C. Dewaele

Stationary phase characterization in high-performance liquid chromatography : A test for trace metal activity in octadecyl bonded silica gel

Abstract In this paper it is shown that metal traces in high-performance liquid chromatographic bonded-phase silica gel interfere with some separations. A test for such activity, based on the chromatography of beer bitter acids, is described. When octadecyl silica get is boiled in concentrated hydrochloric acid-methanol (40:60), part of the phase is dissolved; however, the metal traces are selectively removed and the quality of the phase improved.

Influence of the particle size distribution of the packing material in reversed-phase high-performance liquid chromatography

The influence of a large particle size distribution of reversed-phase silica gel packing materials in high-performance liquid chromatography (HPLC) was studied. Mixtures were prepared with ROSiL-C18-D materials with mean particle sizes of 3, 4, 5, 6, 8 and 10 μm. The efficiency of columns packed with mixtures of different compositions was measured and the usual chromatographic parameters were deduced. The conclusion is that a large particle size distribution has no influence on column efficiency if the eluting speed is kept around the optimum value. At higher solvent flow-rates there is a small negative effect. At all eluting speeds a larger particle size distribution has a negative effect on back-pressure and on separation impedance. A good practical ratio for the diameters of larger over smaller particles in a chromatographic material is about 1.5 to 2 for the ratio dp 90/dp 10. The negative effect of fines or dust in column packing materials is stressed.

Spherical silica gel of 2 μm particle size for reversed-phase high-performance liquid chromatography

Abstract Performance data and applicability of 2-μm spherical octadecylated silica gel (2-μm ROSiL-C 18 -D) are presented. Short columns (25–70 mm) with conventional internal diameter ( ca. 5 mm) generate 200,000 to 270,000 theoretical plates per metre. Even with 2-μm particles, a reduced plate height of 2 d p or a chromatographic efficiency of 50% can thus be obtained. The H/u plot for such 2-μm particles is flat over a very wide range in the C term portion. Analysis time can be shortened considerably through the double effect of this very high efficiency per unit length and the high solvent rate, because of the absence of a minimum in the H/u curve. With such short columns, solvent consumption per analysis is also considerably reduced. The instrument requirements are discussed. The performance of 4 × 0.5 cm I.D. columns, packed with 2-μm material, generating up to 1000 plates/sec is illustrated with some practical examples. In addition to their remarkable speed, these packed columns also give a tenfold increase in sensitivity over conventional columns.

Preparative-scale high-performance liquid chromatography on analytical columns

Abstract The capacity of analytical high-performance liquid chromatographs for preparative-scale separations in the adsorption as well as in the reversed phase mode is shown to be about ten times greater than generally believed. A preparative column of 50 x 0.68 cm which can be handled by an analytical instrument can separate 0.1-1 g of relatively complex mixtures. The displacement and elution chromatographic modes are compared.

Column hardware in preparative liquid chromatography with axial flow

Abstract Scale-up of liquid chromatography (LC) for preparative work poses a number of problems. Problems concerning the column as the carrier of the stationary phase are discussed. Only axial column flow and LC with elution are considered (not radial flow, thick-layer, centrifugal techniques, etc.). The choice, design and technology of columns have mechanical aspects, but they are also dictated by the choice of stationary phase, by sample size requirements and by other chromatographic requirements. A distinction is made between laboratory-size and production-size instrumentation. The design, packing and use of laboratory-size preparative LC columns is not so very different from usual analytical LC practice. Larger-size instrumentation requires different approaches. A bewildering variety of larger-sized column designs is already commercially available. Dry-packing, slurry-packing, axial and radial packing or a combination of the two, chromatography with or without compression all are advocated. About 50 manufacturers of preparative LC columns were asked to provide their latest documentation on larger-scale systems. An analysis and survey of current commercial production-scale LC columns, based on the replies received, is presented.

Quality criteria and structure of silica gel column packing material

Abstract Silica gel and derivatised forms of silica gel are by far the most commonly used column packing materials in high-performance liquid chromatography. Quality criteria of silica gel for chromatography therefore merit attention. Most of them are discussed, and it is suggested that the apparent density is an important one.

Observations and ideas on slurry packing of liquid chromatography columns

Abstract This paper shows that in a graph of reduced plate height against particle size for irregularly shaped and spherical octadecylated silica gel particles, there is a minimum in the curve at 10 μm for irregular and at 5 μm for spherical material. For particles smaller than 10 μm the spherical shape does better, and for particles larger than 10 μm the irregular particles are better. The investigations revealing these facts have led to the conviction that smaller particles are better packed in a low viscosity solvent and with an upward technique. Possible explanations for these assertions are discussed.

High-performance liquid chromatography of beer bitter acids

Abstract The high-performance liquid chromatography of the six beer bitter iso-α-acids on different modified silica gel stationary phases is described. Octadecyl-, nitrophenyl- and cyanopropyl-substituted and buffered silica gels were studied with a variety of solvent systems. The aim was to find a system suitable for the routine isocratic analysis of the beer bitter compounds. Although the six iso-α-acids can be separated, it is concluded that a separation in three groups for each cis and trans pair corresponding to the three α-acids is preferable for this purpose. This is possible with an ion-pairing method on octadecyl-silica gel.

Performance of micro-liquid chromatographic columns in an industrial environment : a case history

Abstract Packed fused-silica capillary liquid chromatography (micro-LC) columns, with an inner wall polymer coating stabilizing the packed column bed, were evaluated in an industrial laboratory environment. Data on their performance and long-term stability in “real-world practice” are reported. The results indicate that micro-LC is a viable proposition for routine application.