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Continuous in situ measurements of stable isotopes in liquid water
Author(s) -
Herbstritt Barbara,
Gralher Benjamin,
Weiler Markus
Publication year - 2012
Publication title -
water resources research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.863
H-Index - 217
eISSN - 1944-7973
pISSN - 0043-1397
DOI - 10.1029/2011wr011369
Subject(s) - isotope , analytical chemistry (journal) , fractionation , isotope fractionation , chemistry , enrichment factor , mass spectrometry , stable isotope ratio , microporous material , water vapor , fraction (chemistry) , chromatography , detection limit , physics , organic chemistry , quantum mechanics
We developed a method to measure in situ the isotopic composition of liquid water with minimal supervision and, most important, with a temporal resolution of less than a minute. For this purpose a microporous hydrophobic membrane contactor (Membrana) was combined with an isotope laser spectrometer (Picarro). The contactor, originally designed for degassing liquids, was used with N 2 as a carrier gas in order to transform a small fraction of liquid water to water vapor. The generated water vapor was then analyzed continuously by the Picarro analyzer. To prove the membrane's applicability, we determined the specific isotope fractionation factor for the phase change through the contactor's membrane across an extended temperature range (8°C–21°C) and with different waters of known isotopic compositions. This fractionation factor is needed to subsequently derive the liquid water isotope ratio from the measured water vapor isotope ratios. The system was tested with a soil column experiment, where the isotope values derived with the new method corresponded well (R 2 = 0.998 for δ 18 O and R 2 = 0.997 for δ 2 H) with those of liquid water samples taken simultaneously and analyzed with a conventional method (cavity ring‐down spectroscopy). The new method supersedes taking liquid samples and employs only relatively cheap and readily available components. This makes it a relatively inexpensive, fast, user‐friendly, and easily reproducible method. It can be applied in both the field and laboratory wherever a water vapor isotope analyzer can be run and whenever real‐time isotope data of liquid water are required at high temporal resolution.