The most commonly used chelate is the N1 chelate. It allows easy labeling and purification, as well as fast dissociation. N1 chelate is best suited in separation assays where the concentration of chelating agents , such as EDTA, DTPA or citrates, is less than 0.05 mmol/L, where pH is greater than 7 and the assay temperature is not more than 38°C.
If stability problems are encountered using N1 chelates, or when assay conditions are more extreme DTPA chelates provide a better alternative. The DTPA chelates can be used in DELFIA separation assays where the concentration of chelating agents is between 0.05 and 10 mmol/L. They also support lower pH values, down to pH 6.5, and assay temperatures as high as 70°C.