Instrumental Analysis 2

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Calibration

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Determines the relationship between the analytical response and the analyte concentration → by using chemical standards.

• Almost all analytical methods require calibration. Exceptions: gravimetric and some coulometric methods (absolute methods).

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Calibration procedures

Back 2

1- Comparison with Standards
2- External-Standard Calibration
3- Standard-Addition Methods
4- Internal-Standard Method

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1- Comparison with Standards

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• Direct Comparison: Involves comparing a property of the analyte with standards such that the property being tested matches that of the standard (e.g. early colorimeters)

• Titrations: Chemical comparison; The analyte reacts with a titrant (standard) in a reaction with known stoichiometry.
The amount of titrant needed to achieve chemical equivalence is related to the amount of analyte.

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2- External-Standard Calibration

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• External Standard: Prepared separately from the sample.
Used to calibrate instruments and procedures when there are no interference effects from matrix components.

• Calibration is accomplished by obtaining the response signal as a function of the [analyte]
- Response signal: absorbance, peak height, peak area… → Calibration curve.

• Multivariate Calibration: measuring multiple responses of an analyte or a mixture of analytes.

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3- Standard-Addition Methods

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• Useful for analyzing complex samples in which matrix effects can occur.

• Involves adding increments of a standard solution to sample aliquots containing identical volumes (=spiking).
Each solution is then diluted to a fixed volume before measurement

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4- Internal-Standard Method

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• Internal Standard: Is added to the sample itself. It is added in a constant amount to all samples, blanks and calibration standards.

• Involves plotting the ratio of the analyte signal to the internal-standard signal as a function of the analyte concentration of standards.