$0.15–3.72THz$ absorption of aqueous salts and saline solutions

It has recently been suggested that near-field terahertz ionic contrast microscopy can be employed to image subtle changes in ionic concentrations arising from neuronal activity. To do so, however, requires that solvated ions exhibit significant absorbance at terahertz frequencies. The authors have investigated this issue and find that, at room temperature, the molar extinctions of both sodium chloride and guanidine hydrochloride are approximately two orders of magnitude below some previous measurements and are, therefore, too low to support the proposed imaging application.