Isotope analyser
Water molecule is form by one atom of oxygen and two atoms of hydrogen; however the presence in nature of stable isotopes of both elements allows the differentiation of specific arrays of isotopes within the same water molecule.
Oxygen has three stable isotopes: (16O, 17O and 18O) with the natural abundances of 99.759%, 0.037% and 0.204 % respectively; while hydrogen has only two: 1H and 2H with the natural abundances 99.985% and 0.015% respectively.
All the stable isotopes are measurement in terms of the proportion between the less and the more abundant isotope (R), and it is referred to a known standard through the delta notation (δ). In the case of the water stable isotopes the standard corresponds to the Vienna Standard Mean Ocean Water (VSMOW) and it is expressed in a per mil notation (‰).
The measurement of water isotopes in the Water Lab of TU Delft is performed with an Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS) device from Los Gatos Research (LGR) with the capacity to measure liquid and vapour samples. The measurements are performed based in the deviation of a laser beam within a vacuum chamber, when the beam hit the different water molecules in the chamber these ones deviate the beam in different angles due to the molecules weight. The number of angle deviations is correlated to the number of molecules with specific weights, which are related to the isotopic composition of each molecule. It is how we are able to obtain the stable isotope composition of water in liquid or vapour phase.
This device is composed by five functional units needed to process both types of samples. Liquid samples are processed with the Unit A and Unit B (LWIA). Unit A corresponds to the autosampler for liquid samples, it manages a mechanic arm with a 1.2 μL needle used to extract a subsample of each bottle and deposit it in a close chamber to vaporize completely the sample before the measurements. Unit B includes the operating system, the vacuum chamber and the laser used to perform the individual measurements. Water vapour measurements are performed with the Units B, C, D and E (WVIA) of the device. Unit C is the multiport inlet unit, a section that allows the system to get up to 8 samples and switch among each other according with the needs of the researchers. Unit D is the dry air source that allows a constant dry air flow within the measurement system. The Unit E is the Standard Source Unit, a device that allows the measurement of vapour with a known isotopic signature. This one is used to provide a calibration method for the continuous measurements. The operating system in the Unit B is able to control all the units from the device. The OA-ICOS system is keep in a room under control temperature and stable power supply, guarantying the best performance of the measuring system.
