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The velocity of the propagation of sound through a gas depends on its density and, thus, the presence of a solute vapor in a gas changes the velocity of sound through it. This velocity change can be utilized as a basis for vapor detection in GC. The frequency of a whistle, consisting of an orifice which directs a stream of gas against a jet edge proximate to a resonant cavity, is related to the velocity of sound in the gas passing through it. A diagram of such a whistle is shown in figure 12. Nyborg et al. (38) showed that the frequency (fn) of the whistle could be described by the following equation.

Testerman and McLeod designed and built a detector based on the whistle principle. In their sensor design, typical values taken for the dimensions in the diagram, and variables in the equation, were (t), 0.064 mm, (d), 0.74 mm, (h), 1.676 mm and (L) 3.81 mm.


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.



Under the flow conditions normally used for GC separations, frequencies ranging from 30-50 kHz (supersonic frequencies) were observed. The sensor contained two sound generators, one operating with pure carrier gas and the other with the eluent from the column. The two frequencies were allowed to beat together, the beat frequency being directly related to the frequency difference between the two whistles and consequently the density difference between the contents of the two sensors. An example of the use of the whistle detector to monitor the separation of a mixture of hydrocarbons is shown in figure 60. The sample size was 7.5 ml of gas mixture and the carrier gas flow rate was 180 ml /min. This chromatogram illustrates the effective use of the detector and the operating conditions shows its limitations. The sensitivity appears somewhat less than that of the katharometer but the very high flow rates necessary to activate the whistle restrict the use of this type of detector very severely. In the original report the linearity was stated to cover 2 orders of magnitude of concentration but with modern electronics it is likely that this linear range could be extended by at least another order of magnitude.


Figure 60 The Separation of a Mixture of Hydrocarbons Monitored by the Supersonic Whistle Detector

The Absolute Mass Detector
The absolute mass detector adsorbs the material as it is eluted from the column onto a suitable adsorbent and continually weighs the mass adsorbed.


This system was devised by Bevan and Thorburn [43,44], who adsorbed the eluent from a GC column on to the coated walls of a vessel supported on a recording balance. A diagram of their apparatus is shown in figure 61. The adsorption vessel was 1.4 cm I.D. and about 5 cm high. The walls of the vessel were coated with a high boiling absorbent such as polyethylene glycol or an appropriate normal hydrocarbon depending on the samples being trapped. Under such circumstances the solutes separated had to be relatively low boiling otherwise they would condense in the capillary connecting tube to the adsorption vessel. The tube dipped to the base of the absorber where a baffle was situated to direct the eluent to the walls of the adsorption vessel. The balance record represented an integral chromatogram, the step height giving directly the mass of solute eluted.


Despite the relative casual arrangement of the adsorbent, it would appear that the adsorption was quite efficient and, with 10 mg charges on the column, an accuracy of 1% could be easily achieved. Later Bevan et al [45,46], reduced the size of the absorber and employed charcoal as the adsorbing material.

Although this improved the performance of the detector and reduced the necessary sample size, the detecting system was never made commercially. Even after modification, its sensitivity was relatively poor and despite it being an absolute detecting system, it placed too many restrictions on the operation of the chromatograph and the samples that could be chromatographed to be generally useful.

The Surface Potential Detector
The surface potential detector was developed by Griffiths and Phillips [47,48] in the early 1950s and consisted of a cell containing two parallel metal plates between which flowed the column eluent. One plate was mechanically attached to an oscillator that vibrated the plate at about 10 kHz. If the plates are identical, the surface charge on each plate is the same and so no potential is induced into the second plate by the vibrating plate. If however the surfaces are dissimilar, then the surface charge on each plate will differ and the vibrating plate will induce a potential on the other plate. A diagram of the detector is shown in figure 62.




Both plates were constructed of the same metal but one plate was coated with a monolayer of a suitable substance that would absorb any vapors present in the column eluent. The absorbing layer caused the charge on the two plates to be dissimilar and thus a potential appeared acrossthetwoplates which was balanced out by the bias potentiometer. When a solute vapor passes through the detector, some is distributed into the absorbent layer, changing the surface charge and thus inducing a change in potential between the electrodes. This produces an AC signal voltage that can then amplified, rectified and the output passed to a recorder (or to a data acquisition system). The signals provided by the detector sensor could be as great as several hundred millivolts.


The sensitivity of the detector was claimed to be similar to that of the katharometer (i.e. about 10-6 g/ml). Its response was partly determined by the distribution coefficient of the solute vapor between the carrier gas and the absorbing layer (and thus the chemical characteristic of the coating) as well as the chemical nature of the solute itself. As a consequence, the response varied considerably between different solutes. Within a given homologous series, however, the response increased with the molecular weight of the solute, but this was probably merely a reflection of the increase in the value of the distribution coefficient with molecular weight. Although an interesting alternative method of detection, this detector has been little used in GC and is not commercially available

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