A method and apparatus are provided for performing in-situ fluid analysis. The method involves obtaining a first and second mixture of uncontaminated oil and a contaminant, wherein a percentage of the uncontaminated oil in the first mixture is different from the second mixture. The method may further include establishing a rate of change of a physical property of the first mixture and the second mixture to estimate a mass density of the uncontaminated oil and a mass density of the contaminant. In addition, the method may include obtaining a volume fraction of the uncontaminated oil for the first mixture and second mixture using the mass density of the uncontaminated oil and the mass density of the contaminant. An optical device may be used to determine a composition of the first and second mixtures in order to calculate a composition of the contaminant and a composition of the uncontaminated oil.