Solid phase extraction (SPE) is a powerful sample preparation tool that makes it possible to extract semi-volatile organic compounds with varying physical and chemical properties. When used properly, this tool will simultaneously extract hundreds of analytes from the most challenging sample matrices. When used improperly – well, this tool can quickly become as effective as using a hammer to paint the walls in your house.
Solid phase extraction is a well-developed technique and there are a huge number of published methods and established workflows. But there are a surprising number of extraction pitfalls that can transform your “brush” into a “hammer.” So keep reading if you want to brush up on your extraction techniques. Below are 7 common mistakes and some tips to avoid them.
- Using the wrong sorbent
Advances in extraction media have left us with an unimaginable number of sorbent materials to choose from. From silica-based sorbents to cross-linked polymers to molecularly imprinted polymers, we can tailor our extraction chemistry to target almost any compound class (or classes). The downside in having an overwhelming number of options is not knowing which media to select for your specific application. Now, there are situations where your options are limited (or removed entirely) by the method you’re following. If you’re performing drinking water extractions per EPA Method 525.2 for example, the guidelines in the method indicate that you should be using “octadecyl bonded silica” – in other words, C18 media. If you’re performing your extractions according to a regulatory method, consult the method guidelines first. You might be surprised to find that your choices are limited – or non-existent. If you’re not strictly following a regulated method, or if the guidelines in your regulated method allow you to choose from a range of media options, keep reading.
The media you choose should be tailored to the analytes you’re targeting for extraction. Silica-based sorbents – C18 in particular – are commonly used because they’re active over a relatively wide pH range, they’re relatively inexpensive, and they will retain a huge number of compounds. Cross-linked, polymer-based sorbents such as divinylbenzene (DVB) or styrene divinylbenzene (SDVB) are suitable alternatives to silica-based sorbents when an extraction involves extreme conditions (harsh solvents, extreme temperatures), or when more chemical capacity is needed. To help guide you to choose a sorbent material for your application, check out this blog post that reviews solid phase extraction media.
- Choosing the wrong media format
Even if you selected a sorbent that’s suited for your application, your extraction could still go awry if your media isn’t packed into the right format. Two of the more commonly used formats include cartridges and disks – each of which presents benefits and challenges for a given sample. A review of disks and cartridges was covered in this blog post so check it out for a comparative overview between the two formats.
It’s important to choose a format that’s suited for your sample – yes, I mean sample. Media should be selected to target your analytes of interest. The media format should be selected to accommodate the sample itself. If your extractions must follow the guidelines in a regulatory method which specifies your media format (EPA Method 537.1 is a good example of this), then your choice is made for you. If you’re following a method which lets you choose your media format, consider both the sample size and the sample matrix. In general, disks are better-suited for larger sample volumes (500 – 1000 mL) and for samples with higher levels of particulate materials. Cartridges are better-suited for smaller sample volumes (<500 mL) and for samples with little or no particulate materials. Media can be packed into either format, so select the one that’s best for your sample.
- Treating SPE as though it’s not a chromatographic technique
Tailoring your extraction can seem complicated and overwhelming, but it becomes more manageable if you treat it like a chromatographic technique. Believe it or not, the retention mechanisms for SPE, LC, GC – and even TLC – are similar. The difference between them is the strength of those interactions. SPE utilizes solvents to completely retain or completely elute target analytes. LC and GC utilize solvent gradients to selectively retain and elute analytes.
The chemical properties of your target analytes influence the retention mechanism/s you should use to optimize your extraction. For example, nonpolar compounds preferentially interact with nonpolar sorbents via van der Waals forces – an optimal situation for these compounds which don’t carry a net dipole moment or a formal charge when they’re in solution. If you want to review some of this solution chemistry, check out this blog post which reviews the retention mechanisms that are commonly used in SPE. If you want an easier method for tailoring your extraction, look to your chromatographic column for guidance. If you’re using an ion exchange column for your LC analysis, chances are good that an ion exchange column would be appropriate for your extraction.
- Processing your sample without considering the matrix
To complete an extraction, the entire sample has to pass through the SPE media. In order to do that, the disk or cartridge must have the physical and chemical capacity to handle your sample matrix. If you followed the first tip, you probably chose media that would provide you with the chemical capacity needed for your application. But what about the physical capacity to filter particles from extremely dirty samples? If you didn’t stop to consider this and your disk or cartridge clogs during the extraction, odds are pretty good that you’ll have to recollect and re-extract that sample.
How do you avoid this pitfall? Evaluate your sample matrix and assemble a stack of pre-filters that will capture all of the particles in your matrix before the solution reaches the SPE media. This blog post provides guidance for building a custom pre-filter stack, based on the challenges of your sample matrix. It might take some experimentation to build the necessary layers of a pre-filter stack, but it’s worth the effort when your analyte recoveries improve and your sample processes without clogging your disk or cartridge.
- Evaporating without drying
Solvent drying is the key to successful solvent evaporation and shouldn’t be overlooked. This is the step in your preparation procedure that removes excess water from your extract, so your evaporator can drive off the organic solvent and reduce your extract volume down to where it needs to be for analysis. Most evaporation systems are set to run at relatively low temperatures to accommodate the boiling point of the organic solvent you’re working with. So if you have water in your extract, most of it will still be there after you’ve evaporated your sample. The extra water will impact the accuracy of your results. If your results are calculated gravimetrically (i.e. by weight), water will severely impact your data quality.
Regardless of how you dry your solvents – chemically or physically – be sure you effectively remove all the water from your extract before trying to evaporate.
- Using the wrong solvent grade
Solvent purity can have a surprising effect on the outcome of an extraction. Almost all published and regulated methods recommend a minimum purity for each of the solvents that are used. You’re certainly welcome to use solvents that are purer than what’s required, but not many labs choose to purchase high purity solvents when they don’t need to. Higher purity usually equates to higher prices.
Given the price of high purity solvents, the temptation to save money can drive some laboratories to experiment with lower grades. This may seem like a harmless swap, but the impurities in lower grade solvents can trigger unwanted reactions, by-products and interferences that reduce your analyte recoveries. If you’re using low-grade drying chemicals (sodium sulfate, for example), they may contain water impurities. This unwanted water reduces the chemical’s effectiveness and can create an opportunity for water-soluble analytes to extract back out of the organic solvent and into the water impurity.
If you need a refresher on reagent purity, check out this blog post on solvent grades.
- Underestimating the importance of flow rates
If you’ve successfully avoided the 6 pitfalls listed above but are still suffering from low analyte recoveries, you may need to look at the solution flow rates you’re using. Using significantly high flow rates can cause solution to break through your disk or cartridge, which will reduce your analyte recoveries. But solution breakthrough is usually visible and your recoveries will be significantly lower than expected. So if you’re using flow rates that are causing breakthrough, chances are good that you’ll catch this error pretty quickly.
But what if your flow rates are just slightly too high? If this is the case, your analyte recoveries might only be slightly low and you probably won’t have visible indications (like solution breaking through the bottom of your disk or cartridge) to tell you that your flow rates are too high. Flow rates affect how well you can recover your target analytes and it only takes a slight increase or decrease to negatively affect your data quality. Your target analytes are exposed to the SPE media as the sample passes through the disk or cartridge. If the interactions with the media become favorable enough, the analytes transition out of the sample and retain on the media.
If you’re using a flow rate that’s too high, the analytes won’t have enough time to interact with the SPE media and some of them – typically the hydrophilic ones – will stay in the original sample and pass straight through to waste as the sample is loaded. Conversely, if you’re using a flow rate that’s too slow, you may suffer from lower recoveries due to analytes that retain (and stay retained) on the media. So double-check your method conditions and consult this blog post on SPE flow rates for additional guidance.
Don’t worry if you’ve made any of these mistakes. Hopefully, you have the tools to avoid making these mistakes in the future and you’ll use these tips to improve your SPE technique.
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