11. Internal Standardization and Isotope Dilution
Part 11 is a continuation of discussions around the recommendations made in Part 10. Please reference Part 10 before you proceed.
Discussion (c and d) - For the sake of convenience, recommendations (c) and (d) are reprinted from Part 10 as follows:
Recommendation (c) - With unknown sample matrices, matching is not possible and is most accurately dealt with using the technique of standard additions. However, this approach is slow as compared to the calibration curve technique with the use of internal standardization.
Recommendation (d) - The use of internal standardization is very effective in many cases but may introduce--or not correct for--all errors. This statement does not apply to isotope dilution
ICP-MS that is considered to be a primary analytical technique.
Matrix effects are arguably the subtlest danger to the ICP-OES analyst. Slight differences in the matrix can cause a considerable systematic error. The most common calibration technique options for ICP measurements are calibration curve and standard additions.
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 礚 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) YI = mx?
where YI = intensity of the sample,
m = slope,
and x? = concentration of the unknown analyte
When a spike addition is made the equation becomes:
(2) Yk = m(x? + xs) = mx? + mxs
Where xs = the concentration contribution from the spike addition to the analyte concentration
and Yk = 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)
Yk - YI = mx? + mxs - mx? = mxs
(Yk - YI) / xs = m
Substitute the value for m into equation (1) along with the intensity (YI) 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.