Question

To pre-concentrate cocaine and benzoylecgonine from river water described at the opening of this chapter, solid-phase extraction was carried out at $\mathit{p}\mathit{H}\mathbf{2}{\mathbf{}}^{}$using the mixed-mode cation-exchange resin in Figure 28-19. After passing $\mathbf{500}\mathit{m}\mathit{L}\mathbf{}$of river water through $\mathbf{60}\mathit{m}\mathit{g}\mathbf{}$of resin, the retained analytes were eluted first with $\mathbf{2}\mathit{m}\mathit{L}\mathbf{}$of $\mathit{C}{\mathit{H}}_{\mathbf{3}\mathbf{}}\mathit{O}\mathit{H}{\mathbf{}}_{}$and then with ammonia solution in. Explain the purpose of using for retention and dilute ammonia for elution.

Short answer

At pH 2: amine is protonated and carboxylic acid is neutral, so analytes are retained by cation exchanger. At high $p{H}^{}$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 

• The $\mathit{p}\mathit{H}$scale is logarithmic, indicating the attention of hydrogen ions in a result in inverse order.
• Acidic results (those containing a larger attention of ions) have a lower $\mathit{p}\mathit{H}$than introductory or alkaline results.

• The pH scale is logarithmic, indicating the attention of hydrogen ions in a result in inverse order.

Determining the purpose of using  pH2 for retention and dilute ammonia for elution.

• In this task we will explain the purpose of using$pH=2$ for retention and also dilute ammonia for elution.

• Here solid- phase extraction was carried out at $pH=2$via mixed-mode cation-exchange resin.

• Considering that cocaine is an amine base it would be a cation at low $pH$such as $pH=2$in our case and also it would be neutral in ammonia.

• The cocaine cation at $pH=2$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 $pH=2$the amine would be protonated and carboxylic acid would be neutral - the compound would be retained by the cation exchange column

• Whereasin contrast, at high $p{H}^{}$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.