Geoscience Reference
In-Depth Information
Table 5.1.
Methods of optical spectroscopy and the measured quantity
Method
Measured quantity
Examples
Absorption
Absorbance or the ratio of
transmitted to incident radiant
power, A = -log(
φ
/
φ
O
)
Atomic absorption, UV-visible molecular
absorption, IR absorption
Optical Emission
Radiant power of emission,
φ
E
ICP and DCP emission, spark emission,
laser-induced breakdown emission,
flame emission, DC arc emission
Photoluminescence
Radiant power of luminescence,
Molecular fluorescence and
phosphorescence, chemi- and
bioluminescence, atomic fluorescence
Scattering
Radiant power of scattering,
φ
S
Raman scattering, Mie scattering,
turbidity
can be defined as the quantitative measurement of the intensity of an optical signal at one
or more wavelengths using a photodetector.
Spectroscopic information about a species and its microenvironment is usually obtained
by stimulating the sample with some energy source such as electrical energy, radiation,
particles, or heat. Before stimulation the sample is normally residing in its lowest energy
level, or ground state. After the stimulation the sample is momentarily induced to a higher
energy level or excited state. Spectroscopic measurements are made from either the radi-
ative emission from the excited species as it returns to the ground state or by the amount
of absorbed radiation. The magnitude of the optical signal as a function of wavelength
describes the sample in terms of its electronic, rotational, and vibrational energy levels
and any associated transitions that have taken place. These, in turn, provide information
on the molecular structure of the molecules that are within the sample that has been ana-
lyzed, such as characteristic absorption frequencies of known specific bonds. The signal
intensity can be described in radiometric terms, now exclusively used in optical spectros-
copy, or in photometric terms that have been based on the human eye response. In the
radiometric system, the basic quantity measured is the actual amount of radiative energy
in joules (J) either being emitted by a source or incident upon a detector. In fluorescence
terms, the radiative energy emitted by a fluorescing sample will be a function (per unit)
of exciting radiation. The four main methods associated with optical spectroscopy are
absorption, emission, luminescence, and scattering (see
Table 5.1
). The emission process
can be stimulated in a variety of ways, including the use of light (photoluminescence) or
by collision with energetic electrons in a plasma (inductively coupled plasma). Scattering
of electromagnetic radiation by matter (molecules and atoms) can be either
elastic
or
inelastic
. Elastic scattering occurs when the energy (and thus wavelength and frequency)
of the light is not substantially changed, such as Mie and Rayleigh scattering. Inelastic
scattering occurs when the kinetic energy of an incident particle, such as a photon, is not
