The F Test
3.2 Methodology, Terms and Definitions
3.2.7 Legal Importance of Results, Traceability and Other Related Concepts
(iii) If there are any multiplicative interferences, their effect for analyte and inter- nal standard should be identical. This condition may be more difficult to ful- fil as compared to the others, since most of the multiplicative interferences are chemical in nature and therefore may be selective for analyte.
(iv) Internal standard should not cause any interference.
Internal standard approach is used best in minimizing the multiplicative interfer- ences which function in a random manner.
In many cases, the absolute amount of the interferant is more effective to induce an observed interference than the interferant/analyte ratio. This behaviour allows one to dilute the sample to a point where the interference is not observed; naturally, at this point analyte concentration should be still higher than LOQ.
In some cases, the interference effect reaches to a saturation point; further increase in interferant concentration does not affect the analytical signal any more. If this is the case, it is possible to add more interferant to the sample until the analytical sig- nal remains constant. At this point, the method of standard additions may be useful.
3.2.7 Legal Importance of Results, Traceability and Other
concentration are sent to the candidate laboratory. After proving an accepted per- formance, the laboratory accreditation is given; however, the control is continuous and the accreditation is valid only as long as the laboratory performance is accept- able by the criteria defined by the upper authority.
Laboratory accreditation is one of the most important elements of traceability.
Probably, the most critical step in evaluation is that the laboratory must produce results within the accepted accuracy limits. This means that the results produced in the laboratory and the known valuesof the samples sent should not be significantly different. For this purpose, the samples with actual (natural) matrices and with known analyte concentrations should be prepared; these are named as standard reference material (SRM) or certified reference material (CRM). Internationally renowned research institutes5–8 prepare homogenized samples with a natural matrix in rather large amounts, such as 30–100 kg. About 50–100 g portions are sent to laboratories selected all over the world. The analytes are pre-determined. After all the results are received, statistical tests are applied to reject some outliers and to determine the mean and standard deviation. Consequently, the certified valuesare listed with well-defined uncertainties such as s, 2s, 3sor confidence limits using a given confidence level. The process of preparing an SRM is difficult and tedious; it may take as long as 2 years.
At the end of the certification process, these SRM samples are made available to indi- viduals who would like to test the accuracy of their own results.
It is generally suggested that for a set of samples to be analyzed in the same run, at least 5% of these samples must be SRM or well-characterized QC samples.
A very comprehensive compilation of SRMs available worldwide has been made by the International Atomic Energy Agency.5Several other national institutes2,6,7are also known to offer SRMs. European Union has a central facility for such purposes located in Belgium.2 Among the other concerted efforts are EURACHEM9 and ILAC10to help the analysts of the world for relevant, updated and useful information regarding accreditation and related concepts and principles.
An official definition of traceability has been made in the document called “ILAC- G2: 1994, Traceability of Measurements”10: “The term traceabilitymeans a process whereby the indication of a measuring system (or a material measure) can be com- pared with a national standard for the measurand in question in one or more stages”.
Important elements of traceability are listed below9:
● An unbroken chain of comparisons should exist between the individual result and a national or international standard.
● Each element in this unbroken chain must have well-defined uncertainties.
● Each step must be performed according to documented and generally acknowl- edged procedures; the results must be equally documented.
● The laboratories must supply evidence for their technical competence, such as laboratory accreditation.
● The results must be referenced to SI units; the appropriate standards must be primary standards for realization of SI units.
● Calibration and recalibrations of methods and instruments must be performed at required intervals, as often as possible.
50 Chapter 3
The Système International (SI) units are shown in Table 3.2.
Among the units shown in Table 3.2,molis the most important one for chemical analysis. The mole is the amount of a substance consisting of elementary entities, as there are atoms in 0.012 kg of Carbon-12. The elementary entities may be atoms, molecules, ions, electrons, other particles or specified groups of such particles. One mole contains 6.022⫻1023(Avogadro’s number) entities as defined. While moleis the name of the concept,mol is the unit.
The efforts to have better definitions for concepts such as traceability, accredi- tation, etc. is continuous; although some references are given in this text, the reader should be aware of the dynamic character of these concepts, definitions and institutes.
References
1. E.P. Wagner II, B.W. Smith and J.D. Winefordner,Anal. Chem., 1996,68, 3199.
2. Institute for Reference Materials and Measurements, Belgium, http://www.
irmm.jrc.be, 2002.
3. B.M. Stewart and D. Darbouret,Int. Lab. News, 1998,28, 25S.
4. B. Welz,Fresenius Z. Anal. Chem., 1986,325, 95.
5. Database of Natural Matrix Reference Materials, A Compilation Prepared by the International Atomic Energy Agency (IAEA), July 2000, http://www.iaea.or.
at/programmes/nahunet/e4/nmrm/index.htm
6. Standard Reference Materials, National Institute of Standards and Technology, USA, http://srmcatalog.nist.gov, 2005.
7. Catalogue of Certified Reference Materials for Environmental Analysis, National Research Council, Canada, http://www.ems.nrc.ca/env2000b.pdf
8. AGAL–National Analytical Reference Laboratory (NARL), Australia, http://
www.agal.gov.au/NARL/index.html, 2005.
9. EURACHEM, A Focus for Analytical Chemistry in Europe, http://www.
eurachem.bam.de, 2005.
10. ILAC Publications, International Laboratory Accreditation Cooperation, http://
www.ilac.org, 2005.
Table 3.2 Système international base units
Quantity Unit Symbol
Length Meter m
Mass Kilogram kg
Time Second s
Electric current Ampere A
Temperature Kelvin K
Amount of substance Mole mol
Luminous intensity Candela candela