Here is a rather nice, although conceptually intricate, way to get both selectivity, linear range, and LOD. The lab keeps on trekking with Gaussian diffusion type curves, but with some asymmetry.

The image is intended to illustrate that when an ion (like lead) is part of a complex, only a small portion moves onto the solid phase thus increasing the retention times. This allows the chromatographer to enhance separation of overlapping peaks thus achieving selectivity.

The exercise is relatively painless. It consists of varying the oxalic acid the viewing what happens to the chromatograms in excel. All you have to do is change the oxalic acid concentration. The rest is done for you. The second exercise is a dry lab with things you already know how to do: consisting of plotting and analyzing chromatograms, determination of retention times, resolution, and LOD. Enjoy!