Ionic liquid SPME: Polycarbazole support for aromatic esters
Improving SPME
Solid-phase microextraction (SPME) is an established technique for removing target compounds from gases and liquids in preparation for analysis but it is also a technique that continues to be developed. One of the more recent innovations is the use of ionic liquids as adsorbents and the durability of the fibre coatings was improved in some cases by using polymeric ionic liquids.
A team of three Chinese scientists from Wuhan University has now produced a novel SPME fibre coating which has good stability but uses monomeric ionic liquids, simplifying the overall process. Faqiong Zhao, Yuanyuan Feng and Baizhao Zeng from Wuhan University based their design on a polycarbazole film onto which the ionic liquid was loaded, as described in Journal of Separation Science.
They were targeting aromatic esters which are a wide group of compounds used in the food industry as flavourings and as fragrances, like cinnamyl anthranilate, and in plasticisers, like the ubiquitous phthalate esters. Many of these esters persist in the environment, so they are regularly monitored by regulatory authorities to ensure that their levels in water courses do not reach levels that might be harmful for humans.
Polycarbazole support for ionic liquid
Clean stainless steel wires were immersed in a solution of carbazole for electrochemical polymerisation on the surface. The polycarbazole formed on the wires had a dense structure with many pinholes and cracks but when the coated wire was immersed in the ionic liquid, the surface became rougher and more porous. Zhao suggested that ionic liquid swelled the polymer particles and became absorbed inside the film.
From four ionic liquids tested, the best extraction efficiency for six aromatic esters was reached for 1-hydroxyethyl-3-methylimidazolium bis[(trifluoromethyl)sulphonyl]imide. The efficiency was also up to eight-fold higher than that of the polycarbazole layer alone, emphasising the importance of the ionic liquid in the extraction process. The increased uptake was attributed to the hydrophobic nature of the ionic liquid and ability of the delocalised electrons on the imidazole cation ring to form π-π interactions with the benzene ring in the esters.
The preferred fibre coating was also stable up to 350°C and was in acidic solution. However, basic solutions and a solution of dimethylformamide reduced the extraction capability due to partial dissolution of the ionic liquid. So, these conditions must be avoided for extractions from real aqueous samples.
One important factor to consider is the reproducibility from fibre to fibre and this was also good with relative standard deviations of up to 9.9%. Each coated fibre could be used up to 160 times without a significant decrease in performance.
Lake water and grape juice
After SPME extraction, the fibre was inserted into the injector port of a gas chromatograph for measuring the aromatic esters. Under optimised extraction time, temperature and stirring rate and in the presence of excess sodium chloride for salting out, the detection limits of methyl and ethyl benzoate, dimethyl and diethyl phthalate, and methyl and ethyl anthranilate were 15.3-61 ng/L.
The ionic liquid/polycarbazole SPME fibre was used to test water from a local lake for the six aromatic esters but none were found. It was a different story for grape juice purchased from a local supermarket for which methyl anthranilate was measured at 211 µg/L. This compound is added to drinks as a grape flavouring and is known to be added to the popular grape Kool-Aid drink.
The recoveries from lake water and grape juice that were spiked with known amounts of the six esters were 99.0-101.9% and 84.5-101.2%, respectively, confirming the excellent extraction capability of the new fibres. The principle of adding an ionic liquid to an electrodeposited polymer on an SPME fibre could also be extended to other types of polymers and ionic liquids to suit particular applications.
