Talanta 46 (1998) 1591–1604 Determination of chloride ion in aqueous samples by isotope-dilution Fourier-transform flame infrared emission (ID-FIRE) spectrometry Kenneth W. Busch a, *, Arvie J. Kuehn a , Marianna A. Busch a , Sandra Yescas a , Ilse Y. Guzman Jimenez b , Christine M. Algozo c a Department of Chemistry, Baylor Uniersity, P.O. Box 97348, Waco, TX 76798 -7348, USA b Instituto Technolo ´gico y de Estudios Superiores de Monterrey, Monterrey, Mexico c Mount Holyoke College, South Hadley, MA, USA Received 5 September 1997; received in revised form 13 January 1998; accepted 14 January 1998 Abstract An isotope dilution method for the determination of chloride ion in aqueous samples is described. The method makes use of the isotopic shift in the rotational lines of the 1–0 band of HCl emitted in the near infrared region of the spectrum by vibrationally excited HCl molecules present in a hydrogen/entrained air flame. Chloride ion in the sample is converted to chlorine gas by electrolysis and swept into a hydrogen/entrained air flame where it is converted into HCl. Because isotope dilution is an absolute method of analysis, matrix effects are minimized, and the chlorine generation step need not be quantitative. With the system described in this paper, samples must contain at least 9 mg of chloride ion per ml, and a 2-ml sample is required. Over the range from 10 to 30 mg Cl - ml -1 , the average error was -0.96%, and the average relative standard deviation was 3.3% for seven samples using seven of the more intense lines in the P branch. Compared with standard silver nitrate titrations, the isotope dilution procedure was not affected by such common interferences as bromide ion and iodide ion. The technique was applied to several seawater samples from different regions. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Isotope dilution; Fourier-transform infrared emission; Chloride analysis; Flame spectrometry; Seawater 1. Introduction The element chlorine occurs widely in nature, primarily as chloride ion, and is generally found combined with sodium, potassium, and magne- sium. In minerals, it is found in halite (NaCl), sylvine (KCl), and carnallite (KCl · MgCl 2 ·6H 2 O), among others [1,2]. Dissolved in water, chloride ion is found in ground water, seawater, mineral springs, and salt brines. Salt brines, which fre- quently contain large amounts of dissolved chlo- ride ion, are an important source of chemicals throughout the world [2]. While the oceans re- main the largest sources of brine, brines are also found in terminal lakes such as the Dead Sea in * Corresponding author. 0039-9140/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0039-9140(98)00044-7