Solid-phase extractions (SPE) can be a long and sometimes complicated process. So, we want to make sure that everything works the first time we extract. In this blog post, I will be discussing some of the benefits that we see when using positive pressure for SPE.
There are several different techniques we can use to extract. We can use a manual vacuum extraction or we can use manual or automated positive pressure to extract.
First, let's go over the basics of using a vacuum manifold to extract. It's a pretty simple concept: you have a manifold for either SPE cartridges or SPE plates. If you're using cartridges, you have to block off all unused cartridge positions on the manifold. Add solvent to the cartridges or wells of the plate. Apply vacuum to pull the solvent through the wells.
Now, let's go over the basics of using positive pressure for SPE. Again, you have a manifold for either SPE cartridges or SPE plates. When using cartridges, you don't have to block off the wells that you aren't using. Add solvent to the cartridges or wells of the plate. Apply positive pressure to push the solvent through the wells. The gas used is typically nitrogen as it is much cleaner than air and it doesn't introduce any contaminants into your samples.
It's also possible to automate a positive pressure method. Automating sample preparation might seem like a major step, but if you follow a few guidelines then you can ensure that your chosen automation solution really does meet your needs.
Advantages and disadvantages
As you can see, both are fairly simple extraction techniques. However, there are some disadvantages to using a vacuum manifold to extract. Since the system is based on vacuum, the wells where solvent flows through faster dry more quickly than the other wells. This could result in full drying of the sorbent in some cartridges or wells in our batch. This could affect our samples and the recovery of our analytes of interest as we don't always want to dry our columns after adding our samples or after washing our columns. The vacuum is also very difficult to regulate. You usually have a gauge on the manifold that will tell you how much vacuum is being applied to your cartridges. However, we don't know that the set vacuum pressure is actually being applied to all of our cartridges equally. As soon as the solvent is pulled through one cartridge, the vacuum applied to the other cartridges changes.
When using positive pressure, we don't have to worry about vacuum changes due to one cartridge having faster flow than other cartridges. The pressure applied to push the sample or solvent through the cartridges remains uniform regardless of whether there is 3 mL of solvent in a cartridge or nothing in a cartridge. Because we can push the solvent through the cartridges more uniformly, our analysis results become more reproducible. We get higher recoveries and decreased matrix effects for the same samples when compared to using a vacuum manifold. The nitrogen we use for positive pressure is a clean gas so we don't have to worry about contaminants being introduced into our samples.
Positive pressure is also much easier to use when trying to analyze viscous samples. The vacuum isn't always strong enough to pull those samples through the cartridges. However, when using positive pressure, the gas flow can be increased to allow those samples to flow through the cartridges.
American Laboratory® magazine published an article in which they compared an automated positive pressure SPE method to a manual vacuum SPE method. They found more reproducibility and higher recoveries when using the positive pressure method, just like we have found.
When developing new methods, I try to stick with positive pressure, whether it is an automated or manual method. It saves me time and I get better, more reproducible results with higher recoveries and lower limits of quantitation.