Quechers dSPE selection - which one is best?20 Apr 2020
Method development for Quechers sample cleanup can be a complicated task. Not only does the analyst need to make sure their analytes of interest can be recovered, but the matrix interferences must be removed enough to make the quantitation possible and reliable. It makes sense that selecting the best dSPE products is highly dependent on the specific matrix that is in the sample.
After you complete the 1st part of the Quechers procedure, which is extraction with a Quechers salt, you will need to take a subsample (aliquot) of the supernatant from that extraction to apply sample cleanup with dSPE. If you are lucky, there may be a validated method an analyst has performed and published that you can use for as a guide for your analysis. Even so, you will need to make sure the conditions are optimized. Usually if there is no existing method to follow, the analyst needs to develop a method for their specific combination of analytes and sample matrix.
The first issue to consider is which size of Quechers tube you need for dSPE, since we sell the sorbents in smaller quantities in 2 mL centrifuge tubes or larger quantities in 15 mL tubes. These are designed for extract aliquot volumes of 1, 6, or 8 mL. If you are only analyzing the final extract one time with one analysis method, a small aliquot volume of 1 would be fine to use. This would call for using the 2 mL tube. If you will need to split the final extract for 2 or more analyses, a sorbent mixture in a 15 mL tube, designed to process aliquot volumes of 6 or 8 mL, will give you more extract to work with. The following table from our instruction sheet shows the aliquot volumes for which each of our dSPE products are intended.
Selecting the best sorbents to use depends on what you need to remove from your sample. Removing interferences is only about two thirds of the battle, though. You also need to determine which combination of sorbents will remove the interferences from the matrix AND still leave your analytes intact enough to give good recoveries. Here is a link to our listing of dSPE products on our website: Q-sep QuEChERS dSPE Tubes for Extract Cleanup
To optimize the removal of interferences, you can select sorbents as follows to fit your sample matrix characteristics.
- To remove residual water from initial extraction- use MgSO4
- To remove organic acids, fatty acids (especially smaller ones), and sugars – use PSA (primary and secondary amines)
- To remove fats/lipids and waxes- use C18 (endcapped C18 bonded to silica)
- To remove pigments- use GCB (graphitized carbon black)
The above listed are the main categories, but be aware there is some crossover, because each sorbent has secondary interactions also that may come into play. (Be on the lookout for more discussion of these in the next blog post.)
HOW MUCH SORBENT
Aim high or low for each sorbent, depending on the levels of each type of interference.
For example, if your sample is a vegetable, it may contain a low amount of sugars or acids. Therefore, I would start with a minimal amount of PSA.
Another example-if your sample does not contain much pigment- let’s say it is something like celery. In that case, you probably do not need GCB, but you may need a small amount of C18 because there will be a bit of waxy material and cellulose. PSA will help with the cellulose also.
A 3rd example- your sample is highly pigmented and a high amount of sugar, like red grapes. You will need PSA and GCB in your sorbent selection. You may need a small amount of C18 as well, but you could try without it to see which way works better.
HOW TO MAXIMIZE RECOVERY OF ANALYTES
The good news is that most of the dSPE sorbents are safe for pesticides because that is what Quechers was originally developed for. There is one exception- GCB does have the potential to interact with planar nonpolar molecules. There are a few pesticides that are planar and prone to such interaction. If those are important targets in your analysis, you might consider not using GCB or keeping it to a minimum. If you are analyzing something like PAHs (Polynuclear aromatic hydrocarbons) instead of pesticides, keep in mind that most of those compounds are planar and would have a strong interaction with GCB. Most of the successful Quechers methods used for PAHs use a minimal amount or no GCB.
The final determination for sorbent selection should be made by experimentation and method development. Once you arrive at a combination or sorbents that you think might work best, you can perform a method validation to ensure reliability and ruggedness. (Validations include processing a set of control samples that are fortified with analytes prior to extraction to mimic levels that one might encounter for real samples. The concentrations and number of replicates required are usually determined by regulatory guidelines or protocols.)
Thank you for reading. Be on the lookout for another blog post regarding Quechers dSPE modification.