Method for quantifying calcium oxalates contained in a surface formation or biomass

Abstract

The present invention relates to a method for quantifying calcium oxalates contained in a sample of a surface formation or biomass, which comprises heating the sample in an inert atmosphere between an initial temperature of between 100 and 300°C and a final temperature of between 575 and 800°C, and measuring a curve (MES) of the temperature-dependent evolution of the amount of CO released by the sample during heating in an inert atmosphere; next, determining a mass of carbon released by the calcium oxalates of the sample in the form of CO during heating in an inert atmosphere on the basis of the measured curve (MES) and an interpolation (INTER) of this curve between a first temperature (T1) of between 390 and 450° C, and a second temperature (T2) of between 550 and 590°C; and then, based on an exponential relationship, deducing therefrom a mass of the calcium oxalates of the sample.

Description

[0001] METHOD FOR QUANTIFYING CALCIUM OXALATES CONTAINED IN A SURFACE FORMATION OR BIOMASS

[0002] Technical field

[0003] The present invention relates to the field of soil science and environmental geosciences, more specifically the present invention relates to the field of quantification of calcium oxalates contained in surface formations, and in particular in soils, or in biomass.

[0004] The evolution of national and international regulatory frameworks to address environmental issues leads to the establishment of "Measurement, Reporting and Verification" (MRV) procedures to monitor progress made against objectives and expected impacts, and to evaluate the effectiveness of the means used. In the field of soil science, the targeted issues are essentially linked to the preservation or improvement of soil quality (ecological issue) while ensuring food security for populations (agronomic issue) and promoting the sequestration of atmospheric carbon (C) and its long-term storage (climate issue).Public authorities, economic actors and the scientific community are therefore paying particular attention to carbon balances and greenhouse gas emissions, which requires the deployment of new standards and new analytical tools to quantify and characterize forms of C according to their nature and properties.

[0005] Calcium oxalates play a specific role in soils fed by plant tissues of oxalogenic plants as they initiate a pathway for the formation of secondary carbonate minerals (Oxalate Carbonate Pathway, OCP) involving oxalotrophic microbial and / or fungal activity. Documented under different species in several geographical areas, these OCP soils arouse particular interest, not only from the scientific community, but also from soil management and exploitation operators as they would be the site of additional sequestration of atmospheric carbon in mineral form. From this economic perspective, the rapid quantification of calcium oxalates present in OCP soils constitutes an essential step to correctly assess their potential for additional carbon storage.

[0006] Prior art

[0007] The following documents will be cited during the description: Misiewicz B, Mencer D, Terzaghi W, VanWert AL. Analytical Methods for Oxalate

[0008] Quantification: The Ubiquitous Organic Anion. Molecules. 2023 Apr 4;28(7):3206. doi:10.3390 / molecules28073206

[0009] Kutaish, N., Poonam, A., Dollimore, D., Thermal analysis of calcium oxalate samples obtained by various preparative routes, Thermochimica Acta, Volume 297, Issues 1—2, 1997, Pages 131-137, ISSN 0040-6031, https: / / doi.org / 10.1016 / S0040-6031 (97)00002-6.

[0010] Muon, R., Ket, P., Sebag, D., Boukbida, HA, Podwojewski, P., Hervé, V., Ann, V., Jouquet, P., Termite constructions as patches of soil fertility in Cambodian paddy fields, Geoderma Regional, Volume 33, 2023, e00640, ISSN 2352-0094, https: / / doi.Org / 10.1016 / j.geodrs.2023.e00640.

[0011] Several techniques can be used to quantify calcium oxalates in solution.

[0012] We know in particular the enzymatic methods ("Enzymatic Methods"), liquid chromatography coupled with mass spectrometry ("Liquid Chromatography-Mass Spectrometry"), electrochemical methods ("Electrochemical Methods"), or even methods based on UV, fluorescence or colorimetric analysis ("UV, Colorimetric, and Fluorescence Methods"). We can refer in particular to the document (Misiewicz et al., 2023) which presents these different methods.

[0013] In the field of thermal analyses, we know in particular the document (Kutaish et al. 1997) which describes a quantification of calcium oxalate by means of a thermogravimetry method, more precisely by quantifying mass losses during thermal decomposition.

[0014] However, existing quantification methods require the isolation or extraction of calcium oxalates, which poses technical problems because these compounds do not preserve well in solution.

[0015] We also know the document (Muon et al., 2023) which describes a thermal method to characterize the organic matter of OOP soils. More precisely, this document describes a method based on measurements of quantities of hydrocarbon compounds (HC), carbon monoxide (CO) and / or carbon dioxide (CO2) released over time by a sample subjected to a heating sequence in an inert atmosphere then to a heating sequence in an oxidizing atmosphere. This document shows that the recorded thermograms (curve of evolution of a measured quantity as a function of temperature) for these samples containing calcium oxalates present a specific signature, more precisely that the recorded thermograms present peaks which are interpreted as being due to calcium oxalates. However, no quantification of calcium oxalates in OCP soils is described in this document.

[0016] The present invention overcomes these drawbacks. In particular, the present invention makes it possible to determine the quantity of calcium oxalates in a sample of a surface formation or biomass simply, quickly and accurately, without requiring the isolation or extraction of the calcium oxalates from this sample. More specifically, the present invention is based on a thermal analysis of a sample comprising (among other things) calcium oxalates, and requires only a single heating phase (in this case under an inert atmosphere), during which only the CO released by the sample is measured. Furthermore, the mass of calcium oxalates determined by the method according to the invention is accurate, as will be demonstrated in the application example below.

[0017] Summary of the invention

[0018] The present invention relates to a method for quantifying calcium oxalates contained in a surface formation or in a biomass, from a representative sample of said surface formation or said biomass.

[0019] The method according to the invention comprises at least the following steps:

[0020] A) said sample is heated in an inert atmosphere according to a sequence of temperatures having an initial temperature between 100 and 300°C and a final temperature between 575 and 800°C, said sequence of temperatures comprising at least one thermal gradient between 1°C / min and 50°C / min, and at least one quantity of CO released during said heating in an inert atmosphere is continuously measured;

[0021] B) from a curve of the evolution as a function of the temperature of said quantity of CO released during said heating in an inert atmosphere by said sample, a first curve is constructed in the following manner:

[0022] - the values ​​of said first curve between said initial temperature and a first temperature and the values ​​of said first curve between a second temperature and said final temperature correspond to the values ​​of said curve of the evolution as a function of the temperature of said quantity of CO released during said heating in an inert atmosphere by said sample, said first temperature being between 390 and 450°C, and said second temperature being between 550 and 590°C;

[0023] - the values ​​of said first curve between said first and second temperatures correspond to an interpolation between said values ​​of said curve of the evolution as a function of the temperature of said quantity of CO released during said heating in an inert atmosphere by said sample at said first and second temperatures; C) a second curve is determined from a difference between said curve of the evolution as a function of the temperature of said quantity of CO released during said heating in an inert atmosphere by said sample and said first curve, and, from said second curve, a mass of carbon released by said calcium oxalates of said sample in the form of CO during said heating in an inert atmosphere is determined;

[0024] D) a mass of said calcium oxalates of said sample is determined according to a formula of the type:

[0025] 128

[0026] Mox = - * a *e P* MCCOox

[0027] 24 where Mox is said mass of said calcium oxalates of said sample, a is a first predetermined coefficient, p is a second predetermined coefficient, and MCCOox is said mass of carbon released by said calcium oxalates of said sample in the form of CO during said heating in an inert atmosphere.

[0028] According to one implementation of the invention, said first coefficient a may be between 40 and 45, and may preferably be 43, and said second coefficient p may be between 1.6 and 1.8, and may preferably be 1.7.

[0029] According to an implementation of the invention, in a prior step and by means of a plurality of reference samples, said mass of said calcium oxalates of each of said reference samples being predetermined, values ​​of said first and second coefficients can be determined in the following manner: steps A), B) and C) are applied to each of said reference samples to determine said mass of carbon released in the form of CO by said calcium oxalates of each of said reference samples during said heating in an inert atmosphere, and said values ​​of said first and second coefficients are determined by means of an exponential regression method, of said mass of carbon released in the form of CO by said calcium oxalates of each of said reference samples during said heating in an inert atmosphere and of said predetermined masses of said calcium oxalates of each of said reference samples.

[0030] According to one implementation of the invention, said mass of carbon released by said calcium oxalates of said sample in the form of CO during said heating in an inert atmosphere can be determined according to a formula of the type: where C2(T) corresponds to said second curve expressed in mV, T1 and T2 correspond respectively to said first temperature and said second temperature, m is the mass of said sample in mg, and k is a conversion coefficient from a measurement in mV to a measurement in mg.

[0031] According to one implementation of the invention, said first temperature may be 400°C and / or said second temperature may be 575°C.

[0032] According to an implementation of the invention, said first temperature and / or said second temperature can be determined by means of a method for searching for minima of a curve implemented so as to determine a local minimum in respectively a first portion of said curve of the evolution as a function of the temperature of said quantity of CO released during said heating in an inert atmosphere by said sample between 390 and 450°C and / or a second portion of said curve of the evolution as a function of the temperature of said quantity of CO released during said heating in an inert atmosphere by said sample between 550 and 590°C.

[0033] According to one implementation of the invention, said interpolation may be a linear interpolation.

[0034] According to one implementation of the invention, said initial temperature may be 200°C and / or said final temperature may be 650°C.

[0035] According to one implementation of the invention, said temperature sequence may comprise an isothermal stage of a predetermined duration at said initial temperature of said temperature sequence, said predetermined duration of said isothermal stage at said initial temperature of said temperature sequence being between 1 and 5 minutes, and preferably being 3 minutes.

[0036] According to one implementation of the invention, at least one of said thermal gradients of said temperature sequence may be between 15°C / min and 40°C / min, and may very preferably be 25°C / min.

[0037] Other characteristics and advantages of the method according to the invention will appear on reading the following description of non-limiting examples of embodiments, with reference to the figures appended and described below.

[0038] List of figures

[0039] Figure 1A schematically illustrates the evolution of the temperature as a function of time of an example of implementation of the temperature sequence according to the invention.

[0040] Figure 1B schematically illustrates the evolution of the temperature as a function of time of another example of implementation of the temperature sequence according to the invention. Figure 2 presents an example of a curve representing the evolution as a function of time t of the quantity of CO measured during step 1) of the method according to the invention, a first curve resulting from the application of step 2.1) of the method according to the invention, a second curve corresponding to the difference between the curve representing the evolution as a function of time t of the quantity of CO measured in step 1) and the first curve resulting from the application of step 2.1), as well as a curve representing the evolution as a function of time of the temperature of the pyrolysis furnace during the temperature sequence implemented for step 1).

[0041] Figure 3 shows the mass of calcium oxalates determined by preliminary weighing for four mixtures of soils and calcium oxalates as a function of the mass of CO released by the calcium oxalates of each of the mixtures determined in step 2.2 of the method according to the invention, as well as curve C of an exponential regression applied to these points.

[0042] Description of the embodiments

[0043] The invention relates to a method for quantifying calcium oxalates present in a surface formation or biomass.

[0044] The term "superficial deposit" means a continental or coastal formation, loose or secondarily consolidated, originating from the mechanical and / or chemical disintegration of pre-existing rocks, and formed at the lithosphere / biosphere / atmosphere interface. A distinction is made between (i) "allochthonous superficial formations" (such as colluvium, alluvium, loess, etc.) which have undergone or are still undergoing near or distant displacements, and no longer rest on their parent material, and "autochthonous superficial formations" (such as arenas, alterities, flint clays, etc.) which have evolved in situ from a parent material which still constitutes their substrate. According to the invention, soils are included in superficial formations.

[0045] Soil refers to the entire set of outer layers of surface formations, the properties of which are directly controlled by the mutual actions of water, air, living and dead organisms, and even human activities in the most recent periods.

[0046] Biomass means a material of an organic nature directly produced by a living organism, and more or less degraded by natural decomposition and / or transformed by a controlled physicochemical process.

[0047] The method according to the invention requires having at least one representative sample of the surface formation or biomass. The surface formation sample may have been taken manually from a pit or by coring using an auger. Advantageously, the surface formation sample as taken is sieved using a sieve whose orifices have, for example, a diameter of 2 mm. The surface formation or biomass sample is then dried at a temperature below 40°C, then crushed until fragments having dimensions of less than 200 μm are obtained.

[0048] The method according to the invention can be advantageously but not limitatively implemented using the ROCK-EVAL® device (IFP Energies nouvelles, France), as described in patents FR 2227797 (US 3953171) and FR 2472754 (US 4352673). Indeed, the ROCK-EVAL® device comprises at least:

[0049] - a pyrolysis oven in an inert atmosphere,

[0050] - means of measuring carbon monoxide (CO).

[0051] The process can alternatively be implemented using any oven allowing heating in an inert atmosphere, cooperating with one or more carbon monoxide measuring devices.

[0052] The method according to the invention comprises at least the following steps:

[0053] 1) Heating sequence under inert atmosphere (pyrolysis)

[0054] 2) Determination of the quantity of calcium oxalates contained in the sample

[0055] Thus, unlike the methods according to the prior art, the method according to the invention does not require a heating sequence under an oxidizing atmosphere, and no measurement of hydrocarbon compounds and / or CO2.

[0056] Indeed, the Applicant observed, through analyses carried out on numerous samples with and without oxalates, that (1) the quantities of CO emitted during pyrolysis were proportional to the quantity of calcium oxalates in the test sample;

[0057] (2) CO2 emissions during pyrolysis and oxidation are confused with those resulting from the thermal decomposition of carbonate minerals in surface formation and soil samples containing limestone and are therefore not exploitable.

[0058] The steps of the method according to the invention are described below in a non-limiting manner for a soil sample. The steps of the method according to the invention can in fact be applied just as well to a sample coming from another layer of a surface formation, or to a biomass sample.

[0059] 1) Heating sequence under inert atmosphere (pyrolysis)

[0060] During this step, the sample is heated under an inert atmosphere (such as for example under a flow of nitrogen or helium) according to a sequence of temperatures whose initial temperature (denoted T0 hereinafter) is between 100 and 300°C and is preferably 200°C, and the final temperature (denoted TF hereinafter) is between 575 and 800°C, and is preferably 650°C. In addition, according to the invention, the sequence of temperatures of step 1) comprises at least one thermal gradient between 1°C / min and 50°C / min.

[0061] According to one implementation of the invention, the temperature sequence under an inert atmosphere may comprise an isothermal plateau at the initial temperature TO, followed by a predetermined thermal gradient so as to raise the temperature of the sample to the final temperature TF. Figure 1A schematically illustrates the evolution of the temperature T as a function of time t of such a temperature sequence, presenting an isothermal plateau at the temperature TO, followed by a thermal gradient until reaching the temperature TF.

[0062] Advantageously, the temperature sequence under an inert atmosphere of this embodiment may further comprise a second isothermal stage, at the final temperature TF. In other words, a second isothermal stage at the final temperature TF follows the phase of the temperature sequence in the form of a thermal gradient. This makes it possible to continue, if necessary, the cracking of the compounds having a cracking temperature close to the final temperature TF of the temperature sequence under an inert atmosphere according to the invention. Figure 1 B schematically illustrates the evolution of the temperature T as a function of time t of such a temperature sequence, having two isothermal stages, at temperatures TO and TF as defined above, and linked together by a thermal gradient.

[0063] According to one implementation of the invention, the initial temperature TO can be 200°C. This temperature is in fact sufficient to release the most labile organic compounds present in most samples of soil, organic amendment or sediment.

[0064] According to one implementation of the invention, the final temperature TF may be 650°C, so as to avoid obtaining a CO curve showing an incomplete peak at the end of pyrolysis, in particular on natural samples (fresh and dried plant tissues, litter, peat, and plant composts, organo-mineral and mineral soils, surface formations).

[0065] According to one implementation of the invention, the isothermal stage(s) of the temperature sequence under an inert atmosphere may have a predetermined non-zero duration (for example, greater than half a minute), preferably between 1 and 5 minutes, and very preferably 3 minutes. Such durations make it possible to consider that the cracking of the compounds having a cracking temperature close to the temperature of the isothermal stage is complete. According to the implementation of the invention in which the temperature sequence under an inert atmosphere according to the invention comprises several isothermal stages and in particular two isothermal stages at temperatures TO and TF, the duration of an isothermal stage may be different from the duration of the other isothermal stages.

[0066] According to one implementation of the invention, the thermal gradient(s) of the temperature sequence under an inert atmosphere may be between 15° and 35°C / min, and are preferably 25°C / min. Such values ​​constitute compromises allowing the thermal cracking of the compounds, while limiting the duration of implementation of the process.

[0067] According to the invention, a quantity of CO contained in an effluent resulting from said heating is continuously measured (i.e. continuously over time). In other words, during this sequence, the quantity of CO released by the sample by thermal cracking of the organic matter and by the thermal decomposition of the carbonaceous minerals can be continuously measured. The measurement of the quantity of CO can be carried out by means of an infrared (IR) type detector or any other means known to those skilled in the art.

[0068] Generally speaking, this particular heating sequence under an inert atmosphere is sufficient to allow thermal cracking of the mineral and organic forms of carbon present in a sample.

[0069] According to one implementation of the invention, the sequence of temperatures under an inert atmosphere according to the invention may be preceded by a phase of temperature rise of the pyrolysis furnace, which may be in the form of a thermal gradient, for example between 1 and 50°C / min, preferably between 20 and 25°C / min, or any other form of temperature rise curve of the pyrolysis furnace. This preliminary phase of temperature rise of the pyrolysis furnace makes it possible to bring the pyrolysis furnace to the initial temperature of the sequence of temperatures under an inert atmosphere according to the invention. This preliminary phase may contribute to starting the thermal cracking of compounds whose cracking temperature is lower than the initial temperature of the sequence of temperatures under an inert atmosphere according to the invention, in particular in the case of fresh biological tissues.

[0070] According to one implementation of the invention, the sequence of temperatures under an inert atmosphere according to the invention may be followed by a phase of lowering the temperature of the pyrolysis furnace, which may be in the form of a thermal gradient, for example between -1 and -50°C / min, preferably between -20 and -25°C / min, or any other form of curve of temperature reduction of the pyrolysis furnace. This final phase of lowering the temperature of the pyrolysis furnace makes it possible, if necessary, to complete the thermal cracking of the compounds associated with the final temperature of the sequence of temperatures under an inert atmosphere according to the invention. According to one implementation of the invention, at the end of this step, a curve representative of the quantity of CO released over time during the pyrolysis phase may be obtained.

[0071] It is quite obvious for a person skilled in the art to move from a curve representing the quantity of CO released over time to a curve representing the quantity of CO released as a function of temperature, since the sequence of temperatures (evolution of temperature as a function of time T(t)) is known.

[0072] 2) Determination of the quantity of calcium oxalates

[0073] During this step, the quantity of calcium oxalates contained in the sample studied is determined from the continuous measurement of the quantity of CO released by the sample during heating in an inert atmosphere in step 1). This step can be divided into three sub-steps:

[0074] 2.1) Determination of a first curve representative of a quantity of CO released by the sample if it did not include calcium oxalates

[0075] 2.2) Determination of a second curve representative of a quantity of CO released by the calcium oxalates of the sample

[0076] 2.3) Determination of the quantity of calcium oxalates contained in the sample

[0077] These sub-steps are detailed below.

[0078] 2.1) Determination of a first curve representative of a quantity of CO released by the sample if it did not include calcium oxalates

[0079] According to the invention, a first curve representative of a quantity of CO released by the sample if it did not include calcium oxalates is constructed from a curve of the evolution as a function of temperature (in other words a thermogram) of the quantity of CO released during said heating in an inert atmosphere by said sample, in the following manner:

[0080] - the values ​​of said first curve between said initial temperature and a first temperature and the values ​​of said first curve between a second temperature and said final temperature correspond to the values ​​of said curve of the evolution as a function of the temperature of said quantity of CO released during said heating in an inert atmosphere by said sample, said first temperature being between 390 and 450°C, and said second temperature being between 550 and 590°C;

[0081] - the values ​​of said first curve between said first and second temperatures correspond to an interpolation between said values ​​of said curve of the evolution as a function of the temperature of said quantity of CO released during said heating in an inert atmosphere by said sample at said first and second temperatures. In other words, the first curve according to the invention corresponds to the CO curve measured in step 1), except between the first and second temperatures according to the invention, where it is modified by interpolation of the values ​​of the CO curve measured in step 1) for the first and second temperatures. This first curve is representative of a quantity of CO which would be released by the sample if it did not include calcium oxalates.

[0082] Figure 2 shows an example of a MES curve representing the evolution as a function of time t of the quantity QCO of CO measured (in mV) during step 1) of the method according to the invention, an INTER curve resulting from the application of step 2.1) of the method according to the invention, an OX curve which corresponds to the difference between the MES curve measured in step 1) and the INTER curve resulting from the application of step 2.1), and a curve T(t) representing the evolution as a function of time t of the temperature T of the pyrolysis furnace during the sequence of temperatures implemented for step 1). It can be observed in this figure that the measured MES curve of CO has a peak in a range of temperatures for the release of calcium oxalates, more precisely between a first temperature T1 of 450°C and a second temperature T2 of 550°C for this sample.This figure also shows the INTER curve which corresponds to the measured CO MES curve, except between temperatures T1 of 450°C and T2 of 550°C, where the INTER curve corresponds to an interpolation (here linear) between the values ​​of the CO MES curve measured at temperatures T1 of 450°C and T2 of 550°C.

[0083] According to one implementation of the invention, the interpolation implemented during step 2.1) of the method according to the invention may be a linear interpolation. Alternatively, the interpolation implemented during step 2.1) of the method according to the invention may be carried out by means of a polynomial interpolation, for example a cubic interpolation (for example cubic interpolation splines).

[0084] According to an implementation of the invention, the first temperature and / or the second temperature according to the invention can be determined by means of a method for searching for minima of a curve implemented so as to determine a local minimum in respectively a first portion of the curve of the evolution as a function of the temperature of said quantity of CO released during heating in an inert atmosphere between 390 and 450°C and / or in a second portion of the curve of the evolution as a function of the temperature of the quantity of CO released during heating in an inert atmosphere between 550 and 590°C. This search can be carried out by means of any method for determining minima of a curve. Preferably, the curve of the evolution as a function of time of the quantity of CO released by the sample considered can be smoothed, prior to searching for the minima(s).At the end of this sub-step, we obtain a first curve which corresponds to a curve representative of a quantity of CO released by the sample if it did not include calcium oxalates.

[0085] 2.2) Determination of a second curve representative of a quantity of CO released by the calcium oxalates of the sample

[0086] During this step, a second curve is first determined from the difference between said curve of the evolution as a function of the temperature of said quantity of CO released during said heating in an inert atmosphere by said sample and said first curve.

[0087] In other words, this step involves calculating the difference between the CO curve measured in step 1) and the CO curve determined in step 2.1). This produces a second curve that is representative of the quantity of CO that would be released by the sample if it only included calcium oxalates. Or, in other words, a second curve is obtained that is representative of the quantity of CO released by the calcium oxalates in the sample during pyrolysis. Figure 2, already described, shows in particular an example of an OX curve resulting from the difference between the CO curve measured in step 1) and the CO curve determined in step 2.1).

[0088] Then from this second curve, according to the invention, we determine a quantity of CO released by the calcium oxalates in the sample.

[0089] According to one implementation of the invention, the mass of CO released by the calcium oxalates of the sample can be determined according to a formula of the type: where C2(T) corresponds to the second curve expressed in mV, T1 and T2 correspond respectively to the first temperature and the second temperature according to the invention, m is the mass of the sample in mg, and k is a conversion coefficient making it possible to convert a quantity of CO measured in mV into a quantity of CO measured in mg. According to one implementation of the invention, the coefficient k can be equal to 1000. Note that the coefficient 12 / 28 of equation (1) is a stoichiometric coefficient, corresponding to the molar mass of carbon divided by the molar mass of CO.

[0090] 2.3) Determination of the quantity of calcium oxalates contained in the sample

[0091] According to the invention, a mass of calcium oxalates present in the sample considered, noted Mox, is determined according to a formula of the type:

[0092] Mox where a is a first predetermined coefficient and p is a second predetermined coefficient. Note that the coefficient 128 / 24 of equation (2) is a stoichiometric coefficient, corresponding to the molar mass of calcium oxalate divided by the molar mass of the two carbon atoms present in the formula of calcium oxalates.

[0093] In other words, according to the invention, the mass of calcium oxalates present in a sample varies exponentially with the mass of CO released by the calcium oxalates in the sample. Indeed, the Applicant was able to determine, by means of numerous laboratory tests carried out on samples having known calcium oxalate contents, that the mass of carbon emitted in the form of CO by the calcium oxalates contained in a sample is linked by an exponential law to the mass of calcium oxalates contained in this same sample.

[0094] According to a preferred implementation of the invention, the first coefficient a may be between 40 and 45, and the second coefficient p may be between 1.6 and 1.8. These preferred values ​​were able to be determined by the Applicant, by means of several tests carried out in the laboratory on a plurality of reference samples for which the mass of calcium oxalates is known. The above ranges of the first and second coefficients a and come from the uncertainties on the measurement of the quantity of CO released (in mV), of the mass of the sample (in mg), and on the estimation of the coefficient of conversion of the measurements in mV to measurements in mg of the quantity of CO released. Preferably, the first coefficient a may be 43 and / or the second coefficient may be 1.7.

[0095] According to an implementation of the invention, in a step prior to step 1) and by means of a plurality of reference samples whose mass of calcium oxalates is predetermined (for example by means of chemical extraction and dosage according to the prior art), values ​​of the first and second coefficients of equation (2) above can be determined in the following manner: steps 1), 2.1) and 2.2) described above are applied to each of the reference samples to determine the mass of carbon released in the form of CO by the calcium oxalates of each of the reference samples during heating in an inert atmosphere.Then the values ​​of the first and second coefficients of equation (2) are determined by means of an exponential regression method, from the mass of carbon released in the form of CO by the calcium oxalates of each of the reference samples, and from the predetermined masses of the calcium oxalates of each of the reference samples.

[0096] Thus, the present invention makes it possible to determine the quantity of calcium oxalates in a sample in a simple, rapid and precise manner. First of all, the method according to the invention does not require isolating or extracting the calcium oxalates from a sample. In addition, the method according to the invention requires measurement only of the CO released by the sample, and only during a pyrolysis phase. It is therefore simple and economical because it does not require a heating sequence under an oxidizing atmosphere and no HC and CO2 detectors (saving an oxidation furnace or a pyrolysis furnace allowing oxidation, HC and CO2 detectors, and energy for heating in an oxidizing atmosphere in particular).

[0097] Examples

[0098] The characteristics and advantages of the method according to the invention will appear more clearly on reading the application example below.

[0099] The application example described below was carried out using cultivated fluviosol soil samples. These soils have been subjected to conventional agriculture with chemical inputs and crop rotation (wheat, sunflower) since 2005. The sample was taken in April 2017 at a depth of between 0 and 30 cm. These soils are free of calcium oxalates.

[0100] From these soil samples, four mixtures denoted M1, M2, M3, and M4 were formed with pure oxalate, added in known concentrations. Table 1 presents the characteristics of each of these mixtures, including the mass of the soil sample, the mass of the added oxalate, and the resulting oxalate content. A 250 mg sample of each of these mixtures was then analyzed using the method according to the invention.

[0101] Figure 3 shows in the form of triangles the mass of calcium oxalates Mox (determined by preliminary weighing) for each of the mixtures M1, M2, M3, and M4, as a function of the mass of CO released by the calcium oxalates MCCOox of each of the mixtures at the end of step 2.2 of the process according to the invention. This figure 3 also shows the curve C resulting from an exponential regression applied to these points and whose first coefficient a is equal to 42.6 and the second coefficient p is equal to 1.7. Thus, as observed in figure 3, the mass of calcium oxalates present in a sample is linked according to an exponential law with the mass of CO released by the calcium oxalates of the sample considered.

Claims

Claims 1. Method for the quantification of calcium oxalates contained in a surface formation or in a biomass, from a representative sample of said surface formation or said biomass, characterized in that at least the following steps are applied: A) said sample is heated in an inert atmosphere according to a sequence of temperatures having an initial temperature (T0) between 100 and 300°C and a final temperature (TF) between 575 and 800°C, said sequence of temperatures comprising at least one thermal gradient between 1°C / min and 50°C / min, and at least one quantity of CO released during said heating in an inert atmosphere is continuously measured; B) from a curve (MES) of the evolution as a function of the temperature of said quantity of CO released during said heating in an inert atmosphere by said sample, a first curve (INTER) is constructed in the following manner: - the values ​​of said first curve (INTER) between said initial temperature (TO) and a first temperature (T1) and the values ​​of said first curve (INTER) between a second temperature (T2) and said final temperature (TF) correspond to the values ​​of said curve (MES) of the evolution as a function of the temperature of said quantity of CO released during said heating in an inert atmosphere by said sample, said first temperature (T1) being between 390 and 450°C, and said second temperature (T2) being between 550 and 590°C; - the values ​​of said first curve (INTER) between said first (T1) and second (T2) temperatures correspond to an interpolation between said values ​​of said curve (MES) of the evolution as a function of the temperature of said quantity of CO released during said heating in an inert atmosphere by said sample at said first (T1) and second (T2) temperatures; C) a second curve (OX) is determined from a difference between said curve (MES) of the evolution as a function of the temperature of said quantity of CO released during said heating in an inert atmosphere by said sample and said first curve (INTER), and, from said second curve (OX), a mass of carbon released by said calcium oxalates of said sample in the form of CO during said heating in an inert atmosphere is determined; D) a mass of said calcium oxalates of said sample is determined according to a formula of the type: Mox where Mox is said mass of said calcium oxalates of said sample, a is a first predetermined coefficient, p is a second predetermined coefficient, and MCCOox is said mass of carbon released by said calcium oxalates of said sample in the form of CO during said heating in an inert atmosphere.

2. Method according to claim 1, in which said first coefficient a is between 40 and 45, and is preferably 43, and said second coefficient p is between 1.6 and 1.8, and is preferably 1.

7. 3.Method according to claim 1, wherein, in a prior step and by means of a plurality of reference samples, said mass of said calcium oxalates of each of said reference samples being predetermined, values ​​of said first and second coefficients are determined in the following manner: steps A), B) and C) are applied to each of said reference samples to determine said mass of carbon released in the form of CO by said calcium oxalates of each of said reference samples during said heating in an inert atmosphere, and said values ​​of said first and second coefficients are determined by means of an exponential regression method, of said mass of carbon released in the form of CO by said calcium oxalates of each of said reference samples during said heating in an inert atmosphere and of said predetermined masses of said calcium oxalates of each of said reference samples.

4. Method according to one of the preceding claims, in which said mass of carbon released by said calcium oxalates of said sample in the form of CO during said heating in an inert atmosphere is determined according to a formula of the type: MCCOox = where C2(T) corresponds to said second curve expressed in mV, T1 and T2 correspond respectively to said first temperature (T1) and to said second temperature (T2), m is the mass of said sample in mg, and k is a conversion coefficient from a measurement in mV to a measurement in mg.

5. Method according to one of the preceding claims, in which said first temperature (T1) is 400°C and / or said second temperature (T2) is 575°C.

6. Method according to one of claims 1 to 4, in which said first temperature (T1) and / or said second temperature (T2) are determined by means of a method for searching for minima of a curve implemented so as to determine a local minimum in respectively a first portion of said curve (MES) of the evolution as a function of the temperature of said quantity of CO released during said heating in an inert atmosphere by said sample between 390 and 450°C and / or a second portion of said curve (MES) of the evolution as a function of the temperature of said quantity of CO released during said heating in an inert atmosphere by said sample between 550 and 590°C.

7. Method according to one of the preceding claims, wherein said interpolation is a linear interpolation.

8. Method according to one of the preceding claims, in which said initial temperature (T0) is 200°C and / or said final temperature (TF) is 650°C.

9. Method according to one of the preceding claims, in which said temperature sequence comprises an isothermal stage of a predetermined duration at said initial temperature (T0) of said temperature sequence, said predetermined duration of said isothermal stage at said initial temperature (T0) of said temperature sequence being between 1 and 5 minutes, and preferably being 3 minutes.

10. Method according to one of the preceding claims, in which at least one of said thermal gradients of said temperature sequence is between 15°C / min and 40°C / min, and is very preferably 25°C / min.

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