Comparison of thick film capillaries with packed columns from the point of view of loadability in combination with high resolving power

The present paper compares the potential of high pressure packed column gas chromatography, with a particle size in the range of 30–80 μm and conventional packed column GC. with that of thick film capillaries for obtaining the maximum loadability at a given performance in efficiency and speed of separation. An alternative treatment, discussing the maximum efficiency of the three column types at normalized loadability and speed of separation is given. Known and established relationships describing plate height, loadability, and linear velocity are used to arrive at the said comparisons. The conclusion of the paper indicates a reconsideration of packed column GC for particular types of analyses where large amounts or high concentrations are required in the detection step.     

Factors affecting reproducibility in the manufacture of glass and siliceous glass open tubular (capillary) columns

Summary Column characteristics affecting the chromatographic behavior of glass and siliceous glass (fused silica) capillary columns include the dimensional uniformity of the column, the physical and chemical characteristics of the column wall and the characteristics of the liquid phase. In the case of the coated column the uniformity and thickness of the liquid phase film are the most important criteria affecting column reproducibility. The paper discusses these factors and their influence on column performance.Presented in part at the Symposium on Standardized Materials for Chromatography, 181 st National American Chemical Society Meeting, Atlanta, Georgia, March 29–April 3, 1981.     

Applications of packed and capillary supercritical fluid chromatography in the separation of tocochromanols

Tocochromanols consisting of tocopherols and tocotrienols, is collectively known as vitamin E. Similarity in their structures, physical and chemical properties rendered the tocochromanols to be subject of chromatography interest. Supercritical fluid chromatography is a highly efficient tool for the separation and analysis of tocochromanols. Separation and analysis of tocochromanols using supercritical fluid chromatography had been carried out in the past using capillary or packed columns. Each of these techniques offer their own advantages and drawbacks. Besides being used for analysis, packed column supercritical fluid chromatography found applications as a purification and content enrichment tool. Emergence of new equipment and stationary phase technologies in recent years also helped in making supercritical fluid chromatography a highly efficient tool for the separation and analysis of tocochromanols. This paper gives an insight into the use of capillary and packed columns in supercritical fluid chromatography for the separation and/or analysis of tocochromanols. The types of stationary phase used, as well as chromatographic conditions are also discussed.     

PoraPLOT Q: A porous layer open tubular column coated with styrene-divinylbenzene copolymer

A porous polymer is deposited on the inner wall of fused silica capillary columns. The retention characteristics of this porous polymer were evaluated and found to be comparable with Porapak Q. The porous polymer has a high retention volume which enables the separation of permanent gases at ambient temperatures or higher. The hydrophobic character of the porous polymer allows the injection of water containing samples without changing retention due to adsorption of water. The inertness of the porous polymer allows the elution of a range of apolar and polar compounds. The maximum temperature of the porous polymer was estimated to be 250°C. With this new type of capillary column, high resolution separations are obtained in combination with short analysis times.     

Considerations of speed and efficiency in capillary gas chromatography

The analysis time for a given resolution is a complex function of stationary phase selectivity, column radius, and thickness of the stationary phase film. Variation of these parameters has a large effect not only on analysis time, but also on the column inlet pressure and other instrumental requirements. The minimum amount that can be reliably detected as well as the maximum sample capacity of a column are strongly related to the selected column dimensions.     

The application of porous silica layers in open tubular columns for liquid chromatography

Summary Two methods to realize a porous retentive silica layer on the inner wall of 10–25 μm fused silica capillaries for OTLC, etching and precipitation of silica from solution, have been investigated. Etching of the fused silica capillaries with 1M KOH, creates an activated surface, but the capacity of the silica layer is too small to serve as retentive layer in OTLC. Better prospects are offered by the precipitation of silica from a solution of polyethoxysiloxane, dynamically coated on the inner wall of the fused silica capillary. It appears to be possible to deposite a porous silica layer up to 0.8 μm thick (in a 25 μm capillary) by this method, which seems to be suitable for liquid-solid an dynamically generated liquid-liquid chromatography in open tubular columns. The performance of these columns are demonstrated by means of efficient separations of test mixtures using on-column fluorescence detection.     

Investigating retention characteristics of a mixed‐mode stationary phase and the enhancement of monolith selectivity for high‐performance liquid chromatography

The synthesis and chromatographic behavior of an analytical size mixed‐mode bonded silica monolith was investigated. The monolith was functionalized by an in situ modification process of a bare silica rod with chloro(3‐cyanopropyl)dimethyl silane and chlorodimethyl propyl phenyl silane solutions. These ligands were selected in order to combine both resonance and nonresonance π‐type bonding within a single separation environment. Selectivity studies were undertaken using n‐alkyl benzenes and polycyclic aromatic hydrocarbons in aqueous methanol and acetonitrile mobile phases to assess the methylene and aromatic selectivities of the column. The results fit with the linear solvent strength theory suggesting excellent selectivity of the column was achieved. Comparison studies were performed on monolithic columns that were functionalized separately with cyano and phenyl ligands, suggesting highly conjugated molecules were able to successfully exploit both of the π‐type selectivities afforded by the two different ligands on the mixed‐mode column.     

Study on the efficiency of assembled packed microbore columns in HPLC

Summary Stainless steel columns (internally mirror-finished, 125 or 250nm in length, of bore 1.0 or 1.6mm) were slurry-packed with 5μm and 4μm reversed phase silicas (Hypersil ODS, LiChrosorb RP-8 and RP-18 and Superspher RP-8. The HPLC equipment consisted of a pump LC5A (Shimadzu) or a pump 2150 (LKB Instruments), a rheodyne valve 7413 with loops of 0.5, 1.0 and 5.0mm³ and a Jasco-Uvidec 100-II UV detector, variously with one of three specially constructed cells of 0.2, 0.4 and 1.3mm³ volume. Columns were assembled using two types of coupling device employing a stainless steel capillary of 0.12mm bore. The effect of sample volume, design of coupling device in assembled column, detector cell volume and geometry on column plate count was examined to optimize the conditions and the instrument compartments. The highest efficiency for the assembled columns was achieved by use of the 0.2mm³ detector cell, a low dead volume coupling device and a 0.5μl sample volume. Sets of 5 assembled columns gave reduced plate height values between 2–3 at a reduced velocity 2<v<3, indicating that extra-column effects were negligibly small; the performance was identical to that of conventional analytical columns. Presented at the 15th International Symposium on Chromatography, Nürnberg, October 1984     

Investigation of capillary micro-packed columns composed of two segments packed with a sorbent of different particle size

Summary For the first time gas chromatographic characteristics of fused-silica capillary micro-packed columns consisting of two segments (i.e., serial columns) and packed with two different particle size fractions of the same sorbent have been investigated. The height equivalent to a theoretical plate (HETP) and the specific separation number vs. carrier gas velocity dependence has been studied. The expediency of empolying serial columns in which the segment containing the finer sorbent fraction is located near the column outlet has been theoretically and practically expalined. Serial columns with such an arrangement of the two segments show a better performance from the view point of efficiency and mass transfer coefficient.     

Capillary columns-from London to London in 25 years

Summary The paper presents an overview of the evolution of open tubular (capillary) columns introduced 25 years ago by M.J.E. Golay. This evolution is divided into five stages: the formulation of the theory and the basic work laying down the foundations for future work; learning the influence of the variables on column performance; development of columns with increased surface area; development of glass and fused-silica columns; and, finally, the introduction of open tubular columns in liquid chromatography. In each stage, the pioneering activities are summarized and the most important references given.Plenary lecture at the 14th International Symposium on Chromatography, London, September 13–17, 1982.     

Large diameter open tubular columns in gas chromatographic analysis

Large diameter open tubular columns provide the packed column chromatographer with a simple route to higher resolution gas chromatography. They can be operated in a high-flow (lower resolution) mode that permits their direct substitution for a packed column, or they can be operated in a low-flow (higher resolution) mode that maximizes separation at the cost of longer analysis times. Inlet design and column installation can influence both the chromatographic results and quantiative reliability. Make-up gas is not required in the high-flow mode, and its benefits in the low-flow mode are restricted to enhanced detector sensitivity, provided the outlet end of the column resides in the detector jet (FID). The columns seem fully compatible with all common modes of detection.     

Capillary columns in series for the gas chromatographic analysis of essential oils

Summary By connecting capillary columns of different polarity in series and by changing the temperature program gradient, a system of variable polarity is obtained. The working conditions can be adjusted to optimize separations of complex mixtures. Application to the analysis of essential oils of: lavender, thyme, lemon petit grain, bergamot, and tangerine, are reported.Dedicated to Prof. Dr. A. Liberti on the occasion of his 70th birthday.     

Factors determining flow rate in chromatographic columns

Summary It is shown that the flow in chromatography is nearly always laminar in nature. Starting from the Darcy equation, expressions are given for the flow rate in both gas and liquid chromatography columns. The concepts of specific permeability, chromatographic permeability and column resistance factor are discussed for packed as well as open tubular columns. The experimental determination of all these factoers is demonstrated. The influence of the shape and pore volume of porous and non-porous supports on the column resistance factor and the chromatographic permeability is discussed.     

Preparation of Fritless Packed Silica Columns for Capillary Electrochromatography

Fritless packed silica gel columns were prepared using sol‐gel technology. A part of a 75 μm i.d. fused silica capillary was filled with a mixture of tetramethoxysilane and poly(ethylene glycol). After gelling at 40°C and heating at 300°C, the resultant silica gel was derivatized with dimethyloctadecylchlorosilane. A scanning electron micrograph of a cross‐section of the capillary column showed that the gel took the form of a spherical particle aggregate and adhered to the column inner wall. The column performance was evaluated for electrochromatography using acetonitrile–50 mM HEPES buffer (pH 6.6) (60/40 or 40/60, v/v) as the mobile phase. An electroosmotic flow of 1.0 mm/s was generated with (60/40, v/v) acetonitrile/HEPES buffer at a field strength of 546 V/cm. Using a sol‐gel‐derived packed column at an electroosmotic flow of 0.5 mm/s, efficiencies of up to 1.1×10⁵ plates/m were obtained for retained solutes.     

Instrumental considerations for the effective operation of short,highly efficient fused-core columns. Investigation of performance at high flow rates and elevated temperatures

Fused core or superficially porous columns offer the advantages of higher efficiency compared with totally porous columns of the same particle size, but similar operating pressures. However, their performance may be adversely affected by extra-column effects that become more significant as the column efficiency increases, and as the diameter of the column is reduced. In this study, we show that 10 cm × 0.46 cm fused-core columns can be used on modified conventional instruments (“microbore systems”) without serious loss in performance, and this approach is at present likely to yield superior results compared with use of 0.21 cm columns (of identical efficiency) on current UHPLC instruments that have minimised extra-column volume. Furthermore, the true efficiency of commercial narrow-bore fused-core columns appears to be reduced compared with those of conventional bore, which may be due to packing difficulties for the former type. The fused-core columns in general gave excellent performance, showing no evidence of an upturn in the Knox plots at high flow velocities and elevated temperatures. Careful control of experimental conditions is necessary to ensure accurate data for these plots.     

Current technological challenges in capillary supercritical fluid chromatography

In cases where high efficiency is required to resolve complex mixtures of either thermally labile or nonvolatile organic compounds, capillary supercritical fluid chromatography may be the most desirable analytical method. While great strides in this new technology have been made over the last few years, several problem areas are requiring increased attention. These include sample introduction systems, pressure reduction at the end of the column, column stability in various supercritical mobile phases, and migration of polar solute molecules. This paper describes the state-of-the-art in capillary SFC with emphasis on the progress made and future needs in the solutions to these specific problems.     

Comparison of commercial organic polymer‐based and silica‐based monolithic columns using mixtures of analytes differing in size and chemistry

Commercially available silica‐based monolithic columns Chromolith RP‐8e, Chromolith RP‐18, and Chromolith HR RP‐18, and polymer‐based monolithic columns ProSwift RP‐1S, ProSwift RP‐2H, and ProSwift RP‐3U varying in pore size and bonded phase have been tested for the fast separation of selected sets of analytes. These mixtures of analytes included small molecules (uracil, caffeine, 1‐phenylethanol, butyl paraben, and anthracene), acylated insulins, and intact proteins (ribonuclease A, cytochrome C, transferrin, apomyoglobin, and thyroglobulin), and covered wide range of chemistries and sizes. Small molecules were well separated with a height equivalent to theoretical plate of 11–26 μm using silica‐based monolithic columns, while organic polymer‐based monoliths excelled in the fast sub 1 min baseline separations of large molecules. A peak capacity of 37 was found for separation of acylated insulins on Chromolith columns using a 3 min gradient at a flow rate of 3 ml/min. Poor recovery of proteins from Chromolith columns and significant peak tailing of small molecules using ProSwift columns were the major obstacles in using monolithic columns in those applications.     

Packed columns vs. wall coated open tubular columns in quantitative analyses of polychlorinated biphenyls

This article describes and critically evaluates a complete method for the quantitation of PCB in sediment samples. The extraction and clean-up procedure is described. Packed column and capillary column results are compared. Capillary column quantitation yields lower values for total PCB loading. Preferences for capillary column quantitation are discussed and explained. Capillary quantitation is based on the combination of an external Aroclor standard and an internal decachlorobiphenyl standard for normalizing data. In light of the recent report of synthesis of all 209 PCB congeners a suggestion is made to use these as absolute standards to establish a bank of primary standard Aroclors.     

Chemically modified teflon wall-coated open tubular columns for gas chromatography

Summary Wall-coated open tubular columns prepared from chemically modified teflon tubing have been developed for gas chromatography. Chemical reaction of the inner walls of teflon tubing allows bonding of an adhesive layer on which a variety of stationary phaes can be coated. Test mixtures of alkanes and alcohols were used to investigate the chromatographic properties of these columns and the stability and mixing of the adhesive and stationary phae layers. the results in dicate that mixing of the layers is negligible and that the column is stable for a long period of time.Principle author