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Calibration Curves 10 alibration Techniques C
Both the accuracy and precision of ICP measurements is dependent, in part, upon the calibration technique used. This section
is focused upon errors (both fixed and random) that can be introduced through the use of different calibration techniques
using accurate calibration standards, samples that have been prepared accurately to within defined error limits, and an
instrument that has been ‘set-up’ correctly using a procedure programmed where there are no spectral/mass interferences that
include background correction. You may believe that if the above errors have been confined to within acceptable and known
limits that there is nothing else to worry about. Unfortunately, this is not the case.
The most common calibration technique options for ICP measurements are calibration curve and standard additions. In
addition, the option of using internal standardization is available for the calibration curve technique and the ability of matrix
matching may also be available. ICP-MS has the added option of using an internal standard that is an enhanced isotope of
the element being measured (i.e., isotope dilution ICP-MS). This discussion will be limited to the above approaches, to the
introduction of the analyte as a nebulized solution, and to the use of Ar as the plasma gas.
Basic Considerations
Before reading ahead, it may be helpful to restate the assumptions made above and make some additional considerations:
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measurement is a comparison process.
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instrumental response that is described by the equation for a straight line.
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uncertainty of the prepared standard solution is known and has been calculated).
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defined limits of time, matrix, concentration, temperature/humidity, and container material(s).
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of the analyst. This assumption is made to allow us to focus completely upon the potential errors involved with the calibration
process.
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errors. It is therefore assumed that the uncertainty in preparation can be described by the random and known sampling,
weighing and volume dilution errors. Again, this is an assumption that is often not the case but is made to allow us to focus
completely upon the potential errors involved with the calibration process.
Calibrations Standards
ICP is a matrix-dependant technique. Based upon the above assumptions and the fact that ICP is a comparative method, the
prime concern is the availability and use of appropriate calibration standards. The problem analysts face is that ICP (ICP-OES
and ICP-MS) is extremely matrix-dependent. Therefore, the ideal situation is that the matrices of the standards and samples
be identical.
Recommendations
This section lists several recommendations. Discussions relating to these recommendations are provided in the next section
for the reader who would like more detail.
Recommendation (a) - Match the acid content of your calibration standards and samples in both the type of acid used and the
concentration of the acid.
Recommendation (b) - Match the elemental matrix components of your calibration standards and samples to the greatest
extent possible. In this situation, the analyst who knows the composition of the sample has this capability.