User guide

Detectors 10
Advanced User Guide 329
• Keep the detector temperature high (320 to 335 °C).
• Turn the hydrogen flow off during solvent peaks and
between runs.
Turning hydrogen off during a solvent peak
When using the NPD, the baseline shifts after a solvent peak and
can take some time to stabilize, especially with chlorinated
solvents. To minimize this effect, turn off the hydrogen flow
during the solvent peak and turn it back on after the solvent
elutes. With this technique, the baseline recovers to its original
value in less than 30 seconds. This also extends the life of the
bead. The hydrogen can be turned on and off automatically as
part of a Run Table. See “Run Time Programming” on page 14.
Turning hydrogen off between runs
To extend bead life, turn off the hydrogen flow between runs.
Leave all other flows and the detector temperature on. Turn on
the hydrogen flow for the next run; the bead will ignite almost
immediately. The process can be automated with Run Table
entries.
Turning off the detector
Setting the initial bead voltage for new beads
Before you turn on the bead for the first time, manually set its
voltage to a safe value so that the new bead is not destroyed.
1 Make sure Adjust Offset is turned Off.
2 After the temperature stabilizes at setpoint, set the initial
Bead Voltage, depending on bead type:
• Blos bead: 0.0 V to 0.5 V
• Ceramic bead (white or black): 0.0 V to 2.0 V
Setting NPD bead voltage manually (optional)
Bead voltage shows the voltage used to heat the bead. It can be a
value derived from the Adjust offset value, or can be entered as a
setpoint. Entering a setpoint causes the voltage to change at
13 mV/second until it reaches the setpoint provided that
CAUTION
If you turn Adjust offset [Off] at any time, the bead voltage,
hydrogen, and air flows all turn off.