The above chromatogram shows the separation of a mixture containing an ester, a carboxylic acid, an alcohol, and an alkane. If normal-phase high-performance liquid chromatography is used to separate the mixture components, which peak corresponds to the alcohol?

Question

The above chromatogram shows the separation of a mixture containing an ester, a carboxylic acid, an alcohol, and an alkane.  If normal-phase high-performance liquid chromatography is used to separate the mixture components, which peak corresponds to the alcohol?

Quizplus · Accepted Answer

11ecba2d_12bc_2f74_a3bf_e5e736f22c8f_MD0008_00 High-performance liquid chromatography (HPLC), a purification technique ideal for small sample sizes, separates compounds based on polarity. The instrumentation consists of a sample injector, column (stationary phase), solvent under pressure (mobile phase), detector, and computer for data acquisition. One of two types of columns-normal-phase (NP) or reverse-phase (RP)-may be used. NP-HPLC consists of a polar stationary phase (typically silica) and a nonpolar mobile phase. In contrast, RP-HPLC has a nonpolar stationary phase (typically C18 alkyl hydrocarbon) and a polar mobile phase. If a mixture is separated by NP-HPLC, the nonpolar compounds elute before more polar compounds because polar compounds interact more with the polar stationary phase, causing them to have a longer retention time. A mixture containing an ester, a carboxylic acid, an alcohol, and an alkane separated by NP-HPLC elutes in the following order (nonpolar to polar): alkane, ester, alcohol, and carboxylic acid. Therefore, peak III (second-longest retention time) corresponds to the alcohol. (Choices A and B) The alkane and ester (peaks I and II, respectively) are more nonpolar than the alcohol. These molecules interact more readily with the mobile phase than with the stationary phase and therefore have a shorter retention time. (Choice D) The carboxylic acid (peak IV) is more polar than the alcohol and therefore has a greater affinity for the stationary phase and longer retention time than alcohols. Educational objective: In high-performance liquid chromatography (HPLC), two types of columns-normal-phase (NP) and reverse-phase (RP)-can be used, depending on the polarity of the compounds being separated. NP-HPLC consists of a polar stationary phase and a nonpolar mobile phase, whereas RP-HPLC consists of a nonpolar stationary phase and a polar mobile phase. Molecules with similar polarity to the stationary phase interact with it more and have longer retention times.

The Above Chromatogram Shows the Separation of a Mixture Containing