The Chemistry of SPE

If you’re like me, you start the new year off with a list of resolutions for the coming months – resolutions to be more fit or to secure a promotion at work, or to reduce your carbon footprint.

Whether you’re trying to improve your health or further your career, these are the types of goals that I like to refer to as getting “back to basics” because they require you to start with a solid foundation that you can build on to achieve success. With that in mind, I’m starting the year with a review of basic concepts around solid-phase extraction (SPE). When used for sample preparation, SPE can be used for a wide range of samples, especially when the chemical media is tailored to meet the requirements of the application. (Need a refresher on SPE? Check out this blog post (Solid Phase Extraction – What is That?) or watch this whiteboard video.

When you think about it, SPE is a pretty powerful technique. Literally hundreds of compounds – organic, inorganic, volatile, semi-volatile – can be extracted and pre-concentrated from a huge number of liquid samples. But it’s important to remember that the success of an extraction depends on the success of the chemical interactions that take place during the extraction – which are dictated by the media itself and how it’s packed. (Disks or Cartridges – Which Should I Use?) In other words, the accuracy and precision are affected by how carefully you choose your chemical media to selectively retain and selectively elute your analytes.

What does that mean? Let me explain. Some of the common media types used for solid-phase extraction include silica-based C8 and C18, divinylbenzene (DVB), and hydrophilic/lipophilic balanced (HLB) media. These media types can be used to extract compounds including dioxins, furans, herbicides, polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), perfluorinated compounds (PFCs), and many, many others.

There are some media types that can be used to extract a huge number of analytes because their retention mechanisms are less specific (surface adsorption via nonpolar interactions, for example), so you may find that you have choices when it comes to selecting your media.

When will you have few (or no) choices?

• -When you are following a regulatory method which dictates the media to be used – EPA Method 525.3 specifies the use of styrene divinylbenzene (SDVB)
• -When you are retaining analytes using a specific retention mechanism – strong cation exchange
• -When your application requires a customized solution – the use of a mixed-mode disk to retain acidic, basic, and neutral semi-volatile organics in compliance with EPA Method 625.1

-If your application requirements give you flexibility around which media you can use, here’s a helpful disk selection guide that pairs common media types with the compounds and EPA Methods for which they are typically used.

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