SERVOPRO CHROMA Gas Analyser
11
04400001A / Revision 7
1.3.2
Plasma Emission Detector (PED)
The plasma emission detector (PED) is based on a spectroscopic emission cell,
which is an established technique to measure impurities from the ppb to ppm level.
The characteristics which make the plasma system stable and selective are the
frequency, intensity, regulation, the coupling technique and the focusing
(stabilizing) electrodes.
The carrier gas flows at atmospheric pressure through a proprietary pure quartz
cell. The cell is submitted to a high frequency high intensity electromagnetic field.
This ionizes the carrier gas which becomes the centre of a luminous phenomenon
(electroluminescence), a collection of charged particles called the plasma.
Once the carrier gas is ionized many spectral lines are emitted. Excitation results
mostly from electron or ion collision; that is, the kinetic energy of electrons or
ions accelerated in an electric field in which the atoms or molecules of a gas are
subjected to, which cause the emission of light.
The presence of the impurities in the sample gas to be analysed will alter the
spectrum of emitted lines. Characteristic emission spectra can be obtained for
carrier gas and each substance in it.
1.3.3
Flame Ionisation Detector (FID)
The low noise flame ionization detector (FID) is designed to measure
hydrocarbons from the sub ppm to percentage level. The sample gas is
introduced into a hydrogen flame. Any hydrocarbons in the sample will produce
ions when they are burnt. The ions are detected using a collector, which
generates a very small current that is proportional to the rate of ionisation, and
which in turn depends on the concentration of hydrocarbons.
The SERVOPRO Chroma uses the logarithm feature of transistors to produce an
electrometer that is not only extremely stable with variations in temperature but
also offers low noise performance. The use of transistors has eliminated the need
for high value resistances, and hence the noise and drift that they cause. The
SERVOPRO Chroma can measure a broad range of currents, from
femtoamperes (10
-15
) to milliamperes (10
-3
).
Information on how to set up and operate the FID is given in Section 12.
