Abstract: In a photo-thermal sensor for determining the concentration of a compound in a sample which includes an excitation light source generating a first light beam of a wave length which is well absorbed by the compound to be determined, a modulator and an optical lens system disposed in the first light beam such that the first light beam is constricted at the location of the sample through which it is directed, a probe light source generating a second light beam extending at a right angle to the first light beam, a beam divider arranged at the intersection of the first and second light beams whereby part of the first light beam is deflected at one side of the beam divider and the second light beam passes through the beam divider such that both beams incide on the sample anti a photo-sensitive detector arranged in the light beam path behind the sample, a diaphragm is arranged in the beam path directly after the sample and an expansion lens adapted to expand the second light beam at the location of the sample to a d

Abstract: The invention relates to a process for continuous or almost continuous separation of cesium ions from aqueous solutions having high concentrations of sodium and/or potassium ions by ion exchange with ammonium molybdophosphate (AMP).A quantitative cesium separation from aqueous solutions which are high in salts, especially from highly radioactive solutions which are strongly nitric and contain nitrate, is achieved, without having to consider bleeding of the AMP and/or undesirable local overheating in the exchanger.