Question

Resources Ex Give Up? Hint 500 The peaks in an unknown mixture are too close together and elute in the range k = 1-3. If you are using a reversed-phase column and a mobile phase that is 50% tetrahydrofuran and 50% water, what change in the solvent composition should be made to improve the resolution and why? decrease the concentration of tetrahydrofuran to decrease the solvent strength O increase the concentration of tetrahydrofuran to increase the solvent strength decrease the concentration of tetrahydrofuran to increase the solvent strength increase the concentration of tetrahydrofuran to decrease the solvent strength If you are using a normal-phase column and a mobile phase that is 50% hexane and 50% diethyl ether, what change in the solvent composition should be made to improve the resolution and why? O increase the concentration of hexane to increase the solvent strength increase the concentration of hexane to decrease the solvent strength decrease the concentration of hexane to decrease the solvent strength decrease the concentration of hexane to increase the solvent strength MacBook Air

Answer 1

The retention (or capacity) factor (k) is a means of measuring the retention of an analyte on the chromatographic column.It is equal to the ratio of retention time of the analyte on the column to the retention time of a non-retained compound.

we have an unknown mixture , k = 1-3 (close peaks )

1.

Reversed phase chromatography is characterized by a situation in which the mobile phase used is more poar than the stationary phase.

k = 1-3

in reversed phase, water is the ‘weakest’ solvent. As water is more polar, it repels the hydrophobic analytes into the stationary phase more than THF.

We can improve resolution by increasing value of k.

The most effective and convenient way to alter the retention factor of a peak is to adjust the ‘solvent strength’ of the chromatographic mobile phase.This is usually achieved in reversed phase chromatography by changing the amount of organic solvent (modifier) in the mobile phase mixture.

Decreasing mobile phase ‘strength’ causes the analyte molecules to be increasingly repelled into the stationary phase. As an increasing proportion of analyte molecules are repelled into the stationary phase the analyte band is more strongly retained on the column. Mobile phase strength is a key parameter used to adjust retention in reversed phase HPLC.

Ans. Decrease in THF Concentration, to decrease solvent strength.

2.

Normal-phase chromatography utilises a stationary phase that is more polar than the mobile phase . The retention mechanism in normal phase is based on polar adsorption of either the solvent molecules or the analyte onto the polar stationary phase surface.

Hexane have negative or low elution strengths.

We can improve resolution by increasing value of k.

The most effective and convenient way to alter the retention factor of a peak is to adjust the ‘solvent strength’ of the chromatographic mobile phase.

Ans. Increase in Concentration of Hexane, to decrease solvent strength.