Mass spectrometry method for polyisoprene

Abstract

To provide a mass analysis method of polyisoprene capable of measuring the molecular weight of polyisoprene contained in a sample by combining thin layer chromatography with a laser desorption ionization flight time type mass analysis method.SOLUTION: A mass analysis method of polyisoprene includes: a separation step of separating polyisoprene contained in a sample by thin layer chromatography; a measurement plate manufacturing step of manufacturing a measurement plate by applying an ionizing agent solution containing a transition metal compound to the polyisoprene separated on the thin layer chromatography plate; and a measurement step of obtaining mass spectrum by measuring the measurement plate using a laser desorption ionization time-of-flight mass spectrometer.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to a mass spectrometry method for polyisoprene that uses a laser desorption ionization time-of-flight mass spectrometer to measure the molecular weight of polyisoprene contained in a sample. [Background technology]

[0002] Natural rubber, whose main component is polyisoprene, has excellent properties such as high tensile strength and low heat generation due to vibration, and is therefore used in a variety of rubber products such as tires, vibration-proof rubber, and belts. Consumption of natural rubber is expected to increase due to the growing demand for rubber products. Because natural rubber is a natural resource extracted from the Para rubber tree, there is a growing need for synthetic rubber (synthetic polyisoprene) with properties equivalent to natural rubber from the perspectives of nature conservation and sustainable use.

[0003] In general, measuring molecular weight and molecular weight distribution is important for understanding the physical properties of polymeric materials such as synthetic rubber. For example, mass spectrometry (MS) is a method for directly measuring the molecular weight of polymeric materials. Mass spectrometry involves ionizing atoms or molecules, moving them in a high vacuum using an electric or magnetic field, and then separating and detecting the ions based on their mass. Methods for ionizing substances include electron ionization (EI), chemical ionization (CI), electrospray ionization (ESI), and laser desorption ionization (LDI). Separation methods for the generated ions include quadrupole (Q), magnetic ionization (BE), and time-of-flight (TOF) spectroscopy. LDI-TOFMS, which combines LDI and TOF, is widely used for measuring polymeric materials. Matrix-assisted laser desorption ionization (MALDI), which ionizes polymeric materials by mixing them with an organic matrix, is particularly useful for measuring the mass of components that are difficult to ionize.

[0004] For example, Patent Document 1 describes a method for analyzing polymer compounds, in which a sample containing a polymer compound is dispersed in an organic matrix reagent such as dithranol, and mass spectrum data is obtained using a matrix-assisted laser desorption / ionization time-of-flight mass spectrometer (MALDI-TOFMS). Patent Document 2 describes a method in which a sample such as a protein is co-crystallized with an organic matrix molecule such as sinapinic acid in the presence of porous fine particles such as silica gel to prepare a sample, and then matrix-assisted laser desorption / ionization mass spectrometry (MALDI-MS) is performed using the sample. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-138273 [Patent Document 2] Japanese Patent Application Laid-Open No. 2006-189391 Summary of the Invention [Problem to be solved by the invention]

[0006] For example, if the sample to be measured is polyisoprene alone, it can be measured using MALDI-TOFMS. However, during the synthesis of polyisoprene, catalysts and other additives are added and various reactions occur, resulting in the resulting product containing many impurities in addition to the target polyisoprene. When such a product is measured using MALDI-TOFMS, the impurities interfere with the measurement, making it impossible to accurately measure the polyisoprene itself. Therefore, the inventors first developed the product using thin-layer chromatography (TLC) to separate polyisoprene from the impurities. They then attempted to directly ionize the polyisoprene separated on the TLC plate and perform mass spectrometry. However, the inventors' investigations revealed that polyisoprene could not be ionized on the TLC plate, even when using the sample alone or the organic matrix agents commonly used in MALDI-TOFMS.

[0007] The aforementioned Patent Documents 1 and 2 do not describe the use of TLC for samples containing impurities. For example, in the analytical method described in Patent Document 1, a desired component is extracted using a solvent before a sample containing a polymer compound is measured using MALDI-TOFMS. In the analytical method described in Patent Document 2, the influence of impurities is suppressed by adding porous microparticles to the sample. Furthermore, Patent Documents 1 and 2 do not consider combining TLC with MALDI-TOFMS or ionizing the object to be measured on the TLC plate.

[0008] The present disclosure has been made in consideration of the above circumstances, and aims to provide a mass spectrometry method for polyisoprene that can measure the molecular weight of polyisoprene contained in a sample by combining thin layer chromatography and laser desorption ionization time-of-flight mass spectrometry. [Means for solving the problem]

[0009] In order to solve the above-mentioned problems, the mass spectrometry method for polyisoprene of the present disclosure is characterized by comprising: a separation step of separating polyisoprene contained in a sample by thin layer chromatography; a measurement plate preparation step of applying an ionizing agent solution containing a transition metal compound to the polyisoprene separated on a thin layer chromatography plate to prepare a measurement plate; and a measurement step of measuring the measurement plate using a laser desorption ionization time-of-flight mass spectrometer to obtain a mass spectrum. [Effects of the Invention]

[0010] According to the present disclosure, the polyisoprene mass spectrometry method first separates the polyisoprene to be measured from a synthetic polyisoprene sample containing contaminants using thin-layer chromatography (TLC). This eliminates the influence of the contaminants, allowing accurate measurement of the molecular weight of the polyisoprene using a laser desorption / ionization time-of-flight mass spectrometer (LDI-TOFMS). Polyisoprene does not readily absorb laser light. Therefore, polyisoprene cannot be ionized simply by irradiating it with laser light. After extensive research, the inventors discovered that polyisoprene can be ionized using an ionizing agent solution containing a transition metal compound. The ionization mechanism of polyisoprene is presumed to be as follows: When an ionizing agent solution containing a transition metal compound is irradiated with laser light, the ionizing agent solution is heated, generating transition metal nanoparticles. The generated transition metal nanoparticles are desorbed, and cations are added to the polyisoprene, causing the polyisoprene to be desorbed and ionized.

[0011] In the polyisoprene mass spectrometry method of the present disclosure, an ionizing agent solution containing a transition metal compound is used to ionize polyisoprene, but the use of a conventionally used organic matrix agent is not excluded. However, according to the polyisoprene mass spectrometry method of the present disclosure, the molecular weight of polyisoprene can be measured by using an ionizing agent solution containing a transition metal compound without using an organic matrix agent. Furthermore, according to the polyisoprene mass spectrometry method of the present disclosure, polyisoprene can be ionized on a thin-layer chromatography plate. Therefore, the thin-layer chromatography plate on which polyisoprene was separated can be used as is, facilitating measurement. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 shows a mass spectrum of Experimental Example 1. [Figure 2] FIG. 1 shows a mass spectrum of Experimental Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, an embodiment of the method for mass spectrometry of polyisoprene according to the present disclosure will be described. Note that the embodiment is not limited to the following embodiment, and various modifications and improvements that can be made by those skilled in the art can be implemented. The method for mass spectrometry of polyisoprene according to the present disclosure includes a separation step, a measurement plate preparation step, and a measurement step.

[0014] <Separation process> This step is a step of separating polyisoprene contained in a sample by thin layer chromatography.

[0015] [sample] The sample can be any liquid containing polyisoprene, such as a solution of polyisoprene dissolved in a solvent or a solution of the product obtained by isoprene polymerization dissolved in a solvent. Examples of solvents that can be used include tetrahydrofuran (THF), butanol, and toluene. The concentration of polyisoprene in the solution (sample) is preferably 2.5 mg / mL or higher. The molecular weight of the polyisoprene contained in the sample should preferably be, for example, a number-average molecular weight of 400 to 2500. If the molecular weight is too large, it will be difficult to desorb during measurement by LDI-TOFMS.

[0016] [Thin-layer chromatography] Thin-layer chromatography separates components in a sample by attaching and drying the sample to a thin-layer chromatography plate and then developing it with a solvent. Thin-layer chromatography plates have a support that has an affinity for polyisoprene and adequately adsorbs polyisoprene. Examples of such supports include silica gel, alumina, and cellulose. Chemically modified silica gel, in which organic moieties are chemically bonded to the silanol groups of silica gel, can also be used. Examples of organic moieties include octadecyl, octyl, and dimethylsilyl groups. As described below, as the molecular weight of polyisoprene increases, interactions with the support can make it difficult for polyisoprene to desorb from the support during LDI-TOFMS analysis. In such cases, it is desirable to change, for example, the type of support or the type of organic moiety chemically bonded to the silica gel.

[0017] The thin layer chromatography plate may have a support for supporting the carrier. Examples of the support include a glass plate, a plastic sheet, and an aluminum sheet. The support may be selected appropriately depending on the type of sample and developing solvent. The sample may be applied to the thin layer chromatography plate in a dotted or planar manner using a capillary, a micropipette, or the like. Alternatively, the sample may be applied by contacting one side of the thin layer chromatography plate with the sample. The developing solvent may be selected appropriately taking into consideration the solubility and separability of polyisoprene. One selected from polar solvents such as water, acetone, dichloromethane, and methanol may be used alone, or two or more may be used in combination.

[0018] <Measurement plate preparation process> This step is a step of preparing a measurement plate by applying an ionizing agent solution containing a transition metal compound to polyisoprene separated on a thin layer chromatography plate.

[0019] The ionizing agent solution is a reagent for ionizing polyisoprene in the subsequent measurement step. The transition metal compound may be any compound capable of absorbing laser light, converting the energy into thermal energy, generating transition metal nanoparticles, and attaching cations to polyisoprene. Examples include silver compounds. Among these, silver trifluoroacetate is preferred. The solvent may be selected appropriately depending on the type of transition metal compound; for example, THF or methanol may be used. The ionizing agent solution may be prepared so that the concentration of the transition metal compound is 5 mg / mL or more and 20 mg / mL or less. The ionizing agent solution may be applied dropwise to the polyisoprene on the thin-layer chromatography plate using a capillary, micropipette, or the like, or the thin-layer chromatography plate may be immersed in the ionizing agent solution.

[0020] In this step, it is not excluded to use a conventionally used organic matrix agent in addition to the ionizing agent solution as a reagent for ionizing polyisoprene. However, if an organic matrix agent is used in combination, the organic matrix agent may be adsorbed to the carrier of the thin-layer chromatography plate, inhibiting the desorption and ionization of polyisoprene. Therefore, when preparing a measurement plate, it is desirable to use a form that does not use an organic matrix agent.

[0021] The higher the molecular weight of polyisoprene, the stronger the interaction with the support of the thin layer chromatography plate. For example, if the support has octadecyl groups (C 18 H 37In the case of silica gel containing octadecyl groups, polyisoprene may be adsorbed to the octadecyl groups, making it difficult for the polyisoprene to be desorbed from the support during measurement by LDI-TOFMS in the subsequent step. Insufficient desorption of polyisoprene makes accurate measurement difficult. Therefore, in order to suppress interactions with the support and accurately measure polyisoprene with a large molecular weight, it is desirable to add a surfactant to the ionizing agent solution. Adding a surfactant inhibits the interaction between polyisoprene and the support, thereby promoting desorption of polyisoprene. For example, the effect of adding a surfactant is most pronounced when the number-average molecular weight of the polyisoprene is 1,800 or higher.

[0022] Examples of surfactants include 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid (CHAPS). The surfactant may be dissolved in a solvent such as THF, methanol, or water and added in the form of a solution prepared to the desired concentration. The surfactant concentration is preferably 10 mg / mL or higher.

[0023] <Measurement process> In this step, the prepared measurement plate is measured using a laser desorption / ionization time-of-flight mass spectrometer (LDI-TOFMS) to obtain a mass spectrum. Measurements using LDI-TOFMS can be performed using commonly used equipment and conditions. In LDI-TOFMS, the measurement plate is placed on the instrument's sample plate and irradiated with a specific laser beam, ionizing the polyisoprene and measuring its mass-to-charge ratio. The horizontal axis of the obtained mass spectrum represents m / z (the ratio of the ion's mass m to its charge number z), and the vertical axis represents the relative peak intensity (relative abundance). From the obtained mass spectrum, the components with different molecular weights that make up polyisoprene and their relative abundances can be determined. [Example]

[0024] (1) Experimental Example 1 A solution containing polyisoprene (PI) with a number-average molecular weight (Mn) of 800 was used as a sample and measured by TLC-LDI-TOFMS.

[0025] <Separation process> The polyisoprene contained in the sample was separated by thin-layer chromatography. The sample used was a polyisoprene solution ("PSS-pio800" manufactured by PPS (Polymer Standard Services), polyisoprene concentration 2.5 mg / mL) in which polyisoprene (Mn = 800) was dissolved in THF. The thin-layer chromatography plate used was a product of Merck's "Silica Gel 60 RP-18 F254S Aluminum Sheet," which has an aluminum sheet support and an octadecyl-modified silica gel carrier, cut into a rectangular shape measuring 70 mm long and 25 mm wide. The developing solvent used was a mixture of dichloromethane and methanol in a volume ratio of 7:3.

[0026] First, 5 μL of sample was dropped onto one of the short sides of the thin layer chromatography plate using a micropipette, deposited as a dot, and then dried. Next, the developing solvent was poured into the development tank, the lid was closed, and the plate was left to stand for 5 minutes. Next, the lid of the development tank was opened, and the thin layer chromatography plate was placed with the short side to which the sample was deposited facing downwards, and left to stand until the developing solvent had migrated to a predetermined height. After the development was complete, the thin layer chromatography plate was removed and dried.

[0027] <Measurement plate preparation process> A measurement plate was prepared by dropping 5 μL of the ionizing agent solution onto the polyisoprene on the obtained thin-layer chromatography plate. The ionizing agent solution used was an AgTFA solution prepared by dissolving silver trifluoroacetate (AgTFA) in THF. The AgTFA concentration in the AgTFA solution was 10 mg / mL.

[0028] <Measurement process> The prepared measurement plate was attached to an ITO (indium tin oxide) glass substrate and measured using LDI-TOFMS in positive mode using a spiral TOFMS instrument (JEOL Ltd., "JMS-S3000", Nd:YLF laser).

[0029] <Measurement results> Figure 1 shows the mass spectrum obtained by measurement at a position of 4.7 cm after thin-layer chromatography development. In Figure 1, the horizontal axis represents m / z and the vertical axis represents relative peak intensity. As shown in Figure 1, ionized polyisoprene [PI+Ag + The peaks observed were those of polyisoprene with a degree of polymerization n of 6 to 11. Thus, it was confirmed that by using an ionizing agent solution containing AgTFA, polyisoprene can be ionized on a thin-layer chromatography plate and subjected to mass spectrometry.

[0030] (2) Experimental Example 2 A solution containing polyisoprene with a number-average molecular weight (Mn) of 1820 was used as a sample, and measurement was carried out by LDI-TOFMS on a thin-layer chromatography plate.

[0031] In Experimental Example 2, a measurement plate was prepared by dropping a sample onto a thin-layer chromatography plate and then dropping an ionizing agent solution onto it without developing it. The sample used was a polyisoprene solution (PSS-pio2.1k, manufactured by PPS, polyisoprene concentration 10 mg / mL) in which polyisoprene (Mn = 1820) was dissolved in THF. First, 5 μL of the sample was dropped onto the same thin-layer chromatography plate as in Experimental Example 1, positioned 10 mm from one short edge, using a micropipette. The sample was then deposited in a dot shape and allowed to dry. Next, 5 μL of the ionizing agent solution was dropped onto the polyisoprene on the thin-layer chromatography plate to prepare a measurement plate. The ionizing agent solution was a mixed solution prepared by adding a CHAPS solution (CHAPS concentration 40 mg / mL) in which the surfactant CHAPS was dissolved in THF to the same AgTFA solution (AgTFA concentration 10 mg / mL) as in Experimental Example 1.

[0032] The prepared measurement plate was attached to an ITO glass substrate and measured using an LDI-TOFMS. The measurement was carried out in positive mode using the same spiral TOFMS apparatus as in Experimental Example 1.

[0033] <Measurement results> The mass spectrum obtained is shown in Figure 2. In Figure 2, the horizontal axis represents m / z and the vertical axis represents relative peak intensity. As shown in Figure 2, in the range of m / z from 1000 to 2000, ionized polyisoprene [PI+Ag + The peaks observed were those of polyisoprene with a degree of polymerization n of 13 to 27. Thus, it was confirmed that by using an ionizing agent solution containing AgTFA and CHAPS, high molecular weight polyisoprene with a number average molecular weight of 1800 or more can be ionized on a thin layer chromatography plate and subjected to mass analysis. [Industrial Applicability]

[0034] The mass spectrometry method for polyisoprene disclosed herein allows accurate measurement of the molecular weight of polyisoprene even when the sample contains impurities, and is useful, for example, for analyzing artificially synthesized polyisoprene and decomposition products of polystyrene.

Claims

1. a separation step of separating polyisoprene contained in the sample by thin layer chromatography; a measurement plate preparation step of applying an ionizing agent solution containing a transition metal compound and a surfactant to the polyisoprene separated on the thin layer chromatography plate to prepare a measurement plate; a measuring step of measuring the measurement plate using a laser desorption ionization time-of-flight mass spectrometer to obtain a mass spectrum; A method for mass spectrometry of polyisoprene, comprising:

2. 2. The method for mass spectrometry of polyisoprene according to claim 1, wherein no organic matrix agent is used in preparing the measurement plate.

3. The method for mass spectrometry of polyisoprene according to claim 1 or 2, wherein the transition metal compound includes a silver compound.

4. The method for mass spectrometry of polyisoprene according to claim 3 , wherein the silver compound comprises silver trifluoroacetate.

5. The thin layer chromatography plate has a support and a carrier, 5. The method for mass spectrometry of polyisoprene according to claim 1, wherein the carrier comprises silica gel modified with an octadecyl group.

6. 6. The method for mass spectrometry of polyisoprene according to claim 1, wherein the surfactant comprises 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid (CHAPS).

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