6-2
2000-OSM, F1
8. A continuous presence of sample at the detector due to sample valve leakage. Perform
Test
Procedure 3
(Sample Valve Leaking/Column Flooding).
9. Filament failure. Perform
Test Procedure 6
(Filament TCD Test).
Cyclic Short Term Drift
Symptom: Baseline (detector signal) is experiencing a cyclic short term drift (see Figure 6-2)
indicated by detector signal, graphics display, or strip chart recorder (if used). If cyclic behavior is
only present on strip chart recorder, verify recorder is functioning properly and that the recorder
connecting wires are properly shielded. Ensure shield is connected to ground at only one end.
BASELINE
BASELINE
TIME
TIME
NORMAL
CYCLIC DRIFT
Figure 6-2. CYCLIC SHORT TERM DRIFT
At the extreme, the signal may indicate a cyclic behavior with the period of each cycle ranging from a
few seconds to several minutes.
Causes:
1. Short term drift, especially cyclic operation, could be due to temperature out of control at the TC
detector or isothermal oven, “dirty” carrier, or defective flow controller. Perform
Test Procedure
30
(Checking Temperature Table Values), steps 1 to 3, 8 and 9;
Test Procedure 8
(Baseline
Cyclic Temperature Controller (TCD));
Test Procedure 9
(Baseline Cyclic Carrier Regulator); or
Test Procedure 10
(Baseline Cyclic Flow Controller), as applicable.
2. Another possible cause is cell vent back pressure changing. Disconnect vents to check. Perform
Test Procedure 34
(Vent Back Pressure).
Continuous Upward or Downward Drift
Symptom: Baseline (detector signal) is slowly drifting upward or downward (see Figure 6-3) as
indicated by the detector signal, graphics display, or strip chart recorder (if used).
BASELINE
BASELINE
TIME
TIME
NORMAL
UPWARD
DRIFT
Figure 6-3. CONTINUOUS UPWARD DRIFT
Cause: This drift characteristic is generally related to detector response. Most common causes are
temperature zone temperature drift, column flooding or contamination by a sample containing heavy
hydrocarbons, leaking sample valve. Temperature zone drift normally occurs during temperature
zone warm up. Column bleed is less pronounced in capillary columns than packed columns.