Helium Detector
The helium detector functions in exactly the same manner as the argon detector except that metastable helium atoms are formed and not metastable argon atoms. As helium is an inert gas, collisions between helium atoms and electrons are perfectly elastic. Thus, if electrons are generated in helium and are then accelerated under a suitable potential, despite collisions, the velocity (and, thus, the energy) of the electrons continually increase. However, when the energy of an electron reaches 19.8 electron volts or more, on collision with an helium atom, energy is adsorbed and one of its electrons changes orbit and a metastable helium atom is formed. These metastable helium atoms, can have energies of 19.8 or 20.6 electron volts and, thus, will ionize the molecules of virtually all substances producing ion and electron pairs. These pairs can be collected by suitably placed electrodes under an appropriate voltage gradient and used as a sensing process for GC detection. The helium detector, functioning in this manner has been shown to have an ionization efficiency of about 2.5 % (cf. 0.0015% for the flame ionization detector and 0.5 % for the argon detector) and a sensitivity of less than 10-13 g/ml. The helium detector, however, has problems that the argon detector dos not have, or at least not to nearly the same extent. The helium must be extremely pure, as any trace impurities quench the production of metastable helium atoms. Initially a long train of adsorption vessels were necessary to reduce the amount of impurities to a sufficiently low level for successful operation which made the apparatus very bulky. Today special helium is available, so the adsorption train may not be necessary but helium of satisfactory purity can be very expensive.
Author: RPW Scott
Book:Gas Chromatography Detectors
Section:GC-Detectors Ionization-Detectors Helium
;
The Helium Detector
The helium detector works on exactly the same
principle as the argon detector, but metastable helium atoms are produced by
the accelerated electrons instead of metastable argon atoms. Metastable helium
atoms, however, have an energy of 19.8 and 20.6 electron volts and thus can
ionize, and consequently detect, the permanent gases and, in fact, the
molecules of all other volatile substances.
As a consequence, contaminants in the helium can be extremely
deleterious and the helium must be extremely pure or the production of the
metastable helium atom production will be quenched by traces of any other
permanent gases that may be present. When first developed a very complicated
helium purifying chain was necessary to ensure its optimum operation. However,
with high purity helium becoming generally available, the detector can now be
used to detect concentrations of
organic vapors at 10-13 g/ml or less.
As an alternative to a radioactive source,
electrons can be
GC-Detectors Ionization-Detectors Helium
Author: RPW Scott
Book:Gas Chromatography Detectors
Section:GC-Detectors Ionization-Detectors Helium
occurs at high collecting voltages, which
may also indicate that electron capturing may also be taking place. This peak
reversal is reported to be controllable by the introduction of traces of neon
in the helium carrier gas. The helium discharge ionization detector is a
relatively new detector and has exhibited high sensitivity to the permanent
gases and is used for the analysis of trace components in ultrapure
gases. Linearity data is a little
scarce as yet, but it would appear that the detector response is linear over at
least two and possible three orders of magnitude with a response index probably
lying between 0.97 and 1.03.
Courtesy of GOW-MAC Instruments
System: Capillary Chromatograph Series 590; Column: GS MoleSieve,
30m x 0.55 mm; Carrier gas: helium, ionizing gas
78.6 ml/min, ionizing flow, 21.1 ml/min. Ionization voltage 524 V, sample
volume 0.25 ml
Figure
35 The Analysis of a Sample of Helium
An example of the use of the detector to
GC-Detectors Ionization-Detectors Helium
Author: RPW Scott
Book:Gas Chromatography Detectors
Section:GC-Detectors Ionization-Detectors Electron-Capture
reported that
the helium must be 99.9995 pure. The base current ranges from 1 x 10-9
to 5 x 10-9 amp, the noise level is about 1.2 x 10-13 amp and the ionization efficiency is
about 0.07%. It is claimed to be about
10 times more sensitive than the flame ionization detector and to have a linear
dynamic range of 105. An example of the use of a pulsed helium
discharge detector for monitoring the separation of some aromatics on a
capillary column is shown in figure 37. The pulsed helium discharge detector
appears to be an attractive alternative to the flame ionization detector and
would eliminate the need for three different gas supplies. It does, however,
require equipment to provide specially purified helium, which diminishes the
advantage of using a single gas.
The Electron Capture Detector
Lovelock's work on ionization detectors
culminated in the invention of the electron
capture detector (25). However,
the electron capture detector operates on an entirely different
GC-Detectors Ionization-Detectors Electron-Capture
Author: RPW Scott
Book:Capillary Chromatography
Section:Capillary Detectors The-Nitrogen-Phosphorus-Detector
The Nitrogen Phosphorus Detector (NPD)
The NPD, is a highly sensitive but very specific detector. It gives a strong response to organic compounds containing nitrogen and/or phosphorus. Despite its appearance it operates on an entirely different principle to that of the FID. A diagram of an NPD detector is shown in figure 14. The sensor of the NPD is a small rubidium or cesium bead contained inside a small heater coil. The helium carrier gas is mixed with hydrogen and passes into the detector through a small jet. The bead, which is heated by passing a current through the coil, is situated above the jet, and the helium-hydrogen mixture (produced by mixing the column carrier gas, helium with a separate stream of hydrogen) passes over it. If the detector is to respond to both nitrogen and phosphorus, then a minimum hydrogen flow is employed to ensure that the gas does not ignite at the jet. In contrast, if the detector is to respond to phosphorus only, a large flow of hydrogen
Capillary Detectors The-Nitrogen-Phosphorus-Detector
Author: RPW Scott
Book:Gas Chromatography Detectors
Section:GC-Detectors Ionization-Detectors Pulsed-Discharge-Electron-Capture-Detector
nbsp;
The Pulsed Discharge Electron Capture Detector
The pulsed discharge electron capture
detector is an extension of the previously discussed pulsed discharge helium
ionization detector, a diagram of which is shown in figure 44. The detector
functions in exactly the same as that of the traditional electron capture
detector but differs in the method of electron production. The sensor consists
of two sections: the upper section has a relatively small diameter and is where
the discharge takes place. The lower section has a much wider diameter and in
this part of the sensor, the column eluent is sensed and electron capturing
occurs. As with the pulsed discharge helium ionization detector, the potential
across the electrodes is
GC-Detectors Ionization-Detectors Pulsed-Discharge-Electron-Capture-Detector
Author: RPW Scott
Book:Gas Chromatography Detectors
Section:GC-Detectors General-Properties
if so desired. Flow programming, attempts to
achieve the same result as temperature programming which is to accelerate the
strongly retained peaks through the column (see
Gas Chromatography). Some detectors require
no other gas than that used as the carrier gas, other require specific gases to
be added to the columns eluent for them to function. In some cases the detector
prescribes a certain gas to be used as the carrier gas (e.g., the sensitivity of the katharometer is greater when helium is
used as the carrier gas). In addition, if the gas chromatograph is being used
for permanent-gas analysis, then helium must be used to differentiate the
carrier gas from the other gases being analyzed.
All gas chromatographs are designed to
operate over relatively wide ranges of temperature (e.g., -20oC
to 400oC). Consequently, to avoid solute condensation in the
detector or detector-connecting tubes, the detector should be capable of
operating at least 20oC higher than the
GC-Detectors General-Properties