A dialysis system comprising: a dialysis fluid circuit; and a device for measuring a concentration of a substance in a sample of dialysate fluid taken from the dialysis fluid circuit, the device comprising a support frame, a first magnet (260, 261) connected to the support frame, a second magnet (280, 281) connected to the support frame in a manner such that the second magnet is disposed within the magnetic field of the first magnet and a magnetic attraction exists between the first magnet and the second magnet, a spacer disposed between the first magnet and the second magnet, the spacer comprising a first side that contacts the first magnet, and a second side that is opposed to the first side and contacts the second magnet, the first side and the second side defining therebetween an interior space configured to receive the sample, and a radio frequency coil supported on the spacer so as enclose a portion of the interior space, the radio frequency coil configured to transmit a radio frequency signal to and receive a radio frequency signal from the sample, wherein the spacer has a peripheral shape that orients the first magnet relative to the second magnet in a manner such that a pole face of the first magnet is maintained substantially parallel to a pole face of the second magnet. a dialysis fluid circuit; and a device for measuring a concentration of a substance in a sample of dialysate fluid taken from the dialysis fluid circuit, the device comprising a support frame, a first magnet (260, 261) connected to the support frame, a second magnet (280, 281) connected to the support frame in a manner such that the second magnet is disposed within the magnetic field of the first magnet and a magnetic attraction exists between the first magnet and the second magnet, a spacer disposed between the first magnet and the second magnet, the spacer comprising a first side that contacts the first magnet, and a second side that is opposed to the first side and contacts the second magnet, the first side and the second side defining therebetween an interior space configured to receive the sample, and a radio frequency coil supported on the spacer so as enclose a portion of the interior space, the radio frequency coil configured to transmit a radio frequency signal to and receive a radio frequency signal from the sample, a support frame, a first magnet (260, 261) connected to the support frame, a second magnet (280, 281) connected to the support frame in a manner such that the second magnet is disposed within the magnetic field of the first magnet and a magnetic attraction exists between the first magnet and the second magnet, a spacer disposed between the first magnet and the second magnet, the spacer comprising a first side that contacts the first magnet, and a second side that is opposed to the first side and contacts the second magnet, the first side and the second side defining therebetween an interior space configured to receive the sample, and a radio frequency coil supported on the spacer so as enclose a portion of the interior space, the radio frequency coil configured to transmit a radio frequency signal to and receive a radio frequency signal from the sample, a first side that contacts the first magnet, and a second side that is opposed to the first side and contacts the second magnet, the first side and the second side defining therebetween an interior space configured to receive the sample, and a radio frequency coil supported on the spacer so as enclose a portion of the interior space, the radio frequency coil configured to transmit a radio frequency signal to and receive a radio frequency signal from the sample, wherein the spacer has a peripheral shape that orients the first magnet relative to the second magnet in a manner such that a pole face of the first magnet is maintained substantially parallel to a pole face of the second magnet.