Carbon nanotubes reinforced hollow fiber microextraction of diethylstilbestrol from milk

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Carbon nanotubes reinforced hollow fiber microextraction (HF-SLPM) was proposed for the first time by Es´haghi et al. in 2010 (1). The original technique consisted of a multi-phase system where the aqueous donor and acceptor phases were separated by a polymeric membrane with a dispersion of carbon nanotubes in 1-octanol immobilized in its pores. The pH gradient established at both sides of the reinforced hollow membrane allowed the extraction of the target analytes. Due to its multiphase nature, HF-SLPM involves a selectivity enhancement compared to traditional hollow fiber liquid phase microextraction.

Diethylstilbestrol
The direct immobilization of carbon nanotubes (CNTs) in the pores of a polypropylene hollow fiber have been recently proposed by researchers from the Chinese Academy of Sciences of Beijing at China (2), for the isolation and preconcentration of diethylstilbestrol from milk. Diethylstilbestrol is a steroid hormone used for animal growth and it presents a negative effect on human health.

In this case, a multi-phase system is also employed as the analyte is transferred from the bulk sample to the immobilized CNTs, located on the pores of the membrane. In order to favor this transference, the CNTs are wet with an appropriate organic solvent. Therefore, the new approach differs from the original in the final acceptor phase. While the classic approach uses an aqueous phase, CNTs are the acceptor phase in the new proposal. The stability of CNTs in the pores of the hollow fiber is achieved thanks to a sol-gel reaction. For this reason, CNTs should be previously derivatized including oxygen-containing functional groups which may participate in such reaction.

The extraction procedure is quite simple and it provides excellent detection limits (5.1 ppb) and good precision levels (relative standard deviation lower than 7.51 %. Moreover, the whole process only requires 40 min (30 min for extraction and 10 min for elution) and 1 mL of sample.

In the original article, the preparation of the reinforced hollow fiber, including the oxidation of CNTs and their final sol-gel reaction, is described in detail. Moreover, the optimization of the extraction attending to different variables may be also useful for the readers.

References:
(1) Carbon nanotubes reinforced hollow fiber solid/liquid phase microextraction: A novel extraction technique for the measurement of caffeic acid in Echinacea purpurea herbal extracts combined with high-performance liquid chromatography. Link
(2) Determination of diethylstilbestrol in milk using carbon nanotube-reinforced hollow fiber solid-phase microextraction combined with high-performance liquid chromatography. Link

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