Chromium speciation by magnetic solid phase extraction on-line with inductively coupled plasma optical emission spectrometry

Background:
As a consequence of its industrial application and its natural occurrence, both natural and anthropogenic chromium enter into the food chain through water, soil and air. The two species determined to be chemically stable are Cr(III) and Cr(VI), and therefore, both species naturally occur in the environment. While chromium is considered to play a helpful role in the carbohydride metabolism in its trivalent form, the hexavalent species is classified as genotoxic and carcinogenic. World regulatory agencies strictly recommend maximum tolerable limits for chromium in drinking water globally.  Because of the important difference in toxicity of the two species, the need for chromium speciation analysis rather than the determination of the total Cr concentration is of paramount importance. In general, the most often applied atomic spectrometric techniques applied for the determination of chromium have to be coupled with a separation technique for the differentiation of the two species.  Such combined techniques can be realized either by on-line or off-line coupling. Since the monitoring of harmful species in the environment requires the processing of many samples to accurately characterize their abundance, automated and uncomplicated methods with high sample throughput are favoured. Apart from chromatographic separation techniques, non-chromatic techniques such as liquid-liquid extraction, capillary microextraction, cloud point extraction and solid phase extraction have been proposed. Flow injection manifolds have often been used to automate such techniques and to interface them to the detection system.

The new method:
A group of Spanish researchers do propose to use a magnetic graphene oxide functionalized with 4-aminobenzenesulfonic acid as a sorbent material for the preconcentration and separation of chromium species. A flow injection manifold was designed, having two collection channels with the sorbent material inside a tubing wrapped around a Nd/Fe/B toroidal magnet and kept in place by polyethylene frits a both ends of the tube. The two collection channels were placed on an eight-port rotary valve of the flow injection system. Pumping water samples adjusted to pH =3 through the collection channels, both Cr(III) and Cr(VI) analyte species are retained on the sorbent material.

Following collection of the chromium from the water sample, the collection channel is purged by pure water. After such cleaning step, the retained Cr(VI) is eluted from the collection channel by using 3.2% NH₄OH solution and is directly send to the sample introduction system of the ICP-AES. During the same time, the second collection channel is loaded with the water sample. Again, the channel is purged for washing out any remaining matrix with pure water. Following the cleaning step, elution of this channel is done with 2.3% HNO₃, so that total chromium is sent to the ICP-AES.

The whole procedure was optimized with respect to collection pH, eluent concentration, FIAS and ICP-AES parameters. Using the optimized conditions and a loading time of 3 min., preconcentration factors of 15 and 23 for Cr(VI and Cr(total) were obtained, leading to detection limits in the order of 0.1 µg/L. A complete cycle of the FI-operation was about 480 s per sample, allowing for about 7.5 samples per hour.  

The method was validated by using two certified reference materials and applied for the analysis of different water samples, including seawater and tap water. Also spike recoveries for all water samples were close to 100%.



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