1.Introduction

Reference materials are substances with sufficient well-established properties, which are used for calibration of equipment, assessing measurement methods, chemical analysis and materials testing. The chemical measurement system that hast evolved over the years involves production, purification and characterization of chemical substances as reference materials. These may be classified as Standard Reference Materials, Certified Reference Materials and Reference Materials depending on certain established norms. Institutions such as NIST of USA, EURALAB, BAM of Germany and the Laboratory of the Government. Chemist, U.K. etc., are major providers of reference materials but there are an increasing number of commercial sources, e.g. RTC, who sell directly and also through the official organizations noted above.

2.Certified reference Materials (CRMs)

Reference materials, accompanied by a certificate,showing that one or more property values are certified by a procedure, which establishes their traceability to an accurate realization of the unit in which the property values are expressed, Each certified value is accompanied by an uncertainty at stated level of confidence.

3.Reference Materials (RMs)

Materials or substances, one or more of whose property values are sufficiently homogeneous and well established to be used for calibration of an apparatus, the assessment of a measurement method, or for assigning values to materials. These do not provide formal teraceability but are useful and less expensive alternatives for initial method development.

4.Calibration

Calibration is a set of operation that establish, under specified conditions, the relationship between the values of quantities indicated by a measurement instrument or measuring system or values represented by a material measure or a reference material , and the corresponding values realized by standards.

5.ISO/IEC 17025 & CRMs& RMs

Clause 5.6.1 " All equipment used for tests /or calibrations, including equipment for subsidiary measurements (e.g. for environmental conditions) having significant effect on the accuracy or validity of the results of the test, calibration or sampling shall be calibrated before being put into service."

Cause 5.6.2.1.2 "There are certain calibrations that currently, cannot be strictly made in SI Units, in these cases calibration shall provide confidence in measurements by establishing traceability to appropriate measurement standards such as: - the use of certified materials, provided by a competent supplier to give a reliable physical or chemical characterization of a material."

6.The International Standard / documents on CRMs and RMs

- ISO REMCO, the international committees provide the relevant information.

- ISO Guide 30: 1992 Terms and definitions used in connection with reference materials

- ISO Guide 31: 1996 Contents of the certificates of reference materials certificates of reference materials is are documents that contain summarized descriptions of all information on the reference material. This guide shows all the items to be written in the table of contents of the certificate of reference materials.

• Identity of producer and identification of the reference material
• A description of the material and its intended use
• Instructions on the correct use of the material
• The assigned property values and the method used to derive these values
• The date of certification and the period of validity of the certificate
• Safety instructions
• An indication of the level of homogeneity of the material
• Signature and the name of the certifying officer(s) signing an analysis certificate; legal consideration

In the case of certified reference materials, the traceability and a statement of the uncertainty interval at a stated level of confidence must also be provided to clients.

- ISO Guide 32: 1997 "Calibration of chemical analysis and use of certified reference materials"

This guide contains general recommendations for those who are involved in quality assurance of testing and/or calibration laboratories,

- ISO Guide 33: 2000(E) "Uses of certified reference materials" In this guide, various terms concerning statistics are defined to express the uncertainty of the result of measurement or analysis in the form of confidence in the certified value.

ISO Guide 34: 2000 "Quality system requirements for reference material producers"

Since confidence in chemical measurements depends on the confidence in the reference material used for the measurements, it is necessary to assure the reference material. The basic policy of this guide is not to prove the quality of each reference material, but to assure the quality of the reference material by proving the scientific and technical competence of the producer of the material. It contains the requirements for the producers of reference materials.

Describes the role of reference materials in measurement science, measurement uncertainty, homogeneity of materials, general principles of certification, certification by definitive methods, certification by inter-laboratory testing, certification based on the metrological approach. ISO REMCO Document N 330.

"Contains a list of producers of certified reference materials, Information by task force 3 "promotion"

Increasingly accreditation bodies are developing accreditation of reference materials providers, to ISO Guide 34 soon to become ISO 17034. this is beginning to lead to a situation where the only reference materials recognized for supporting accredited tests will be those from ISO Guide 34 accredited providers.

7.Types of reference materials

• Pure substances characterized for chemical purity and/or trace impurities.
• Standard solutions and gas mixtures, often prepared gravimetrically from pure substances and used for calibration purposes.
• Matrix reference materials, characterized for the composition of specified major, minor trace chemical constituents. Such materials may be prepared from matrices containing the components of interest, or by preparing synthetic mixtures
• Physico-chemical reference materials characterized for properties such as melting point, viscosity, and optical density.
• Reference objects or artefacts characterized for functional properties such as taste, odour, octane number, flash point and hardness. This type also includes microscopy specimens characterized for properties ranging from fibre type to microbiological speciation.

8.Classes of Reference Materials

If the materials are available from other sources it may not be cost effective for laboratories do make working standards of their own. However when this is necessary , the guidance given in (i) ILAC requirement for the competence of reference material producers & (ii) X R Pan , Metrologia, 34, 35-39, 1997 must be complied with by non specialist laboratories, to ensure acceptance by accreditation bodies.A few of the issues that need to be addressed are: selection of materials (appropriateness, native materials versus spikes, material preparation etc.), homogeneity testing, preparation and packaging( homogeneity, contamination, stability etc.) , stability testing, certification studies, uncertainty estimation, documentation and QA, certification approval, storage and distribution.

9.Traceability of reference materials

The use of appropriate reference material can provide essential traceability for chemical measurements and enable analysts to demonstrate the accuracy of results,satisfactory calibration of equipment and methods, enable monitoring of laboratory performance and validity of methods.

Two types of materials are recognized by ISO REMCO, ( i ) Certified reference materials (CRMs) and (ii) Reference materials (RMs).

CRMs, must by definition be traceable to an accurate realization of the unit in which the property values are expressed, and for which each certified value is accompanied by an uncertainty at a stated level of confidence. Compared to CRMs, RMs are of lesser Quality and by definition are materials whose property values are sufficiently homogeneous and well established to be used for calibration of an apparatus, the assessment of a measurement method, or for assigning values to materials.

10.Uses/role of CRMs and RMs

• To calibrate measurement systems/analytical instruments
• CRMs are used for method validation, development and evaluation

• CRMs are used in quality assurance programs.
  Laboratories also use reference materials for the quality assurance of analytical measurements. When unacceptable discrepancies arise in sample results between analysts using different methodologies, or the same method, the use of reference material cal help identify and resolve the analytical problem. If analytical results are to be meaningful, the laboratory must adhere to a defined quality assurance program. Quality assurance in this context will be the abidance to a set of procedures meant to ensure validity of analytical results for intended purpose.

• CRMs are sometimes used in PT program/inter laboratory comparison or round robin programs

• CRMs for development of in-house standards
  Reference materials that are produced by laboratories (in-house) are generally used on day to day basis. They can be verified by analysis alongside CRMs. The attraction of using in house standards is that they provide a relatively cheap option as compared to using certified reference material. It must be remembered that in-house reference materials do not replace CRMs, but only enable CRMs to be used on less frequent basis.

11.Selection of Reference material

In selecting a reference marital for a particular application, it is the responsibility of the analyst to assess its suitability. As such, it is necessary to consider all or some of the following factors before selecting a material:

• Matrix match and potential interferences
• Measurands (analytes)
• Measurement range concentration
• Measurement uncertainty
• Certification procedure used by the producer and accreditations held
• Documentation supplied with the material ( certificate report etc.)

Reference:

• ISO Guide 31.
• Use of reference materials in laboratories, by John Eames, NATA NEWS Volume 94.
• The selection and use of reference material by Bernard King, NATA NEWS VOLUME 95.