Question

To pre-concentrate cocaine and benzoylecgonine from river water described at the opening of this chapter, solid-phase extraction was carried out at $\mathrm{p}\mathrm{H}2$ using the mixed-mode cation-exchange resin in Figure 28-19. After passing $500\mathrm{m}\mathrm{L}$of river water through $60\mathrm{m}\mathrm{g}$of resin, the retained analytes were eluted first with $2\mathrm{m}\mathrm{L}$of $\mathrm{C}{\mathrm{H}}_3\mathrm{O}\mathrm{H}$and then with $2\mathrm{m}\mathrm{L}of2\mathrm{\%}$ ammonia solution in$\mathrm{C}{\mathrm{H}}_3\mathrm{O}\mathrm{H}$. Explain the purpose of using $\mathrm{p}\mathrm{H}2$ for retention and dilute ammonia for elution.

Short answer

At pH 2: amine is protonated and carboxylic acid is neutral, so analytesare retained by cation exchanger.At high $\mathrm{p}\mathrm{H}$ the amine would be neutral and carboxylic acid (carboxylate) would be negative,the anion isn't retained by the cation exchange column and would be eluted with methanol

Step-by-step solution

Definition of$\mathrm{p}\mathrm{H}$.

• $pH$ is a scale used in chemistry to quantify the acidity or stipulation of awaterless result. It's preliminarily denoted as" eventuality of hydrogen" (or" power of hydrogen").
• Acidic results (those containing a larger attention of $H$ ions) have a lower $pH$ than introductory or alkaline results.
• The $pH$ scale is logarithmic, indicating the attention of hydrogen ions in a result in inverse order.

Determine the purpose of using $\mathrm{p}\mathrm{H}2$ for retention and dilute ammonia for elution.

• In this task we will explain the purpose of using$\mathrm{p}\mathrm{H}=2$ for retention and also dilute ammonia for elution.
• Here solid- phase extraction was carried out at$\mathrm{p}\mathrm{H}=2$via mixed-mode cation-exchange resin.
• Considering that cocaine is an amine base it would be a cation at low$\mathrm{p}\mathrm{H}$such as$\mathrm{p}\mathrm{H}=2$in our case and also it would be neutral in ammonia.
• The cocaine cation at $\mathrm{p}\mathrm{H}=2$would be retained by cation-exchange resin and the neutral cocaine would be easily eluted by methanol
• Benzoylecgonine (metabolite of cocaine) contains both amine and carboxylate functionality which is seen on the picture below

• At $\mathrm{p}\mathrm{H}=2$ the amine would be protonated and carboxylic acid would be neutral - the compound would be retained by the cation exchange column
• Whereas, at high $\mathrm{p}\mathrm{H}$ the amine would be neutral and carboxylic acid(carboxylate) would be negative - the anion isn't retained by the cation exchange column and would be eluted with methanol.