ICE concentration for improving stir bar sorptive extraction

Freeze concentration (FC) is a separation technique scarcely exploited for sample treatment in Analytical Chemistry although it has been successfully applied in the industrial field. For example, FC is used in the food industry to extract water avoiding losses of (semi)volatile and thermolabile compounds producing extracts enriched in the solutes of interest. FC is based on the fact that solutions (water plus solutes) have lower freezing points than pure water. Therefore, when a solution is cooled down at an appropriate temperature, ice crystals start to be formed excluding solutes which are concentrated in the remaining liquid phase.

Prof. Logues´s research group at South Dakota State University (USA) have applied this principle in an innovative way to improve the extraction yield of stir bar sorptive extraction (SBSE). The overall extraction procedure is quite simple as it only requires cooling the vial that contains the sample while SBSE is performed. The cooling of the samples induces the ice formation which grows bottom-up leaving a more concentrated aqueous phase in close contact with the stir-bar. This procedure, therefore, increases the concentration gradient between the liquid phase and the stir-bar improving the transference of the analytes. No doubt, the low temperature is also beneficial from the thermodynamic point of view as sorption is exothermic. After the extraction, the stir bar is collected from the vial for the final elution or thermal desorption of the analytes.

 

Figure 1. SBSE and ICECLES of methyl violet (MV) in water

Figure 1 shows the different performance of SBSE for the extraction of methyl violet (MV) as model compound when it is used alone or in combination with FC. The purple color on the stir bar indicates the amount of MV extracted during sample preparation, with ICECLES clearly increasing the amount of MV on the stir bar.

Prof Logue states that "ICECLES holds the potential to more easily permit currently difficult, and sometimes unachievable, ultratrace analysis and to expand the reach of comprehensive analysis (such as flavor analysis) to a wider range of compounds. We have seen evidence of this with signal increases of over 1000 times (ICECLES versus SBSE) for single analytes, the ability to highly concentrate difficult to extract polar compounds, and the ability to detect many more compounds (up to 50% more versus SBSE) during comprehensive analysis"

 

Figure 2.  GC/MS chromatograms of benzaldehyde prepared via SBSE and ICECLES

The combination of FC-SBSE improves the sensitivity over simple SBSE in the range from 1.5 to 168 times. As a way of example, Figure 2 shows GC/MS chromatograms of benzaldehyde prepared via SBSE and ICECLES. ICECLES produced a signal increase of about 150 times over SBSE for this analysis.

We encourage our followers to read the complete article which is available at the journal webpage. It is really inspiring and it explains in detail all the potential of this technique.

Reference

(1) ICE Concentration Linked with Extractive Stirrer (ICECLES). Link to the article

Special acknowledgment. We would like to thank Prof Logue for his kind help, providing the figures and sharing his thoughts with our followers. Prof. Logue is the copyright owner of the Figures 1 and 2.