When developing an analytical workflow, it is helpful to keep in mind three common roles solid phase extraction (SPE) can play:
From previous posts, remember that the principles of chromatography apply when trying to achieve your analysis goals, and there are a wide variety of products available that can help you to set up successful SPE methods that meet your analytical requirements.
Sample purification or cleanup is the principal goal of most SPE methods. These methods prioritize selective separation of analytes from other matrix components that could interfere with the analysis. Selectively separating your analytes from interfering and/or contaminating matrix components before performing the analytical separation can make a world of difference.
It’s never ideal to have another compound elute from the analytical column at the same time as a target analyte. But circumstances will affect just how serious that scenario may be.
In the case of GC-FID or LC-UV analyses, for instance, co-elution may make it impossible to quantify the analyte of interest. Worse still, it may lead to a false identification because your instrument can’t tell the interfering peak from the target analyte.
Even MS and MS/MS users can be negatively impacted. Even though an MS may be able to resolve a compound’s spectrum from co-eluting interferences, these co-eluting matrix components can still suppress or enhance the ionization of target analytes. This can result in signals that are biased low or high.
Co-eluting matrix components can also cause problems for baselines. This effect can emerge even when matrix interference does not obscure the peak of interest. Co-eluting matrix components can make baselines unstable enough to render efficient automated integration difficult. Unstable baselines may make it necessary to manually set integration parameters, peak by peak. This creates additional time and labor burdens.
Co-eluting matrix components are, at best, a nuisance. At worst, they can lead to undetected inaccuracies in your reported results. So how do you address their presence in your analysis?
If you’re looking at a sample with relatively few target analytes in a relatively clean matrix, then it may be enough to optimize your analytical separations. Sometimes, though, the matrix is just too complex and/or the list of target analytes is too long. In such cases, optimization won’t be enough. When the chromatogram gets too crowded, you need a way to selectively thin the herd before the analytical separation. SPE may be just the answer.
Preventing Unwanted Interactions:
While some matrix elutes with your target analytes, causing problems during detection, other may never elute in the first place. They stick around, waiting for your next sample injection. In these cases, matrix components from a previous sample may introduce a new, unwanted opportunity for interaction with your target analytes. That built-up matrix can cause the retention of target compounds. That retention can slow their movement through the chromatographic system and can cause distorted peak shapes (i.e peak tailing). These unwanted interlopers can even react with target compounds. These chemical reactions can create whole new compounds that were never in your sample to begin with. When this breakdown occurs prior to the analytical column, it can give rise to new peaks. When the breakdown occurs within the analytical column, it can lead to peak distortion. Such matrix-related system contamination jeopardizes data quality, so it’s best to avoid introducing it in the first place. It may be possible to mitigate these effect by diluting the sample, but when you do that, you dilute your target analytes, too. If you’re finding that dilution is no longer the solution to pollution, solid phase extraction may be your next step.
Avoiding Instrument Downtime:
The accumulation of matrix contamination in your instrument leads to problems over time. In the short term, you may be lucky enough to avoid unwanted interactions between target analytes and matrix contamination, but in the long term, problems will likely develop that will require instrument downtime and maintenance.
In the best case scenario, your instrument consumables and hardware perform well between routine preventative maintenance events. But it’s not uncommon for a batch of samples to fail in between scheduled maintenance. In these cases, maintenance and recalibration must be performed, taking down an instrument that has work to do.
The previous section dealt with the need to separate the target analytes from potentially interfering or contaminating matrix components. In other cases, however, it can be helpful to separate the analytes from each other before performing the quantitative analysis.
What if a sample contains two different compound classes (e.g. aliphatic and aromatic compounds)? Each class might need its own analytical method. In this case, using solid phase extraction can help make gross separations. Using SPE to fractionate a sample allows you to prepare your sample for the fine-tuned separations of the analytical chromatograph.
Chromatographers are frequently faced with the challenge of trying to monitor extremely small concentrations of target compounds. Sometimes those concentrations are well below the reliable quantitation limits of available instrumentation. In these cases, you will likely need some way to concentrate the sample prior to analysis. Assuming you have enough of the sample to work with, tailoring an SPE method that will strongly retain the analytes but allow most of the matrix to be eluted may be the solution you’re looking for. You can collect a detectable mass of your target analytes and then switch up the elution solvent to transfer them to a vial for analysis.
The Benefits Often Outweigh the Cost
In a perfect world, chromatographers would be able to simply inject their sample directly into their instrument with no negative effects. In reality, of course, we live in labs that are pretty far from that dream. It is still tempting, however, to get by with as little sample preparation as possible. Sample preparation has its own cost in supplies, time and labor. Manual sample preparation introduces a possible risk of error into the analytical workflow. There’s also the additional risk that target analytes might be lost along with the matrix components you’re trying to remove.
The problem is, lack of sample preparation generally has negative consequences in the end. You may save time on the front end by not developing an appropriate sample preparation process, but what do you risk?
- Decreased instrument consumable lifetimes
- Increased instrument downtime for routine or unexpected maintenance
- Wasted time reanalyzing samples
- Unnecessary time spent manually integrating peaks
The good news is that there are many different types of SPE products available. There is likely a solution out there for you! The challenge is finding the right combination of products and the right method for your application. Restek is here to help. Don’t hesitate to contact us with your questions about SPE. Remember, solid phase extraction is still chromatography, and chromatographic separations are our business. We can help you find the right solution for your analytical challenges.