; 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

The Pulsed Helium Discharge Detector The pulsed helium discharge detector [7,8] is an extension of the helium detector, a diagram of which is shown in figure 36. Courtesy of Valco Instruments Company Inc. Figure 36 The Pulsed Helium Discharge Detector

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

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

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

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