<img src='images/depthcomparison.jpg' alt='comparison of the sampling depth of XPS and contact angle tools' />

X-ray photoelectron spectroscopy (XPS) is a powerful tool for surface analysis, and combined with ion etching it can be used to compile a depth profile - an evaluation of the variation of composition with depth. The "sampling depth" of the technique, dependent upon a number of factors including the sample matrix and the photoelectron kinetic energy, is typically in the 5-10 nm range. One ought to ask, therefore, if the sample composition is homogeneous within the analyzed volume, because, if it is not, the XPS analysis will sample a range of compositions and return a weighted average.

As an example, whereas a water contact angle analysis "samples" to a depth of 0.5 nm, the XPS analysis samples much deeper, to a depth of 5-10 nm. Therefore, if the sample composition is not homogeneous to 10 nm, the two techniques will sample different compositions, as showned to the right.

<img src='images/graphconccomp.jpg' alt='measured % of oxygen for XPS and water contact angle techniques for a non-homogeneous composition' width="375" height="292"/>

In the case of this hypothetical oxygen concentration depth profile, a contact angle analysis will detect 25% oxygen, but the XPS analysis will sample the complete oxygen profile and mix in a contribution from deeper down, returning an apparent oxygen concentration, in an analysis made normal to the sample surface, of 13.9% (a consequence of the Beer-Lambert law).

Ideally, therefore, every XPS analysis should consist of the determination of the complete concentration depth profile within the analyzed volume. In many samples, this can be accomplished by means of ion etching. There are occasions, however, when ion etching is inappropriate, for example in the case of polymers, when the sample would undergo chemical degradation. In such cases one might employ angle-resolved XPS, in which the sample is tilted so as to vary the angle between the axis of the photoelectron analyzer and the normal to the sample surface (the photoemission angle  θ.)

XPS data is collected at a series of angles θ from 0° to 75° photoemission angle. Five or six angles are usually sufficient. Cumpson has recommended acquiring data at 0°, 40°, 55°, 63° and 70°, thereby concentrating the data acquisition at the higher photoemission angles, where the analysis is most sensitive to the sample surface.

<img src='images/analyser.jpg' alt='' width="271" height="320"/>