The variation of elemental composition in as-grown GaInNAs/GaAs quantum well structures has been investigated by energy dispersive X-ray analysis of cross-sections using a high resolution scanning transmission electron microscope. The formation of quaternary GaInNAs dot structures is indicated by low temperature photoluminescence measurements and by the correlation of indium and nitrogen distributions. The distributions of arsenic and nitrogen across the well structure suggest the presence of a continuous nitride-like layer formed at the surface of the GaAs buffer layer before the GaInNAs dots. The influence of this nitride-like interlayer on the mechanism of GaInNAs dot formation is discussed.