Hollow cathodes for OES

The energetically important parts of the discharge (cathode layer, dark space, and negative glow) as well as the sample are inside the cathode cavity. The volatilization results from cathodic sputtering and/or thermal evaporation. This depends on whether the whole cathode with its outer and inner wall is subjected to sputtering without any cooling (hot hollow cathode), the outer wall is shielded by a quartz 

The characteristic depends on the discharge gas (at a few mbar of argon i = 0.2-2 A, V = 1-2 kV at 10-20 mbar of helium i = 0.2-2 A, V 1 kV) as well as on the cathode mounting. Indeed, in a cooled cathode the characteristic is normal and the analyte volatilizes by cathodic sputtering only, whereas in the hot hollow cathode thermal evaporation also takes place, by which the characteristic, especially at high currents, may become normal. In this case, thermal effects could even lead to a strong selective volatilization, which can be made use of analytically. 

The latter was shown to occur in the case of brass, as could be demonstrated by electron probe micrographs of partly molten brass samples, the outer layers of which are less rich in Zn [551]. 

In this sense, it has found wide use for analysis of dry solution residues, e.g. in the case of the rare earth elements, or after matrix separation in the analysis of high-purity substances (see Refs, in [552]). The Doppler widths of the lines are low and accordingly this source has even been used for isotopic analysis (determination of u235 jj238) -j-q jg g.(.j]] employed for the determination of volatile elements 

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