Provided is a method for purifying a-thrombin and for quantifying a-thrombin and its degradation polypeptides in a liquid proteinatious solution. The method employs a one- step anion exchange chromatography method. The method allows purification and/or quantification of a homogenous post-translationally modified a-thrombin. The method can also be used for purification and/or quantification of ß-thrombin.