Dance of the light brigade

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  • Published: Apr 19, 2010
  • Author: Jon Evans
  • Channels: Detectors
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Canadian physicists have come up with a totally novel way to detect and identify analytes separated by gas chromatography (GC), by measuring their speed as they travel through an optical fiber.

Optical fibers guide the passage of light by taking advantage of the fact that light travels at different speeds in different substances. As a beam of light passes from one substance to another, this change of speed deflects the beam slightly. This process is known as refraction and different substances have different refractive indexes, depending on how much they slow light. So, for instance, water has a higher refractive index than air, which explains why an object such as a pencil appears bent when placed in water.

An optical fiber consists of a substance with a high refractive index surrounding by a substance with a lower refractive index, such as pure glass surrounded by plastic. This set up induces a state of total internal reflection within the fibre, meaning that a beam of light introduced into the fibre is always reflected away from the interface between the two substances. As such, the beam is trapped within the fibre and forced to travel along it.

Optical fibers are regularly used to transmit information over large distances and to peer into hard-to-see places, such as inside our bodies. But now a team of Canadian physicists from the Institut National d'Optique in Québec, jointly led by Serge Caron and Claude Paré, has shown that optical fibers can also be used to perform gas chromatography.

Caron and Paré took a 250µm-wide optical fiber consisting of a 9µm-wide glass core with a high refractive index surrounded by plastic with a lower refractive index. They then drilled a hole through the plastic material to create a 100µm-wide capillary and coated this capillary with poly(dimethylsiloxane) as the stationary phase.

But this was no ordinary optical fiber, because it had been designed to induce a special kind of refraction known as double refraction or birefringence, in which a single beam of light is split into two. This produces an ordinary beam and an extraordinary beam, with the extraordinary beam polarized at right angles to the ordinary beam. In other words, if the light beams are considered as waves, then the ordinary light wave is essentially vibrating vertically and the extraordinary light wave is vibrating horizontally.

The idea is that the migration of analytes down the capillary continually interferes with the polarization state of the ordinary and extraordinary rays as they travel through the core. 'A birefringent waveguide, as the one we are using, can support the propagation of two orthogonal polarization states,' explains Caron. 'For this sensor, light is launched in only one polarization component; when it encounters an analyte, this analyte gives a "side kick" that couples this polarization state to the other one and this gives rise to a transfer of a small quantity of light power to the other polarization state.'

The altered polarization states of the two beams are detected by a device known as a polarimetric interferometer as they exit the fiber. The extent of the alteration depends on where along the capillary the light interacted with the analytes, which changes as the analytes migrate through the capillary. Because of this, the polarization state recorded by the interferometer appears to oscillate, with the frequency of this oscillation dependent on the speed of the analytes through the capillary.

Testing this novel version of GC on a mixture of pentane, hexane, heptane and octane, the physicists were able to separate them and accurately determine their velocities. Because these velocities were different for each analyte, they could be used to identify them, as well as to calculate their retention times.

But this novel version offers a lot more than merely a new way to identify analytes, because it also provides a way to monitor the passage of analytes through the capillary, potentially allowing GC parameters to be adjusted during the separation process. 'Another application we discovered recently is that we can observe local defects of the stationary phase,' Caron told separationsNOW.

The two physicists are now looking to collaborate with analytical chemists to determine other possible applications.



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

Optical fibers

 

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