Question

(a) When you try separating an unknown mixture byreversed-phase chromatography with 48%acetonitrile 50%water,the peaks are too close together and are eluted in the range k = 2- 6Should you use a higher or lower concentration of acetonitrile in thenext run?

(b) When you try separating an unknown mixture by normal-phasechromatography with 50%hexane50% methyl t-butyl ether, thepeaks are too close together and are eluted in the range k = 2 - 6Should you use a higher or lower concentration of hexane in thenext run?

Short answer

(a) Lower concentration of acetonitrile should be used because lower solvent strength increases the difference in retention between compounds.

(b) Solvent that would lower the solvent strength is recommended for the use. In this case it is hexane.

Step-by-step solution

Define Acetonitrile:

Acetonitrile, often abbreviated MeCN (methyl cyanide), is the chemical compound with the formula CH3CN. This colourless liquid is the simplest organic nitrile (hydrogen cyanide is a simpler nitrile, but the cyanide anion is not classed as organic). It is produced mainly as abyproduct of acrylonitrile manufacture.

Concentration of Acetonitrile:

We should use lower concentration of acetonitrile because lower solvent strength increases the difference in retention between compounds.

DefineChromatography

Chromatography is a group of laboratory techniques used to separate the components of a mixture by passing the mixture through a stationary phase.

The Concentration of Hexane:

We should use a higher concentration of hexane in the next run because in normal-phase chromatography, solvent strength increases as the solvent becomes more polar (in this case methyl t-butyl ether is more polar). So, to improve resolution and increase the retention time,such solvent should be used that would lower the solvent strength. In this case it is hexane.