Faraday Detector; Introduction Of A Gas Sample - Thermo Scientific Prima PRO User Manual

Technical Description: System

Introduction of a Gas Sample

Ions collected in the Faraday bucket result in a current that is sensed
and then amplified to a suitable level in the instrument amplifier. The

Faraday Detector

Faraday bucket is light tight and coated with colloidal graphite to
reduce emission of secondary electrons that would otherwise cause a
false indication of ion current. Further suppression is provided at the
entrance to the detector by a suppressor plate, biased at
approximately -40 volts and magnets that create a field around the
bucket to ensure any electrons emitted remain confined.
Introduction
Gas samples are introduced into the ion source through the inlet
of a Gas
probe assembly. For a Prima PRO, gas is sampled through a micro
Sample
capillary in the stream selector assembly (RMS, solenoid, or single
point), passes along a transfer line, into the inlet probe, and finally
down a bypass line to the rotary vacuum pump. In the inlet probe, a
small portion of the gas flowing through the assembly passes through
a further leak element into the ion source.
The flow of gas through the capillary and inlet probe is viscous in
nature. It is desirable that the flow through the leak element should
be molecular, i.e. of the same characteristics as the flow out of the
ion source. Failure to achieve this can result in a distortion to the
composition of the gas as presented to the source, specifically a
depletion of light gases with respect to heavier gases. The leak
elements used are typically a pinhole type (commonly 70 µm
diameter, but other diameters may be used depending on
application). For leaks of this diameter, flow is predominately
molecular if the pressure of gas at the leak is around 1 mbar. The
length and diameter of the bypass line are chosen such that this
pressure is achieved. The leak element therefore forms the transition
point between viscous and molecular flow.
The flow through the capillary is 7 to 10 ml/min, while the flow rate
through the leak element is around 5  10
-4
mbar l/s, i.e. only a small
fraction (<1%) of the gas entering the system through the capillary
passes to the ion source, the bulk of the sample passes to exhaust
through the rotary pump. The flow path through the capillary,
transfer line, inlet probe, and bypass line to the pump can be
considered as a fast loop, so that changes in sample gas composition
are rapidly transmitted to the inlet probe.
A typical arrangement (in the case of an RMS inlet system) is shown
in the following figure.
Thermo Fisher Scientific Prima PRO & Sentinel PRO Mass Spectrometers User Guide B-13