You’ve just decided to develop a solid phase extraction (SPE) method for a new assay in your laboratory. However, there are so many decisions to be made! Which company do I choose? Which product do I use? Polymer based sorbent or silica based? Cation or anion exchange? While this seems incredibly difficult, it doesn’t have to be that hard! Today, we’re going to walk through this process of figuring out how to develop a starting point for our SPE method.
The first things we need to do is learn a bit about our compounds. For this example, let’s discuss an opiate panel with morphine and codeine as two of our compounds. We need to know what the pKas of our compounds are so that we can figure out what kind of sorbent we used. (Stephanie did a great post about what a pKa is and where to find that information.) Using The Human Metabolome Database, morphine has a basic pKa of 9.12 and codeine has a basic pKa of 9.19. Knowing this, we can figure out what kind of sorbent to use!
Biotage has an EVOLUTE Express User Guide that has a chart to help us pick a sorbent based on the pKas of our compounds. According to this chart, basic pKas of 2-10 are optimally extracted using EVOLUTE Express CX, which is a strong cation exchange sorbent. Great! Now we have a sorbent to try out that should work for our compounds since we picked it based on our compounds.
So how do we figure out an optimal pretreatment? And washes? And elution solvents? We can go back to the EVOLUTE Express User Guide as a starting place. There are several recommendations for this sorbent based on compounds. However, there are a ton of application notes that we can also use as resources! Biotage also has applications chemists that can help to optimize methods specifically for your compounds and instruments.
Developing a new method can be difficult but there’s no need to reinvent the wheel! There are user guides, application notes, and chemists that can help you figure out exactly what will work for your method.