Without an anionic competitor, nitrate would be totally retained on the column. Sodium sulfate was used by Takeuchi et al. to separate inorganic anions including nitrate and nitrite on a PGC column , but we decided to use diluted sulfuric acid (0.005 M H2SO4) instead to keep the pH as low as possible for subsequent reduction (pH ~ 1). However, due to the broad peak widths produced in our modified interface and the LC-IRMS interface in general, efficient separation of nitrite and nitrate from the injection peak was not possible simultaneously (Figure ). Here, Blank injections produce a measurable nitrogen peak (A).