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Chapter 25: Q2TY (page 696)
25-2Why does the retention onder of peaks 2 and 3 change on the polar embedded column?
Compound 2 has polar functionalities that could interact with the polar embedded groups, increasing retention. Compound 3 cannot interact with these groups
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(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?
Retention factors for three solutes separated on anon-polar stationary phase are listed in the table. Eluent was a 70 : 30 (vol/vol) mixture of 50 mM citrate buffer (adjusted to pH with) plus methanol. Draw the dominant species of each compound at each pH in the table and explain the behavior of the retention factors.
A bonded stationary phase for the separation of optical isomers has the structure
To resolve the enantiomers of amines, alcohols, or thiols, the compounds are first derivatized with a nitroaromatic group that increases their interaction with the bonded phase and makes them observable with a spectrophotometric detector.
When the mixture is eluted with 20 vol% 2-propanol in hexane, the (R)-enantiomer is eluted before the (S)-enantiomer, with the following chromatographic parameters:
Resolution
Relative retention
k for (R)-isomer =1.35
whereis the average width of the two Gaussian peaks at their base.
(a) Findandwith units of minutes.
(b) The width of a peak at half-height isrole="math" localid="1663582749865" (Figure 23-9). If the plate number for cach peak is the same, findfor each peak.
(c) The area of a Gaussian peak is. Given that the areas under the two bands should be equal, find the relative peak heights.
A mixture of 14compounds was subjected to a reversed-phase gradient separation going from 5%to 100%acetonitrile with
a gradient time of 60min. The sample was injected at t =time. All peaks were eluted between 22and 50min.
(a) Is the mixture more suitable for isocratic or gradient elution?
(b) If the next run is a gradient, select the starting and ending %acetonitrile
and the gradient time.
Why are the relative eluent strengths of solvents in adsorption chromatography fairly independent of solute?
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