No abstract

No abstract

No abstract

SUMMARY Some problems of the quantitative analysis of diffusible elements in cryosections are reviewed. The two prevalent methods for obtaining concentrations from X‐ray data, one based on characteristic radiation alone and the other on continuum‐normalization, are recapitulated. Both methods seem suitable at cellular level while the latter seems preferable at finer spatial resolution. Recourse to both methods together is desirable in the analysis of frozen‐hydrated sections especially when there is no peripheral standard. Selective local contamination is a particular hazard in the analysis of chlorine. In the case of sodium, physical parameters set restrictive limits to the minimum concentration measurable by ‘energy‐dispersive’ X‐ray spectrometry (about 20 mm kg−1) and to the spatial resolution attainable by diffractive X‐ray spectrometry (∼0·2 μm). One obvious danger to meaningful quantitative analysis is inadvertent redistribution of diffusible elements during the moments preceding the freeze‐quenching of a tiny piece of tissue. Data are presented to show that concentration changes due to simple evaporation are a real hazard prior to the quenching of sub‐millimetre size samples.

The principle of the microprobe analysis of chemical elements is illustrated in Fig. i. Some kind of radiation is directed on to the specimen, generating signals characteristic of the elements present. Local analysis in situ is achieved in one of two ways. Most often the impinging beam is finely focused so that the signal at any moment comes only from a selected microregion. Alternatively, in some instruments, the impinging beam floods a larger region but the emergent signals characteristic of a particular element may be selected and focused to give an elemental ‘map’ or ‘image’.