The errors occurred when α, the parameter that accounts for the non- ideal behavior of the epithermal flux, are neglected, have been evaluated for the determination of twenty elements in biological materials, using gold and sodium comparators, and zirconium to measure the thermal to epithermal flux ratio (f). Sodium allows the determination of 15 elements, and up to 18 elements can be determined in positions relatively well thermalized by the simultaneous use of gold and sodium comparators, in all the cases with errors ≤2%, when α is not considered. Results obtained for reference materials are presented.