By Marianne R. Metzger
When having drinking water samples analyzed by a laboratory, it is important to collect samples properly to ensure the most accurate results. While sample collection (simply put) is just filling sample bottles with water, there are many things that need to be considered to get the most out of an analysis. Questions need to be answered before collection begins, including: For what is this being analyzed? Where should samples be collected and how many? What analysis should be done in the field? For answers, other questions may need to be asked, such as what symptoms are being experienced, are there taste and/or odor issues, is the water discolored or is corrosion present and are there equipment failures.
What type of analysis is needed?
First, it is best to determine what analysis will be run because this will help determine which sample bottles will need to be used. The type of bottle is important, as some analyses require glass bottles while others may call for plastic bottles. Certain contaminants will break down in sunlight; those samples need to be collected in amber-colored bottles to help preserve them. The size of the bottle is often dictated by how much water is actually needed to run an analysis. Keep in mind, the detection level needed can also play a role in how much water should be collected. In order to get down to low-detection levels, especially when testing synthetic organics, a minimum of one liter and sometimes two (about one-quarter to one-half gallon, respectively) are needed to run the analysis and achieve a meaningful detection level.
The type of analysis will also dictate what (if any) chemical preservatives are required. There are basically two ways to chemically preserve samples. One is to have the chemical already in the bottle that will be used to collect the sample. This is called a pre-preserved bottle and most laboratories try to use these to make it as easy as possible to collect samples. Most bacteria bottles are pre-preserved with sodium thiosulfate, a dechlorinating agent needed when testing water that has chlorine. The second type of chemical preservation is to add the preservative in the field when the sample is collected. For example, when collecting samples for volatile organic chemicals, acid should be added to the samples in the field, until the sample has a pH < 2. Another type of preservation is temperature. Many analyses require samples to be chilled down to 4-6°C (39.2°F), especially if samples are not delivered to the laboratory promptly after sample collection. If shipping samples, one may need to use a combination of blue ice and wet ice to keep samples cool enough. When collecting samples during summertime or in warmer climates, it may be necessary to ice samples before shipping to ensure they arrive at the laboratory within the appropriate temperature requirement. This can be especially important for samples that are intended to meet regulatory requirements or for potential litigation.
Once you have the sampling supplies, you need to determine the logistics of collecting the samples. In addition to bottles and preservatives, there are holding times that must be considered when collecting samples for laboratory analysis. The holding time refers to the amount of time when the sample is collected until the time analysis is started. The holding time is determined by the method that will be used for analyzing the sample.
In some cases the holding time is immediate and testing should be done onsite. Some examples of analyses that should be done on site include pH, chlorine and dissolved oxygen. The reason these should be analyzed immediately is they can easily change and results may not be accurate when run after the specified holding time. If delivering samples directly to the lab, make sure the lab will be open to receive the samples and have enough time to start analysis. If samples need to be shipped to the lab, a little more planning may be needed. Samples may need to be collected later in the day and shipped the same day via overnight to ensure the samples meet the holding time requirements. When shipping samples overnight, it is a good idea to determine which carrier and which service to use in order to get samples to the lab in time. Find out what time samples need to be picked up or dropped off at the shipping office in order for them to ship that same day. Missing this deadline can be disastrous, so a little planning ahead of time can assure prompt delivery.
Now it is time to determine the best place to collect the sample. This can differ from sample to sample, depending on circumstances for testing. When collecting residential water, samples are typically collected from the kitchen sink, where water is most often used for drinking and cooking. There may be reasons to collect from other areas, however. Problems occurring in specific areas of the house may prompt sampling to occur only in that specific area. For example, if discolored water is coming from a bathroom faucet, this is where a sample should be collected.
A question I’m commonly asked is if the sample should be collected before any water treatment or after. The answer depends upon the use of the testing results. If a homeowner is concerned their water treatment equipment is not functioning properly, the sample should be collected after treatment to validate what the systems is doing to the water quality. If a homeowner is considering new water treatment equipment, the water samples should be collected ahead of any pre-existing treatment equipment. In some cases, more than one sample may be necessary to get the answers needed for proper water treatment. Samples should be collected from a point that makes the most sense for why the samples are being collected in the first place.
When collecting bacteria samples, it is best to collect samples from a faucet that is stationary and preferably not a goose neck. Faucets that are designed to move have O-rings that may accumulate bacteria; it is best to collect these samples from a non-moving faucet. Goose-neck faucets can cause problems as well; bacteria can grow back through the goose neck, making it nearly impossible to fully disinfect the faucet before collecting the sample. It is best to avoid these faucets when collecting a sample for bacteria testing.
Collecting samples properly adds to the accuracy of the results. It’s important to plan for testing in order to have the appropriate bottles and supplies on hand to collect and return the samples to the lab. If you have questions or are not sure about something, call the laboratory doing the analysis and ask for advice or help to ensure the results you get are meaningful. Laboratory analysis can be expensive; take the time to do the sampling correctly, read any instructions provided before you collect samples and plan ahead to avoid problems.
About the author
Marianne Metzger is a sales professional who has worked with water treatment professionals for 18 years. She holds a Bachelor’s Degree in environmental geology and political science. Most of Metzger’s career has been spent at National Testing Laboratories, helping clients with water quality analysis. She has also worked as a sales engineer for Accent Control Systems, a manufacturer representative for chemical feed pumps, flowmeters and inline analytical equipment. Metzger can be reached via phone, (800) 458-3330, ext 217 or email, [email protected]
About the company
National Testing Laboratories is celebrating 30 years of providing water testing services to water treatment professionals. NTL’s testing products help water treatment professionals properly diagnosis and size equipment to exceed their customer’s expectations. For more information, call (800) 458-3330 or visit the company’s websites: www.ntllabs.com and www.watercheck.com