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.
