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8 Analytical Chemistry and Quality
8.4.4 Specific Quality Systems Used in Analytical Chemistry
Some aspects of Quality Assurance elements are directly related to analytical
quality. Thus, the methods used by an analytical laboratory subject to (or
benefiting from) a quality control system can be ranked according to accuracy,
precision and institutor. Boxes 8.7 and 8.8 classify analytical methods according
to quality. Only by proper, controlled application of a good method can quality
in the results be achieved, which entails using the method within the framework
of Quality Assurance.
Box 8.7
A Primary Quantitation Method
Recently, the Consultative Committee for Ouantity of Matter (CCOM) issued the following
tentative definition of primary method:
A primary methad is one that passesses the highest levels of metrological quality. The operations of the eMP involved must be accurately described and be understood and justified in such
a way that a comprehensive, detailed expression of its uncertainty can be written in terms of 51
base units. Therefore, its results should be acceptable without the need to use a reference to a
standard of compositional quantity (an analytical standard containing the analyte).
A primary method must thus possess the following four essential features: (1) metrological quality, (2) an accurate description and understanding, (3) an uncertainty defined in terms
of 51 base standards and (4) the need for no analyte standard. Two essential requisites for a
primary method in chemical metrology are specificity to an accurately defined analyte/
sample couple and an accurate knowledge or the abi lity to calculate with a specified, acceptabe uncertainty, every parameter and correction depending on other species and/or the
sample matrix. According to CCOM, there are four types of chemical metrological methods
with the potential to be considered primary methods, namely:gravimetry, titrimetry, coulometry and isotope dilution mass spectrometry (IDMS).
The following notes complement the previous definition of primary method:
(a) "Calcu lable" quantitation methods have the potential to be considered primary methods.
Before they are categorized as such, however, they must be checked in other respects.
Thus, whether they possess the requ ired high metrological qualification should be confirmed via intercomparison studies of international scope at the highest quality levels.
Also, a primary method requires not only monitoring of its procedure but also resting on
an appropriate metrological infrastructure. For this reason, primary methods tend to be
the patrimony of reference (e. g. national) laboratories rather than routine laboratories.
(b) No signal provided by a real sample standard containing the analyte (measurand) is
requ ired to express the result of a primary method. Coulometry and gravimetry use
no analytical chemical standards; in titrimetry, the standard is the titrant, which is reacted with the analyte; in IDMS, the sample is supplied with a standard containing
the pure analyte but in a different isotope ratio. This is also the case in physical metrology (e. g. in measuring length via the time light takes to travel the distance to be
measured or in measuring temperature by means of a noise thermometer or from the
Plank equation).
8 Analytical Chemistry and Quality
8.4.4 Specific Quality Systems Used in Analytical Chemistry
Some aspects of Quality Assurance elements are directly related to analytical
quality. Thus, the methods used by an analytical laboratory subject to (or
benefiting from) a quality control system can be ranked according to accuracy,
precision and institutor. Boxes 8.7 and 8.8 classify analytical methods according
to quality. Only by proper, controlled application of a good method can quality
in the results be achieved, which entails using the method within the framework
of Quality Assurance.
Box 8.7
A Primary Quantitation Method
Recently, the Consultative Committee for Ouantity of Matter (CCOM) issued the following
tentative definition of primary method:
A primary methad is one that passesses the highest levels of metrological quality. The operations of the eMP involved must be accurately described and be understood and justified in such
a way that a comprehensive, detailed expression of its uncertainty can be written in terms of 51
base units. Therefore, its results should be acceptable without the need to use a reference to a
standard of compositional quantity (an analytical standard containing the analyte).
A primary method must thus possess the following four essential features: (1) metrological quality, (2) an accurate description and understanding, (3) an uncertainty defined in terms
of 51 base standards and (4) the need for no analyte standard. Two essential requisites for a
primary method in chemical metrology are specificity to an accurately defined analyte/
sample couple and an accurate knowledge or the abi lity to calculate with a specified, acceptabe uncertainty, every parameter and correction depending on other species and/or the
sample matrix. According to CCOM, there are four types of chemical metrological methods
with the potential to be considered primary methods, namely:gravimetry, titrimetry, coulometry and isotope dilution mass spectrometry (IDMS).
The following notes complement the previous definition of primary method:
(a) "Calcu lable" quantitation methods have the potential to be considered primary methods.
Before they are categorized as such, however, they must be checked in other respects.
Thus, whether they possess the requ ired high metrological qualification should be confirmed via intercomparison studies of international scope at the highest quality levels.
Also, a primary method requires not only monitoring of its procedure but also resting on
an appropriate metrological infrastructure. For this reason, primary methods tend to be
the patrimony of reference (e. g. national) laboratories rather than routine laboratories.
(b) No signal provided by a real sample standard containing the analyte (measurand) is
requ ired to express the result of a primary method. Coulometry and gravimetry use
no analytical chemical standards; in titrimetry, the standard is the titrant, which is reacted with the analyte; in IDMS, the sample is supplied with a standard containing
the pure analyte but in a different isotope ratio. This is also the case in physical metrology (e. g. in measuring length via the time light takes to travel the distance to be
measured or in measuring temperature by means of a noise thermometer or from the
Plank equation).