Box 4-2 Using a Nonlinear Calibration Curve

To find the quantity of protein, substitute the measured absorbance into Equation A  

Before using your calculator or computer to find the least-squares straight line, make a graph of your data. The graph gives you an opportunity to reject bad data or stimulus to repeat a measurement or to decide that a straight line is not an appropriate function. Examine your data for sensibility. 

It is not reliable to extrapolate any calibration curve, linear or nonlinear, beyond the measured range of standards. Measure standards in the entire concentration range of interest. 

In the preceding example, an unknown with a corrected absorbance of y = 0.302 had a protein content of = 18.24 pg. What is the uncertainty in the number 18.24 A full treatment of the propagation of uncertainty gives the following results 1-9 

To find values of t that are not in Table 4-2, use the Excel function TINV. For 12 degrees of freedom and 95% confidence, the function TINV(0.05.12) returns f = 2.179. 

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