A method and apparatus for quantitatively determining chlorine content, chloride ion content, and fractional abundance of chlorine isotopes in unknown samples. An unknown sample is combined with a known sample enriched in a specific chlorine isotope which may be <35>Cl or <37>Cl to form a mixture which is excited to emit an infrared spectrum. An unknown aqueous sample is combined with a known aqueous sample with a known chloride ion content enriched in a specific chlorine isotope which may be <35>Cl or <37>Cl to form a mixture which is subject to electrolysis, reaction with hydrogen, and heating to excite the emission of an infrared spectrum. An unknown sample is excited to emit a distinctive, characteristic infrared spectrum from a portion. The quantitative determination of chlorine content, chloride ion content, and fractional abundance of chlorine isotopes may be derived from the measurements of the infrared emissions using infrared spectrometry.