Sampling, Containers & Sample Preservation

Laboratory results are only useful when the sample represents the process condition being evaluated. A perfectly performed analysis can still produce misleading information if the sample was collected from the wrong location, contaminated, stored incorrectly, preserved improperly, or analyzed too late.

Operators should understand sampling objectives, sample types, container requirements, preservation, holding time, labeling, and documentation.

Why Sampling Matters

Sampling is the first step in laboratory testing.

The purpose of sampling is to collect a portion of water, wastewater, sludge, or another process stream that represents the condition being evaluated.

A Representative Sample

A representative sample should reflect the actual characteristics of the material or process condition of interest.

Representativeness depends on:

• sampling location;
• sampling time;
• sample type;
• flow conditions;
• mixing;
• collection method;
• sample handling.

Define the Sampling Objective

Before collecting a sample, determine what question the sample is supposed to answer.

Examples include:

• What is the influent BOD concentration?
• What is the chlorine residual leaving the contact basin?
• Is a filter producing acceptable turbidity?
• What is the ammonia concentration in final effluent?
• What is the pH in an aeration basin?

Sampling Location

The sampling point should match the process condition being evaluated.

A poor sampling location can produce misleading results even when the laboratory analysis is correct.

Examples of Sampling Locations

Common locations include:

• raw water;
• plant influent;
• process basins;
• filter effluent;
• secondary clarifier effluent;
• final effluent;
• distribution system;
• recycle streams.

Avoid Unrepresentative Locations

Avoid locations that are not representative of the main process stream unless that location is specifically the target of the investigation.

Examples include:

• dead-end piping;
• stagnant sample lines;
• poorly mixed corners;
• areas immediately next to chemical injection when bulk-process concentration is desired.

Flush Sample Lines When Required

A sample line may contain water that has been stagnant and no longer represents the current process.

Where the sampling procedure requires it, the line should be flushed sufficiently before collection.

Grab Samples

A grab sample is collected at one location and one point in time.

It represents the process condition at approximately that moment.

When Grab Samples Are Useful

Grab samples are useful for measurements that can change rapidly or that must be evaluated immediately.

Examples may include:

• pH;
• temperature;
• dissolved oxygen;
• chlorine residual;
• some microbiological samples;
• investigation of a short-term process upset.

Composite Samples

A composite sample combines multiple individual samples collected over time or flow conditions.

A composite sample can provide a better estimate of average conditions over a longer period.

Time-Proportional Composite

A time-proportional composite collects equal sample portions at equal time intervals.

This method gives each sampling time similar weight.

Flow-Proportional Composite

A flow-proportional composite adjusts sample contribution according to flow.

Higher-flow periods contribute more sample volume than lower-flow periods.

This can provide a more representative average concentration when flow varies significantly.

Grab Versus Composite

Use the sample type required by the analytical or compliance procedure.

In general:

• grab samples describe conditions at a specific time;
• composite samples describe average conditions over a defined period.

Automatic Samplers

Automatic samplers can collect samples according to:

• time;
• flow;
• programmed events.

Automatic Sampler Maintenance

Check:

• tubing condition;
• intake location;
• refrigeration;
• sample volume;
• programming;
• cleanliness;
• power supply.

Sampler Tubing

Dirty or damaged tubing can:

• contaminate samples;
• restrict flow;
• change collected volume;
• retain material from previous samples.

Sampling Containers

Container material can affect sample quality.

Depending on the analysis, containers may be made of:

• glass;
• plastic;
• specialized laboratory materials.

Use the Correct Container

The correct container depends on the analyte and analytical method.

Using the wrong container can cause:

• contamination;
• adsorption;
• chemical reaction;
• loss of analyte.

Precleaned Containers

Some analyses require containers prepared or supplied by the laboratory.

Do not assume a visibly clean household container is suitable for laboratory sampling.

Do Not Rinse Every Sample Bottle

Whether a container should be rinsed depends on the sampling procedure.

Some bottles contain:

• preservative;
• dechlorinating agent;
• other prepared reagents.

Rinsing such a bottle can remove the material required for proper sample collection.

Microbiological Sample Bottles

Microbiological sample bottles are often specially prepared and should be handled carefully to avoid contamination.

Do not touch:

• inside of the bottle;
• inside of the cap;
• bottle opening.

Sample Contamination

Contamination can occur from:

• dirty hands or gloves;
• unclean containers;
• sampling equipment;
• dust;
• chemicals;
• contact with the ground;
• cross-contamination from another sample.

Cross-Contamination

Cross-contamination occurs when material from one sample or location enters another sample.

Preventive practices include:

• clean sampling equipment;
• separate containers;
• proper glove changes;
• controlled sample handling.

Sample Preservation

Preservation slows physical, chemical, or biological changes after sample collection.

Preservation methods may include:

• cooling;
• chemical addition;
• pH adjustment;
• protection from light;
• immediate analysis.

Why Samples Change After Collection

After collection:

• microorganisms can continue reacting;
• gases can escape;
• solids can settle;
• chemicals can oxidize or reduce;
• pH can change;
• analytes can adsorb onto container surfaces.

Cooling

Cooling slows many biological and chemical reactions.

Samples that require cooling should be placed under the required temperature conditions as soon as practical after collection.

Chemical Preservation

Some samples require a chemical preservative.

The preservative may:

• control pH;
• inhibit biological activity;
• stabilize the analyte;
• prevent precipitation or adsorption.

Preservatives Must Match the Method

Do not add a preservative simply because another sample uses it.

The required preservative depends on:

• analyte;
• analytical method;
• regulatory procedure.

Immediate Analysis

Some parameters change so rapidly that preservation cannot adequately maintain the original condition.

These parameters may require immediate or very prompt measurement.

Common examples include:

• temperature;
• pH;
• dissolved oxygen;
• chlorine residual.

Holding Time

Holding time is the maximum allowed or recommended time between sample collection and analysis under the specified preservation conditions.

Why Holding Time Matters

If holding time is exceeded:

• sample chemistry may change;
• organisms may grow or die;
• analyte concentration may change;
• results may no longer represent the original sample.

Holding Times Differ by Analysis

There is no single holding time that applies to all laboratory tests.

Operators should use the holding time specified by:

• approved analytical method;
• laboratory procedure;
• regulatory requirement.

Do Not Guess Holding Times

If the required holding time is uncertain, verify it before sample collection.

A sample collected correctly but analyzed after its allowable holding time may be unusable.

Sample Volume

The laboratory needs enough sample volume to complete:

• analysis;
• quality-control testing;
• repeat analysis if required.

Collect the amount specified by the laboratory or method.

Headspace

Some analyses require:

• no headspace;
• controlled headspace;
• normal bottle filling.

Headspace requirements depend on the analyte.

Do not apply one filling technique to every sample.

Volatile Compounds

Volatile compounds can escape from a sample into the air.

Samples for volatile analyses often require special collection methods designed to minimize loss.

Sample Mixing Before Subsampling

Some samples containing suspended material must be mixed appropriately before a smaller portion is removed for analysis.

Otherwise, settled solids can cause the subsample to be unrepresentative.

Do Not Mix Samples That Should Remain Undisturbed

Mixing requirements depend on the analytical method.

Follow the specified procedure rather than applying the same handling to all samples.

Sample Labeling

Every sample should be identified clearly.

Useful label information may include:

• sample location;
• date;
• time;
• sample type;
• collector identification;
• preservative where appropriate.

Use Unique Sample Identification

Unique sample IDs reduce the risk of mixing up samples from:

• different locations;
• different times;
• different processes.

Field Records

Field sampling records may include:

• sample ID;
• location;
• date and time;
• collector;
• sample type;
• weather or process condition where relevant;
• field measurements;
• preservation;
• unusual observations.

Document Unusual Conditions

Useful observations may include:

• sample unusually turbid;
• strong odor;
• process upset occurring;
• sample line recently repaired;
• automatic sampler malfunctioned.

Chain of Custody

Chain of custody documents sample possession and transfer from collection through laboratory receipt and analysis.

It is especially important for compliance, enforcement, or legally significant samples.

Chain-of-Custody Information

A chain-of-custody form may include:

• sample identification;
• collection location;
• date and time;
• requested analyses;
• collector;
• preservation;
• transfer signatures;
• dates and times of transfer.

Why Chain of Custody Matters

It demonstrates:

• which sample was collected;
• who handled it;
• when custody changed;
• whether documentation remained traceable.

Sample Transport

Samples should be transported under conditions required by the method.

Important considerations include:

• temperature;
• protection from breakage;
• protection from sunlight;
• holding time;
• custody documentation.

Coolers

Sample coolers should be:

• clean;
• able to maintain required temperature;
• organized to prevent container damage;
• properly labeled when required.

Protect Glass Containers

Glass bottles should be packed so they do not:

• break;
• leak;
• damage other samples.

Sample Receipt at the Laboratory

The laboratory may check:

• sample identification;
• container type;
• temperature;
• preservation;
• sample volume;
• holding time;
• chain-of-custody documentation.

Rejected Samples

A laboratory may reject or qualify a sample if:

• container is wrong;
• bottle is broken;
• preservation is incorrect;
• holding time is exceeded;
• sample identification is unclear;
• volume is insufficient;
• required temperature was not maintained.

Field Blanks

A field blank is a quality-control sample used to evaluate contamination introduced during:

• sampling;
• transport;
• field handling.

Equipment Blanks

An equipment blank can be used to evaluate whether reusable sampling equipment has been cleaned adequately.

Trip Blanks

A trip blank travels with sample containers and may be used for specified volatile analyses to identify contamination during transport and handling.

Duplicate Samples

Duplicate samples are collected to evaluate sampling and analytical variability.

Large differences between duplicates can indicate:

• sample heterogeneity;
• collection problems;
• analytical variability.

Composite Sample Refrigeration

Composite samplers may require refrigeration during the collection period.

Failure of refrigeration can affect sample validity for analyses requiring temperature control.

Check the Sampler Before the Sampling Period

Before starting a composite sample, verify:

• correct bottle;
• correct program;
• correct intake location;
• tubing condition;
• refrigeration;
• power;
• flow signal if flow pacing is used.

Check the Sampler After Collection

After the sampling period, verify:

• sample was actually collected;
• expected volume is present;
• sampler remained cold if required;
• program completed correctly;
• no obvious equipment failure occurred.

Flow-Proportional Sampling Problems

A flow-proportional composite can become unrepresentative if:

• flow meter signal is incorrect;
• sampler programming is wrong;
• intake tubing plugs;
• sample volumes are inconsistent.

Sampling During Process Upsets

When investigating a process upset, record:

• exact sampling time;
• process conditions;
• equipment status;
• chemical-feed conditions;
• relevant alarms.

This allows laboratory results to be connected with actual operating conditions.

Sampling and Process Detention Time

When comparing influent and effluent data, remember that the water entering a process now may not be the same water leaving it now.

Detention time can create a time delay between:

• influent change;
• downstream laboratory response.

Sampling for Process Control

Process-control sampling should answer operational questions.

Examples include:

• Is nitrification occurring?
• Is phosphorus being removed?
• Is a filter breaking through?
• Is chlorine demand changing?
• Is a recycle stream increasing nutrient loading?

Compliance Sampling

Compliance sampling must follow applicable permit, regulatory, and approved-method requirements.

Operators should not substitute a convenient sampling location or sample type for the required one.

Safety During Sampling

Sampling can expose operators to:

• traffic;
• open water or basins;
• chemicals;
• biological material;
• slips and falls;
• confined spaces.

Sampling procedures must include appropriate safety controls.

Do Not Enter a Confined Space Just to Obtain a Sample

If the designated sampling activity requires confined-space entry, applicable confined-space procedures must be followed.

Alternative safe sampling methods should be used when appropriate.

Biological Exposure

Wastewater samples may contain pathogens.

Good practices include:

• appropriate gloves;
• handwashing;
• avoiding hand-to-face contact;
• cleaning contaminated equipment;
• proper sample transport.

Common Sampling Mistakes

• Collecting from the wrong location.
• Using a grab sample when a composite is required.
• Using the wrong container.
• Rinsing a bottle that contains preservative or dechlorinating agent.
• Touching the inside of a sterile bottle or cap.
• Failing to flush a stagnant sample line when required.
• Using dirty sampling equipment.
• Failing to preserve or cool a sample promptly.
• Exceeding the analytical holding time.
• Failing to label the sample clearly.
• Failing to document collection time.
• Allowing an automatic sampler to operate with plugged tubing or failed refrigeration.
• Failing to connect laboratory results with actual process conditions.

A Practical Grab-Sampling Sequence

1. Confirm the sampling objective.
2. Confirm the correct sampling location.
3. Confirm container type and preservative requirements.
4. Use appropriate PPE.
5. Flush the sample point if required.
6. Collect the sample without contaminating the container.
7. Measure immediate field parameters where required.
8. Preserve or cool the sample as specified.
9. Label the container completely.
10. Record date, time, location, and unusual conditions.
11. Transport the sample within required conditions and holding time.

A Practical Composite-Sampling Sequence

1. Confirm whether the composite is time-proportional or flow-proportional.
2. Inspect and clean the automatic sampler.
3. Confirm correct intake location.
4. Install the correct container.
5. Verify sampler programming.
6. Verify refrigeration if required.
7. Verify flow signal if flow pacing is used.
8. Start the sampling program.
9. Review sampler condition after collection.
10. Mix the composite appropriately if the method requires it.
11. Transfer and preserve the sample according to procedure.
12. Document collection period and sampler problems.

What to Remember for the Exam

• A laboratory result is only as useful as the sample used to produce it.
• A representative sample must match the process location, time, and condition being evaluated.
• A grab sample represents approximately one point in time.
• A composite sample combines multiple samples to represent conditions over a longer period.
• Flow-proportional composites give greater sample weight to higher-flow periods.
• Use the sample type, location, and frequency required by the applicable procedure.
• Container material and preparation depend on the analyte and analytical method.
• Do not rinse bottles that contain required preservatives or dechlorinating agents.
• Sample preservation slows biological, physical, or chemical changes after collection.
• Cooling is a common preservation method, but not every analysis uses the same preservation requirement.
• Holding time is the allowable or recommended period between collection and analysis under specified conditions.
• Holding times differ by analytical method and should not be guessed.
• Some rapidly changing parameters require immediate or prompt analysis.
• Sample labels should clearly identify location, date, time, sample type, and other required information.
• Chain of custody documents sample possession and transfer.
• Field blanks, equipment blanks, trip blanks, and duplicates are quality-control tools used for specific purposes.
• Automatic samplers require correct programming, tubing, intake location, and refrigeration where applicable.
• Compliance sampling must follow the required location, sample type, preservation, and analytical procedure.
• Sampling records should document unusual process or collection conditions.
• Sampling must be performed safely, including protection from biological, chemical, fall, traffic, and confined-space hazards.