Table 2.

Results of preliminary optimization.

Column	Mobile phase	Elution mode	Flow rate	Observation	Result
Luna C8	Methanol-0.002 M solution of tetrabutylammonium sulfate (20 : 80, v/v)	Isocratic	1.0 mL/min	Elution force was too weak for sodium benzoate (retention time of sodium benzoate was more than 30 minutes)	Rejected
Luna C8	Acetonitrile-0.002 M solution of tetrabutylammonium sulfate (20 : 80, v/v)	Isocratic	1.0 mL/min	The two peaks of potassium guaiacolsulfonate were not completely resolute; retention time too long for sodium benzoate (more than 20 minutes)	Rejected
Luna C8	Methanol-0.002 M solution of tetrabutylammonium sulfate (50 : 50, v/v)	Isocratic	1.0 mL/min	Unable to distinguish the two components of potassium guaiacolsulfonate	Rejected
Luna C8	Methanol-0.005 M solution of tetrabutylammonium hydrogen sulfate (20 : 80, v/v)	Isocratic	1.0 mL/min	Poor and unstable peak shape for both analytes	Rejected
Luna C8	Methanol-0.002 M solution of tetrabutylammonium sulfate (20 : 80, v/v)	Gradient (the gradient program was presented in Table 1)	1.0 mL/min	Good resolution and peak shape for components of potassium guaiacolsulfonate; good resolution between potassium guaiacolsulfonate and sodium benzoate; acceptable analysis time (15 minutes)	Accepted