Particulate column packings

Advances in column technology have improved the selectivity, stability, and reproducibility of LC analytical columns. For example, analytical columns are packed with a variety of stationary phases, providing enormous versatility in the separation process. This section describes (1) column dimensions, (2) particulate column packings, (3) monolithic column packings, and (4) the use of guard columns. 

The modification of the sand surface allows the grains to simultaneously adsorb soluble heavy metals and remove particulate metals by filtration in a column packed with the media. Important factors to the performance of the adsorbent include pH of the solution to be treated, empty bed detention time (EBDT), and the presence of complexing agents, oil, surfactant, and biodegradable substances. 

All chromatographic separations were made using an ES Industries 0.46 x 15 cm reverse-phase column packed with E. Merck 5 nm Lichrosorb RP-18. A Whatman pre-column (0.21 x 7 cm) packed with C0 PELL ODS (Reeve-Angel) was used to protect the analytical column from both particulate matter and irreversible bonding of injected components onto the stationary phase. The pre-column is also used to dampen mobile phase pulsations from the chromatographic pumps. 

It is interesting to compare the flow characteristics of both types of silica columns with respect to the column pressure drop and column permeability. The column pressure drop / flow rate curves of both types indicate a much lower slope of the monolithic columns (not shown) than the particulate columns. In other words, the column pressure drop of the monolithic columns corresponds to that of a column packed with 15 pm silica particles [6]. The column permeability values are listed in Table 3. 

An advantage of this sampling configuration is that particulate matter can be trapped on the column packing or removed by filtration prior to chelation. This allows analysis of either the total trace element concentration or the dissolved trace metal concentration. 

Correct sample preparation ensures good resolution and extends the life of the column. Sample buffer composition does not directly affect resolution. During separation the sample buffer is exchanged with buffer in the column. Viscous samples, which could cause an increase in back pressure and affect column packing, should be diluted. Samples must be free from particulate matter, particularly when working with bead sizes of 34pm or less (see Chapter 8 for details of sample clarification procedures)... 

Usually, a short guard column is introduced before the analytical column to increase the life of the analytical column by removing not only particulate matter and contaminants from the solvents but also sample components that bind irreversibly to the stationary phase. The composition of the guard-column packing should be similar to that of the analytical column the particle size is usually larger, however, to minimize pressure drop. When the guard column has become contaminated. it is repacked or discarded and replaced with a new one of the same type. Thus, the guard column is sacrificed to protect the more expensive analytical column. 

The use of monolithic columns in LC has advanced rapidly since their first introduction in the 1990s [18-21]. In contrast to capillary columns packed with particulate stationary phases, monolithic columns consist of a single continuous support. Monolithic stationary phases can be subdivided in two classes, i.e., polymer-and silica-based materials. 

From the emission intensity, an equivalent spherical particle size can be calculated for sample particulates. One material which is available as pure, spherical particles of well-controlled particle size is sihca, used for chromatography column packing. A micrograph of 3 p.m diameter Si02 beads collected on a polycarbonate filter is shown in Fig. 7.51(a). While relatively uniform in diameter (monodisperse), you can see that not all the beads are perfect single spheres. The silica particles are aspirated into the He... 

CEC columns are generally made of fused-silica tubing, usually packed with the appropriate stationary phase. Today, the most commonly used CEC columns have i.d. of 100 p,m or less, with 50 and 75 p,m i.d. being the most popular. The stationary phase is retained in the column by two frits. Column designs can be categorized into two major types OT columns and packed structures, which include packed columns, monolithic columns, and microfabricated stractures (open or continuous beds). Packed capillary columns are most commonly used, as has been demonstrated in numerous papers [9-11]. They can be subdivided into three different categories columns packed with particles, columns with continuous beds fabricated in situ creating a rod-like monolithic structure, and columns with immobilized or entrapped particulate materials. 

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