Mighty metal SPME fibres with carbon nanotubes

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  • Published: Jun 8, 2009
  • Author: Steve Down
  • Channels: Sample Preparation
thumbnail image: Mighty metal SPME fibres with carbon nanotubes

Since solid-phase microextraction was announced in 1990, there have been many variations on the theme, producing spin-off techniques and different fibre coatings. But in conventional SPME, a few types of coating tend to dominate, depending on the nature of the target compounds.

Polydimethylsiloxane (PDMS) performs well for non-polar analytes and polycyclic aromatic hydrocarbons whereas polyacrylate coatings have high affinity for polar compounds. Both of these coatings are liquids so the trapping process proceeds via absorption. On the other hand, the mixed-phase coatings such as Carboxen/PDMS and Carbowax/dinylbenzene are solid sorbents so they extract by adsorption.

All of these types have limitations due to their relatively low operating temperatures (up to 240°C), instability in organic solvents and questionable reproducibility. In addition, the fibres are generally made from thin fused silica and are very fragile.

A different approach introduced for preparing SPME fibre coatings involved the use of sol-gels. They have the advantage of being able to mix inorganic and organic components and appear to have better thermal stability. In general, sol-gel coatings have been applied to fused silica fibres but there have been some instances where metallic fibres were employed, giving the obvious advantage of improved strength.

Using the advantages of both the sol-gel techniques and metal fibres, scientists from China have now prepared stainless steel fibres coated with multi-walled carbon nanotubes (MWCNTs) by the sol-gel process. Fang Zhu, Gangfeng Ouyang and colleagues from Sun Yat-Sen University in Guangzhou, China, along with Janusz Pawliszyn from the University of Waterloo, Canada drew on the fact that CNTs have already been proven to be good adsorbents for a number of different types of analytes.

Native MWCNTs are insoluble in organic solvents so they worked with hydroxy- and carboxyl-functionalised MWCNTs. A stable homogeneous sol solution was prepared in methyltrimethoxysilane solution containing poly(methylhydrosiloxane) and poured into a glass tube which had the bottom opening covered with a parafilm membrane.

A stainless steel wire (diameter 125 µm) which had been extensively degreased and cleaned was inserted into bottom of the tube through the membrane. The end of the wire was attached to a thread connected to a motor which pulled the fibre slowly up through the sol at 20 cm/min to allow the sol-gel coating to form on the surface. The coating was cured and the process repeated until the required thickness was reached. Before use, the fibres were cut into 2-cm lengths and conditioned in a GC injector port.

Scanning electron microscopy revealed that the CNTs were distributed homogeneously within the coating. Their SPME performances were tested for the headspace extraction from solutions of benzene, toluene, ethylbenzene and o-xylene (BTEX) with subsequent GC/MS analysis. A batch of fibres cut from the same coated wire was examined.

The fibre-to-fibre and between fibre reproducibilities were 1.4-2.5 and 1.9-6.5% r.s.d, respectively, better than Carboxen/PDMS-coated fused silica fibres. The improvement reflects the fact that the fibres were all prepared at the same time under the same conditions, since they originated from the same wire.

The fibres were also conditioned at 250-350°C or soaked in methanol or acetonitrile for 7 hours before use. In both cases, the treatment had no effect on the extraction yields for BTEX, illustrating the good thermal stability and solvent resistance as well as the strong chemical bonding between the fibre and the coating.

For BTEX and 5 phenols, representing non-polar and polar analytes, respectively, the fibres performed better in every case than a commercial PDMS fibre or a pure sol-gel PDMS fibre prepared in the absence of CNTs.

No competition between the different analytes was observed over 2 hours of extraction, demonstrating that the removal process is absorption, compared with adsorption for the Carboxen/PDMS fibre. A broad linear range from 100 µg/L to 2.5 mg/L was found for BTEX.

A final characterisation was carried out using 5 of the 10 McReynolds probe solutes, benzene, butanol, pyridine, 2-pentanone and nitropropane, which represent compounds of differing functionality. The distribution coefficients of a 3-µm sol-gel fibre were all far better than those measured on a commercial 7-µm fibre. The highest distribution coefficient was observed for pyridine, representing bases and aromatic N-heterocycles. The performance indicates that the coatings have good potential as GC stationary phases, say the researchers.

The overall physical properties and performance of the sol-gel CNT stainless steel fibres and their ability to extract efficiently a broad range of analytes suggests that they will find extensive applications in SPME in the future.

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


Image: Courtesy Chris Ewels
 © Chris Ewels

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