Calibration Curve and Standard Additions Techniques

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.

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