Indirect gold trap–MC-ICP-MS coupling for Hg stable isotope analysis using a syringe injection interface

Abstract

Gold traps are part of standard instrumentation to quantitatively pre-concentrate gaseous elemental mercury, Hg⁰_(g), and are a pre-requisite for atmospheric Hg⁰_(g) analysis. High precision mercury stable isotope ratio analysis by on-line coupling of gold traps to multi collector-inductively coupled plasma-mass spectrometry (MC-ICP-MS) has up to now been problematic due to the transient nature of the signal. We present a new off-line technique, where amalgamated elemental Hg⁰ is volatilized from the gold trap in an argon gas stream and temporarily stored in a large volume gas tight syringe. Hg⁰_(g) contained in the syringe is then injected on-line into the MC-ICP-MS interface using a syringe pump at a continuous flow rate and resulting in a steady state Hg signal. Different syringe injection schemes are discussed, as well as matrix effects. The 2SD external precision on an in-house Hg⁰ vapor standard was found to be 0.24‰ for δ²⁰²Hg. The method is illustrated for fumerole Hg⁰_(g) emissions at Vulcano (Italy) with δ²⁰²Hg of −1.74‰ relative to NIST SRM 3133.