With the development of photomultiplier tubes in the middle of the twentieth
century, spectroscopy, especially absorption spectroscopy in the UV, visible and
(near) infrared wavelength ranges, became firmly established in analytical laboratories, both for the analysis of whole samples as well as single components after
separation by chromatography. By the mid-1990s, the online at-site spectrometer
instrument had reached a mature development stage and is since seeing increasing
use in real-time quality monitoring and process control. Applications include monitoring feed and product composition and quality in such diverse industries as
pharmaceuticals, petrochemistry, food, as well as water quality monitoring. The
most commonly used types of spectroscopy in such at-site, sometimes even in situ,
devices are UV/Vis absorbance, near-infrared (NIR) absorbance and fluorescence
spectroscopy. Further methodologies include refractive index measurement, Raman
spectroscopy, laser-induced breakdown spectroscopy (LIBS) and image analysis.
3 Interaction of Light and Matter
All spectroscopic methods rely on the interaction of light with atoms and molecules.
The interaction of light and matter can be described by two different models, one
assuming light as a wave phenomenon and the other assuming light to consist of
particles. The wave approach is most appropriate to describe such interactions as
reflection, refraction and interference. For spectroscopic methods, the interaction of
light with atoms and molecules can best be described using the particle approach,
with the light particles being called photons. The important parameters of a photon
are its energy E, wavelength λ and frequency f, which are related according to
Eq. (1):
E ¼
hc
λ
¼ h f
ð1Þ
where h is the Planck constant (6.63 Â 10
À34 Js). From Eq. (1), it follows that the
energy of a photon is proportional to its frequency and reciprocal to its wavelength.
For the discussion herein, the most important parameter is the wavelength, often
expressed in units of [nm] or [μm]. Another parameter regularly used in spectroscopy is the reciprocal of the wavelength, the wavenumber ν, often expressed in
[cm
À1 ].
Although the electromagnetic spectrum is broadly divided into eight regions,
ranging from highly energetic γ-radiation to radio waves (Fig. 1), spectroscopy of
aqueous samples is focused on the ultraviolet (UV) (200–400 nm), visible (Vis)
(400–700 nm) and near-infrared (NIR) (750–1,400 nm) domain, with lower wavelengths corresponding to higher photon energies. The primary reasons for the
prevalence of these domains in water analysis are the transparency of water to
radiation at these wavelengths and the fact that spectrometers using these wavelengths do not require exotic materials or extreme operating conditions.
286
J. van den Broeke and T. Koster
century, spectroscopy, especially absorption spectroscopy in the UV, visible and
(near) infrared wavelength ranges, became firmly established in analytical laboratories, both for the analysis of whole samples as well as single components after
separation by chromatography. By the mid-1990s, the online at-site spectrometer
instrument had reached a mature development stage and is since seeing increasing
use in real-time quality monitoring and process control. Applications include monitoring feed and product composition and quality in such diverse industries as
pharmaceuticals, petrochemistry, food, as well as water quality monitoring. The
most commonly used types of spectroscopy in such at-site, sometimes even in situ,
devices are UV/Vis absorbance, near-infrared (NIR) absorbance and fluorescence
spectroscopy. Further methodologies include refractive index measurement, Raman
spectroscopy, laser-induced breakdown spectroscopy (LIBS) and image analysis.
3 Interaction of Light and Matter
All spectroscopic methods rely on the interaction of light with atoms and molecules.
The interaction of light and matter can be described by two different models, one
assuming light as a wave phenomenon and the other assuming light to consist of
particles. The wave approach is most appropriate to describe such interactions as
reflection, refraction and interference. For spectroscopic methods, the interaction of
light with atoms and molecules can best be described using the particle approach,
with the light particles being called photons. The important parameters of a photon
are its energy E, wavelength λ and frequency f, which are related according to
Eq. (1):
E ¼
hc
λ
¼ h f
ð1Þ
where h is the Planck constant (6.63 Â 10
À34 Js). From Eq. (1), it follows that the
energy of a photon is proportional to its frequency and reciprocal to its wavelength.
For the discussion herein, the most important parameter is the wavelength, often
expressed in units of [nm] or [μm]. Another parameter regularly used in spectroscopy is the reciprocal of the wavelength, the wavenumber ν, often expressed in
[cm
À1 ].
Although the electromagnetic spectrum is broadly divided into eight regions,
ranging from highly energetic γ-radiation to radio waves (Fig. 1), spectroscopy of
aqueous samples is focused on the ultraviolet (UV) (200–400 nm), visible (Vis)
(400–700 nm) and near-infrared (NIR) (750–1,400 nm) domain, with lower wavelengths corresponding to higher photon energies. The primary reasons for the
prevalence of these domains in water analysis are the transparency of water to
radiation at these wavelengths and the fact that spectrometers using these wavelengths do not require exotic materials or extreme operating conditions.
286
J. van den Broeke and T. Koster