2.1.1. Sampling by Complete Cryogenic Trapping
In this method ambient atmospheric water vapor is cryogenically trapped
at -78 °C from a stream of ambient air, without any isotopic
fractionation during sampling, so that isotopic composition of liquified
ambient water vapor represents the true isotopic composition of ambient
vapor. A stream of ambient air is diverted into a glass condenser
immersed in a liquid Nitrogen + Alcohol slush maintained at -78 °C. The
liquid Nitrogen + Alcohol slush temperature is carefully chosen to be
slightly greater than -78 °C so that the ambient CO2
does not condense along with the water vapor. The air-flow rate is also
maintained at around 450 cc/ minute to ensure that there is no isotopic
fractionation due to preferential loss of lighter isotopic molecular
species of water vapor. Detailed sampling procedure and sampling devices
for complete cryogenic trapping is discussed by R. D. Deshpande et al.
(2013).
With an average Rh of about 65 % and temperature of 27 °C, it typically
took approximately 4-5 hours to collect about 2ml of the liquefied vapor
by complete cryogenic trapping. Depending on the specific humidity of
ambient air, sampling duration for each sampling day was estimated to
ensure that minimum of 2 ml of liquified vapor could be sampled. After
an adequate duration of trapping of moisture in the condenser, the
condenser was taken outside the Dewar flask for it to attain room
temperature. This liquid was then transferred into 15 ml sample bottles.
Thus, the true ambient vapor was trapped and sampled using this method
and for the purpose of this study we shall from now on refer to this
sample as ‘vapor’ representing true isotopic composition of vapor.