Helium
Helium is an inert, monatomic gas, having an atomic weight of 4. In gas chromatography, helium is commonly used as the carrier gas as it is very inert and solutes diffuse rapidly in it. The high solute diffusivity reduces the resistance to mass transfer in the mobile phase and the column efficiency is consequently higher. Helium must be used as the mobile phase, when the helium detector is employed. As 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 a helium atom, energy is adsorbed, 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, however, is difficult to operate as the helium must be extremely pure, any trace impurities quench the production of metastable helium atoms and so the detector does not function. The helium detector, functioning in this manner has been shown to have a high ionization efficiency and is probably the most sensitive detector available. Nevertheless, the main use of helium in gas chromatography is as a carrier gas.
Author: RPW Scott
Book:Gas Chromatography Detectors
Section:GC-Detectors Ionization-Detectors Helium
advantage for certain applications.
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
GC-Detectors Ionization-Detectors Helium
Author: RPW Scott
Book:Gas Chromatography Detectors
Section:GC-Detectors Ionization-Detectors Helium
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
analyze a sample of helium is shown in figure 35. The high sensitivity of the
detector to traces of the permanent gases is clearly demonstrated
GC-Detectors Ionization-Detectors Helium
Author: RPW Scott
Book:Gas Chromatography Detectors
Section:GC-Detectors Ionization-Detectors Electron-Capture
1.5 mm. It is 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
GC-Detectors Ionization-Detectors Electron-Capture
Author: RPW Scott
Book:Gas Chromatography Detectors
Section:GC-Detectors Ionization-Detectors Pulsed-Helium-Discharge
takes place) and the lower section, a tube 3 mm I.D. (where
reaction with metastable helium atoms and photons takes place). Helium makeup
gas enters the top of the sensor and passes into the discharge section. A
potential (about 20 V) applied across the discharge electrodes is pulsed at
about 3 kHz with a discharge pulse-width of about 45 ms for optimum performance. The discharge
produces electrons and high energy photons (that can also produce electrons),
and probably some metastable helium atoms.
Column; J & W DB1701, 10 m x 0.05 mm, film thickness 0.05 mm; Flow rate 20 ml/min. Sample split
1:150;1, benzene; 2, toluene; 3, ethylbenzene; 4, m and p xylene; 5, o-xylene
Figure
37 The Separation of Some Aromatic Hydrocarbons Monitored by the Pulsed Helium
Discharge Detector
GC-Detectors Ionization-Detectors Pulsed-Helium-Discharge
Author: RPW Scott
Book:Gas Chromatography - Tandem Techniques
Section:GC-Tandem GC-MS Ion-Generation
of ten and depended somewhat on the jet arrangement. The sample recovery was usually significantly in excess of 25%. Even today, the Ryhage concentrator is still used in GC/MS systems when packed columns are employed.
The Bieman Concentrator
The Bieman concentrator worked on an entirely different principle and was somewhat simpler but equally effective. A diagram of the Bieman concentrator is shown in figure 52. It consisted of a heated glass jacket surrounding a sintered glass tube. Helium was again used as the carrier gas which, after leaving the column, passed directly through the sintered glass tube and the helium diffuses radially through the porous walls and was pumped away. The helium stream enriched with solute vapor then passed on to the mass spectrometer.
Figure 52. The Bieman Concentrator
The concentration factor and sample recovery was similar to the Ryhage device. The apparatus, however, was more bulky but easier to operate. The sintered tube could be
GC-Tandem GC-MS Ion-Generation
Author: RPW Scott
Book:Gas Chromatography - Tandem Techniques
Section:GC-Tandem Atomic-Spectroscopy Atomic-Emission
is now very sensitive and, in conjunction with the gas chromatograph, has been used successfully for a number of years as a combination system for specific element detection.
The Atomic Emission Spectrometer
The atomic emission spectrometer is an extremely versatile device, with a very high sensitivity and excellent selectivity. The model described here was originally designed and manufactured by the Hewlett-Packard Corporation. Basically, atomic emission is achieved by means of a helium plasma, and the light emitted is analyzed by a diode array spectrometer. A diagram showing the basic principles of the helium plasma atomic emission spectrometer is shown in figure 21. The plasma is microwave induced into a helium stream employing a water-cooled transducer. The sample, mixed with the pure helium make-up gas, enters the plasma and the elements present in the solute emit light, the wavelength of which is characteristic for each element. The sample residue subsequently
GC-Tandem Atomic-Spectroscopy Atomic-Emission