These procedures provided important structural data about the analyzed samples. It was possible to detect a reduction in the percentage of aldehyde, carboxylic acid, and ether compounds, while the level of aromatic, alkene, and alkane compounds increased. In other words, it was possible to use NMR spectroscopy in the investigation of structural changes of pyrolysis oil compounds before and after an accelerated aging process.
The NMR technique has an advantage with respect to other common methods, such as infrared spectroscopy or chromatographic techniques, as it has high resolution compared to the others, and it also offers the possibility of obtaining valuable structural information. Besides that, the accelerated aging process of pyrolysis oil is carried out inside the NMR equipment, so the analysis is made in situ and in real time [ 96 ]. NMR spectroscopy can also be used for quality control either of the vegetable oils used as raw material for biofuel production or the final obtained product [ 38 ].
The quality of the vegetable oil influences many properties in the produced biodiesel, such as viscosity, lubricity oxidative stability, cold flow, ignition quality, and the heat of combustion.
The type of fatty acid also influences the final product. Monounsaturated and saturated fatty acids are more stable than highly unsaturated ones, although they show higher viscosity and a bigger tendency to solidify in cold weather. Analytical techniques for fatty acid determination, such as gas chromatography and near-infrared spectroscopy, need calibration models using some standards with similar chemical composition related to the analyzed sample [ 38 ].
High-resolution NMR has an advantage above the others because calibration models are not necessary. It is possible to quantify compounds in the sample through observing the integrated area of peaks with the chemical shift corresponding to a specific type of fatty acid [ 97 ]. Usually, the acquisition time for this type of analysis is quite long. However, Prestes et al.
It was possible to analyze more than a thousand samples per hour, thus working as a powerful tool to speed up the selection of oilseeds that are suitable for biodiesel applications [ 98 ].
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Garro Linck et al. Another NMR application is as the PAT tool as described for infrared spectroscopy , which can be used for the in-situ monitoring of the biodiesel production process, where sampling is not necessary [ 85 ]. As was discussed, 1 H NMR spectroscopy can be a robust, rapid, and quantitative method that can be applied for determining the presence of multiple components due to specific chemical shifts in the spectrum, and reaction monitoring can be applied over time, based on the integration of individual proton signals.
An example of this application is the work of Anderson and Franz, where they used high-resolution NMR equipment for the monitoring of the biodiesel production reaction, evaluating the transesterification of triacylglycerol TAG and the resulting products, including diacylglycerol DAG , monoacylglycerol MAG , glycerol, and fatty acid methyl esters FAME [ ]. Due to different molecular structures and different environments and different forms of hydrogen, it is possible to differentiate the signals from each molecule and evaluate them over time, as shown in Figure Anderson and Franz were able to see, then, the decrease of the TAG bands due to the biodiesel production and also the isomers of intermediates, such as DAG, throughout the reaction.
This is a statement of the sensibility and sensitivity of the NMR spectroscopy technique in achieving information from the molecules, which could not be achieved by other techniques.
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This technique is very useful in identifying functional groups in a molecule because a correlation between the absorption bands and the functional group can be done. While inorganic compounds normally absorb light in the visible part of the spectrum, organic molecules usually present some functional groups capable of absorbing radiation from UV light sources.
These functional groups must be unsaturated or have a heteroatom with non-bonding electrons, such as oxygen, sulfur, or halogens. Table 3 shows some functional groups and the radiation wavelength absorbed [ 1 ]. According to the Lambert-Beer law, the intensity of the absorption of radiation by the species present in the sample is directly proportional to its concentration in the system.
Thus, quantitative determination of compounds containing absorbing groups can be easily made. The utilization of blends between biodiesel and conventional diesel is very common, and the blend level can vary. The blend level directly influences some important characteristics of the fuel, such as its lubricity and tail pipe emissions [ ]. Thus, it is very important to develop adequate methods for such applications. For example, Zawadzki et al. The proposed method is robust, even with changes in the biodiesel feedstock and the fuel diesel origin. An interesting example is a statistical study performed by Foglia et al.
This system was employed to determinate the level of simple alkyl esters of fats and oils biodiesel blended in petroleum diesel and to validate this method for this kind of analysis using different biodiesel feedstocks. A study performed by Fernandes et al.
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Another application that is indirectly correlated to the concept of visible light spectroscopy and that has been applied throughout the past few years is image analysis. The simplest definition for the word image can be understood as an optical replica of a luminous or illuminated object formed by a mirror of lens. Therefore, the object that gives rise to an image does so through an interaction with electromagnetic radiation, either by emission or absorption processes.
Over the years, a wide variety of analytical methods involving image analysis as the main tool have been introduced, due to the ease with which they are carried out in a fast, noninvasive, and inexpensive way compared to more advanced spectroscopic techniques. New fields were opened with the introduction of chemometric methods to analyze image analysis results, such as exploratory image analysis, multivariate statistical process monitoring, multivariate regression, and image resolution [ ].
The versatility of this technique has been explored in several areas, from flame determination of elements [ ] to biochemistry analysis [ ], providing fast and cheap results in a simple way. In the field of renewable fuels, image analysis has been also explored for applications in various stages of production.
For example, for the application of microalgae for biofuel production, an area that has attracted increasing attention recently, digital image processing has been successfully used as a way for monitoring and quantifying the amount of biomass present on photobioreactors, using the RGB color system [ ], and also to build the light distribution profile in microalgae cultivation [ ], a key factor for mobile productivity. Another interesting application related to microorganisms with potential use in biodiesel production is the determination of the intracellular accumulation level of lipids in yeast cells, which can be done in a dynamic and nondestructive manner via high-content images [ ].
Because biodiesel quality control is becoming increasingly important, given the high market demand for renewable energy sources, methods such as image analysis have emerged as fast and reliable alternatives for the evaluation of some parameters. Another example is the application of image analysis together with thin layer chromatography, a very simple method, for glycerol detection in biodiesel [ ].
This organic compound is a byproduct of the manufacturing process and is considered a contaminant in the final product. Similarly to fossil fuels, the burning of biodiesel, for example, causes the emission of pollutants, such as NOx species. Image analysis has proved useful in the prediction of the level of these emissions by the use of flame radical imaging to monitor the biomass combustion process [ ]. In the coming years, researchers expect that image analysis will be explored in other stages of the production process because of its potential for easily obtaining relevant results for the biofuel area.
The determination of trace elements in biofuels is a key parameter for their use, and the importance of conducting such analyses reliably and quickly grows as the global demand for renewable energy sources becomes greater. The main spectroscopic techniques currently used for trace elements determination in biofuels are based on the phenomena of emission and absorption of electromagnetic radiation by atoms or elementary ions.
In the case of the atomic absorption principle, electronic excitation occurs in atoms from the ground state to more energetic electronic levels because of energy absorption at specific wavelengths that are characteristic of each element. The excited electrons tend to quickly return to the ground state, releasing energy at wavelengths also characteristic of each element.
This is the concept explored by atomic emission methods. Both of the concepts are shown in Figure ICP-OES employs a highly energetic plasma, formed by an electrically neutral gas, usually argon, converted to positive ions and electrons. Such plasma has enough energy to atomize, ionize, and virtually excite all the elements of the periodic table to more energetic electronic states [ ]. These species tend to return rapidly to the ground state, releasing energy at characteristic wavelengths depending on the elements present in the analyzed sample, and the radiation intensity is directly proportional to the concentration of the element in the sample [ 1 ].
AAS also employs atomization methods as well as emission techniques.
Commonly, this process is carried out by means of a flame, in which desolvation, evaporation, and dissociation of the molecules into atoms take place [ 1 ]. Another common atomizing method is electrothermal atomization, in which the energy for volatilization and atomization is provided by means of an electric current applied to a graphite furnace [ ]. Two more specific atomization techniques are hydride atomization by heating a quartz tube, a very common method for the analysis of some metals, such as arsenic, and the cold vapor technique, which is widely used for mercury determination.
After atomization, the analytes are submitted to radiation from a source, whether a single emission line source e. The analytes in the ground state absorb energy at specific wavelengths corresponding to their more favorable electronic transitions, thus generating an absorption spectrum whose intensity depends on the population of the atoms in the ground state, directly proportional to their concentration in the sample. Trace elements may cause significant problems with serious implications for the use of biofuels, such as biodiesel and bioethanol.
Sodium and potassium, for example, are used in the form of hydroxides in the synthesis of biodiesel, and together with Al, Ca, Fe, Mg, and Ti, they form a group of elements that tend to form a large amount of ash metal oxides after the fuel is burned [ , ]. It causes difficulties for the operation of the gears, reducing the longevity of the engines.
Furthermore, these elements are also involved in corrosion processes [ ]. Some transition metals, such as Cu, Pb, Cd, and Zn, can cause biodiesel oxidation [ ], resulting in residues that may be deposited in the engines.
Moreover, they can contribute to air pollution and cause environmental damage due to their toxicity [ ]. The sulfur level in the produced biofuels also must be carefully monitored because of emission legislation. Low sulfur levels are also needed for good performance in modern engines [ , ].
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Iron and vanadium can act as catalyst poisoners and may reduce the efficiency of advanced catalysts that are commonly used in gears [ ]. Some additives that are added to biofuels to improve physical or burning characteristics also contain metals, and their levels must be monitored. Because of these problems, strict laws have been adopted for the maximum level of metal contaminants in commercial biodiesel.
Table 4 presents the tolerated level of some metals established by ASTM standards [ ]. It has become essential to develop precise, accurate, and sensitive methods that are applicable for monitoring metal contaminants to qualify biofuels according to the established standards for their use. In general, organic matter decomposition procedures often microwave assisted can be applied to convert the matter to a simple aqueous matrix, therefore reducing the chances of spectral interference with the measurements and avoiding difficulties in the injection of these samples [ — ].
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This process is also necessary because of the difficulties in introducing the samples in atomizers and in selecting appropriating calibration standards for the analyzed system [ ]. In some cases, however, direct injection of samples of biodiesel, bioethanol or fuel ethanol may be advantageous in order to reduce the number of steps of the analysis procedure. This operation brings some drastic consequences.
The injection, for example, can be greatly hindered due to the physical-chemical characteristics of the sample, such as viscosity and surface tension, thus modifying the ease of suction of the components. Both in the case of the ICP-OES technique in which a nebulizer is used for generating an aerosol [ ] and in the case of the sample introduction systems used in flame atomic absorption equipment, problems in the injection of organic samples affect the efficiency and reproducibility of the analytical methods.
Burning organic compounds normally generates instability in the plasma with ICP-OES [ ], and the generated pyrolysis products cause some spectral interference. Another problem may be the deposition of material originating in the pyrolysis processes on the torch or other spectrometer facilities [ , ].