The most common contaminants are the mono- and diphosphate forms of the nucleotide. Our purification procedures are designed to minimize the amount of other phosphates present, but since they result from hydrolysis that occurs at some finite rate with all nucleotides, they are generally present in the 0.5-3% range as detected by anion exchange HPLC. We routinely analyze older lots to identify these species and repurify the NTP if necessary to ensure it meets our purity specification (>90% for catalog products) before shipping. Two other potential contaminants are salts and inorganic phosphate. Salts are removed during the reverse phase HPLC and precipitation steps. Inorganic phosphate can be particularly problematic in some applications, therefore we take special measures to remove as much as possible. Every lot of NTP we manufacture is analyzed for the presence of inorganic phosphate using phosphorous NMR.