Chlorine isotope effects in mass spectrometry by multiple reaction monitoring

Procedures are developed in which the reversed-sector double-focusing mass spectrometer is employed to monitor signals due to specified ionic reactions. These procedures are employed to determine chlorine isotope effects for positive and negative ions fragmenting both unimolecularly and on collision at high velocity. Values of the /sup 35/Cl//sup 37/Cl isotope effect are measured for reactions involving Cl. and HCl elimination and are found to range from unity to 1.2. A dependence is noted upon the internal energy of the ion and, in the case of chlorobenzenes, on the nature of the para substituent. Fragmentations of positive ions which are subject only to secondary chlorine isotope effects are monitored to provide internal standards and as a check upon possible isotope effects in the chemical ionization source. The magnitudes of the kinetic heavy atom isotope effects for positive ions are rationalized in terms of the strength of the C--Cl bond as determined chiefly by the contribution of the chloronium (>C=Cl/sup +/) resonance form. A value of 1.19 +- 0.01 (95% confidence) is recorded as the secondary chlorine isotope effect for HCl elimination from the negative ions formed by chloride ion attachment to benzoic acid. The error in this determination is no more than 5%. 2 tables, 2 figures.