An apparatus for measuring the light absorbance of a substance in a solution, comprising: - at least one sample cell arranged to contain said solution that is at least partially transparent to light of a predefined wavelength spectrum; - at least two light passages through said at least one sample cell each of said light passages having a known light path length; - at least two photodetectors and - an LED light source arrangement comprising at least two UV LEDs each arranged to emit a light output with a UV wavelength different to the other UV LED or LEDs within said predefined wavelength spectrum, wherein each of the at least two UV LEDs includes an associated optical fiber bundle , the optical fiber bundle comprising plural optical fibers, one for each light passage , each optical fiber bundle providing a plurality of discrete light guides from each respective UV LED to each of the light passages , wherein the apparatus is configured to pass light from the LED light source arrangement through each of the light passage to be received by respective photodetectors , wherein the LED light source arrangement further comprises at least one optical filter for each UV LED , said optical filter being arranged between the UV LED and the optical fiber bundle , wherein said at least one sample cell is a dual path length flow cell comprising said at least two light passages , and wherein each of said light passages has a path length differing from each of the other light passages, and wherein glass rods are provided and inserted into the sample cell to create a first and second light passage therein and where the first light passage has a smaller path length than a path length of the second light passage . - at least one sample cell arranged to contain said solution that is at least partially transparent to light of a predefined wavelength spectrum; - at least two light passages through said at least one sample cell each of said light passages having a known light path length; - at least two photodetectors and - an LED light source arrangement comprising at least two UV LEDs each arranged to emit a light output with a UV wavelength different to the other UV LED or LEDs within said predefined wavelength spectrum, wherein each of the at least two UV LEDs includes an associated optical fiber bundle , the optical fiber bundle comprising plural optical fibers, one for each light passage , each optical fiber bundle providing a plurality of discrete light guides from each respective UV LED to each of the light passages , wherein the apparatus is configured to pass light from the LED light source arrangement through each of the light passage to be received by respective photodetectors , wherein the LED light source arrangement further comprises at least one optical filter for each UV LED , said optical filter being arranged between the UV LED and the optical fiber bundle , wherein said at least one sample cell is a dual path length flow cell comprising said at least two light passages , and wherein each of said light passages has a path length differing from each of the other light passages, and wherein glass rods are provided and inserted into the sample cell to create a first and second light passage therein and where the first light passage has a smaller path length than a path length of the second light passage . <EOS>