Impactful Sample Prep Series: EME Turns 20

In the late 1990s, my former colleague Knut Rasmussen and I were working on hollow-fiber liquid-phase microextraction (HF-LPME) (1). The idea was to develop a robust formate for miniaturized liquid-liquid extraction, where the analytes of interest were extracted from aqueous sample, through an oil membrane (organic solvent immobilized in a porous polymeric membrane), and into acceptor. The acceptor was either an organic solvent or an aqueous buffer. We mainly worked with pharmaceuticals, and these were extracted into aqueous buffer based on a pH gradient. For basic pharmaceuticals, the sample was made alkaline, while the acceptor was acidic. The basic analytes were extracted as neutral species across the liquid membrane, and into acidic acceptor. Here the molecules were trapped due to protonation. HF-LPME provided excellent cleanup and enrichment, acceptors were injected directly into LC-MS after extraction (no evaporation and reconstitution), and the consumption of organic solvent was limited to a few microliters per sample. However, a lengthy (30-45 minutes) extraction was required to reach equilibrium.

In the autumn 2004, we began discussing the prospect of controlling and accelerating the extraction by applying an electrical field across the liquid membrane, provided that the analyte was charged both in the sample and acceptor. In our old introductory organic chemistry courses, we were told that organic substances with charge have no partition across water-oil interfaces. But what happens, if a strong electrical field is applied across the water-oil interface? As curious experimentalists, we tested different oil membranes. Initially, we weren’t successful. But, when we tested 2-nitrophenyl octyl ether (NPOE) – one of the solvents available in our laboratory – as an oil membrane, we surprisingly obtained very fast extraction.

After a couple of more days of successful experiments, including extraction from human plasma and urine samples, we understood a new extraction technique was born. We then checked the literature for similar extractions, but we were unable to find any. We then contacted the technology transfer office at the University of Oslo, and they decided to file a patent application. This was followed up by our first article on EME published in 2006 (2). We initially termed the technique “electromembrane isolation” but this was later replaced by “electromembrane extraction” (EME).

On November 4 this year, we can celebrate that EME is 20 years old. Over the past two decades, more than 500 papers have been published on EME – spanning applications, fundamental understanding, and technical formats. Applications include pharmaceutical substances, environmental pollutants, and contaminants in food and beverages. These papers contain optimized extraction conditions and performance data for a large variety of analytes, and are extremely important for future development and use of EME. The papers on fundamental understanding are the scientific anchor of EME, and are mandatory for the acceptance of the technique. The papers devoted to technical formats illustrate that EME can be adapted to 96-well plates and to microfluidic devices. Thus, among others, EME is amenable to high-throughput operation and to operation in-field in combination with smartphones and other handheld detection devices.

About 20 percent of the EME papers originate from our laboratory in Oslo; the rest have been published by more than 200 scientists from 45 different countries. All experiments up to date have been done with laboratory-built systems. Very recently, however, a Norwegian company launched commercial equipment for EME, based on the use of conductive vials. Thus, the traditional use of platinum electrodes has been omitted, and the electrical field is now coupled directly through the containers. From my point of view, this first-generation commercial equipment represents a huge step forward for EME, because the technique now becomes available for all laboratories, and experiments reported with this equipment of industrial standard can easily be repeated by other laboratories.

Based on the papers in the literature, and due to the release of commercial equipment, I foresee increased interest and use of EME in the near future. The new activities may go in two different directions. In one direction, EME may be considered as a new and alternative extraction tool for laboratory use in areas such as pharmaceutical, environmental, food, and beverages analysis. Development of EME in this direction may be justified by greenness (due to its low solvent and sample consumption), and next-generation analytical scientists will definitely have a strong focus on this. Very efficient sample cleanup, and acceptors directly injectable in LC-MS are additional advantages in favor of EME for pharmaceutical, environmental, food, and beverages analysis. Development of 96-well plates for the commercial EME equipment is in progress, and I expect this will accelerate the implementation of EME in the laboratory direction.

In a second and highly innovative direction, one can look into EME much more fundamentally, as a separation principle based on transfer across a liquid-liquid interface under the influence of an electrical field. My feeling, after 20 years of EME experiments, is that this principle can be used for much more sophisticated separations and applications than reported up to date. This is a core part of our research – taking EME to the next level of sophistication via new and innovative experiments. The rest of our research is currently in the laboratory direction, devoted to the development of robust generic EME methods for a variety of chemical substances and biomolecules, according to their charge and polarity.

I hope many scientists will join in the years to come, either using EME as a green sample preparation technique in routine applications, or developing EME into systems of very high sophistication.

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