A new method is presented to analyze recombinative desorption from surfaces in an isothermal mode. The activation energy for desorption obtained this way is accurate as long as it is coverage independent. Second order recombinative desorption experiments of N-15(2) and D-2 from Ru(001) were used to demonstrate this method. The activation energies were E-a(N-2) = 48 +/- 2 kcal/mol and E-a(D-2) = 22 +/- 1 kcal/mol for coverages below 0.1 and 0.2 of saturation coverage, respectively. Studying N/Ru(001) provides evidence for bulk nitrogen atoms that slowly diffuse to the surface leading to isotope scrambling.