A technique facilitates obtaining samples of well fluid or other fluid. A sampling tool comprises a sample chamber for collecting the fluid sample. Access to the sample chamber is controlled with a rod shiftable within the sampling tool. Additionally, a locking mechanism works in cooperation with the rod to lock the rod against undesirable movement at various stages of the sampling operation. The locking mechanism may comprise a pair of locking features which engage a groove in the rod to initially block inadvertent closure of access to the sample chamber and subsequently to block inadvertent opening of access to the sample chamber after collection of the fluid sample.