A statistical procedure is presented for calibrating chromatographic methods that generate negative chromatographic peaks. The technique is illustrated via data collected during the ion-chromatographic analysis of the ammonium ion in methansulfonic-acid solutions. A peak-integration protocol is explained and subsequent regression data are presented. For the concentration range of 1 to 5 ppb, a quadratic model could be fitted to the calibration data, using ordinary least squares as the fitting technique. The precision was approximately ± 0.5 ppb, at 95% confidence.