Stick to the track: Tapered turns increase separation efficiency on microchip CE

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  • Published: Feb 21, 2011
  • Author: Jon Evans
  • Channels: Electrophoresis
thumbnail image: Stick to the track: Tapered turns increase separation efficiency on microchip CE

Racetrack effect

By designing separation channels with the look and feel of a racetrack, US chemists have managed to avoid the 'racetrack' effect when separating different sugar molecules on a capillary electrophoresis (CE) microchip.

One of the main challenges in shrinking CE down to the scale of a single microchip is separating analytes in very short separation channels, usually just a few centimetres long at most. This has led scientists to develop a whole host of new, more efficient versions of electrophoresis, involving novel polymers, tiny pillars, self-assembling walls and even bumpy surfaces. But an alternative approach is simply to introduce a few turns to the separation channel.

So rather than separate analytes in a long, straight channel, they are separated on a channel with several 180° turns, allowing a long separation distance to be fitted onto a small microchip. But, as ever, there is a problem.

In the same way that the back of a race car will slide out when going at speed around a bend, the back of an analyte band travelling down the separation channel will also tend to slide out while going round the turn. As a result, the band will spread and extend, potentially mixing with other analyte bands and thereby ruining the separation efficiency of the whole system. This is known as the 'racetrack' effect.

One potential solution to this problem is to reduce the width of the separation channel at the turns, producing so-called tapered turns. Not only does this reduce the space available for the band to slide out as it speeds around the turn, but the tapered turns also actively rotate the band in the opposite direction to its normal slide. Because of this, the analyte band spreads out far less, with the only sign being a slight outward bulge.

Racing glycans

A team of chemists from Indiana University in Bloomington, led by Stephen Jacobson, has now shown that such tapered turns can be used to separate various sugar molecules commonly attached to proteins, collectively known as glycans. What is more, they also discovered that the degree of tapering is more important than the number of turns.

To do this, Jacobson and his team fabricated several microchip CE systems containing separation channels of varying lengths, number of turns and degree of tapering. So they fabricated three microchips each containing 22cm-long channels with two turns, but differing in the degree of tapering. On one microchip, the channels didn't taper at all; on another, the turns were half as wide as the straight channels; and on the last, the turns were a third as wide as the straight channels.

They also fabricated a microchip containing an 11cm-long channel with two turns, and two microchips with 18cm-long and 36cm-long channels, both with four turns. The turns on all these microchips were a third as wide as the straight channels.

Lots of narrow turns

As would be expected, when they separated various different glycans found on an enzyme known as ribonuclease B, they found that the longest channel had the best separation efficiency. Nevertheless, the 22cm-long channel with the narrowest turns wasn't far behind. Both microchips were also very quick, separating the molecules in 3.1 minutes and 1.25 minutes respectively.

Also as expected, the separation efficiency increased with the degree of tapering, such that microchips containing the channels with the narrowest turns performed best. Perhaps more surprising, however, was the finding that the number of turns had little or no effect on the efficiency. This suggests that the separation efficiency of microchip CE systems can be maximised by fabricating long channels with lots of narrow turns.

To demonstrate a potential application of these tapered turns, Jacobson and his team used the microchip containing a 22cm-long channel with two turns a third as wide as the straight channels to analyse the blood of a patient with ovarian cancer and a healthy control. They did this because cancer cells often contain proteins with a different mix of glycans to normal cells, potentially offering a novel route to diagnosis.

They found that the microchip could separate the different glycans in the blood samples, with the cancer patient's blood showing a different mix of glycans to the healthy patient.

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

Stick to the track: tapered turns increase separation efficiency on microchip CE

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