53
Chemical and Optical Analysis of Petroleum in the Marine Environment
Definitive analysis of petroleum and its by-products in water, sediments, and organisms is today performed using coupled gas chromatography and mass spectrometry,
complex techniques requiring compressed gases, high vacuum, and powerful magnetic fields. Chemical separation of most of the individual compounds is achieved
by gas chromatography and their unequivocal identification and quantification by
mass spectrometry. Extreme care is required in sample collection to avoid contamination and painstaking sample preparation is required prior to analysis. Nonetheless,
the more toxic cyclic aromatic fractions of petroleum are optically active in the UV
region and thus may be detected and quantified by optical means very similar to
those described above for CDOM.
Crude petroleum light absorption is greatest in the UV A, B, and C bands
(200–400 nm) but strong also in the visible band and thus appears as brown or black
to the eye (black gold). Aromatic hydrocarbons may contain different numbers of
conjugated aromatic (benzene) rings such that optical absorption is spread out in the
spectrum since each individual ring absorbs in concert with its neighboring electron
clouds. The resulting UV/Vis absorption spectrum from about 300 nm conforms to
a rather featureless exponential decay function as is the case for CDOM albeit
incorporating small peaks attributed to optically active heavy metal chelates. In the
absence of clear diagnostic absorption peaks, parameters such as specific absorption
coefficients at key wavelengths and spectral slope within narrow bands (about
20 nm) provide objective numerical characterization.
Each component ring of a polynuclear aromatic hydrocarbon (PAH) constitutes
a fluorophore with different excitation/emission properties responding to its immediate electron distribution cloud. Mathematical techniques are required to deconvolute the complex ex/em spectra resulting from the number of PAH fluorophores in a
crude oil sample and their spectral overlap. Parallel factor analysis applied to data
arising from instrumental excitation-emission matrix spectroscopy analysis, much
as used for CDOM, enables discrimination and quantification of individual PAH
components (Zhou et al. 2015). Less precise estimates of petroleum PAH dissolved
and dispersed in seawater can be obtained using ex/em pairs tuned to abundant
known aromatic compounds. Chrysene, for example, a fused four ring member PAH
Optical Activity of Aromatic Compounds
Aromatic compounds derive their optical activity in the UV/Vis bands to
delocalization of electrons into clouds of toroidal probability distribution to
either side of the planar ring surface of a six member carbon ring, the socalled delocalized pi electron orbitals. Concatenation of adjacent rings further
delocalizes the cloud shifting absorption to lower wavelengths encompassing
the visible region.
2.4 Electro-Optical Sensors for Measurement of Organic Matter in Seawater
Chemical and Optical Analysis of Petroleum in the Marine Environment
Definitive analysis of petroleum and its by-products in water, sediments, and organisms is today performed using coupled gas chromatography and mass spectrometry,
complex techniques requiring compressed gases, high vacuum, and powerful magnetic fields. Chemical separation of most of the individual compounds is achieved
by gas chromatography and their unequivocal identification and quantification by
mass spectrometry. Extreme care is required in sample collection to avoid contamination and painstaking sample preparation is required prior to analysis. Nonetheless,
the more toxic cyclic aromatic fractions of petroleum are optically active in the UV
region and thus may be detected and quantified by optical means very similar to
those described above for CDOM.
Crude petroleum light absorption is greatest in the UV A, B, and C bands
(200–400 nm) but strong also in the visible band and thus appears as brown or black
to the eye (black gold). Aromatic hydrocarbons may contain different numbers of
conjugated aromatic (benzene) rings such that optical absorption is spread out in the
spectrum since each individual ring absorbs in concert with its neighboring electron
clouds. The resulting UV/Vis absorption spectrum from about 300 nm conforms to
a rather featureless exponential decay function as is the case for CDOM albeit
incorporating small peaks attributed to optically active heavy metal chelates. In the
absence of clear diagnostic absorption peaks, parameters such as specific absorption
coefficients at key wavelengths and spectral slope within narrow bands (about
20 nm) provide objective numerical characterization.
Each component ring of a polynuclear aromatic hydrocarbon (PAH) constitutes
a fluorophore with different excitation/emission properties responding to its immediate electron distribution cloud. Mathematical techniques are required to deconvolute the complex ex/em spectra resulting from the number of PAH fluorophores in a
crude oil sample and their spectral overlap. Parallel factor analysis applied to data
arising from instrumental excitation-emission matrix spectroscopy analysis, much
as used for CDOM, enables discrimination and quantification of individual PAH
components (Zhou et al. 2015). Less precise estimates of petroleum PAH dissolved
and dispersed in seawater can be obtained using ex/em pairs tuned to abundant
known aromatic compounds. Chrysene, for example, a fused four ring member PAH
Optical Activity of Aromatic Compounds
Aromatic compounds derive their optical activity in the UV/Vis bands to
delocalization of electrons into clouds of toroidal probability distribution to
either side of the planar ring surface of a six member carbon ring, the socalled delocalized pi electron orbitals. Concatenation of adjacent rings further
delocalizes the cloud shifting absorption to lower wavelengths encompassing
the visible region.
2.4 Electro-Optical Sensors for Measurement of Organic Matter in Seawater