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The Flame Ionization detector
A detector is considered to be composed of a sensor and associated electronics and it is the sensor unit that is commonly referred to as the FID. A diagram of the FID sensor is shown in figure 16. The body and the cylindrical electrode are usually made of stainless steel and stainless steel fittings connect the detector to the appropriate gas supplies. The jet and the electrodes are insulated from the main body of the sensor with appropriate high temperature insulators. Care must be taken in selecting these insulators as many glasses (with the exception of fused quartz) and some ceramic materials become conducting at high temperatures (200-300℃) .


The use of high voltages in conjunction with the very small ionic currents require that all connections to the jet or electrode must be well insulated and electrically screened. In addition, the screening and insulating materials must be stable at the elevated temperature of the detector oven. In order to accommodate the high temperatures that exist at the jet-tip, the jet is usually constructed of a metal that is not easily oxidized such as stainless steel, platinum or platinum/rhodium.

The Nitrogen Phosphorus Detector (NPD)
The nitrogen phosphorus detector (NPD) (sometimes called the thermionic detector) is a very sensitive, specific detector the design of which, is based on the FID. Physically the sensor appears to be very similar to the FID but, in fact, operates on an entirely different principle. A diagram of an NPD detector is shown in figure 22.



The NPD sensor differs from that of the FID by a rubidium or cesium chloride bead contained inside a heater coil situated close to the hydrogen jet. The bead is situated above a jet and heated by a coil, over which the nitrogen carrier gas mixed with hydrogen passes. If the detector is to respond to both nitrogen and phosphorus, then the hydrogen flow should be minimal so that the gas does not ignite at the jet. If the detector is to respond to phosphorus, only, however, a large flow of hydrogen can be used and the mixture burnt at the jet. The heated alkali bead emits electrons by thermionic emission which are collected at the anode and provides background current through the electrode system. When a solute that contains nitrogen or phosphorusiseluted, thepartiallycombustednitrogenandphosphorusmaterials are adsorbed on the surface of the bead.

The adsorbed material reduces the work function of the surface and, thus, electron emission is increased and the current collected at the anode rises. The NPD has a very high sensitivity, i.e., about an order of magnitude less than that of the electron capture detector (ca.10-12 g/ml for phosphorus and 10-11 g/ml for nitrogen).

The Electron Capture Detector
Lovelock's work on ionization detectors culminated in the invention of the electron capture detector. However, the electron capture detector operates on an entirely different principle from that of the argon detector. A low energy b-ray source is used in the sensor to produce electrons and ions. The first source to be used was tritium absorbed into a silver foil but, due to its relative instability at high temperatures, this was quickly replaced by the far more thermally stable 63Ni source.

The Thermionic Ionization Detector
Electrons produced by a heated filament can be accelerated by an appropriate potential so that they attain sufficient energy to ionize any gas or vapor molecules in their path. In 1957, the early days of gas chromatography, Ryce and Bryce modified a standard vacuum ionization gauge to examine its possibilities as a GC detector. A diagram of the device is shown in figure 47.

The sensor consisted of a vacuum tube containing a filament, grid and anode, very similar in form to the thermionic triode valve. The tube was operated under reduced pressure and an adjustable leak was arranged to feed a portion of the column eluent into the gauge. The sensor was fitted with its own pumping system and vacuum gauge and the usual necessary cold traps. Helium was used as a carrier gas and the grid collector–electrode was set at +18 V with respect to the cathode and the plate at -20 V to collect any positive ions that are formed. As the ionization potential of helium is 24.5 volts, the electrons would not have sufficient energy to ionize the helium gas. However, most organic compounds have ionization voltages lying between 9.5 and 11.5 V and consequently would be ionized by the 18 V electrons and provide a plate current. The plate current was measured by an impedance converter in much the same way as the FID ionization current. The detection limit was reported to be 5 x 10-11 moles, but unfortunately the actual sensitivity in terms of g/ml is not known and is difficult to estimate.


The sensitivity is likely to be fairly high, probably approaching that of the FID. The response of the detector is proportional to the pressure of the gas in the sensor from about 0.02 mm to 1.5 mm of mercury. In this region of pressure it was claimed that the response of the detector was linear . Hinkle et al. who also examined the performance of the detector, suggested the sensor must be operated under conditions of molecular flow i.e. where the mean freepathofthe molecules is about the same as the electrode separation. Very pure helium was necessary to ensure a low noise and base signal. The detector had a "fast" response but its main disadvantage was the need to operate at very low pressures so that it required a vacuum pump; furthermore, forstability, thesensorpressureneeded to be very precisely controlled.

The Discharge Detector
( I can't paste the picture , if you want to see , pls go to the web address
http://www.bbioo.com/instrument/2007/instrument_19149_6.htm )
About the same time that Ryce and Bryce were developing the thermionic ionization detector, Harley and Pretorious and (independently) Pitkethly and his co-workers were developing the discharge detector. By applying the appropriate potential, a discharge can be maintained between two electrodes situated in a gas providing the pressure is maintained between 0.1–10 mm of mercury. After the discharge has been initiated, the electrode potential can be reduced and the discharge will still continue. Under stable discharge, the electrode potential remains constant and independent of the gas pressure and the electrode current.The electrode potential, however, depends strongly on the composition of the gas. It follows, that the system could function as a GC detector. Pitkethly modified a small domestic neon lamp for this purpose and a diagram of his sensor is shown in figure 48.The lamp was operated at about 3 mm of mercury pressure with a current of 1.5. Under these conditions the potential across the electrodes was 220 V. Pitkethly reported that a concentration of 10-6 g/l gave an electrode voltage change of 0.3 V.
The noise level was reported to be about 10 mV thus at a signal–to–noise level of 2 the minimum detectable concentration would be about 3 x 10-11g/ml. This sensitivity is comparable to that of the FID and the argon ionization detector. The detector was claimed to be moderately linear with a linear dynamic range of three orders of magnitude but values for the response index were not reported. It was not apparent whether the associated electronics contained non linear signal modifying circuitry or not. Unfortunately, there were several disadvantages to this detector. One disadvantage was the erosion of the electrodes due to "spluttering" In addition, the electrodes were contaminated by sample decomposition and it was essential that it was used with a well–controlled vacuum system.


The Spark Discharge Detector
Lovelock noted that the voltage at which a spark will occur between two electrodes situated in a gas will depend on the composition of the gas between the electrode tips and suggested that this could form the basis for a GC detector. The system suggested by Lovelock is shown in figure 49.



The sensor consists of a glass tube in which two electrodes are sealed. The electrodes are connected in the circuit depicted in figure 49. The voltage across the electrodes is adjusted to a value that is just less than that required to produce a spark. When a solvent vapor enters the sensor, the sparking voltage is reduced and a spark discharge occurs. This discharges the capacitor until its voltage falls below that which will maintain the spark discharge. The capacitor is then charged up through the charging resistor until the breakdown voltage is again reached and another spark is initiated. Thus the spark frequency will be proportional to (or at least be a monotonic function of) the vapor concentration. The total counts in a peak will be proportional to the peak area and, if a digital–to–analog converter is also employed, the output will be proportional to the concentration in the detector and thus, plotted against time, will provide the normal chromatogram. This detector does not appear to have been developed further but is an interesting example of a sensor that, in effect, produces a digital output.

The Radio Frequency Discharge Detector
When an RF discharge occurs across two electrodes between which the field is diverging (i.e. within a coaxial electrode orientation) a DC potential appears across the electrodes, the magnitude of which depends on the composition of the gas through which the discharge is passing. Karman and Bowman developed a detector based on this principle. A diagram of their detector is shown in figure 50.


The sensor consisted of a metal cylinder that acted as one electrode with a coaxial wire passing down the center that acted as the other. A 40 MHz radio frequency was applied across the electrodes and the DC potential that developed across them was fed via simple electronic circuit to a potentiometric recorder. The resistance capacity decoupling shown in their circuit appears hardly sufficient to achieve the removal of the AC signal in a satisfactory manner and consequently, the circuit shown in figure 50 may be only schematic. The column was connected directly to the sensor and the eluent passed through the annular channel between the central electrode and the sensor wall.


The response of the radio frequency discharge detector was reported as 106mV for a concentration change of 10-3 g/ml of methyl laureate. The noise level was reported to be 0.05 mV, which would give the minimum detectable concentration for a signal–to–noise ratio of 2 as about 6×10-10 g/ml. This detector had the advantage of operating at atmospheric pressure and so no vacuum system was required. The effect of temperature on the detector performance was not reported, nor was its linearity over a significant concentration range. This detector appears not to have been made commercially

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