3.4 Types of Standards and their Traceability
107
the analytes, special calibration methods or reference matrices (standards),
all of which are redundant in PMPs.
(d) Although both PMPs and CMPs require calibrating the measuring equipment, some chemical methodologies additionally entail relating the concentration (amount) of analyte to the signal it gives. The differences between
equipment and method (analytical) calibration, both of which are closely
related to traceability, are discussed in Sect. 3.5.4.
__ Box3.4
The characterization of a given sea water sample in physica l (e. g. temperature),chemical (e. g.
pH. concentration of mercury species) and biological terms (e.g. presence or absence of
microbes) differs strongly in com plexity and difficulty.
Measuring the water temperature is fairly simple: it suffices to use a pre-calibrated
thermocouple. Measuring its pH is also quite simple: a pH-meter is also a sensor equipped
with a combined potentiometric electrode which, similarly to the thermocouple, must be
ca librated with standard solutions of known pH prior to immersion in the sample. On the
other hand, the speciation of mercury is a longer process that involves using various types of
conta iners to store the sa mple, withd rawing an accurately measured aliquot from it (e. g. by
means of a calibrated pipette) and conditioning it (e. g. by preconcentration, clean -up), inserting the treated aliquot into a complex instrumental system (e.g. a gas chromatograph
coupled on-line to an atomic spectroscopic detector the response of which should previously
have been cali brated with standards of usually limited commercial availability) and processing the transient signals obtained. Obviously, the speciation of mercury will be subject to
many more sources of va riability and to larger systematic and random errors than will the
previous measurements. Finally, the scarcity of certified reference materials for speciation will
normally pose an additional difficulty.
The microbiological characterization of sea water entails the use of a scarcely
robust measuring system for which available references (measurement standards) are
also scant.
3.4 Types of Standards and their Traceability
Standards are closely related to traceability in all metrological fields. In fact, they
are the basis for measuring physical, chemical, biochemical and biological
quantities. It is thus important to know not only what types of standards are
available, but also how they relate to one another.
Standards for chemical measurements can be ranked in terms of two different
criteria, namely (Fig. 3.4); (a) nearness of their value in the chemical parameter
of interest to the value held as true; and (b) availability or tangibility. Uncertainty in the reference value increases as one climbs down the ranking of Fig. 3.4. All
the steps in this hierarchy are mutually related via traceability; thus, analytical
standards are related to SI base units through chemical standards. Therefore,
chemical and analytical chemical standards can also be referred to as "transfer
standards", similarly as in physical metrology.
107
the analytes, special calibration methods or reference matrices (standards),
all of which are redundant in PMPs.
(d) Although both PMPs and CMPs require calibrating the measuring equipment, some chemical methodologies additionally entail relating the concentration (amount) of analyte to the signal it gives. The differences between
equipment and method (analytical) calibration, both of which are closely
related to traceability, are discussed in Sect. 3.5.4.
__ Box3.4
The characterization of a given sea water sample in physica l (e. g. temperature),chemical (e. g.
pH. concentration of mercury species) and biological terms (e.g. presence or absence of
microbes) differs strongly in com plexity and difficulty.
Measuring the water temperature is fairly simple: it suffices to use a pre-calibrated
thermocouple. Measuring its pH is also quite simple: a pH-meter is also a sensor equipped
with a combined potentiometric electrode which, similarly to the thermocouple, must be
ca librated with standard solutions of known pH prior to immersion in the sample. On the
other hand, the speciation of mercury is a longer process that involves using various types of
conta iners to store the sa mple, withd rawing an accurately measured aliquot from it (e. g. by
means of a calibrated pipette) and conditioning it (e. g. by preconcentration, clean -up), inserting the treated aliquot into a complex instrumental system (e.g. a gas chromatograph
coupled on-line to an atomic spectroscopic detector the response of which should previously
have been cali brated with standards of usually limited commercial availability) and processing the transient signals obtained. Obviously, the speciation of mercury will be subject to
many more sources of va riability and to larger systematic and random errors than will the
previous measurements. Finally, the scarcity of certified reference materials for speciation will
normally pose an additional difficulty.
The microbiological characterization of sea water entails the use of a scarcely
robust measuring system for which available references (measurement standards) are
also scant.
3.4 Types of Standards and their Traceability
Standards are closely related to traceability in all metrological fields. In fact, they
are the basis for measuring physical, chemical, biochemical and biological
quantities. It is thus important to know not only what types of standards are
available, but also how they relate to one another.
Standards for chemical measurements can be ranked in terms of two different
criteria, namely (Fig. 3.4); (a) nearness of their value in the chemical parameter
of interest to the value held as true; and (b) availability or tangibility. Uncertainty in the reference value increases as one climbs down the ranking of Fig. 3.4. All
the steps in this hierarchy are mutually related via traceability; thus, analytical
standards are related to SI base units through chemical standards. Therefore,
chemical and analytical chemical standards can also be referred to as "transfer
standards", similarly as in physical metrology.