Tutorial: Standard Additions

Quick and Dirty: Single Standard Additions

It is possible to calculate the analyte concentration with only a single standard addition. If you do so, you must realize that

• the linearity of the signal cannot be checked
• precision suffers, since only two measurements are used to determine analyte concentration
• you cannot determine the standard deviation of the point estimate by analysis of residuals, because there are no residuals in a two-point standard additions plot

Despite these disadvantages, there are definitely times when a single addition is tempting. The standard additions procedure can be tedious, especially if there is a long line of samples waiting to be analyzed. If measurement noise is only a small component of the overall analysis uncertainty - for example, if sampling or sample preparation is the dominant error source, as is common - then the loss of measurement precision, and the ability to estimate its magnitude from the residuals of a standard addition plot, is acceptible. Since linearity is assumed in the data analysis procedure, you must be certain that the measurements are within the linear dynamic range of the technique when using this procedure.

Example

The concentration of copper in brandy may be measured by atomic emission spectroscopy using an inductively coupled plasma. After measuing the emission signal from pure brandy, a 50.00 mL aliquot is spiked with 1.00 mL of 100.0 ppm copper standard. The emission signal for the pure sample was 2.11 units, and for the spiked sample it was 6.37 units. Calculate the concentration of copper in the brandy.

Answer: 0.912 ppm

The procedure in the previous example only results in a point estimate of the analyte concentration, rather than a confidence interval. If measurement error is a small component of the overall error, that may be acceptible - since other samples would have to be analyzed to determine the overall uncertainty in the analytical procedure.