B.Scott Broyles

Visualization of bed compression in an axial compression liquid chromatography column

The consolidation of a packed bed undergoing axial compression was studied in glass columns using an on-column visualization process. In this visualization process the refractive indices of the mobile phase (carbon tetrachloride) and the stationary phase (YMC C18 silica) matched perfectly, hence the otherwise opaque stationary phase became transparent to the eye. Alumina layers, which have a different refractive index, were placed at regular intervals along the column bed. These layers were therefore visible and their movement could be tracked during the axial compression of the bed. Consequently, the Youngs modulus could be measured at three radial locations and at four bed depths below the head fitting. Theresults showed that the bed was heterogeneous in packing density, both radially (with the bed density increasing from the column center toward the wall) and axially (with the density increasing from the column top toward its center). Furhermore, the bed was shown to be non-symmetrical about the column axis. This was thought to be due to the column inlet head fitting making contact with the packing material on one side of the column first, rather than making contact with the entire cross section of the packing simultaneously.

Axial and radial diffusion coefficients in a liquid chromatography column and bed heterogeneity

The axial and transverse diffusion coefficients of a band of iodine in a chromatographic column were measured optically as a function of time. It was found that the axial diffusion coefficient remains constant even when the edges of the sample band get close to the wall. By contrast, the radial diffusion coefficient decreases progressively with increasing time when the edges of the sample band leave the core region and begin to diffuse inside the wall region. The local axial and transverse diffusion coefficients of the band decrease from the column center toward the wall. Hence, the increase in local height equivalent to a theoretical plate observed in the region close to the wall must be explained by increasing mass transfer resistances and degree of heterogeneity of the bed.

Visualization of sample introduction in liquid chromatography columns: The effect of the frit diameter

A previously developed on-column detection technique using 35 mm SLR cameras [J. Chromatogr. A 826 (1998) 1] was employed to visualize colored sample bands as they elute through frits of differing diameter. Head fittings containing a 4.0 mm frit and a 15.9 mm frit were mounted in a 17 mm I.D. glass column packed with C18 silica with an average particle size of 21 microns. A carbon tetrachloride mobile phase of matching refractive index to that of the silica provides clarity along the column diameter during band migration. The photographs of the migrating sample zones were scanned and analyzed with appropriate imaging software. The smallest diameter frit induced severely parabolic sample distributions at the column inlet compared to the larger frit. Local axial dispersion coefficient values, expressed as local reduced plate height, were calculated as well as local zone velocities at the column inlet. The results demonstrate clearly the need to match the diameter of the inlet frit to the I.D. of the column so as to avoid the initial onset of zone broadening due to the frit.

Visualization of solute migration in chromatographic columns influence of the frit porosity.

The dual-perspective, on-column detection method previously described was used to observe the effects of the inlet frit on the profiles of chromatographic bands. Visualization of bands of iodine was achieved by injecting its dilute solutions in carbon tetrachloride into a glass column packed with a C18-bonded silica and eluted with carbon tetrachloride, which has the same refractive index as the packing material. The bands were photographed on-column with two standard 35-mm SLR cameras oriented at right angle. The photographs were scanned and the digitized images of the sample bands analyzed with proper software. A number of columns, as similar as possible, were fitted with different 2- and 10-microm porosity stainless steel frits. Subsequent analysis of the digitized band images revealed irregularities in the band shape resulting from frit contributions to band dispersion. The 2-microm frits produced more dramatic effects overall than the coarser frits. Local axial dispersion coefficient values, expressed as local reduced plate height, were calculated. The results demonstrate the possibly damaging effects of the frit on the band shape.

Visualization of solute migration in liquid chromatography columns

Abstract By matching the refractive indices of the mobile phase, the stationary phase and the material of the bed enclosure, one can render transparent to the eye the chromatographic column which is normally opaque in nature. As a result, band visualization is readily obtained. High-definition on-column detection becomes feasible by using a photographic detector instead of the conventional post-column, on-line (UV–Vis or similar) detector. Quantitative information regarding the concentration distribution in the band is obtained by utilizing optical scanners to obtain a digital image and computer imaging software. The processes of data collection and image analysis are discussed in detail and are illustrated by observing the concept of the infinite diameter column following a central point injection. The performance of the photographic detection method is compared to that of regular detection procedures. The efficiency of the column was determined with both on-column measurements and regular post-column measurements. A minimum reduced plate height ( h ) of 1.9 was recorded with post-column detection, in agreement with the average of the results given by on-column detection. On-column analysis allowed the determination of the local plate height which was found to vary across the central region of the column between 2.7 and 0.95.

Consolidation and column performance of several packing materials for liquid chromatography in a dynamic axial compression column

The mechanical properties of four different packing materials for preparative liquid chromatography were investigated. Their compression kinetics and consolidation behavior were measured under dynamic compression stress in a 5-cm I.D. axial compression column. The internal and external porosities, the packing density, the permeability, and the efficiency of these columns were also measured. The materials studied included two spherical silica-based materials for preparative chromatography, NovaPak C81 (6 μm) and Hyperprep HS BDS C18 (12 μm), another octadecyl bonded silica material made of irregular shaped particles, Baker 1-130 C18 (130 μm), and an unmodified alumina (50 μm). The first two packing materials showed much higher mechanical stability than the third one which experienced extensive particle breakage under high compression stress. The material with the smallest spherical particles (average size, 6 μm) gave a stable, rigid column, but, either because it contains fine particles or because of a small degree of particle breakage, it caused problems. Maintenance after unpacking a column and cleaning the piston frit was difficult. This could lead to a significant decrease of the productivity. The behavior of the fourth column material, unmodified alumina, is very different from the ones of all silica-based materials so far studied; it is not superior to that of the three silica-based materials presented here.

Evaluation of the Secondary Consolidation of Columns for Liquid Chromatography by Ultrasonic Irradiation

The consolidation of packed analytical chromatography columns was carried out under ultrasonic irradiation. Columns were first packed using a conventional high pressure downward slurry method. Then, they were subjected to further bed consolidation in the presence of ultrasonic vibration. This process of further bed consolidation is referred to as secondary consolidation. Secondary consolidation was observed to occur more readily in solvents of low viscosity and at low flow-rates (low pressures). Column efficiency was not observed to be a factor affecting the process of secondary consolidation of the packed bed.

Visualization of Solute Migration in Chromatographic Columns. Quantitation of the Concentration in a Migrating Zone.

The concentration distribution across a zone of iodine migrating along a column made of glass, packed with C18-bonded silica, and eluted with carbon tetrachloride was derived from a quantitative analysis of the photographs of the zone. The photographs were scanned and turned into digital images. The intensity distributions obtained from the measurement of the grayscale intensity were converted into concentration profiles using a calibration method. This procedure is illustrated and suitable corrections are introduced to account for the transverse variation of the optical path length, as a result of using a cylindrical detector cell (the column itself), and for the refraction of light due to the differences between the refraction indices of the glass wall and the liquids involved. An error analysis is also reported. It shows that the method can reliably produce results with a precision of a few percent, allowing on-column evaluation of column performance and the derivation of the radial distributions of the column efficiency, the migration velocity of the zone, and the sample distribution at the head of the column.

Modeling of the influence of the experimental conditions on the separation of the components of a binary mixture in isocratic overloaded elution preparative chromatography

Abstract The elution band profiles of the components of a binary mixture were calculated for systematic variations of the experimental conditions. Three values of the separation factor were used, 1.8, 1.3 and 1.1. The column efficiency was 3000 theoretical plates, except for α = 1.1, in which case efficiencies of both 3000 and 5000 plates were used. The relative compositions of 3:1, 1:1, and 1:3 were selected. The values of the loading factor were chosen to illustrate the increasing degrees of interference and band interaction which take place with increasing column loading. In each case, the cutting volumes for the production of each component at a 98% purity were calculated and tabulated. The influence of the experimental conditions on the recovery yield is discussed.