Radial compression packing

The Waters system uses a plastic cartridge which is inserted into a device (the Z-module) that subjects the column to radial compression, ie pressure is applied along the radial axis of the column tube. The flexible wall of the column then moulds itself into the voids that are present in the wall regions of the column. This method is claimed to produce an improvement in the packed bed structure, better column performance and longer useful column life. 

The majority of published HPLC techniques used in fat-soluble vitamin assays have utilized 5- or 10-/zm particles of porous silica or derivatized silica packed into stainless steel tubes of typical length of 250 mm and standard internal diameter (ID) of 4.6 mm. Radially compressed... 

Larger scale chromatography was run on a J.Y. Chromatospac Prep 100 (J.Y. Instruments) or a Waters Prep-500A equipped with a Whatman Magnum 40 (4.8 X 50 cm) column dry packed with Whatman Partisil Prep 40 ODS-3 (37-60 urn) and mounted in place of the radial compression chambers. Injections were carried out by pumping on dilute solutions of samples in a solvent of lower eluting power. Detection was by the Gow Mac model 80-850 U.V. detector at 210 or 254 nm. 

A major requirement for column tubes in HPLC is constancy of the column diameter when changing the pressure drop along them. If this is not so, the stability of the packing is affected on changing the pressure drop, which destroys the column efficiency. All available polymeric tubes, such as PTFE, nylon or plastic tubes, show a definite pressure dependence of the tube diameter. Apart from this, polymeric materials, except PTFE, are easily attacked by organic solvents. For these reasons polymeric materials are unsuitable as column tube materials in HPLC. Recently, however, the application of a radial compressed PTFE column for preparative LC was reported (Waters Assoc.). 

A better approach to the problem is the active compression of the bed through axial compression (13). This technique has not been used for analytical columns, but is widely practiced for large-diameter preparative columns. As is the case for radially compressed column, the permeability of an axially compressed column is lower than the permeability of an uncompressed column (14), but the effect is much smaller because of the lower compression factor. Axial compression maintains column efficiency. In repeated packing trials, reproducible efficiencies can be obtained, but the peak shapes reported do not indicate a good uniformity of the packed bed. There are no studies available yet that allow one to judge whether this is an intrinsic property of axial compression or if this is due to the technical difficulties associated with the packing of large-diameter columns. 

In radial compression technology, the packed bed is contained in a flexible-wall tubing. Controlled hydraulic or mechanical forces are applied umformly to the outside wall of the column (Fig. 3.11). This compresses the packed bed, resulting in a denser and—when applied properly—more uniform structure. The technology is derived from isostatic compaction, which is frequently used in the ceramics industry, for example, in the mass production of spark plugs. 

The compaction of the bed can be carried out both mechanically or hydraulically. In a mechanical scheme, the bed is compressed to a controlled volume, while the hydraulic compression uses a controlled pressure. Most of the initial studies of radial compression were carried out with a controlled compression pressure, applied either to preparative columns packed with irregularly shaped particles with a particle size of about 80/rni or analytical columns packed with spherical lO- tm particles (12). 

The interstitial fraction of an uncompressed bed of hard spherical particles packed by conventional packing technologies is 40 2%. When hydraulic radial compression is applied to such a bed, it readily densities until an interstitial fraction of 34 1% is obtained at 3 MPa. A further increase in... 

The principle radial compression has successfully solved the problems of packed-bed uniformity and of bed stability. However, radial compression has its Umitations in the strength of the partied If excessive radial compression is applied, and particles start to fracture, the homogeneity of the bed is lost. This sets an upper limit to the hydraulic compression that can be applied to soft or fragile particles. Since the column backpressure cannot exceed the compression pressure, this limits the applicability of hydraulic compression. Mechanical compression does not suffer from this problem, but it is more difficult to achieve a uniform compression, and therefore a good implementation of... 

Also shown in Figure 3.13 are the actual results obtained for a steel column and a radially compressed column packed with the same particles. The results for the steel column follow closely the standard curve. The radially compressed column shows inferior results at high velocity. This is due to the convention used here that the apparent particle diameter in the column is determined from the permeability. Ra ally compressed columns have a denser bed, and thus a higher pressure drop. The particle diameter determined via the measurement of the permeability was S.7 fim instead of the true diameter of 8 /expected performance of a SJ-fim column is not met at high velocity. On the other band, the maximum of the curve for the radially compressed column matches the expectation. This is because the increased unfformity of the packed... 

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