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Tutorial: Standard Additions |
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It is important to choose an appropriate concentration for the standard spikes that are added to the sample in the standard additions procedure. The volume should be chosen first: the volume should be large enough to be convenient to pipet but should be small enough to have negligible effects on the sample matrix. Addressing this last point: we want the same matrix to be present in every solution that is measured, so that the same interferences affect each measurement obtained.
Once the volume is chosen, the concentration must be calculated. The concentration should be small enough so that all measurements remain within the linear dynamic range of the analytical technique, but large enough to have a significant effect on the signal. If possible, I try to double the analyte concentration at some point during the addition procedure. Note that it may be necessary to dilute the original sample so that we are still within the linear response region after the standard is added.
The concentration of phosphorus in a 5mL sample of blood serum is to be determined using a colorimetric procedure. The analyst suspects that multiplicative matrix effects might bias her results, so she wishes to use a standards addition approach. How should she proceed? The analyte concentration is expected to be approximately 0.1 ppm, and the method is linear at least to 50 ppm.
Notice that some information is needed (linear range, typical analyte concentrations) in order to make an intelligent guess for the standard concentration. It may well be that you may have to adjust the standard additions procedure slightly once you use it for a particular application.
