Internal Standardization and Isotope Dilution

Standard Additions

The technique of standard additions is used when the matrix is quite variable and/or when an internal standard that corrects for plasma related effects couldn't be found. This technique is also useful in confirming the ability of an internal standard calibration curve technique to correct for both nebulizer and plasma related effects (see part 10 of this series for more on nebulizer and plasma related matrix effects). The following considerations may prove useful in performing the technique of standard additions:

• Split the analytical (sample) solution accurately. For example, if the final sample solution is made to 100.00 grams, then remove exactly 50.00 grams of solution to a separate clean container for spiking.
• The technique of standard additions requires a linear response. It is therefore important to work within the linear working range for each analyte.
• It is beneficial to perform a quick semi-quantitative analysis of the unknown to estimate analyte levels so that the analyst can spike the unknown solution with a concentrate of the analyte(s) of interest to levels of between2x_? and 3x_? where x_? represents the unknown concentration(s) of the analyte(s) of interest.
• Many analysts prefer to make more than one spiked level (i.e., 2x_?, 3x_?, 4x_?, and 5x_?). As with all techniques, a primary concern is in making an accurate spiked addition. For ICP, an additional concern is drift. The objective is to make an accurate measurement. Rather than making multiple spiked additions where drift is given more ground to introduce error, it is suggested that the analyst measure the sample along with a single spiked sample several times to account for drift. A reasonable measurement sequence would be:

  blank sample blank spiked sample blank sample blank spiked sample blank sample blank

  —where an average of all measurements is taken for the final calculation. The above analysis sequence assumes linear drift that should be confirmed before acceptance of the data.

• Attempt to keep the spiking volumes low. For example, a spike of 100 µL to a 50.00 gram sample aliquot represents a 0.2% relative error. If larger spiking aliquots are required then an equal volume of 18 MO water should be added to the unspiked sample portion to cancel out volume dilution errors.
• The technique of standard additions assumes that the instrumental response is described by the equation of a straight line with x,y coordinates of 0,0 as follows:

(1)  Y_I = mx_?

where Y_I = intensity of the sample,
m = slope,
and x_? = concentration of the unknown analyte

When a spike addition is made the equation becomes:

(2)  Y_k = m(x_? + x_s) = mx_? + mx_s

Where x_s = the concentration contribution from the spike addition to the analyte concentration
and Y_k = intensity for the spiked sample

• Note that the above equation (1) relating intensity (Y) to concentration (x) requires that the intensity is zero at an analyte concentration of zero. It is therefore necessary that the signal intensities be background corrected.
• The analyte concentration is determined as follows:

Subtract the intensity of the spiked from the unspiked sample solution and divide this by the concentration of the analyte spike to calculate the slope (m)

Y_k - Y_I = mx_? + mx_s - mx_? = mx_s

(Y_k - Y_I) / x_s = m

Substitute the value for m into equation (1) along with the intensity (Y_I) to calculate the unknown analyte concentration (x_?)

The technique of standard additions offers the best possible solution to matrix interference through plasma related effects. The technique it requires an accurate background correction of the analytical signal intensities and does not account for instrument drift. For unknown matrices, it may well be the fastest approach. When using standard additions on unknown matrices, it is possible to have severe spectral and background correction problems. It is cautioned here that at least two spectral lines should be used and the spectral region carefully scanned and studied.

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