This document provides a protocol for measuring the mass fraction values of gold (Au) nanoparticles in suspensions of Au nanoparticles similar to National Institute of Standards and Technology (NIST) Reference Materials, RM 8011 Gold Nanoparticles Nominal 10 nm Diameter, RM 8012 Gold Nanoparticles Nominal 30 nm Diameter, and RM 8013 Gold Nanoparticles Nominal 60 nm Diameter.