Light-element analysis

For present purposes ‘light’ elements are defined as those below 10 in atomic number. Of these, H and He do not produce characteristic X-rays, and the Li K line is outside the accessible wavelength range: the elements (and atomic numbers) concerned are thus Be (4), B (5), C (6), N (7), O (8) and F (9). The K lines of these elements are listed in Table 8.1. Light-element analysis requires an approach that differs in certain respects from that described in the previous chapter for ‘ordinary’ (Z > 10) elements.

The F Kα peak lies easily within the range of the TAP crystal, but O Kα is close to (for some instruments beyond) the upper limit of θ. Lead stearate (2d = 100 Å) covers atomic numbers 5 (B) to 8 (O), but has been superseded by evaporated multilayers (Section 5.3.1), which give more intensity but poorer resolution. To detect long-wavelength X-rays a proportional counter with a thin window must be used (Section 5.3.4). There are potential interferences from high-order lines and the L and M lines of heavier elements, but these are small. Light-element K lines can be detected by thin-window ED detectors (Section 5.2.1) and are reasonably well resolved from each other.

Contamination with carbon (Section 3.10.1) gives rise to a spurious C peak, as well as causing additional absorption of emerging low-energy X-rays (especially O Kα, which is just above the absorption edge of carbon). Anti-contamination measures should therefore be used routinely for light-element analysis.