The fluorescence intensity of propidium iodide bound to DNA depends on the concentration of sodium chloride.

While performing DNA analysis with propidium iodide using the FACS-II cell sorter, it was noted that the fluorescence intensity was not constant during the measurements. When different sheath and sample fluids were used, the fluorescence intensity of a given cell population increased or decreased during the measurement, depending on the concentration of sodium chloride in the fluids. By selecting the appropriate sheath and sample fluid combination, the drift in fluorescence intensity can be avoided.