Flash Chromatography

“Flash Chromatography" is a rapid form of preparative column
chromatography- Prep LC based upon “an air pressure driven
hybrid of medium and short column chromatography optimized
for rapid separation.” This approach was pioneered by W.C.
Still at Columbia University, and described in J. Org Chem 43,
2923 (1978). Separation was based upon the relatively
inexpensive apparatus used.

Flash Chromatography is typically used to prepare 0.1-10.0 g
of material in less than 15 minutes and is especially useful
when the differences on TLC are greater than 0.15 Rf units.
Clearly, Flash Chromatography is a simple and economical
approach to Prep LC.

Flash Chromatography is a type of preparative liquid
chromatography used for the separation of organic compounds.
This is adsorption chromatography for the routine purification
of organic compounds. By using the flash technique
chromatographers can scale up normal phase chemistries
from thin layer chromatography (TLC) helping to satisfy the
demands of the pharmaceutical and biotech industries in the
transition to large scale purification of organic compounds
and peptides. The technique utilizes an air pressure driven
hybrid of medium pressure and short column chromatography
optimized for particularly rapid separations. 1
Flash is very similar to traditional column chromatography
except that solvent is driven through the column by applying
positive pressure. Resolution is measured in terms of the
ratio of retention time (r) to peak width (w, w/2). The technique
simply uses a set of chromatography columns and flow
controller valves. Modern flash chromatography systems are
very convenient, being sold as prepackaged plastic cartridges
with solvent being pumped through the cartridge..

Column chromatography (which is the basis for Flash
Chromatography) follows the same principles as thin layer
chromatography (TLC). The main difference is that TLC
separates miniscule amounts of material whereas column
chromatography can be used to separate large amounts of
material. If the solvent flows down the column by gravity or
percolation the technique is called gravity column
chromatography. If the solvent is forced down the column by
positive air pressure it is called Flash Chromatography. The
term flash chromatography was first used by Dr. W. Clark at
Columbia University because the technique allows organic
compounds to be purified “in a flash”.
Column chromatography involves stationary and mobile
phases. In column chromatography the stationary phase (a
solid absorbent) is placed in a vertical column and the mobile
phase (liquid) is added to the top and flows down through the
column by either gravity or external pressure. In column
chromatography the stationary phase is most commonly either
silica (Si02) or alumina (Al2O3). The columns packed with silica
usually have a defined particle size of 40-60 microns. The
mobile phase is normally a mixture of hexane and ethylacetate.
Mobile phases with low viscosity require smaller
particle sizes. The stationary phase is normally more polar
than the mobile phase.
By increasing the polarity of the solvent system all components
of the mixture move faster. By lowering the polarity all
components move more slowly.

The eluting power of organic solvents
The highest polarity being the most powerful eluters (at thetop of the list)

Acetic acid
Alcohol
Acetone
Ethyl acetate
Diethyl ether
Halogenated hydrocarbons (methylene chloride)
Toluene
Alkanes (hexanes, petroleum ether)

The impure mixture to be analyzed by column chromatography

is applied to the top of the column. The liquid solvent (eluent)

is passed through the column by gravity or by the application

of gas pressure (normally nitrogen or compressed air).

The chromatography column is filled with the stationary phase

adsorbent and impure product is placed as a solution on the

top of the stationary phase. As solvent (the mobile phase) is

flushed through the column compounds the impure product

passes slowly down through the stationary phase. The speed

at which each compound travels down the column is

determined by a number of factors including the particle size

of the stationary phase, the polarity of the mobile phase and

solvent flow rate. Each compound will partition between the

mobile and the stationary phases differently. They will take

different times to pass through the column and each of the

partitions is then collected separately. The advantage of flash

chromatography is that pressure is used to rapidly push all

the air from the stationary phase material (silica or alumina)

and to speed up the purification process.

Component retention on TLC plates is measured in terms of

retention factor (Rf). Using Flash chromatography retention

is measured in column volumes (CV). There is a reciprocal

relationship between Rf and CV:

CV = 1/Rf

Therefore methods developed using TLC are generally

transferred to flash chromatography.

A low Rf (0.15-0.35) is preferred because a lower Rf means a

greater CV due to the reciprocal relationship. Large CV’s

indicate an increased contact time with the stationary phase,

improving the changes of component resolution. Since CV is

a measure of compound retention, then CV is a measure of

compound resolution. Using flash purification, CV dictates

the sample load range possible for any given cartridge size.

For two adjacent components a large column volume is

desirable.

Silica based Flash Chromatography demands using materials

consistent in grade, particle size and quality. In response to

the demands and requests of chromatographers DAI has

developed a superior Flash Grade Silica. This new product

ensures more uniform silica packed columns and cartridges,

providing separation chemists and chromatographers with

enhanced resolution and separation capabilities.

The particle size for the uniform DAI Flash Grade Silica

measures 32-63 microns. More than 90% of all silica particles

in the DAI Flash Silica product lie within this defined size

range. For you the chromatographer the benefits are obvious.

This product contains a very low level of fines, which are small

particles measuring less than 32 microns in size. Fines cause

back pressure increases and column clogging, particularly

dangerous when performing MPLC (medium pressure liquid

chromatography) or when using glass columns for product

separation. Small particles (fines) may pass through filters,

and as such can contaminate final product purification,

rendering product isolation useless. DAI has the lowest level

of fines in any silica offered for the chromatography market

today. Offering Flash grade silica with less fines provides a

regular, stable and reproducible chromatography bed with a

fast, even flow rate.

Just as bad as small particle fines are large particles for

product isolation and purification. Large particle size allows

solvent to flow quickly through the column which impairs

separation. Within a column solvent will take the path of least

resistance, flowing around pockets of small particles. Uneven

flow greatly affects chromatographic separation, because yield

peaks will have different retention times depending on the

flow path through the column. As the product being isolated

exits the column, the compound gives peaks which may be

broad and poorly separated. The goal for the chromatographer

is to achieve product yield as well defined as possible.

It is very import to start with a clean particle size distribution

silica gel when performing separations. Uneven flow of solvent

through a column leads to broad peaks which are poorly

separated from other components. More even particle

distribution provides better defined Gaussian peaks, yielding

purer products.