Question

Why is the “standard additions” a better method compared to the external calibration method?

Answer 1

The method is usually presented as the separate addition of several different equally-spaced amounts of analyte to separate aliquots of test solution. Measurement is followed by extrapolation of the calibration line to zero response.The use of several spiking concentrations is justified in the standard paradigm by the idea that it helps to check that the calibration is truly linear. However, this rationale is not compelling in a routine, quality-assured laboratory, because:

• you should not attempt standard additions unless you are quite convinced at the validation stage that the analytical calibration is truly linear over the whole relevant concentration range (nonlinear extrapolation being an unwise enterprise);

• tests for non-linearity require a large number of measurements to have a useful degree of statistical power. That would require far more work for the analyst than is reasonable for obtaining a single measurement result.

Standard addition method:

1.Most convenient when a small number of samples are to be analyzed.

2.Useful when the analyte is present in a complicated matrix and no ideal blank is available.

-Add one or more increments of a standard solution to sample aliquots of the same size. Each mixture is then diluted to the same volume.

-Prepare a plot of Analytical Signal versus:

a)volume of standard solution added, or

b)concentration of analyte added.

The method of standard additions is usually followed to eliminate matrix effects. Experimentally, equal volumes of the sample solution are taken, all but one are separately ‘spiked’ with known and different amounts of the analyte, and all are then diluted to the same volume. The instrument signals are then determined for all these solutions and the results plotted. As usual, the signal is plotted on the y-axis; in this case the x-axis is graduated in terms of the amounts of analyte added (either as an absolute weight or as a concentration). The (unweighted) regression line is calculated in the normal way, but space is provided for it to be extrapolated to the point on the x-axis at which y = 0. This negative intercept on the x-axis corresponds to the amount of the analyte in the test sample. This value is given by a/b, the ratio of the intercept and the slope of the regression line.