SLE (supported liquid extraction) is a sample prep technique that has been in use for over ten years now, but many analytical chemists don’t know about, or understand the best way to do an SLE extraction. In this post, I’m going to talk about how SLE works and the proper way to do an SLE extraction for sample clean up.
It happens to all of us. We’re getting a new method developed and validated and then it comes time to run our negative urines. And everything comes up as positive! There are peaks for our analytes of interest in every urine that we run! How is that possible? In this blog post, I’m going to discuss some of the more troublesome analytes that I’ve encountered as far as finding an actual blank sample and what I’ve done to try and fix the issue.
When running through the exhaustive process of method development, most of us put the majority of our focus on validation and how to complete our crazy validation checklists. Throughout this process, the last thing we want to see is some random hiccup in our workflow. But a whole validation without a hiccup is just wishful thinking, right? Whether your analytes are on back order or you have the misfortune of catastrophic instrument failure, we all experience some type of complication or mishap. One in particular, sample carryover, seems rather innocuous, but without some type of preventive action, it can really ruin a good method. Nonetheless, if you have preventative measures in place, this can be easily preventable.
In my last blog post we talked about LogP and its role in sample prep. Today we are going to discuss the acid dissociation constant, pKa, and how it affects method development. Knowing and understanding the pKa of your compounds tells you if the compound can be ionized, and under what conditions, so you can use this property to develop better sample prep methods.
The goal of sample preparation is to create cleaner samples, collecting the compounds of interest and eliminating interferences – that “junk” we don’t care about the can cause ion suppression and matrix effects that affect sensitivity, accuracy and precision. The ideal sample prep method removes all interfering compounds and produces 100% recovery of all analytes of interest. The problem is that many interfering compounds have properties that are similar to the compounds we need to detect and quantitate. It takes some skill and knowledge to develop a method that washes interfering compounds away and elutes the analytes of interest in a separate step. You need to make sure your washes don’t elute the compounds you care about, and you don’t want interfering junk eluting with your analytes.
Understanding the chemical properties of the compounds in your sample matrix, both the ones you want to detect and the ones you want to eliminate, is necessary for successful method development. Is the molecule acidic or basic? What functional groups are present? Can they be ionized? Is the molecule hydrophilic or hydrophobic? Polar or non-polar? In this post, I am going to discuss the octanol-water partition coefficient, or LogP and its role in sample preparation using supported liquid extraction (SLE) and solid phase extraction (SPE).