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, in which the sample is heated in an inert atmosphere from an initial temperature of 100 to 300°C to a final temperature of 575 to 800°C, and a curve (MES) is measured showing the temperature evolution of the amount of CO released by the sample during heating in an inert atmosphere. Then, the mass of carbon released by the calcium oxalates in the sample as CO during heating in an inert atmosphere is determined from the measured curve (MES) and an interpolation (INTER) of this curve between a first temperature (T1) of 390 to 450°C and a second temperature (T2) of 550 to 590°C. Then, using an exponential relationship, we deduce the mass of calcium oxalates in the sample. Figure 2 to be published.

Description

Title of the invention: Method for quantifying calcium oxalates contained in a surface formation or biomass. Technical field

[0001] 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.

[0002] The evolution of national and international regulatory frameworks to address environmental challenges has led to the implementation of Measurement, Reporting and Verification (MRV) procedures to monitor progress made towards objectives and expected impacts, and to assess the effectiveness of the resources deployed. In the field of soil science, the targeted challenges are essentially related to preserving or improving soil quality (ecological challenge) while ensuring food security for populations (agronomic challenge) and promoting the sequestration of atmospheric carbon (C) and its long-term storage (climate challenge).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 the forms of C according to their nature and properties.

[0003] Calcium oxalates play a specific role in soils nourished by the plant tissues of oxalogenic plants, as they initiate a secondary carbonate mineral formation pathway (Oxalate Carbonate Pathway, OCP) involving oxalotrophic microbial and / or fungal activity. Documented in various tree species across several geographical areas, these OCP soils are of particular interest, not only to the scientific community but also to soil management and farming operators, as they are considered sites of additional atmospheric carbon sequestration in mineral form. From this economic perspective, the rapid quantification of calcium oxalates present in OCP soils is an essential step in properly assessing their potential for additional carbon storage. Prior art

[0004] The following documents will be cited during the description:

[0005] Misiewicz B, Mencer D, Terzaghi W, VanWert AL. Analytical Methods for Oxalate Quantification: The Ubiquitous Organic Anion. Molecules. 2023 Apr 4;28(7):3206. doi:10.3390 / molecules28073206

[0006] 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.

[0007] 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 Régional, Volume 33, 2023, e00640, ISSN 2352-0094, https : / / doi.org / 10.1016 / j.geodrs.2023.e00640.

[0008] Several techniques allow for the quantification of calcium oxalates in solution.

[0009] Enzymatic methods, liquid chromatography-mass spectrometry, electrochemical methods, and methods based on UV, fluorescence, or colorimetry analysis are well known. Reference may be made, in particular, to the document (Misiewicz et al., 2023) which presents these different methods.

[0010] 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 thermogravimetric method, more precisely by quantifying the mass losses during thermal decomposition.

[0011] However, existing quantification methods require isolating or extracting calcium oxalates, which poses technical problems because these compounds do not keep well in solution.

[0012] We also know of the document (Muon et al., 2023) which describes a thermal method for characterizing the organic matter of OCP soils. More specifically, this document describes a method based on measurements of the 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 followed by a heating sequence in an oxidizing atmosphere. This document shows that the recorded thermograms (curve showing the evolution of a measured quantity as a function of temperature) for these samples containing calcium oxalates exhibit a specific signature; more precisely, the recorded thermograms show peaks that are interpreted as being due to calcium oxalates. However, no quantification of calcium oxalates in OCP soils is described in this document.

[0013] 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 in a simple, rapid, and precise manner, without requiring the isolation or extraction of calcium oxalates from the sample. More specifically, the present invention is based on a thermal analysis of a sample containing (among other things) calcium oxalates, and requires only a single heating phase (in this case, under an inert atmosphere), during which only the CO₂ released by the sample is measured. Furthermore, the mass of calcium oxalates determined by the method according to the invention is precise, as will be demonstrated in the application example below. Summary of the invention

[0014] The present invention relates to a method for quantifying calcium oxalates contained in a surface formation or in biomass, from a representative sample of said surface formation or of said biomass.

[0015] The method according to the invention comprises at least the following steps:

[0016] 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 a thermal gradient between 1°C / min and 50°C / min, and at least a quantity of CO released during said heating in an inert atmosphere is continuously measured;

[0017] B) from a curve of the evolution as a function of 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:

[0018] - 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 temperature of said quantity of CO released during said heating in inert atmosphere by said sample, said first temperature being between 390 and 450°C, and said second temperature being between 550 and 590°C;

[0019] - the values ​​of said first curve between said first and second times pératures correspond to an interpolation between the said values ​​of the said curve of the evolution as a function of temperature of the said quantity of CO released during the said heating in an inert atmosphere by the said sample at the said first and second temperatures;

[0020] C) a second curve is determined from a difference between said curve of the evolution as a function of temperature of said quantity of CO released during said heating in an inert atmosphere by said sample and said first curve, and, from From the said second curve, a mass of carbon released by the said calcium oxalates of the said sample is determined in the form of CO during the said heating in an inert atmosphere;

[0021] D) a mass of said calcium oxalates of said sample is determined according to a formula of the type:

[0022] Mox =

[0023] 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.

[0024] According to one embodiment of the invention, said first coefficient a may be between 40 and 45, and may preferably be 43, and said second coefficient 0 may be between 1.6 and 1.8, and may preferably be 1.7.

[0025] According to one embodiment of the invention, in a preliminary step and using a plurality of reference samples, the mass of calcium oxalates in each of the reference samples being predetermined, values ​​of the first and second coefficients can be determined as follows: steps A), B), and C) are applied to each of the reference samples to determine the mass of carbon released as CO by the calcium oxalates in each of the reference samples during heating in an inert atmosphere, and the values ​​of the first and second coefficients are determined using an exponential regression method.of the said mass of carbon released as CO by the said calcium oxalates of each of the said reference samples during said heating in an inert atmosphere and of the said predetermined masses of the said calcium oxalates of each of the said reference samples.

[0026] According to one embodiment of the invention, the mass of carbon released by the calcium oxalates of the sample in the form of CO during heating in an inert atmosphere can be determined according to a formula of the type:

[0027] MCCOox = H 4^2 ( T )

[0028] 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.

[0029] According to one embodiment of the invention, said first temperature may be 400°C and / or said second temperature may be 575°C.

[0030] According to one embodiment of the invention, said first temperature and / or said second temperature can be determined by means of a method for finding minimum 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 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 temperature of said quantity of CO released during said heating in an inert atmosphere by said sample between 550 and 590°C.

[0031] According to one embodiment of the invention, said interpolation may be a linear interpolation.

[0032] According to one embodiment of the invention, said initial temperature may be 200°C and / or said final temperature may be 650°C.

[0033] According to one embodiment of the invention, said temperature sequence may include an isothermal plateau of a predetermined duration at said initial temperature of said temperature sequence, said predetermined duration of said isothermal plateau at said initial temperature of said temperature sequence being between 1 and 5 minutes, and preferably 3 minutes.

[0034] According to one embodiment 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.

[0035] Other features and advantages of the process according to the invention will become apparent from the following description of non-limiting examples of embodiments, with reference to the figures attached and described below. List of figures [Fig AI]

[0036] Fig. 1A schematically illustrates the evolution of temperature over time in an example of implementation of the temperature sequence according to the invention. [Fig IB]

[0037] Fig. 1B schematically illustrates the evolution of temperature over time in another example of implementing the temperature sequence according to the invention. [Fig 2]

[0038] Figure 2 shows an example of a curve representing the evolution over time t of the quantity of CO measured during step 1) of the process according to the invention, a first curve resulting from the application of step 2.1) of the process according to the invention, a second curve corresponding to the difference between the curve representing the evolution over time t of the quantity of CO measured at step 1) and the first curve resulting from the application of step 2.1), as well as a curve representing the evolution over time of the temperature of the pyrolysis oven during the temperature sequence implemented for step 1). [Fig 3]

[0039] Figure 3 shows the mass of calcium oxalates determined by prior 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 process according to the invention, as well as the curve C of an exponential regression applied to these points. Description of the implementation methods

[0040] The invention relates to a method for quantifying calcium oxalates present in a surface formation or biomass.

[0041] The term "superficial deposit" refers to a continental or coastal formation, loose or secondarily consolidated, resulting from the mechanical and / or chemical weathering of pre-existing rocks, and formed at the lithosphere / biosphere / atmosphere interface. A distinction is made between (i) "allochthonous superficial deposits" (such as colluvium, alluvium, loess, etc.) which have undergone or are still undergoing local or long-distance transport and no longer rest on their parent material, and "autochthonous superficial deposits" (such as sandy soils, weathered material, flinty clays, etc.) which evolved in situ from parent material that still constitutes their substrate. According to the invention, soils are included in the superficial deposits.

[0042] By soil, we mean all the external layers of surface formations, the properties of which are directly controlled by the mutual actions of water, air and living and dead organisms, or even human activities for the most recent periods.

[0043] Biomass is understood to mean 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 physico-chemical process.

[0044] The process according to the invention requires 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 with an auger. Advantageously, the surface formation sample as taken is sieved using a sieve with orifices having, for example, a diameter of 2 mm. The surface formation or biomass sample is then dried at a temperature below 40°C and then ground into fragments with dimensions less than 200 µm.

[0045] The process according to the invention can advantageously, but not exclusively, be 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: - a pyrolysis furnace in an inert atmosphere, - means of measuring carbon monoxide (CO).

[0046] The process can alternatively be implemented using any furnace allowing heating in an inert atmosphere, cooperating with one or more carbon monoxide measurement devices.

[0047] The method according to the invention comprises at least the following steps:

[0048] 1) Heating sequence under an inert atmosphere (pyrolysis)

[0049] 2) Determination of the quantity of calcium oxalates contained in the sample

[0050] Thus, unlike prior art processes, the process according to the invention does not require a heating sequence under an oxidizing atmosphere, and no measurement of hydrocarbon compounds and / or CO2.

[0051] 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 portion; (2) the CO2 emissions during pyrolysis and oxidation are indistinguishable from those resulting from the thermal decomposition of carbonate minerals in the surface formation and soil samples containing limestone and are therefore not usable.

[0052] The steps of the process according to the invention are described below in a non-limiting manner for a soil sample. The steps of the process according to the invention can in fact be applied equally well to a sample from another layer of a surface formation, or to a biomass sample.

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

[0054] During this step, the sample is heated under an inert atmosphere (as by (example under a flow of nitrogen or helium) according to a temperature sequence in which the initial temperature (hereafter denoted T0) is between 100 and 300 °C and preferably 200 °C, and the final temperature (hereafter denoted TF) is between 575 and 800 °C, and preferably 650 °C. Furthermore, according to the invention, the temperature sequence of step 1) includes at least one thermal gradient between 1 °C / min and 50 °C / min.

[0055] According to one embodiment of the invention, the temperature sequence under an inert atmosphere may include an isothermal plateau at the initial temperature T0, followed by a predetermined thermal gradient to raise the sample temperature to the final temperature TF. Figure 1A illustrates this in a manner schematically the evolution of the temperature T as a function of time t of such a sequence of temperatures, showing an isothermal plateau at the temperature TO, followed by a thermal gradient until reaching the temperature TF.

[0056] Advantageously, the temperature sequence under an inert atmosphere in this embodiment may further include a second isothermal plateau at the final temperature TF. In other words, a second isothermal plateau at the final temperature TF follows the phase of the temperature sequence that takes the form of a thermal gradient. This allows the cracking of compounds with a cracking temperature close to the final temperature TF of the temperature sequence under an inert atmosphere according to the invention to continue, if necessary. Figure 1B schematically illustrates the evolution of the temperature T as a function of time t in such a temperature sequence, exhibiting two isothermal plateaus at temperatures TO and TF as defined above, and linked by a thermal gradient.

[0057] According to one embodiment of the invention, the initial temperature TO can be 200°C. This temperature is indeed sufficient to release the most labile organic compounds present in most soil, organic amendment or sediment samples.

[0058] According to one embodiment of the invention, the final temperature TF can be 650°C, so as to avoid obtaining a CO curve with 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).

[0059] According to one embodiment of the invention, the isothermal plateau(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 most preferably 3 minutes. Such durations allow the cracking of compounds having a cracking temperature close to the temperature of the isothermal plateau to be considered complete. According to the embodiment of the invention in which the temperature sequence under an inert atmosphere comprises several isothermal plateaus, and in particular two isothermal plateaus at temperatures T0 and TF, the duration of one isothermal plateau may differ from the duration of the other isothermal plateau(s).

[0060] According to one embodiment of the invention, the thermal gradient(s) of the temperature sequence under an inert atmosphere can be between 15° and 35°C / min, and preferably 25°C / min. Such values ​​represent a compromise that allows for the thermal cracking of the compounds while limiting the implementation time of the process.

[0061] According to the invention, measurement is taken continuously (i.e., continuously in the time), the amount of CO₂ contained in an effluent resulting from said heating. In other words, during this sequence, the amount of CO₂ released by the sample through thermal cracking of organic matter and thermal decomposition of carbonaceous minerals can be continuously measured. The measurement of the amount of CO₂ can be carried out using an infrared (IR) detector or any other means known to those skilled in the art.

[0062] In general, this particular heating sequence under an inert atmosphere is sufficient to allow the thermal cracking of mineral and organic forms of carbon present in a sample.

[0063] According to one embodiment of the invention, the inert atmosphere temperature sequence according to the invention may be preceded by a heating phase 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 type of heating curve for the pyrolysis furnace. This preliminary heating phase of the pyrolysis furnace allows the pyrolysis furnace to be brought to the initial temperature of the inert atmosphere temperature sequence according to the invention. This preliminary phase can help to initiate the thermal cracking of compounds whose cracking temperature is lower than the initial temperature of the inert atmosphere temperature sequence according to the invention, particularly in the case of fresh biological tissues.

[0064] According to one embodiment of the invention, the temperature sequence under an inert atmosphere according to the invention can be followed by a phase of lowering the temperature of the pyrolysis furnace, which can 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 decrease curve for the pyrolysis furnace. This final phase of lowering the temperature of the pyrolysis furnace allows, if necessary, the completion of the thermal cracking of the compounds associated with the final temperature of the temperature sequence under an inert atmosphere according to the invention.

[0065] According to one embodiment of the invention, at the end of this step, a curve representing the amount of CO released over time during the pyrolysis phase can be obtained. It is readily apparent to a person skilled in the art to transform a curve representing the amount of CO released over time into a curve representing the amount of CO released as a function of temperature, since the temperature sequence (evolution of the temperature as a function of time T(t)) is known.

[0066] 2) Determination of the quantity of calcium oxalates

[0067] In this step, the quantity of calcium oxalates contained in the sample studied is determined by continuously measuring the quantity of CO released by the sample during heating in an inert atmosphere in step 1). This This step can be broken down into three sub-steps:

[0068] 2.1) Determination of a first curve representing a quantity of CO released by the sample if it did not contain calcium oxalates

[0069] 2.2) Determination of a second curve representing a quantity of CO released by calcium oxalates in the sample

[0070] 2.3) Determination of the quantity of calcium oxalates contained in the sample

[0071] These sub-steps are detailed below.

[0072] 2.1) Determination of a first curve representing a quantity of CO released by the sample if it did not contain calcium oxalates

[0073] According to the invention, a first curve representing the quantity of CO released by the sample if it did not contain 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:

[0074] - 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 temperature of said quantity of CO released during said heating in inert atmosphere by said sample, said first temperature being between 390 and 450°C, and said second temperature being between 550 and 590°C;

[0075] - the values ​​of said first curve between said first and second times pératures correspond to an interpolation between the said values ​​of the said curve of the evolution as a function of temperature of the said quantity of CO released during the said heating in an inert atmosphere by the said sample at the said first and second temperatures.

[0076] 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 interpolating the values ​​of the CO curve measured in step 1) for the first and second temperatures. This first curve is representative of the amount of CO that would be released by the sample if it did not contain calcium oxalates.

[0077] Figure 2 shows an example of a MES curve representing the evolution over time t of the quantity QCO of CO measured (in mV) during step 1) of the process according to the invention, an INTER curve resulting from the application of step 2.1) of the process 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 T(t) curve representing the evolution over time t of the The temperature T of the pyrolysis furnace during the temperature sequence implemented for step 1). This figure shows that the measured CO₂ TSS curve exhibits a peak within a range of calcium oxalate release temperatures, specifically between a first temperature T1 of 450°C and a second temperature T2 of 550°C for this sample. This figure also presents the INTER curve, which corresponds to the measured CO₂ TSS curve, except between temperatures T1 of 450°C and T2 of 550°C, where the INTER curve represents an interpolation (here linear) between the values ​​of the measured CO₂ TSS curve at temperatures T1 of 450°C and T2 of 550°C.

[0078] According to one embodiment of the invention, the interpolation performed in step 2.1) of the process according to the invention may be linear interpolation. Alternatively, the interpolation performed in step 2.1) of the process according to the invention may be carried out by means of polynomial interpolation, for example cubic interpolation (e.g. cubic interpolation splines).

[0079] According to one embodiment of the invention, the first temperature and / or the second temperature according to the invention can be determined by means of a method for finding the minima of a curve implemented so as to determine a local minimum in, respectively, a first portion of the curve showing the evolution as a function of temperature of the quantity of CO released during heating in an inert atmosphere between 390 and 450°C and / or in a second portion of the curve showing the evolution as a function of temperature of the quantity of CO released during heating in an inert atmosphere between 550 and 590°C. This search can be carried out using any method for determining the minima of a curve. Preferably, the curve showing the evolution as a function of time of the quantity of CO released by the sample under consideration can be smoothed prior to finding the minima(s).

[0080] At the end of this sub-step, we obtain a first curve which corresponds to a curve representing the quantity of CO released by the sample if it did not contain calcium oxalates.

[0081] 2.2) Determination of a second curve representing a quantity of CO released by calcium oxalates in the sample

[0082] During this step, a second curve is first determined from the difference between said curve of the evolution as a function of temperature of said quantity of CO released during said heating in an inert atmosphere by said sample and said first curve.

[0083] 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 yields a second curve that represents the amount of CO that would be released by the sample if it contained only calcium oxalates. Or, as stated Alternatively, a second curve is obtained representing the amount of CO released by the calcium oxalates in the sample during pyrolysis. Figure 2, already described, notably shows 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).

[0084] Then from this second curve, according to the invention, a quantity of CO released by the calcium oxalates of the sample is determined.

[0085] According to one embodiment of the invention, the mass of CO released by the calcium oxalates in the sample can be determined according to a formula of the type:

[0086] MCCOox = *Çc2( T) (1)

[0087] where C2(T) corresponds to the second curve expressed in mV, T1 and T2 correspond respectively to the first and second temperatures according to the invention, m is the mass of the sample in mg, and k is a conversion factor allowing the conversion of a quantity of CO measured in mV to a quantity of CO measured in mg. According to one embodiment of the invention, the factor k can be 1000. Note that the factor 12 / 28 of equation (1) is a stoichiometric coefficient, corresponding to the molar mass of carbon divided by the molar mass of CO.

[0088] 2.3) Determination of the quantity of calcium oxalates contained in the sample

[0089] According to the invention, a mass of calcium oxalates present in the sample considered, denoted Mox, is determined according to a formula of the type:

[0090] Mox = (2)

[0091] where a is a first predetermined coefficient and [3 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.

[0092] 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 has been able to determine, through numerous laboratory tests carried out on samples with known calcium oxalate contents, that the mass of carbon emitted as CO by the calcium oxalates contained in a sample is related by an exponential law to the mass of the calcium oxalates contained in that same sample.

[0093] According to a preferred embodiment of the invention, the first coefficient a may be between 40 and 45, and the second coefficient [3] may be between 1.6 and 1.8. These preferred values ​​were determined by the Applicant through several laboratory tests carried out on a plurality of reference samples. for which the mass of calcium oxalates is known. The above ranges for the first and second coefficients u and P are derived from uncertainties in the measurement of the amount of CO released (in mV), the sample mass (in mg), and the estimation of the conversion factor from mV measurements to mg measurements of the amount of CO released. Preferably, the first coefficient u can be 43 and / or the second coefficient P can be 1.7.

[0094] According to one embodiment of the invention, in a step prior to step 1) and by means of a plurality of reference samples whose mass in calcium oxalates is predetermined (for example by means of chemical extraction and assay 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 as 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 using an exponential regression method, of the mass of carbon released as CO by the calcium oxalates of each of the reference samples, and of the predetermined masses of the calcium oxalates of each of the reference samples.

[0095] 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, the method according to the invention does not require isolating or extracting the calcium oxalates from a sample. Furthermore, the method according to the invention requires measuring only 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 the need for an oxidation furnace or a pyrolysis furnace for oxidation, HC and CO2 detectors, and energy for heating in an oxidizing atmosphere, in particular). Examples

[0096] The characteristics and advantages of the method according to the invention will become clearer upon reading the application example below.

[0097] The application example described below was carried out using soil samples of cultivated fluviosols. These soils have been subjected to conventional agriculture with chemical inputs and to 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.

[0098] [Tables 1] Mixtures Mass of soil (g) Mass of calcium oxalates (mg) Concentration of calcium oxalates (mg / g) Ml 45 10.8 0.24 M2 22.02 5.28 0.12 M3 21.03 2.52 0.06 M4 19.06 1.14 0.03

[0099] From these soil samples, four mixtures, labeled M1, M2, M3, and M4, were formed with pure oxalate added in known concentrations. Table 1 shows the characteristics of each of these mixtures, including the mass of the soil sample, the mass of oxalate added, and the resulting oxalate content. A 250 mg sample of each of these mixtures was then analyzed using the method according to the invention.

[0100] Figure 3 shows, in the form of triangles, the mass of calcium oxalates Mox (determined by prior 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 from 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, where the first coefficient a is 42.6 and the second coefficient P is 1.7.

[0101] Thus, as observed in [Fig.3], the mass of calcium oxalates present in a sample is related according to an exponential law with the mass of CO released by the calcium oxalates of the sample considered.

Claims

Demands

1. A method for quantifying calcium oxalates contained in a surface formation or in biomass, from a representative sample of said surface formation or 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 (TO) between 100 and 300°C and a final temperature (TF) between 575 and 800°C, said sequence of temperatures including 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 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 (Tl) 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 temperature of said quantity of CO released during said heating in an inert atmosphere by said sample, said first temperature (Tl) 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 temperature of said quantity of CO released during said heating in inert atmosphere by said sample at said first (T1) and second (T2) temperatures; C) A second curve (OX) is determined from the difference between said curve (MES) of the evolution as a function of 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; 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, 0 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. A method according to claim 1, wherein said first coefficient a is between 40 and 45, and preferably 43, and said second coefficient P is between 1.6 and 1.8, and preferably 1.

7.

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

4. A method according to any one of the preceding claims, wherein 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: MCCOm ^C2(T) where C2(T) corresponds to said second curve expressed in mV, T1 and T2 correspond respectively to said first temperature (T1) and 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. A method according to any one of the preceding claims, wherein said first temperature (T1) is 400°C and / or said second temperature (T2) is 575°C.

6. A method according to any one of claims 1 to 4, wherein said first temperature (T1) and / or said second temperature (T2) are determined by means of a method for finding 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 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 temperature of said quantity of CO released during said heating in an inert atmosphere by said sample between 550 and 590°C.

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

8. A method according to any one of the preceding claims, wherein said initial temperature (T0) is 200°C and / or said final temperature (TF) is 650°C.

9. A method according to any one of the preceding claims, wherein said temperature sequence includes an isothermal plateau of predetermined duration at said initial temperature (T0) of said temperature sequence, said predetermined duration of said isothermal plateau at said initial temperature (T0) of said temperature sequence being between 1 and 5 minutes, and preferably 3 minutes.

10. A method according to any one of the preceding claims, wherein at least one of said thermal gradients of said temperature sequence is between 15°C / min and 40°C / min, and is most preferably 25°C / min.

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