Sampling and Preservation
Title 40 Code of Federal Regulations Part 136 (40 CFR 136)
Sampling and Preservation
The primary objective of a sampling program is to collect a small portion of a material so that analyses can be made which will accurately represent the entire body of the material. Proper sampling and preservation techniques are important in order to maintain the integrity of the sample. Improper sample handling can invalidate the results of any analysis that a laboratory performs. In other words, the results are only as good as the sample. A table with the sampling, preservation, and holding times for various analytical parameters can be found in Appendix D.
This will be an overview of various topics concerning sampling and preservation. For further information, consult the reference listed at the beginning of this section.
Standard Conditions for NPDES Permits, an attachment to the NPDES permit, – Part 1 – General Conditions – Section A – Monitoring and Reporting – Paragraph 5 requires sampling records. This is a log of all the daily samples collected in the treatment plant. Regulations vary regarding how long these records must be kept, but the permit requiring the sampling log will also identify how long these records must be retained. Items to record are:
Include a spot to record any remarks concerning the sample collection that may have an effect on test results (e.g. heavy rain, unusual color or odor, heavy recycle stream to head of plant, refrigeration of sampler off, etc.).
Types of Samples
There are two types of samples based upon their method of collection: grab samples and composite samples. The choice of method depends upon the analysis that is required on the sample.
A grab sample is an individual sample collected at a selected time.
A composite sample is composed of one of the following three:
Items that Influence Grab and Composite Samples
A grab sample is one discrete sample where all of the material is collected at once and can only represent the conditions at a particular time. It can be taken manually by using a pump or scoop or by dipping the container directly into the water to be sampled.
Certain tests require the use of grab samples and are to be analyzed immediately upon collection. These tests include pH, temperature, dissolved oxygen, chlorine residual and fecal coliform. Sometimes it is necessary to collect a grab sample for tests that usually require composite samples. This can include instances when an unusual discharge occurs and an operator must determine if appropriate action needs to be taken immediately. A grab sample may also be necessary if there is an intermittent discharge, as in industrial monitoring situations.
A composite sample is made up of a number of grab samples collected over a period of time and mixed together. It represents the average conditions of a site over a specific period of time. For most purposes, a twenty-four hour time period is considered standard. Aliquots are usually collected at one site, but can be collected at multiple sites, such as in soil analyses, and then mixed together. Composite samples are used only when the parameters to be determined are shown to remain unchanged during collection and preservation.
Composite samples can be collected either manually or with an automatic sampler. Manual compositing of samples can present some problems. They are time–consuming and too dependent on the person(s) collecting the samples, especially if more than one person is collecting them. The margin of error increases due to the differences between the methods of collecting the sample – that is, the depth where the sample is collected, how well it is stirred or shaken, the length of time between samples, etc.
A better way to collect composite samples is by using an automatic sampler. The automatic sampler eliminates many of the errors associated with manual collection. Advantages include more consistent sampling, less sample handling and the ability to collect samples in various modes – time proportional or flow proportional with either fixed or variable volumes. Disadvantages include increased maintenance, the possibility of lines or probes becoming clogged or frozen and possible sample contamination. Problems in collection from an automatic sampler can still arise if the sample is not thoroughly mixed before pouring into smaller containers and if the sampler jug and hoses are not kept clean.
A time proportional sample with a fixed volume is probably the most common kind of composite sample. Both the time interval and the size of the sample remain constant. This is used when the flow is fairly constant throughout the entire sampling period. To calculate the volume for each sample, the time interval and the final volume need to be established. For example, if one wishes to collect six liters of a sample in half hour increments during a 24-hour sampling period, first calculate the number of samples needed per day: 24/0.5 = 48 samples/day. Then, calculate the volume of each sample.
A container marked at 125 mL can then be used for manual collection or, following the manufacturer’s instructions, an automatic sampler can be programmed for a 125 mL sample every 30 minutes.
A flow-proportional composite can be collected two ways. One way is to keep the sample volume constant and vary the time interval. As the flow increases, the time interval between samples decreases. The other way is to keep the time interval constant and vary the sample volume with the flow. With either method, a sampling chart listing flow rates and their corresponding sample volumes must be established and closely followed for manual collection. It is far easier to use an automatic sampler which is tied into a flow measuring device so that it will collect a fixed volume of sample that is flow-proportional. A signal from the flowmeter starts the sample pump and a sample is taken. In this way, the sample is dependent upon flow rather than time.
There are also automatic samplers, known as sequential samplers, available that can collect a series of discrete samples in individual containers, which can then be composited at a later time. These are useful when one wishes to analyze different portions during the sampling period based on color, shift time, etc. Some industries are required to be monitored this way in order to separate the discharges between operational and non-operational hours.
The techniques for collecting representative samples are as varied as their sampling locations. In general, the samples should be collected in an accessible location in an area of high turbulence to insure good mixing. The sampling site should be kept constant so that results from repeated testing can be compared. If possible, collect the sample in the center of the flow, about halfway from the bottom in order to avoid the debris on the bottom and any large amounts of floating materials. When sampling manually with a bottle, place the mouth of the container below the liquid surface and facing the flow, keeping ones hand from blocking the mouth of the bottle. It is a good practice to wear waterproof gloves while sampling by hand so as to lessen the chance of infection by pathogenic organisms, especially if one has cuts or abrasions on the hand.
Collect enough sample to allow sufficient volume for analysis and for any duplicate testing. Some analyses, such as for volatile organics, dissolved oxygen, etc., require the containers to be completely full with no air trapped in them. Other analyses, such as fecal coliform and total suspended solids, need airspace to allow adequate mixing before analysis. Consult the individual analytical methods for proper collection techniques.
If samples are to be taken from a closed pipe through a valve or faucet, the valve should be opened and allowed to flush thoroughly before the sample is collected. The time needed for the flushing will depend upon the diameter and length of the pipe and the velocity of the flow.
For solid samples such as soils and filter cakes, several small portions are collected from over the entire area, placed in a wide mouth container, and mixed thoroughly before taking subsamples for analysis.
As each sample is collected, it must be clearly identified so there is no chance of confusing it with any other sample. Samples should be tagged or labeled with the following information:
Fill in the information on the tag with waterproof ink and attach to the container immediately after collection. This is important when more than one sample is taken or when a composite sample is divided into different containers at one sampling location.
At the same time the samples are being tagged, a chain-of-custody document needs to be filled out and kept with the samples. This is used to trace the possession and handling of a sample from the point of collection through the final reporting of data. A chain-of-custody is a record that shows the samples are collected, transferred, stored and analyzed properly, thereby ensuring the validity of the resulting data, and that documentation exists for the samples if the resulting data is ever used in enforcement cases. The information on the chain-of-custody needs to include all of the items listed above that are on the sample tag, along with the signature of the collector, the signatures of all involved with the possession of the sample and dates and times of each possession. The samples should then be taken to the laboratory as soon as possible.
Sampling and Preservation; the Tips for Techs version