unacceptable impurity, excessive water, or an unapproved manufacturing-source change.

Define what “purity” means before ordering tests

Technical material is the manufactured active substance intended for formulation, not the finished pesticide product. Technical-grade active ingredient, commonly designated TC, should also be distinguished from a technical concentrate, or TK, which may contain intended solvents or diluents. Applying a TC specification to a TK can produce a misleading conclusion.

For an incoming batch, purity verification should answer four separate questions:

• Identity: Is the material the specified active substance, including the required salt, ester, or stereochemical form?
• Content: Does the active ingredient meet its specified concentration on the stated reporting basis?
• Impurities: Are relevant impurities within their individual limits, and is the broader impurity profile consistent with the approved source?
• Condition: Have moisture, degradation, contamination, or physical changes made the batch unsuitable?

“Relevant impurity” has a specific significance in pesticide specifications: it concerns impurities with toxicological, environmental, or other consequential effects, rather than simply the largest chromatographic peaks. A small peak may matter more than a much larger, less hazardous process by-product.

Build the acceptance criteria around the registered material

Before sampling, assemble the current approved specification, purchase specification, supplier certificate of analysis, safety data sheet, manufacturing-source information, and applicable analytical methods. Check that the batch number, manufacturing date, container identification, and declared source agree across the records.

The controlling requirements depend on the destination market, pesticide registration, and approved manufacturing source. FAO/WHO pesticide specifications and associated evaluation reports can provide important technical references, while CIPAC methods may provide recognized analytical procedures. Neither should be treated as a universal substitute for the applicable registration requirements.

A published FAO specification does not automatically establish that material from every manufacturer is equivalent. A different synthesis route or manufacturing site can introduce a different impurity profile even when the active ingredient assay looks identical. Source equivalence and batch conformity are related, but they are not the same assessment.

Resolve discrepancies before the laboratory starts. A contract that says “98% purity” without defining the method, reporting basis, and impurity requirements leaves too much room for disagreement.

Sample the batch—not just the easiest drum to open

Sampling can introduce more uncertainty than the instrument. Powders may segregate during transport, liquids may stratify or crystallize, and moisture ingress may affect only damaged containers. One convenient grab sample cannot reliably exclude these problems.

Use a documented sampling plan based on batch size, container configuration, expected homogeneity, supplier history, and material hazards. Define how containers are selected, where increments are taken, how much material is collected, and whether individual samples or a composite will be tested. There is no single container-count formula suitable for every agrochemical shipment.

Composite samples can estimate average batch composition, but they may dilute a localized contamination problem. Where container damage, inconsistent appearance, or segregation is suspected, retain and test individual samples rather than relying solely on a pooled result.

Sampling also requires an exposure-control plan. Consult the safety data sheet and site risk assessment for containment, ventilation, compatible tools, personal protective equipment, and waste handling. Avoid open sampling where toxic dust or vapor could escape, and do not homogenize reactive or unstable material without an approved procedure.

Record the sampling locations, container condition, sampler, date, and environmental conditions where relevant. Seal laboratory and retention samples in compatible containers, protect them from light or moisture as necessary, and maintain chain-of-custody records. These details become essential if buyer and supplier results disagree.

Choose analytical tests that answer different questions

Confirm identity independently of the supplier’s label

Identity testing may use infrared spectroscopy, chromatographic comparison with a qualified reference standard, mass spectrometry, or another validated technique. Matching retention time alone can be insufficient when closely related compounds co-elute. Where substitution or interference is plausible, an orthogonal technique provides stronger evidence.

For stereoisomer-sensitive active ingredients, an ordinary achiral assay may report acceptable total content while missing an incorrect isomer ratio. Use stereoselective analysis when required by the specification or when the manufacturing process creates a meaningful isomer risk.

Measure active ingredient content with calibrated quantitation

HPLC is frequently appropriate for nonvolatile or thermally sensitive substances; GC can suit compounds that are sufficiently volatile and thermally stable under the method conditions. Other materials may require titration or a different established assay. Selection should follow the analyte’s properties and the applicable specification, not instrument availability alone.

Chromatographic area percentage is not automatically mass purity. Different substances can have different detector responses, some impurities may not be detected, and water or inorganic residues may be invisible to the chosen system. A chromatogram showing 99% main-peak area does not, by itself, establish 99% active ingredient by mass.

Use the prescribed calibration approach with a qualified reference standard. Account for its assigned potency, sample mass, dilution factors, and any required chemical-form conversion. Reference-standard storage, expiry, and traceability matter because an incorrect standard value can bias every batch result.

Target impurities according to process and degradation risks

Build the impurity panel from the approved specification, manufacturing chemistry, source characterization, and known degradation pathways. Depending on the material, testing may include residual starting materials, reaction by-products, hazardous trace impurities, residual solvents, or selected elemental contaminants. Not every batch requires every possible analytical technique.

Targeted testing and broader profiling serve different purposes. A sensitive targeted method checks a known impurity against its limit. A comparative chromatographic profile can reveal unexpected peaks or shifts that targeted assays miss. An unexplained new peak should trigger investigation, even when routine assay results pass.

Measure water where it is specified or affects interpretation. Karl Fischer analysis can be suitable, provided the material does not interfere with the selected procedure. Loss on drying is not interchangeable with water determination: it may also measure solvent loss or thermal decomposition.

Make sure the method works in this material

A recognized method still needs documented implementation checks in the receiving laboratory. An in-house, modified, or transferred method requires appropriate validation or verification for its intended use. Particular attention is needed when the supplier changes synthesis route, because new impurities may interfere with an established separation.

• Selectivity: Demonstrate that the active ingredient and specified impurities can be measured without meaningful interference.
• Accuracy and recovery: Check performance in the actual matrix, not only in clean solvent.
• Precision: Evaluate independent sample preparations; repeated injections alone do not assess preparation variability.
• Range and quantitation capability: Cover the assay range and quantify impurities sufficiently below their acceptance limits.
• Robustness and solution stability: Establish whether routine operating variation or storage before analysis changes the result.

Each analytical sequence should include the controls required by the method, such as blanks, calibration checks, system-suitability tests, and appropriate quality-control samples. Carryover and incomplete dissolution deserve particular attention: both can create apparently convincing but incorrect results.

An ISO/IEC 17025-accredited laboratory can provide useful assurance, but check the scope of accreditation. Accreditation does not mean every pesticide, matrix, and method offered by that laboratory is covered.

Read the results as a chemical profile, not a single number

Compare each result with its own criterion and reporting basis. An assay reported on a dry basis should not be compared directly with an as-received minimum. Likewise, an impurity expressed relative to active ingredient cannot be compared directly with a limit expressed per kilogram of technical material without the appropriate conversion.

For illustration, consider a specification requiring at least 980 g/kg active ingredient and no more than 1.0 g/kg of a relevant impurity. A batch containing 985 g/kg active ingredient but 1.2 g/kg of that impurity does not meet the specification. Its higher assay does not compensate for the impurity failure. These figures are illustrative, not universal pesticide limits.

Near-limit results need the laboratory’s established decision rule, including how measurement uncertainty is handled where applicable. Agree this rule before testing rather than choosing a favorable interpretation afterward. Do not round results selectively to turn a failure into a pass.

Also compare the batch with historical data. A result can remain within specification while showing a significant shift from the established process profile. Such a shift may justify additional testing, supplier clarification, or increased monitoring, even when it does not itself establish nonconformity.

Release, hold, or investigate—with an auditable reason

A defensible release package links the received material to the samples, methods, raw data, calculations, specifications, and authorized review. It should demonstrate identity, acceptable active ingredient content, compliance with relevant impurity limits, and resolution of any significant anomalies.

If a result is out of specification, place the affected material on hold and investigate under a written procedure. Review calculations, standard preparation, instrument performance, sample preparation, and sampling integrity. Retesting should address a scientifically justified question—not continue until a passing value appears. Do not average away a failing result without a valid, documented basis.

When supplier and buyer results differ, examine methods, reference-standard potency, reporting bases, sample stability, and sampling locations before assuming either laboratory is wrong. A sealed retention sample and an agreed independent laboratory can help resolve the dispute.

The practical release question is therefore broader than “Does the certificate say 98%?” It is: Does representative, traceable evidence show that this batch of agrochemical technical material meets the approved chemical and safety requirements? That distinction protects formulation performance, worker safety, and regulatory compliance—and makes the decision easier to defend long after the shipment has arrived.