Arrangement for determining the concentration of a substance in a

Abstract

A mixture of substances is analyzed by spectral analysis to establish the concentration of one component of the mixture. For this purpose signals U.sub.1 (.lambda.) representative of the spectrum of the substance under investigation and U.sub.2 (.lambda.) representative of the spectrum of the mixture of substances are manipulated electronically so as to remove the effects of cross-sensitivity and to yield an accurate value for the concentration of the substance in the mixture of substances. This concentration is normally difficult to determine because of the effects of cross-sensitivity, i.e. because other components in the mixture of substances have similar spectral lines to those of the substance under investigation. In one arrangement the two signals U.sub.1 (.lambda.) and U.sub.2 (.lambda.) are first differentiated in respective differentiating stages 13 and 14 and the differentiated signals are multiplied in a multiplier 17. The output signal from the multiplier is subsequently integrated in an integrator 20 and the output of the integration is a measure of the concentration of the substance under investigation. The signal F derived from the multiplier 17 has two components F.sub.N +F.sub.S. The component F.sub.N is a product of the wanted signal in respect of the substance under investigation from the reference source 11 and of the wanted component from the measured signal U.sub.2 (.lambda.). This product is always positive and thus leads to a maximum value at the output of the integrator. The component F.sub.S is equivalent to a product of the wanted signal from the reference source with unwanted signals from the measurement and is in fact a value which statistically flucutates about zero. The result of integrating this component is thus a value which approximates to zero. In other words the result of the integration provides exclusively information about the substance under investigation.