Variations of thermal-lens spectrophotoetry with helium-cadmium laser excitation for the determination of traces of phosphorus

Single-beam, differential single-beam and dual-beam thermal-lens measurements are compared for the determination of phosphorus traces by means of a helium-cadmium laser. Phosphate is determined as the molybdophosphate/auramine ion-pair which is extracted into 1:2 isobutanol/hexane; acetone is added to dissolve any ion-pair precipitate. In dual-beam thermal lens experiments a low-power HeNe laser is uded as the probe laser. The detection limit, restricted by variable blank measurement, is 0.6 ng cm^?3 phosphorus in all cases. The advantages and drawbacks of each measurement technique are discussed. The dual-beam system of measurement is more sensitive than the single-beam system and is appropriate for measurements on samples of small volume. The dual-beam system is applied for layer-by-layer determination of phosphorus in semiconductor silicon over the concentratin range 5 × 10^?8 ?5 × 10^?4%. The data obtained by the dual-beam method compare well with those obtained by a neutron activation method.