91Ó°ÊÓ

Determine unknowns from a hand-drawn calibration curve. The following data are for a set of calibration standards for \(\mathrm{Zn}\), measured by atomic absorption spectrophotometry. Absorbance measurements for a series of standard solutions containing different amounts of zinc \begin{tabular}{ll} \hline Zinc concentration \(\left(\mu g ~ m L^{-1}\right)\) & Absorbance \\ \hline 0 & \(0.000\) \\ 1 & \(0.082\) \\ 2 & \(0.174\) \\ 3 & \(0.257\) \\ 4 & \(0.340\) \\ 5 & \(0.408\) \\ 6 & \(0.463\) \\ 7 & \(0.511\) \\ 8 & \(0.543\) \\ 9 & \(0.561\) \\ 10 & \(0.575\) \\ \hline \end{tabular} Draw a calibration curve by hand using graph paper and estimate the concentration of zinc in the following water samples: (a) an undiluted sample, giving an absorbance of \(0.157\) (b) a 20 -fold dilution, giving an absorbance of \(0.304\); (c) a fivefold dilution, giving an absorbance of \(0.550\). Give your answer to three significant figures in each case.

Determine unknowns from a calibration curve produced in Excel. The following data are for a set of calibration standards for protein content, measured by the Lowry (Folin-Ciocalteau) method (p. 384). Absorbance measurements for a series of protein standards \begin{tabular}{ll} \hline Protein (mg per tube) & Absorbance \(\left(A_{600}\right)\) \\ \hline \(0.00\) & \(0.000\) \\ \(0.02\) & \(0.161\) \\ \(0.04\) & \(0.284\) \\ \(0.06\) & \(0.438\) \\ \(0.08\) & \(0.572\) \\ \(0.10\) & \(0.762\) \\ \hline \end{tabular} Using PC-based software (e.g. Excel), fit a trend line (linear regression) and determine the protein content of solutions with the following absorbances: a) \(0.225\) b) \(0.465\) c) \(0.682\) Give your answers to three significant figures in each case.