A membrane inlet laser spectrometer for in situ measurement of triple water isotopologues

We describe a novel compact autonomous in situ sensor for semi-continuous measurement of water isotopes (δD, δ¹⁸O, and δ¹⁷O) in liquid water. The sensor relies on a dual-inlet water vapor injection system based on the pervaporation through a semi-permeable membrane, and on the water vapor composition analysis using a dedicated optical feedback cavity enhanced absorption spectrometer. The sensor has dimensions of 165 mm diameter and 550 mm long, for a weight of ∼ 8 kg. A titanium casing allows applications down to 6000 m deep for a total effective weight of 45 (23) kg in air (water). It has a power consumption of ∼ 40 W, and an autonomy of 10–12 h which is ensured by a dedicated Li-ion battery pack. The sensor is equipped with single-pair high-speed digital subscriber line communication for telemetry purposes. The instrument provides an accuracy of 0.3‰ (2σ) for all water isotopes with a 9-min integration time. The instrument is suitable for investigating the freshwater cycle in the ocean, and in particular the transformation of ocean water masses related to iceberg and ice shelf melting.