Gas Chromatography Detectors - Ionization Detectors > The Helium Detector > Page 66
The column eluent enters the top of the ionization chamber and mixes with the helium from the discharge chamber and exits at the base of the ionization chamber. Ionization probably occurs as a result of a number of ionization processes. The electric discharge produces both electrons and photons. The electrons can be accelerated to produce metastable helium atoms, which in turn can ionize the components in the column eluent. However, the photons generated in the discharge have, themselves, sufficient energy to ionize many eluent components and so ions will probably be produced by both mechanisms. Other ionization processes may also be involved, but the two mentioned are likely to account for the majority of ions produced. The response of the detector depends on the collecting voltage and, as one might expect, is very sensitive to traces of inert gases. Peak reversal often 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 ultra–pure 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 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.