A device and method for sampling a fluid, especially a fluid of varying densities, are provided. The device includes a retention tube configured to receive a sample of the fluid, and can include a support jacket to support the retention tube. A control member is positioned within the retention tube with a plug attached to the distal end. The plug is movable away from the opening to facilitate laminar flow of the fluid into the retention tube. The plug can sealably engage the retention tube by affixing a portion of the control member. The device can be jolted to cause the control member to stretch, which disengages the plug temporarily, thereby releasing a lab sample portion from the retention tube. The rest of the sample of fluid can be released by completely releasing the plug from sealing contact.