An oily solution for arsenicals: Ultrasonic-assisted DDLME for extraction from edible oils

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  • Published: Apr 11, 2011
  • Author: Steve Down
  • Channels: Sample Preparation
thumbnail image: An oily solution for arsenicals: Ultrasonic-assisted DDLME for extraction from edible oils

Combining extraction techniques for improved performance

As more effort is placed in attempting to devise increasingly efficient extraction techniques, many research groups have adopted what might be termed a combinatorial approach. Rather than devising new techniques, they are looking at individual techniques and combining the best aspects to give improved performance.

This has been illustrated by a recent article in Chinese scientists integrated multiwalled carbon nanotubes, molecularly imprinted polymers and a porous polymeric membrane to combine the advantages of molecular imprinting solid-phase extraction and liquid-phase microextraction. Their novel technique was illustrated for the extraction of a herbicide from milk, river water and wastewater.

Now, a second team of Asian scientists has announced the amalgamation of two different processes for the removal of three organo-arsenic compounds from edible oils. The main technique was dispersive liquid-liquid microextraction (DLLME), which they boosted with ultrasonic assistance.

DLLME was designed to merge extraction and enrichment into one step. The extraction solvent is added to the sample and transformed into small droplets by a dispersant liquid to increase the contact area with the sample solution. After an appropriate time, the solution is centrifuged so that the extractant coalesces and can be drawn off for analysis.

The vast majority of DLLME applications are reported for aqueous solutions but mixing the extractant with oils proved far more difficult. This is where ultrasonic excitation came in, improving the mixing process and realising good recoveries.

The ultrasound-DLLME combo for edible oils

The technique was devised and optimised by Maw-Rong Lee and colleagues from the National Chung Hsing University, Taichung, Taiwan, and Zhejiang University of Technology, PR China. They specifically targeted monomethylarsonic acid (MMA), dimethylarsinic acid (DMA) and 3-nitro-1-hydroxyphenylarsinic acid, also known as roxarsone.

The three compounds were added to edible oils in known amounts along with an amine N-oxide as an internal standard. Then aqueous ammonium formate was added as the extractant along with the dispersant. Hexane was found to give far better recoveries than 1-octanol or acetonitrile, so it was employed as the dispersant.

Following extraction, the solution was centrifuged and the aqueous layer was removed with a syringe for LC/MS analysis of the arsenic compounds, using the peak areas under selected ion monitoring to estimate the recoveries.

The value of ultrasonic-assisted DDLME was illustrated by comparison with sample shaking. The recoveries were increased 1.5-2.7-fold by the application of ultrasound, increasing emulsion formation to enhance analyte transfer from the oils to the extractant.

The extraction parameters were optimised using an orthogonal array design, setting the volumes of extraction solvent and dispersant, the extraction time and pH as the variables. Each factor influenced the recovery for the three arsenic compounds, with the extractant and dispersant volumes having the greatest effect. In practice, the researchers optimised the parameters for MMA because it had the lowest signal.

The LC/MS detection limits achieved with the optimum conditions were 1.0, 3.0 and 5.8 ng/g for DMA, MMA and roxarsone, respectively, with linear ranges of 25-500 ng/g for MMA and 10-500 n/g for the other two compounds.

Arsenicals in commercial edible oils

Six types of edible oil purchased from a local market were extracted and analysed by the new procedure. Sunflower oil, olive oil, grapeseed oil, soybean oil, palm oil and frying oil were all examined.

The recoveries of the three arsenicals from spiked samples were 79-95% for all oils except frying oil, which had lower values at 64-78%. This was attributed to the higher viscosity of frying oils following heating.

DMA was the only target compound detected, being found at 6 ng/g in frying oil. MMA and roxarsone were undetected.

In practice, the ultrasonic-assisted DLLME procedure was simple and inexpensive to operate and gave good recoveries of three organo-arsenic compounds from edible oils. It could also be applied to other media where mixing with the extractant and dispersant with the sample is problematical, to increase the enrichment factors and improve overall recoveries.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

Arsenic compounds have been extracted from commercial edible oils by a new technique that combines the benefits of ultrasonic extraction with dispersive liquid-liquid microextraction to give good recoveries and ppb detection limits by LC/MS

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