Abstract: A common problem with reverse circulation downhole hammers is retention of the sample tube that extends through the actual hammer component., There is a tendency for the sample tube to become loose in its mounting, and the resulting movement through vibration results in accelerated wear which rapidly leads to failure of the component In this application, the sample tube for use in a reverse circulation downhole hammer comprises a sample tube that has a lower end connected to a drill bit sample delivery aperture and an upper end connected to a drill string sample delivery tube. The sample tube has at least one projection intermediate the upper and lower ends of the sample tube that has a non-circular cross section extending radially from the sample tube. The projection locates within a recess that has a corresponding cross sectional shape, the recess being fixed with respect to the hammer to prevent rotation of the sample tube with respect to the hammer.