“…1), and 15 N 2 -labeled 2,4-dinitrotoluene-3-sulfonic acid were synthesized, purified, and characterized by Dr. Klaus Steinbach, Department of Chemistry, PhilippsUniversity of Germany [21,22]. Stock solutions (1 µg µL -1 ) of 2-amino-4-nitrotoluene-5-sulfonic acid (2A4NTSA-5), 4-amino-2-nitrotoluene-3-sulfonic acid (4A2NTSA-3), 2,4-dinitrotoluene-3-sulfonic acid (2,4DNTSA-3), 2,4-dinitrotoluene-5-sulfonic acid (2,4DNTSA-5), 2,6-dinitrotoluene-3-sulfonic acid (2,6DNTSA-3), and 2,6-dinitrotoluene-4-sulfonic acid (2,6DNTSA-4), and 6.4 ng µL -1 of 15 N 2 -labeled 2,4-dinitrotoluene-3-sulfonic acid were prepared in methanol.…”