Use of Long Cavities for Aqueous Samples in an Electron Spin Resonance Spectrometer

Abstract

IN a comment on the electron spin resonance spectrometer designed especially for aqueous samples which has been described recently by Cook and Mallard¹, Stoodley² has calculated that the method of using a multimode microwave cavity tends to degrade the sensitivity rather than improve it. I would like to point out that support for Stoodley's result is provided by a spectrum recorded by Varian Associates on their aqueous sample spectrometer and published in some recent advertising material³. The signal-to-noise for this spectrum is some thirty times better than for that presented by Cook and Mallard. Further, although the sensitivity was said to be comparable with that obtained by Commoner et al.⁴, in fact these workers claim a figure of 2.5 to 5 × 10⁻⁷ molar of Mn⁺⁺ while that of Cook and Mallard is 3 × 10⁻⁶ molar. The sensitivity of the Varian spectrometer expressed in these terms is 1 × 10⁻⁷. molar. The difference in sensitivity between that achieved by Cook and Mallard and by Varian can be attributed to three causes. First, sensitivity is proportional to the square root of the microwave power and, while Varian quote 180 mW, Cook and Mallard appear to be using a 723 A/B klystron which gives 20 mW. Hence the sensitivity drops by a factor of three. Secondly according to Stoodley, the cavity design should give a sensitivity which is 4.4 times worse than optimum. The remaining factor of 2.2 is due to deficiencies in the preamplifier used by Cook and Mallard.