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

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

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

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

takes place) and the lower section, a tube 3 mm I.D. (where reaction with metastable helium atoms and photons takes place). Helium make–up 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 DB–1701, 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

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