In the world of solid phase extraction (SPE), the list of media that is available seems to be ever-growing. From polymeric stationary phases, to silica-based media, and even molecularly imprinted polymers specifically designed for target analytes. The possibilities seem endless. Luckily for us, most EPA methods specify which media type is required for analysis, but what about methods that don’t specify?
For the methods which don’t specify the media you must use, how do you select your media type?
The answer is, it depends! It depends on the compound (or compounds) you’re trying to extract. The principle of SPE is based on retention, so you need to identify which retention mechanism is best suited for your target analytes. For many EPA methods that involve the analysis of aqueous samples, the best approach is the use of a reverse phase retention mechanism. This narrows down your media choices a bit, but there are still a handful of media options that are very commonly used. Octadecyl bonded silica (more commonly called C18) and polymeric divinylbenzene are the two most common reverse phase media types.
C18 media has dominated the reverse phase separation market and probably will continue to because it works great and it’s cheap. C18 is even used as the stationary phase for most reverse phase HPLC columns. However, just because it is the predominant media in reverse phase separations, that does not mean it’s the best choice for your application. This media has some glaring downfalls that must be dealt with accordingly. First and foremost, C18 media requires activation using methanol. Unfortunately, once the media is conditioned with methanol, it cannot go dry – otherwise, the active sites will not retain any compounds. Another challenge when using C18 is that the surface of the bead contains residual silanol groups (in other words, free –OH groups on the surface of the silica material) which can interfere with the retention mechanism and, therefore, the extraction process. For this reason, C18 media is strongly advised for use between pH 2 and pH 8 only. Below pH 2, there is measurable silanol activity which interferes with the retention of your target analytes. Above pH 8, the silanol groups become deprotonated, which creates an ion exchange effect and changes the retention mechanism. To reduce this silanol activity, the media is typically “endcapped” (most commonly with trimethylsilylchloride) to form O-Si(CH3)3 groups instead of OH groups. This renders the media non-polar and helps to reduce silanol activity on the surface of the media; however, it’s important to note that some interactions will still occur which interfere with the retention of your target analytes.
Polymeric divinylbenzene (DVB) is a more robust media than C18. DVB is cross-linked in order to make sure that the polymeric shape is fixed, which ensures that the polymer is resistant to solvents and extreme temperatures. Cross-linking also increases the durability of the polymer. The only atoms that make up the structure of DVB are carbon and hydrogen so you will not get the same interferences you do with silica-based media. DVB has more surface area and, therefore, more active sites than C18 because it does not contain sterically hindered octadecyl groups. DVB media is more expensive than C18, however, less media is needed to achieve the same retention as C18, so your cost per sample doesn’t change significantly.
Another option is a hydrophilic-lipophilic balanced (HLB) media. This type of media is made up of DVB which is copolymerized with N-vinylpyrrolidone. The DVB captures the compounds that are separated using a reverse-phase mechanism, where the N-vinylpyrrolidone captures the more polar analytes. This multi-functional media allows a greater range of compounds to be captured.
Mixed-mode media such as weak or strong anion exchange (WAX, SAX) as well as weak or strong cation exchange (WCX, SCX), combine a reverse phase mechanism with an anion or cation exchanging group to retain compounds based on their charge.
With all these options, solid phase extraction can be a very powerful tool in separating out the analytes of interest. However, it requires an understanding of which retention mechanism best fits your analyte of interest. Please feel free to share what type of media that your lab uses for certain performance-based methods.