Preparation, characterisation and modification of porous carbon monolithic materials for chromatographic and electrochemical applications
Eltmimi, Ali (2009) Preparation, characterisation and modification of porous carbon monolithic materials for chromatographic and electrochemical applications. PhD thesis, Dublin City University.
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Chromatographic column technology in HPLC is an ever-growing field. One area in column technology that has captivated attention in recent years is the use of
monolithic columns. Monolithic stationary phases for use in liquid chromatography possess unique characteristics, the most significant of which is a relatively high flow
through porosity leading to low column pressure drops, with the potential for use with elevated flow rates leading to faster separations. Silica-based monolithic columns are the most popular phases currently for small molecule separations, although organic polymer-based monoliths have also been studies extensively. Within this thesis the preparation of carbon monoliths of varying flow
through porosity for potential chromatographic and electrochemical applications is described. Carbon based monoliths can provide excellent chemical and thermal
stability, surpassing those of silica. Unlike alternative materials, carbon monoliths have the advantage of being resistant to swelling and hydrolysis.
Carbon monolithic rods produced within this work were characterised using diverse techniques such as SEM/EDX, BET, dilatometry, conductivity & backpressure determination and investigated for chromatographic application.
Surface modification of the resultant carbon monoliths with gold nano-particles and nano-layers was carried out for further modification with thiol based affinity and
exchange chemistries. Retention on the carbon monolithic phases was investigated using reversedand
normal-phase high performance liquid chromatography. Preliminary separations of mixtures of hydrophobic molecules (alkylbenzenes) on a short 5 μm pore sized
glassy carbon monolithic column were carried out using a mobile phase of 100% hexane (or pentane), flow rate 0.7 ml/min, UV detection 270 nm. The backpressure
for the monolithic phase was 2 bar. Also, preliminary separations of mixtures of hydrophilic molecules (caffeine, theophylline and benzoic acid) using various
concentrations of acetonitrile as a mobile phase were investigated on the same column. Furthermore, preliminary separations of mixtures of butylbenzene–
acetophenone, and butylbenzene-nitrobenzene, respectively, on a same column column were also carried out using a mobile phase of 100% hexane (or pentane),
flow rate 1 ml/min, UV detection 254 nm, with an injection volume 15 μL. The backpressure for the monolithic phase ranged between 5 and 8 bar. To improve retention, surface modification (conditioning) of the glassy
carbon monolith was performed using 0.5 and 2 M solutions of sulphuric acid, followed by a similar strength solution of 50:50 sulphuric:nitric acid. Following such
conditioning the retention factors for the mixture of butylbenzene and nitrobenzenene were significantly increased under normal phase conditions,
indicating increased surface polarity on the glassy carbon monolith. The formation of metallic monolithic structures within the actual larger flow through pores (5-10 μm) of the carbon rods was also studied, leading to the
formation of novel gold modified carbon monolithic phases. The carbon/gold composite monolithic columns were further modified with mercaptohexanoic acid and evaluated for retention characteristics in ion exchange chromatography.
Imidazole (1 μg/mL) was injected onto the 5 μm pore sized gold modified glassy carbon monolithic column and was shown to be retained through ion-exchange
chromatography using a 1 mM acetate buffer mobile phase (pH 3.96) at a flow rate of 1 mL/min, with UV detection at 280 nm. The column backpressure was 24 bar.
Carbon monoliths have the advantage of being conductive materials and were therefore also investigated as electrode materials for electrochemical analysis.
Electrochemical measurements obtained on the porous glassy carbon monolithic electrode and gold modified porous glassy carbon monolithic electrode showed a more reproducible response relative to the standard glassy carbon electrode. The electrode was investigated using cyclic voltammetry following electrochemically induced oxidation to impart hydrophilicity. Cyclic voltammetry was investigated
using a solution containing K3Fe(CN)6 and KCl between -0.2 and +1.1 V. The scan rate used for the above characterisations was 20–100 mV/s.
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