170
4 The Measurement Process in Chemistry
The human body (specifically, senses such as sight and smell) can serve as an
instrument in applications such as classical qualitative analysis and titrimetries
(e.g. when reading the volume of titrant used from a graduated scale). More
often, however, signals are measured by using an instrument proper (e. g. an
atomic absorption spectrometer for optical signals, a balance or a mass spectrometer for mass signals, a differential thermal analyser for thermal signals, a
magnetic resonance spectrometer for magnetic signals, etc.).
Depending on their relationship to the analytes (or their reaction products),
instruments can be of the active or passive type (Fig. 4.16, 3). In passive instruments, the signal is not induced; it arises from physico-chemical properties
of the analytes or their products (e. g. mass in balances, the luminous energy of
chemiluminescence produced by photomultiplier tubes) or from the reactivity
of the former (e.g. in the visual location of the liquid meniscus on the volumetric scale of a burette once the end-point of the main titrimetric reaction has been
reached). Active instruments induce a signal by imposing some type of energy
(e.g. luminous, electric) on the analytes or their products (e.g. in photometry,
fluorimetry, polarography).
Depending on their relationship to the previous and following step of the
CMP (Fig.4.16, 4), measuring instruments can be of the stand-alone (off-line) or
integrated type (on-line). Thus, with respect to preliminary operations (Sect.
4.3), an instrument can be (a) stand-alone when it receives an aliquot of a collected, treated sample that is manually or automatically (discretely) inserted into
it; and (b) integrated or coupled on-line (e.g. a chromatograph or a capillary
electrophoresis system including a detector connected to the output of the capillary separation column). With respect to the third step of the CMP (Sect. 4.5), the
instrument is most often interfaced on-line to a computer; some instruments,
however, provide digital or, less often, analog readings that are processed by the
operator or input into a computer for calculation of the results.
The relationship of an instrument to chemical and SI base standards (Chap.
3) dictates the type of quantitative calibration to be applied (Fig. 4.16, 5). Socalled primary instruments (and the balance) require "equipment calibration"
only (see Fig. 4.5, 11); on the other hand, relative instruments, which operate by
comparing the signals for standards and the sample, call for "method calibration" in addition to equipment calibration (Fig. 4.5, MC1).
Depending on the objective of the analysis (Fig. l.20), instruments can
provide responses for - preferentially - qualitative, quantitative, structural or
combined purposes (Fig. 4.16, 6). Instruments for preferentially qualitative
purposes provide a wealth of multivariate information that allows the reliable
identification of the analytes; typical examples include Fourier transform infrared (FTIR) spectrophotometers and mass spectrometers. This, however, does not
exclude their use to quantify amounts or concentrations of analyte. Instruments
for essentially quantitative processes provide a reliable signal-concentration
relation but cannot deliver - by themselves - dependable qualitative information; such is the case with molecular absorption and emission spectrometers
(photometers and fluorimeters, respectively). Some instruments (e.g. atomic
absorption and emission spectrometers) are used for both purposes. Others,
4 The Measurement Process in Chemistry
The human body (specifically, senses such as sight and smell) can serve as an
instrument in applications such as classical qualitative analysis and titrimetries
(e.g. when reading the volume of titrant used from a graduated scale). More
often, however, signals are measured by using an instrument proper (e. g. an
atomic absorption spectrometer for optical signals, a balance or a mass spectrometer for mass signals, a differential thermal analyser for thermal signals, a
magnetic resonance spectrometer for magnetic signals, etc.).
Depending on their relationship to the analytes (or their reaction products),
instruments can be of the active or passive type (Fig. 4.16, 3). In passive instruments, the signal is not induced; it arises from physico-chemical properties
of the analytes or their products (e. g. mass in balances, the luminous energy of
chemiluminescence produced by photomultiplier tubes) or from the reactivity
of the former (e.g. in the visual location of the liquid meniscus on the volumetric scale of a burette once the end-point of the main titrimetric reaction has been
reached). Active instruments induce a signal by imposing some type of energy
(e.g. luminous, electric) on the analytes or their products (e.g. in photometry,
fluorimetry, polarography).
Depending on their relationship to the previous and following step of the
CMP (Fig.4.16, 4), measuring instruments can be of the stand-alone (off-line) or
integrated type (on-line). Thus, with respect to preliminary operations (Sect.
4.3), an instrument can be (a) stand-alone when it receives an aliquot of a collected, treated sample that is manually or automatically (discretely) inserted into
it; and (b) integrated or coupled on-line (e.g. a chromatograph or a capillary
electrophoresis system including a detector connected to the output of the capillary separation column). With respect to the third step of the CMP (Sect. 4.5), the
instrument is most often interfaced on-line to a computer; some instruments,
however, provide digital or, less often, analog readings that are processed by the
operator or input into a computer for calculation of the results.
The relationship of an instrument to chemical and SI base standards (Chap.
3) dictates the type of quantitative calibration to be applied (Fig. 4.16, 5). Socalled primary instruments (and the balance) require "equipment calibration"
only (see Fig. 4.5, 11); on the other hand, relative instruments, which operate by
comparing the signals for standards and the sample, call for "method calibration" in addition to equipment calibration (Fig. 4.5, MC1).
Depending on the objective of the analysis (Fig. l.20), instruments can
provide responses for - preferentially - qualitative, quantitative, structural or
combined purposes (Fig. 4.16, 6). Instruments for preferentially qualitative
purposes provide a wealth of multivariate information that allows the reliable
identification of the analytes; typical examples include Fourier transform infrared (FTIR) spectrophotometers and mass spectrometers. This, however, does not
exclude their use to quantify amounts or concentrations of analyte. Instruments
for essentially quantitative processes provide a reliable signal-concentration
relation but cannot deliver - by themselves - dependable qualitative information; such is the case with molecular absorption and emission spectrometers
(photometers and fluorimeters, respectively). Some instruments (e.g. atomic
absorption and emission spectrometers) are used for both purposes. Others,