An online chemical analysis instrument comprising: a chemical analyser in which a single mechanism introduces a sample, adds reagent, and cleans and flushes a measurement cell, the analyser incorporating: a longitudinal chamber comprising the measurement cell incorporating a first inlet port to introduce sample fluid, a second inlet port to allow injection of fluid reagent, and an outlet port to allow expulsion of sample/reagent mixtures; a detector located in or adjacent to said chamber between the inlets and said outlet; and a piston movable in said chamber carrying a series of seals (20,21,22) such that movement of the piston sequentially introduces sample fluid, injects fluid reagent, and expels sample/reagent mixtures; and reagent delivery means for delivering a volume of reagent into the measurement cell, characterised in that the reagent delivery means includes: a compressible reservoir of reagent which is positioned within a rigid container and is surrounded by a fluid which substantially fills the space between said reservoir and the inside wall of said container; wherein the piston is arranged such that the movement of the piston in conjunction with the seals (20,21,22) generates a pressure differential between said surrounding fluid and said reservoir to effect partial collapse of said reservoir in order to inject reagent into said measurement cell. a chemical analyser in which a single mechanism introduces a sample, adds reagent, and cleans and flushes a measurement cell, the analyser incorporating: a longitudinal chamber comprising the measurement cell incorporating a first inlet port to introduce sample fluid, a second inlet port to allow injection of fluid reagent, and an outlet port to allow expulsion of sample/reagent mixtures; a detector located in or adjacent to said chamber between the inlets and said outlet; and a piston movable in said chamber carrying a series of seals (20,21,22) such that movement of the piston sequentially introduces sample fluid, injects fluid reagent, and expels sample/reagent mixtures; and a longitudinal chamber comprising the measurement cell incorporating a first inlet port to introduce sample fluid, a second inlet port to allow injection of fluid reagent, and an outlet port to allow expulsion of sample/reagent mixtures; a detector located in or adjacent to said chamber between the inlets and said outlet; and a piston movable in said chamber carrying a series of seals (20,21,22) such that movement of the piston sequentially introduces sample fluid, injects fluid reagent, and expels sample/reagent mixtures; and reagent delivery means for delivering a volume of reagent into the measurement cell, characterised in that the reagent delivery means includes: a compressible reservoir of reagent which is positioned within a rigid container and is surrounded by a fluid which substantially fills the space between said reservoir and the inside wall of said container; wherein the piston is arranged such that the movement of the piston in conjunction with the seals (20,21,22) generates a pressure differential between said surrounding fluid and said reservoir to effect partial collapse of said reservoir in order to inject reagent into said measurement cell. a compressible reservoir of reagent which is positioned within a rigid container and is surrounded by a fluid which substantially fills the space between said reservoir and the inside wall of said container; wherein the piston is arranged such that the movement of the piston in conjunction with the seals (20,21,22) generates a pressure differential between said surrounding fluid and said reservoir to effect partial collapse of said reservoir in order to inject reagent into said measurement cell.