The accurate determination of fluid properties and contamination while sampling with a wireline pump-out formation tester is essential to achieve the primary objective of obtaining representative reservoir fluid samples with minimum rig time. Despite advancements in fluid identification sensors, sampling in mixed phases, especially immiscible fluids, still poses a great challenge. It often happens that apparent erratic sensor responses are attributed to sensor noise, but careful study reveals that the sensors are actually showing the true nature of the multi-phase fluid flow. However, if this multi-phase behavior is not considered, it can be difficult to determine fluid type and contamination. This work addresses the development of a new numerical and analytical investigation that makes it possible to not only understand the cleaning behavior of formation fluids but also quantitatively determine fluid qualities in real time. This newly developed technique highlights the variables that play an important role in guiding the clean-up process and, at the same time, provides the temporal characteristics of the contamination level versus both time and fluid volume. Furthermore, uncertainties in the pumping time required to achieve the desired level of contamination are also calculated with this method. Field examples from Gulf of Mexico, South America and North Sea are provided to demonstrate the efficiency of this technique in oil-based mud and water-based mud contamination examples for both hydrocarbon fluid and formation water samples, with comparisons to PVT laboratory measurements. An error analysis is performed for each example, and results are presented in this research.The new analysis technique is applied to a high-resolution fluid density sensor that monitors the change of resonance frequency of a vibrating tube-carrying fluid sample. The same interpretation method is also applied to a capacitance sensor and a resistivity sensor to further confirm the results derived from the density sensor.