Fizzy function: Effervescent tablet for dispersive extraction

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  • Published: Nov 28, 2011
  • Author: Steve Down
  • Channels: Sample Preparation
thumbnail image: Fizzy function: Effervescent tablet for dispersive extraction

Dispersed for better extraction

The extraction technique known as dispersive solid-phase extraction (SPE) that was introduced in 2003 quickly became widely accepted as a rapid and robust procedure that was easy to operate and used low volumes of solvent. It was soon adopted by the Association of Official Analytical Chemists as a method for extracting pesticides from fruits and vegetables.

The technique is sometimes referred to as QuEChERS, based on its recognised characteristics: Quick, Easy, Cheap, Effective, Rugged and Safe. It can be regarded as a two-stage procedure.

A homogenised sample is first extracted with a solvent such as acetonitrile in the presence of magnesium sulphate to aid phase separation between water and acetonitrile. Then this extract is mixed with the sorbent using external means such as vortexing to aid dispersion and enhance the extraction procedure.

A team of scientists from the University of Cordoba in Spain has now added a modification to dispersive SPE which allows the process to occur in a single syringe, while assisting sorbent dispersal into the sample. Miguel Valcarcel, Guillermo Lasarte-Aragones, Rafael Lucena and Soledad Cardenas introduced effervescent-assisted dispersive micro-SPE and illustrated it with the removal of a series of explosives-related nitroaromatic compounds from water.

Putting the fizz in to extraction

The effervescence is produced by carbon dioxide which is produced in situ and acts as the dispersant. A commercial sorbent comprising a poly(vinylpyrrolidone-divinylbenzene) copolymer was mixed with sodium carbonate as the carbon dioxide source and sodium dihydrogen phosphate as acid reagent. The mixture was pressed into tablets for storage and use.

When the tablet was added to an aqueous solution, it dissolved and released carbon dioxide which dispersed the sorbent throughout the solution to increase the contact area. This process took place within a 10 mL glass syringe.

After a suitable period, the solution was expressed from the syringe and the sorbent recovered on a filter. The target analytes were eluted from the filter for analysis by HPLC with UV detection.

Two carbon dioxide sources (sodium carbonate and sodium bicarbonate) and three acid reagents (sodium dihydrogen phosphate, citric acid and ascorbic acid) were tested together in all six combinations but sodium carbonate and sodium dihydrogen phosphate gave the best performance.

This was attributed in part to the relative hygroscopicities of the reagents in the tablets, the presence of moisture inhibiting effervescence when the tablet is contacted with water. For this reason, it was important that the reagents were completely dried before tablet formation and were stored in an inert atmosphere.

Factors such as the ratio of reagents and sorbent, effervescence time, solution pH and final eluent were all optimised to give excellent relative recoveries from spiked river water samples. They ranged from 94-103% for five nitroaromatic compounds: 4-nitrobenzaldehyde, 2,4-dinitrotoluene, nitrobenzene, 1,4-dinitrobenzene and 4-nitrotoluene. The detection limits ranged from 1.8-7.0 µg/L.

Fizzy features in solid-phase extraction

Effervescent-aided dispersion SPE enhanced the removal of the pesticides compared with conventional SPE, increasing the HPLC-UV signal three-fold. However, the better sensitivity was achieved by the conventional technique, due to the ability to extract large sample volumes up to 500 mL.

The researchers pointed out that the sensitivity of effervescent dispersive SPE would be improved by starting with larger sample volumes, although that would need a new design of extraction vessel, and by lower elution volumes.

The effervescent technique performed better than conventional dispersive SPE carried out with the same sorbent and using a vortex stirrer to aid dispersion, giving higher signal intensities.

The principal disadvantage of this novel process is the requirement for tablet production prior to the extraction, which took about one hour. However, several tablets were produced at the same time and could be stored for future use.

This time constraint is countered by the ability to perform the whole extraction procedure in a simple syringe with the aid of a filter, lending itself to on-site extraction of environmental water samples.

So, although there is room for improvement, effervescence-aided dispersion SPE offers a simple and effective alternative for environmental analysis, improving upon conventional dispersion SPE.

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

An effervescent tablet boosts a dispersive extraction procedure by rapidly increasing the contact area between analytes and absorbent, say Spanish researchers, who demonstrated this novel approach for the removal of nitroaromatic compounds from water

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