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Extraneous peaks
Air peaks are sometimes seen in a chromatogram
when leakage occurs because the valve rotor
does not seal properly. These leaks may not be
detectable using the soap-bubble method.
If a leak is suspected but cannot be located with
soap bubbles, a pressure check will determine
definitely if a leak exists. Extraneous peaks can
occur due to contamination or improper condi-
tioning of the valve. If leaks are not apparent,
clean or condition the valve.
Other causes, totally unrelated to the valve, may
produce similar symptoms. Impure carrier gas
(that is, containing water) can cause extraneous
peaks.
Released: April 2004
Peak broadening and tailing
Voids in the flow system (valve and connecting
tubing) cause tailing and peak broadening. Use
inlets and liners with small internal diameters
and connect the valve to the inlet or column with
short lengths of connecting tubing of narrow
inner diameter.
If early-eluting peaks are too broad, stationary
phase or thermal focusing effects should be used
with packed-column ports or increased split
flows when capillary split inlets are used. Inlets
should be equipped with narrow inner diameter
liners, and narrow-bore connecting tubing should
be used between the valve and inlet.
6850 Series Gas Chromatograph Service Procedures
335
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