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Designer trap: Ionic liquids for lead

发布时间:2014-06-17 10:13:47        阅读次数:

Modified SPE

The first solid-phase extraction medium containing functionalised ionic liquids has been developed by Iranian scientists who used it to extract lead from various materials, including leaves and lipstick.

A group of scientists in Iran has produced a new solid-phase extraction material incorporating a modified ionic liquid that is designed especially for one specific task: the extraction and preconcentration of lead from a variety of materials. Maryam Mohamadi, Asma Saljooqi, Tayebeh Shamspur and Ali Mostafavi from the Shahid Bahonar University of Kerman claim that this is the first time that such a designer system has been prepared for a particular metal cation.

ionic liquids are one of the greenest types of extraction media available these days, their low vapour pressures reducing the potential environmental pollution compared with volatile organic solvents. They comprise an organic cation with an organic or inorganic anion which can be selected to tune the properties of a material in a certain way, for a particular analyte.

In this case, the researchers modified an imidazolium cation so that it would be specific for lead and immobilised the resultant ionic liquid on the surfaces of multiwalled carbon nanotubes to prepare a novel SPE phase, as they described in Journal of Separation Science. The whole preparation procedure was fairly simple and the resultant extraction process is as quick as any conventional SPE method

Immobilised ionic liquid

The cation was prepared from 1-(3-aminopropyl)imidazole, a common ionic liquid component, that was modified with ethyl isothiocyanate to incorporate the thiourea group. The anionic component was the bis(trifluoromethylsulphonyl)imide ion.

The ionic liquid was added to multiwalled carbon nanotubes that had been treated with nitric acid to produce binding sites on the surface and packed into a glass tube with a funnel tip. This simple procedure produced an immobilised layer of ionic liquid on the surface of the nanotubes which was ready to extract lead ions from solution.

Using an aqueous solution of lead ions, the extraction variables were optimised. The best recoveries were achieved with a sample pH of 4.7 using 3% thiourea in dilute hydrochloric acid at 1 ml/min as the eluent.

An enrichment factor of 200 was achieved, which is better than many other published methods involving a variety of techniques, including hollow fiber-supported liquid membrane extraction, hollow-fiber liquid-liquid-solid micro-extraction and online SPE.

When the extracts were analysed by graphite electrothermal atomic absorption spectroscopy, the detection limits were 0.13 ng/mL, the selectivity was good and interferences from other metal ions were negligible. The accuracy of the whole procedure was confirmed by the analysis of a reference material before real-world samples were investigated.
Leaves and lipstick

Following the addition of 1.0 ng/mL of a standard solution of lead to well water, the sample was subjected to the ionic liquid-SPE procedure to remove lead for analysis. Lead concentrations were 3.85 and 4.72 ng/mL for unspiked and spiked water, respectively, confirming the excellent recovery in the presence of other ions.

The level of lead in a red lipstick bought locally was found to be 68.5 ng/g. It was confirmed again by spiking and a recovery of 98.6% was recorded. The final example was pine leaves, which contained 34.7 ng/g lead and delivered a recovery of 97.8%.

This novel phase, consisting of a functionalised ionic liquid bound to multiwalled carbon nanotubes for solid phase extraction, provided good selectivity for lead ions. It could lead to the development of a new range of extraction media targeting specific analytes. The green credentials are boosted by the use of an ionic liquid as well as carbon nanotubes, increasing its attraction.