Electricity to extract charged analytes

15 Nov 2014 Relatively low electrical potential powered by a common battery can be used to increase the efficiency of charged analyte extraction from aqueous samples to a sorbent.

Microscale extractions of analytes from aqueous samples became popular in the mid-1990s due to their environmental friendliness (minimized or no solvent use, generation of little or even no waste), operational simplicity, and true applicability to real-world samples. In most of these procedures, the extraction can be considered to occur passively, with diffusion of analytes from the water sample (which is agitated to improve mass transfer) to a solid material (the sorbent) being the main driving force. A more active mode of driving the extraction is by using low voltage (15V); this applies specifically to charged analytes which can be made so by simply adjusting the pH of the sample.

A team led by Prof LEE Hian Kee from the Department of Chemistry in NUS noticed that by using a voltage adapter and alligator clips holding platinum wires as electrodes (positive electrode in the water sample; negative electrode attached to a metal tube in the vicinity of the sorbent material which is coated on a support) to set up a close circuit with a voltmeter to monitor voltage, charged analytes can be electrokinetically driven to migrate from the water sample to the sorbent onto which the analytes then adsorb. At the end of the extraction, the support for the sorbent (now containing the analytes) can be removed, so that the analytes can be desorbed thermally for analysis by gas chromatography.

For charged analytes, the use of electrical potential allows for a much more rapid extraction than having their neutral forms passively approach the sorbent (by the agitation of the sample to bring them to the vicinity of the sorbent so that adsorption can occur). The speed of extraction can be up to several times faster. Just consider that in conventional passive extraction procedure, it may take 30 minutes. However, with a more active extraction driving force, only 3 – 5 minutes are needed.

In their recent work in applying the approach described as electro-enhanced solid-phase microextraction (EE-SPME) (see Figure), the team has extracted tricyclic antidepressant drugs in environmental water samples. The widespread use of pharmaceuticals has led to their discharge to the sewer. If wastewater treatment is found wanting, it is possible for such substances to be released to the environment. Most importantly, in cases where wastewater is reclaimed for potable use, it is critical to ensure that these substances (amongst others that can be found in wastewater) are completely removed by treatment processes. Thus, the analytical chemistry of wastewater before and after treatment is necessary. The procedure developed can be used for monitoring water quality.

The figure shows the setup of the described EE-SPME procedure. [Image credit: LEE Hian Kee]

Reference

Xu R, Lee HK. “Application of electro-enhanced solid-phase microextraction combined with gas chromatography-mass spectrometry for the determination of tricyclic antidepressants in environmental water samples.” Journal of Chromatography A 1350 (2014) 15.