Ionic liquid boost for AAS

Researchers in Argentina have used room temperature ionic liquids (RTILs) as an online pre-concentration medium for flame atomic absorption spectrometry analysis of trace cadmium in plastic food packaging materials.

Rodolfo Wuilloud and Estefania Martinis of the Laboratory of Environmental Research and Services of Mendoza (LISAMEN) and colleagues there and at CONICET and INQUISAL-CONICET, explain how common plastic packaging has revolutionised the food industry. Not only does it make transportation and shelf-stacking so much more convenient, it also reduces waste as food is preserved effectively in essentially impermeable polymers.

However, there are concerns that such convenience comes at a price. "Polymers contain additives such as plasticizers, lubricants, stabilizers, and antioxidants" the researchers explain. These are all chemicals necessary either for the processing of the packaging or to maintain its stability in use, but there are worries that certain additives and components of plastic food packaging may leach into the food.

Cadmium is one such additive used extensively in the manufacture of plastics, the team points out. Cadmium content of packaging materials, of course, succumbs to European Community regulations under Directive 94/62/EC, which limits the concentration of cadmium to 100 milligrams per kilogram.

Nevertheless, because cadmium has such a low excretion rate, its biological half-life is between ten and thirty years, it can accumulate in body tissues and bone. Therefore, the ingested cadmium is a problem even if it is present only at very low concentrations. For the sake of health, "sensitive, accurate, and fast analytical methods for trace metal determination in a variety of plastic materials are required," the team asserts.

Elemental analysis by spectrophotometric techniques usually involves eluting any target metal ion from a polymer sample in aqueous solution and then applying the likes of inductively coupled plasma-mass spectrometry (ICP-MS). The necessary instrumentation is often prohibitively expensive for many laboratories. Besides, flame atomic absorption spectrometry (FAAS) or electrothermal atomic absorption spectrometry (ETAAS) are more commonly used analytical techniques for the determination of cadmium content in a sample.

"FAAS is widely applied in routine laboratories due to its low cost and greater simplicity as compared to ETAAS," the team says, "However, conventional FAAS has a detection limit, which is not low enough to determine cadmium at trace levels in plastics." The Argentinian team has now turned to a novel pre-concentration method that allows them to achieve accurate, sensitive and reliable results even at trace levels using room temperature ionic liquids.

RTILs are a little-known nineteenth century chemical discovery that have attracted considerable attention in the last decade or so as chemists have sought environmentally benign alternatives to toxic, volatile organic compounds (VOCs) as solvents. Essentially, they are formed by using bulky organic cations and various anions that present an energetic hurdle to crystallisation that is not surmounted at room temperature forcing what would otherwise be a solid to remain in the liquid phase at temperatures around 23 Celsius.

These ionic materials are potent solvents of many disparate materials, have very low volatility, are insensitive to air and moisture and have low toxicity. They can also be mixed with VOCs and water and so are particularly attractive as separation solvents. "Recently, numerous studies have shown their good extractability for various organic compounds and metal ions," the team explains. "However, up to now, all the extraction/preconcentration methodologies based on RTILs involve batch procedures and no on-line system has been developed so far for RTIL phase separation."

The researchers point out that batch mode preconcentration is notorious for cross-contaminating samples, and so an online approach is favoured, particularly where trace quantities of metal ions, including cadmium, are involved.

The team has now demonstrated how effective an RTIL based on 1-butyl-3-methylimidazolium hexafluorophosphate can be in acting as the preconcentration medium for the chelation of cadmium in a sample using 5-Br-PADAP as a chelating agent to sequester the cadmium ions.

"On-line retention and separation of the RTIL phase was achieved with a microcolumn filled with silica gel," the team explains, "The method was successfully applied to cadmium determination in plastic materials, demonstrating the possibility of using RTILs for metal extraction from complex matrices."