Choosing Your Method

Jan. 19, 2010

About the author: Marianne Metzger is GPG business unit manager for National Testing Laboratories, Ltd. Metzger can be reached at 800.458.3330 or by e-mail at [email protected].

Related search terms from testing, contaminants, instrumentation

There are thousands of testing methods used to analyze water for various contaminants, with more than one way to analyze for a single contaminant. For example, you can test the pH of water using test strips, other colorimetric methods or by using an electrochemical method, which utilizes an electrode. It can be difficult to determine what method is going to be best for your application.

Once you have picked a method, it is important you take the time to do something called method development. When laboratories begin running a new method, they do method development, which ensures they are running the testing method correctly so as to yield the most accurate results.

Testing Methods: Where Do They Come From?

Methods can be developed by governmental agencies such as the U.S. Environmental Protection Agency, Department of Energy and the U.S. Geological Survey.

Other acceptable methods are commonly referred to as standard methods, which have been published in several editions called Standard Methods for the Examination of Water and Wastewater. The American Public Health Assn., the American Water Works Assn. and the Water Environment Federation have published these methods collectively. Most were developed for laboratory analysis but have been adapted by instrument manufacturers in order to make them more easily run in the field.

Choosing the Right Method

With so many methods available, you need to look at methods that will meet your specific needs. First, you need to determine what kind of detection level you require. For example, testing for iron at a minimum level of 5 parts per million may not be low enough for drinking water applications. When you are looking for a minimum detection level, you should also be aware of the maximum detection level. Methods are only so accurate within a working range, so if you are above the maximum level you may need to perform a dilution and rerun the method. Consider the levels in which you are commonly working so you do not need to perform dilutions in the field. When running analysis in the laboratory, dilutions are commonly run, which raises the minimum detection levels.

The other thing you must consider is the precision, which refers to the reproducibility of the method. Reproducibility of a method refers to the ability to repeatedly run the method under unchanged conditions and produce the same results. Many commercially developed methods that are used in the water treatment industry have a high level of reproducibility, but not all. For example, if you commonly test for pH and precision is important, using the electrode method can yield highly precise results versus colorimetric tests, which may only indicate a range. The method used in the field for running nitrate analysis is based upon the cadmium reduction method, and this method is not as easily reproducible because technique is extremely important when running this method. You can get results that are up to 50% off the true value because of the difference in testing techniques making it extremely important for you to use standards to ensure you are obtaining an accurate result.

Cost is very important when choosing your method. Some methods can require expensive equipment as well as reagents. For example, there are a number of colorimeters available; some can be quite expensive while the reagents are inexpensive, and other colorimeters are less expensive while the reagents are more expensive. Depending on how many tests you plan on running, the more expensive meter may be worth it in the long run.

It is important to remember the cost of standards or calibration solutions. For example, you can run pH using strips for a couple of cents per test or you can invest in a pH meter, which can cost $100 for an inexpensive pen-style meter or more than $500 for a more sophisticated meter. You must also replace the electrode once or twice per year. Also, there is some cost in the electrode storage solution required for the pH meter as well as the pH buffers needed to calibrate the meter.

Finally, you should determine the level of accuracy required. To ensure you are getting accurate results, you need to properly maintain your testing equipment, as well as calibrate on a regular schedule. Maintaining equipment can include proper storage, such as keeping equipment away from extreme heat or cold. Proper storage also includes storing pH electrodes in the proper storage solution.

When first working with a new piece of equipment, you should practice running samples using what is known as a standard. A standard is a solution with a known concentration of whatever contaminant you are testing. For example, if you are running an analysis for iron, you use an iron standard such as 5 mg/L. When ordering standards, you may want to order one that you can dilute several times so you have many samples to analyze and practice your testing technique.

Sometimes it may be to your advantage to have the water analyzed at a laboratory, which can utilize equipment that is more accurate than other methods run in the field.

Ensuring Accuracy

Once you have determined your method, there are a couple of things you need to do to ensure you are running the test correctly and getting the most accurate result. First, read the method completely, making sure to take note of things that may interfere with the method. Every method has some kind of interference; for example, when running a chlorine test using a DPD method, there are several things that can cause interference. The presence of other oxidizers such as chlorine dioxide, ozone, iodine or bromine will cause the chlorine level you are measuring to increase by the amount of the other oxidizers present.

Another contaminant that interferes with the chlorine DPD method is manganese. Manganese has several different oxidation states. Mn +2 through Mn +7 and free chlorine will readily oxidize soluble manganese. This oxidized manganese will react directly with the DPD reagent, causing skewed results. Now that you are aware of what can cause interferences, you can take measures to alleviate the interference or you can utilize another method that does not have the interference you may be experiencing.

Once you are familiar with the instructions for the running the test, it is time to run some standards. You should run five to 10 standards depending on the difficulty of the analysis. You need to run as many standards as it takes to make you comfortable that you are achieving accurate results. You are now ready to start testing some unknown samples. If you are having more than one person run the tests, it is important that each individual run several standards because testing techniques will differ from person to person. You may need to run standards again whenever you experience unexpected or suspect results, as there may be a problem with your testing equipment. It is also important to use standards within their shelf life, so be diligent about discarding outdated standards or solutions, as they do expire and lose their accuracy.

With certain electronic equipment it may be necessary to run standards more frequently. For example, pH electrodes will slowly deteriorate over time, so it is important to calibrate these on a regular basis. When calibrating a pH meter it is also important to use more than one standard, commonly called buffers. It is most common to use three buffers when calibrating a pH meter. The electrode will eventually go bad and will need to be replaced. When you cannot get your pH meter to calibrate correctly, it is probably time to replace the electrode. Again, all of this information should be provided by the manufacturer in the instructions, so become familiar with the instructions.

Choosing the right method can be intimidating, but with a little research you can find the right method to suit your needs. Taking care of the equipment is necessary to ensure a higher level of accuracy, so make sure anyone using the equipment is fully trained on care and maintenance as well as running the methods. Once you have set up your testing program you may want to randomly monitor the results by sending in samples to a laboratory to spot-check your results. If you are testing in the field for any health-based contaminant, you should always send a sample into a laboratory for verification, as the equipment used in most laboratories is significantly more accurate than field methods available.

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About the Author

Marianne Metzger

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