No abstract

Amperometric electrochemical gas sensors have the advantage of combining good sensitivity and selectivity at relatively low cost. However, their use is restricted to the detection of gases that are very reactive at potentials positive of that at which oxygen is reduced. We describe here a new detection method for less reactive species such as benzene and halogenated hydrocarbons, which cannot be directly oxidized. Instead, the species is continuously adsorbed at a potential below 0.5 V and then oxidized at potentials where oxygen coadsorbs. The oxidation current is corrected for the oxygen adsorption current by performing an additional potential cycle, in which no adsorption is allowed to occur. We have so far obtained sensitivities in the lower ppm range for benzene, toluene, tetrachloroethene, epichlorohydrin, and vinyl acetate; for saturated halogenated hydrocarbons such as CFCl 3 , it is still below 1000 ppm.

No abstract

Quantitative electron energy-loss spectroscopy (EELS) needs a correction for the limited collection aperture α and a deconvolution of recorded spectra for eliminating the influence of multiple inelastic scattering. Reversely, it is of interest to calculate the influence of multiple scattering on EELS. The distribution f(w,θ,z) of scattered electrons as a function of energy loss w, scattering angle θ and reduced specimen thickness z=t/Λ (Λ=total mean-free-path) can either be recorded by angular-resolved EELS or calculated by a convolution of a normalized single-scattering function ϕ(w,θ). For rotational symmetry in angle (amorphous or polycrystalline specimens) this can be realised by the following sequence of operations :(1)where the two-dimensional distribution in angle is reduced to a one-dimensional function by a projection P, T is a two-dimensional Fourier transform in angle θ and energy loss w and the exponent -1 indicates a deprojection and inverse Fourier transform, respectively.