Recently the first observation of a channelling effect in oxygen K-she
ll X-ray emission was reported in this journal by Qian et al. [Ultrami
croscopy 41 (1992) 147], who pointed out that this would allow oxygen
to be used as a reference site in atom location by channelling enhance
d microanalysis (ALCHEMI) applied to oxides. Experimental results for
characteristic X-ray emission in spinel are reported which confirm the
ir results. Calculations of ionization cross sections and the effectiv
e ionization potential for each of the constituent atoms of the spinel
MgAl2O4 have been carried out from first principles. This allows a qu
antitative theoretical insight into the channelling effect for the lig
ht elements Al, Mg and O which incorporates delocalization of the ioni
zation interaction.