Peak Broadening And Tailing; Baseline Shifts; Baseline Upsets - Agilent Technologies 7820A Service Manual

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Agilent 7820 GC Service
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.

Baseline upsets

Frequently, baseline upsets are seen on chromatograms when
valves are switched. These upsets are caused by pressure
changes within the system, injections of large volume samples,
or by changing the amount of restriction in the flow path. These
upsets will become more of a problem when high sensitivity is
required. Addition of a fixed restriction downstream from the
valve may help minimize the upset. Changes in column length
may also help reduce the upsets.