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Case Studies - Particle
Size Analysis
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.
| Figure 1: |
| The syringe barrel on the left
shows the uneven flow paths caused by fines. The syringe barrel
on the right shows the straight flow paths that are achieved
when fines are eliminated. |
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.
| Figure 2: |
| The particle distribution curves of 3 Manufacturers
compared to SiliCycle Flash Gel. |
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.
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References
iStill, W. Clark et al, J. Org. Chem, 43, (1978)
2923-2925
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