Method for characterizing particle size and composition of soil nanoparticle by using asymmetrical flow field-flow fractionation combined with inductively coupled plasma mass spectrometry technology

Abstract

The present invention provides a method for characterizing a particle size and composition of a soil nanoparticle by using asymmetrical flow field-flow fractionation combined with inductively coupled plasma mass spectrometry technology. The method comprises: S1, pretreating an asymmetrical flow field-flow fractionation instrument in an anaerobic environment; and S2, loading a soil nanoparticle suspension sample in the asymmetrical flow field-flow fractionation instrument, and after a sample injection phase, an elution phase, and a wash stage, obtaining a characterization result. The present invention develops an application of soil nanoparticles in an asymmetrical flow field-flow fractionation instrument, adopting a method combining the asymmetrical flow field-flow fractionation instrument with ICP-MS to fractionate and characterize the soil nanoparticles. The present invention offers advantages such as ease of operation, simultaneous multi-element determination, simple pretreatment, low destructiveness, low detection limits and the like.

Description

A method for characterizing the particle size and composition of soil nanoparticles using asymmetric flow-field coupled inductively coupled plasma mass spectrometry. Technical Field

[0001] This invention relates to a method of using an asymmetric flow field analyzer, and more particularly to a method for characterizing the particle size and composition of soil nanoparticles using asymmetric flow-field coupled inductively coupled plasma mass spectrometry. Background Technology

[0002] Soil nanoparticles are complex nanomaterials, including aluminosilicates (clay minerals), oxides and hydroxides of aluminum, iron, and other minerals, enzymes, humic substances, viruses, and non-fixed colloids. Their structural diversity and complexity far exceed those of artificial nanomaterials. Most migrations of nutrients, pollutants, organic matter, and heavy metals occur at the nanoscale, either within or around nanoparticles.

[0003] Currently, there is no single method capable of qualitatively and quantitatively characterizing soil nanoparticles of various sizes. However, several well-known nanoparticle characterization methods exist, which can be compared and contrasted with existing methods. Existing nanoparticle characterization methods include: transmission electron microscopy (TEM), dynamic light scattering (DLS), inductively coupled plasma optical emission spectrometry (ICP-OES), and inductively coupled plasma mass spectrometry (ICP-MS).

[0004] Transmission electron microscopy (TEM) possesses atomic-level resolution, enabling the observation of not only the microscopic morphology of samples but also the characterization of the internal structure of the observed region. The principle of TEM is roughly as follows: a high-voltage electron source releases high-speed electrons, which, after passing through a condenser lens system, form a parallel electron beam that is incident on and passes through a localized region of the sample. The electron beam interacts with soil nanoparticles, modulating the phase and amplitude of the incident electron beam to form an outgoing electron beam. This outgoing electron beam passes through the objective lens system, converging on the focal plane of the objective lens to form a diffraction spot, which then propagates downwards to the image plane of the objective lens, forming the first magnified image of the sample. TEM allows the study of size, shape, and crystal structure at the single-particle level.

[0005] Dynamic light scattering (DLS) is a technique that derives particle size information from samples by measuring the fluctuations in the intensity of scattered light. Because molecules in the sample are constantly undergoing Brownian motion, this motion causes changes in the scattered light. By utilizing the fluctuations in scattered light over time, a relevant equation is derived, leading to the average velocity (Dt) of the particles in the sample, and ultimately, the average hydrodynamic diameter. DLS offers advantages in particle size detection due to its speed and simplicity, and it has statistical significance. However, because the intensity of scattered light is proportional to the sixth power of the particle size, larger particles are scattered more strongly, thus the average particle size result for widely distributed samples tends to favor larger particles. Therefore, DLS is suitable for particle size detection in monodisperse systems with relatively narrow distributions. Furthermore, DLS experiments are susceptible to the effects of dust or impurities, making sample filtration and centrifugation crucial.

[0006] Nanoparticle tracking analysis involves irradiating a solution of suspended particles with a concentrated laser beam through a glass prism. A chrome-plated glass surface minimizes background signals, allowing the intensity of scattered light from each particle to be detected. This enables observation of the Brownian motion of the nanoparticles in the solution and image capture. By tracking and analyzing the Brownian motion of the particles, the particle size is calculated using the Stokes-Einstein equation, and the concentration is derived from the number of particles. Nanoparticle tracking analysis offers higher accuracy than dynamic light scattering analysis. However, like dynamic light scattering, the signals from particles can be blocked, leading to a bias towards larger particles. Furthermore, the limited focal plane of the image means that the three-dimensional motion of the particles is ultimately represented as a two-dimensional trajectory on the instrument, resulting in some degree of inaccuracy in the final results.

[0007] Inductively coupled plasma atomic emission spectrometry (ICP-OES) utilizes the high temperature generated by ICP plasma to atomize or ionize the analyte, creating an excited state that produces a characteristic emission spectrum. By detecting the wavelength and intensity of the spectral lines, the presence and concentration of the analyte in the sample can be determined. Although ICP-OES is widely used, significant interference can occur when trace elements encounter high-concentration matrices, particularly affecting Ca, Al, and Fe.

[0008] Inductively coupled plasma mass spectrometry (ICP-MS) utilizes the high temperature generated by plasma to ionize most elements in a sample, creating monovalent positive ions. Mass spectrometry then filters ions with different mass-to-charge ratios, allowing specific ions to pass through and reach the detector. This allows for the detection of a particular ion and its intensity, enabling the analysis and calculation of the element's concentration. ICP-MS can simultaneously determine multiple elements and offers advantages over ICP-OES, such as lower detection limits and higher precision.

[0009] Current transmission electron microscopy (TEM) methods suffer from limitations. The limited amount of nanoparticles used in electron microscopy can lead to a lack of overall statistical significance in the measurement results. Furthermore, the high surface activity and tendency of nanoparticles to aggregate necessitate ultrasonic dispersion before sample preparation. Accurate results are also difficult to obtain for nanoparticle samples that are susceptible to strong electron beam bombardment. Additionally, the nanoparticles must be electronically transparent and able to withstand the high vacuum and beam energy used in the characterization process. Sample damage is particularly common due to the presence of high-energy electron beams.

[0010] The existing dynamic light scattering method, because the intensity of the scattered light is proportional to the sixth power of the particle size, has a stronger scattering effect on large particles. This results in an average particle size result that is biased towards large particles for samples with a wide distribution. The result may not be accurate or representative enough, and it is easily affected by dust or impurities, which will increase the error of the result and increase the complexity and difficulty of operation.

[0011] Existing nanoparticle tracking analysis methods suffer from light scattering by particles, which causes signals between particles to be blocked, resulting in a bias towards larger particles in the obtained particle size. In addition, due to the limitation of the limited focal plane of the imaging, the three-dimensional motion of the particles is actually presented as a two-dimensional motion trajectory on the instrument, leading to a certain deviation in the final result.

[0012] Existing inductively coupled plasma atomic emission spectrometry (ICP-AES) suffers from complex spectral lines, significant spectral interference, and requires large sample quantities. Furthermore, when used alone, it necessitates pretreatment such as acidification, which can damage the sample structure.

[0013] Existing inductively coupled plasma mass spectrometry (ICP-MS) is greatly affected by matrix effects, requires internal standard correction, and has poor tolerance to salt content in the analyte, typically less than 0.2%.

[0014] To overcome the shortcomings of existing technologies, such as complex operation, over-reliance on separation and extraction techniques, and relatively limited characterization capabilities, and to achieve efficient separation and characterization of soil nanoparticles, this method uses an asymmetric flow field analyzer (AF4, hereinafter referred to as the flow field analyzer) coupled with ICP-MS to separate and characterize soil nanoparticles. This method has the advantages of convenient operation, simultaneous determination of multiple elements, simple pretreatment, minimal destructiveness, and low detection limit. Summary of the Invention

[0015] This invention provides a method for characterizing the particle size and composition of soil nanoparticles using inductively coupled plasma mass spectrometry (ICP-MS) with asymmetric flow-field coupling, so as to enable the use of soil nanoparticles in an asymmetric flow field instrument.

[0016] This invention provides a method for characterizing the particle size and composition of soil nanoparticles using asymmetric flow-field coupled inductively coupled plasma mass spectrometry, comprising:

[0017] S1. Pre-treat the asymmetric flow field instrument in an oxygen-free environment;

[0018] S2. The soil nanoparticle suspension is loaded into an asymmetric flow field analyzer and subjected to an injection stage, a flushing stage, and a rinsing stage to obtain characterization results. Further, the soil nanoparticle suspension to be tested also contains metal ions, specifically Fe. 2+ Fe 3+ Cu 2+ Al 3+ One or more of them.

[0019] Furthermore, the concentration of the metal ions is 100-400 μmol·L⁻¹.

[0020] Furthermore, the method for preparing the soil nanoparticle suspension to be tested is as follows:

[0021] S101. Adjust the pH of 100-300 mg·L⁻¹ soil nanoparticle suspension and 300-500 μmol·L⁻¹ nitrate metal ions to 6.0 respectively;

[0022] S102. Mix the two solutions separately at a volume ratio of 1:1 and react with horizontal shaking for 24 hours to obtain the sample to be tested.

[0023] Furthermore, the nitrate metal ion is specifically Cu(NO3)2.

[0024] Furthermore, the horizontal oscillating reaction is specifically carried out at 31°C and 150 rpm for 24 hours.

[0025] Further, S1. pre-treating the asymmetric flow field instrument in an oxygen-free environment includes:

[0026] Prepare a 25 μM NaCl solution with oxygen-free water, select a low flow rate of 0.05 mL / min, and control the operation of the asymmetric flow field analyzer for 12-36 hours to ensure that the asymmetric flow field analyzer is in an anaerobic environment.

[0027] Further, in step S2, the soil nanoparticle suspension is loaded into an asymmetric flow field analyzer, and after a sample introduction stage, a flushing stage, and a rinsing stage, the characterization results are obtained, including:

[0028] S201. In the anaerobic workstation, take 2.5 mL of the soil nanoparticle suspension after 24 h of reaction and put it into a 1 mL sample bottle for determination;

[0029] S202. After the sample introduction stage, flushing stage and rinsing stage, the characterization results are obtained.

[0030] Furthermore, the sample introduction stage includes: taking a soil nanoparticle suspension from the sample bottle and setting the program of the asymmetric flow field instrument to a detector flow rate of 0.5 mL / min; during the focusing process: sample introduction speed: 0.20 mL / min, sample introduction time: 5 min, sample introduction volume: 200 μL, lateral flow rate: 1.00 mL / min, focusing pump: 1.3 mL / min.

[0031] Furthermore, the elution stage includes setting the elution process conditions as follows: (1) lateral flow rate 1.00 mL / min, mode constant, time 15 min; (2) lateral flow rate 1.00 mL / min, mode Power, Exponent 0.20, time 30 min; (3) lateral flow rate 0.05 mL / min, mode Power, Exponent 0.80, time 10 min; (4) lateral flow rate 0.00 mL / min, mode constant, time 30 min.

[0032] Compared with the prior art, this invention develops a system for the use of soil nanoparticles in an asymmetric flow field instrument, which can effectively study and characterize soil nanoparticles. Attached Figure Description

[0033] Figure 1 shows soil nanoparticles and Cu. 2+ Graph showing the changes in AF4-UV curves before and after the reaction;

[0034] Figure 2 shows the changes in component distribution at different particle sizes before and after the reaction of soil nanoparticles with Cu2+; where (a) UV signal distribution before reaction; (b) UV signal distribution after reaction; (c) Fe and Cu mass spectrometry signal distribution before reaction; (d) Fe and Cu mass spectrometry signal distribution after reaction; (e) Al and Si mass spectrometry signal distribution before reaction; and (f) Al and Si mass spectrometry signal distribution after reaction. Detailed Implementation

[0035] To enable those skilled in the art to better understand the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0036] The asymmetric flow field analyzer (Poatnova, AF2000MT, GERMAN), hereinafter referred to as the flow field analyzer, involves asymmetric flow fields in separation. Under the influence of asymmetric flow field forces and opposing diffusion fields, the sample forms different equilibrium layers within the channel. During the sample introduction stage, before separation begins, the sample enters the channel from the inlet and is subjected to a unidirectional cross-flow force from the top of the channel. The pressure generated by the cross-flow causes the colloidal particles of the sample to be retained on a dialysis membrane that only allows small molecule solutes to pass through. At this point, the sample diffuses in the vertical direction of the channel, and the diffusion coefficient depends on factors such as the size and density of different colloidal particles. Colloidal particles with larger particle sizes or densities have smaller diffusion coefficients and aggregate near the lower part of the channel near the membrane, while particles with smaller particle sizes or densities tend to be closer to the middle of the channel space. Simultaneously, the sample is also subjected to a focusing flow field force opposite to the elution direction. Under the combined action of the channel flow and the focusing flow, the sample is concentrated near the inlet, and only small molecule solutes can be eluted through the dialysis membrane. After sample introduction, the focusing flow velocity returns to zero, and the colloidal particles are eluted by the channel flow and then flow towards the detector. According to boundary layer theory, within the same cross-section of the channel, the fluid velocity near the boundary is slower, while the fluid velocity near the center of the channel is faster, exhibiting a parabolic streamline along the cross-section. Therefore, smaller particles are flushed out first by the faster fluid and preferentially enter the detector, while larger particles are gradually flushed out as the cross-flow decreases.

[0037] The field flow meter can be coupled with many detectors, such as ultraviolet-visible spectrophotometer (UV-vis), multi-angle static laser particle size analyzer (MALS), fluorescence detector, differential detector, ICP-MS, etc. By analyzing different characteristics of colloidal particles through different detectors, it is possible to simultaneously detect, separate and characterize colloidal particles online.

[0038] By using a combined field flow analyzer and ICP-OES to separate nanoparticles of different sizes and then characterize their elemental composition and particle size, the relationship between the particle size and elemental composition of nanoparticles can be explored. This has important reference value for the study of the environmental effects and biotoxicology of nanoparticles at the nanoscale.

[0039] Although ICP-MS can characterize the elemental composition of soil nanoparticles, the complex interactions between colloidal particles can easily lead to their adsorption and aggregation, forming colloidal particles of different sizes. These particles may have different elemental ratios and properties. Therefore, by using a field flow analyzer and ICP-MS to separate soil nanoparticles of different purity and then characterizing their elemental composition and particle size, the relationship between the particle size and elemental composition of soil nanoparticles can be explored. This has important reference value for the study of the environmental effects and biotoxicology of nanoparticles at the nanoscale.

[0040] Specifically, the embodiment of the present invention is as follows: Step 1. 200 mg·L⁻¹ -1 Soil nanoparticle suspension with 400 μmol·L -1 The pH of Cu(NO3)2 was adjusted to 6.0, and then added to a 50 mL centrifuge tube at a volume ratio of 1:1 (20 mL each). The mixture was shaken by hand and then oscillated horizontally for 24 h (31 °C, 150 rpm).

[0041] Step 2. Before separation, prepare a 25 μM NaCl background solution with oxygen-free water and run it overnight at a low flow rate of 0.05 mL / min to ensure an anaerobic environment during separation.

[0042] Step 3. In the anaerobic workstation, take 2.5 mL of the sample after 24 hours of reaction and put it into a 1 mL sample vial for analysis. The sample is separated using an asymmetric flow field analyzer (AF2000MT, Postnova, Germany). The operation method is set on the flow field analyzer software, which consists of three steps: sample introduction, flushing, and rinsing.

[0043] The specific operating parameters are set as follows: detector flow rate 0.5 mL / min. During focusing: injection rate: 0.20 mL / min, injection time 5 min, injection volume: 200 μL, lateral flow rate: 1.00 mL / min, focusing pump: 1.3 mL / min. Elution process: (1) lateral flow rate 1.00 mL / min, constant mode, time 15 min. (2) lateral flow rate 1.00 mL / min, power mode, Exponent 0.20, time 30 min. (3) lateral flow rate 0.05 mL / min, power mode, Exponent 0.80, time 10 min. (4) lateral flow rate 0.00 mL / min, constant mode, time 30 min. The total running time for one sample is 90 min.

[0044] The ultraviolet and mass spectrometry signals were acquired by using a UV-Vis and inductively coupled plasma mass spectrometer (ICP-MS, NexION 350D, PerkinElmer, USA). The correlation between elution time and particle size was obtained by running latex standard samples for calibration. The particle size distribution of nanoparticles in the sample and the elemental signal intensities of Fe, Cu, Al and Si were then determined.

[0045] This invention embodiment analyzed soil nanoparticles and Cu using AF4-UV coupled analysis. 2+The changes in UV signal in the suspension system before and after the reaction with elution time were investigated. The relationship between elution time and particle size was corrected by running latex standard samples, thus semi-quantitatively determining the particle size distribution characteristics of soil nanoparticles and the particle size redistribution behavior after the reaction. As shown in Figure 1, the initial state of soil nanoparticles before the reaction exhibits two absorption peaks: 1–20 nm and 200–300 nm, indicating that it contains at least two types of nanoparticle components with different particle sizes. After the reaction with Cu²⁺, the particle size distribution of the soil nanoparticles changed significantly, with three absorption peaks appearing in the UV curve. Based on the relatively independent relationship between the absorption peaks, it can be seen that three particle size ranges (20 nm, 50 nm, and 300–400 nm) are present in the system at this time. The original 200–300 nm particle component essentially disappeared, replaced by a smaller particle size (50 nm) and a larger particle size (300–400 nm) component.

[0046] AF4-UV-MALS-ICP-MS was used to further analyze the distribution characteristics of major elements such as Fe, Cu, Al, and Si across particle size, thereby exploring the main components of soil nanoparticles of different particle sizes before and after the reaction. It is important to emphasize that the ordinate of the element distribution curves with particle size is the mass spectrometry signal intensity value (cps), which does not represent the actual concentration. Therefore, it only provides reference information on the presence and relative changes of the element. As shown in Figure 2, before the reaction, Fe, Al, and Si mineral elements were distributed in both particle size fractions of soil nanoparticles, with relatively higher concentrations in particles of 200–300 nm, while Cu was mainly concentrated in the smaller particle size fraction. After the reaction, with the redistribution of particle size, the particle composition changed. Fe, Al, and Si were mainly concentrated in newly formed aggregates, while the distribution of Cu remained relatively unchanged.

[0047] The distribution of Cu showed little correlation with Fe, Al, and Si, primarily tending to allocate to smaller particle sizes (mainly organic components). This indicates that Cu in soil nanoparticles mainly exists in the form of organic matter bound together, with a weaker relationship to minerals. The changes in Fe, Cu, Al, and Si after the reaction were largely consistent with the particle size redistribution trend, with most Fe, Al, and Si concentrated in the 300–400 nm particle size range. This clearly demonstrates that the stability of mineral-dominated particles in soil nanoparticles is more susceptible to disturbance, leading to aggregation. While the particle size of Cu distribution slightly increased with deteriorating system stability and enhanced aggregation, it still showed a weak correlation with mineral components overall. This further confirms that the distribution, adsorption, and binding of Cu in soil nanoparticles are dominated by organic components.

[0048] In summary, this invention has the following beneficial effects: 1. For the first time, an asymmetric flow field analyzer coupled with inductively coupled plasma mass spectrometry (ICP-MS) is used to characterize soil nanoparticles, and good characterization results are obtained, which can be used as a means to separate and characterize soil nanoparticles; 2. Suitable flow field analyzer methods for samples are explored, including axial flow velocity, crossflow velocity, injection time, rinsing time, etc., to improve the elution and detection effects of samples; 3. Practice has proven that the method parameters of the asymmetric flow field analyzer are effective method parameters for separating colloidal particles and nanoparticles.

[0049] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that after reading this application specification, they can still modify or make equivalent substitutions to the specific implementation of the present invention, but these modifications or changes do not depart from the protection scope of the pending claims of the present invention.

Claims

1. A method for characterizing the particle size and composition of soil nanoparticles using asymmetric flow-field coupled inductively coupled plasma mass spectrometry, characterized in that, include: S1. Pre-treat the asymmetric flow field instrument in an oxygen-free environment; S2. The soil nanoparticle suspension was loaded into an asymmetric flow field analyzer and subjected to a sample introduction stage, a flushing stage, and a rinsing stage to obtain the characterization results.

2. The method for characterizing the particle size and composition of soil nanoparticles using asymmetric flow-field coupled inductively coupled plasma mass spectrometry according to claim 1, characterized in that, The soil nanoparticle suspension to be tested also contains metal ions, specifically Fe. 2+ Fe 3+ Cu 2+ Al 3+ One or more of them.

3. The method for characterizing the particle size and composition of soil nanoparticles using asymmetric flow-field coupled inductively coupled plasma mass spectrometry according to claim 2, characterized in that, The concentration of the metal ions is 100-400 μmol·L⁻¹. -1 .

4. The method for characterizing the particle size and composition of soil nanoparticles using asymmetric flow-field coupled inductively coupled plasma mass spectrometry according to claim 3, characterized in that, The preparation method of the soil nanoparticle suspension to be tested is as follows: S101. Adjust the pH of 100-300 mg·L-1 soil nanoparticle suspension and 300-500 μmol·L-1 nitrate metal ions to 6.0 respectively; S102. Mix the two solutions at a volume ratio of 1:1 and react with horizontal shaking for 24 h to obtain the sample to be tested.

5. The method for characterizing the particle size and composition of soil nanoparticles using asymmetric flow-field coupled inductively coupled plasma mass spectrometry according to claim 4, characterized in that, The nitrate metal ion is specifically Cu(NO3). 2 .

6. The method for characterizing the particle size and composition of soil nanoparticles using asymmetric flow-field coupled inductively coupled plasma mass spectrometry according to claim 4, characterized in that, The horizontal oscillating reaction is specifically carried out at 31°C and 150 rpm for 24 hours.

7. The method for characterizing the particle size and composition of soil nanoparticles using asymmetric flow-field coupled inductively coupled plasma mass spectrometry according to claim 1, characterized in that, S1. Pre-treatment of the asymmetric flow field instrument in an oxygen-free environment includes: Prepare a 25 μM NaCl solution with oxygen-free water, select a low flow rate of 0.05 mL / min, and control the operation of the asymmetric flow field analyzer for 12-36 hours to ensure that the asymmetric flow field analyzer is in an anaerobic environment.

8. The method for characterizing the particle size and composition of soil nanoparticles using asymmetric flow-field coupled inductively coupled plasma mass spectrometry according to claim 1, characterized in that, S2. The soil nanoparticle suspension is loaded into an asymmetric flow field analyzer and, after the sample introduction stage, flushing stage, and rinsing stage, the characterization results are obtained, including: S201. In the anaerobic workstation, take 2.5 mL of the soil nanoparticle suspension after 24 h of reaction and put it into a 1 mL sample bottle for determination; S202. After the sample introduction stage, flushing stage and rinsing stage, the characterization results are obtained.

9. The method for characterizing the particle size and composition of soil nanoparticles using asymmetric flow-field coupled inductively coupled plasma mass spectrometry according to claim 8, characterized in that, The sample introduction stage includes: taking soil nanoparticle suspension from the sample bottle and setting the program of the asymmetric flow field flowmeter to a detector flow rate of 0.5 mL / min; during the focusing process: sample introduction speed: 0.20 mL / min, sample introduction time: 5 min, sample introduction volume: 200 μL, lateral flow rate: 1.00 mL / min, focusing pump: 1.3 mL / min.

10. The method for characterizing the particle size and composition of soil nanoparticles using asymmetric flow-field coupled inductively coupled plasma mass spectrometry according to claim 9, characterized in that, The elution phase includes setting the elution process conditions as follows: (1) lateral flow rate 1.00 mL / min, mode constant, time 15 min; (2) lateral flow rate 1.00 mL / min, mode Power, Exponent 0.20, time 30 min; (3) lateral flow rate 0.05 mL / min, mode Power, Exponent 0.80, time 10 min; (4) lateral flow rate 0.00 mL / min, mode constant, time 30 min.

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