INFICON HAPSITE ER Chemical Identification System - Manual (Page 437)

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HAPSITE ER Operating Manual
Chapter 14
SituProbe
14.1 SituProbe
The SituProbe™ Purge and Trap System is a sampling accessory for the HAPSITE
portable GC/MS used for in-situ testing of volatile organic compounds (VOCs) in
water. When used in combination with the HAPSITE, the SituProbe accessory
performs water analyses for qualitative and quantitative results in the field. The
SituProbe can be configured for unattended continuous sampling as well as
user-initiated, manual sampling.
The HAPSITE GC/MS is designed to analyze volatile organic compounds in air.
Therefore, the water sample VOCs must also be introduced to the HAPSITE in the
gas phase. The SituProbe uses Nitrogen to create a headspace in the probe and
to purge VOCs from the water sample. The Nitrogen purge gas collects in the
headspace at the top of the probe head. A portion of the headspace, containing the
VOCs, is sampled by the HAPSITE for analysis.
Unknown VOCs are tentatively identified using the National Institute of Standards
and Technology (NIST) mass spectral library. Known VOC compounds can be
quantified when a calibrated target compound library is included in the analytical
method.
14.2 Theory
The SituProbe and HAPSITE is a dynamic purge and trap system. The SituProbe
purges the VOCs from the water sample while the HAPSITE collects those VOCs
on an absorbent trap or in a fixed volume Sampling Loop within the HAPSITE. The
accumulated VOCs are subsequently desorbed from the trap or flushed from the
sampling loop and analyzed by the HAPSITE.
To purge VOCs from the water sample, Nitrogen gas bubbles are introduced at the
bottom of the SituProbe purge tube. The bubbles rise approximately five inches
through the water and collect in the headspace at the top of the purge tube. As the
bubbles rise through the water sample, a portion of the VOCs pass from the water
phase to the gas phase. The technical term for this transition is "partitioning". The
ratio of the VOC concentration in the water phase, VOC
water
, to the concentration
in the gas phase, VOC
gas
, is the partition coefficient. See equation [1] .
[1] Partition Coefficient
VOC
water
VOC
gas
---------------------- =