Since its release in 1995, EPA method 525.2 has been one of the most widely used methods for quantifying semi-volatile compounds in drinking water. Chances are, if you work for or own a drinking water lab, you probably analyze for compounds in this method – at the very least, you’re probably at least familiar with the method. This is a widely accepted method for quantifying semi-volatile organic compounds; however, there are some glaring issues with the method that the EPA has recognized and addressed. These changes have been collected and implemented in a new revision – Method 525.3 – which was published in 2012. Method 525.2 is still more frequently used by laboratories processing drinking water samples; however, I would argue that Method 525.3 is more scientifically sound. In this 2-part blog series, I will address multiple aspects of Method 525.2 that have been modified to improve the collection, preservation, and processing of drinking water samples. In this first part, I will focus on the improvements that have been made with respect to the sample preservation process.
Per Method 525.2, sodium sulfite is used to dechlorinate the samples and they are preserved with hydrochloric acid to a pH of approximately 2. The addition of hydrochloric acid is rather confusing since the method states that some of the target compounds are subject to hydrolysis, yet the samples are allowed to sit in an acidic environment – for a number of days if needed – prior to the extraction. The EPA realized this did not make much sense. After all, how can you be expected to recover analytes that have degraded in solution? This question forced the EPA to look for improvements – which have been published in Method 525.3.
In the updated (dare I say, “improved”) method, a buffer is used for preserving samples instead of a strong acid. Rather than using concentrated HCl, 9.8 g/L of potassium hydrogen citrate is added to the samples, which creates a buffered solution at pH 3.8. This environment is acidic enough to restrict microbial growth but not harsh enough to hydrolyze the analytes.
The dechlorination step still occurs when this procedure is followed, however, the dechlorinating agent is changed from sodium sulfite to L-ascorbic acid. L-ascorbic acid is relatively inexpensive and safe to work with (compared to sodium sulfite) so this substitution benefits laboratories which are looking to lower operating costs and maximize technician safety.
The final sample preservation change that was made was in the use of ethylenediaminetetraacetic acid (EDTA), trisodium salt samples. If you recall your inorganic chemistry classes from college, you remember that EDTA is a chelating agent – more specifically a hexadentate ligand. This means that EDTA has a high affinity for binding to metals, and many active sites for metals to bind to (which means a little goes a long way). The removal of metal ions is important because metals can act as catalysts in hydrolysis reactions. EDTA is also a very safe reagent to work with. One of its more common uses is to treat heavy metal poisoning in humans!
The EPA has made great strides toward improving the sample preservation process in the steps outlined in Method 525.3. They have built a method that achieves the goal of improving the process while making the samples safer to work with! Stay tuned for the second part of this 2-part blog series and don’t forget to share this post!