This Information Applies To: Agilent 7890A and 6890 Gas Chromatography Systems
Agilent Nitrogen Phosphorus Detectors (NPDs) use three different types of beads with different characteristics. The NPD method settings within the GC method are specific to the type of bead used in the NPD. Damage to the newly replaced bead can occur if incorrect method settings are used for the type of installed bead.
This article details the process to change the bead in Agilent NPD detectors.
Steps to follow:
Figure 1. Exploded view of the NPD
1. Bead Assembly, 2. Lid 3. Metal C-ring, top, 4. Ceramic Insulator, upper, 5. Collector, 6. Metal C-ring, bottom, 7. Ceramic Insulator, bottom, 8. Metal C-ring, bottom, 9. Jet, 10. NPD cover, 11.Column Adapter, 12. Insulation, 13. Insulation, 14. Insulation cup, 15. Ferrule, 16. Column nut, 17. Bead assembly cable, 18. NPD lid standoff, 19. J-clamp, 20. Electrometer
Identify and order the correct bead for the NPD. The three types of beads are:
Set the NPD bead voltage to 0.0. (Do not set it to Off.)
Set Adjust Offset to Off.
Cool the detector to 60 °C or lower. Leave all gas flows on. To cool the detector faster, raise the GC detector cover and open the hinged NPD cover.
Put on lint-free gloves before touching any of the detector parts.
Twist the ring to disconnect the bead assembly cable. Push and twist the lock so that the button slides up in the groove, then pull the cable ends apart.
Figure 3. Disconnecting the bead assembly
Remove the 3 T- 10 Torx screws from the bead assembly.
Figure 4. Removing the screws from the bead assembly
Gently lift and remove the old bead assembly. Avoid bumping the bead on the sides of the collector.
Figure 5. Removing of the bead assembly
Remove the protective cap covering the new bead.
Mount the new bead assembly on the NPD lid. Be careful not to bump the bead on the sides of the lid or collector.
Replace the screws. Finger-tighten the first screw; tighten the remaining screws normally and then completely tighten the first screw. Do not overtighten the screws.
Carefully bend the bead assembly cable 90°.
Figure 6. Installing of the bead assembly.
Reconnect the bead assembly cable to the NPD cable and twist the ring to lock the connection.
Figure 7. Reconnecting the bead assembly cable.
Close the NPD cover and reinstall all GC covers. All covers must be closed to get a stable NPD baseline.
Caution: Before you turn on the new NPD bead for the first time, manually set its voltage to a safe value so that the new bead is not destroyed. See the Turning on the NPD bead chapter in the Replacing the NPD Bead instruction guide for complete instructions.
For the 7890A GC, configure the new bead.
Set the bead type
Review the Maximum Bead Voltage setting and adjust, if necessary.
Review the Dry Bead and Auto Adjust Bead settings.
Restore normal NPD operating gas flows.
With all gases on, heat the detector to 150 °C and hold for about 15 minutes, then increase the temperature to 250 °C and hold for 15 minutes.
Increase the temperature to operating value (310 to 320 °C recommended). Allow 15 minutes for equilibration.
Check the NPD leakage current.
Display the detector output.
Verify that the output (leakage current) is stable and < 2.0 pA. If > 2.0 pA, verify bead installation and recheck. If still high, replace the ceramic insulators; inspect/replace the collector.
Make sure that Adjust Offset is turned Off.
After the temperature stabilizes at setpoint, set the initial Bead Voltage, depending on bead type:
Blos bead: 0.0 to 0.5 V
Ceramic bead (white or black): 0.0 to 2.0 V
After the bead reaches the initial voltage, begin to increase the voltage value in 0.05 V increments until the bead ignites. Wait about 10 seconds between each voltage adjustment. Monitor the detector output.
A new Blos bead typically ignites between 0.5 and 1.1 V.
A new ceramic bead typically ignites between 2.8 to 3.2 V.
When the bead ignites, the output will rise suddenly, then decrease towards a more stable value. It is best to allow the NPD to remain in this state without further adjustment for about 24 hours.
Then you may adjust the bead voltage in small increments (0.05 to 0.1 V) until reaching the desired offset. With a clean environment, clean gas supplies, and low bleed column, a typical offset may decrease 6- 12 pA during a 24-hour period.
|Learn how to effectively maintain your Agilent NPD:
GC-MULTI-2251r - NPD Maintenance e-learning course available from Agilent education