Precise Determination of the Absolute Isotopic Abundance Ratio and the Atomic Weight of Chlorine in Three International Reference Materials by the Positive Thermal Ionization Mass Spectrometer-Cs2Cl+-Graphite Method

Because the variation in chlorine isotopic abundances of naturally occurring chlorine bearing substances is significant, the IUPAC Inorganic Chemistry Division, Commission on Isotopic Abundances and Atomic Weights (CIAAW-IUPAC) decided that the uncertainty of atomic weight of chlorine (A(r)(CI)) should be increased so that the implied range was related to terrestrial variability in 1999 (Coplen, T. B. Atomic weights of the elements 1999 (IUPAC Technical Report), Pure Appl. Chem. 2001, 73(4), 667-683; and then, it emphasized that the standard atomic weights of ten elements including chlorine were not constants of nature but depend upon the physical, chemical, and nuclear history of the materials in 2009 (Wieser, M. E.; Coplen, T. B. Pure Appl. Chem. 2011, 83(2), 359-396). According to the agreement by CIAAW that an atomic weight could be defined for one specified sample of terrestrial origin (Wieser, M. E.; Coplen, T. B. Pure Appl. Chem, 2011, 83(2), 359-396), the absolute isotope ratios and atomic weight of chlorine in standard reference materials (NIST 975, NIST 975a, ISL 354) were accurately determined using the high-precision positive thermal ionization mass spectrometer (PTIMS)-Cs2Cl+-graphite method. After eliminating the weighing error caused from evaporation by designing a special weighing container and accurately determining the chlorine contents in two highly enriched (NaCl)-Cl-37 and (NaCl)-Cl-35 salts by the current constant coulometric titration, one series of gravimetric synthetic mixtures prepared from two highly enriched (NaCl)-Cl-37 and (NaCl)-Cl-35 salts was used to calibrate two thermal ionization mass spectrometers in two individual laboratories. The correction factors (i.e., K-37/35 = R-37/35meas/R-37/35calc were obtained from five cycles of iterative calculations on the basis of calculated and determined R(Cl-37/Cl-35) values in gravimetric synthetic mixtures. The absolute R(Cl-37/Cl-35) ratios for NIST SRM 975, NIST 975a, and ISL 354 by the precise calibrated isotopic composition measurements are 0.319876 +/- 0.000067, 0.319768 +/- 0.000187, and 0.319549 +/- 0.000044, respectively. As a result, the atomic weights of chlorine in NIST 975, NIST 975a, and ISL 354 are derived as 35,45284(8), 35.45272(21), and 35.45252(2) individually, which are consistent with the issued values of 35.453(2) by IUPAC in 1999.