Luminescence spectra obtained by electron bombardment of carbon nanotubes bundles show an UV signal centered at about 380 nm. We show that these transitions are extrinsic to nanotubes, in the sense that they are not linked to any carbon compound or structure, but are caused by the residual catalyst material used in the growth process. A possible role of such luminescence attributable to impurities is also discussed. In fact our luminescence measurements, in association with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and X-ray photoelectron spectroscopy (XPS), permit us to assign these spectral bands to Zn and Al oxides. Our conclusions, supported by data in literature, and the comparison with the quantitative results obtained by LA-ICP-MS spectrometry (which detects ppm) allows us to define cathode-luminescence analysis as a good technique to control cleanness and purity of carbon nanotube samples.