ABSTRACT
We demonstrate an improved method based on continuous-flow elemental analyser pyrolysis isotopic ratio mass spectrometry (CF-EA-PY-IRMS) to measure the 2 H/1 H ratios of water trapped in halite crystals. Two challenges to overcome are the low hydrogen concentration of samples (10-50 µmol H2 ·g-1 ) and the high chloride concentration released when reacting halite in an elemental analyser. We describe an optimization procedure for determining the 2 H/1 H ratio of this trapped water with an acceptable accuracy. This technique involves the use of a high-temperature Cr reactor to quantitatively convert H2 O into H2 . The initial step was performed on halite crystals precipitated from a water reservoir where 2 H/1 H ratios were monitored from its initial stage until the end of evaporation. The 2 H/1 H isotopic analyses were automated online in continuous-flow mode. Precision of the method was determined for those "synthetic" samples with hydrogen concentrations ranging from 0.2 to 0.5 wt%. 2 H/1 H isotopic ratios of evaporating waters bracket the compositions of water inclusions. The formation of fluid inclusions is not instantaneous and records the isotopic signature of the residual waters across a time range during which the isotopic values of the water still evolve. This property explains why the δ2 HVSMOW standard deviation of ±5 (2σ) observed for 10-mg aliquots of halite exceeds the instrumental error (about ±1.5 2σ) determined on the basis of IAEA-CH7, NBS 30, and NBS 22 references along with calibrated waters with and without added halite crystals. We also applied this method to Mesoproterozoic (1.4 Ga) and Neoproterozoic (0.8 Ga) halite samples with relatively low hydrogen concentrations (300-1500 ppm). The measured δ2 HVSMOW values for Precambrian waters range from -89 to -54. We propose that this technique offers a new perspective and great potential for palaeoenvironmental reconstructions based on the 2 H/1 H analyses of water trapped in halite.
ABSTRACT
RATIONALE: The evolution of stable isotope applications has demonstrated the increasing need for the determination of more than one isotopic signature from the same sample. Simultaneous determinations of (13)C/(12)C and (15)N/(14)N have become a widespread technique but up to now very few fully automated systems have offered the possibility of also measuring (34)S/(32)S from the same sample aliquot. This could be critical when sample amounts are limited, but it could also represent a significant gain of analytical time or cost. The technique that we are presenting provides these multiple isotopic signatures on small sample aliquots with high precisions, especially for sulfur determinations. METHODS: A high-precision, easy and rapid method for the simultaneous measurement of carbon ((13)C/(12)C), nitrogen ((15)N/(14)N) and sulfur ((34)S/(32)S) ratios as well as elemental concentrations was employed, using a new combination of an elemental analyzer and an isotope ratio mass spectrometer. The elemental analyzer was based on 'purge and trap' technology rather than conventional packed-gas chromatography (GC) gas separation. Emphasis was put on the efficiency of the system to reliably combust sulfur-bearing compounds of both organic and inorganic origin with high conversion yields. RESULTS: High-quality measurement of (34)S/(32)S ratios was obtained using various international reference materials. A working calibrated material was also selected and characterized for all three isotopic signatures in order to fully use the capacities of the system in future work. CONCLUSIONS: The possibilities of such a system for the reliable measurement of S isotope ratios as well as N and C isotope ratios within the same aliquot of sample opens up new fields of investigation in many domains where multi-isotopic approaches are required.
