Regardless of the particular product, ion exchange solid phase extraction (SPE) works on the same basic principal. When it comes to retaining species and subsequently eluting them from the ion exchange SPE sorbent, you start with a charged species in your sample and an oppositely charged sorbent surface. This will create an electrostatic attraction that will result in the charged species being retained by the oppositely charged surface. Neutral and oppositely charged species won’t be retained (at least not by the ion exchange mechanism at play – more on this later, though). When the time is right, you “turn off” the charge at one of the two places – target compound or sorbent surface - breaking the electrostatic attraction and the previously retained species can now be eluted from the SPE product. So, start with charged species and surfaces for retention, then turn the charge off for one or the other to elute. Sounds simple, right?
Matching Sample Type to Ion Exchange SPE Product
|Compound to be retained by Ion Exchange SPE||Charge state of the retained compound||Matching ion exchange product||Charge state of the ion exchange product|
|Strong acid||Negative (anion)||Weak Anion Exchange||Positive (cationic) under low pH conditions|
|Weak acid||Negative (anion) under high pH conditions||Strong Anion Exchange||Positive (cationic)|
|Strong base||Positive (cation)||Weak Cation Exchange||Negative (anionic) under high pH conditions|
|Weak base||Postive (cation) under low pH conditions||Strong Cation Exchange||Negative (anionic)|
In actual execution it may not always seem that simple, though. Where it might get a little tricky is making sure that you actually have started with charges in all the right places, that you’ve matched the charged species with the right charged sorbent surface. Then, you have to make sure that you change conditions sufficiently to effectively “turn off” the charge someplace, and where that place should be will depend on the sample, as we’ll discuss.
To help make these method development choices a little easier, it’s helpful to remember that ion exchange SPE is a world of opposites. Positive charges attract negative charges. Weak ionic character should be paired with strong ionic character. Weak acids need to be turned into their conjugate bases, and vice versa. Up is down…you get the picture. :)
Let’s start with your sample, which is always a good place to start for any sample preparation method development. If your sample contains species that carry permanent charge under SPE-friendly conditions (i.e. not operating at the extremes of the pH range), that compound has a “strong” ionic character. “Strong” ionic species can’t have their charge “turned off” by simply changing experimental conditions. Assuming this is a compound we want to retain initially but then eventually elute, we’ll need to be able to “turn off” the charge someplace. In case of the compound having a strong ionic character that can’t be turned off, we’ll need to rely on turning off the charge at the sorbent surface instead.
This leads us to using a “weak” ion exchange sorbent when we have “strongly” ionic compounds in our sample. The “weak” characteristic describes compounds or sorbent surfaces that can have their charge state changed by a relatively simple change in the surrounding conditions.
Let’s continue our example with a few more specifics. What if our strongly ionic compound is a strong base and carries a positive charge, making it a cation. We’ll want to pair that compound with a weak ion exchange sorbent that has a surface with an opposite, negative (or anionic), charge. That weak ion exchange sorbent’s charged surface will already have a positively charged species present at the surface. Nature hates to leave a charge unbalanced, after all. But when the cation in the sample shows up, it can change places with that other cation and thus be retained – this is the “ion exchange” process that gives the products’ their name. So, even though we’re looking for an ion exchange sorbent with a weakly anionic surface, we’ll be looking for an SPE product that’s described as a “weak cation exchange” sorbent. The name has two parts, one part describing the ionic strength of the sorbent (weak) and the other part defining the kind of ion the sorbent attracts (cation), not the surface charge of the sorbent itself. The resulting retention between the weak cation exchange sorbent and the strong cation in our sample is likely going to be strong enough that, when/if the time comes when we to want to elute that strong cation from the surface of the SPE sorbent and collect it for analysis, we’re going to need to turn off the charge at the sorbent surface, releasing the cation for subsequent collection. We’d do this by lowering the prevailing pH, which will flood the system with positively charged species that will eject the compound of interest. We need to make more competition for those charged sites on the sorbent, in other words, and likely also help to repel our positively charged compound away from the surface and back into solution.
Were we to use a strong cation exchange sorbent instead, then we would have a strong cation paired with a strong cation exchange sorbent, and we wouldn’t have a way to turn off the charge at either the sorbent surface or the compound (at least not without creating other problems under the aggressive condition changes that would be necessary). We would have effectively trapped the charged species in the sorbent. This is not generally the goal, which is why the recommendation is to avoid using SPE products described as “strong” ion exchange sorbents when your sample contains permanently charged compounds of interest – your recoveries will suffer if you pair “strong” with “strong.”
If we turn the tables, and instead consider a sample that contains a weakly ionic species, like a weak base, we’re going to see that pairing it with a strong ion exchange sorbent is going to give us the retention we need when both parties are in their charged state. This is the “strong-to-weak” pairing that is important for successful ion exchange SPE methods at play again. When we want to elute our weak base analyte, we will be able to “turn off” its charge with a change in the prevailing conditions, similar to how we turned off the charge state of the surface of the sorbent in the previous example. In this instance, though, part of the trick is starting with the analyte almost fully converted into its charged state.
In our example of a weak base, we can look up the acid dissociation constant (pKa) for the compound of interest (hopefully it’s available), which will give us the pH condition at which half of the compound’s population will exist in solution in the charged state and the other half will exist in the neutral state. Starting with sample conditions whose pH value is at the pKa value of the compound is not a good idea. We’d only be retaining half of our analyte population on the ion exchange sorbent (the charged half) while we wash away the neutral half! Our recoveries would not be good! Instead, we would want to drive the majority of the analyte population into the charge state, which we would accomplish by adjusting the pH. Typically, the advice is to change sample pH conditions two pH units away from a compound’s pKa in the appropriate direction to drive the compound into the charge state (as opposed to the neutral state). For our weak base example, we want to convert the weak base into its charged state (aka the weak base’s “conjugate acid”), which will have a positive charge, so we want to lower the pH by around two units from the compound’s pKa value.
Under those conditions, most of the analyte will be in the positively charged cation state and will be ready to interact with the negatively charged strong ion exchange sorbent surface. Again, matching “strong to weak” is important, so the strong cation exchange sorbent would be the best choice to ensure good retention of the weakly charged analyte. It’s generally not advised to pair a weakly ionic compound with a weak ion exchange sorbent. The retention may not be strong enough to allow for effective sample cleanup with high enough recoveries.
Once the time comes to elute the retained target compound cation, we’ll need to turn off the charge of the compound (the weakly ionic member in the interaction), which we will do by changing the elution solvent pH, now swinging in the opposite direction (there’s another case of those “opposites” at play again!). By raising the pH above the compound’s pKa value (again, by two units if possible), we’ll convert the compound to its neutral molecular state which will break the electrostatic attraction with the permanently charged surface of the strong ion exchange sorbent and allow the compound to be eluted.
And in the final example of our rule of opposites, the opposite would apply if the above examples dealt with strong or weak acids!
It feels like a lot of keep track of, honestly, but if you keep the principals of “opposites” in mind, it might help when you are deciding if an ion exchange SPE product is right for your application.
One type of compound that we haven’t discussed so far is a zwitterion. Zwitterions are compounds that have positively and negatively charged groups at mid-pH ranges. It’s best to make sure the pH conditions are such that the zwitterion is charged one way or the other, depending on your ion exchange sorbent and your other analytical objectives.
Also, as promised at the beginning of the article, you can find essentially purely ion exchange sorbents (e.g. silica-based ion exchange SPE products) or you can find ion exchange sorbents that also have additional retention mechanisms built-in, like the polymeric-based ion exchange products that also have reversed phase retention characteristics, making them capable of retaining neutral, non-polar compounds as well as charged species. For samples containing a wide variety of compound chemistries, these polymer ion exchange sorbents are often a good choice.
Hopefully this blog post helped you identify which ion exchange SPE product might be a good fit for your analysis. And don’t worry, if you feel like you know which product to pick, that product will come with a set of instructions that provide details for developing the actual SPE method. If you have any questions or comments, your Restek Sales Representative is ready to consult with you about your sample preparation needs.