How to collect an accurate sample

To ensure accurate testing, there are three keys needed for an effective program.

  • A trained technician
  • Proper (and clean) instruments or equipment
  • Fresh chemicals or reagents.

An issue in one area can cause issues for your entire testing program; however, proper testing begins with the operator. Most of the time, when an error occurs, it is because of user error or a problem with how a sample was taken or handled. This is why proper training and procedures are important.

Proper sample collection: Is your sample representative?

Water testing measures key parts of a water system quality and chemistry to make sure the water treatment program is working properly. It is important when taking water samples that the sample represents the whole system and not a smaller, non-representative portion of the system. If a sample isn’t representative of the whole system, or if it is handled improperly, it can ‘lie to you’. By this we mean the test results will be true of the sample, but not the entire system and could lead to decisions affecting the water treatment program. Improper handling can consume certain chemicals or introduce contaminants which affect tests and your response to the result. An important key to a representative sample, is flushing the sample line before you take your sample. This will clear out impurities or chemical concentrations at the sample point.

You can learn a lot from your sample. By looking for clues in your samples, you can get a head start on understanding what is going on in the system. Look for things like color, smell and quality (cloudiness or suspended solids). Was the sample line clogged? These observations can help direct your testing.

Finally, in certain situations a chain of custody – that is, a record of who took or handled a sample, and when, is important for regulatory or legal reasons. In these cases, proper sampling and records are critical.

Preparing your sample

Once you have selected your sampling point, follow flushing guidelines and grab a sample. Doing this properly helps you prepare it for testing. 

  1. Use a clean, non-reactive bottle such as glass or plastic Nalgene. Also, make sure you have a cap.
  2. Clear contaminates by rinsing the sample bottle at least three times with the water sample to be tested.
  3. Fill the bottle to the brim to remove air and cap it. This keeps the sample fresh and removes air that can change the sample over time. 

Assume you will make mistakes, or a test result doesn’t make sense, so take enough of a sample to duplicate your tests if needed. It’s also important to label your samples for your records. On the bottle, write the sample location and application.

Once you’ve collected your sample, it’s important to test immediately!

Sample Accuracy

You’ve collected your sample. Great! Now you need to ensure an accurate result. This begins with having the proper sample size in your testing vial. Rinse the sample vial three times with the sample to be tested. Then, refer to the test procedure to determine the sample size you need. When filling your sample vial, hold the vial close to eye level. Verify the sample volume by placing the vial on a flat, level surface and bend down to eye level. If you simply hold the vial you may be tilting it slightly and skew your results. For example, if your test requires a 5 mL sample and you fill to 4 mL, your test will start off with a 20% error! Mitigate this error by placing the vial on a flat, level surface and bend down to eye level to confirm you have the correct sample size. Remember, always read from the bottom of the meniscus!


More Resources

For a library of testing videos and proper testing technique, please visit our resources page or contact our technical team if you have any questions about how you can improve your testing program.