IsoLyser

IsoLyser is rather a work horse of many uses – rather like the power take-off on a tractor.  The basic machine is an oven, designed to be used at fairly low temperatures, usually less than 150 degrees Celsius.  Tubes pass through the oven and can be used to dry samples or convert liquid to vapour, in both cases the vapour comes out in a dry nitrogen carrier and typically passes through a Picarro cavity-ring down isotope analyser.  Water can be injected through the septum and vapour produced.  A dew point generator can be used to ensure that the concentration limits of the laser are not exceeded.  Especially when the dew point generator is saturated, the isotopic composition of the vapour pulse varies over time, either due to fractionation as the injected liquid evaporates or due to Rayleigh fractionation in the dew point generator.  Thus the whole of the pulse must be integrated by software – an instant analysis is not adequate.  We post-process the Picarro output file to do this.  Again, the 2130 produces a better result than the 1102 as it samples more frequently.  Solid samples (such as soil and leaf) can be dropped into the vertical tube by removing the septum, and their vapour analysed.

The whole IsoLyser set up comprises the oven and the pressurised cylinder water injector.  The cylinder is first filled with a standard water.  This is then pressurised using a cylinder of nitrogen with a regulator.  The gas pressure forces the water through the capillary and out through the needle (a length of fine HPLC steel tubing), which pierces the septum on the top of the oven and introduces the water into the stream of dry gas.  The vapour is then homogenised in the flowing gas and ideally should emerge with a constant concentration and isotopic composition: the latter should be identical to that of the parent water.  Vapour concentration is widely variable and is controlled by varying the pressure of the driving gas.  The filter limits the maximum pressure (it bursts) so the capillary must be chosen to match the flow to the amount of gas/vapour required.

The system is not ideal but we continue to work on it.  The capillary and needle system is a bit tricky and tends to block.  It is also difficult to get a very steady concentration without playing around.  On the other hand, with several cylinders and needles, vapour of varying isotopic composition and concentration can be produced on demand.  This can be useful for calibrating vapour from experiments as required, and is useful if you don’t have a PAL.  We (especially HSW) continue to work on this, both from the hardware and software angles.  The oven and due point generator continue to be an attractive option for extracting water from leaves and soils as the method offers very precise temperature control unlike, for example, induction heating.