Have you ever wondered why solution flow rates are so important when performing sample preparation with solid phase extraction (SPE)? If you have, read on – I have the answer for you!
Throughout my college career, the phrase “like dissolves like” was referred to quite frequently. This phrase was particularly relevant when we did solubility experiments and for good reason – it’s 100% true! Solvents tend to dissolve solutes with physical and chemical properties that are similar to theirs. Other factors such as temperature, pressure and pH can affect the solubility of solutes as well, but let’s just keep it simple for the purposes of this discussion and keep it focused on physical and chemical properties. Given this simplistic definition of solubility, the opposite stands true as well – solutes don’t tend to dissolve into solvents with differing physical and chemical properties. These solvents and solutes want to stay as far from each other as is possible.
This same principle applies to a water matrix with a mixture of hydrophobic and hydrophilic compounds. When performing an extraction by SPE, hydrophobic compounds adhere to the media within the disk because the media is hydrophobic – which allows us to use hydrophobic media to extract hydrophobic analyte compounds from water matrices without too much difficulty.
Unfortunately, hydrophilic compounds are a bit more challenging to extract from the water matrix due to the fact they are hydrophilic. These compounds want to stay in their original water matrix when you’re loading your sample onto an SPE disk.
How do you overcome this issue?
Believe it or not, increasing the amount of time that the hydrophilic compounds are in contact with the media, increases the recoveries of these compounds. This statement doesn’t just apply to chemistry, though. There are many examples within nature where flow rates play an important role. If water moves too quickly through rivers and streams, certain animals would have a hard time making a home or would have to adapt in order to survive. For instance, beavers won’t build their dams in a fast flowing river because their homes would be destroyed or the entrance would be blocked by the moving river bed below. Therefore, to protect themselves and their families, they build their dams in water that flows slowly.
Another example of this is with those water filter pitchers that people use to filter their household tap water. Those pitchers can take some time to fill, because the water passes through the cartridge somewhat slowly. This is by design, though. If the water passes through those cartridges too quickly, the water would not be filtered as well, and the odd taste you were trying to get rid of would still be in your water. The media in the cartridge needs to be exposed to the tap water for a longer time to effectively retain all the target analytes (which includes those hydrophilic compounds).
With that example in mind, we apply this to solid phase extraction. If you have a clean one-liter sample that passes through a solid phase extraction disk in three minutes, the recovery of the hydrophilic compounds in that sample will be very poor. These compounds do not have enough contact time with the media to ensure adequate retention. Instead, they go straight to water waste, much like the beaver dam that would flow downstream in a current that is too strong. On the other hand, if you have a clean one-liter sample that passes through a solid phase extraction disk in twelve minutes, the recoveries of those same hydrophilic compounds will be greatly improved. In this scenario, the media has more time to interact with the analytes, allowing them to be retained on the disk instead of passing straight through to waste. This is much like the beaver dam that will stay in the place it was built in slower-moving water.
At this point, you’ve probably figured out that since flow rates affect the recovery of your analytes, you can use your experimental data to optimize the flow rates in your method, and to verify that your pumps are delivering solution at the flow rate they are supposed to be (for those of you using automated extractors). An example of this would be in measuring the recovery of dimethoate and caffeine, when performing drinking water extractions per EPA Method 525.2. These analytes aren’t included in the compound list that’s published in that method, but they are both hydrophilic and are good indicators of how quickly solution is passing through your C18 disk. If these compounds do not have enough contact time with the media, they will head straight to waste and you’ll end up with poor recoveries for these 2 analytes.
Check out this application note with examples of optimal flow rates and remember to double check all your flow rates, particularly when your analyte recoveries seem low. You might be losing some important compounds!