The methodology adopted at the Laboratory of Analytical Techniques of Instituto Peruano de Energía Nuclear for preparation of monitors and comparators intended for application in the k0-INAA method is described. The basic procedure consists of depositing a known amount of a primary or secondary standard solution of the element of interest onto a filter paper disc, which is subsequently dried and prepared as small tablets, to obtain geometry similar to that of the unknown samples. Sodium and zinc are regularly used as comparators, whereas molybdenum, gold, cobalt and lutetium are selected for determination of the reactor fluxes and neutron temperature. Two different methods are used for measurement of the masses delivered on the paper discs. On the basis of the protocols of weight or volume determination, the results of the analysis of certified reference materials and their associated uncertainty, using sodium comparator, are presented and discussed in the present work.