High Baseline; Drifting Baseline; Spikes; Shifting Retention Times - ABB PGC5000 Series Service Instructions Manual

PGC5000
PGC5000 Series Service Instructions

5.10.5 High baseline

Bad gas: When operating in the HID mode, background signal is directly linked to the quality of the gases being used.
Therefore, at a minimum, it is important to have an oxygen scrubber, a water scrubber, and a hydrocarbon scrubber in line
when you are operating in HID mode. When the highest sensitivity is desired, the detector should be operated with a
getter to remove the residual oxygen and nitrogen often found in UHP helium.
Air leak: If there is an air leak in the system before the detector, the presence of oxygen and nitrogen will cause a high baseline
and a noisy signal. Generally, an air leak at the detector will cause a low signal (due to the presence of nitrogen in the
plasma) rather than a high signal. If a high signal is encountered and a leak is suspected, verify the fittings before the
detector (i.e., the injector and the injector gas supply lines).

5.10.6 Drifting baseline

Pressure control problems: Pressure controls not providing a consistent, pulse free, flow rate to the detector will lead to
detector drift and noise. Monitor the pressure gauges on the supply cylinder and measure the flow out of the exhaust port
over a one hour period to verify correct flow conditions.
Column bleed: If the system is running isothermally, the changing baseline may be due to column bleed. This is especially
problematic when using packed columns. Verify that the system is leak free. Ensure that there is on oxygen in the
system. Re-condition, rinse, or replace the column.
Plasma cell leak: If the base of the plasma cell is not well sealed, it is possible to see a rise in the baseline. Initially this may
appear as column bleed, but the signal will settle down once the oven returns to an isothermal mode. Reset the plasma
cell in the Swagelok fitting and tighten to make it leak free.
Contamination or late-eluting components: Alter the cycle time of injections to determine if the drifting baseline changes
location. If it does, it is possible that there is a late eluting component or contamination that is coming off the column
several runs later. If this is the case:
1. Install filters to eliminate the contamination.
2. Extend the run time to get the material off the column.
3. Extend the instrument cycle time to elute the material prior to the next analysis.

5.10.7 Spikes

Loose connections: Verify the signal cable is securely attached to the electrometer and to the BNC fitting on the collector
shield. Verify that the signal spring contacts the exhaust tube.
Missing ground leads: Verify that the ground lead from the plasma center electrode is connected. If not, turn off the power to
the plasma and re-connect the center electrode ground lead. Verify that the main power supply ground lead is connected
to an appropriate oven chassis ground point.
Defective bias voltage: If the bias voltage is not stable, it is possible to get signal spikes. Verify the bias voltage and stability
using a VOM connected between the bias voltage rod and the oven chassis ground.

5.10.8 Shifting retention times

If compounds elute early, the wrong restrictor may be in place. If the restrictor has recently been replaced, it is likely that a
restrictor is being used that has an inside diameter that is too large. Replace it with a new restrictor of know dimensions and
verify performance.
SI/PGC5000-EN, Rev B
5 Diagnostics and troubleshooting
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