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.