Figure 2.

Calibration curve depicting the net XRF signal (a.u., arbitrary units) as a function of GNP concentration at 0, 0.01, 0.02, 0.03, 0.05, 0.06, 0.08, 0.1, 0.3, 0.5, and 1.0 wt. %. The source was operated at 125 kVp, 24 mA with a 1.8‐mm Sn filter. The XRF signal acquisition time was 10 s. The shaded area represents signal less than 1.96 times the standard deviation of the background (noise level). The lowest GNP concentration that was definitively detectable under these irradiation conditions was 0.03 wt. % or 0.3 mg/cm³. [Color figure can be viewed at wileyonlinelibrary.com]