- Sampling and preservation are part of the method; a wrong sample gives a wrong result whatever the lab does.
- Choose container, preservative and holding time per parameter, using the method you follow (for example Standard Methods 1060 or US EPA 40 CFR 136).
- Field-measure pH, temperature, DO and residual chlorine; they change within minutes.
- Keep an unbroken chain of custody from collection to disposal.
- Field blanks, trip blanks and duplicates show whether problems came from the field or the lab.
- Routine lab QC — blanks, check standards, LCS, duplicates, spikes, proficiency testing — keeps results defensible.
Why sampling matters more than the analysis
In environmental testing, the largest source of error is often not the instrument but the sample. A river changes along its length and through the day. A wastewater discharge changes with the production shift. Soil varies from one corner of a field to another. Air quality changes hour by hour.
A perfect analysis of an unrepresentative or badly preserved sample still gives a wrong answer. The sampling plan, the container, the preservative, the temperature during transport and the time before analysis are all part of the method.
If the sample was not collected, preserved and documented properly, the result cannot be trusted, however good the laboratory work.
Planning: the sampling plan
Before going to the field, write down why the samples are being taken (compliance, process control, survey, complaint), which parameters are needed, where and when to sample, how many samples, and which methods the lab will use. The parameters decide the containers, volumes, preservatives and holding times.
- Objective and the limits or standards the results will be compared with
- Sampling points with GPS coordinates or clear descriptions, and photographs
- Sample type: grab, composite (time- or flow-proportional) or integrated
- Parameter list with container, volume, preservative and holding time for each
- QC samples to be collected: field blanks, trip blanks, duplicates, spikes
- Safety: confined spaces, traffic, water bodies, chemical hazards, PPE
- Transport plan so samples reach the lab within holding time
Representative sampling by matrix
Representative means the sample has the same composition as the material it stands for, at the time and place of interest. Each matrix needs its own approach.
| Matrix | Typical approach | Key points |
|---|---|---|
| Drinking water (tap) | Grab sample after flushing; separate sterile sample for microbiology | Flush to get mains water (or take first-draw if the aim is plumbing metals such as lead); disinfect the tap for microbiology; do not rinse bottles containing preservative or thiosulphate |
| Surface water / groundwater | Grab samples at defined depth; purge wells before sampling | Avoid surface film and bottom sediment; for wells, purge until field pH, EC and temperature stabilise |
| Wastewater / effluent | Composite samples (time- or flow-proportional over 24 h) plus grab samples for unstable parameters | Sample where flow is well mixed; keep composites cool during collection; take pH, DO, residual chlorine, oil and grease and volatiles as grabs |
| Soil (fertility) | Composite of many sub-samples (for example 10–20 cores in a zig-zag pattern) from a uniform area | Sample to the stated depth (often 0–15 cm for plough layer); avoid field edges, manure heaps, fertiliser bands and wet spots; mix, quarter and keep about 0.5–1 kg |
| Soil (contamination) | Grid or targeted sampling; discrete samples at defined depths | Clean tools between points; use glass jars for organics; minimise disturbance for volatiles |
| Ambient air (PM) | Continuous 24-hour sampling at a sited monitoring station | Inlet height and distance from obstructions and local sources as per siting guidance; calibrated flow; filters handled with forceps |
Containers, preservatives and holding times
Many analytes change after collection. Microbes grow or die, organic matter degrades, ammonia and nitrate inter-convert, metals stick to container walls, and volatile compounds escape. Preservation slows these changes; it does not stop them. Holding time is the maximum time allowed between collection and analysis (or extraction) for the result to remain valid.
The table shows typical practice, of the kind listed in APHA Standard Methods section 1060 (Collection and Preservation of Samples) and in US EPA 40 CFR 136 (Table II). Values vary between methods and regulators; always confirm with the method you are following.
| Parameter | Container | Preservation | Typical maximum holding time |
|---|---|---|---|
| pH, temperature, dissolved oxygen (probe), residual chlorine | Plastic or glass | None — measure in the field | Analyse immediately (about 15 minutes) |
| BOD | Plastic or glass, filled | Cool ≤ 6 °C, dark | 48 hours |
| COD | Plastic or glass | H₂SO₄ to pH < 2; cool | 28 days |
| Ammonia, TKN | Plastic or glass | H₂SO₄ to pH < 2; cool | 28 days |
| Nitrate, nitrite | Plastic or glass | Cool ≤ 6 °C | 48 hours |
| Suspended / dissolved solids | Plastic or glass | Cool ≤ 6 °C | 7 days |
| Metals (except mercury) | Plastic (acid-washed) | HNO₃ to pH < 2 | 6 months |
| Mercury | Plastic or glass (acid-washed) | HNO₃ to pH < 2 | 28 days |
| Oil and grease | Wide-mouth glass, not pre-rinsed | HCl or H₂SO₄ to pH < 2; cool | 28 days |
| Volatile organics | Glass vials with PTFE-lined septum, no headspace | HCl to pH < 2; cool; dechlorinate if needed | 14 days |
| Coliforms / E. coli | Sterile bottle with sodium thiosulphate | Cool (below about 10 °C), dark | Short — typically 8 hours for source and wastewater; up to about 24–30 hours for drinking water in some methods |
| Soil (nutrients, pH, OC) | Clean polythene bag or jar, labelled | Air-dry in the lab; store dry | Weeks to months once air-dried (field-moist nitrate/ammonium needs cooling and quick analysis) |
| PM filters | Labelled filter cassette or petri slide | Protect from heat; cool storage for chemical analysis | Post-weigh as per method (typically within days of retrieval) |
Preserved bottles from the lab must not be rinsed with sample, or the preservative is lost. Fill sterile bottles without touching the inside of the cap or neck.
Labelling and field records
- Unique sample ID on every container, written in waterproof ink or printed
- Site and point, date and time of collection, sampler's name
- Sample type (grab or composite) and preservative added
- Field measurements: pH, temperature, EC, DO, residual chlorine, flow
- Weather and any unusual observations (colour, odour, discharge events, construction nearby)
- For air: sampler ID, filter ID, start and end times, flow readings, temperature and pressure
Chain of custody
Chain of custody is the documented record of who had the sample, from collection until disposal. It proves the sample tested is the sample collected and that nobody could have tampered with it. It matters most for regulatory, legal and dispute samples, but good labs use it routinely.
- 1The sampler fills in the chain-of-custody form with sample IDs, parameters, preservatives and collection details.
- 2Containers or cool boxes are sealed with custody seals where required.
- 3Each time the samples change hands (sampler → courier → lab reception), both parties sign and record date and time.
- 4At reception the lab checks seals, labels against the form, container integrity, temperature on arrival (often with a temperature blank) and pH of preserved samples, and records any deviation.
- 5The lab assigns its own IDs in the LIMS, linked to the client IDs, and records storage location.
- 6Samples and records are retained and disposed of as per the retention policy.
Field QC samples
Field QC samples show whether contamination or variability came from sampling, transport or the lab. They are analysed like normal samples, ideally without the analyst knowing which they are.
| QC sample | What it is | What it tells you |
|---|---|---|
| Field blank | Analyte-free water poured into a sample container at the site and preserved like a sample | Contamination from the field environment, containers, preservatives or handling |
| Trip blank | Analyte-free water sealed in the lab, carried to the site and back unopened (typically for volatiles) | Contamination during transport and storage |
| Equipment (rinsate) blank | Analyte-free water passed through cleaned sampling equipment | Carry-over from samplers, pumps, augers or bailers |
| Field duplicate | A second sample collected at the same place and time | Combined sampling and analytical precision; reported as relative percent difference (RPD) |
| Matrix spike / spike duplicate | Sample portion spiked with a known amount of analyte | Matrix effects on recovery and precision |
| Temperature blank | A bottle of water travelling in the cool box | Whether samples were kept cool enough |
| Filter field blank (air) | A filter taken to the site, handled but not sampled | Mass gain from handling and transport |
QA/QC inside the environmental laboratory
Laboratories accredited to ISO/IEC 17025 must show that results are valid and traceable. In environmental testing this is done with a routine set of checks in every batch, often described in APHA Standard Methods section 1020 (Quality Assurance) and in regulator guidance.
- Method blank (reagent blank) with every batch, to detect lab contamination
- Calibration with fresh standards, plus an independent check standard from a second source
- Continuing calibration verification at intervals (for example every 10–20 samples)
- Laboratory control sample (LCS) or certified reference material, with recovery tracked on control charts
- Laboratory duplicates and matrix spikes at a set frequency (for example 1 in 10 or 1 in 20)
- Detection and quantitation limits established and periodically verified
- Proficiency testing (inter-laboratory comparisons) for each accredited parameter
- Holding-time and preservation checks at sample reception, with deviations flagged on the report
- Instrument calibration and maintenance records, including balances, pH meters, incubators and samplers
Common mistakes
- Sampling a convenient point rather than a representative one (dead legs, edges of fields, stagnant pools)
- Rinsing pre-preserved bottles, or using the wrong preservative for the parameter
- Leaving headspace in volatile-organic vials
- Exceeding holding times because samples waited over a weekend
- No field blank, so contamination cannot be traced
- Missing or incomplete chain-of-custody signatures
- Air-drying soil in an oven at high temperature, which alters pH, nitrogen forms and organic matter
- Touching PM filters with fingers or serrated forceps
Related in the Lab Explorer
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