Mark M. Robson

Capillary electrochromatography using columns packed with a supercritical fluid carrier

SummaryColumns for capillary electrochromatography may be prepared by packing reversed phase silica-based material using a supercritical fluid carbon dioxide carrier. Procedures for the in-situ manufacture of frits and UV detection windows, and the wetting of columns are described. The columns were employed in two commercial instruments (and a home-built system), and their properties investigated during the separation of standard mixtures of test compounds. The columns are highly efficient and durable, with reduced plate heights of 1.0–1.4. The repeatability of retention time, peak area and peak height was measured. The influence of applied voltage and column temperature and of electrokinetic injection parameters was explored, along with other practical considerations.

Investigation into the Formation of Bubbles in Capillary Electrochromatography

The formation of bubbles in capillary electrochromatography (CEC) is well documented: possible origins include Joule heating and variations in EOF velocity on passing from the stationary phase through the frit and into the open tube. Methods for the prevention of bubble formation are discussed which are confirmed by experimental results. Using frit lengths varying from 1 mm to 6 mm it is shown how frit length is directly related to the likelihood of bubble formation and how this is affected by applied voltage. It is shown that the change in applied voltage across a capillary affects the formation of bubbles and also that rebonding octadecylsilane (ODS) onto the silica frit after formation of the frit can minimize the formation of bubbles and how this effects the chromatography. A method is also described for increasing the robustness of silica capillaries using a column coupler along with modifications made to conventional capillary electrophoresis equipment to cater for CEC.

Capillary electrochromatography on silica columns: factors influencing performance

Much capillary electrochromatography (CEC) work is carried out on bonded silica packings which offer many advantages: the number of such packings which are available; the fact that the chemistry of bonding and the separation process are fairly well understood; and the possibility of the transfer to CEC of existing HPLC methods. Packing methods for the preparation of CEC columns have been investigated. The problems inherent in the use of burned-in frits remains an obstacle, but can be at least partially overcome by minimising the length and by silanisation. The influence of a variety of mobile phase variables on aspects of CEC is in agreement with theory for: ionic strength, organic content (including isoeluotropy), and pH. Temperature can be used as a variable to change column selectivity in CEC. The influence of pH on electroosmotic flow (EOF) by changing the degree of ionisation of residual silanol groups is similar for a wide range of neutral bonded groups, but is much less marked for bonded sulphonic acid groups. The EOF may be reversed for bonded groups containing nitrogen.

Hydrocarbon group-type separations for high aromatic fuels by supercritical fluid chromatography on packed capillary columns

Abstract The contents of total aromatics and of mono-, di-, and polyaromatics in fuels may be rapidly and conveniently determined by supercritical fluid chromatography on capillary columns packed with silica using a supercritical carrier. The use of carbon dioxide mobile phase permits detection by flame ionisation. A study of the retention behaviour of test compounds allowed the separation temperature and pressure to be optimised. The aromatic content of a range of fuels (17–88%) determined in this way was in good agreement with fluorescent indicator analysis.

Development of a universal Raman detector for microchromatography

SummaryA detector for microchromatography in which Raman spectroscopy is used to identify the eluted species has been developed. The detector is designed to be applicable to a wide range of compounds without requiring the presence of a chromophore. Its use is illustrated in the analysis of nitro compounds on a 250 μm i.d. column. Raman spectra of each of the compounds could be identified as they passed the detector. The advatages of the use of fully deuterated solvents are demonstrated by the analysis of nitrobenzenes in methanol/water mobile phases. The detection limit for nitrobenzene using the Raman line at 1342 cm−1 was 75ng.

Simple interface for gradient elution CEC and coupled HPLC-CEC

SummaryAn interface which permits both gradient elution capillary electrochromatography (CEC) and the coupling of high performance liquid chromatography (HPLC) to CEC is described. Mobile phase from an HPLC system with continuously variable composition is supplied on demand to the inlet of a CEC column, without the posibility of pressure driven flow. The interface also allows fractions separated by either reverse phase or normal phase HPLC to be transferred to a CEC column operated in reverse phase mode for further separation.

Supercritical Fluid Chromatography of Polycyclic Aromatic Compounds: A Review

Abstract An overview of supercritical fluid chromatography as a method of analysing complex mixtures of polycyclic aromatic compounds is given.

Capillary electrochromatography. Tutorial Review

Capillary electrochromatography (CEC) has seen a resurgence of interest during the 1990s, despite having origins in the 1970s. The technique combines the desirable features of both high-performance liquid chromatography (HPLC) and capillary electrophoresis (CE): the separation process is based on differential interactions between the stationary and mobile phases, whilst the electroosmotic flow transports the mobile phase through the capillary. Thus, it has demonstrated advantages over both HPLC and CE, which are yet to be fully exploited over a wide field of application; already the popularity of CEC is on the increase, as reflected in the number of scientific publications and seminars held. The aim of this tutorial review is to increase awareness and understanding of both theoretical and practical aspects of CEC. Whilst it does not provide an in-depth account of CEC, it does attempt to cover the more important, relevant work available in the open literature: only major advancements associated with CEC applications are highlighted. Material presented in the review was typically obtained by literature searches involving Analytical Abstracts, Chemical Abstracts and ‘BIDS’ (for academic use only).