The discrepancy between the hydride chemical shifts of the ATP standard and ATP in the RBC extract can be attributed to sample matrix effects. The Chemical shift changes are a common problem in NMR metabolomics, and polyanionic molecules are especially good at chelating cations, causing their chemical shifts to change substantially. In addition to metal ions blood extract also contains various organic compounds with different pKa, which can shift ATP hydride signals by forming salts. This is usually overcome by buffering the NMR sample to keep the pH and the environment constant from sample to sample.