Agilent Technologies 6890 Series Service Manual page 672

Troubleshooting 1150
Chromatographic symptoms
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 conditioning 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 (i.e., containing water) can cause extraneous peaks.
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.
Baseline shifts
Baseline perturbations are caused by changes in column flow as the valve is
rotated and as the sample loop equilibrates to system pressure. Slow valve
rotation momentarily stops carrier gas flow and, when the valve stops
rotating, a sudden increase in flow occurs which slowly returns to the set
point. Check actuator pressure (usually 40 to 75 psi), valve rotor tension, and
valve temperature to ensure that the valve rotates as quickly as possible. A
restrictor or backpressure regulator can be added to the sample vent line to
maintain the sample loop at system pressure. This will reduce the time it
takes for the flow to stabilize after the valve is switched.
3 of 6
Valves
Jun 2001
Agilent 6890 Gas Chromatograph Service Manual