Picarro 1102i

This was our first laser.  We quickly formed the impression that the machine was pretty good, although the documentation and support were pretty poor.  As time went by, reports of the difficulties caused by organic interference became more frequent.  But despite that, it’s been a good machine and ~ 1 uL injections of standards vs accepted values typically have a slope of 1.  With lined vials we can easily get enough water to get a good analysis from a sample of leaf held within a small Licor 6400 cuvette.

There are two main problems with the machine.  The first is concentration dependence.  When injecting samples the vapour concentration is fairly uniform and values are directly comparable with each other.  This is not true when examining things like vapour in the stream from a Licor.  The value reported by the 1102 will not be the same if the concentration was different from the injected standard.  In practice you can run concentration curves by injecting a range of amounts and calibrating your vapour against them.  We also have a machine which converts water to vapour (IsoLyser) and this can extend the injected range.  Also, while it seems reasonable to assume that injected liquid is fine for calibrating vapour, there have been times when doubts have arisen.

And this all ties into the second problem: calibrating the isotope values.  Injecting standards (international or lab) is pretty easy and we typically use three or more different waters in any run injected via the PAL.  But what about vapour?  The isotopic composition of vapour over liquid depends on temperature, so this is not a reliable way of creating vapour.  Both our methods rely on total evaporation of a small amount of water, so that delta_vapour = delta_water.  The first technique relies on filling a bottle (Dewey) with dry nitrogen, then injecting about 30 uL of standard water through a small portal in the top and then waiting for it all to evaporate.  The other method uses water injected into an oven – IsoLyser.

We have the small induction module for leaf and soil water but have not used it much.

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