In this paper, several factors that could lead to the appearance of false peak were investigated by using on-line sample sweeping technique under different experimental conditions. The tested analytes were buflomedil hydrochloride, ephedrine hydrochloride, benzyl alcohol, vanillin, p-hydroxy~benzald~ehyde and m-methylphenol. Results showed that among the six compounds, three of them, i.e., buflomedil hydrochloride, ephedrine hydrochloride and benzyl alcohol will cause false peaks to appear when sample injection time is long, sodium dodecyl sulfate (SDS) concentration is high and there is pH gradient between cathode and anode. In order to avoid the appearance of false peak, the pH gradient should be avoided. In this paper, several factors that could lead to the appearance of false peak were investigated by using on-line sample sweeping technique under different experimental conditions. The tested analytes were buflomedil hydrochloride, ephedrine hydrochloride, benzyl alcohol, vanillin, p-hydroxy ~ benzald ~ ehyde and m-methylphenol. Results showed that among the six compounds, three of them, ie, buflomedil hydrochloride, ephedrine hydrochloride and benzyl alcohol will cause false peaks to appear when sample injection time is long, sodium dodecyl sulfate (SDS) concentration is high and there is pH gradient between cathode and anode. In order to avoid the appearance of false peak, the pH gradient should be avoided.