A sampling system for an optical cell containing a process fluid comprises a first bi-directional pump in fluid communication with a sampling path and a second bi-directional pump in fluid communication with the first bi-directional pump and a storage vessel. The first bi-directional pump is configured to withdraw a first sample of the process fluid and to cause the first sample to flow towards the first bi-directional pump. The second bi-directional pump is configured to withdraw a second sample from the storage vessel and to cause the second sample to move toward and mix with the first sample. A first rate of withdraw of the first sample is greater than a second rate of movement of the second sample toward the first sample, and the difference between the first rate and the second rate correspond to a pre-determined ratio of the first sample mixed with the second sample.