Kromasil technical evidence

• Introduction
• Mechanical stability
• Chemical stability
• Surface properties
• Loadability
• Scalability

Mechanical strength

Why high mechanical strength?

“Mechanical strength is crucial for the lifetime of the packing and for a problem-free process”

In modern HPLC, SFC and SMB systems small efficient particles are used for the best total economy. Columns have to be packed at high piston pressure to achieve optimum efficiency. Further, high mobile phase velocity is used for high throughput. As a result, high back pressure is developed in the system. In large diameter dynamic axial compression (DAC) columns, the mechanical stress on the particles can be significant, and a very high mechanical strength is crucial. In addition, one column is frequently used for several pharmaceutical products, where the packing has to be dedicated to one product. The column therefore has to be unpacked and repacked for every new campaign, putting high demands on the mechanical strength of the packing. Less strong packings break under these conditions. They form fines which clog the frits and cause an uneven flow distribution in the column which lower the efficiency. At the end, very high pressure is developed in the system, and the column has to be repacked with new material.

What influences mechanical strength?

Mechanical strength of silica HPLC packings is influenced by:

• particle shape
• pore volume
• pore diameter
• treatment of the material

Spherical particles are stronger than irregular ones. Large pore volume and large pore diameter mean a weaker silica matrix and fewer contact points between the primary particles giving lower mechanical strength. The larger the pore, the lower the mechanical strength.  

In Kromasil particles, the pore volume is optimized to give the highest surface area without loosing mechanical strength. This unique combination is achieved by special treatment of the material in several production steps.

Proof of Kromasil mechanical strength

The mechanical strength of every batch of Kromasil silica is tested by a unique method developed by Eka Chemicals.

The particles are packed in a preparative column (50 mm I.D.) with a short bed (50 mm) and axial compression. The piston pressure is increased step by step from 50 bar all the way up to 300 bar. Piston pressure is then released completely and a new series is repeated. In figure 1, the increase in bed pressure drop between two series, of Kromasil 60 Å and 100 Å and other commercial materials, is illustrated.

Fig. 1 - Bed pressure drop increase after one compression cycle for Kromasil and some other commercial materials. Materials showing a higher measured bed pressure drop increase than expected, crash during the test. The four materials to the right show a very high pressure drop increase of more than 200%.

In figures 2 and 3 particle size analysis, of Kromasil and other commercial materials after performed mechanical strength test, is shown. 

Fig. 2 - Particle diameter (median by number) for the used packing materials, compared to the virgin material value. 100% means no degradation. The particle size analysis was performed on a Coulter Counter.

Fig. 3 - Particle size analysis of virgin (dotted) and used (solid) silica.
Kromasil: The used material is very slightly affected, as seen in the lower diameter region.
Silica H: The used material consists mainly of crushed particles, giving rise to an extremely high pressure drop.