VICI D-2-I - Manual (Page 25)

21
Installation
Troubleshooting High Background Current
If the background current does not drop below 2 nA even after a 12 hour
bakeout, there is either a leak in the system or the column effluent is not
clean. To see if the high background current is due to the column:
1. Make sure the controller is in the un-zeroed condition. (Refer to the
discussion about the ZERO push button on page 7.)
2. For a PD-D2, loosen the knurled nut and pull the column out ~20 mm.
Secure the nut.
For a PD-D2-IS, completely disconnect the column from the column inlet
tube, leaving the inlet open.
3. Watch the detector standing/background current, indicated in the control-
ler DISPLAY. If the current remains high, then either the system has a
leak in the discharge gas supply line or the discharge gas has impurities
in it. Proceed to the next section, “Checking for Leaks in the Discharge
Gas Plumbing”. If the current decreases dramatically, then either the
carrier gas supply has leaks and/or contaminants, or the column is the
source of contamination and needs a bakeout. Read the “Column
Bakeout Precautions” on the next page before proceeding.
Checking for Leaks in the Discharge Gas Plumbing
Leaks can be detected with hydrogen. A small lecture bottle of hydrogen
with a regulated flow of 10-15 mL/min through a small outlet tube is all that
is required. (This method can only be used to detect leaks on the discharge
gas side of the plumbing and between the column and detector inlet.)
1. Make sure the controller is in its un-zeroed condition. (Refer to the
discussion about the ZERO push button on page 7.)
2. Hold the hydrogen outlet tube at a fitting connection for ten seconds
while monitoring the standing current display on the controller.
3. If the current remains the same, proceed to the next fitting. If the current
increases, there is a leak at that connection. Tighten the fitting and test
it again, repeating as necessary until there is no change in the standing
current.
4. Repeat the test for every fitting in the discharge gas plumbing.
5. Reinstall the column according to the instruction on page 17. When the
standing current reaches an acceptable level, the detector is ready for
use. Proceed to the next chapter, “Mode Selection and Setup”. If the
current stays high, use this method to check for leaks at the column/
detector connection.
NOTE: Some stationary phases will have a higher bleed than others, but
are still suitable for this detector. However, the lower the bleed, the lower
the chances of contaminating the detector cell.
From this point, the standing current should be observed and logged after
any system change. In addition, logging the standing current (with and
without the column) on a regular basis is an effective monitor of system
integrity (leak-tightness and cleanliness). We also recommend tracking the
internal standard (quantity on column/area count) for sensitivity continuity.