10. Calibration Curve and Standard Additions Techniques
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:
ICP is a 'Comparative Method' where the measurement of an unknown sample is based upon chemical standards i.e. the measurement is a comparison process.
It is not assumed that the calibration standards and samples have identical matrices.
It is assumed that the calibration is linear. This means that the standard and sample elemental concentrations give an instrumental response that is described by the equation for a straight line.
It is assumed that the analyst has prepared the chemical standards accurately to within defined error limits (i.e., the uncertainty of the prepared standard solution is known and has been calculated).
It is assumed that the stability of the standards, however and by whoever prepared, is known and are only used within these defined limits of time, matrix, concentration, temperature/humidity, and container material(s).
The uncertainty of the measurement of an unknown can only be worse (greater) than the uncertainty of the calibration.
It is assumed that there are no spectral/mass interferences. This of course is an area of great concern and effort on the part of the analyst. This assumption is made to allow us to focus completely upon the potential errors involved with the calibration process.
It is assumed that the sample prepared for analysis involves no positive or negative contamination errors and no sampling 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 you 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 you calibration standards and samples to the greatest extent possible. In this situation, the analyst who knows the composition of the sample has this capability.
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.
Recommendation (e) - "[C]hemical calibration is an approximation at best. The analytical chemist must be constantly aware of the possibility of bias introduced by the nature of the standards used, which may be the major source of bias in the analytical data. Appropriate reference materials should be used to evaluate this and other aspects of the measurement process."1
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