Trap current limiting problem on AutoSpec 2 - WKB96181

SYMPTOMS

• The EI Trap Current reads back a significant current but a lower current than that requested
• The EI Filament Current is maximized at 5 A
  • Typical EI Source reads back with an open circuit fault condition
• The EI Source may regulate at lower requested Trap currents
• When the source is not regulating, there may be a very poor beam or no beam at all
• There is no response on the beam to Ion Repeller slider adjustment/tuning

ENVIRONMENT

• AutoSpec Premier
• AutoSpec Ultima

CAUSE

An open circuit repeller, or very high resistance on the repeller contacts

• An open circuit repeller inside the EI ion source will charge up due to the electron beam passing through the inner source. This disturbs the electron beam's passage through the inner ion source and onto the trap, causing the source to lose regulation. 

FIX or WORKAROUND

1. When the issue shows itself, set the trap to a lower current than readback, typically between 100 and 300 µA.
2. Set the Magnet mass to transmit a strong ion from PFK 219Da, 331Da, or from Air, 28Da.
3. Move the Repeller slider strongly from left to right, -25 to +25 V, and observe the response to the voltage readback and the tuning peak.
   • On a normally working system, the Ion Repeller voltage readback will be seen to follow the requested values, swinging from -25 to +25 V.
   • There should also be a strong reaction seen on the tuning signal with the peak seen to increase and decrease in intensity with the tuning slider.
4. In the above fault condition, Open Circuit Ion Repeller contact:
   • The Ion Repeller readback will follow the requested tuning values.
   • However, NO REACTION will be observed on the tuning peak, which will remain at the same intensity.
5. Set the system into Standby and prepare to remove the outer source following normal instrument procedures:
   • Cool the Ion Source - Set the Ion Source temperature to 0 °C.
   • Close the Reference Inlet, open the Reference Pump Valve for 30 seconds before again closing it.
   • Close the Analyzer Isolation Valve.
   • Set the GC oven temperature to 20 °C; when it reaches room temperature, turn the Oven off.
   •  When the source is below 100 °C, switch off the Source Ion Gauge, close the Source Isolation Valve, and vent the source housing.
   • Pull back the GC column about 2.5 cm (1 inch) to ensure that it is not protruding into the Inner/Outer Source.
   • Pull back the Inner Source.
   • Remove the Outer Source when it is cool enough to handle safely and comfortably.
6. Check the wiring from the right-hand side contact rail contact 8 to the Ion Repeller contact. Any looseness must be cleared; tighten connections as necessary. 
7. Fit the inner source into the outer source and check the spring contact connection between inner and outer sources.
8. The contact may be very gently adjusted using a small screwdriver to ensure good contact.
9. Refit the Inner/Outer sources and the GC column and pump the Source region, opening the Source Isolation Valve when pre-vacuum is sufficient.
10. Before setting the source temperature back to the normal value, test the system as above by going into Operate with a beam present and strongly adjusting the Ion Repeller slider.
11. A strong reaction should be seen on the tune peak, and the trap current may now be returned to normal values.
12. Allow the system to pump fully and stabilize at normal analytical source and GC temperatures.

ADDITIONAL INFORMATION

Closely related KCS articles are WKB8643, WKB11064, WKB96173, WKB124360, and WKB124362.

See also WKB62609 for conditioning filaments and WKB62805 for tips on increasing their lifetime.