Open access peer-reviewed chapter
Abstract
Pyrolysis techniques use thermal energy to degrade large molecules into smaller and volatile molecules, which can be analyzed by gas chromatography to determine the composition or structure of the initial sample. A laser system coupled to a GC/MSMS, for analysis of these species, gave rise to the technique called Laser MicroPyrolysis-GC/MSMS (LmPy-GC/MSMS). The laser micropyrolyzer is a hyphenated system with a laser source, an optical microscope, and a GC/MSMS technique. In this hyphenated technique, a collimated, short-duration laser beam with a large amount of energy can be focused on isolated components, or a macromolecule inside a heterogeneous matrix, promoting the thermal cleavage and producing smaller components, which are finally analyzed and characterized by GC/MSMS. The LmPy-GC/MSMS has been proven to be able to supply important molecular data on organic fossils, coals, source rocks, etc., improving the knowledge and the understanding of the compositions and opening a horizon in the study of several heterogeneous materials.
Keywords
- laser micropyrolysis
- organic matter
- instrumental development
- gas chromatography
- mass spectrometry
1. Introduction
The pyrolysis technique has been extensively used for many years as an analytical technique in which large molecules are degraded into smaller volatile species using solely thermal energy under an oxygen-free environment. The coupling of this thermal energy as a laser source (pyrolyzer), an optical microscope, and the gas chromatographic tandem Mass Spectrometry (GC/MSMS) system compound a hyphenated analytical technique called Laser Micropyrolysis GC/MSMS (LmPy-GC/MSMS). This coupling contributes to improving the analytical processes during sample characterization, and it is considered a useful tool for the determination of the composition or structure of the organic compounds whether in individual components or samples of complex mixtures.
Molecular information on organic fossils in samples such as coals [1, 2, 3, 4], source rocks [5, 6, 7], kerogen [8, 9, 10, 11, 12, 13, 14]; oil shales [4, 15, 16], and synthetic organic polymers [17, 18] had been successfully afforded by LmPy-GC/MSMS among existing pyrolysis technique, proving to be an excellent way to characterize the chemical composition of heterogeneous materials.
The analytical temperature is a crucial variable in pyrolysis techniques, being low temperatures are not analytically useful for efficient degradation while high temperatures are more efficient for breaking molecule bonds, however, caution needs to be taken not to destroy the molecule. In this sense selecting the adequate temperature at which macromolecules can be degraded in a wide array of products has great significance.
In the LmPy-GC/MSMS system is not possible to control the temperature. This laser provides a thermal flux as high as 1000°C/s which is enough to heat the sample in a very short time. The combination of high temperature and short time minimizes secondary pyrolysis reactions when laser pyrolysis is used and generates pyrolysis fragments characteristic of the original sample. This short-duration laser beam energy (1000°C/s) is collimated and coherent allowing the spread of very large amounts of thermal energy on a specific area of slim dimension, enabling the thermal cleavage of samples (e.g., isolated components or a macromolecule) into smaller components which in turn are analyzed by GC/MSMS.
The collimated and coherent laser irradiation allows focus on tiny or small areas of heterogeneous materials, which enables analysis of non-volatile, thermally labile components at the microscopic level. The focused and irradiated area can be less than 100 μm, using the appropriate lenses in the optical microscope. These microscopic areas permit isolated analysis of individual components into complex mixtures. In conclusion, the LmPy-GC/MSMS shows great potential to improve the understanding of organic composition in heterogeneous materials as well as isolated organic-walled microfossils.
Several authors [15, 16, 18, 19, 20] lay emphasis on their research on instrumental development and list diverse factors to be considered, such as (i) little knowledge about laser-material interactions; (ii) limitations in the sensibility of chromatographic techniques needed for products with low concentrations; (iii) need for interdisciplinary skill; (iv) possible interfacing-instruments difficulties; (v) the issue that all samples are not compatible with laser radiation to produce pyrolysis products and; (vi) the financial expense due to the acquisition of diverse instruments. On the other hand, the applicability of the LmPy-GC/MSMS as well as the use of other detection methods, such as NMR or FTIR, to characterize the molecular composition of distinctive organic samples, have been studied by many authors.
The kerogen of the Ordovician Estonian Kukersite sample was analyzed by Derenne et al. using spectroscopic NMR and FTIR, and off-line and flash pyrolytic methods [5]. Stout and Hall assayed two synthetic organic polymer samples by LmPy-GCMS [17] while Arouri et al. characterized Acritarch specimens [8], Chitinozoan specimens from Silurian marine rocks were studied by Jacob et al. [11], and, Silva et al. compared the chemical composition of
The favorable comparison of laser-derived molecular data with corresponding data from traditional methods, reported above, suggests that pyrolysis methods are suitable for molecular characterization of organic macromolecules. In addition, these researchers have also evidenced the possibility of a couple of combinations of LmPy-GC/MSMS with other micro or macroscale spectroscopic methods such as NMR, CP-Py, or FTIR. All of these results are useful to improve knowledge and understanding, allowing individual analysis of an isolated particle or smaller components inside complex mixtures in heterogeneous organic materials.
2. The LmPy-GC/MSMS hyphenated system
LmPy-GC/MSMS is a hyphenated analytical technique composed by: a neodymium-doped yttrium aluminum garnet (Nd-YAG; Nd: Y3Al5O12) laser source, an optical device containing the sample chamber, a transfer line from the optical device to the gas chromatography equipment coupled to a triple quadrupole mass spectrometer (GC/MSMS) with a cryogenic entrapment (Liquid Nitrogen, −80°C) or Cold Trap in the PTV (programmed temperature vaporization) injector. Figure 1 shows the schematic system of the LmPy-GC/MSMS technique and the Palynofacies and Organic Facies Laboratory (
Figure 1.
Schematic LmPy-GC/MSMS system (a) and LmPy-GC/MSMS hyphenated system located at
The most common laser type used to emit light in the infrared spectrum (λ = 1064 nm) consists of an yttrium oxide and crystalline aluminum as a “host” doped with neodymium as a “guest.” This device is known as the Nd-YAG laser and forms a variety of garnets. In the LmPy-GC/MSMS system, the Nd-YAG laser is the thermal energy source used to degrade large molecular-weight polymer carbon chains present in organic matter into smaller volatile species, under an oxygen-free environment. While the GC/MSMS technique separates and details the composition of molecular pyrolysis products (i.e., molecular fingerprinting). The association of these techniques has been useful to determine the composition or structure of original samples.
3. The LmPy-GC/MSMS system performance
An efficient and appropriate sample preparation method is necessary for good work involving the LmPy-GC/MSMS technique. Results interpretation quality is strongly influenced by factors such as standard sample collection, features of the samples, and preparation proceedings. Additionally, all these factors also have a significant effect on the utility of the LmPy-GC/MSMS analysis. The LmPy-GC/MSMS analysis can be carried out in samples such as whole rock (WR), kerogen concentrated (KC), individual organic components, or solid bitumen. However, analyses of isolated components are strongly recommended to avoid the influence of bordering material products.
3.1 Sample preparation for particulate organic components analysis
For analyzing individual organic components, a standard non-oxidative method is applied for the isolation of kerogen [24, 25]. The crushed (2–5 mm) sample is pre-extracted in the Soxhlet extraction apparatus using dichloromethane (CH2Cl2) for at least 24 hours. The crushed fraction must be treated with HCl and HF acids to remove carbonates and silicates. Heavy solution of ZnCl2 (ρ = 1.95 at 20°C) is applied in order to concentrate the kerogen. The organic residue is sieved through a 10 mm nylon mesh using distilled water, isolated kerogen is the fraction retained in the sieve.
Particulate organic components are separated by size using a set of nylon membranes. Set of nylon membranes with pore diameters ranging from 5 to 200 μm. The sieving process follows the arrangement of the sieves (membranes) in a sequential nest: smallest mesh number (larger aperture, 120 to 200 μm) at the top and largest mesh number (smallest aperture, 20– 100 μm) at the bottom. The set of smallest aperture nylon membranes is used for the filtration process using a Kitasato flask and a vacuum pump. Organic residues separated by size after the filtration process are spread on slides using different solutions (water, detergent, and alcohol). The hand-picking technique is performed on the kerogen concentrate (KC) strewn slide. This slide is placed under the microscope and the organic particles are separated from each other, one by one, with the aid of a histological needle (Figure 2a). The components of the heterogeneous mixture are then identified and individually separated from each other (Figure 2b–e).
Figure 2.
Hand-picking technique (a). Hand-picked
3.2 Assembling the LmPy-GC/MSMS system for analysis performing
3.2.1 Sample chamber assembly
The sample chamber is the module for the insertion of the sample. The chamber is connected to a gas injection system for capturing and transferring the generated products during pyrolysis. Samples (e.g., hand-picked organic material or solid bitumen) are placed in a glass slide and transferred inside the sample chamber compartment which has an inner diameter of 6 mm. The chamber is sealed with a copper gasket seal then, closed with a fused silica coating (VAR coating reduces reflectance from around 10–1%), and finally, connects to the gas line. The steps to assemble the sample chamber are shown in Figure 3.
Figure 3.
Assembling the sample chamber: Sample in strewn slide (a), seal the chamber with copper gasket seal (b), close the chamber with a fused silica coating (c), and connecting the gas line (d).
3.2.2 Thermal extraction
Organic material is subjected to thermal extraction inside the sample chamber by heating at 100°C. This procedure is performed to remove adsorbed organic material on the surface of the samples. Finally, these extracted products are online transferred to the PTV injector via a transfer line heated at 300°C, using He as carrier gas.
3.2.3 Establishment of laser power conditions
In order to establish the most favorable parameters for the generation of primary pyrolysates, the laser energy, and irradiation time, among other laser conditions must be varied. These variations also provide a characterization of the performance and influence on the technique. Figure 4 shows the spot size diameters obtained from a test performed in a Torbanite sample using different laser energy (13–20 A) at the same irradiation time (0.5 s).
Figure 4.
Laser test in a Torbanite sample (Split mode) showing the spot size diameters obtained with different laser energy 20A (a), 17A (b), and 13A (c) using the irradiation time of 0.5 s.
3.2.4 Identification and selection of organic particles for pyrolysis
The organic particle is first identified through its optical properties (translucency, fluorescence, etc.), using a 20x objective and a 10x magnification front lens, and subsequently selected and marked with a spot size of range diameter of up to 100 μm for laser micropyrolysis analysis (Figure 5).
Figure 5.
Optical microscope equipment used to identify organic particles (a). Example of identification, selecting, and marking organic particles of
3.2.5 Analysis running
Due to the laser spot size depending on the sample size, this spot is controlled by adjusting the magnification in the microscope objective lens (10x, 20x, 40x, or 50x) and the laser power. Laser radiation is triggered, and the laser beam is directed throughout the prisms (lens) of the system until the entrance of the microscope as shown in Figure 6. The samples placed inside the sample chamber are pyrolyzed by focusing the laser radiation directly through the objective of the microscope and, generating the pyrolysis products. Multiple shots are necessary on the particles to obtain enough pyrolysis products (Figure 7) to be then transferred online to interface between the sample chamber and GC system. This transfer is carried out via a heated transfer line at 300°C, using He as carrier gas. Pyrolysis products are continuously transferred to the PTV injector and then cryogenically entrapped in a cold trap (cryol collecting) using Liquid Nitrogen at −80°C.
Figure 6.
Microscope receiving the laser beam (a) directed throughout lens (b) of the LmPy-GC/MSMS hyphenated system.
Figure 7.
Sample chamber at 300°C connected to an He flux interfacing (a) to transfer the pyrolysis products. Photomicrographs taken under TWL (b-d) and FM (e-g) showing the multiple shots on
Pyrolysis products are kept in the cold trapping long enough to ensure the complete trapping of the products from the sample chamber. After this time, the inlet temperature rapidly rises to vaporize the compounds and transfer the products to the Capillary GC Column. In this way, the pyrolysis products that were cryogenically entrapped at −80°C are heated up to 350°C and finally analyzed by GC/MSMS system (Figure 8).
Figure 8.
Transfer line (300°C) of pyrolysis products from the microscope to the PTV injector in GC/MSMS equipment (a). PTV injector (b) cooled at −80°C with liquid nitrogen (c).
4. Study cases
Applying examples of LmPy-GC/MSMS systems located at LAFO/UFRJ performed with samples of hand-picked
4.1 LmPy-GC/MSMS of hand-picked Botryococcus braunii samples
The hand-picked
Figure 9.
Photomicrographs of hand-picked
Figure 10.
Total ion chromatograms (TICs) from LmPy-GC/MSMS analysis of the
To perform LmPy-GC/MSMS analyses on isolated particles (
The experimental results indicated the presence of higher molecular weight n-alkanes, ranging from C25 to C31. These compounds were also identified in the bitumen, as illustrated in Figure 10A, suggesting that they were not completely removed in the thermal extraction stage. It is important to note that the presence of aromatic compounds was not detected. This relevant information indicates that the temperature conditions were not severe, avoiding secondary reactions and cleavage of alkyl groups.
4.2 LmPy-GC/MSMS of solid bitumen samples
An isolated solid bitumen particle was analyzed to check its molecular organic composition. Figure 11 shows the identification and selection of solid bitumen particles for pyrolysis and, of the craters and halo, and oil droplets produced in the sample after laser irradiation. The most favorable parameters, 25A, and 0.8 s, for primary pyrolysate generation, were obtained after testing different laser energy and irradiation time.
Figure 11.
Selected isolated solid bitumen particle after LmPy-GC/MSMS analysis showing craters and halo and oil droplets being produced by laser irradiation. Photomicrographs were taken under TWL (a) and FM (b).
4.2.1 Geochemical characterization of the original bitumen sample
The total ion chromatogram (TIC) in Figure 12, shows the predominance of 8,14-Secohopane C29 hydrocarbon (m/z 123) and absence of other members of this major hydrocarbon series such as D-ring monoaromatic 8,14- secohopane (m/z 365) and D, E-ring aromatic 8,14-secohopane (m/z 363). Results showed low concentration of
Figure 12.
TIC and profile of
4.2.2 Geochemical characterization of the compounds released during the LmPy-GC/MSMS analysis of bitumen sample
Results of chromatographic characterization (Figure 13) of compounds released during LmPy-GC/MSMS analysis show that the most abundant compound is the monoaromatic 8,14 Secohopanoid D-ring (m/z 365), besides the secondary presence of the aromatic D, E-ring 8,14-secohopanoids (m/z 363) and 8,14, Secohopane C29 hydrocarbon (m/z 123). These aromatic compounds were absent in the original sample characterization. The laser experiments also produced
Figure 13.
TIC and profile of
4.2.3 Geochemical characterization of residual bitumen sample after LmPy-GC/MSMS analysis
In Figure 14 can be observed, for the sample of residual bitumen after LmPy-GC/MSMS analysis, the predominance of the D-ring monoaromatic 8,14 Secohopanoid (m/z 365) and secondarily the presence of D, E-ring aromatic 8,14 secohopanoids (m/z 363), and other secohopane hydrocarbons (m/z 123). The presence of
Figure 14.
TIC and profile of
5. LmPy-GC/MSMS system performance
The analyses of both types of samples showed in the chapter it was possibly observed that apart from the variation of laser parameters no significant difference, referent to the relative abundance of the hydrocarbon products, was observed between analyses of the same sample. Nevertheless, it was observed that parameters such as; the size of the formed craters and thus the intensity of the pyrolyzate distribution are degree focusing dependent as well as span time, and laser’s energy settings (Figure 15).
Figure 15.
Isolated
6. Final remarks
The laser beam, in the LmPy system, is focused on smaller areas (than 100 μm) due to the use of focusing lenses. This makes it possible to analyze individual microscopic particles among a wide variety of small components in complex mixtures. The short duration of the laser beam enables the thermal cleavage of single particles or even macromolecules, using a huge amount of energy. This thermal interaction between the laser and matter triggers a sequence of pyrolysis reactions releasing compounds that, in turn, are examined by GC/MSMS.
The level of temperature is critical to any pyrolysis analysis. The temperatures must be high enough to break molecular bonds without destroying the material. Hence, caution needs to be taken when pyrolysis temperature is selected. Overly high temperatures result in molecular degradation, which generates non-specific products while low temperatures are not analytically useful due to their low degradation potential. In this way, to obtain successful analytical pyrolysis one must select a temperature that allows the controlled degradation of macromolecules, producing a wide variety of products, quickly enough to be compatible with GC [26]. However, the control temperature in the LmPy system is not possible. On the other hand, the laser beam also provides a thermal flux (1000°C/s) enough to allow heating of polymeric materials in less than 2 seconds. This extremely fast laser incidence is a unique feature of micropyrolysis and has the fundamental purpose of reducing or annulling secondary reactions, allowing the production of original substances [15, 17].
Another characteristic of LmPy is the temperature applied. This temperature corresponds to the required energy to break covalent bonds, which can modify the molecular stereochemical features during the pyrolysis analysis. Consequently, the highly energetic, collimated, and coherent laser radiation is able to supply significant amounts of thermal energy in tiny and specific areas. These properties of directional coherent and monochromatic high-powered laser irradiation allow its use to thoroughly investigate non-volatile and thermally labile materials on a microscopic scale [3].
The use of the technique still has major hurdles to overcome, especially the high costs involved. Other difficulties are that the laser-material interactions are not fully understood, and not all samples couple efficiently to the laser to produce thermal degradation products. However, LmPy and other related methods have shown success in characterizing several organic components, significantly improving the possibility of studying the chemical composition of heterogeneous and small-sized materials, as well as identifying specific biomarkers in isolated OWM.
The destination of waste produced at the end of the consumption chain for industrialized products has attracted greater attention from various sectors of the economy, the scientific community, and public authorities. This concern with the reuse of raw materials is aimed at generating less environmental impact and making the consumption of products more sustainable [27].
In the field of pyrolysis, there are various initiatives to incorporate certain products into the circular economy. A good example is plastic, the mass production of which causes a number of environmental problems, and its final disposal is inefficient. Chemical recycling, a thermal process for breaking polymer bonds, has therefore been gaining ground in studies to increase the useful life of plastics [28]. One application widely used by researchers is pyrolysis as a technique for converting plastic waste into economically noble materials such as oils, gases, and ashes [29]. These measures do not close the loop, but they ensure more sustainable use of fossil energy to produce this waste and reduce greenhouse gas emissions.
LmPy-GC/MSMS is an experimental technique that can contribute to the development of circular models by enabling detailed analysis of individual elements in complex mixtures. It was initially developed to meet the specific demands of the oil exploration industry. However, its methodological proposal can be used in various economic or scientific segments with the aim of identifying substances and understanding the molecular structures of pyrolyzed products, offering alternatives for improving recycling processes, creating materials with greater added value, and eliminating undesirable waste. One of the advantages of LmPy-GC/MSMS is the low amount of sample required to pyrolyze these components in the order of micrograms.
Acknowledgments
The authors would like to thank CSIRO (Commonwealth Scientific and Industrial Research Organization), Division of Petroleum Resources (AUS) for assembling LmPy-GC/MSMS system, and PETROBRAS/Brazil for financial support. Special thanks to
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