A method and apparatus is provided for use in connection with a downhole formation testing instrument for acquisition of a phase intact sample of connate fluid for delivery via a pressure containing sample tank to a laboratory facility. One or more fluid sample tanks contained within the instrument are pressure balanced with respect to the wellbore at formation level and are filled with a connate fluid sample in such manner that during filling of the sample tanks the pressure of the connate fluid is maintained within the predetermined range above the bubble point of the fluid sample. The sample tank incorporates an internal free-floating piston which separates the sample tank into sample containing and pressure balancing chambers with the pressure balancing chamber being in communication with borehole pressure. The sample tank is provided with a cut-off valve enabling the pressure of the fluid sample to be maintained after the formation testing instrument has been retrieved from the wellbore for transportation to a laboratory facility. To compensate for pressure decrease upon cooling of the sample tank and its contents, the piston pump mechanism of the instrument has the capability of increasing the pressure of the sample sufficiently above the bubble point of the sample that any pressure reduction that occurs upon cooling will not decrease the pressure of the fluid sample below its bubble point.