An improved method of introducing sample into a low-pressure flow stream of a high-pressure chromatography system to improve efficiency and significantly shorten the time of the separation process through a chromatography column is described. The method comprises injecting samples in a modifier liquid flow stream upstream of a mixing column that combines two independent flow streams of a liquid modifier with a highly compressed solvating fluid. Focusing can be controlled at the column head by changing the ratios of the two flow streams. Total flow may also be adjusted to speed the application of the sample onto the column during injection. The method improves the timing and efficiency of a supercritical fluid chromatography system and can reduce the amount of modifier added to the mobile phase flow stream.