On the application of the high frequency resonance method in the analysis of flame ionization

Abstract

The advantages and disadvantages of the h.f. resonance method in comparison with the microwave absorption methods are discussed. In particular, the capability of the h.f. resonance method to detect heavier ions may be noted. Besides, this method is more sensitive in the detection of electrons, in general, than the microwave method. A detailed theoretical analysis is given in order to understand the behavior of the resonant circuit, containing a parallel-plate condenser with a flame as dielectric. In this analysis two dimensionless parameters are introduced, both being functions (a) of the real and the imaginary part of the complex conductivity in the flame, and (b) of the ratio between the flame thickness and the interplate distance of the parallel-plate condenser. Under certain conditions these parameters enable us to calibrate the resonant circuit in terms of the electrical properties of the flame, without knowing exactly the values of the circuit elements. If the concentration of heavier ions exceeds that of the electrons, it seems to be possible to determine the mean collision frequency of the ions by varying the field frequency. In the experimental work described here, the electron concentration (as a function of the concentration of alkali metal introduced into the flame) was derived from the shift in resonant frequency as well as from the damping of the circuit. The results with both methods proved to agree with each other. The lower limit for the detection of electrons by measurements of the frequency bandwidth was found to be 3×1014 electrons/m3. From combined h.f. and additional optical measurements an electron collision frequency of about 8×1010 sec−1 was found. A discrepancy was noted between the degree of ionization found experimentally with several alkali elements and that predicted by the Saha formula at the measured flame temperature.