  With the development of affordable aberration-correctors, analytical scanning
transmission electron microscopy (STEM) studies of complex interfaces can now
be conducted at high spatial resolution at laboratories worldwide.
Energy-dispersive X-ray spectroscopy (STEM-EDS) in particular has grown in
popularity, since it enables elemental mapping over a wide range of ionization
energies. However, the interpretation of atomically-resolved data is greatly
complicated by beam-sample interactions that are often overlooked by novice
users. Here we describe the practical factors---namely, sample thickness and
the choice of ionization edge---that affect the quantification of a model
perovskite oxide interface. Our measurements of the same sample in regions of
different thickness indicate that interface profiles can vary by as much as 2-5
unit cells, depending on the spectral feature. This finding is supported by
multislice simulations, which reveal that on-axis maps of even perfectly abrupt
interfaces exhibit significant delocalization. Quantification of thicker
samples is further complicated by channeling to heavier sites across the
interface, as well as an increased signal background. We show that extreme care
must be taken to prepare samples to minimize channeling effects and argue that
it may not be possible to extract atomically-resolved information from many
chemical maps.
