Methods for the quantitative determination of the components of a solution

Abstract

The invention relates to a method for the quantitative determination of the components of a solution. The method comprises at least the following steps: - Providing a sample, - Providing a reference solution comprising at least a first standard in the form of a 1H-NMR active substance and a second standard in the form of a metal salt of a metal with an atomic number ≥ 10, wherein the solvent is in particular a deuterated liquid, - Dissolving the entire sample in the reference solution to obtain an analytical solution, - Analysis of the analytical solution using 1H-NMR spectroscopy and - Applying a defined volume of the analytical solution to a carrier and allowing the analytical solution to dry, and then - Analysis of the dried analytical solution using XRF spectroscopy, - Evaluation of the data obtained in the analyses. The method according to the invention is advantageously simple and requires only one sampling. Weighing the sample taken is not necessary to determine the composition.

Description

[0001] The present invention relates to a method for the quantitative determination of components of a solution, such as such a method is used in chemical analysis.

[0002] Precise knowledge of the quantitative composition of liquid or solid substances plays a crucial role in many areas of material manufacturing. This knowledge allows, for example, the optimization of separation processes, quality control, and adherence to limit values. Both solids and solids in solution, gases, and liquids are accessible for chemical analysis. For quantitative analysis in particular, it is sometimes necessary to dissolve solids. Furthermore, the analysis of elements from the light and heavy fractions of a material sample sometimes necessitates separate procedures and sample preparations for each fraction.

[0003] The fraction of light elements according to the invention comprises those elements from which organic molecules are predominantly formed, i.e., the group consisting of C, H, O, N, S, and P. The fraction of heavy elements according to the invention comprises all elements from the fourth period onward. The separation according to the invention is particularly relevant for chemical substances containing organic cations and metal cations of the heavy fraction, for which group the inventive method is particularly advantageous. In addition to the metal cations, anions can also be determined in the heavy fraction. Individual elements of the fractions can be evaluated as representatives of organic cations or metal cations.

[0004] Sampling that is only possible with a small sample volume or sample mass places additional demands on the analytical method, e.g. if the proportion of an element to be detected is already small or if an analytical method itself requires a certain sample quantity.

[0005] For the purposes of this invention and description, a sample is understood to be a separated or extracted quantity of a material whose composition is to be analyzed; this can also be referred to as an analysis portion. A sample serves as a representative of the composition of the entire material, in its raw or processed form. In extreme cases, a sample can also comprise an entire workpiece or a complete, specific part of an object, such as an applied layer from a mass-produced item for quality control purposes.

[0006] One analytical method accessible to light elements is, for example, nuclear magnetic resonance spectroscopy (NMR spectroscopy), with 1H-NMR spectroscopy being particularly noteworthy, as it allows access to hydrogen nuclei. NMR spectroscopy typically requires samples in solution. The measurement signals always refer to an internal standard in the solution in which the sample is dissolved. To analyze the target element or compound (molecule), e.g., hydrogen, the so-called chemical shift relative to the internal standard is determined, and quantitative analysis is possible via the integral of the signals. Tetramethylsilane or residual protons in a deuterated solvent are used as standards, for example. In the review article by V. Rizzo and V.Pinciroli (Quantitative NMR in synthetic and combinatorial chemistry, Journal of Pharmaceutical and Biomedical Analysis, Vol. 38, 2005, pp. 851-857) explains the basic requirements, procedures, applications and evaluation methods of quantitative NMR spectroscopy.

[0007] One analytical method accessible to heavy elements is, for example, X-ray fluorescence analysis (XRF), also called X-ray fluorescence spectroscopy. Depending on the X-ray beam's orientation to the sample, XRF exists as total reflection XRF (T-XRF), grazing-incidence XRF (GIXRF), using a micrometer-scale X-ray beam (µ-XRF), and X-ray absorption spectroscopy (XAS), which uses the fluorescence signal and employs a transmission geometry. For quantitative analysis, an internal standard or reference is also required. In the review article by E. Marguí et al. (X-ray fluorescence spectrometry for environmental analysis: Basic principles, instrumentation, applications and recent trends, Chemosphere, Vol. 303, 2022, 135006-1-18) are application, instrumentation, sample preparation, etcAspects for the execution and evaluation of the RFA were discussed.

[0008] An example of the need for quantitative analysis of a material's composition for quality control is the production of perovskite layers for perovskite solar cells. In particular, this requires the determination of elements from both the light and heavy fractions for perovskites containing organic cations.

[0009] The challenges of analyzing the composition of perovskites with mixed cations are discussed, for example, in article 1 by C. Pareja-Rivera et al. (On the True Composition of Mixed-Cation Perovskite Films, ACS Energy Letters, Vol. 3, 2018, pp. 2366-2367). This article employs, among other techniques, NMR spectroscopy. Article 2 by J. Teuscher et al. (Control and Study of the Stoichiometry in Evaporated Perovskite Solar Cells, ChemSusChem, Vol. 8, 2015, pp. 3847-3852) addresses the same topic, but with a focus on different analytical techniques.

[0010] CN 1 18 209 576 A discloses a method for analyzing electrolytes from lithium batteries, in particular the elements phosphorus, sulfur, and fluorine contained therein. In this method, a qualitative analysis of a first, dried sample of the electrolyte is performed using X-ray fluorescence analysis. Subsequently, the elements phosphorus, sulfur, or fluorine in the electrolyte are quantitatively determined using magnetic resonance spectroscopy. For the second step, a deuterated solvent and an internal standard are added to a further, liquid sample of the electrolyte.

[0011] German patent DE 101 38 428 A1 discloses a process in which, in particular, organic chlorine, bromine, iodine, and sulfur compounds are separated from aqueous solutions, sludges, sediments, or solids and, after washing out interfering inorganic compounds, converted into a volatile solvent or solvent mixture. This solution is treated with an internal standard and evaporated onto a support. The elements chlorine, bromine, iodine, and / or sulfur are quantitatively determined in the solution using total reflection X-ray fluorescence analysis.

[0012] The object of the present invention is to provide a method by which both so-called light and heavier elements can be quantitatively determined using a reduced sample quantity compared to the prior art, and this in a simplified form with a single sampling procedure.

[0013] The problem is solved by the method in claim 1. Advantageous embodiments are the subject of the dependent claims.

[0014] The applicant was able to demonstrate that the object of the invention can be solved by a method in which the preparation of a sample in a solution with two standards, one for quantitative analysis using 1H-NMR spectroscopy and one for XRF, is carried out.

[0015] The inventive method for the quantitative determination of components of a solution comprises at least the following steps.

[0016] A sample is taken from the material or workpiece to be tested. The sample may need to be prepared for further processing, e.g., ground. Preparation is unnecessary for samples consisting of easily soluble materials. In extreme cases, a sample can also comprise an entire workpiece or a complete, specific part that forms a unit, such as an entire applied layer of a coating stack in a mass production run for quality control purposes. The advantage of the latter type of sampling (a complete, specific part) is that the information obtained relates to the entire volume and not, for example, only to the surface or sections that may be affected by inhomogeneities.

[0017] Furthermore, a reference solution is provided which includes at least a first standard in the form of a 1H-NMR active substance and a second standard in the form of a metal salt of a metal with an atomic number ≥ 10, and wherein the solvent is in particular a deuterated liquid.

[0018] In principle, all stable organic compounds can be used as standards for 1H-NMR. A selection should be made based on the following criteria: -The proton signal should show a chemical shift on the order of the sample being analyzed. -The standard must not react with the solvent or the substances being analyzed. -The standard must be completely soluble in the solvent. -Overlapping of the signals with those of the sample being analyzed should be avoided.

[0019] The fulfillment of the conditions to be met by the standard may need to be verified experimentally.

[0020] The standard in the form of a 1H-NMR active substance can be, for example, one of the following groups: Acetanilide, benzhydrol, benzoic acid, (Na)-benzoate, benzyl methyl ether, dibromomethane, dichloromethane, 1,2-dichloroethane, diethyl phthalate, 1,4-dimethoxybenzene, dimethoxymethane, dimethyl fumarate, 2,2-dimethylmalonic acid, dimethyl phthalate, dimethyl terephthalate, dimethyl sulfone, 1,4-Dinitrobenzene, diphenylmethane, 1,4-dioxane, furmaric acid, hexamethylcyclotrisiloxane, potassium hydrogen phthalate, maleic acid, mesitylene (1,3,5-trimethylbenzene), sodium acetate, tetradecane, toluene, trichlorethylene, 1,3,5-trichloro-2-nitrobenzene, 2,4,5-trichloropyrimidine, 3,4,5-Trichloropyridine, 2,3,5-Triiodobenzoic acid, 2,4,6-Triiodophenol, 3,4,5-Trimethoxybenzaldehyde, 1,3,5-Trioxane. In particular, the standards dimethyl sulfone and potassium hydrogen phthalate, among others, are especially advantageous because they are easy to handle and pose no risks to health or the environment.

[0021] The primary requirement for XRF standards is that they are soluble in the deuterated solvent used for the NMR reference solution. The XRF standard is, for example, a soluble metal or metal salt with an atomic number ≥ 10, and can be, for example, a metal or metal salt from the following group: yttrium, copper, metal sulfate, metal nitrate (except bismuth nitrate), metal perchlorate (except alkali perchlorates), or metal fluoride (except alkaline earth fluorides). The metal (of the metal salt or elemental) must be matched to the metal under investigation in the sample with respect to its absorption edges.The standard metal must not correspond to the metal being analyzed in the sample and must have at least one non-overlapping absorption edge at the intended absorption edge under investigation, with at least one absorption edge of the analyte also being non-overlapping. Furthermore, it is advantageous to use a metal (the metal salt or elemental) with an absorption edge of the same type—that is, an absorption edge resulting from the same orbital—as that of the metal being analyzed in the sample as the standard. The positions of the absorption edges of the metal in the standard and in the sample should be optimized to be no more than 7 keV apart in the spectrum of the X-rays used for XRF. This simplifies standardization. An overview of the absorption edges of a large number of elements can be found, for example, in article 4 by RD Deslattes et al.(X-ray transition energies: new approach to a comprehensive evaluation, Reviews of Modern Physics, Vol. 75, 2003, pp. 35-99). The NIST data collection on X-ray absorption edges of a wide variety of elements is available on the website "https: / / www.nist.gov / pml / x-ray-transitionenergies-database" (last accessed on June 10, 2024) of the National Institute of Standards and Technology (NIST), US Department of Commerce, Radiation Physics Division.

[0022] In principle, any stable deuterated solvent can be used as the reference solution. The selection should be made based on the following criteria: -No reactions may occur with the standards used or the substances to be analyzed. -Both the standards and the sample must be completely soluble in the solvent. -Overlapping of the non-deuterated solvent signals with those of the sample to be analyzed and the standards should be avoided.

[0023] The deuterated solvent can be given, for example, by one of the following groups: MeOD (methanol-d4), EtOD (ethanol-d6), DMSO (dimethyl sulfoxide-d6), D2O, CDCl3 (chloroformd), C6D6 (benzene-d6), Me2CO (CD3COCD3, acetone-d6), MeCN (CD3CN, acetonitrile-d3), CD3COOD (acetic acid-d4).

[0024] The standards, the solvent, and the sample to be dissolved in it may need to be matched. This sometimes requires preparing test solutions and consulting relevant reference works.

[0025] Article 3 by T. Rundlöf et al. (Survey and qualification of internal standards for quantification by 1H NMR spectroscopy, Journal of Pharmaceutical and Biomedical Analysis, Vol. 52, 2010, pp. 645-651) discusses frequently used standards and solvents. An overview of NMR standards and solubilities in various solvents can be found in the table "AK Hintermann List of qNMR Standards - Version 1.7" available at "https: / / www.ch.nat.tum.de / fileadmin / w00bzu / oca / Ressources / AK_Hintermann_List_of_qNMR_Standards.pdf" (accessed on June 7, 2024), from the research group of Prof. Dr. L. Hintermann, Chair of Organic Chemistry, TUM School of Natural Sciences, Technical University of Munich.

[0026] The provided sample is dissolved in the provided reference solution. If necessary, the kinetics of the dissolution process can be influenced to accelerate it, for example by heating and stirring. Advantageously, the sample does not need to be weighed in the method according to the invention. By using the dual standard, the absolute masses of a sample containing elements of the light and heavy fractions can be directly quantified. This is achieved by combining information about the light fraction of a compound, e.g., organic cations in a perovskite, with information about the heavy fraction, e.g., the remaining cations such as lead or tin and the halides, e.g., iodide or bromide in a perovskite.

[0027] The analytical solution is analyzed using 1H-NMR spectroscopy in a sample vessel, also determined by the 1H-NMR spectrometer, within a volume predetermined by the spectrometer. The volume is usually defined by the sample vessel, which is typically a thin-walled glass tube. Performing 1H-NMR spectroscopic analysis requires expert knowledge.

[0028] 1H-NMR spectrometers, sample containers and standards as well as solvents are well known and commercially available.

[0029] 1H-NMR spectroscopic analysis leaves the analytical solution physically and chemically unchanged. Further analysis using the same analytical solution, already analyzed by 1H-NMR, can be directly traced back to one and the same sample, just like the 1H-NMR spectroscopic analysis itself. Therefore, only one sample is required.

[0030] The analytical solution, analyzed by 1H-NMR spectroscopy, is then applied wholly or partially to a support, e.g., by pipetting, which advantageously allows for adjustment of the applied volume. The applied analytical solution is then allowed to dry on the support. Drying can be accelerated by exposure to air, evacuation, or heating. Overflow with air or an inert gas is also possible.

[0031] Suitable supports for use in the inventive method are objects with at least one flat surface, such as plates, discs, disks, blocks, etc., wherein the analytical solution is applied to the at least one flat surface of the support. The support material must not contain any element to be analyzed, nor any elements that exhibit absorption edges at the intended energy of the analysis. In particular, the support is advantageously made of a plastic. Plastics have a negligible interaction cross-section in the energy ranges where metals with atomic number ≥ 10 absorb, which is particularly true for T-XRF.

[0032] X-ray fluorescence analysis (XRF), particularly under total internal reflection (T-XRF), is performed on the dried analytical solution. T-XRF offers the advantage that, due to the extremely small angle of incidence, no or only very small amounts of a carrier are excited, thus preventing interference in the analysis and rendering it negligible. Experimental stations at synchrotron radiation sources, such as BESSY II at the Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, can be used for T-XRF, as well as laboratory equipment, which is also advantageous. Synchrotron radiation sources provide X-rays over a wide energy range (~100 eV to 50 keV) with sufficient intensity and tunability. Laboratory equipment can also be used if the required energies are available with sufficient intensity.

[0033] The analytical data obtained using the 1H-NMR spectroscopy and XRF methods are evaluated after acquisition to obtain information about the quantitative proportions of, for example, organic cations or metals in the sampled material, such as a perovskite. The results are to be used for process and quality control.

[0034] The relevant algorithms or software programs are used for the evaluation. An algorithm for the quantitative evaluation of 1H-NMR spectra is given by the following formula (1) for calculating an amount of substance, n. i , of a questionable analyte: n(analyte) = n(standard) ⋅ peak area (analyte) / number of hydrogen atoms (analyte) / peak area (standard) / number of hydrogen atoms (standard)

[0035] An algorithm for the quantitative evaluation of T-XRF spectra is given by formula (2) for calculating an amount of substance, n i , of a questionable analyte: n(analyte)=n(standard)⋅peak area(analyte)⋅S(analyte)peak area(standard)⋅S(standard)

[0036] S(Analyte) = relative sensitivity of the analyte, S(Standard) = relative sensitivity of the standard. A selection of sensitivities is listed, for example, in article 5 by E. Towett et al. (Quantification of total element concentrations in soils using total X-ray fluorescence spectroscopy (TXRF), Science of the Total Environment, Vol. 463-464, 2013, pp. 374-388) and is usually included in commercially available instruments for performing T-XRF for use with the instrument's proprietary software.

[0037] To determine the absolute composition of a sample without weighing it, the amounts of substance or masses of all elements or compounds, as well as the light and heavy fractions, must be determined. Knowing the light fraction of a compound, e.g., organic cations in a perovskite, together with information about the heavy fraction, e.g., the other cations such as lead or tin and the halides, e.g., iodide or bromide in a perovskite, allows for the creation of a chemical formula. Furthermore, the total mass of the sample can be determined by summing all the individual masses.

[0038] The information obtained after analysis can be used, for example, to optimize manufacturing processes.

[0039] The inventive method enables a simple and economical analysis of both the light and heavy fraction elements with minimal sample material and does not require weighing the sample material.

[0040] In one embodiment of the inventive method for the quantitative determination of components of a solution, the 1H-NMR active substance is dimethyl sulfone.

[0041] In a further embodiment of the inventive method for the quantitative determination of components of a solution, the metal salt nickel nitrate hexahydrate, Ni(NO3)2·6 H2O, is used.

[0042] The two embodiments can be advantageously combined. Examples

[0043] The invention in an exemplary embodiment is described in more detail below by means of an example of the inventive method for the quantitative determination of components of a solution and 2 figures.

[0044] The figures show: Fig. 1: 1H-NMR spectrum of the analytical solution of the exemplary embodiment. Fig. 2: XRF spectrum of the same analytical solution as in Fig. 1, in its dried state,

[0045] In this embodiment, the reference solution consists of deuterated dimethyl sulfoxide (D6-DMSO) as the solvent, dimethyl sulfone as the 1H-NMR active substance (standard), and nickel nitrate hexahydrate, Ni(NO3)2·6 H2O, as the standard for XRF. The concentration of the 1H-NMR standard dimethyl sulfone is 3.378 mmol / L. The concentration of the nickel nitrate hexahydrate standard is 7.594 mmol / L, which corresponds to a nickel concentration of 0.28 mg / mL.

[0046] In this example, the material to be sampled is a perovskite deposited on a layer stack for the fabrication of a perovskite solar cell. The perovskite is a formaidinium cesium lead iodide bromide perovskite with the formal composition Cs a FA b PbI x Br yThe perovskite is deposited by co-evaporation on a 5 cm x 2.5 cm glass substrate coated with indium tin oxide and a perforated contact layer, with a layer thickness of approximately 550 nm. The deposited perovskite is detached from the layer stack by multiple extractions with a defined volume of the reference solution (see above). The resulting analytical solution now contains the entire sample material, i.e., the entire perovskite layer, in a defined volume of, for example, 1 ml. Dissolving the entire sample prevents erroneous sampling caused by localized inhomogeneities. Enabling this type of sampling is an advantage of the invention.

[0047] 0.6 ml of the analytical solution is placed into a 5 mm precision NMR tube. In this example, the analytical solution is analyzed in a standard 500 MHz spectrometer (AVANCE III) from Bruker.

[0048] In Fig. Figure 1 shows the 1H NMR spectrum of the sample solution. The signals of the analyte (formamidinium, FA) are highlighted. + ), A, between 9.5 and 7.5 ppm and the signal of the standard (dimethyl sulfone), S, at 3 ppm. For evaluation, the signals of the corresponding CH functions or the NH2 / NH2 are used. + Groups are integrated. A simple proportion allows the direct calculation of the amount of analyte by comparing the integrals of the analyte and the standard, normalized to the number of hydrogen atoms involved, with the known amount of substance of the standard. In principle, both the signal of the CH functions and the signals of the NH2 / NH2 can be used. + Groups are used for quantification due to exchange processes that lead to peak broadening of NH2 / NH2. + Signals are used, but the CH signal is preferred. To calculate the amount of substance of the analyte in question, n i, the evaluation is carried out according to formula (1), in which hydrogen is taken as a representative of the analyte, assuming an ideal composition of the organic cation FA:

[0049] In the exemplary embodiment, the following results: n(FA+)=3.378 mmoll⋅1 ml⋅114.386=0.004627 mmol

[0050] The molar mass (45.07 g / mol) also determines the mass of FA. + accessible in the sample taken, which yields 209 mg.

[0051] From the same analytical solution used for 1H-NMR analysis, 0.015 ml is pipetted onto an acrylic glass sample holder and allowed to air dry. Since the analytical solution remains unchanged during the 1H-NMR measurement, both the previously analyzed solution and the solution not used for 1H-NMR can be used for XRF. XRF is performed using molybdenum excitation in total internal reflection mode (T-XRF). The T-XRF spectrum is shown in the Fig. Figure 2 shows the signal of the nickel standard between 7.5 and 8.5 keV and the characteristic lines of the elements to be quantified—I, Br, Cs, and Pb—which were used for the evaluation. A nickel concentration of 0.28 mg / ml in a 0.015 ml sample yields a mass of 4201 ng of nickel per sample carrier. This mass can be used directly, based on formula (2), with the software installed on the T-XRF instrument (Bruker S4 T-STAR), to determine the amount of substance or the masses of the elements to be determined (Br, I, Cs, Pb). The evaluation shows that 14140 ng of lead, 1011 ng of cesium, 28620 ng of iodine, and 1363 ng of bromine are present on the sample carrier. Since only very small amounts of analytical solution are sampled for T-XRF, it is recommended to perform several measurements to reduce measurement uncertainty.

[0052] For the perovskite sample of the example, taking into account the dilution for T-XRF (1 ml to 0.015 ml), the following masses are found on the sample: 0.943 mg lead, 1.908 mg iodine, 0.0674 mg cesium, 0.0909 mg bromine and 0.209 mg FA. + This corresponds to a composition, represented in the molecular formula normalized to lead, of Cs 0.11 FA 1.02 PbI 3.30 Br 0.25 .

[0053] As shown in the example, the method according to the invention is advantageously simple and requires only one sampling. Weighing the sample taken is not necessary to determine the composition, which is an advantage of the invention.

Claims

[1] Method for the quantitative determination of the components of a solution, comprising at least the following steps: - Providing a sample, - Providing a reference solution comprising at least a first standard in the form of a 1H-NMR active substance and a second standard in the form of a metal salt of a metal with an atomic number ≥ 10, wherein the solvent is in particular a deuterated liquid, - Dissolving the entire sample in the reference solution to obtain an analytical solution, - Analysis of the analytical solution using 1H-NMR spectroscopy and - Applying a defined volume of the analytical solution to a carrier and allowing the analytical solution to dry, and then - Analysis of the dried analytical solution using XRF spectroscopy, - Evaluation of the data obtained in the analyses. [2] Method for the quantitative determination of components of a solution according to claim 1, characterized by that the 1H-NMR active substance is dimethyl sulfone. [3] Method for the quantitative determination of components of a solution according to claim 1, characterized by , that the metal salt is nickel nitrate hexahydrate, Ni(NO3)2·6 H2O. [4] Method for the quantitative determination of components of a solution according to claim 1, characterized by that the sample is a perovskite.

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