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The outer phase in tube radial distribution chromatography (TRDC) operated as a stationary phase in chromatographic separation. Chromatographic peak information from model analytes 1-naphthol, 1-naphthalenesulfonic acid, 2,6-naphthalenedisulfonic acid, and 1,3,6-naphthalenetrisulfonic acid was examined using TRDC at various flow rates, using a fused-silica capillary tube (inner diameter, 75 μm; length, 120 cm; effective length, 100 cm) as the separation column and water/acetonitrile/ethyl acetate (3:8:4, v/v/v) as eluent. From the data, the relationships between average linear velocities and values of height equivalent to one theoretical plate of the analytes were investigated, and the separation efficiency by TRDC, comprising an inner phase (mobile phase) and outer phase (stationary phase) in an open-tubular capillary tube, was examined using the van Deemter equation.

High performance liquid chromatography (HPLC) and ultrahigh performance liquid chromatography (UHPLC or UPLC) have been the most widely used tools for research and routine quality control of active pharmaceutical ingredients (API). The most important challenge in these techniques is fast and efficient separation. Both techniques are preferred due to their selectivity, high accuracy and remarkable precision. On the other hand, they have some limitations: In some cases, traditional HPLC uses high amounts of organic solvents with longer analysis time, and furthermore UHPLC has high back pressure and frictional heating. To overcome these limitations, scientists have developed new type of column particles. In general, two different silica types of column packing material based on their backbone have been used for HPLC and UHPLC. Stationary phases that have fully porous silica particles comply with the essential criteria of analysis, but these show all the limitations of HPLC. However, in recent years, core–shell silica particles (a combination of solid core and porous shell) have been increasingly used for highly efficient separation with reduced run times. Thus, core–shell technology provides the same efficient separations as the sub 2 µm particles that are used in UHPLC, while eliminating the disadvantages (potentially lower backpressure). The key factors for core–shell particles are size and thickness of porous shell layer, the latter of which can be explained using the Van Deemter equation. The columns packed with core–shell particles have been employed in a wide range of applications for analysis and quality control of pharmaceutical active substances. This review will underline the advantages of core–shell silica particles in the analysis of pharmaceutically active ingredients based on liquid chromatography from the perspective of column properties, system suitability test parameter results and validation steps.

Only slices are measured of very long segments of solute molecules that are injected into columns in chromatographic systems. Since the concentration of solute molecules changes during elution as a function of time, the observed signal should be re-constructed for a comparison of experimental results with theory. The influence of response time on the observed signal in high-performance liquid chromatography experiments was investigated in detail using a C-18 column for the measurement of caffeine in pure methanol as eluent. The response function influenced the observed signal by converting the square signal of pulse flow into a chromatographic peak. At faster linear flow rates (LFRs), the response function influenced the observed shape of the chromatographic peak, whereas diffusion dominated at slow LFRs. By convolution of the square signal with the response function, it was possible to predict the shape of the observed signal and chromatographic parameters and to provide an alternative explanation to the van Deemter equation. By using the shape of a known response function and modelling the new theory to data, it was proposed that the injected solute molecules were eluted over long distances through the chromatographic column, distances that are much longer than the physical length of the system.

The moment analysis of R-limonene was implemented using chromatograms that were obtained with the pulse input method from an octadecyl silica (ODS) high-performance liquid chromatography (HPLC) column. The particle diameter of the stationary phase was 5 µm. The general rate (GR) model was employed to calculate the first absolute moment and the second central moment. Two important coefficients for moment analysis, molecular diffusivity and external mass transfer coefficient, were estimated by the Wilke-Chang and Wilson-Geankoplis equations, respectively. The intra particle diffusivity was obtained by comparing theoretical plate number equation with the van Deemter equation. Experiments were conducted by varying flow rate, volume ratio of the mobile phase, and solute concentration. After The moment analysis results based on the experimental elution curve, van Deemter plots were drawn. And van Deemter coefficients were compared each other in order to investigate the effects of Hax, Hf, and Hd on Height equivalent to a theoretical plate (HETP, Htotal). Intra-particle diffusion (Hd) was the most dominant factor contributing to HETP, while external mass transfer (Hf) was negligible factor.

The van Deemter equation and moment equations were used for analyzing the flow rate dependence of HETP experimentally measured in order to clarify chromatographic behavior from the kinetic points of view in columns packed with C18-silica monolith and C18-core-shell particles under RPLC conditions. They provided some information about molecular diffusion, eddy diffusion, and mass transfer kinetics in the columns. Additionally, the value of intra-stationary phase diffusivity was determined in the range of 10(-7)-10(-6) cm(2) s(-1) by the moment analysis. Information about the contribution of the mass transfer resistance in the external liquid film around the stationary phase to HETP was also obtained. Its contribution increases with an increase in the flow velocity of the mobile phase. It was concluded that the moment equations can provide more detailed information about the kinetic behavior in the columns than the van Deemter equation.

The enhanced leachability of rare-earth ore from the lean weathered crust elution-deposited rare-earth ores with ammonium sulfate solution as lixiviant and sesbania gum as a filter-aid reagent was elucidated and optimized with chromatographic plate theory. The effects of the flow rate, the concentration of lixiviant, the sesbania gum filter-aid reagent, and the lixiviant consumption on the leaching process were examined with column elution. The results show that the sesbania gum has a positive effect on the leaching process; the relationship between the height equivalent to a theoretical plate (HETP) and the leaching flow rate can be described by the van Deemter equation. There is an optimum flow rate in the leaching process. The optimum concentration of the sesbania gum in lixiviant was discovered. Moreover, sesbania gum added in lixiviant also reduces lixiviant consumption and promotes the optimum leaching flow rate. It provides a new approach to leaching the lean weathered crust elution-deposited rare-earth ores with high efficiency and low reagent consumption. The approach also can reduce the NH4 +-N pollution.

The study is concerned with addressing hydrodynamic dispersion of an electroneutral non-adsorbed solute being transported by electroosmotic flow through a slit channel formed by walls with different zeta potentials. The analysis is conducted in terms of the plate height which, using the Van Deemter equation, can be expressed through the cross-sectional mean flow velocity, the solute molecular diffusion coefficient and a length scale param- eter having meaning of the minimum achievable plate height and depending on the velocity distribution within the channel cross-section. The minimum plate height is determined by substituting distribution of electroosmotic velocity into the preliminary derived integral expression that is valid for any given velocity distribution within a slit channel cross-section. The electroosmotic velocity distri- bution within the slit channel cross-section is obtained by solving one-dimensional version of the Stokes equation accounting for electric force exerted on the local equilib- rium electric space charge. The major obtained result is an analytical expression which represents the minimum plate height normalized by half of channel width as a function of two dimensionless parameters, namely, half of channel width normalized by the Debye length, and the ratio of the wall zeta potentials. The obtained result reveals a sub- stantial increase in the minimum plate height compared with the case of equal wall zeta potentials. Different lim- iting cases of the obtained relationships are analyzed and possible applications are discussed.

Efficiency in HPLC can be enhanced by increasing the column length and/or decreasing the particle size. The use of high temperature in HPLC has emerged as a valuable tool to overcome the increase in column backpressure when using small packing particles, as it allows for reduction in mobile phase viscosity. In this study, high plate count was obtained by coupling sub-2 μm columns at elevated temperature to reduce the viscosity of the mobile phase, thus reducing the column backpressure. At 80 °C, up to three columns of 15 cm × 4.6 mm I.D. packed with 1.8 μm particles could be coupled generating ~84,000 theoretical plates for the last eluting compound. The number of theoretical plates was increased on average by a factor of ~3.6 when three columns were coupled at 80 °C compared with one column at 30 °C. The relationships between separation efficiency and column length were examined using Van Deemter plots constructed at 30 °C and 80 °C for different column lengths. The advantages of using coupled columns in combination with elevated temperature for the environmental analysis were illustrated using test mixtures comprised of eight sulfonamides separated on one column at 30 °C and three coupled columns at 80 °C by isocratic elution. Sample clean up was carried out by employing solid-phase extraction (SPE) using Oasis HLB cartridges. The method developed was validated based on parameters such as linearity, precision, accuracy, detection and quantification limits. Recoveries generally ranged from 71.7 to 99% (with the exception of sulfanilamide), with standard deviations not higher than 4.7%. The detection limits of the method ranged from 0.6–2 μg L−1, while limits of quantification were in the range 2–6.7 μg L−1 with UV detection.

Wide use of a combination of gas chromatographic (GC) separations with mass spectroscopic detection causes the necessity to study the effect of the relative pressure in a capillary column on its efficiency and separation properties. Using n-decane as a sorbate, a decrease in the relative pressure was shown to induce a slight increase in the efficiency of polar capillary columns but results in a considerable decrease in the optimal flow rate of the mobile phase and strongly constricts the Van-Deemter curve profile. In the presence of the restrictor, even minor deviations from the optimal flow rate can result in a considerable decrease in the column efficiency. Since the capillary restrictor is often a component of the interface between the column and mass spectrometer, it is necessary to exactly maintain the optimal operation conditions of the GC column to achieve an optimal efficiency of separation.

This paper addresses the problem of maximizing the first Euler load of a column of constant aspect ratio (moment of inertia varies with the square of the cross-sectional area), considering the column self-weight, subject to the constraint of fixed volume of column material and height. The coupled nonlinear integro-differential equations of optimality and stability, generated through variational principles, have been solved using the method of parameter perturbation. The optimal column has cross-sectional area that follows a perturbed two-thirds power law; the first Euler load is up to 87% larger than that of the corresponding column of uniform cross-section having the same volume and height.

Mobile phase chromatographic conditions for the selective separation of metoprolol from related amino alcohols have been evaluated using Hypercarb as support and carbon dioxide with addition of methanol as mobile phase. The objective for the presented study was to show the unique ability of the porous graphitized carbon surface to separate closely structurally related substances. Experimental results, using Hypercarb and “chromatographic normal phase conditions” (SFC), are presented with focus on how to control retention and to improve peak performance. A high concentration of basic aliphatic amine additive was required in order to elute the amine analytes as symmetrical peaks. N,N-Dimethyloctylamine was preferred over triethylamine since the retention was markedly shorter though the selectivity and resolution were virtually the same. The selectivity can be optimized by altering the temperature of the column. A high selectivity was demonstrated between metoprolol and two homologues with one and two extra methylene groups inserted between the secondary nitrogen atom and the carbon atom with a hydroxyl group attached. The mobile phase flow rate effect the column efficiency only to a minor extent as the steepness of the van Deemter curve for metoprolol was virtually flat in the range studied: 0.5–3.0 mL min−1.

An improved visualization device made of polymethyl methacrylate (PMMA) was used to study the 3D flow profile inside conical columns of a 10° opening angle packed with C18 phase. The outside wall of the conical columns was rectangular in shape in order to improve the transparency property of the column wall and reduce the deformation of the image for better observation of the flow profiles of colored solutes inside the column. The influence of flow rate, particle size and shape on the flow profile of a colored band were studied on a 5-cm-long column and a scaled-up column of four fold in volume. It was found that the flow rates of the mobile phase had little influence on the flat flow profile of the iodine band while the properties of the stationary phase had a certain influence on them. We observed that the flow profiles of the scaled-up column were flat during the whole chromatographic process, and the efficiency and resolution of the column were also increased in accordance with theoretical prediction. The experimental results proved that the 10° conical columns can be proportionally scaled up while still keeping the flat flow profile, sample load per unit volume of packing material, and column efficiency, which are superior to the conventional column.

Fused-silica capillary columns (100 pm I.D.) englobing a porous monolithic stationary phase were prepared by in situ copolymerization of 2-ethylhexyl methacrylate, ethylene glycol dimethacrylate and 2-acrylamido-2-methylpropanesulfonic acid (AMPS) in the presence of a porogenic mixture containing 1-propanol, 1,4 butanediol and water. The influence of the monomers ratio and the porogen solvent composition as well as the content of AMPS in the polymerization mixture on column total porosity and efficiency was investigated to attain minimum HETP values for the reversed-phase capillary electrochromatography separation of bioflavonoids. For the most promising column, the van Deemter plots, in both mu-HPLC and CEC, were also evaluated. In CEC the reduced plate height was found almost constant (1.6-2.0) within the range of linear mobile phase velocity between 0.2-2.0 mm s(-1). The chemical and mechanical stabilities of the monolithic column over a wide range of buffer pH (2-10) and time were satisfactory. Furthermore, the effects of mobile phase parameters, such as buffer concentration and organic modifier content, on the electroosmotic flow were studied systematically. CEC separations of standard mixtures of polyphenols, including flavonols, flavanones and flavanones-7-O-glycosides, were accomplished in less than 8 min. The CEC separation of the major flavanone glycoside constituents in the extract from a freshly squeezed grapefruit juice was also reported.

Separation of rare earth elements (REE) by ion chromatography on a Dionex Ion Pac CS3 column (sulfonic acid type) has been studied at different temperatures (10–70 °C) and with both isocratic and gradient elution with α-hydroxyisobutyric acid (α-HIBA). Changes of column efficiency as a function of distribution coefficient and temperature could not be explained on the basis of classical Van Deemter or Glueckauf-Hamilton equations and possible explanation indicating the contribution of longitudinal diffusion in the resin phase to total zone spreading was provided. It was shown that the plate height for yttrium was distinctly smaller that those for lanthanides of similar ionic radii. An improved combined isocratic-gradient elution procedure was devised for separation of Sc, Y, La, and the lanthanides in one run.

The effect of hydrodynamic parameters and the specific features of instrument design on the efficiency of substance separation in countercurrent liquid chromatography (CCC) was studied using a constant retention factor of the stationary phase in the column. The study was conducted with the separation of benzyl alcohol and p-cresol in a two-phase liquid system heptane–ethyl acetate–methanol–water (1.4 : 0.6 : 1 : 1) in as an example. It was shown that the peak resolution is improved with an increase in the rotational speed of the column and a decrease in the flow rate of the mobile phase. The best peak separation was attained using columns for which the ratio of the column rotation radius to the radius of column revolution was 0.615. It was shown that countercurrent chromatography allows the separation of substances with low partition constants (K < 1) in dilute solutions. The volume of the test sample may be up to 15% of the total volume of the chromatography column.

Fused silica capillary columns of the internal diameter of 320 μm were packed with the Nucleosil C18 stationary phase of 5 μm using the slurry packing method. The time of the bed compaction phase, packing pressure, and the use of ultrasound varied to study their influence on the column performance. Van Deemter curves were measured and separation impedance values were calculated in order to assess both separation efficiency and kinetic performance of the columns. Selected columns were tested again after nine months to evaluate the stability of their beds. Separation efficiencies of all columns were similar, but a major difference, caused by the use of ultrasound, was observed in the bed stability. Columns sonicated for 25 minutes during the bed compaction phase exhibited unchanged performance in the course of several months, while the performance of non-sonicated columns decreased.

The separation of homologous maltodextrins using methyl-, ethyl- and n-butyl esters of aminobenzoic acid and n-heptyloxyaniline by means of reversed phase nano-HPLC is presented. Fused silica capillaries packed with four different stationary phases were tested, the separation of the derivatized sugars was optimized and the separation properties of these columns were tested on the basis of the van Deemter plots. UV/VIS detection was used (285 nm for the aminobenzoic esters and 275 nm for the aniline derivatives) in combination with on-line ESI triple quadrupole mass spectrometry and off-line MALDI-TOF mass spectrometry. The choice of the stationary phase had a tremendous influence on the separation. Optimal results were obtained with an encapsulated ODS stationary phase and n-butyl aminebenzoate as label for the oligosaccharides. Malto-oligosaccharides with a degree of polymerization of up to of 25 could be resolved to baseline. On-line coupling with ESI-MS yielded additional information as a result of induced fragmentation and increased sensitivity by monitoring precursor ion. Off-line mass detection with MALDI-TOF was performed on the separated derivatized saccharides collected from nano-HPLC runs. The MALDI-TOF mass spectra confirm the ESI-MS data.

The theory of positive column of gas discharge in the plasma approximation is developed with due regard for the ion inertia, ionization friction, and directed component of the ion velocity in the frequency of collisions between ions and atoms. In the diffusion approximation, these factors are not taken into account. The problem is reduced to two essentially nonlinear first-order differential equations for the velocity and concentration of ions. In dimensionless variables, the behavior of distributions of the velocity and concentration, as well as the characteristics of the positive column, depends on two parameters, namely, the number of ion collisions over the characteristic length of the system and the ratio of this length to the Debye length. It is shown that the transition to the Schottky theory takes place at the values of parameters exceeding 104. In discharges of low and moderate pressure, the use of ambipolar diffusion approximation may, in some cases, lead to a considerable error. The characteristics of the positive column in a wide range of variation of parameters are calculated.

Continuous macroporous silica gel networks were prepared in a fused silica capillary, and evaluated in reversed-phase liquid chromatography. Under pressure-driven conditions, considerable dependence of column efficiency on the linear velocity of the mobile phase was observed in spite of the small size of the silica skeletons. A major source of band broadening in the pressure-driven mode was found in the A-term of van Deemter equation. The performance of the continuous silica capillary column in the electro-driven mode was much better than that in the pressure-driven mode.

A systematic derivation of corresponding equations shows that averaging the pressure and the flow rate of the mobile phase in a gas chromatographic column over the column length and over the time that an unretained component resides in the column, requires the use of three different compressibility correction factors,j 2 1 ,j 3 2 , andj 4 3 . When multiplied by the adjusted retention volume,V R-VM, the Martin and James mobile phase compressibility correction factorj 3 2 , only, produces the value of specific retention volume,V g T , which is connected unambiguously with the thermodynamic phase distribution coefficient,K, of the sorbate.

An experimental study of the collision of a column of N beads ( ) with a fixed wall is presented. For a fixed height of fall and a rigid wall, we show that the maximum force felt by the wall is independent of the number of beads N. The duration of impact, the velocity of the deformation wave in the column and an effective restitution coefficient of the column are also measured as a function of N. For a soft wall, we show that the maximum force depends on N. A non-dissipative numerical model, based on a nonlinear interaction law between nearest neighbours, gives results in agreement with the experimental data. Moreover, we show that, after the compression phase, the beads of the top of the column separate one after the other from the column with a velocity greater than the initial one. The beads at the bottom then bounce upwards in block, with a velocity smaller than the initial one. We emphasize that this detachment effect results from the energy redistribution within the whole system during the collision and not from any dissipative effect.

Silica gels retaining double pore structure in the size ranges of micrometer and nanometer have been applied to the rod-shaped monolithic column for liquid chromatography. The macropore structure was designed by controlling the phase separation process induced by the hydrolysis and polycondensation of alkoxysilane, whereas the mesopore structure was tailored by the solvent exchange treatments on wet gels. The size exclusion chromatograms on polystyrene standards exhibited almost similar features for octadecyl-modified rod and conventional packed beads columns. The dependence of plate height on the velocity of mobile phase determined for amylbenzene was by far weaker in the rod column than in the packed beads column, suggesting that additional geometrical factors should be considered in describing the separation mechanism in the rod column.

Characteristics of a column with continuously varying properties were investigated. The velocity change was induced by column's geometry, with a continuously varying diameter inside the column. This column was compared to a standard analytical column and to a narrower classical column in view of separation of acyclovir from the human plasma samples. Typical performance of all columns operating under the same conditions was examined. The proposed column configuration reduces retention times compared to widely used analytical columns. Compared to the standard column, a higher linear velocity at the same flow-rate was achieved. Therefore, the retention times are shorter and the consumption of the mobile phase was smaller.

This paper reports the qualitative and quantitative effects of the column pressure drop on the retention of lipid components in a serially coupled capillary column SFC system. The contribution of the pressure drop consists of two components, the density effect and the flow effect. The magnitude of the flow effect,i. e. the change in retention which results from changes in the flow-rate when column pressures are changed, is determined by the difference in single column analyte k values. The effect will be positive compared with the uncorrected retention values when the column with largest k value is closest to the injector. With the columns in reversed order, the effect will be negative. The contribution from the density effect always resulted in larger coupled column k values and was in most instances of more significance than the flow effect component. Values calculated with and without pressure drop correction have been compared and it has been shown that for most of the eighteen model lipid compounds investigated, the deviations from the experimental retention factors were smaller when pressure drop corrections were made.

This report attempts to link several previously unassociated phenomena to the formation of a thick film of adsorbed mobile phase acting as part of the effective stationary phase in SFC. Plots of log k′ vs density were found to have a different shape than is generally accepted. The plots of highly retained solutes appear linear at both low and high densities with a change in slope at intermediate densities where adsorption is know to be most extensive. Efficiency degrades under identical conditions that produce significant adsorption of the mobile phase. Van Deemter curves change shape consistent with an additional “C” term, suggesting the formation of a thick stationary phase under the same conditions known to result in multi-layer adsorption of carbon dioxide onto stationary phases.

A mixture of two amides has been used as a probe to achieve van Deemter plots for both packed and capillary columns employing as nearly as possible the same stationary phase, 100% CO2 as the mobile phase and isothermal/isoconfertic conditions. Linear velocity was varied via the use of various flow restrictor inner diameters. Chromatographic operating conditions were not optimized, but were those that have become routine in many laboratories. Plots of both capacity factor vs density and resolution vs density at different temperatures are presented for each column. Based on this study, various conclusions are drawn regarding the optimum performance of each column under different operating conditions. Direct injection with the capillary column yielded different optimum chromatographic parameters relative to split injection wherein the column was not overloaded.

A model is proposed that avoids the need for the controversial labyrinth (or multipath) term γ when dealing with packed columns. In accordance with early assumptions of Purnell [3] it is shown that molecules in the mobile phase diffuse longitudinally for only part of their residence time in the column. The fraction of residence time is a simple function of the specific pore volume of the support and the interparticle volume. From this function it can be concluded that the magnitude of γ must be in the range 0.4

High efficiency, glass micropacked columns are evaluated in terms of Van Deemter plots showing that the same number of theoretical plates is obtained for hydrocarbons and polar compounds, such as aliphatic primary amines, alcohols and carboxylic acids. A direct GC-MS coupling system is employed showing no loss in resolution in the ion source even for very polar compounds. It is shown that the high capacity of GLSC micropacked columns is very effective in obtaining interpretable mass spectra of minor compounds in complex organic mixtures. Mass spectra obtained with the direct coupling are compared with those obtained with a conventional packed column of the same efficiency making use of a Biemann separator.

To match the need of high efficiency capillary column (plate number N2, Plate height H2 and carrier gas linear velocity U2), a new high efficiency packed column (N1, H1 and U1) at high carrier gas velocity with small capacity factor has been developed in the light of theoretical discussion of factors effecting the column efficiency utilization ratio.

A suitable criterion of performance for a production chromatograph is the total separation cost, including capital and operating costs, per unit mass of product. A semi-empirical expression for this parameter is developed for gas-liquid chromatography. The equation is used to find the effect of chromatographic design and operating variables on performance and, where relevant, to find the optimum values of these variables. Optimum values exist for feed band width, column length, degree of band overlap and column temperature. In general, the carrier velocity and feed concentration should be as high as possible and the theoretical plate height as low as possible. The stationary phase loading has only a small, if any, net effect on performance. The stationary phase should be chosen for its chemical stability, involatility and enhancement of solute relative volatility.

The most recent interest in liquid chromatography (LC) is the trend of column miniaturization, i.e., in reducing the diameter of the columns. Capillary columns used in LC can be classified into three types, based on the flow resistance factor ϕ defined by Knox as $$\phi = d^2 /K_0 = \Delta Pd^2 /\eta \mu L$$ where d is a characteristic dimension of the column (the internal diameter of the tube of the mean particle diameter of the packing), Ko is the specific column permeability, ΔP is the pressure drop along the column, η is the viscosity and μ the linear velocity of the mobile phase, and L is the column length. The first type of LC capillary columns have a ϕ value of about 30. These are theopen-tubular columns, with a retentive film or layer on the inside wall of the column tubing the diameter of which should be as small as possible. The second type of LC capillary columns (ϕ≃100) is made by losely packing a glass tube which is then drawn to smaller diameter. During the drawing process the particles will be partially inbedded into the tube wall. Finally the third type of LC capillary columns (ϕ≃500–1000) are made by slurry-packing 0.2–0.5 mm ID fusedsilica capillary tubes under high pressure. These columns are well packed. In the paper various aspects of these columns are reviewed and their relative performance discussed.

Liquid chromatography in open tubes at flow rates around 0.1 cm · s−1 and less can generate an enormous number of theoretical plates but suffers from long analysis times. The Golay and van Deemter equations are also obeyed with high accuracy.

Time variance is better suited than volume variance to compare performances of columns of different diameters but of same length and working at the same linear velocity of the mobile phase. When a make-up flow is used at the detector inlet, only column and detector time variances remain additive. Make-up flow enables reduction of the detector contribution to the total variance. However, in some practical cases, the dispersion effects of the detector inlet tubing or electronic time constant can prevail over that of the flow cell. Owing to their different flow dependence, the decrease in detector variance due to make-up is often much lower than expected. Theoretical equations and curves are given for a rapid evaluation of make-up flow.

Mutual diffusion coefficients of two gases A and B can be determined in an empty gas chromatographic column by letting component B enter at an intermediate position of the column and continuously flow through a part only of it, as a carrier gas. The other component A is injected in a small amount instantaneously at the closed end of the column with the detector placed at the other end. By repeatedly stopping and then restoring after a short time the flow of B, narrow extra peaks are produced on the chromatographic elution curve, owing to diffusion of A into B. An equation is derived giving the area under the curve of each stop-peak as a function of time of the corresponding stop. Plotting the experimental data according to this equation permits the determination of the diffusion coefficient of A into B. Some results obtained by this method show negligible variations with changes in the experimental parameters.

1. Experiments on a pilot plant for the purification of lubricating oils with phenol showed that the process of purification is more efficient in a column with vibrating plates than in a packed column. 2. In columns with vibrating plates, according to the intensity of vibration, there are three characteristic types of conditions: “velocity”, “transitional”, and “inertia”. 3. As the parameter characterizing the inertia conditions we can use the dimensionless vibration criterion of Froude or the acceleration of the vibrations. 4. By dimensional methods we found the general form of a criterial equation of column flooding for an extraction column with vibrating plates. 5. From the experimental data we found an explicit form of the equation for the limiting flow rates and the equation for the limiting flow rates and the equation for the onset of inertia conditions. 6. An equation is given for the HETP of the system oil-phenol.

The van Deemter polts for the same glass capillary column one coupled to an FID detector and once to an MS instrument are compared. With full vacuum at the capillary end the column still produces 75% of the maximum attinable separation efficiency.

The cationic exchange reaction between bentonite clays and the appropriate quaternary ammonium salt produces the organo-clay known as Bentone 34*). This material was first used for gas chromatography by White and Cowan who obtained symmetrical elution peaks for paraffins but asymmetrical peaks for aromatic hydrocarbons [ 1 ]. Hughes, White, and Roberts subsequently studied the selectivity of Bentone 34 towards various metaand para-substituted isomers and commented on the asymmetry of the peaks [2]. Mort imer and Gent found that the addition of liquid modifiers to the Bentone gave symmetrical peaks for a number of hydrocarbons and chlorinated hydrocarbons but some asymmetry was experienced with more polar compounds [3]. Taramasso and Timidei also commented that the use of Bentone 34 alone gave asymmetrical peaks and recommended the use of alternative organo-clays for selective hydrocarbon separations [4]. It is significant to note that all previous published work on this topic recommends the use of diatomaceous earth or firebrick as a support medium for the organo-clay. We have found that by using glass beads as a support, highly symmetrical elution peaks are obtained for a wide range of compounds on both unmodified and liquid modified Bentone 34. In addition, compounds such as aromatic hydrocarbons, ketones, aldehydes, esters, primary amines, etc. can be selectively separated at a much higher theoretical plate speed than can be achieved in diatomaceous earth packed columns, and, appropriate liquid modifiers can be added to adjust the retention characteristics. Figure 1 shows some Van Deemter plots for columns of this type containing (a) Bentone 34 alone, (b) 50:50 Bentone 34/polyphenyl ether, and (c) 50:50 Bentone 34/Trimer acid. Each column was 150 cm long x 3 mm I.D. and packed with 100 mesh BDH glass beads coated with .08 % by weight of the total stationary phase. Before use the beads were washed thoroughly in water containing a detergent, followed by distilled water, acetone, and finally ether. The beads were dried over a water bath and then heated to 400 ~ for 1 hour. The requisite weights of Bentone 34 and liquid modifier were dispersed in 5 ml benzene and the solution mixed with 50 g glass beads. 35

IN a gas-liquid chromatographic column the assumption of equilibrium demands that the rate constants for solute transfer between the gas and the liquid phases must be infinitely large and the flow-rates extremely small. In practical columns neither of these conditions is ever fulfilled; the kinetic rate constants are finite, and the use of extremely slow flow-rates is out of the question because of undesirably long retention times and pronounced longitudinal diffusion in the gas phase which enhances band broadening. This has led various workers to advance theories which take account of the effect of non-equilibrium conditions on band structure and column efficiencies1–4. Of particular importance is the van Deemter theory4, which uses the driving force concept to describe the departure from equilibrium in terms of resistances to mass transfer in the gas and the liquid phases. The hypothesis of interfacial equilibrium involved in the theory is one the validity of which cannot be demonstrated directly by experiment. The existence of equilibrium at the interface would mean that if the concentration in the liquid phase at the interface is specified, the concentration in the gas phase at the interface is the same as if the two phases had been in contact for an indefinite length of time. Owing to continuous flow of carrier gas it is scarcely likely that the molecules once evaporated from the surface will get to the same spot again, and this, obviously, introduces conceptual difficulties regarding the nature of dynamic equilibrium at any point within the column. A natural consequence of the assumption of interfacial equilibrium is that once the diffusing molecules strike the interface, they pass into the other phase without experiencing any resistance at the interface itself. On the contrary, discrepancies between the calculated and observed rates of absorption suggest that there should be resistance to mass transfer at the interface5–7. From the point of view of the theory of absolute reaction-rates, there are sound theoretical grounds for postulating the existence of interfacial resistance. One can imagine an energy barrier which may involve the formation of an ‘intermediate-activated state’; of all the molecules which strike the boundary surface only those which have the required energy of activation for diffusion surmount the energy barrier while the rest rebound into the gas phase, and furthermore the orientation of molecules may play a part.