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Thermal Analysis of the Essential Oil of Aniba rosaeodora Ducke by TGA and DSC

Thermal analysis comprises a group of techniques to measure a physical property of a sample subjected to controlled temperature programming, including thermogravimetry (TGA), derivative thermogravimetry (DTG) and differential scanning calorimetry (DSC). The species Aniba duckei Kostermans, synonymous with Aniba rosaeodora Ducke, occurs in the Amazon and is also known as rosewood. Essential oils are complex oily aromatic liquids obtained by extraction from highly volatile aromatic plant materials as secondary metabolites. This research aimed to chemically and thermally characterize the essential oil of Aniba duckei. Gas chromatography was used for the quantification, by the external standard method, of the main component. The thermal study of the oil and the linalool standard was carried out using DSC and TG-DTG. The essential oil yield was 1.93% and the analysis by GC-MS confirmed the majority presence of linalool, with a concentration of 89.34% in the oil. The technical analysis revealed some similarity between the curves of pure linalool and essential oil, which is attributed to the high content of linalool in the oil. The differences between the boiling points and enthalpies of the linalool and oil pattern are justified by the presence of minor components, their concentrations in the oil and because they are substances of different polarities, molecular masses and intermolecular forces. The techniques proved to be efficient to characterize the oil. The results of the thermal analysis of the essential oil of the Aniba rosaeodora plant species are unprecedented, enabling the determination of linalool in by DSC.


Introduction Essencial oils
Brazilian biodiversity is one of the richest on the planet, with a high rate of biological endemism, dispersed in unique biomes (Maia et al., 2009;Barata, 2012;Joly et al., 2011). Natural products are produced by plants, as the specimens produce their own food and also have the ability to deal with adverse situations. In an irregular environment, they produce certain chemicals called secondary metabolites, which are used as an attack and defense mechanism (Nazir et al., 2021).
Among these secondary metabolites are essential oils, which are volatile, lipophilic, odorous and liquid products extracted from different parts of aromatic plants, which have different and complex chemical composition and guarantee adaptive advantages to the plants in the environment in which they are inserted (Oussalah et al., 2007;Hasani et al., 2018;Tuğçe et al., 2021). Essential oils are mainly composed of monoterpenes, sesquiterpenes and phenylpropanoids, metabolites that confer their organoleptic characteristics. Essential oils from various plant species are used in perfumery, cosmetics, food and as adjuvants in medicines, being considered important in commercial and monetary aspects (Bizzo et al., 2009;Kreutz et al., 2021).

The Vegetal Species Aniba rosaeodora Ducke
The botanical species Aniba duckei Kostermans, synonymous with Aniba rosaeodora Ducke (Maia, 2000;Teles et al., 2018), of the Lauraceae family, commonly known as rosewood, was discovered in Brazil in Juriti Velho, in the state of Pará in 1925 (Correa et al., 1975;Siani et al., 2000). Its tree can reach up to 30 meters in height and its trunk 2 meters in diameter. The flowers are ferruginous and the fruit is a drupe, 2 to 3 cm long (Sampaio et al., 2005). The extraction of native essences such as Aniba rosaeodora, began to consolidate during the Second World War (Vivan et al., 2011).
Linalol has wide application in several areas, being used as a starting point in important syntheses such as that of linanyl acetate and tested as an acaricide, bactericide and fungicide, sedative and anticonvulsant. A recent application for the oil is in aromatherapy. It is a product in great demand internationally for its use as a fixative in perfumes, for example the famous Chanel number 5 (Chaar, 2000;Sudam, 1971;Teles, Mouchrek Filho et al., 2017).

Thermogravimetry (TGA) and Derived Thermogravimetry (DTG)
Thermogravimetry is a technique in which the mass variation (loss or gain) that occurs in the sample is monitored as a function of time (constant temperature) or as a function of temperature. The measurement is performed using an equipment called a thermobalance, which consists of a combination of an electronic microbalance coupled to an oven and a linear temperature programmer, allowing the continuous weighing of a sample as a function of temperature, as the sample is heated or cold (Santos et al., 2004;Aslan et al., 2018;Mallick et al., 2018).
Derived thermogravimetry (DTG) is the first derivative of the thermogravimetric curve, that is, the derivative of mass variation in relation to time or temperature. The DTG curve presents the information in a clearer way, with the area being directly proportional to the mass variation, leading to the prompt determination of the peak temperature and indicating the initial and final temperatures of the process (Gonçalves, Teixeira & Teixeira, 2003).

Differential Scanning Calorimetry (DSC)
Differential Scanning Calorimetry (DSC) is the technique in which the difference in energy released or absorbed by the sample is measured, in relation to a thermally inert reference material, as a function of temperature, while the sample and the reference are subjected to a temperature programming (Bernal et al., 2002).
When a material undergoes a change of physical state or chemical reaction, heat is released or absorbed. During the thermal event, the DSC measures change in the thermal energy of the sample. In general, phase transition, dehydration, reduction and some decomposition reactions produce endothermic effects, while crystallization, oxidation and some decomposition reactions produce exothermic effects (Dantas, 2006).

Thermal Analysis of Essential Oils
There are vast and diverse applications of thermal analysis in industrial activities, such as chemical, pharmaceutical, cosmetics, food and petrochemicals, among others. There is also the literature on thermal analysis of essential oils, for characterization, evaluation of their thermal behavior, stability and investigation of possible adulteration (Hazra et al., 2002;Hazra et al., 2004;Monteiro et al., 2011;Cremasco et al., 2011;Siqueira et al., 2016).
However, there is no research in the literature on thermal analysis of essential oil of the plant species aniba rosaeodora Ducke nor of species in its family -Lauraceae. Thus, the present work reports the research in which the chemical characterization and thermal analysis of the essential oil of the plant species Aniba rosaeodora Ducke and its major component linaloolwas carried out by Thermogravimetric Analysis (TGA) and by Differential Scanning Calorimetry (DSC).
The essential oil was extracted from 30 grams of the sample with 300 mL of distilled water, by hydrodistillation, in a Clevenger system, and the oil was dried by percolation in anhydrous Na2SO4. The yield was calculated in the mass/mass ratio. Research, Society and Development, v. 11, n. 3, e3411326085, 2022 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v11i3.26085 4 GC-MS essential oil analysis was performed on a Varian chromatograph, model 3900, using helium as carrier gas with flow in the column of 1 mL min -1 ; Injector temperature: 270°C, split 1:50; capillary column (30m x 25 mm) by stationary phase VF-1ms (100% methylsiloxane 0.25 μm) and oven temperature programmed to 60°C and then increased to 220°C at a rate of 4°C min -1 and then increased again to 260°C, this time at a rate of 1°C min -1 , with total running time of 100 minutes.
For mass spectrometer, the manifold, ion trap and transfer line temperatures were set to 50, 190 and 200°C, respectively. 1.0 μL (automatic injector CP-8410) aliquots of the samples diluted were injected in proportion of 20 μL for 1.5 mL of hexane.
Linalool was quantified by the external standard method, considering its high concentration in the samples.
For identification of the compounds the spectral databases of the spectral libraries NIST105, NIST21, WILEY139 and AMSDIS (Automated Mass Spectral De-convolution Mass & Identification System) software were used, as well as references (Teles et al., 2018;Namara et al., 2007;Adams, 2001). For linalool, confirmation was also by addition of standard.

Thermal analysis
The calorimetric curves were obtained in a Thermal Analyzer, brand TA INSTRUMENTS, model SDT 2920 through the non-isothermal method of analysis, at a heating ratio of 10 °C min -1 and temperature range of 25-350 °C, in order to verify the thermal decomposition profile. The DSC (Differential Scanning Calorimetry) curves of the essential oil of Aniba rosaeodora Ducke and of the linalool standard, stored in closed aluminum (Al) sample holders, with and without holes, in atmospheres of air or nitrogen gas (N2), were obtained with a heating ratio of 10 ºC min -1 .

Results
From the essential oil chromatogram, the substances listed on Table 1 were identified, which include: the peak number in the order of elution, the retention time (tRET) of each substance in the column (in minutes), the name of the substance identified, the percentage of normalized area (%A) which indicates the relative distribution of the compounds in the sample.   In nitrogen atmosphere, the DSC curve for 5.55 mg of the standard of linalool and the sample of 10.00 mg of essential oil of Aniba rosaeodora resulted in grafics showed in Fig. 2, A e B, respectively.

Thermal analysis
For the linalool standard, the DSC curve shows a single peak with a temperature of 206.24 °C and an enthalpy of 253.6 J g -1 . The sample of the essential oil of Aniba rosaeodora Ducke showed two endothermic transitions, the first with peak temperature of 106.12 °C and enthalpy of 360.5 J g -1 , and the second with peak temperature of 209.90 °C and enthalpy of 62.88 J g -1 .   Research, Society andDevelopment, v. 11, n. 3, e3411326085, 2022 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v11i3.26085

Discussion
Comparing the values for the essential oil of the branches of the Aniba rosaeodora species with those in the literature, it is observed that there is a strong similarity. The substances identified from the chromatogram of the essential oil of Aniba rosaeodora Ducke showed that the major component is linalool, with 89.34%, which is also in agreement with the literature.
The DSC curves for the determination of standard linalool boiling temperatures in air and nitrogen show little variation in the peak temperatures and vaporization enthalpies of linalool. Lower values in an oxidizing atmosphere occur through the reaction of alcohol, forming less polar substances. The difference in peak temperatures and vaporization enthalpies verified in the DSC curves of the air atmosphere in essential oil samples and nitrogen gas is due to the presence of minor components in the oil, whereas the fact to this variation be small is justified by the high content of linalool in the essential oil.
The DSC curves obtained from the linalool standard demonstrated the decomposition of linalool and confirmed the absence of water of hydration. The DSC curves obtained with the essential oil, on the other hand, show an endothermic transition around 100 °C, which may show that the essential oil has water of hydration.
The TG-DTG curves in air ( Fig. 3 and Fig. 4) showed a single decomposition step for both standard and essential oil, resulting from the volatilization process of the linalool or the oil. This analysis also showed that they both lost more than 99% of their mass. The oil starts to lose mass at a temperature lower than the standard temperature and ends up at a slightly higher temperature, which can be explained by the presence of minor components in the oil, as some are more volatile than linalool and others are less.
The difference between the temperature values of the essential oil and the standard is explained by the fact that linalool is the major component in the oil, with 89.34%. The temperature value attributed to the boiling temperature of linalool is similar to that found in the literature (Merk, 1996;Cavalheiros, Chaar, Breviglieri & Chierice, 2004).
The differences between the boiling points and enthalpies of the measured linalool and essential oil standard are justified by the presence of the minor components, their concentrations in the essential oil and their interactions. It should be considered that the oil has other substances of different polarities, molecular masses and intermolecular forces, which contribute to these differences.
The thermal analysis of essential oil, using TG-DTG e DSC, opened a new path for essential oil analysis. The results obtained were unprecedented for the essential oil of Aniba rosaeodora Ducke, even allowing to suggest the quantitative determination of linalool by DSC.
As this is a new and efficient technique for determining the boiling temperatures of essential oils, further studies should be carried out in order to broaden its spectrum of scientific investigation of essential oils, as well as envisioning that the technique can be used in certification and quantification of these oils, considering that many of them with high economic value are frequently adulterated.