I devised a method of electron beam lithography for patterning thin-film silicon- nitride membranes with nanoscale metal arrays of known dimensions. Employing SEM image-based metrology and ellipsometry to determine all three spatial dimensions of the metal nanostructures in the array, the calculated mass of each individual one can be used as a standard for elemental X-ray microanalysis. Thereafter, the net X-ray counts from the patterned nanostructures on a thin silicon-nitride substrate and nanoparticle unknowns can be compared directly for a "standards-based" quantitative analysis. The minimum mass detectable by this technique is ~10-15 gm, supporting the determination of the absolute mass of nanoparticles with diameters as small as 20 to 30 nm. I then used a JEOL 6500F SEM equipped with a Bruker Quantax XFlash 4030 silicon drift detector (SDD) to assess the absolute masses of nickel nanoparticles contained in agglomerates of single-walled carbon nanotubes.