Replacing the NPD Bead
Instruction Guide 15
To Manually Adjust the Bead Voltage
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. 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.
Blos Beads
• A new Blos bead typically ignites between 0.5 and 1.1 V.
• A Blos bead typically requires a lower offset than the white
and black beads to achieve a similar response.
• A new Blos bead shows an increase in baseline during a
temperature programmed analysis. This effect minimizes
after a few cycles.
Recommended flow settings (starting values):
Capillary makeup flow: 1 to 20 mL/min
If using He makeup gas,
recommend < 10 mL/min to
avoid cooling effects.
Hydrogen flow: 1 to 3 mL/min
Air flow: 120 mL/min
NPD Bead Replacement.book Page 15 Monday, October 13, 2008 2:32 PM
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