Setting Up the Detectors
4-44
WARNING
Turn off the hydrogen gas before disconnecting a column
from the NPD.
When two NPDs are installed, but only one NPD has a
column connected to it, plug the inlet of the NPD without a
column with a 1/8-inch stainless steel plug (P/N N930-
0061) to prevent hydrogen from back-diffusing into the
oven.
NOTE: The detector gas modules have been set to OFF at the factory for both manual and
PPC pneumatics.
NPD Overview
The nitrogen phosphorus detector (NPD) is a highly specific thermionic detector for
organically bound nitrogen and phosphorus.
NOTE: When using capillary columns with the NPD, Hydrogen is NOT a recommended
carrier gas. The additive effect of the hydrogen carrier gas and the hydrogen
detector gas affects the sensitivity and selectivity because it produces an overall
higher hydrogen gas flow. If you are temperature programming, column flow
changes may produce a negative baseline drift.
The detector operates by electrically heating a glass bead, that contains an alkali
metal, to the point where it emits electrons. A mixture of hydrogen gas and air flows
around the bead to produce a hydrogen plasma. Stable intermediates are formed in the
hydrogen plasma which then capture the electrons emitted from the bead to produce
ions. A polarizing field directs these ions to the collector electrode, thereby creating a
current which is amplified. Sensitivity is affected primarily by the air flow, and
selectivity is affected primarily by the hydrogen flow.
A cross-sectional view of a NPD is shown in the following figure.
NOTE:
CHLORINATED SOLVENTS MUST NOT BE USED WITH THIS DETECTOR. The
life and performance of the alkali bead will be severely impaired by contact with
chlorinated compounds. In general, any halogenated compounds will degrade bead
performance; therefore, avoid using stationary phases containing these compounds
whenever possible. The use of solvents such as water, methanol or ethanol will also
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