According to British comedian Harry Hill, the problem with heroin is that it's very more-ish. Another problem is the general lack of capillary electrophoresis (CE) methods for sensitively detecting heroin and its metabolites in bodily fluids.

Various other detection methods have been developed, based on analytical technologies such as enzyme immunoassays and high performance liquid chromatography. But CE offers the possibility of a method that is both quicker and simpler.

The stumbling block is that heroin and its metabolites, such as morphine and codeine, tend to be present at low concentrations in bodily fluids like urine. This means that although they can be separated by CE, detecting them in tiny capillaries with UV detection is much more difficult.

As other detection methods, such as laser-induced fluorescence, tend to be more expensive, one option is to concentrate all the traces of heroin and its metabolites in a sample of bodily fluid into a small plug, which is then separated by CE. Concentrating the analytes in this way should make them easier to spot by UV detection.

This what a team of Taiwanese scientists led by Shou-Mei Wu from Kaohsiung Medical University has now done by taking advantage of a technique known as cation-selective exhaustive injection and sweeping micellar electrokinetic chromatography (CSEI-sweep-MEKC).

Developed by Joselito Quirino and Shigeru Terabe at the Himeji Institute of Technology in Japan around 10 years ago, CSEI-sweep-MEKC combines two different sample concentration techniques - stacking and sweeping. These are used in conjunction with MEKC, which is a form of CE that involves mixing a detergent such as sodium dodecyl sulphate (SDS) with the separation buffer to form lots of charged bubbles known as micelles.

The analytes in the sample interact with these micelles to varying extents while the micelles migrate through the capillary under the influence of the electric field. By acting as a kind of moving stationary phase, these micelles allow neutral analytes to be separated via an electrophoresis-like process. If the micelles move in the opposite direction to the electroosmotic flow (EOF), then those analytes that interact strongly with the micelles move more slowly than those that don't. If the micelles move in the same direction as the EOF then the situation is reversed.

In CSEI-sweep-MEKC, the capillary is half-filled with a high-concentration phosphate buffer, which is sandwiched between a small plug of low-concentration phosphate buffer on one side and a much larger expanse of low-concentration buffer on the other side. The secret to this set-up is that analytes move more slowly through the high-concentration buffer than the low-concentration buffer.

So the idea is to introduce the sample into the small plug of low-concentration buffer and then apply an electric field. This creates an EOF that carries the analytes to the high-concentration buffer where they all bunch up together as they migrate at a slower rate. It's similar to when a crowd of people have to pass through barriers at a train station: slowing down to pass through the barriers causes everyone to crowd together. This is the stacking stage of the concentration process.

Next, a mixture of phosphate buffer and SDS is inserted into the capillary, forming anionic micelles that begin to migrate down the capillary. These micelles also bunch together at the high-concentration buffer, where they sweep up all the neutral analytes, providing the second stage of the concentration process.

Under the influence of the electric field, the micelles travel through the high-concentration buffer to the low-concentration buffer, where they act as a stationary phase for separating the analytes.

Wu and her team had already used CSEI-sweep-MEKC to separate various forensic drugs in urine and hair samples, and they found that the technique was equally effective a separating heroin and its metabolites. After demonstrating that the technique worked with specially-prepared mixtures at various concentrations, Wu and his team showed it could also detect heroin metabolites in urine samples from known drug users.

Heroin may still be bit more-ish, but at least there's now a simple CE method for detecting it.

Related links:

• Journal of Chromatography A (Article in Press): "On-line stacking and sweeping capillary electrophoresis for detecting heroin metabolites in human urine"

Article by Jon Evans

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.