The present invention provides method and apparatus for quantifying sample clean up in real time by providing curve-fitting measurements of optical or other physical properties of fluid downhole. Fluid is extracted from the formation surrounding a borehole. As fluid continues to be extracted the composition of the extracted fluid changes, altering the measured values of optical and physical properties of the fluid. Measurements are made of optical or physical properties of the sampled fluid, analysis is performed on the acquired measured data points.