A system for providing constant flow on-line dilution comprising a first fluid introduction line carrying a first fluid, a second fluid introduction line carrying a second fluid, a flow path selection assembly coupled to the fluid introduction line and the second fluid introduction line, a mixing assembly suitable for mixing the first fluid, the second fluid and a sample, and a plurality of fluid transport lines coupled to the flow path selection assembly suitable for transporting the first fluid, the second fluid and a sample to the mixing assembly. The flow path selection assembly is suitable for selecting a flow path for each of the first fluid introduction line and second fluid introduction lines and providing changeable coupling of each of the first fluid introduction line and second fluid introduction line to one of the plurality of fluid transport lines through the flow path selection assembly.