Silke Mechernich, Tibor Dunai, Steven A. Binnie, Tomasz Goral, Stefan Heinze, Alfred Dewald, Irene Schimmelpfennig, Karim Keddadouche, Georges Aumaître, Didien Bourlès

Two natural mineral separates, labeled CoCal-N and CoFsp-N, have been prepared to serve as intercomparison
material (ICM) for in situ-produced cosmogenic 36Cl and natural chlorine (Clnat) analysis. The sample CoCal-N is
derived from calcite crystals in a Namibian lag deposit, while the sample CoFsp-N is derived from a single crystal
of alkali-feldspar from a Namibian pegmatite. The sample preparation took place at the University of Cologne
and a rotating splitter was used to obtain homogeneous splits of both ICMs. Forty-five measurements of CoCal-N
(between 1 and 16 per facility) and forty-four measurements of CoFsp-N (between 2 and 20 per facility) have
been undertaken by ten target preparation laboratories measured by seven different AMS facilities. The internal
laboratory scatter of the 36Cl concentrations indicates no overdispersion for half of the laboratories and 3.9 to
7.3% (1σ) overdispersion for the others. We show that the CoCal-N and CoFsp-N splits are homogeneous regarding
their 36Cl and Clnat concentrations. The grand average (average calculated from the average of each
laboratory) yields initial consensus 36Cl concentrations of (3.74 ± 0.10)×106 at 36Cl/g (CoCal-N) and
(2.93 ± 0.07)×106 at 36Cl/g (CoFsp-N) at 95% confidence intervals. The coefficient of variation is 5.1% and
4.2% for CoCal-N and CoFsp-N, respectively. The Clnat concentration corresponds to the lower and intermediate
range of typical rock samples with (0.73 ± 0.18) μg/g in CoCal-N and (73.9 ± 6.8) μg/g in CoFsp-N. We
discuss the most relevant points of the sample preparation and measurement and the chlorine concentration
calculation to further approach inter-laboratory comparability. We propose to use continuous measurements of
the ICMs to provide a valuable quality control for future determination of 36Cl and Clnat concentrations.