Direct equilibration of soil water for δ18O analysis and its application to tracer studies

Abstract

Current methods for stable oxygen isotopic (delta (18)O) analysis of soil water rely on separation of water from the soil matrix before analysis. These separation procedures are not only time consuming and require relatively large samples of soil, but also have been shown to introduce a large potential source of error. Current research at Queen's University Belfast is focused on using direct equilibration of CO(2) with the pore water to eliminate this extraction step using the automated Multiprep system and a Micromass Prism III isotope ratio mass spectrometer (IRMS). The findings of this research indicate the method is less time consuming, more reliable, and reproducible to within accepted limits (+/-0.1% per thousand delta (18)O). In this study the direct equilibration method is used to analyse delta (18)O tracer profiles in the unsaturated zone of field soils, concurrently with chloride tracer profiles, which can be used to assess infiltration rates and mechanisms through the unsaturated zone. Copyright 1999 John Wiley & Sons, Ltd.

An inexpensive, fast, and reliable method for vacuum extraction of soil and plant water for stable isotope analyses by mass spectrometry

“…Because organic solvents can alter CO 2 equilibration during IRMS measurements of 18 O as well as bias analyses with laser-based spectroscopy instruments through interference with the absorption spectra of water, [7][8][9] azeotropic distillation can be problematic for such instruments. Direct equilibration methods developed for IRMS [10][11][12][13] have the advantages of simplicity and good reproducibility when applied to saturated substrates but some of these methods are restricted to 18 O analyses. For micro-distillation methods and offline preparation of water samples, an isotope ratio mass spectrometer equipped with a dual-inlet system is necessary, which is a technical restriction for some laboratories.…”

“…Commonly, there is a need to process large sample numbers with high resolution in time and space, e.g., to characterize rainfall patterns, runoff processes, soil water movement, or water isotope footprints in plant, animal and human tissue, or the fossilized remains thereof . Various methods have been developed to enable water isotope analyses by mass spectrometry from substrates after extraction, e.g., centrifugation, squeezing, distillation and equilibration techniques …”

“…[1] Various methods have been developed to enable water isotope analyses by mass spectrometry from substrates after extraction, e.g., centrifugation, squeezing, distillation and equilibration techniques. [2][3][4][5][6][7][8][9][10][11][12][13][14] Vacuum extraction is the most commonly used technique for water extractions from unsaturated soil samples, xylem, plant water, and cell water (e.g. [5,[15][16][17][18] ).…”

Validation and application of a cryogenic vacuum extraction system for soil and plant water extraction for isotope analysis

“…To determine the isotopic signature of water, it is necessary to separate or extract the water from other components of the sample material (plant roots, stems and leaves, soil). In recent decades, several extraction methods have been developed: azeotropic distillation with various toxic substances such as toluene, hexane, and kerosene (Revesz and Woods, 1990;Thorburn et al, 1993), mechanical squeezing (Wershaw et al, 1966;White et al, 1985), cryogenic vacuum extraction (Dalton, 1988;Dawson and Ehleringer, 1993;Sala et al, 2000;West et al, 2006;Goebel and Lascano, 2012), the batch-method for stem water extraction (Vendramini and Sternberg, 2007), the modified vacuum extraction technique of Koeniger et al (2011), centrifugation with or without immiscible heavy liquids (Mubarak and Olsen, 1976;Batley and Giles, 1979;Barrow and Whelan, 1980;Peters and Yakir, 2008) as well as different equilibrium techniques especially for soil samples (Scrimgeour, 1995;Hsieh et al, 1998;McConville et al, 1999;Koehler et al, 2000;. Out of these, cryogenic vacuum extraction is the most widely utilized method (Ingraham and Shadel, 1992;West et al, 2006;Vendramini and Sternberg, 2007;Koeniger et al, 2011).…”

Leaf water oxygen isotope measurement by direct equilibration

“…As has been discussed in previous studies, direct equilibration is advantageous over the standard vacuum distillation method in that it requires only a small amount of operator time, and may be accomplished with a minimum of laboratory equipment (Scrimgeour, 1995;Hsieh et al, 1998;McConville et al, 1999), except for the obvious requirement for an instrument capable of d 18 O measurement of CO 2 (this could be an IRMS, a TDL, or one of the significantly cheaper laser absorption-based spectrometers now available). It may be argued that the equilibration method described here is less efficient compared with the recently developed ME-IRIS method in which the simultaneous extraction and measurement system allows complete analysis of a single leaf sample in < 20 min (Munksgaard et al, 2014).…”