Packed-Column Diameter Tolerances

With random packing, the tolerance on column diameter is similar to that for tray columns (refer back to Fig. 7.4). With structured packing, tighter tolerances are often specified to ease installation. Typical diameter tolerances for columns containing structured packings (168) are shown in Table 8.1. 

TABLE 8.1 Typical Column Tolerances for Structureci Packings 

Column Wire mesh, in diameter Corrugated sheet, in Wire mesh, in Corrugated sheet, in 

Notes These are plus and minus tolerances Some dimensions are rounded off upon con-  

Source G.V. Horner, The Chemical Engineer Supplement, September 1987, reprinted courtesy of the Institution of Chemical Engineers, UK. 

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In a packed column the HETP depends on the particle diameter and is not related to the column radius. As a result, an expression for the optimum particle diameter is independently derived, and then the column radius determined from the extracolumn dispersion. This is not true for the open tubular column, as the HETP is determined by the column radius. It follows that a converse procedure must be employed. Firstly the optimum column radius is determined and then the maximum extra-column dispersion that the column can tolerate calculated. Thus, with open tubular columns, the chromatographic system, in particular the detector dispersion and the maximum sample volume, is dictated by the column design which, in turn, is governed by the nature of the separation. 

Recently, 530-p.m capillaries, sometimes called megabore columns, have appeared on the market. These columns will tolerate sample sizes that are similar to those for packed columns. The performance characteristics of megabore open tubular columns are not as good as those of smaller diameter columns but are significantly better than those of packed columns. 

The packed GC column has a value for (og) of about 55 pi, whereas the high efficiency microbore LC column only 0.23 pi. It is clear that problems of extracolumn dispersion with packed GC columns are not very severe. However, shorter GC capillary columns with small diameters will have a very poor tolerance to extracolumn dispersion. In the same way, short microbore LC columns packed with small... 

The promise of monolith is the achievement of a higher performance at a lower backpressure than a packed bed. While this is true in principle, current implementations are limited by the fact that the external wall to the structure is made from PEEK. At the time of this writing, the commercially available monoliths can only be used up to a pressure of 20MPa (200 atm, 3000 psi), while packed bed steel columns can be used up to double this pressure and higher. Also, the preparation of the monolith appears to be cumbersome. At the current time, the silica-based monoliths are available only with an internal diameter of 4.6mm. The speed is thus also limited by the flow rate achievable by the HPLC instrument. At the same time, the detector of choice today is the mass spectrometer, which can tolerate only much... 

As an example of the use of equation (8) in detector design, let us calculate the standard deviation of the extracolumn dispersion that can be tolerated by an LC column 10 cm long 4.6 mm I.D packed with particles 10 p in diameter. 

If efficiencies could be increased sufficiently, perhaps by using particles 1 pm in diameter, and techniques were developed to efficiently and reproducibly pack them, a single chiral stationary phase might be all that is necessary to separate the majority of enantiomeric mixtures. Such a system is, no doubt, part of the future. It must, however, be preempted by pumps and sample valves that can provide and tolerate the necessary high pressures. In addition, there must be detectors with sufficient sensitivity to furnish an adequate linear dynamic range of response to accommodate the inherent low loading capacity of such columns. 

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