A blood treatment machine comprises a cartridge 30 having a rigid chassis 30 with a network of walls located between a pair of flexible membranes 32,36 which all together define a chamber 50 including a fluid pathway which, in use, direct dialysate solution into and out of a fluid sensing region bounded within a toroidal coil 45 on a platen 40. The coil 45 extends into the cartridge 30 through an aperture 33 in membrane 36 which is configured to releasably receive the sensing coil 45 and the composition of the fluid within the coil sensing region is determined using magnetic resonance spectrometry by applying radiofrequency alternating potential difference to the coil 45 to generate a pulsed alternating magnetic field in combination with a fixed magnetic field.
