In the following paper, the mean dry weight % of sediment-bound iron (0.15%) is equated to the concentration in μM (29.0 μM). I'm wondering what the calculations/equations behind this are, as I'd like to do the same for the whole dataset.

https://www.nature.com/articles/ncomms4929#Abs1 - Ice sheets as a significant source of highly reactive nanoparticulate iron to the oceans (Hawkings et al., 2014).

Here's the part I'm referring to - 'Mean ascorbate-extractable Fe was 0.15% (dry weight; Table 1), equating to 29.0 μM of potentially bioavailable Fe.'

Any help would be very much appreciated!