Peter Myers

Theory of capillary electrochromatography

The present state of the theory of capillary electrochromatography (CEC) is reviewed. Emphasis is placed on electroosmosis and the electrical double layer, and the generally good understanding of the factors affecting the electroosmotic flow in CEC columns. The relation of CEC to other electrically driven separations are described, along with band broadening, and the influence of column temperature in CEC. The theoretical potential of CEC is assessed from the standpoint of current and future column technology, and likely future application areas are described.

Tapers and restrictors for capillary electrochromatography and capillary electrochromatography-mass spectrometry

Tapered and narrow restrictor capillaries have been investigated for use in capillary electrochromatography (CEC) and have been shown to offer viable, more reproducible, alternatives to silica frits and may aid in suppression of bubble formation, a recognised problem in CEC. In addition, these capillaries have been coupled to mass spectrometry, especially for low-flow and nano-flow electrospray ionisation, where results have shown better maintenance of chromatographic efficiency and higher sensitivity, compared with more conventional interfaces. The use of tapered CEC column inlets is suggested as being useful in sampling from micro-environments, such as cell contents.

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.

Influence of the electrical double-layer on electroosmotic flow in capillary electrochromatography

Abstract An intensive investigation into the electroosmotic flow in capillary electrochromatography has been performed on C 18 packed columns. Linear velocities obtained when varying voltage and electrolyte characteristics were comparable to those found in capillary electrophoresis. Only at low ionic strength was a deviation from the expected relationship observed; this was attributed to double-layer overlap in the particle pores and turbulent flow. Van Deemter studies on C 18 phases confirmed that linear velocity was unrelated to particle size, but did not support current theory that smaller particles will result in improved efficiency. The role of particle porosity remains ambiguous; lower flow-rates were generated on a C 18 material with 30 nm pores compared to an 8 nm pore material, yet silica did not display the same trend. Our results suggest that high efficiencies can be obtained on 3 μm particles although the columns may be poorly packed. Reproducible separations are most likely to be achieved on well-packed columns of larger particle size. Studies on silica highlighted problems in column manufacture; duplicate columns gave comparable flows but not separation parameters. New procedures for the use of very small particles (1.5 μm) are desirable.

History of gas chromatography

Modern gas chromatography (GC) was invented by Martin and James in 1952 [1], and has become one of the most important and widely applied analytical techniques in modern chemistry. Major milestones in the development of GC, especially in column technology, detection and sample introduction are described in this historical review. Many trends in current progress can be seen to originate in the first two decades of the history of GC, but the invention of fused-silica capillary columns greatly increased the application of high-resolution GC across the field of organic analysis; the development of low-cost, bench-top mass spectrometers led to further advances. Progress continues to be rapid in comprehensive 2D GC, fast analysis, detection by atomic emission and time-of-flight mass spectrometry, and in applications to process analysis.

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.

Electro-osmotic and pressure-driven flow properties of frits for packed column capillary electrochromatography prepared from functionalised and bare silica packings

Changes in electro-osmotic flow (EOF) induced in an open fused silica capillary by the introduction of a single packing retainer (referred to as a ‘frit’ throughout the article) are investigated. Frits located in a 34.5 cm long capillary close to the detection window, as used commonly in packed column capillary electro-chromatography, were made by sintering silica based packing materials having varying functionalities (bare silica, ODS-silica, a strong cation exchange packing (SCX) and a strong anion exchanger (SAX) at 430 °C for 15 s. The contributions of such frits to the total EOF measured in the capillary are discussed. Despite the high temperature of heating of the packing, sufficient residual functional groups remained on the frit such that distinctively different EOF behaviour was observed for each type of packing. Frits made from materials providing negative surface charges, such as silica, ODS-silica and SCX, increased the magnitude of the cathodic EOF compared to the open capillary. On the other hand, a substantial anodic EOF of −1.88 × 10−8 m2 V−1 s−1 was produced by introduction of a single frit made from the SAX material. An explanation of this behaviour is given, based on the hypothesis that the EOF generated by the frit determines the overall flow in the whole open capillary. The frit is considered to work as a pump and overrides the EOF generated at the capillary wall.

The Use of the Helium Ionization Detector for Gas Chromatographic Monitoring of Trace Atmospheric Components

Two different gas chromatographic detectors, the helium ionization detector (HID) and the more commonly used flame ionization detector (FID), were used in parallel to compare their responses to a number of organic compounds. Atmospherically important oxygenated species were analyzed, as well as hydrocarbons and chlorinated and sulfur containing organics. The HID exhibited the better response to all compounds investigated, most notably to formaldehyde and higher oxygenates. A gas chromatographic system was developed to trap and analyze atmospheric organic compounds with HID detection. This required careful choice of the adsorbent material and removal of inorganic components (namely nitrogen and oxygen) before analysis. Real air samples were then taken and analyzed qualitatively for a range of olefinic and aromatic compounds.

Comparison of different packing methods for capillary electrochromatography columns

A study was carried out in which 50 microm I.D. fused-silica capillaries were packed with 3 microm octadecylsilane bonded silica, from the same batch, by four methods; liquid slurry and carbon dioxide supercritical carrier, each with and without the use of an ultrasonic probe. A neutral test mixture was analysed by capillary column in reversed-phase mode, and the reproducibility of the electroosmotic flow and of migration time, column efficiency and retention factors, was determined. Initially results suggested that there was no significant difference between properties of columns packed by different methods, and a more thorough statistical evaluation confirmed this; differences observed in the column performance were attributed to random variations between replicate columns, and not between packing methods. However, the variation was least when applying the ultrasonication during liquid slurry.