Figure 6
Compression
fitting
with two-piece ferrule design
Fittings
Compression fittings provide gas-tight, leak-free connections
without the use of Teflon tape or adhesives. A Swagelok-
type compression fitting consists of a nut, a back ferrule, a
front ferrule, and the fitting (union, elbow, tee, etc.) as shown
inFigure 6. Slide the nut and ferrules onto clean, deburred
tubing and insert the tubing into the fitting as far as possible.
Hand tighten the nut, then use awrench to tighten further. For
t/s” tubing, tighten the nut 3/4-turn past finger-tight. For
‘14”
tubing, tighten the nut liLturns past finger-tight.When
tightened, the back ferrule forces itself into the front ferrule
causing it to compress and grip the tubing forming a leak-free
seal. Be careful not to over-tighten the nut, or the tubing and
ferrules can become deformed and not seal. A properly
tightened compression fitting usually shows one thread from
the back of the nut (Figure 7). Overtightened fittings show no
thread and are prone to leakage.
I
Figure 7
-
A properly tightened compression
fitting
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Valves
To expedite troubleshooting, the
entireGC network should incorporate
valving. 1/8'” brass plug valves (cat#
21889) are recommended to isolate
the system to check for leaks or to
allow theGC to be taken off-line for
repairs. Plug valves use a rotating
cylinder to control gas flow in one
direction only. Ball valves use a ball
encased in Teflon packing and allow
flow in either direction. One drawback of ball valves is the
potential for Teflon to flow form and cause the valve to leak
when used under fluctuating temperature conditions.
Valves should be placed before gas purification traps to allow
simple trap replacement without shutting down otherGCs on
the same line. For easy identification and troubleshooting,
label or color code each valve throughout the system to help
identify each gas type. After the system is pressurized, leak
check valves in all possible positions using a thermal conduc-
tivity leak detector (cat.# 21607).
Caution:
Two different gas types should never be connected
together by a tee or a valve to allow easy change-over of
carrier gases. Mixing will inevitably occur making trouble-
shooting very difJicult.
GCGas Purification
Clean carrier gas is the
key to longer column
lifetime and less
detector noise. Oxygen
andmoisture can enter
downstream of the
carrier gas cylinder
through fitting leaks or
connectors that utilize
rubber o-rings. Also,
contamination of the
tubing with solvents or
lubricating oils can increase background noise and cause
ghost peaks with GC systems. Therefore, traps should always
be used (evenwith ultra high purity gases) to prevent
impurities from entering the GC system. Individual traps are
designed to removemoisture, oxygen, hydrocarbons and
other contaminants from the gas supply. Traps are available
with either 1/4 or 1/8" compression fittings and are typically
constructed with metal or glass bodies. Plastic bodied traps
should never be used since oxygen andmoisture permeation
will occur. Several common carrier,make-up, and detector
gas purifiers are discussed in the following section.
Themost common contaminants in carrier gas are oxygen,
water, and hydrocarbons. Both oxygen andmoisture degrade
the stationary phase and shorten column lifetime. Hydrocar-
bons cause ghost peaks or increase detector noise. Oxygen
contamination in carrier gas can produce excessive column
bleed at high temperatures (Figure 8). Although some
Figure 8 - 02
damage results in
high bleed and peak tailing.
1,
A
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min.4 8
12 16 Al
:4 A? :2 :6 4’0 14
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8
8
1998
