US Environmental Protection Agency (EPA) and Department of Defense (DoD) methods for testing per- and polyfluoroalkyl substances (PFAS) in drinking water require using solid phase extraction (SPE). SPE has been used extensively in environmental contaminant analysis both for concentrating large sample volumes (improving method sensitivity) and removing matrix interferences (sample cleanup).

Although SPE is a highly selective method for sample cleanup, there are some instances where testing laboratories might prefer to use direct injection, also known as large volume injection. Direct injection can be a much quicker and simpler option for high-throughput applications such as testing drinking water. Direct injection methods benefit from minimal sample preparation and decreased risk of lab-based PFAS contamination.

Both sample preparation methods are useful as the need for rapid and robust PFAS testing increases. So which method is right for your application? Here, we look at the pros and cons of using SPE vs. using large volume injection.

Using SPE
Using SPE for testing drinking water ensures that your methods meet EPA and DoD requirements. SPE is also required by some US states. This makes SPE a great option when testing for any governmental requirements. SPE is also a well-proven method that delivers robust results for large sample volumes, and SPE cartridges are widely available commercially.

There are some downsides to using SPE, however. SPE requires longer sample preparation times than direct injection, and it requires additional training for sample prep technicians to ensure that samples are not contaminated. SPE can also add some quantitative method variability. This can be largely mitigated by the use of internal standards, but LOQs for PFAS analysis are often limited not by the analytical LC-MS/MS method, but by the variability and background contamination introduced by performing SPE. This can limit the usefulness of SPE.

Using direct injection/large volume injection
Direct injection is less time consuming than SPE and requires less sample preparation, which decreases the risk of lab-based contamination. Direct injection also requires no extraction. This method is suitable for cleaner samples, such as drinking water and some surface and ground waters, and it allows testing laboratories to provide more high-throughput offerings to clients and therefore more environmental testing options.

There are also some cons to using direct injection. It is not allowed in many regulated methods, which limits its application, and it has not been evaluated by the EPA. In addition, dirtier, more complex sample matrices can result in LC system and column clogging or high matrix effects.

When to use which
The method best suited to your application will depend on your unique situation and the needs of your lab. Learn more from our experts about the benefits of using direct injection or SPE methods for PFAS testing to help you make the right choice.