8 September 2005: Polymerised ionic liquids make stable GC stationary phases

The high-temperature performance of gas chromatography (GC) stationary phases made from ionic liquids has been dramatically improved by polymerisation, say US researchers.

Ionic liquids are a general class of fluids with melting points below 100?C. They are characterised by high-temperature stability and very low vapour pressure, properties that make them ideal materials for stationary phases in gas chromatography. The added bonus is their dual mode of operation, with the ability to separate mixtures of polar and nonpolar compounds.

But the use of ionic liquids as GC phases is still limited by variable wettability of the capillaries and break up of the coated film at high temperatures. These shortcomings have been eliminated by Daniel Armstrong and Jared Anderson from Iowa State University, US, who have developed a crosslinked form of ionic liquids with improved properties.

Novel ionic liquids containing two cations based on bis(vinylimidazolium)alkanes were synthesised and subjected to radical polymerisation within fused-silica GC capillaries, to produce crosslinked stationary phases. Two types were prepared. The first was only partially crosslinked and was suitable for separation at 30-280?C, whereas the second was highly crosslinked and operated successfully at 300-400?C.

Both types gave minimal column bleed, were highly selective and displayed increased efficiencies at high temperatures. Despite the structural modifications induced by crosslinking, they also retained their dual separation ability.

A highly crosslinked ionic liquid phase was used to separate a mixture of 20 polycyclic aromatic hydrocarbons (nonpolar) and chlorinated pesticides (polar) within nine minutes, with a temperature ramp up to 335?C.

’Our [earlier] development of dicationic liquids improved their thermal stability, but they still tended to flow when high temperatures were reached. Adding appropriate double bonds to their structure for crosslinking worked quite well at preventing unwanted flow,’ Armstrong told Chemistry World. ’We expect that there will be a commercialized product within a few months.’
Robert Synovec from the University of Seattle, US, welcomed the new phases. ’They have outstanding useful properties and should greatly broaden the scope of applicability for GC. In particular the high thermal stability capability to 350 ?C, especially for separating polar compounds, will be a very important tool for a wide range of applications. It will be a welcome sight to have these novel GC phases commercialised,’ said Synovec. Steve Down