Figure 21.

Chemically specific imaging of the fern Pteris vittata. The X-ray absorption spectra of standard solutions of arsenite and arsenate are compared in (a), and the incident X-ray energies selected for imaging are indicated as E₁ and E₂. The tip of one pinna (leaf) was selected for imaging (b), and the raw imaging data are shown in (d)–(f) with a micrograph (c), X-ray absorbance (d), and X-ray fluorescence at E₁ (e) and at E₂ (f). Using eqs 4 and 5, the analyzed data in (g)–(i) are obtained, with the concentration of arsenite (g) and arsenate (h), with maxima of 40 and 2 μM, respectively. The thickness (i) has a maximum value of 0.2 mm. Adapted with permission from ref (28). Copyright 2006 American Chemical Society.