Measurements of internal hydrogen isotopic composition of plasmas in JET and elsewhere have been lacking. In this report we show that it is feasible to determine the profile of relative hydrogen isotope density, using a neutral particle analyzer. Using detailed modeling of the formation of atomic flux and its measurement, we have quantified sensitivity of the deduced plasma composition to uncertainty of plasma parameters that impact on the measurement. We have tested application of the proposed methodology by experimentally determining the perturbed deuteron density profile in a hydrogen plasma, when the deuteron density at the plasma edge was modulated using a short pulse of deuterium gas. We have also determined the deuteron density diffusivity. The determinations are very crude at present; however this exercise is intended to enable us to design better experiments to determine plasma isotopic composition and the radial isotope ion diffusivities.