Certification of the chlorine content of the isotopic reference materials IRMM-641 and IRMM-642

The Slovak Institute of Metrology and the Institute for Reference Materials and Measurements have collaborated in the certification of the two chlorin e reference materials IRMM-641 and IRMM-642. Until now no isotopically enri ched chlorine isotopic reference material certified for isotopic compositio n and content has been available commercially. The isotopic reference mater ials IRMM-641 and IRMM-642 described herein are certified for isotopic comp osition and for chlorine content. The chlorine content of the reference mat erial IRMM-641 was certified by use of high-precision argentometric coulome tric titration at the Slovak Institute of Metrology. The base material used for IRMM-641 is NIST Standard Reference Material 975. The chlorine content of the reference material IRMM-642 was measured by isotope dilution, using negative thermal ionization mass spectrometry at the Institute for Referen ce Materials and Measurements. Both standard reference materials were prepa red by dissolving NaCl in water. The reference material IRMM-641 contains 0 .025022 +/-0.00011 mol kg(-1) chlorine of natural isotopic composition with an n(Cl-37)/n(Cl-35) ratio of 0.31977 +/-0.00082. The reference material I RMM-642 contains 0.004458 +/-0.000028 mol kg(-1) chlorine with an n(Cl-37)/ n(Cl-35) ratio of 0.01914 +/-0.00088.