Soil testing lab, food and environmental laboratories: the sampling decides the result

Getting a usable result

1. Design the sampling plan first. Number of increments, location, depth or point of collection, and how they are composited. For heterogeneous material this dominates the uncertainty and it is decided before anyone contacts a laboratory.
2. Preserve and transport to the method's requirement. Container type, temperature, preservative and holding time are specified by the method. Exceeding a holding time invalidates the result regardless of how well the analysis was run.
3. Name the method and check the scope. Different standards give different numbers for the same property. State the method and confirm it appears on the laboratory's published accreditation schedule.
4. Ask about matrix recovery. Detection limits assume a recovery from a matrix. Ask for recovery data on a matrix like yours rather than accepting a figure generated on a clean one.
5. Establish chain of custody where it matters. For forensic, regulatory and dispute related work the custody record is part of the evidence. Agree the documentation before the first sample is taken.
6. Specify the report contents. Method, sample description and condition on receipt, holding times, result with uncertainty, any deviations and a signature. Agree it in advance rather than discovering an omission when it is needed.

Sampling dominates the uncertainty

For heterogeneous matrices, the variation between samples taken metres apart routinely exceeds the analytical uncertainty by a large factor. Improving the analysis while sampling one point does not improve the answer.

Spend the effort on increments and compositing. It is cheaper than a better method and it changes the result more.

Accreditation is method by method

A laboratory is accredited for named methods on named matrices, published in a schedule. Reading it takes minutes and prevents a report that cannot be used for its purpose.

Where your method is outside the scope, the work may still be good and the report will say something different. Know which you are buying before the samples go.

An asbestos testing lab, and what the analysis is

Asbestos identification is a microscopy method, not a chemical one: bulk samples are examined by polarised light microscopy with dispersion staining against reference fibres, and air samples are counted by phase contrast microscopy on a filter, with electron microscopy where fibre type or very low concentrations matter. Accreditation is per method and the analyst's own proficiency is part of it. Ask which methods the laboratory holds, what the reporting limit is for air, and how a sample that cannot be identified by light microscopy is escalated.

Third party lab testing, and what to agree first

Sending work out works when four things are agreed in writing: the method or standard, the deliverable and its reporting limits, the sample handling from collection to receipt, and who owns the data and the raw records. Accreditation scope matters more than the laboratory's size, and a specimen report tells you more than a capability list. Ask about turnaround from receipt rather than from dispatch, and about the policy when a result is out of specification, since that is when the relationship is tested.

food testing laboratories and what accreditation covers

food testing laboratories are chosen by the scope of their accreditation rather than their equipment, because a method outside that scope produces a report a buyer can reject. Turnaround and sampling support matter commercially, and for a contaminant with a regulated limit the reported limit of quantification per matrix is what decides whether the result is usable.

environmental lab equipment and the matrices it must handle

environmental lab equipment is chosen by matrix as much as by analyte, because water, soil, air and waste each need their own preparation, and the digestion and extraction systems are where the capital and the labour go. The regulatory method names the technique, so the instrument follows the method rather than a preference.

A milk testing laboratory and the panel it runs

A milk testing laboratory measures composition, somatic cell count, bacterial count, antibiotic residues and adulterants, and the methods are prescribed by regulation and by payment schemes, which is why the laboratory's accreditation scope rather than its instruments decides whether a result is accepted. Sampling and chilling before analysis change the microbiology more than the method does.

food analyzers and the split in the market

food analyzers divide between dedicated instruments that report one parameter to a prescribed method, such as fat or nitrogen, and general analytical instruments used with a validated method. Dedicated instruments are faster and defensible against the method they implement; general ones are flexible and need validation per matrix, which is the real cost.

Common questions

Why do soil testing labs' results vary so much between samples?

Heterogeneity, almost always. Soil, food and environmental matrices vary over short distances, and a single grab sample can differ substantially from the average. A proper increment plan is the remedy.

Does holding time really invalidate a result?

For many analytes, yes, because they degrade or transform. Methods specify holding times for that reason, and exceeding one makes the number unusable for a regulatory purpose.

What should I check about a local laboratory?

That its accreditation schedule includes your method and matrix, what it subcontracts, and its turnaround to a signed report. Proximity matters mainly because it shortens holding time exposure.

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