6.2 Calculable Quantitation Methods
257
the Faraday constant, the number of electrons exchanged in an electrochemical reaction, stoichiometric coefficients for a chemical reaction) to
primary data provided by the instruments used in the CMP (e. g. the mass of
a precipitate in grams, the amount of electricity used in coulombs, the
volume of titrant employed in mL).
(b) The equipment used should be calibrated using physical standards such as
transfer weights, timer and ammeter checking systems, measurements of
solution masses, etc.
(c) No tangible reference materials containing the analyte are employed.
(d) Chemical standards (e.g. atomic weights, the Faraday) and SI base units play
a prominent role in the process by which the result is obtained.
(e) Calculable methods can be implemented both through classical (e.g. gravimetry, titrimetry) and instrumental techniques (e.g. isotope dilution mass
spectrometry, coulometry).
(f) They are potential primary methods (see Box 8.7).
As stated in Sect. 6.1.5, there are two different types of calculable methods,
namely: absolute methods involving no analytical standard, which require no
analytical chemical reference materials (e. g. pure substances or sample
standards) to express the result; and absolute methods involving analytical
standards, which use such standards in addition to others in order to express the
result. This is the criterion adopted in this book to deal with these two types of
methods separately.
I 6.2.1 Absolute Methods Involving no Analytical Standard
These quanti tat ion methods provide results without the need to use analytical
chemical reference materials such as pure substances or sample standards. Consequently, they require equipment calibration but not method calibration. This
type of method is similar to quantitation methods (PMPs) for physical parameters (e.g. temperature, length, time, current intensity, pressure), which require
calibration of the measuring instrument alone. As a result, this type of CMP is
that with the shortest traceability chain; also, the last link in the chain can be an
SI unit (a base standard). Both PMPs and CMPs use physical standards (e.g.
transfer weights) for equipment calibration. However, CMPs are usually longer
(multistep) as they entail conditioning the sample (see Sect. 4.3) prior to
measurement (second step of the analytical process); by contrast, PMPs involve
direct application of the instrument to the object or sample to be measured (see
Box 6.4).
__ Box6.4
Absolute quantitation methods (absolute (MPs) using no tangible analytical chemical
standards (e.g. gravimetry and coulometry) resemble physical metrological methodologies
in that they only requ ire calibration of the measuring instrument (typically, using physical
standards). Th is box highlights the practical differences between absolute (MPs and PMPs.
257
the Faraday constant, the number of electrons exchanged in an electrochemical reaction, stoichiometric coefficients for a chemical reaction) to
primary data provided by the instruments used in the CMP (e. g. the mass of
a precipitate in grams, the amount of electricity used in coulombs, the
volume of titrant employed in mL).
(b) The equipment used should be calibrated using physical standards such as
transfer weights, timer and ammeter checking systems, measurements of
solution masses, etc.
(c) No tangible reference materials containing the analyte are employed.
(d) Chemical standards (e.g. atomic weights, the Faraday) and SI base units play
a prominent role in the process by which the result is obtained.
(e) Calculable methods can be implemented both through classical (e.g. gravimetry, titrimetry) and instrumental techniques (e.g. isotope dilution mass
spectrometry, coulometry).
(f) They are potential primary methods (see Box 8.7).
As stated in Sect. 6.1.5, there are two different types of calculable methods,
namely: absolute methods involving no analytical standard, which require no
analytical chemical reference materials (e. g. pure substances or sample
standards) to express the result; and absolute methods involving analytical
standards, which use such standards in addition to others in order to express the
result. This is the criterion adopted in this book to deal with these two types of
methods separately.
I 6.2.1 Absolute Methods Involving no Analytical Standard
These quanti tat ion methods provide results without the need to use analytical
chemical reference materials such as pure substances or sample standards. Consequently, they require equipment calibration but not method calibration. This
type of method is similar to quantitation methods (PMPs) for physical parameters (e.g. temperature, length, time, current intensity, pressure), which require
calibration of the measuring instrument alone. As a result, this type of CMP is
that with the shortest traceability chain; also, the last link in the chain can be an
SI unit (a base standard). Both PMPs and CMPs use physical standards (e.g.
transfer weights) for equipment calibration. However, CMPs are usually longer
(multistep) as they entail conditioning the sample (see Sect. 4.3) prior to
measurement (second step of the analytical process); by contrast, PMPs involve
direct application of the instrument to the object or sample to be measured (see
Box 6.4).
__ Box6.4
Absolute quantitation methods (absolute (MPs) using no tangible analytical chemical
standards (e.g. gravimetry and coulometry) resemble physical metrological methodologies
in that they only requ ire calibration of the measuring instrument (typically, using physical
standards). Th is box highlights the practical differences between absolute (MPs and PMPs.