Research aiming at developing new insecticides carried out in the 1930s led to the discovery of a number of highly toxic organophosphorus compounds. These were weaponized and classified as nerve agents due to its action in the nervous system. Nerve agents are listed in the Chemical Weapons Convention and the manufacturing, storage and use of them are prohibited. Their degradation products, phosphonic acids and phosphoramidic acids, can be analyzed as markers of nerve agent contamination and use. Because these acids are non-volatile and very polar, their identification by the most commonly available analytical technique, GC-MS, requires derivatization prior to analysis. Standard derivatization methods for GC-MS analysis give very similar spectra for many alkyl phosphonic acid isomers, which hampers the identification process. In this report we present a new reagent, 3-pyridyldiazomethane, for preparing picolinyl ester derivatives of phosphonic and phosphoramidic acids. The new derivative facilitates the structure determination of alkylphosphonic acids by enhancing predictable fragmentation of the O-alkyl chain. Diagnostics ions and mechanisms for their formation are suggested. The reported method will improve the unambiguous structural determination process for nerve agent hydrolysis products.