A method for preventing diffusion-caused contamination within a microfluidic circuit including at least two inlet ports (202, 204, 206) through which at least two reactive fluids are introduced into the circuit, a non-reactive fluid port through which non-reactive fluid is introduced into the circuit, at least one microchannel for fluid flow in fluid communication with the inlet ports (202, 204, 206) and the non-reactive fluid port, an outlet port in fluid communication with the microchannel through which the reactive and non-reactive fluid from the microchannel are collected, and inlet channels (212, 214, 216, 220) connecting the inlet port (202, 204, 206, 218) to the microchannel, said method comprising the steps of: a. causing a reactive fluid flow into the microchannel from the inlet port by applying a negative pressure differential between the outlet port and the inlet port by applying a negative pressure at the outlet port by connecting the outlet port to a negative pressure source via a valve while substantially preventing non-reactive fluid from flowing from the non-reactive fluid port; b. substantially stopping the reactive fluid flow into the microchannel from the inlet port by closing the valve thereby removing the negative pressure differential between the outlet port and the inlet port; and c. causing non-reactive fluid flow into at least one of the inlet channels (212, 216) from the non-reactive fluid port by opening the inlet ports (202, 206) to a negative pressure source via a valve (224, 228) to apply a negative pressure differential between at least one of the inlet ports (202, 206) and the non-reactive fluid port, wherein the at least one of the inlet channels (212, 216) associated with the inlet ports (202, 206) is partially filled with the non-reactive fluid drawn from the inlet channel preventing diffusion-caused contamination of the main channel having reactive fluid from the inlet channel with the at least one reactive fluid from the at least one of the inlet channels (212, 216). a. causing a reactive fluid flow into the microchannel from the inlet port by applying a negative pressure differential between the outlet port and the inlet port by applying a negative pressure at the outlet port by connecting the outlet port to a negative pressure source via a valve while substantially preventing non-reactive fluid from flowing from the non-reactive fluid port; b. substantially stopping the reactive fluid flow into the microchannel from the inlet port by closing the valve thereby removing the negative pressure differential between the outlet port and the inlet port; and c. causing non-reactive fluid flow into at least one of the inlet channels (212, 216) from the non-reactive fluid port by opening the inlet ports (202, 206) to a negative pressure source via a valve (224, 228) to apply a negative pressure differential between at least one of the inlet ports (202, 206) and the non-reactive fluid port, wherein the at least one of the inlet channels (212, 216) associated with the inlet ports (202, 206) is partially filled with the non-reactive fluid drawn from the inlet channel preventing diffusion-caused contamination of the main channel having reactive fluid from the inlet channel with the at least one reactive fluid from the at least one of the inlet channels (212, 216).