Sampling and prep

Why it matters

The most precise instrument in the world cannot rescue a sample that does not represent the material it came from. In many analyses, sampling and sample preparation contribute more uncertainty than the measurement itself. A truckload of ore, a batch of tablets, a lake, a field of soil: the analyst may test a gram, or a milliliter, and the result is supposed to describe all of it. Planning how to get from "the lot" to "the test portion" without bias is part of the analytical method, not an afterthought.

Safety and scope. Real sampling and preparation use hazardous reagents, solvents, digestion acids and sometimes hazardous materials. They are done only in a teaching or employer lab, with your instructor or supervisor and the lab's safety training. The activity uses dry kitchen foods only (rice and lentils), nothing hazardous.

From lot to test portion

1. Lot (population): everything the result must describe.
2. Gross (bulk) sample: increments taken from the lot according to a sampling plan.
3. Laboratory sample: the gross sample reduced to a size the lab can handle.
4. Test portion: the amount actually weighed or measured into the analysis.

Each reduction step must keep the composition. That requires homogenizing (grinding, mixing, blending) and dividing properly.

Representative sampling

• Random sampling: every part of the lot has an equal chance of being sampled. Avoids the "grab the top of the drum" bias.
• Systematic sampling: at regular intervals (every 10th container, every 15 minutes from a pipe). Simple, but can miss periodic variation.
• Stratified sampling: the lot is divided into known layers or groups (top, middle, bottom of a tank; each production shift) and each is sampled. Useful when the lot is known to be heterogeneous.
• Composite sample: several increments combined into one for an average result. Cheaper, but hides variation between increments.

Heterogeneity drives everything. A well-mixed solution is homogeneous, so one sample is enough. A powder blend, a soil or an ore is particulate: the more variable the particles and the larger they are, the bigger the sample needed to keep sampling error small. That is why grinding to a smaller particle size before sub-sampling reduces error.

Dividing a sample

• Coning and quartering: pile the material into a cone, flatten it, divide it into four quarters, keep two opposite quarters, and repeat.
• Riffle splitter: a box of alternating chutes that splits a stream of particles into two halves.
• Rotary divider: the most reproducible method for powders.

Scooping from the top of a jar is not dividing: fine particles settle and large ones rise when a powder is shaken.

Preservation and holding times

Samples change after collection: volatile compounds escape, microbes consume organics, metals stick to container walls. Methods specify containers (glass vs plastic), preservatives (for example acidification for dissolved metals), temperature (often around 4 degrees C) and maximum holding times. A sample analyzed after its holding time is usually reported as compromised.

Sample preparation

The goal is to get the analyte into a form the instrument can measure, free of interferences:

• Dissolution and dilution of soluble samples.
• Acid digestion (including microwave digestion) to destroy organic matter for metals analysis.
• Liquid-liquid extraction and solid-phase extraction (SPE) to isolate and concentrate an analyte.
• Filtration and centrifugation to remove particles that would block a column.
• Derivatization to make a compound volatile for GC or detectable for UV.

Quality controls in sampling and prep

• Field blank: clean matrix taken to the site and handled like a sample; detects contamination from containers and transport.
• Method blank: carried through the whole preparation; detects lab contamination.
• Matrix spike: a known amount added to a real sample; shows recovery in that matrix.
• Duplicates: two samples or two preparations; show sampling and preparation precision.

Worked example: a tablet assay

The lot is 500,000 tablets. The plan takes 20 tablets at random across the batch, weighs them, grinds them to a uniform powder, and weighs out a test portion equal to about one average tablet weight, in duplicate. The result is reported per tablet using the average tablet weight. Testing a single tablet would tell you about that tablet (content uniformity is a separate test), not the batch average.

Common mistakes

• Sampling only the most accessible part of a lot.
• Sub-sampling a powder without mixing or proper division.
• Ignoring holding times or preservation.
• Skipping blanks and spikes, so contamination and losses go unseen.

Quick recap

• Lot, gross sample, laboratory sample, test portion: each step must keep the composition.
• Random, systematic, stratified and composite sampling suit different lots.
• Reduce particle size and divide properly (coning and quartering, riffles).
• Preserve samples, respect holding times, and use blanks, spikes and duplicates.

Try it

Open the Do tab. You will see sampling error for yourself with rice and lentils.