256
cyclotron equation υ(Hz) = 1.535611 · 10
7
B 0 · (m/z)
−1
, where B 0 is the magnetic field
strength of the instrument, in Tesla (Marshall et al. 1998).
In order to compensate for non-ideal interactions such as space charge effects
and other ion suppression effects, the spectrum is calibrated, and the masses of each
peak are adjusted using standards with a priori chemical information or known compounds. The accurate mass to charge information for each ion signal is then converted to molecular formulas for each ion accurate mass assessed. This is possible
due to the nonintegral unique atomic masses of most elements (unique mass defects).
Data processing of complex, organic matter ultrahigh-resolution mass spectra
yields a long list of molecular formula assignments and intensities related to peaks
found in the spectra. In order to be able to efficiently interpret and compare such
“big” datasets, compositional sorting and other visualization strategies are needed,
which could reveal the compositional characteristics of the investigated sample set.
Typical petroleum geochemistry-related investigations use sequential data layers
based on the heteroatom content, double bond equivalent (DBE, a measure of unsaturation due to the presence of rings and/or double bonds in the molecular structure),
and carbon number (C#) to create plots (Marshall and Rodgers 2008). Elemental
ratios (e.g., H/C vs. O/C) calculated from molecular formulas are also frequently
used in organic matter studies, especially in the form of van Krevelen plots (Wu
et al. 2004). While detailed explanation of FTICR-MS terminology is out of this
chapter’s scope, more information can be found in other publications (Marshall and
Rodgers 2008; McKenna et al. 2013; Oldenburg et al. 2014, 2017). Again, the use
of software packages, either developed in-house or commercially available, makes
the investigation of the several layers of data easier.
15.3 Characterization of Source Oils and Weathered Oil
Residues Using FTICR-MS
The oil released during the DWH spill was a sweet, light Louisiana crude, rich in
volatile and semi-volatile saturated and aromatic hydrocarbon compounds, readily
amenable for standard gas chromatography analyses (Overton et al. 2016).
Notwithstanding, when characterized with FTICR-MS, a plethora of additional,
high molecular weight, and/or non-hydrocarbon polar oil constituents of MW oil
also becomes accessible. McKenna et al. (2013) have used atmospheric pressure
photoionization (APPI) and electrospray ionization (ESI) to inventory more than
16,000 unique molecular monoisotopic elemental compositions for the acidic,
basic, and nonpolar components of the source MW oil. These included carbazoles
and benzocarbazoles, indoles, carboxylic (naphthenic) acids, pyridines, and quinolines (McKenna et al. 2013).
However, weathering processes such as photooxidation and biodegradation have
transformed the parent oil compounds in the spilled MW oil during the months and
years after the release, to a complex mixture of oxygen-containing products, which
escape the detection window of conventional gas chromatography tools (Aeppli
et al. 2012, 2014; Radović et al. 2014; Ward et al. 2018). In this case, the capability
J. R. Radović et al.
cyclotron equation υ(Hz) = 1.535611 · 10
7
B 0 · (m/z)
−1
, where B 0 is the magnetic field
strength of the instrument, in Tesla (Marshall et al. 1998).
In order to compensate for non-ideal interactions such as space charge effects
and other ion suppression effects, the spectrum is calibrated, and the masses of each
peak are adjusted using standards with a priori chemical information or known compounds. The accurate mass to charge information for each ion signal is then converted to molecular formulas for each ion accurate mass assessed. This is possible
due to the nonintegral unique atomic masses of most elements (unique mass defects).
Data processing of complex, organic matter ultrahigh-resolution mass spectra
yields a long list of molecular formula assignments and intensities related to peaks
found in the spectra. In order to be able to efficiently interpret and compare such
“big” datasets, compositional sorting and other visualization strategies are needed,
which could reveal the compositional characteristics of the investigated sample set.
Typical petroleum geochemistry-related investigations use sequential data layers
based on the heteroatom content, double bond equivalent (DBE, a measure of unsaturation due to the presence of rings and/or double bonds in the molecular structure),
and carbon number (C#) to create plots (Marshall and Rodgers 2008). Elemental
ratios (e.g., H/C vs. O/C) calculated from molecular formulas are also frequently
used in organic matter studies, especially in the form of van Krevelen plots (Wu
et al. 2004). While detailed explanation of FTICR-MS terminology is out of this
chapter’s scope, more information can be found in other publications (Marshall and
Rodgers 2008; McKenna et al. 2013; Oldenburg et al. 2014, 2017). Again, the use
of software packages, either developed in-house or commercially available, makes
the investigation of the several layers of data easier.
15.3 Characterization of Source Oils and Weathered Oil
Residues Using FTICR-MS
The oil released during the DWH spill was a sweet, light Louisiana crude, rich in
volatile and semi-volatile saturated and aromatic hydrocarbon compounds, readily
amenable for standard gas chromatography analyses (Overton et al. 2016).
Notwithstanding, when characterized with FTICR-MS, a plethora of additional,
high molecular weight, and/or non-hydrocarbon polar oil constituents of MW oil
also becomes accessible. McKenna et al. (2013) have used atmospheric pressure
photoionization (APPI) and electrospray ionization (ESI) to inventory more than
16,000 unique molecular monoisotopic elemental compositions for the acidic,
basic, and nonpolar components of the source MW oil. These included carbazoles
and benzocarbazoles, indoles, carboxylic (naphthenic) acids, pyridines, and quinolines (McKenna et al. 2013).
However, weathering processes such as photooxidation and biodegradation have
transformed the parent oil compounds in the spilled MW oil during the months and
years after the release, to a complex mixture of oxygen-containing products, which
escape the detection window of conventional gas chromatography tools (Aeppli
et al. 2012, 2014; Radović et al. 2014; Ward et al. 2018). In this case, the capability
J. R. Radović et al.