(a) Explain how anions and cations can be separated by hydrophilic interaction chromatography in Figure 25-5, using the zwitterionic stationary phase in Figure 24-14. Why does the gradient go from high CH3CN to low CH3CN content?
(b) Isocratic elution with 20 vol% acetonitrile plus 80 vol% aqueous buffer did not separate Na+ from Cl-. Eluent with 40 vol% acetonitrile plus 60 vol% aqueous buffer gave a baseline separation. Suggest an explanation.