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Tutorial: Standard Additions |
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The method of standard addition assumes a linear response between measured signal and analyte concentration (except in the case of applying the standard additions method to potentiometric analysis); to be more specific, it is assumed that the signal measured for the sample (with no spike) is within the linear region of the calibration function. This assumption is not as easy to justify when the standard additions plot is nonlinear. Nevertheless, if the nonlinearity is not too severe, a reasonably accurate confidence interval can be calculated for the analyte concentration. In this section, I will demonstrate a method to obtain a confidence interval for nonlinear standard addition plots. If the nonlinearity of the plot is severe, this result should be viewed with great skepticism.
Cadmium in soil was analyzed by extracting a 1.2901g soil sample with acid followed by measurement with graphite furnace atomic absorption spectroscopy. The soil extract was diluted to 100.00 mL and then separated into five 20.00 mL aliquots. The concentration of cadmium was determined using the standard addition procedure: after spiking three of the aliquots with varying volumes of 10.0 ppm cadmium solution, the following measurements were obtained.
| Volume of spike, mL | Peak Area, AU |
|---|---|
| 0.00 | 0.238 |
| 0.10 | 0.445 |
| 0.20 | 0.622 |
| 0.30 | 0.776 |
| 0.40 | 0.895 |
Based on these measurements, report the concentration of extractable cadmium in the original soil sample as a 95% confidence interval.
Answer:
3.31 +/- 0.18 ppm (95% CL) extractable cadmium in the soil sample.