Method of Standard Addition
Source: Laboratory of Dr. Paul Bower - Purdue University
The method of standard additions is a quantitative analysis method, which is often used when the sample of interest has multiple components that result in matrix effects, where the additional components may either reduce or enhance the analyte absorbance signal. That results in significant errors in the analysis results.
Standard additions are commonly used to eliminate matrix effects from a measurement, since it is assumed that the matrix affects all of the solutions equally. Additionally, it is used to correct for the chemical phase separations performed in the extraction process.
The method is performed by reading the experimental (in this case fluorescent) intensity of the unknown solution and then by measuring the intensity of the unknown with varying amounts of known standard added. The data are plotted as fluorescence intensity vs. the amount of the standard added (the unknown itself, with no standard added, is plotted ON the y-axis). The least squares line intersects the x-axis at the negative of the concentration of the unknown, as shown in Figure 1.
Figure 1. Graphic representation of method of standard addition.
1. Preparing the Reagents
- 100 ppm standard Al3+ solution: Dissolve 0.9151 g aluminum nitrate (Al(NO3)3•9H2O) into a 1-L volumetric flask with DI water.
- 8HQ solution in 1 M acetic acid (2% wt/vol): Add 2.0 g of 8-hydroxyquinoline to a 100-mL volumetric flask.
- Carefully add 5.74 mL glacial acetic acid to the 100-mL flask, then dilute to the mark with DI water. This allows the 8-hydroxyquinoline to dissolve in aqueous phase.
- 1 M NH
A scan of the excitation wavelength from 335–435 showed the highest absorption at 399 nm, so the excitation monochromator was set for that value. Then the emission scan was performed from 450–550 nm, and the strongest signal was found to be at 520 nm. These are the wavelengths that are used for all of the samples.
