FIGURE 3. Plotted net peak area vs. concentration of the spiked and unknown expanded vermiculite samples. The triangles are the spiked calibration samples used to create the regression equation: amphibole ppm = 14632(702) x 110 peak area. The squares are the Libby samples fit to the regression line based upon their measured 110 amphibole net peak areas. All the Libby samples contain less than 10 000 ppm (or 1%) amphibole.