Cobalt is an essential element for humans, being part of vitamin B12, one of the few natural compounds that possess a carbon-metal bond. It is also an important industrial chemical and finds its way into the environment via effluents, where it can pose a danger to wildlife and humans. Cobalt is a suspected carcinogen and some of its compounds are toxic.
The conventional method for measuring levels of cobalt in environmental and biological samples is electrothermal atomic absorption spectrometry (ETAAS), which has good sensitivity and relatively low cost. It is a routine and well-established technique but sometimes needs a little help when cobalt levels are very low. Preconcentration and sample clean up of trace cobalt is often required before analysis can proceed.
One extraction technique for cobalt that has found favour recently involves the use of room temperature ionic liquids (RTILs). They have been used in single drop microextraction, where a complexing agent traps the metal ions before transfer to the extraction drop. However, the limited reproducibility encountered in published methods, combined with break-up of the drops led some research groups to examine the use of ionic liquids in the larger-volume dispersive liquid-liquid microextraction (DLLME).
One complexing agent used to trap cobalt ions was 1-(2-pyridylazo)-2-naphthol but it showed limited sensitivity towards metals, despite having favourable stability constants. An alternative naphthol has now been tested by researchers in Argentina, who found that 1-nitroso-2-naphthol (1N2N) has excellent selectivity and sensitivity.
Rodolfo Wuilloud and Paula Berton from the Laboratory of Environmental Research and Services of Mendoza (LISAMEN), part of the National Scientific and Technical Research Council (CONICET), and the National University of Cuyo, Mendoza, optimised the extraction of cobalt for subsequent analysis by ETAAS.
The RTIL was selected from three candidates on the basis of its viscosity and solubility in water. 1-Hexyl-3-methylimidazolium hexafluorophosphate was preferred over the butyl and octyl analogues.
For method optimisation, cobalt(II) standard solutions were mixed with 1N2N, acetate buffer (pH 4.0), sodium nitrate and surfactant to form the Co-1N2N complex. The solution was cooled over ice and adjusted to pH 2 with hydrochloric acid before addition of the ionic liquid as extractant and methanol as dispersant for DLLME. Dispersion of the RTIL into the aqueous sample expanded the contact area between the two phases.
After a suitable extraction period in the region of 7 minutes, the solutions were centrifuged to separate the phases and the RTIL was withdrawn, dissolved in methanol and analysed.
The amount of the RTIL required for 100% extraction efficiency was minimised to avoid the possibility of interfering in the ETAAS assay. For 40-80 mg cobalt and 0.5 ml methanol, recoveries of 100% were achieved with 60 mg of the ionic liquid.
The other extraction conditions were also optimised for the extraction of cobalt, including the sample volume (7.5 ml), the pH for complex formation (4.0) and the temperature for complex formation (50°C). The pH for extraction of the cobalt-1N2N complex was set to 2.0 to eliminate the co-extraction of other metal species that might form complexes with 1N2N.
Under these conditions, the cobalt detection limit in ETAAS was 3.8 ng/l with a r.s.d. value of 3.4%.
The optimised procedure was used to extract cobalt from domestic tap water, river water, human saliva and, following digestion with peroxide under UV light, human urine. Saliva is an important and non-invasive medium for monitoring cobalt that has leached out from dental cast alloys and devices such as orthodontic wires.
Recoveries over the whole range of samples were 97-103% and cobalt was found in all cases apart from two saliva samples. For 4 samples of each type, cobalt levels were 0.45-0.57 (tap water), 0.53-0.65 (river water), 0-0.15 (saliva) and 0.32-0.92 µg/l (urine).
The DLLME method for extracting cobalt from aqueous samples following complexation with 1-nitroso-2-naphthol presents a new, green procedure that provides high efficiency with short extraction times. It is suitable for ETAAS, the standard technique for measuring trace levels of metals in biological samples.
Related links:
* Analytica Chimica Acta 2010, 662, 155-162: "Highly selective ionic liquid-based microextraction method for sensitive trace cobalt determination in environmental and biological samples"
No comments:
Post a Comment