Q17P (a) Nonpolar aromatic compounds ... [FREE SOLUTION]

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Chapter 25: Q17P (page 708)

(a) Nonpolar aromatic compounds were separated by HPLC on an octadecyl $(C_{18})$ bonded phase. The eluent was 65 vol% methanol in water. How would the retention times be affected if 90% methanol were used instead?
(b) Octanoic acid and 1-aminooctane were passed through the same column described in (a), using an eluent of 20% methanol/80% buffer (pH 3.0). State which compound is expected to be eluted first and why.
role="math" localid="1656416023291" ${CH}_{3} {CH}_{2} {CH}_{2} {CH}_{2} {CH}_{2} {CH}_{2} {CH}_{2} {CO}_{2} H Octanoic acid {CH}_{3} {CH}_{2} {CH}_{2} {CH}_{2} {CH}_{2} {CH}_{2} {CH}_{2} {CH}_{2} {NH}_{2} 1 鈥� Aminooctane$
(c) Polar solutes were separated by hydrophilic interaction chromatography (HILIC) with a strongly polar bonded phase. How would retention times be affected if eluent were changed from 80 vol% to 90 vol% acetonitrile in water?
(d) Polar solutes were separated by normal-phase chromatographyon bare silica using methyl t-butyl ether and 2-propanol solvent. How would retention times be affected if eluent were changed from 40 vol% to 60 vol% 2-propanol? (Hint: See Table 25-4.)

Short Answer

Expert verified

(a) The non-polar compounds would be more soluble in the mobile phase.

(b) The 1-aminooctane would eluated first.

(d) Less polar mobile phase.

Step by step solution

Step 1:The retention times be affected if 90% methanol

Part (a)

The retention time will be shorter if 90% methanol in water were used instead of 65%methanol in water because methanol is less polar than water.

So, the non-polar compounds would be more soluble in the mobile phase.

Step 2:Which compound is expected to be eluted first and why

Part (b)

At pH=3, the octanoic is in neutral form, while the 1-aminooctane is in the form of cation. Cation form is insoluble in the non-polar phase. So, if octanoic acid and 1- aminooctane passed through the same column using an eluent of 20% methanol/80% buffer (at pH 3.0), the 1-aminooctane would eluated first.

Step 3:The retention times be affected if eluent were changed from 80 vol% to 90 vol% acetonitrile in water

Part (c)

If polar solutes were separated by hydrophilic interaction chromatography (HILIC) with a strongly polar bonded phase, the retention time would be longer if eluent were changed from 80 % to 90 % acetonitrile in water because the polarity of mobile phase decreases and the polar analytes are more strongly retained than non-polars.

The retention times be affected if eluent were changed from 40 vol% to 60 vol% 2-propanol

Part (d)

If polar solutes were separated by normal-phase chromatography on bare silica (polar stationary phase) using methyl t-butyl ether and 2-propanol solvent (less polar mobile phase), the retention time would be shorter if eluent were changed from 40% to 60 % 2-propanol because 2-propanol has higher eluent strength than methyl t-butyl