A sleeve or cover for preventing the production of secondary x-ray signal contamination from an analytical x-ray tube is disclosed. The x-ray tube includes an evacuated enclosure in which is disposed a cathode and anode. The sleeve or cover is useful in applications such as x-ray fluorescence spectroscopy for improving the spectral purity of the primary stream of x-rays produced by electron bombardment of the anode target surface by the cathode. In one embodiment, the sleeve is disposed about a portion of the anode substrate, and is comprised of beryllium. Electrons back-scattered from the target surface are attracted to the anode substrate and impact the beryllium sleeve, producing secondary x-rays that are not detected by spectroscopic detectors and are therefore not contaminating to the primary x-ray stream.