PROCESSING OF WEAK PHOTOTHERMAL SIGNALS FROM THE OPTOTHERMAL WINDOW (OW) CELL - APPLICATION TO THE DETERMINATION OF AMMONIUM ION IN WATER

The optothermal window (OW) cell is a compact photothermal device simi lar to an open photoacoustic cell. Using the c.w. He-Ne laser for exci tation and a piezoelectric transducer, the concentrations of ammonium ion (range 2 to 400 mmol/m(3)) in water were determined via the measur ement of the amplitude of the OW signal. Although theoretical signal p hase saturates in case of weak absorption, it was demonstrated here th at at specific modulation frequencies the uncorrected signal phase car ries spectrophotometric information as well. This is due to the fact t hat the phase of the background signal is shifted relative to that of the absorption signal. Simpler calibration procedure is an advantage i n phase measurements.