Sample preparation-free calibration method for rapidly detecting heavy metals in rice, reference calibration sample and detection method
By employing a sample-free method, combining citric acid pretreatment and gradient immersion with LIBS/XRF technology, the problems of matrix matching and cumbersome sample preparation in heavy metal detection of rice have been solved. This enables rapid and non-destructive calibration sample preparation and detection, meeting the needs of high-throughput on-site detection.
Patent Information
- Application Number
- CN202511295295.4
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-09-11
- Publication Date
- 2025-11-28
AI Technical Summary
Existing technologies for heavy metal detection in rice suffer from problems such as the lack of standard samples with matching matrix and cumbersome and destructive sample preparation processes, leading to decreased detection accuracy and making it difficult to meet the needs of rapid on-site detection.
A sample-free method was adopted, in which rice was pretreated with citric acid solution, soaked in heavy metal solutions of varying concentrations, and spectral intensity was detected by LIBS or XRF technology to establish a calibration curve. Qualified samples were selected for digestion to prepare calibration samples.
It enables rapid, non-destructive, in-situ high-throughput detection of heavy metals in rice, simplifies the sample preparation process, and improves detection accuracy and ease of on-site application.
Smart Images
Figure CN121027188A_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of heavy metal detection technology, specifically relating to a calibration method for rapid detection of heavy metals in rice without sample preparation, a reference calibration sample, and a detection method. Background Technology
[0002] Rapid analytical techniques such as laser-induced breakdown spectroscopy (LIBS), X-ray fluorescence spectroscopy (XRF), and Raman spectroscopy can quickly determine the content of heavy metals by directly detecting the characteristic spectral intensities of heavy metal elements in samples, and have been widely used in the detection of heavy metal contamination in rice. These techniques, with their rapid response, simultaneous multi-element analysis, and in-situ detection potential, provide important tools for on-site screening of food safety.
[0003] However, existing technologies still face two major challenges in practical applications: 1) A severe lack of matrix-matched standard samples: Rice grains have unique physical structures (such as starch matrix and moisture distribution) and chemical compositions (such as proteins and trace elements), resulting in a significant difference in their spectral matrix effects compared to powdered tablets or solution standards. Currently, commercial standard materials are all in rice powder form, and the difference from the actual matrix of rice leads to a decrease in the accuracy of calibration models. 2) Cumbersome sample preparation process and significant sample damage: Existing spectroscopic detection methods still cannot avoid destructive processing of rice grain structures, such as grinding rice into powder and pressing it into tablets or converting it into films, or extracting heavy metals with acid and solidifying them onto a substrate. Although these steps can improve detection accuracy, they are time-consuming (e.g., ultrasonic extraction takes more than 1 hour) and rely on laboratory equipment, making it difficult to meet the needs of rapid on-site detection.
[0004] In summary, due to limitations imposed by standard samples, the detection performance of spectroscopic techniques, which are typically capable of rapid detection, is not ideal in practical applications. Therefore, developing a standard sample with a highly matched matrix suitable for in-situ detection of heavy metals in rice samples will save significant time in sample preparation, enabling high-throughput in-situ detection of heavy metals in rice, which is of great importance for rice quality supervision. Summary of the Invention
[0005] To address the aforementioned technical problems, one objective of this invention is to provide a calibration method for rapid detection of heavy metals in rice without sample preparation.
[0006] The present invention adopts the following technical solution:
[0007] A calibration method for rapid detection of heavy metals in rice without sample preparation includes the following steps:
[0008] S1. Sample pretreatment: After washing the rice, soak it in 0.1 mol / L citric acid solution for 30 min. After soaking, take it out and rinse it with clean water.
[0009] S2. Sample preparation: Prepare heavy metal solutions of the required gradient concentrations using 20% ethanol aqueous solution. Divide the rice treated in step S1 into equal portions and soak them in the heavy metal solutions of the gradient concentrations respectively. After soaking for 3 days, take them out, wash them, and dry them to obtain rice samples with different heavy metal concentrations.
[0010] S3. Spectral intensity detection: The rice sample from step S2 is detected using laser-induced breakdown spectroscopy or X-ray fluorescence spectroscopy to obtain the spectral intensity of each grain of rice as the spectral intensity of the heavy metal element at its corresponding gradient concentration.
[0011] S4. Heavy metal content detection: Select at least 5 grains of rice from the samples corresponding to each heavy metal gradient concentration as calibration samples, and use the digestion method to obtain the heavy metal content of each grain of rice as the true value;
[0012] S5. Create a calibration curve: Use the true values of heavy metal content obtained in step S4 and the spectral intensities of heavy metal elements at corresponding gradient concentrations obtained in step S3 to create a calibration curve.
[0013] Preferably, the heavy metal solution is set with at least 5 gradient concentrations, and the number of rice grains soaked in each gradient concentration is not less than 30 grains.
[0014] Preferably, the heavy metal solution is provided with five gradient concentrations: 0 mg / L, 1 mg / L, 5 mg / L, 10 mg / L, 20 mg / L, and 50 mg / L. In actual use, the concentration of the soaking solution can be adjusted appropriately according to the approximate concentration range of the sample to be tested.
[0015] Preferably, in step S3, the spectrum of each grain of rice is detected at three points on its surface: both ends and the middle. The average value of the detected values is taken as the spectral intensity of the rice.
[0016] Preferably, when using laser-induced breakdown spectroscopy for detection, 10 spectral data points are detected at each location; when using X-ray fluorescence spectroscopy for detection, 5 spectral data points are detected at each location; after detection, the relative standard deviation is calculated, and rice samples with a relative standard deviation within a set threshold are selected as qualified samples and proceed to step S4.
[0017] Preferably, in step S4, the method for selecting calibration samples is as follows: after sorting all qualified samples according to spectral intensity, samples are taken from all samples at equal intervals according to the number of samples or stratified sampling is performed according to the spectral intensity value, and the first and last samples are mandatory samples.
[0018] Preferably, in step S4, the digestion method is to add 2 ml of concentrated nitric acid, leave overnight to remove the acid, and then dilute to 20 ml with 2% dilute nitric acid.
[0019] Preferably, in step S4, the heavy metal content is detected using ICP-MS.
[0020] Preferably, the rice sample is prepared from any variety or strain according to the testing requirements.
[0021] Preferably, the method also includes a calibration method for rapid detection of heavy metals in rice without sample preparation. Rice is soaked in a heavy metal solution with a gradient concentration of multiple elements. After soaking, rice samples with different concentrations of heavy metals are obtained. Steps S3-S5 are repeated with the concentration of one of the heavy metal elements to establish a calibration curve.
[0022] The second objective of this invention is to provide a reference calibration sample for rapid detection of heavy metals in rice without sample preparation, which is prepared using the calibration method described above.
[0023] The third objective of this invention is to provide a reference calibration sample for rapid detection of heavy metals in rice without sample preparation, which is prepared using the calibration method for multi-element reference standard samples described above.
[0024] The fourth objective of this invention is to provide the application of the reference calibration sample or multi-element reference calibration sample for rapid detection of heavy metals in rice without sample preparation, as described above, in the rapid detection of heavy metal content in rice.
[0025] The fifth objective of this invention is to provide a method for rapid detection of heavy metals in rice without sample preparation, comprising the following steps:
[0026] S1. Establish the calibration curve of the test sample based on the reference calibration sample as described above;
[0027] S2. Detect the spectrum of the sample to be tested, and directly read the content of the target heavy element in the sample according to the established calibration curve.
[0028] The beneficial effects of this invention are as follows:
[0029] This application provides a calibration method for rapid detection of heavy metals in rice without sample preparation, including steps such as sample pretreatment, preparation of rice samples containing heavy metals, spectral intensity detection, standard method detection, and concentration calibration.
[0030] This application improves the pretreatment and soaking methods. First, the rice is washed to remove surface starch. Then, the rice is soaked in a citric acid solution to dissolve surface lipids, exposing the microporous structure and facilitating heavy metal penetration. Finally, the rice sample is soaked in an ethanol-containing solution, effectively ensuring that the rice does not swell or deform and does not affect the elemental spectra of the sample itself. This soaking method yields calibration samples containing all heavy metals quickly and with good uniformity. When selecting calibration samples, they are first sorted by spectral intensity before selection, allowing for control over the horizontal or vertical uniform distribution of points on the calibration curve.
[0031] The digestion method used in this application can completely digest rice samples, and requires the least amount and type of acid. It does not require heating or microwaves and is easy to operate.
[0032] This application first measures the spectral data of rice, then uses standard methods to obtain the true values of its heavy metal content, ensuring that the physical properties of the sample to be tested are consistent with those of the calibration sample. Therefore, the established calibration curve can directly perform rapid detection of rice samples without sample preparation. According to this method, a reference calibration sample can be obtained for detection. After the reference calibration sample is prepared in the laboratory, it can be applied to the rapid on-site detection of heavy metals in rice samples without any pretreatment steps, making it simple and fast.
[0033] Since LIBS and XRF detection cause minimal damage to rice samples, samples prepared in one go can be properly preserved and reused for calibration. Attached Figure Description
[0034] Figure 1 This is a schematic diagram illustrating the sorting and sampling of rice samples according to the spectral intensity of heavy metals.
[0035] Figure 2 LIBS spectra of phosphorus in unsoaked rice, rice soaked in pure water, and rice soaked in ethanol of different concentrations.
[0036] Figure 3 The figure shows the changes in the LIBS spectral intensity of Mo in rice after citric acid pretreatment. Figure A represents the result without citric acid pretreatment, and figure B represents the result with citric acid pretreatment.
[0037] Figure 4 The figures show the results of the Cd soaking experiment. In the figure, A represents the ascending order of the spectral intensities of the soaked rice, B represents the obtained calibration curve, and C represents the comparison between the LIBS rapid detection results and the ICP-MS detection results of the sample to be tested.
[0038] Figure 5 The figures show the results of immersion experiments for Mo, Cr, and Cd. Figure A compares the LIBS and ICP-MS results for Mo, Figure B compares the LIBS and ICP-MS results for Cr, and Figure C compares the LIBS and ICP-MS results for Cd. Detailed Implementation
[0039] To facilitate understanding, the technical solution of the present invention will be described in more detail below with reference to the embodiments.
[0040] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this article can be purchased from the market or prepared by existing methods.
[0041] Example 1
[0042] Single-element reference standard sample
[0043] A calibration method for rapid detection of heavy metals in rice without sample preparation includes the following steps:
[0044] S1. Sample preparation: Wash the rice to be prepared with deionized water to remove the starch attached to the surface. Then, prepare a 0.1 mol / L citric acid solution and immerse the washed rice sample in it to dissolve the surface lipids and expose the microporous structure to facilitate the penetration of heavy metals. After soaking for 30 minutes, take it out and rinse it with deionized water.
[0045] S2. Sample preparation: Prepare a 20% ethanol aqueous solution. Use this solution to prepare heavy metal solutions with the required gradient concentrations. Divide the rice treated in step S1 into equal portions and soak them in the heavy metal solutions with gradient concentrations respectively. After soaking for 3 days, take them out and rinse them with deionized water to remove the heavy metal solutions remaining on the surface. Let them air dry naturally to obtain rice samples with different heavy metal concentrations.
[0046] In this method, the amount of rice used in a single reference sample preparation is no less than 180 grains, which can be set as needed. The more grains used, the more reference calibration samples can be prepared in one go. In this embodiment, five gradient concentrations of heavy metal solution are set: 0 mg / L, 1 mg / L, 5 mg / L, 10 mg / L, 20 mg / L, and 50 mg / L. The amount of rice used is 180 grains, which are divided into 6 equal portions for soaking.
[0047] S3. Spectral detection: The rice sample from step S2 is detected using laser-induced breakdown spectroscopy (LIBS) or X-ray fluorescence spectroscopy (XRF) to obtain the spectral intensity of each grain of rice as the spectral intensity of the heavy metal element at its corresponding gradient concentration.
[0048] Specifically, the spectra of each grain of rice were measured at three points on its surface: both ends and the middle. When using LIBS detection, 10 spectral data points were collected at each point; when using XRF detection, 5 spectral data points were collected at each point. After detection, the fluctuation of the spectral intensity of each grain of rice (relative standard deviation, RSD) was calculated. Samples with RSD exceeding the threshold were removed. An excessively large RSD indicates poor uniformity of heavy metal distribution within the rice grain, making it unsuitable as a reference calibration sample. The threshold setting was adjusted according to actual conditions; in this embodiment, it was set to 30%. Rice samples with a relative standard deviation within the set threshold were selected as qualified samples, and the average value was taken as the spectral intensity of that rice sample, proceeding to step S4.
[0049] After testing, the samples are stored in a sample tray in units of individual particles. It is essential to store each particle separately and label it accordingly.
[0050] S4. Heavy metal content detection: Select at least 5 grains of rice from all qualified samples as calibration samples, use the digestion method to obtain the heavy metal content of each grain of rice as the true value, and use the digestion solution as the detection reference;
[0051] Specifically, the sampling method is as follows: sampling and digestion are performed based on the spectral intensity distribution. All qualified samples are sorted according to their spectral intensity, with the horizontal axis representing the sample number and the vertical axis representing the spectral intensity of heavy metal elements. Figure 1 As shown. Sampling shall be performed according to any of the following methods:
[0052] First, samples are taken from all samples at equal intervals according to the sample quantity, that is, samples are taken evenly distributed according to the horizontal axis, such as... Figure 1 First, take one sample from each of the five red circles; second, sample according to a uniform distribution on the vertical axis, that is, stratify according to spectral intensity values, such as... Figure 1 As indicated by the green arrow in the middle.
[0053] A minimum of 5 samples must be taken, with the first and last samples being mandatory. The larger the sample size, i.e., the larger the sample size for the calibration curve, the more accurate the results.
[0054] In this method, the digestion and detection steps are as follows: weigh the contents separately, add 2 ml of concentrated nitric acid, incubate overnight, remove the acid, and dilute to 20 mL with 2% dilute nitric acid. The heavy metal content is then determined by ICP-MS. This digestion method is simpler than the standard laboratory method, requiring the least amount and types of acid.
[0055] However, it should be understood that the method of digesting and detecting heavy metal content is existing technology, and other digestion methods can also be used in this method. In actual use, this application does not make any special requirements for this.
[0056] S5. Create a calibration curve: Create a calibration curve using the true value of heavy metal content obtained in step S4 and the spectral intensity of heavy metal elements at the corresponding gradient concentrations obtained in step S3. This calibration curve can be used for rapid detection of heavy metals in rice without sample preparation. It is suitable for rapid on-site detection and does not require any pretreatment of the sample to be tested.
[0057] In this method, the rice used can be any variety (such as indica rice, glutinous rice, or japonica rice) or strain (such as Longdao 18, Wuyoudao No. 4, or Songjing 28) selected according to the testing requirements to further improve the detection accuracy.
[0058] It should be understood that in this method, soaking is only for quickly obtaining samples containing gradient heavy metals. If there are naturally occurring samples with excessive heavy metals, they can also be directly detected according to steps S3-S5 to obtain calibration curves.
[0059] When this method is applied to the on-site detection of heavy metals in rice, first adjust the instrument, then test 4-10 calibration samples to establish a calibration curve, and then test the spectrum of the rice to be tested. Based on the calibration curve, the content of heavy metals of the element to be tested can be obtained.
[0060] Example 2
[0061] The effects of different pretreatments on rice
[0062] P is an element naturally present in rice. This study focuses on the effect of different soaking solutions on the spectrum of P.
[0063] like Figure 2 As shown, Figure 2 The LIBS spectra of phosphorus (P) in unsoaked rice and rice soaked in pure water and different concentrations of ethanol (20%, 40%, 60%, 80%, and 100%) are shown. It can be seen that the P spectrum of rice soaked in pure water is lower because the rice expands and deforms significantly after soaking, which greatly affects the spectrum. The P content of rice soaked in 20%–100% ethanol is not significantly different from that of unsoaked rice; therefore, setting the ethanol content of the soaking solution to 20% will not affect the on-site calibration.
[0064] Figure 3 Unused ( Figure 3 a) and use ( Figure 3 (b) Changes in the LIBS spectral intensity of Mo at the same point on the surface of rice pretreated with citric acid after soaking in a 100 mg / L Mo solution. In Figure a, the spectral intensity of the first two pulses is significantly higher than that of subsequent pulses. After 10 pulses, the spectral intensity of Mo is very weak, indicating that in untreated rice, Mo exists only on the surface and cannot penetrate into the interior. In Figure b, however, there is no significant change in Mo intensity before and after treatment, indicating that after citric acid treatment, Mo effectively penetrates into the interior of the rice.
[0065] Example 3
[0066] Cd element experiment
[0067] The method is the same as in Example 1, except that the concentration gradients of Cd in the soaking solution are set to 0, 2, 5, and 10 mg / L. Figure 4 As shown, Figure 4 In the diagram, 'a' represents the result of the spectral intensities arranged in ascending order. Figure 4 In the figure, b represents the obtained calibration curve, and R... 2 The linearity is 0.8598, indicating good linearity. In Figure 4, c represents a comparison between the LIBS rapid detection results and the ICP-MS detection results of the sample. The horizontal axis represents the ICP-MS detection results, and the vertical axis represents the LIBS detection results. The slope is close to 1, and R0 is relatively stable. 2The value is 0.8405, which proves that the detection results of the method in this application are well correlated with those of the ICP-MS method, and the difference in detection results is not significant under the premise of simplifying the detection steps.
[0068] Example 4
[0069] Experiments with Mo, Cr and Cd elements
[0070] The method is the same as in Example 1, with the concentrations of Mo, Cr and Cd in the soaking solution being 0, 1, 2, 5, 10, 25, 50 and 100 mg / L, respectively.
[0071] Figure 5 To compare the LIBS and ICP-MS results for Mo, Cr, and Cd elements, all three elements were immersed and detected simultaneously. It can be seen that the correlation coefficients Ra for the LIBS and ICP-MS results for Mo (Figure a), Cr (Figure b), and Cd (Figure c) are high compared to those for Mo and Cd. 2 The values were 0.8097, 0.8965, and 0.8177, respectively, demonstrating that the detection results of the method in this application are well correlated with those of the ICP-MS method, and that the difference in detection results is not significant under the premise of simplifying the detection steps.
[0072] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.
Claims
1. A calibration method for rapid detection of heavy metals in rice without sample preparation, characterized in that, Includes the following steps: S1. Sample pretreatment: After washing the rice, soak it in 0.1 mol / L citric acid solution for 30 min. After soaking, take it out and rinse it with clean water. S2. Sample preparation: Prepare heavy metal solutions of the required gradient concentrations using 20% ethanol aqueous solution. Divide the rice treated in step S1 into equal portions and soak them in the heavy metal solutions of the gradient concentrations respectively. After soaking for 3 days, take them out, wash them, and dry them to obtain rice samples with different heavy metal concentrations. S3. Spectral intensity detection: The rice sample from step S2 is detected using laser-induced breakdown spectroscopy or X-ray fluorescence spectroscopy to obtain the spectral intensity of each grain of rice as the spectral intensity of the heavy metal element at its corresponding gradient concentration. S4. Heavy metal content detection: Select at least 5 grains of rice from the samples corresponding to each heavy metal gradient concentration as calibration samples, and use the digestion method to obtain the heavy metal content of each grain of rice as the true value; S5. Create a calibration curve: Use the true values of heavy metal content obtained in step S4 and the spectral intensities of heavy metal elements at corresponding gradient concentrations obtained in step S3 to create a calibration curve.
2. The calibration method for rapid detection of heavy metals in rice without sample preparation as described in claim 1, characterized in that, The heavy metal solution is set with at least 5 gradient concentrations, and the number of rice grains soaked in each gradient concentration is no less than 30 grains.
3. The calibration method for rapid detection of heavy metals in rice without sample preparation as described in claim 1, characterized in that, The heavy metal solution was set with five gradient concentrations: 0 mg / L, 1 mg / L, 5 mg / L, 10 mg / L, 20 mg / L and 50 mg / L.
4. The calibration method for rapid detection of heavy metals in rice without sample preparation as described in claim 1, characterized in that, In step S3, the spectrum of each grain of rice is detected at three points on its surface: both ends and the middle. The average value of the detected values is taken as the spectral intensity of the rice.
5. The calibration method for rapid detection of heavy metals in rice without sample preparation as described in claim 4, characterized in that, When using laser-induced breakdown spectroscopy for detection, 10 spectral data points are collected at each location; when using X-ray fluorescence spectroscopy for detection, 5 spectral data points are collected at each location. After detection, the relative standard deviation is calculated, and rice samples with a relative standard deviation within the set threshold are selected as qualified samples and proceed to step S4.
6. The calibration method for rapid detection of heavy metals in rice without sample preparation as described in claim 5, characterized in that, In step S4, the method for selecting calibration samples is as follows: after sorting all qualified samples according to spectral intensity, samples are taken from all samples at equal intervals according to the number of samples or stratified sampling according to spectral intensity values, and the first and last samples are mandatory samples.
7. The calibration method for rapid detection of heavy metals in rice without sample preparation as described in claim 1, characterized in that, In step S4, the digestion method is to add 2 ml of concentrated nitric acid, let it sit overnight to remove the acid, and then make up to 20 ml with 2% dilute nitric acid. In step S4, the heavy metal content is detected by ICP-MS.
8. The calibration method for rapid detection of heavy metals in rice without sample preparation as described in claim 1, characterized in that, The rice samples can be prepared from any variety or strain, depending on the testing requirements.
9. A calibration method for rapid detection of heavy metals in rice without sample preparation, as described in any one of claims 1-8, characterized in that, It also includes a calibration method for rapid detection of heavy metals in rice without sample preparation. Rice is soaked in a heavy metal solution with a gradient concentration of multiple elements. After soaking, rice samples with different concentrations of heavy metals are obtained. Steps S3-S5 are repeated with the concentration of one of the heavy metal elements to establish a calibration curve.
10. A reference calibration sample for rapid detection of heavy metals in rice without sample preparation, characterized in that, It was prepared using the calibration method described in any one of claims 1-8.
11. A reference calibration sample for rapid multi-element, sample-free detection of heavy metals in rice, characterized in that, It was prepared using the calibration method described in claim 9.
12. The application of the reference calibration sample for rapid detection of heavy metals in rice without sample preparation as described in claim 10 or the reference calibration sample for rapid detection of heavy metals in rice without sample preparation as described in claim 11 in the rapid detection of heavy metal content in rice.
13. A method for rapid detection of heavy metals in rice without sample preparation, characterized in that, Includes the following steps: S1. Establish a calibration curve for the sample to be tested using the reference calibration sample as described in claim 10 or 11; S2. Detect the spectrum of the sample to be tested, and directly read the content of the target heavy element in the sample according to the established calibration curve.