Cooling the drop, improving the yield

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  • Published: Nov 2, 2016
  • Author: Rafael Lucena
  • Channels: HPLC / Gas Chromatography / Electrophoresis / Detectors / Proteomics & Genomics / Ion Chromatography / Laboratory Informatics / Sample Preparation / Chemometrics & Informatics / Raman / Proteomics / NMR Knowledge Base / Atomic / Base Peak / X-ray Spectrometry / UV/Vis Spectroscopy / MRI Spectroscopy / Infrared Spectroscopy

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Headspace (HS) analysis is a simple and robust procedure for the determination of volatile and semivolatile analytes in liquid or solid samples. As the reader will know, it is based on the controlled heating of the samples (previously located in a tightly closed vial) which induces the transference of the target compounds to the empty volume (headspace) of the vial. After a defined incubation time, the headspace volume is sampled to be analyzed usually by gas chromatography, although direct mass spectrometric analysis is also possible. In special circumstances, when the concentration of the targets is too low, a preconcentration step is required. In these cases, solid phase microextraction or single drop microextraction (SDME) may be an option.

Working temperature is key in headspace analysis but it becomes critical in HS-SDME. In the latter case, temperature plays two contradictory roles. On the one hand, the use of high temperatures favors the volatilization of the analytes (transference to the HS) and the extraction kinetics. On the other hand, the transference of the analytes from the HS to the drop is hindered at high temperatures as partitioning is exothermic. In addition, high temperature may induce the partial or total volatilization of the organic drop. In this context, one question arises: How should I select the working temperature in HS-SDME? The simplest answer would be: Optimize this temperature as a compromise between the above mentioned contradictory effects. This approach works although it does not provide the best results. In fact, the best way to boost the extraction efficiency is selecting one temperature (high) for the HS generation and another (low) for SDME. Several research groups have been working in this idea during the last years. This post tries to highlight a recent contribution proposed by researchers from Shanghai Jiao Tong University at China. In the article, the authors proposed a novel device that allows the heating of the vial (80 ºC) while the drop is cooled (25 ºC). This temperature gradient explains why the extraction yields are 3-3.5 times better over the conventional (one fixed temperature) approach.

The core of the device is the designed vial cap that plays several roles. It seals the vial but allowing the introduction of a micro syringe for droplet depletion. Additionally, the cap is designed in such a way that permits the continuous flowing of a cooling liquid.

The article is worthy not only for the results (enrichment factors from 302 to 388, limits of detection in the low ng/L range) but also for the mathematical development and simulation presented. The figures included in the supplementary information of the article (free access) are really inspiring.

Here below you will find further resources if you are interested in the potential of SDME.

Main reference:

  1. Anal. Chem, 2016, 88, 10490. Jahan et al. In-Vial Temperature Gradient Headspace Single Drop Microextraction Designed by Multiphysics Simulation.

Other effervescence techniques sources:

  1. Steve Down, Don’t drop it: Stabilising suspended air bubbles for microextraction, separationsNOW, March 24, 2014.
  2. Rafael Lucena, Droplet microextraction for single cell mass spectrometric analysis, Microextraction Tech blog.

Blog post by: Rafael Lucena

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