PerkinElmer CLARUS 590 GC, Руководство пользователя

Страница: 151 / 284

NPD Overview . 151
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 reduce
bead performance and life. Although these solvents may still be used, they should be avoided whenever
possible.
Setting Up a PPC NPD
To set up a NPD using PPC modules, set the hydrogen and air flow as described below, then condition the
bead.
Set the Hydrogen Flow and Air Flow for PPC
The pneumatics consist of PPC modules where the plasma gases are adjusted and set from the Clarus GC
keyboard.
The PPC modules are calibrated at the factory; therefore, to set the flows for an analysis, simply follow this
procedure.
1. Turn on the hydrogen at the tank and adjust the line pressure to 60 - 90 psig.
2. Turn on the air at the source and adjust the pressure to 70 - 90 psig.
3. In the System Status Screen touch the NPD icon. The following screen appears.