INFICON HAPSITE Smart Plus Chemical Identification System - Manual (Page 576)

B - 6
IP
N 07
4-
47
2-
P1
C
HAPSITE Smart Plus Operating Manual
B.1.3 Gas Cylinder Safety, Contamination Checks, and Corrective Steps
WARNING
Safety of operations should always take precedence in
the working environment. Gas cylinders should be
properly affixed to lab benches with clamps, or chained
to the wall for safety. A safety certified gas cylinder cart
should be available in the vicinity of where the cylinders
are normally used, for moving them and replacing empty
cylinders. Gas cylinders should never be transported
with the regulator attached!
Tedlar bags may be cleaned for reuse, or replaced with new bags. To clean a Tedlar
bag for use with different VOC's or concentrations, partially fill with VOC-free N
2
or
VOC-free air, heat it to 40-50 °C by wrapping the bag with an electric blanket for
several minutes, then evacuate the bag contents through the open valve with a
clean transfer line to a diaphragm vacuum pump. This operation should be
repeated 3 times for a normal cleaning. Then the bag may be stored filled with VOC
free N
2
or VOC-free air until needed.
To protect the concentration integrity in the bag, care should be taken to purge and
refill any standard gas mix after two weeks. Keep the standard gas mix at room
temperature between samplings with the bag valve securely closed,
This is especially important for concentrations below 1 ppmv and for VOC's of
limited stability in the Tedlar sample bags. Depending on the compounds (and their
concentrations) that were previously present in the bags, and if the bags are to
contain less than 1 ppmv standard gas mixes, pre-analyze the bag (after the
cleaning process above) while the bag is still filled with the VOC-free N
2
, or
VOC-free air, using the HAPSITE. The detection of any target compounds in the
bag should necessitate recleaning or replacement with a bag known to be "clean".
A supply of clean Tedlar bags can be useful for quick standards preparation by
direct liquid injection of VOC's not regularly analyzed into an N
2
or air matrix in the
bags. This allows a more convenient and rapid alternative to gaseous cylinder
mixes in such uses as new applications development or verification of unknown
VOCs by component spiking. This should be qualified by saying that accurate gas
standard preparation by direct liquid injection is only recommended at levels
greater than 5 ppmv, because the minimum liquid volume deliverable by syringe at
an acceptable accuracy and precision is about 0.5
 L. This corresponds to
approximately 10 ppmv in a 12 liter Tedlar bag, or approximately 3 ppmv in a 40
liter Tedlar bag. Larger Tedlar bags are available but convenience in regular
handling and the possibility of target compound adsorption on the larger interior
surface area may be matters of concern.