Strengths and Weaknesses of the Analytical Techniques Used for Measuring Low Mercury Mass Fractions (&lt; 10 ng g^‐1) in Crystalline Rocks: Direct Mercury Analyser <i>Versus</i> Cold Vapour‐Atomic Fluorescence Spectroscopy

Elemental mercury (Hg) is routinely determined in crystalline rocks with mass fractions lower than 10 ng g ‐1 by thermal decomposition using Direct Mercury Analyzer (DMA‐80) or Lumex RA‐915+ (equipped with a PYRO‐915+ attachment) instruments, both based on atomic absorption spectroscopy. However, 223 analyses over the course of one year with DMA‐80 and cold vapour‐atomic fluorescence spectroscopy (CV‐AFS) on three reference materials (RMs) and six crystalline rocks (granite, diorite, gabbro, spinel peridotite, phlogopite‐rich peridotite, and sulfide‐rich orthogneiss) from the exposed transcrustal section of the Ivrea‐Verbano Zone and upper crustal Serie dei Laghi unit (western Alps, Italy) reveal that rock analyses using the DMA‐80 are variably affected by different internal and external biases when Hg mass fractions are below 10 ng g ‐1 . Conversely, CV‐AFS analyses are more precise, providing homogenous and repeatable results, even at ultra‐low Hg mass fractions (&lt; 1 pg g ‐1 ). Furthermore, CV‐AFS analyses show that gabbro and spinel peridotite powders roasted for analysis by DMA‐80 still contain ~ 0.6 to ~ 1.4 ng g ‐1 of Hg, implying inefficient release of Hg from basic/ultrabasic lithologies. Therefore, we recommend the use of CV‐AFS for Hg measurements in crystalline rocks. We also propose a new Hg reference value of 3.9 ± 1.5 ng g ‐1 for the GSJ granodiorite reference material JG‐1a.