Hydrogen-isotope analysis of nanomole (picoliter) quantities of H2O
Introduction
The D/H ratios of hydrous phases constrain the sources and geochemical pathways of volatiles in terrestrial and meteoritic samples (e.g., Craig and Lupton 1976, Taylor and Forester 1979, Schidlowski et al 1983, Leshin et al 1996. Analysis of D/H ratios by conventional techniques (i.e., vacuum reduction of H2O followed by analysis of H2 on a dual-inlet, dynamically pumped gas-source mass spectrometer) requires samples containing ∼10−4 to 10−6 moles of hydrogen as H2Bigeleisen et al 1952, Coleman et al 1982, Horita et al 1989, Gehre et al 1996, Halas and Jasinska 1996, Tanweer and Han 1996, restricting minimum sample sizes to microliters for liquids, milliliters to liters for gases, and milligrams to grams for common hydrous solids. These limits generally do not permit analysis of D/H ratios of individual mineral grains or fluid inclusions or study of isotopic zonation in solids—key capabilities for many problems in geochemistry and meteoritics.
Recently, methods have been developed for on-line pyrolysis or reduction of organic compounds combined with continuous-flow mass spectrometry of evolved H2 gas; these methods are capable of determining D/H ratios of hydrocarbon compounds with external precision of approximately ±1 to 10‰ (1σ) for sample sizes of ∼10−9 moles Merritt et al 1992, Tobias and Brenna 1996, Hilkert et al 1999. These techniques have been adapted for analysis of microliter-sized samples of liquid H2O or alcohols Prosser and Scrimgeour 1995, Tobias et al 1995, Begley and Scrimgeour 1997 and milligram-sized samples of hydrous solids (Sharp et al., 2001), significantly lowering effort and potentially improving precision but not reducing minimum sample size relative to conventional techniques. Ion microprobe methods are capable of analyzing the D/H ratios of exceedingly small amounts of H-bearing solids (∼10−12 moles of hydrogen as H2) with external precision on the order of ∼10‰ but are not appropriate for study of gases and liquids and involve large (100s of per mil) and poorly understood analytical corrections (Valley et al., 1998, and references therein).
We describe here a method of on-line reduction of water vapor and continuous-flow mass spectrometry of evolved hydrogen for sample sizes down to ∼10−9 moles. This method is suitable for analysis of trace water vapor (∼10−5 ccSTP), tens of picoliters of aqueous liquids, and micrograms of hydrous minerals. This article emphasizes the principles and equipment used in the first and simplest of these sample types (water vapor). Analysis of liquids and solids involves combination of this method with techniques for extracting water from condensed phases; examples are presented involving stepped heating of solids followed by analysis of liberated water.
Section snippets
Principles
The technique we describe is an adaptation of methods for analysis of D/H ratios in microliter-sized samples of water and alcohols described by Begley and Scrimgeour, (1997), which in turn derive from continuous-flow techniques for analysis of 13C/12C and D/H ratios in organic compounds Hayes et al 1977, Matthews and Hayes 1978, Merritt et al 1992: A stream of He gas containing a small quantity of water vapor is introduced into a high-temperature furnace containing elemental carbon deposited on
Reaction products
The products of the water-gas reaction (H2O + C = H2 + CO ± CO2) in systems such as that we describe here have been previously discussed (Begley and Scrimgeour, 1997); therefore, we only summarize their first-order features and add other details that have not been previously described. For reactor temperatures between ∼750 and 1050°C, the dominant products of passing H2O through the reactor are H2, CO, and CO2. The ratio of CO to CO2 increases with increasing reactor temperature over this range
Application to study of water evolved from pyrolysis of an SNC meteorite and terrestrial control
The methods described in the preceding sections are applicable to study of the D/H ratios and quantities of hydrogen in a variety of materials, provided that hydrogen can be extracted from those materials as H2O vapor and entrained in a stream of He. Here we illustrate a relatively straightforward sample preparation strategy that can be used for this purpose: Heating of hydrous samples under a He stream and collection of evolved H2O vapor by means of the He-purged step heating apparatus
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