. The concentration of arsenic in each standard solution corresponds to the verticalgrid lines of the calibration curve. This curve is then used to determine the concentration of the two unknown solutions shown on the right. We first find the absor bances of the unknowns on the absorbance axis of the plot and then read the corresponding concentrations on the concentration axis. The lines leading from the cuvettes to the calibration curve show that the concentrations of arsenic in the two deer were 16 ppm and 22 ppm respectively. Arsenic in the kidney tissue of an animal is toxic at lev els above about 10 ppm, so it was probable that the deel were killed by ingesting an arsenic compound.