Antimony analysed

A simple, yet sensitive, method for detecting inorganic antimony in food packaging has been developed using cloud point extraction combined with electrothermal atomic absorption spectrometry (ETAAS).

Xiuming Jiang, Shengping Wen, and Guoqiang Xiang of the School of Chemistry and Chemical Engineering, at Henan University of Technology, in Zhengzhou City, China, describe details in the Journal of Hazardous Materials.

Antimony is one of those trace elements with no known biological role that simply accumulates in the body over the course of a lifetime and plays its toxic part depending on what it binds to and its oxidation state. It sits below arsenic in the periodic table and above bismuth.

The toxicity of antimony(III) ion is ten times that of antimony(V) ion, for instance. Sb(III) has also been shown to cause lung cancer and occupational exposure to antimony trioxide in the glassware and ceramics industry represents a known hazard. The compound is added to molten glass to make it clear but can also be used as an additive for pigments, paints and textile dyes. Antimony has gained popular notoriety in recent years because they are also used as metal coatings and added to rubber and other materials used in soft furnishings as flame retardants. An antimony(V) complex of citrate has also been identified in orange juice contained in poly(ethyleneterephthalate) (PET) bottles.

"There is a great risk of antimony leaching from food packaging materials, such as plastic, enamel and porcelain containers into the food chain," explain Xiang and colleagues. They point out that the various concerns emphasize the importance of identification and quantification of antimony in its various chemical forms so that a clearer picture of its toxicity and relevance to human health can be described.

Xiang and colleagues explain that there are several techniques such as flame and electrothermal atomic absorption spectrometry (FAAS and ETAAS), atomic fluorescence spectrometry (AFS), inductively coupled plasma atomic emission spectrometry (ICP-OES), inductively coupled plasma mass spectrometry (ICP-MS) that have been investigated for determination of antimony species in different samples. The main issue facing those hoping to analyse antimony, which recurs throughout modern analytical chemistry, is that the quantities of the complexes that might be released from food packaging materials are very small, mere nanograms per millilitre. As such separation and pre-concentration methods have been developed to complement the various analytical procedures.

One such technique is cloud point extraction (CPE). This utilises non-ionic surfactants in a separation step that allows metallic elements to be pulled into a distinct surfactant-rich phase, trapped in hydrophobic micelles. An appropriate chelating reagent mops up the metal for analysis. "CPE offers many advantages over traditional liquid-liquid extraction, such as the fact that it is simple, cheap, rapid, and uses no organic solvents," the team says. It also has a high capacity to concentrate a range of analytes effectively. Various teams have combined CPE with AAS for antimony analysis. However, Xiang and colleagues point out that ETAAS detection could lower the detection limits significantly beyond that possible with FAAS, with the added advantage that only a very small injection volume is needed.

The team has now successfully demonstrated CPE pre-concentration in combination with ETAAS for determining inorganic antimony species in solutions leached from various food packaging materials. Ammonium pyrrolidine dithiocarbamate (APDC) was used as chelating agent to form a suitable hydrophobic complex of antimony(III) at pH 5.0, which can enter the surfactant-rich phase. Antimony(V) does not respond to the advances of APDC and is retained in the aqueous phase. The antimony(III) could then be released using nitric acid and analysed with ETAAS. Under optimum conditions, the detection limit was 0.02 nanograms per millilitre for antimony(III).