Assurance In Action: Episode 45 Testing

Assurance In Action: Episode 45 Testing combines ELISA rapid screening with LC-MS/MS confirmation to detect chemical residues in food, feed, water, and environmental samples. The screening rules out negatives at speed; confirmation turns presumptive positives into defensible numbers. This article walks through principle, sampling, workflow, performance metrics, interpretation, and acceptance criteria as a practical checklist for residue testing.

Principle & mechanism

The screening stage uses competitive ELISA. Antibodies pre-coated on a microplate bind target molecules in the extract, and an enzyme conjugate competes for the same sites. A chromogenic substrate is added, then absorbance is read on a microplate spectrophotometer. Optical density falls as analyte concentration rises. This inverse signal supports a cutoff decision against calibrators run on the same plate. The confirmation stage relies on liquid chromatography coupled to tandem mass spectrometry. The extract is separated on a reversed-phase column, and the detector monitors compound-specific precursor-to-product ion transitions. Retention time and ion ratios identify the analyte. Peak area measured against an internal standard yields the quantitative result.

Sample types & collection

The workflow applies to many sample types: foodstuffs of plant and animal origin, compound feed, drinking and surface water, and soil. Matrices differ in water and fat content, so the extraction protocol is selected per matrix during method validation. Collection starts with a representative composite drawn from defined sampling points. Samples go into clean, inert containers with airtight seals, labeled with batch, date, and location. A cold chain at 0–4 °C, or freezing for unstable residues, preserves integrity until analysis. Holding times and storage conditions are logged, and chain-of-custody records accompany every sample. In the laboratory, the whole sample is homogenized before subsampling. This way the analytical portion reflects the batch rather than a local hotspot.

Stepwise test method workflow

Registration and review come first. The laboratory checks sample condition, assigns a laboratory number, and confirms the target analyte list with the client. The homogenized portion is weighed, then extracted with a solvent suited to the target class. Acetonitrile is common for many pesticide residues. The extract is centrifuged and filtered. A cleanup step on a solid-phase extraction cartridge removes co-extracted fats and pigments where the matrix demands it. One aliquot is transferred to the ELISA plate alongside blanks, calibrators, and controls. After incubation, washing, substrate addition, and stopping, the plate is read. Results are compared with the cutoff. Presumptive positives proceed to LC-MS/MS: matrix-matched calibration, internal standard addition, instrument sequence, and verification of ion ratios before quantification and reporting.

Sensitivity,

linear range & repeatability
Method sensitivity is expressed as the limit of detection and the limit of quantification. Both are established during validation from blank matrices fortified at low levels, and the reporting limit is set at or above the LOQ. The linear range is the calibrated interval over which response stays proportional to concentration. Laboratories verify it with matrix-matched calibration curves. They check that back-calculated calibrator concentrations stay within the validated tolerance and that residuals scatter without systematic drift. Repeatability is assessed through replicate measurements of a homogeneous material within one run. Across days, intermediate precision applies. Relative standard deviation values must fall inside the acceptance window defined by the validated method and the laboratory quality system. Control charts track reference materials over time and flag drift before it reaches client data.

Co-testable parameters & interpretation

A single LC-MS/MS run can cover an entire multi-residue panel. Clients therefore request related analyte classes together, and the cost per parameter falls. Where the workflow touches water matrices, routine physicochemical parameters such as pH can be measured alongside. Gravimetric moisture and ash determinations apply to solid food samples. Interpretation follows a fixed decision tree. A screening result below the cutoff clears the batch without further work. A presumptive positive moves to confirmation. Only a confirmed identity with matching ion ratios and retention time, above the applicable regulatory limit, is reported as non-conforming. Results between the LOQ and the limit are reported as trace-level detections. Clients use trace findings to judge trends rather than to reject a batch outright.

Acceptance

criteria & application scenarios
A reportable result must pass a defined gate. Procedural blanks stay clean. Internal standard recoveries sit inside the method window. Calibration verification standards pass before and after the sample sequence. Confirmatory ion ratios match reference values within tolerance. Replicate results must meet the repeatability limit. Any recovery check outside the validated range invalidates the affected batch for re-extraction. Typical application scenarios include incoming raw material checks, in-process control during production, finished product release, supplier audits, and investigation of a suspected contamination event. Importers and retailers commission the combined screening-plus-confirmation package when they need fast negative clearance with defensible numbers for flagged lots. Environmental and water operators apply the same logic to periodic monitoring against applicable limits.