Screening Devices product range

The Effect of Particle Size Distribution in Flash Chromatography

In 1978 W. Clark Still published the first paper on Flash Chromatography in Journal of Organic Chemistry, he reported that the grade of silica gel, which gave the best performance, was 40-63 µm. At that time the most popular grade of silica gel was 63-200 µm, which gave, Still the poorest separation this is was partially due to the broadness of the particle size range.

The connection between particle size distribution and column performance is very simple. When the range is broad the column packing is uneven. Some parts are composed of only large particles where the solvent will flow fast and meet little resistance and there sections composed of small particles where the solvent flows slowly and meet great resistance. The solvent will take the path of least resistance through the column and flow around the pockets of small particles and not straight through the column. This uneven flow greatly effects the separation because the peaks will have different retention times depending its flow path through the column when they merge as they exit the column the peaks will be broad and poorly separated. Narrow particle size distributions results in a column that is packed evenly so the solvent flows straight and evenly resulting in optimal separation. Figure 1 visually illustrates the effect of a wide versus a narrow particle size distribution.

The importance of the particle size distribution varies depending on the type of chromatography being done. In HPLC it is so important that spherical particles of a very narrow size distribution are used. It is not cost effective to used spherical particles for flash chromatography but in selecting a silica chemists need to remember that not all 40-63µm gels are the same. Figure 2 shows the distribution curves of SiliCycle Flash gel compared to three other manufacturers flash silica gels, all products were sold as 40-63µm gel. The two key points of the graph are the height of the volume differential (diff) and percentage of particles below 40µm. The SiliCycle curve has a much higher percentage of particles between 40-60µm and a very low level of small particles below 40µm or “fines”.

Fines are the small particles below 40µm, they cause backpressure and clogging which is particularly dangerous when using glass columns. Fines can also pass through filters and contaminate final products rendering it useless. SiliCycle has the lowest level of fines available on the market today.

Today almost all silica gel manufacturers sell a form of 40-63µm gel but not all gels are equal. The narrow particle size distribution and low level of fines offered by SiliCycle optimizes your separating power saving you time and money.

References
Still, W. Clark et al, J. Org. Chem, 43, (1978) 2923-2925