To determine the limitations of the method, the ULF relaxation profile of human whole blood and blood plasma was measured. As shown in Fig. , the T 1 and T 2 relaxation times of the protons in whole blood varied with the storage field (10-510 µT) without a specific pattern. This can be a result of a residual cellular metabolism which can change the protein content over time . In blood plasma, where no cellular metabolism is expected, the content of metabolites should remain stable over time.