To preconcentrate cocaine and benzoylecgonine from river water at the opening of this chapter, solid-phase extraction was carried out at pH 2 using the mixed-mode cation exchange resin in Figure 27-18. After passing 500 mL of river water through 60 mg of resin, the retained analytes were eluted first with 2 mL of CH3OH and then with 2 mL of 2% ammonia solution in CH3OH. Explain the purpose of using pH 2 for retention and dilute ammonia for elution.