Elsevier

Talanta

Volume 82, Issue 4, 15 September 2010, Pages 1439-1447
Talanta

Preparation, characterization, and analytical applications of a novel polymer stationary phase with embedded or grafted carbon fibers

https://doi.org/10.1016/j.talanta.2010.07.019Get rights and content

Abstract

A polymer-based chromatographic stationary phase with embedded or grafted multi-walled carbon nanotubes (MWCNTs) has been developed. Three different synthetic methods were utilized to combine the nano-fibers with the substrate of polystyrene–divinylbenzene (PS–DVB). After optimizing the synthetic conditions, this novel polystyrene–divinylbenzene–carbon nanotube (PS–DVB–CNT) stationary phase was characterized by scanning electron microscopy, Raman spectroscopy, thermogravimetric analysis, chemical adsorption and desorption measurement, and mechanical stability test. Compared to PS–DVB particles, PS–DVB–CNT particles have certain improvement in physical and chromatographic performances because the addition of MWCNTs has altered the structures of the particles. The novel stationary phase owns satisfactory resolution, wide pH endurance, and long lifetime, which can be used as an extent to normal HPLC.

Introduction

To date, high-performance liquid chromatography (HPLC) has become an indispensable technique for the analysis of samples, the determination of physical constants and the isolation of purified components from complex mixtures in different scientific areas [1], [2], [3]. Columns of packed particles are still the most popular devices for liquid chromatographic separations because of their great utility, excellent performance and wide variety. The design of novel stationary phase is a permanent demanding challenge in chromatographic separation science to enable analysis with enhanced selectivity, specificity and speed.

Although new types of stationary phases are continually developed, most columns for reversed phase liquid chromatography (RP-LC) separations are manufactured from silica materials [1], [2]. However, silica-based packings are less stable under extreme pH conditions and show some retention abnormalities on old or non-silanol-endcapped reversed phase-columns (RP-column). To overcome these drawbacks, other inorganic materials such as zirconia [4], titania [5] and alumina [6] have been developed as HPLC stationary phase with improved chemical and thermal stabilities. However, they have residual metal hydroxyl and metal-oxo-metal groups and represent amphoteric materials with both anion- and cation-exchange properties. In view of the drawbacks of inorganic RP-columns, effort has been focused on the replacement of inorganic-based RP packings by polymer phases. Organic beaded polymer supports based on polystyrene–divinylbenzene (PS–DVB) [7] and polymethacrylates [8] are mostly investigated and applied polymer phases for RP-HPLC. And yet polymer phases are somewhat less efficient compared to inorganic-based counterparts because of the relatively low rigidity.

In order to overcome the defects of traditional packings, novel composite materials used as chromatographic stationary phases have been developed for years, which maintain the advantages of both organic polymers and inorganic supporters. Coating and coupling techniques are most frequently adopted for the preparation of composite materials, as it is rather difficult to combine macromolecules together. For instance, polymer-coated silica [9] or polymer-grafted silica [10] on pre-formed silica materials and polymer-coated metal oxides [11] are all used as RP-stationary phases. The major advantage of polymer-coated inorganic stationary phases lies in pH-stable layer that may easily be derivatized for various purposes [12]. At the same time, the significant loss of specific surface area that occurs in course of such coating procedures as well as the inhomogeneous distribution of polymer coating represents major drawbacks.

Fortunately, with the development of material science, especially the great advancement of nano-materials, new stationary phases are coming up continually. Carbon-based nano-materials [13], [14], [15], especially carbon nanotubes (CNTs) play an important role in analytical chemistry. CNTs can be described as a graphite sheet rolled up into a nanoscale-tube (single-wall carbon nanotubes (SWCNTs)), or with additional graphene tubes around the core of an SWCNT (multi-wall carbon nanotubes (MWCNTs)) [16]. They have diameters in the range between fractions of nanometers and tens of nanometers, and lengths up to several centimeters with both their ends normally capped by fullerene-like structures. The addition of this nanostructure in traditional materials have impacts on resulting properties, such as tensile strength, modulus, impact resistance, electrical conductivity, thermal stability and mechanical stability [17].

The potential of CNTs has been evaluated in separation techniques with their high thermal and mechanical stability, high surface area available for chemical interactions and the possibility of direct synthesis. The use of CNTs in stationary phases has already been reported as: (i) a novel monolithic stationary phase for μ-HPLC and capillary electrochromatography [18], (ii) a new stationary phase for gas chromatography [19], [20], (iii) a solid phase extraction adsorbent for organic [21] and inorganic compounds [22], (iv) a stationary phase in situ grows in microfluidic channels on a microfabricated chip for liquid chromatography [23], gas chromatography [24], [25] and electrophoresis [26]. Besides, CNTs are used as the stationary phases of HPLC as well [27], [28], [29], [30]. Nevertheless, to the best of our knowledge, all the papers describing the CNT composite stationary phases of HPLC referred to the silica-based stationary phases, regardless of which way was chosen to prepare the Si–CNTs material. Here, we firstly report the preparation of polymer–CNT composite material used as column packings of HPLC and expect to find out some improvements in chromatographic behaviors. Three different synthetic paths have been developed and characterizations were used for comparison with normal polymer packings. Besides, applications of this novel stationary phase are described as well.

Section snippets

Reagents

Styrene (ST) (99+%, Lingfeng Chemical Reagent Co. Ltd., Shanghai, China) and divinylbenzene (DVB) (55+%, Zhengguang Chemical Plant, Hangzhou, China) were used after distillation under reduced pressure. MWCNTs (sample purities: 95+%) with the average outer diameter (OD) about 10 nm were supplied by the institute of material physics and microstructure of Zhejiang University, China.

Polyvinylpyrrolidone (PVP, K-30) was purchased from Aldrich (USA). Azobisisobutyronitrile (AIBN) (Shanghai Chemical

Surface characterization of the composite particles

PS–DVB–CNT particles were taken into consideration because they are suitable for LC, with great stability against many oxidizing reagents; high rigidity and wide tolerance of pH values. Particle size and morphology were determined by SEM, which showed different synthetic methods resulted in different morphologies. The sample was coated with a thin conducting layer of gold by sputter coating before sampling. From the SEM photography of PS–DVB (Fig. 3(a)), it can be seen that the particles are

Conclusion

The synthesis of this novel stationary phase was based on classical swelling and polymerization method. As both MWCNTs and PS–DVB are macromolecules, the combination was not so easy to realize. We developed three different synthetic methods to combine MWCNTs with PS–DVB particles. After characterization and comparison, the first method was considered to be the best choice for the synthesis of PS–DVB–CNT particles. The results show that the beads have the uniformity in particle size and

Acknowledgements

This research was financially supported by National Natural Science Foundation of China (Nos. 20775070, J0830413), Zhejiang Provincial Natural Science Foundation of China (Nos. R4080124, Y4090104), and Zhejiang Qianjiang Project of Science and Technology for Competent People (No. 2008R10028).

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