Columns axial compression packing

Correct column packing is obtained by using preparative columns equipped with dynamic axial compression (DAC) technology [10, 11]. The column includes a movable piston attached to a hydraulic jack. The piston is used to pack and unpack the column and to maintain the stationary phase under dynamic compression, ensuring perfect particle stacking (Figure 12.5) and bed stability through time. 

Since efficiency can be increased by improving the density of the packing, several commercial units use special means for compressing the column after it is packed. Both radial and axial compression are available. Other columns have been designed for use with soft gels that tend to swell and shrink during use. They have movable end pieces that can be adjusted... 

Until recently, one company dominated the market for preparative units. The success of prep LC has encouraged other manufacturers to enter the market, and a recent review77 lists eight commercially available units, three of which can be operated automatically and unattended. These instruments can also accommodate larger columns and hence larger samples. Capacities up to 750 g and flows of 5 L/min are claimed. Columns are relatively expensive and overload easily. In addition to normal slurry packing techniques, radial and axial compression devices are used in some of them to improve performance. 

Analytical columns are packed by the slurry technique, where a dilute suspension of the packing is pumped at a high flow rate and a high pressure through the column. Particles are retained by a porous frit at the end of the column. Preparative columns with dp between 10 and 30 xm are, preferably, packed by the dynamically axial compression technique (Unger, 1994). The column contains a movable piston that keeps the packing under an external pressure during operation. The operation pressure should be always less than the piston pressure. 

Table 3.15 Column performance of several types of silicas packed into dc = 50 mm columns by the dynamic axial compression technique (Marme, 1991).

Cherrak, D. E., Al-Bokari, M., Drumm, E. C., Guiochon, G. Behavior of packing materials in axially compressed chromatographic column, J. Chromatogr. A, 2002, 943, 15-31. 

Column packing - dynamic axial compression technology... 

T ay, the major manufacturers routinely produce in the order of 100,000 columns per year, and experienced manufacturers have learned to pack columns to a high reproducibility of their performance and stability. Thus the technologies designed to avoid or eliminate column voiding are not needed any more and are found only in niches where the problems have not yet been solved. The primary example is preparative chromatography with large-diameter columns. Because of their importance in preparative HPLC, we will discuss here the principles of radial- and axial-compression technologies. 

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 Section 3.4 we discuss alternatives to the conventional column design. We into the characteristics of radially and axially compressed columns. The Hhsequent examination of alternatives to packed beds of partides leads us to [-reflection of ways to evaluate the merits of these alternative designs. 

Small particle size material requires a high-pressure slurry packing procedure. It is preferable to work with spherical particles since they have better mechanical stabiUty and thus longer life than irregular ones. Axial compression columns allow for efficient in-house packing of columns with bulk stationary phases. Systems utilizing radially compressed, preloaded disposable cartridges are commercially available. 

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