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SPE Sorbent Screening

We have previously considered the first stages of solid phase extraction (SPE) method development, that is, analyte assessment and mechanism selection.1,2  Once a primary extraction mechanism has been selected, e.g. polar, non-polar, ion exchange etc., several sorbents which exhibit the chosen mechanism should be screened – this screening process provides the flexibility of having multiple effective sorbent choices to ensure the cleanest extracts for the final protocol.

There are many different sorbent choices for non-polar, polar, and cation exchange mechanisms, whereas, the choices for anion exchange and mixed mode are more limited.  Commercial SPE suppliers offer kits for sorbent screening which facilitates the process while also allowing the use of many different sorbent chemistries without investing in single boxes of each sorbent, making these kits efficient and cost effective.  Examples of appropriate choices for each of the extraction mechanisms are as follows:

  • Non-polar – C18 (multiple varieties if available), C8, C4, C2, cyano, and phenyl silica-based phases and no-polar polymeric phases
  • Polar – unbonded silica, aminopropyl, diol, non-endcapped sort chain hydrocarbon (e.g. cyano, C2, C4, and phenyl) silica-based phases.  Ion exchangers – both silica and polymer based.  Ion exchangers typically perform as polar phases in non-polar organic solvents
  • Cation exchange – weak and strong exchangers such as carboxylic acids, sulfonic acids in silica and polymer phases, plus non-endcapped short chain hydrocarbons (e.g. cyano, C2, C4, and phenyl, in which unbonded silanols act as cation exchangers)
  • Anion exchange – quaternary amines, aminopropyl, diethylamino, and diamine phases, in both silica and polymer versions
  • Mixed-mode – first decide which ion exchange mechanism to use then screen all available mixed mode phases in both silica and polymer formats
  • Chelation – these phases are typically created by the user by starting with either and aminopropyl surface or a dicarboxylic acid sorbent, then treating with copper sulfate to give a sorbent that will complex amines or carboxylic acid containing analytes

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The steps used in the screening process are specific to the respective mechanism being tested.  In general, the screening process is as follows:

  • All the sorbents are conditioned and pre-equilibrated identically (Figure 1, Screen 1)
  • Analyte standards in a matrix (modified to make it suitable for the mechanism being explored) are applied to the sorbents, and fractions collected to screen for breakthrough (Figure 1, Screen 2)
  • Sorbents exhibiting adequate retention are then screened for elution potential using appropriate solvents; fractions are collected to verify proper elution
    (Figure 1, Screen 3)
  • Analytes spiked into actual sample matrices are put through initial protocols, and extract analysed to verify adequate cleanliness (Figure 1, Screen 4)
  • If clean up is insufficient, wash steps are added to improve extract quality

Throughout this process careful notes should be taken, particularly of solvents that do or do not elute analytes from the sorbents.  Solvents that do not elute analytes may be very good candidates later I the process as wash solvents.

Figure 1. Sorbent screening process.

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Dr. Dawn Watson

This article was written by Dr. Dawn Watson.

Dawn received her PhD in synthetic inorganic chemistry from the University of Strathclyde, Glasgow. The focus of her PhD thesis was the synthesis and application of soft scorpionate ligands. As well as synthetic skills, this work relied on the use of a wide variety of analytical techniques, such as, NMR, mass spectrometry (MS), Raman spectroscopy, infrared spectroscopy (IR), UV-visible spectroscopy, electrochemistry, and thermogravimetric analysis.

Following her PhD she spent two years as a postdoctoral research fellow at Princeton University studying the reaction kinetics of small molecule oxidation by catalysts based on Cytochrome P450. In order to monitor these reactions stopped-flow kinetics, NMR, HPLC, GC-MS, and LC-MS techniques were utilized.

Prior to joining the Crawford Scientific and CHROMacademy technical team she worked for Gilson providing sales and support for the entire product range including, HPLC (both analytical and preparative), solid phase extraction, automated liquid handling, mass spec, pipettes, and laboratory consumables.

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