Understanding Sample Collection
Thursday, 05 December 2013 15:49

Introduction – Everyone that has worked in an environmental lab has seen it; the bacteria sample that is collected in a general use plastic container, the trace metals or anions sample that comes in a Gatorade bottle (with the label removed of course), or the soil sample that arrives in an ordinary zip seal bag. We all smirk when we see these come across our lab benches, but do we really stop to think what kind of effect that could have on the results of the test? The collection method for our samples is just as important as the method used for the analysis. Everything from the type of container, to what should or should not be added, to how to store it plays a vital role in assuring that what you analyze will give an accurate measure of what is in the sample.

Approved Methods – The table that includes all of the appropriate containers, their preservative, and the holding time is much too large to be included in the body of this text. You can find it in 40 CFR part 136 Table II. There is also a link directly to the relevant pages that have been extracted from the Method Update Rule published in 2012. Method Summary – Sample locations are often specified in permits for regulatory compliance. Samples that are pulled for a profile generation must be planned to give a good representation of the area. An appropriate amount of sample is collected into an appropriate container. This aliquot is then transferred to the lab where it must be analyzed within a specified amount of time.

What You Should Know – Sample collection is an often overlooked part of the sample analysis procedure. Many lab people, including myself when I first began my career in this field, think that the science begins only when the analytical procedure starts. While your SOP may start with pouring an aliquot into a digestion vial the real quest for Quality (with a capital Q) extends back through receipt of the sample in the lab, to collection of the sample in the field and often as far back as the preparation of the sample container.

The reasons the examples given in the introduction are potentially bad for the sample are that the containers have not been properly prepared. Collection containers for bacteria should be sterile prior to sample introduction. Gatorade contains levels of electrolytes designed to replenish your body during and after strenuous work. This includes sodium, potassium, phosphate, and chloride salts in levels that can compromise the analysis of those elements in typical environmental samples. With a zip seal bag you neither know what might be on the inner surface of the bag nor how well that bag is truly sealed.

The standards used in providing a clean and contaminate free sample container arise from an EPA document titled “Specifications and Guidance for Contaminant-Free Sample Containers”. This document was originally intended for use within the EPA CLP and Superfund programs. It has since been adopted for use with all environmental sampling containers. The document gives a specific cleaning procedure that is recommended for use in order to produce containers of desired cleanliness. It also contains the detection limits needed to demonstrate that the bottles are clean enough for use.

Once the bottle has been demonstrated to be clean enough for use you must determine the proper material to use. Different parameters have different requirements for the sample material. As a general rule most organic parameters must be collected in glass while the inorganic parameters can use plastics or fluoropolymers in addition to glass. Because not all plastics have the same properties or inherent background levels you are typically limited to polypropylene, LDPE, or polyethylene. Bacteria samples are generally limited to materials that can be sterilized. Additionally there may be requirements dealing with fluorescence or clarity that may be taken into consideration depending on the analytical method used.

After collection, many samples require a chemical preservative to extend the holding time to a workable frame. The holding time is the maximum allowable time that may pass in which the sample can be expected to give accurate results for the parameter in question. These holding times can range from as little as 15 minutes to as long as one year. The most common preservation is that of temperature. Most samples are required to be held ≤ 6°C but not frozen. The bacteria tests have the requirement of <10°C while a select few, such as pH or some anions, do not have a temperature storage requirement. The next most common preservation is chemical in nature. Many analytes call for acidic storage. Oftentimes a choice of acids is given where the only real requirement is that the pH be lowered. For others, such as the nitrogen tests, there are specific acids to avoid (nitric acid in this case). Some parameters including bacteria and some volatile organics, are sensitive to oxidizers (chlorine is the most common one) and must be preserved with sodium thiosulfate in order to neutralize any that may be in the sample.

The final sampling consideration is the holding time. Every parameter has a defined holding time within which the sample analysis must commence. All sample analysis must commence prior to the expiration of the holding time in order to ensure valid results. Interpretation of results obtained after the holding time has expired is generally not allowed for regulatory results. In most cases if some form of data is needed from an expired sample you can presume that the result will represent a minimum for what the true value was at collection. This is because the most typical fate of analytes is loss due to bacterial activity or possible adsorption onto container walls. One exception to this would be bacteria tests. For those samples you could have either an increase due to normal biological activity or a decrease due to lack of sufficient growth conditions. How long you wait after the holding time would be the determining factor in which it is.

Method Procedure

Note – This is not intended to be a standalone method and does not address all safety or quality control aspects that may be required. Please consult your local regulations to comply with all requirements.

  1. Collect your sample in the appropriate size and material bottle for your analysis needs. Samples for soil volatiles or low level mercury have special consideration.
  2. Preserve with chemical additives as needed.
  3. Transport your sample to the lab in enough time to meet the sample analysis holding time.
  4. We all like things that make life easier. Was this document helpful? Or do you…disagree with something? Have something to add? Contact me at This e-mail address is being protected from spambots. You need JavaScript enabled to view it to let me know what you think.

    Click here for a downloadable pdf of Understanding Sample Collection.

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