Calibration and Standards

Calibration:

With the exception of absolute methods of analysis that involve chemical reactions of known stoichiometry (e.g., gravimetric and titrimetric determinations), a calibration or standardization procedure is required to establish the relation between a measured physico-chemical response to an analyte and the amount or concentration of the analyte producing the response. Techniques and methods where calibration is necessary are frequently instrumental, and the detector response is in the form of an electrical signal. An important consideration is the effect of matrix components on the analyte detector signal, which may be supressed or enhanced, this being known as the matrix effect. When this is known to occur, matrix matching of the calibration standards to simulate the gross composition expected in the samples is essential (i.e. matrix components are added to all the analyte standards in the same amounts as are expected in the samples).

There are several methods of calibration, the choice of the most suitable depending on the characteristics of the analytical technique to be employed, the nature of the sample and the level of analyte(s) expected. These include: ● External standardization. A series of at least four calibration standards containing known amounts or concentrations of the analyte and matrix components, if required, is either prepared from laboratory chemicals of guaranteed purity (AnalaR or an equivalent grade) or purchased as a concentrated standard ready to use. The response of the detection system is recorded for each standard under specified and stable conditions and additionally for a blank, sometimes called a reagent blank (a standard prepared in an identical fashion to the other standards but omitting the analyte). The data is either plotted as a calibration graph or used to calculate a factor to convert detector responses measured for the analyte in samples into corresponding masses or concentrations (Topic B4).

● Standard addition.

● Internal standardization.

The last two methods of calibration are described in Topic B4.

Instruments and apparatus used for analytical work must be correctly maintained and calibrated against reference values to ensure that measurements are accurate and reliable. Performance should be checked regularly and records kept so that any deterioration can be quickly detected and remedied. Microcomputer and microprocessor controlled instrumentation often has built-in performance checks that are automatically initiated each time an instrument is turned on.

Some examples of instrument or apparatus calibration are

● manual calibration of an electronic balance with certified weights;

● calibration of volumetric glassware by weighing volumes of pure water;

● calibration of the wavelength and absorbance scales of spectrophotometers with certified emission or absorption characteristics;

● calibration of temperature scales and electrical voltage or current readouts with certified measurement equipment.

Chemical Standard:

Materials or substances suitable for use as chemical standards are generally single compounds or elements. They must be of known composition, and high purity and stability. Many are available commercially under the name AnalaR. Primary standards, which are used principally in titrimetry (Section C) to standardize a reagent (titrant) (i.e. to establish its exact concentration) must be internationally recognized and should fulfil the following requirements:

● be easy to obtain and preserve in a high state of purity and of known chemical composition;

● be non-hygroscopic and stable in air allowing accurate weighing;

● have impurities not normally exceeding 0.02% by weight;

● be readily soluble in water or another suitable solvent;

● react rapidly with an analyte in solution;

● other than pure elements, to have a high relative molar mass to minimize weighing errors.

Primary standards are used directly in titrimetric methods or to standardize solutions of secondary or working standards (i.e. materials or substances that do not fulfill all of the above criteria, that are to be used subsequently as the titrant in a particular method). Chemical standards are also used as reagents to effect reactions with analytes before completing the analysis by techniques other than titrimetry.

Some approved primary standards for titrimetric analysis are given in Table 1.

Reference materials

Reference materials are used to demonstrate the accuracy, reliability and comparability of analytical results. A certified or standard reference material (CRM or SRM) is a reference material, the values of one or more properties of which have been certified by a technically valid procedure and accompanied by a traceable certificate or other documentation issued by a certifying body such as the

Table 1: Some primary standards used in titrimetric analysis

Type of titration	Primary standard
Acid-base	Sodium carbonate, Na2CO3
	Sodium tetraborate, Na2B4O7.10H2O
	Potassium hydrogen phthalate, KH(C8H4O4)
	Benzoic acid, C6H5COOH
Redox	Potassium dichromate, K2Cr2O7
	Potassium iodate, KIO3
	Sodium oxalate, Na2C2O4
Precipitation (silver halide)	Silver nitrate, AgNO3
	Sodium chloride, NaCl
Complexometric (EDTA)	Zinc, Zn
	Magnesium, Mg
	EDTA (disodium salt), C10H14N2O8Na2

Bureau of Analytical Standards. CRMs or SRMs are produced in various forms and for different purposes and they may contain one or more certified components, such as

● pure substances or solutions for calibration or identification;

● materials of known matrix composition to facilitate comparisons of analytical data;

● materials with approximately known matrix composition and specified components.

They have a number of principal uses, including

● validation of new methods of analysis;

● standardization/calibration of other reference materials;

● confirmation of the validity of standardized methods;

● support of quality control and quality assurance schemes.