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Calcjum in a sample solution is determined by atomic absorption spectrometry. A stock solution of calcium is prepared by dissolving 1.834 g CaCl,, 2H,0 in water and diluting to 1 L. This is diluted 1:10. Working standards are prepared by diluting the second solution, respectively, 1:20, 1-10, and 1:5. The sample is diluted 1:25. Strontium chloride is added to all solutions before dilution, sufficient to give 1% (wt/vol) to avoid phosphate interference. A blank is prepared, to cive 1% SrCl,. Absorbance signals are acquired by a data system as the solutions are aspirated into an air-acetylene flame, as follows: blank, 1.5 units; standards, 10.6, 20.1, and 38.5 units; sample, 29.6 units. What is the concentration of calcium in the sample in parts per million?