Method for detecting heavy metals in wheat by LIBS (laser-induced breakdown spectroscopy) based on circular hole constrained ultrasonic extraction

Abstract

The invention discloses an LIBS (Laser-induced Breakdown Spectroscopy) method for detecting heavy metals in wheat based on round hole constrained ultrasonic extraction, which comprises the following steps: mixing wheat powder and a hydrochloric acid solution, performing ultrasonic oscillation and centrifugal separation, and extracting supernate; dropwise adding the supernatant into a circular hole restraint sheet attached to a glass slide, and drying to obtain a sample; and measuring the LIBS spectrum of the sample in an inert gas atmosphere by adopting an LIBS method, taking the characteristic spectral line intensity of the Cd element in the LIBS spectrum as input, and obtaining the Cd element concentration of the sample based on the established calibration curve, thereby completing the detection of the Cd element in the wheat. According to the method, the coffee ring effect of the UAE method is improved, and the spectrum stability is improved. By adopting the CHC-UAE method, the LIBS technology can realize high-sensitivity detection of harmful heavy metal Cd in wheat, the stability of the LIBS spectrum is improved, and the requirements of actual application of heavy metal detection in wheat are met.

Description

Technical Field

[0001] The invention belongs to the technical field of laser spectral chemical analysis, and specifically relates to a LIBS method for detecting heavy metals in wheat based on circular hole constrained ultrasonic extraction. Background Art

[0002] In recent years, the continuous improvement of wheat breeding and cultivation technology has led to a continuous increase in wheat yield per unit area, but it has also brought about a series of ecological and food safety issues such as soil environment, fertilizer use, and heavy metal accumulation. During its growth, wheat has a certain ability to absorb and enrich heavy metal elements in the soil environment. In particular, after cadmium (Cd) accumulates in farmland soil, it is more easily absorbed by the wheat root system and transported to the aboveground part, thus forming potential heavy metal pollution in the grain. Cd is listed as a Class I human carcinogen by the International Agency for Research on Cancer (IARC) due to its strong toxicity and bioaccumulation. When consumers consume wheat products contaminated with Cd for a long time, it may lead to chronic health problems such as impaired kidney function, osteoporosis, and decreased immunity, causing persistent toxic hazards to the human body. Therefore, high-sensitivity, rapid and accurate analysis and monitoring of heavy metal pollution such as Cd in wheat has become an important part of ensuring food security and public health.

[0003] In the field of heavy metal detection in food, traditional methods mainly include inductively coupled plasma mass spectrometry (ICP-MS), atomic absorption spectroscopy (AAS), scanning electron microscope-energy dispersive spectrometry (SEM-EDS), etc. Although these methods have high sensitivity and accuracy, they have many limitations in practical applications.

[0004] Laser-Induced Breakdown Spectroscopy (LIBS) is an atomic emission spectroscopy analysis technique. It uses high-energy laser pulses to generate plasma on the sample surface and analyzes the spectral characteristics of the plasma emission to achieve qualitative and quantitative detection of multiple elements in the sample. LIBS technology has the advantages of small sample requirements, fast detection, simultaneous analysis of multiple elements, and simple sample pretreatment. It is widely used in environmental monitoring, food safety, metallurgical industry, biomedicine, deep space exploration and other fields.

[0005] However, LIBS still faces the problem of high element detection limit (LOD) in the detection of heavy metal elements in agricultural products. It is mainly reflected in the following two aspects: 1. Most agricultural products have complex matrices of starch, protein, cellulose and various organic compounds, which will interfere with the generation and stability of plasma and affect the intensity and repeatability of characteristic spectral lines. 2. Heavy metal elements usually exist in extremely low concentrations (μg / kg or mg / kg level). LIBS technology itself is more sensitive to high-concentration elements, but has limited sensitivity to trace element signals. These factors limit the sensitivity and accuracy of LIBS technology in low-concentration heavy metal detection.

[0006] Ultrasound-assisted extraction (UAE) is a solid-liquid-solid transformation (SLST) sample processing method. This method can significantly reduce the detection limit by destroying the sample matrix and analyzing the heavy metals that escape from the matrix. It has been proven to be effective in detecting trace heavy metals in rice. However, this method has a "coffee ring effect" during the liquid-solid transformation process, that is, the concentration of the analyte in the edge area is higher than that in the center area. This effect is caused by the difference in evaporation rate between the edge area and the center area. The higher evaporation rate at the edge causes the liquid from the center to flow to the edge, bringing the analyte to the edge for precipitation, making the analyte uneven. During the LIBS acquisition process, the inhomogeneity of the analyte will affect the temperature of the plasma excited by the laser during the ablation process and the content of the ablated sample, thereby affecting the spectral stability, thereby affecting the precision of elemental quantification in LIBS analysis and the accuracy of the results during the application process.

[0007] According to the food safety limit value for cereals stipulated in the Chinese national standard (GB2762-2017), the Cd content in wheat cannot exceed 0.1 mg / kg. So far, there has been no report on the use of LIBS technology to detect heavy metal Cd in wheat. Summary of the invention

[0008] The present invention provides a LIBS method for detecting heavy metals in wheat based on circular hole-constrained ultrasonic-assisted extraction (CHC-UAE), which improves the "coffee ring effect" of the UAE method and improves the spectral stability. The LIBS technology can detect the harmful heavy metal Cd in wheat with high sensitivity through the CHC-UAE method, and improve the stability of the LIBS spectrum, meeting the needs of actual application of heavy metal detection in wheat.

[0009] The present invention provides a LIBS method for detecting heavy metals in wheat based on circular hole constrained ultrasonic extraction, comprising: (1) Mix wheat powder and hydrochloric acid solution, perform ultrasonic vibration, centrifuge and extract the supernatant; (2) dropping the supernatant into a circular hole constraint sheet attached to a glass slide, and obtaining a sample after drying; (3) The LIBS spectrum of the sample is measured in an inert gas atmosphere using the LIBS method. The characteristic line intensity of the Cd element in the LIBS spectrum is used as input, and the Cd element concentration of the sample is obtained based on the established calibration curve, thereby completing the detection of the Cd element in wheat.

[0010] Preferably, the inner diameter of the circular hole constraint sheet is 10-12 mm. The present invention can achieve uniform distribution of the supernatant containing heavy metal Cd while providing a sufficiently large space for LIBS detection by controlling the inner diameter of the circular hole constraint sheet.

[0011] Preferably, the mass / volume ratio of the wheat powder to the hydrochloric acid solution is 1:9 - 1:11 g / mL, and the particle size of the wheat powder is <125 μm. The present invention controls the concentration and size of the wheat powder so that the hydrochloric acid solution can fully react with the heavy metal Cd in the wheat powder to obtain CdCl2.

[0012] Preferably, the pH value of the hydrochloric acid is 1, and the ultrasonic oscillation time is 8-12 min. The present invention controls the pH value at a lower level to accelerate the reaction, and can achieve a full reaction between the hydrochloric acid solution and the heavy metal Cd in the wheat powder within a shorter ultrasonic oscillation time, thereby improving the reaction efficiency.

[0013] Preferably, the rotation speed of the centrifugal separation is 4000-4500 rpm, and the time of the centrifugal separation is 4-6 min. The present invention can obtain a supernatant with a suitable concentration of heavy metal Cd by controlling the parameters of the centrifugal separation.

[0014] Preferably, the drying temperature is 60-65°C and the drying time is 4-6 min. The present invention controls the drying temperature and drying time to evaporate the liquid, while avoiding the sputtering of the solution when the temperature is too high, which affects the uniformity of the formed solid and affects the concentration of heavy metal Cd. When the temperature is too low, the drying time increases, which reduces the efficiency.

[0015] Preferably, the inert gas includes Ar or N2. LIBS detection is performed in an inert gas atmosphere to avoid the incorporation of C, H, and O elements in the air that may affect the accuracy of the test.

[0016] Preferably, in the LIBS method, the cumulative number of pulses is 90-110.

[0017] Preferably, the LIBS method collects data from the central area of ​​the sample, further improving the uniformity of the spectral area collected from the sample.

[0018] Preferably, in the LIBS method, the calibration curve is a linear relationship between the characteristic spectral line intensity of Cd and the concentration of Cd element, and the detection limit is 0.0816 ± 0.08 mg / kg The method provided by the present invention can meet the wheat food safety limit value (0.1 mg / kg) specified in the Chinese national standard (GB2762-2017) in terms of detection sensitivity, providing a new solution for efficiently realizing wheat food safety detection.

[0019] On the one hand, the present invention uses an ultrasonic-assisted extraction sample pretreatment method to improve the "coffee ring effect" in the solid-liquid conversion process using a circular hole-constrained droplet morphology, thereby improving the stability of LIBS spectrum acquisition; on the other hand, the LIBS technology is effectively combined with the circular hole-constrained ultrasonic extraction pretreatment method to achieve high sensitivity and high stability detection of Cd heavy metals in wheat. This working principle is mainly reflected in the following aspects: 1. The circular hole constrained ultrasound-assisted extraction method transforms the sample into solid-liquid-solid and extracts Cd heavy metal.

[0020] Solid-liquid conversion process: Cd metal in sample powder and Cl in HCl under ultrasound-assisted dissolution - The ions react to form CdCl2, while removing the organic substrate and extracting the heavy metal Cd. The lower layer is separated in a centrifuge, and the liquid is collected as the supernatant.

[0021] Liquid-solid conversion process: Under the constraint of the circular hole, the free diffusion of the droplet is restricted by the tension and the contact angle becomes larger, which makes the curvature of the droplet edge larger, the amount of droplet in the edge area decreases, and the concentration of the edge area after high-temperature drying decreases accordingly, thereby improving the uniformity of the sample.

[0022] 2. Spectral analysis capability of LIBS technology: LIBS generates plasma on the sample surface through high-energy laser pulses. The plasma emits a characteristic spectrum during the cooling process, which reflects the composition and concentration information of the chemical elements in the sample. During the acquisition process, the central rectangular area is selected for acquisition to further improve the uniformity of the spectral area collected by the sample. At the same time, the sample is placed in an Ar atmosphere during the acquisition process to improve the stability of the spectrum.

[0023] Compared with the prior art, the present invention has the following beneficial effects: The present invention utilizes ultrasound-assisted dissolution to dissolve Cd metal in wheat powder and Cl in HCl. -The ions react to form CdCl2, and at the same time the organic substrate is removed, and the Cd heavy metal is extracted. The lower interlayer is separated in a centrifuge, and the collected liquid containing CdCl2 is the supernatant. The above method can obtain a CdCl2 supernatant with a higher concentration, thereby enabling LIBS detection.

[0024] The present invention utilizes a circular hole constraint sheet to constrain the dropped supernatant. Due to the tension, the free diffusion of the supernatant is restricted and the contact angle becomes larger. The diffusion speed of the supernatant in the middle area to the edge decreases, so that the amount of solution in the middle and at the edge is more uniform, which effectively improves the "coffee ring effect" of the sample. After drying, the uniformity of the Cd heavy metal in the sample is significantly improved, and the stability of the spectrum is improved. Therefore, after LIBS detection, the relative standard deviation (RSD) value of the calibration curve is lower, so that the heavy metals in wheat can be detected more sensitively through LIBS.

[0025] Compared with nanoparticle enhanced (NE) LIBS, electrochemical (EC) LIBS, and LIBS methods with physical pretreatment of samples, the present invention does not require a long sample processing time. The sample preparation time of the CHC-UAE-LIBS method is generally 15-20 minutes, which reduces the influence of human factors and sample processing process to a certain extent, and can complete the spectrum acquisition of the sample in a short time. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The present invention will be further described below in conjunction with the accompanying drawings.

[0027] Figure 1 A preparation flow chart of a method for detecting heavy metals in wheat by LIBS based on circular hole constrained ultrasonic extraction (CHC-UAE-LIBS) provided in a specific embodiment of the present invention; Figure 2 The Cd element spectrum of the sample treated with CHC-UAE and without CHC-UAE before LIBS detection provided in the specific embodiment of the present invention, detected by LIBS in the wavelength range of 210-235 nm; Figure 3 A relative standard deviation diagram of the intensity of three characteristic spectral lines of Cd element in samples treated with CHC-UAE, UAE only, and no CHC-UAE before LIBS detection provided in a specific embodiment of the present invention; Figure 4 A calibration curve of Cd Ⅱ 226.50 nm obtained by the CHC-UAE-LIBS method provided in a specific embodiment of the present invention; Figure 5 A graph showing the intensity variation of the Cd Ⅱ 226.50 nm spectral line at different ultrasonic times according to the CHC-UAE-LIBS method provided in a specific embodiment of the present invention; Figure 6 A graph showing the intensity variation of the Cd II 226.50 nm spectral line at different mass / volume ratios of wheat powder and hydrochloric acid solution using the CHC-UAE-LIBS method provided in a specific embodiment of the present invention. DETAILED DESCRIPTION

[0028] In order to enable those skilled in the art to better understand the scheme of the present invention, an embodiment of the present invention is specifically described below with reference to the accompanying drawings. Obviously, the described embodiments are only partial embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without making creative work should belong to the protection scope of the present invention.

[0029] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other. The present invention will be described in detail below in conjunction with the embodiments.

[0030] Example 1: In the specific embodiment of the present invention, wheat samples collected from farmers were selected, and the Cd content measured by ICP-MS was 0.804 ± 0.02 mg / kg. The LIBS method for detecting heavy metals in wheat based on circular hole constrained ultrasonic extraction provided by the present invention was used to prepare the samples and collect LIBS. Figure 1 As shown, including: (1) Mix 1 g of wheat powder and 10 ml of hydrochloric acid solution (HCl, pH = 1) in a glass bottle and ultrasonically shake it in an ultrasonic cleaner for 10 min.

[0031] (2) Take out the glass bottle, extract 5 ml of the mixture into a centrifuge tube, put it into a centrifuge, and centrifuge it at 4000 rpm for 5 minutes.

[0032] (3) After cleaning the glass slide with ethanol solution, stick a high-temperature resistant circular hole constraint sheet with an inner diameter of 10 mm on the glass slide, and use a micropipette to absorb 50ul of supernatant and drop it in the center of the circular hole.

[0033] (4) Place the glass slide on a heating table at a constant temperature of 60°C and dry it for 5 min to obtain a sample after solid-liquid-solid conversion.

[0034] (5) The LIBS spectrum of the sample was measured in an inert gas atmosphere using the LIBS method: first, the sample was placed on an XYZ mobile platform in the sample chamber, and then high-purity Ar gas was introduced into the sample chamber through a gas bottle and a transmission tube. This process lasted for more than 30 minutes, and a stable gas flow was continuously introduced during the experiment to ensure the Ar atmosphere in the sample chamber. The pulse energy provided in the specific embodiment of the present invention was optimized to 80 mJ, the spectrometer gate width was 10 μs, and the ICCD delay time was 1 μs, which to a certain extent avoided the influence of bremsstrahlung and composite radiation on the spectrum and improved the signal-to-noise ratio; the mobile three-dimensional platform allowed the pulse to act on the rectangular area in the middle of the sample during the acquisition process, and a total of 100 pulses were accumulated into a spectrum. The experimental work was carried out under standard atmospheric pressure, indoor temperature of 25°C, and relative air humidity of 37%.

[0035] The LIBS equipment used for sampling in the specific embodiment of the present invention is configured as follows: the ablation light source is a Nd:YAG solid laser (pulse width is 6ns, repetition frequency is 10 Hz); a fiber optic spectrometer (spectral range is 190~430nm); an enhanced charge coupled detector (ICCD), an integrated electronically controlled XYZ mobile platform and a sample chamber that can create an Ar atmosphere environment and a computer system.

[0036] According to the database of the National Institute of Standards and Technology (NIST), Cd Ⅱ 214.44 nm, Cd Ⅱ 226.50 nm and Cd Ⅰ 228.80 nm are the three main characteristic spectral lines of Cd element, such as Figure 2 As shown, the LIBS spectrum collected from the control group compressed sample 1 prepared by the CHC-UAE method hardly detects the spectral line signals of Cd Ⅱ 214.44 nm, Cd Ⅱ 226.50 nm and Cd Ⅰ 228.80 nm, while the sample prepared by the CHC-UAE method provided by the present invention has a strong signal intensity of the three Cd characteristic spectral lines after LIBS spectrum collection. This is due to the strong matrix effect of the control group compressed sample 1. The complex organic matter or other interfering substances in the sample may absorb part of the laser energy or interfere with the breakdown efficiency of the laser, resulting in a weak signal. The matrix properties of the sample prepared by the CHC-UAE method are optimized through extraction and transformation, reducing the interference of the complex matrix on the LIBS signal. At the same time, the uniformity of the matrix improves the formation efficiency of the plasma, thereby significantly enhancing the emission signal of Cd.

[0037] like Figure 3The same experimental parameters were used for the control group compressed sample 1 not prepared by the CHC-UAE method, the control group sample 2 prepared by the UAE method, and the sample prepared by the CHC-UAE method provided by the present invention to collect spectra 5 times, and the relative standard deviation (RSD) of the characteristic spectra of Cd Ⅱ 214.44 nm, Cd Ⅱ 226.50 nm and Cd Ⅰ 228.80 nm was obtained, wherein the characteristic line RSD of sample 1 was larger, the characteristic line RSD of sample 2 was lower, and the CHC-UAE method sample RSD was the smallest. This is because the characteristic line signal intensity of sample 1 caused by the matrix effect during the LIBS acquisition process is weak, and the slight changes in external factors (airflow, background stray light, laser energy fluctuations) will have a greater impact on the weak signal, increasing the instability of the signal. Sample 2 uses the UAE method to extract the Cd heavy metal element, and obtains a higher characteristic element signal intensity, but the "coffee ring effect" caused by the solid-liquid conversion process makes the sample concentration in the collection area different, the signal intensity changes accordingly, and the signal instability increases. The samples prepared by the CHC-UAE method used a circular hole constrained droplet morphology to ensure that the concentration of the sample area tended to be consistent during the solid-liquid conversion process. After drying, the uniformity of Cd heavy metal in the sample was significantly improved, the stability of the spectrum was improved, and the RSD was significantly reduced.

[0038] like Figure 2 and Figure 3 As shown, among the three characteristic spectral lines of Cd heavy metal, the spectral line intensity of Cd II 226.50 nm is the highest and the RSD is relatively low. In the specific embodiment of the present invention, Cd II 226.50 nm is selected as the characteristic spectral line for qualitative and quantitative analysis.

[0039] (6) Preparing a calibration curve: In the specific embodiment of the present invention, a sample with a gradient concentration of Cd element was prepared according to the standard addition method. The concentration of the prepared Cd element is shown in Table 1.

[0040] Table 1 Cd element gradient concentration samples prepared by standard addition method

[0041] The samples with sample numbers 1-10 were subjected to LIBS acquisition test to obtain the characteristic spectral line intensities of Cd elements with different concentrations. The concentration of Cd elements and the corresponding characteristic spectral line intensities were fitted to obtain the calibration curve, such as Figure 4As shown in the figure, the characteristic spectral intensity of CdⅡ 226.50 nm was used to fit the standard curve. The error bar represents the uncertainty of the spectral intensity, which is equivalent to the standard deviation at different concentrations. The slope of the calibration curve is 12.0199, and the determination coefficient (R2) is 0.9996, indicating good linearity. The relative standard deviation (RSD) is 5.6%, indicating that the method has good stability and precision. The limit of detection (LOD) is 0.0816 mg / kg, which can meet the national standard requirement of 0.1 mg / kg for the detection of Cd content in wheat.

[0042] (7) Finally, the Cd element in the wheat sample was determined according to the calibration curve. Table 2 shows the comparison of the detection results of the two techniques. Using the relative error (RE) as the accuracy evaluation standard, it can be seen that the RE of the Cd element is less than 5.85%, reflecting that the detection results using the CHC-UAE method are more accurate.

[0043] Table 2 Cd element concentration and relative error obtained by CHC-UAE-LIBS and ICP-MS methods

[0044] Example 2: Different from Example 1, the specific embodiment of the present invention provides different ultrasonic oscillation times. The LIBS line intensity of Cd II 226.50 nm of the prepared sample changes with the ultrasonic duration as shown in Figure 5 As shown in the figure, with the increase of ultrasonic time, the intensity of the characteristic peak is positively correlated with time in the first 10 min, and tends to be stable after 10 min, indicating that the Cd metal element in the solid is more fully dissolved by ultrasonic in 8-12 min and is effectively extracted into the solution.

[0045] Example 3: Different from Example 1, the specific example of the present invention provides different mass / volume ratios of wheat powder and hydrochloric acid solution. The LIBS line intensity of Cd II 226.50 nm of the prepared sample changes with the mass / volume ratio of wheat powder and hydrochloric acid solution as shown in the figure. Figure 6 As shown, it shows that when wheat / HCL changes from 1:6 to 1:14 g / mL, the spectral intensity of Cd II at 226.50 nm increases. The spectral intensity reaches a maximum at 1:10 g / mL and then decreases.

Claims

1. A LIBS method for detecting heavy metals in wheat based on circular hole constrained ultrasonic extraction, characterized in that: include: (1) Mix wheat powder and hydrochloric acid solution, perform ultrasonic vibration, centrifuge and extract the supernatant; (2) dropping the supernatant into a circular hole constraint sheet attached to a glass slide, and obtaining a sample after drying; (3) The LIBS spectrum of the sample is measured in an inert gas atmosphere using the LIBS method. The characteristic line intensity of the Cd element in the LIBS spectrum is used as input, and the Cd element concentration of the sample is obtained based on the established calibration curve, thereby completing the detection of the Cd element in wheat.

2. The LIBS method for detecting heavy metals in wheat based on circular hole constrained ultrasonic extraction according to claim 1 is characterized in that: The inner diameter of the circular hole constraint sheet is 10-12 mm.

3. The LIBS method for detecting heavy metals in wheat based on circular hole constrained ultrasonic extraction according to claim 1 is characterized in that: The mass / volume ratio of the wheat powder to the hydrochloric acid solution is 1:9-1:11 g / mL, and the particle size of the wheat powder is <125 μm.

4. The method for detecting heavy metals in wheat by LIBS based on circular hole constrained ultrasonic extraction according to claim 1, characterized in that: The pH value of the hydrochloric acid is 1, and the ultrasonic oscillation time is 8-12 min.

5. The method for detecting heavy metals in wheat by LIBS based on circular hole constrained ultrasonic extraction according to claim 1, characterized in that: The rotation speed of the centrifugal separation is 4000-4500 rpm, and the time of the centrifugal separation is 4-6 min.

6. The method for detecting heavy metals in wheat by LIBS based on circular hole constrained ultrasonic extraction according to claim 1, characterized in that: The drying temperature is 60-65° C. and the drying time is 4-6 min.

7. The method for detecting heavy metals in wheat by LIBS based on circular hole constrained ultrasonic extraction according to claim 1, characterized in that: The inert gas includes Ar or N2.

8. The method for detecting heavy metals in wheat by LIBS based on circular hole constrained ultrasonic extraction according to claim 1, characterized in that: In the LIBS method, the cumulative number of pulses is 90-110.

9. The method for detecting heavy metals in wheat by LIBS based on circular hole constrained ultrasonic extraction according to claim 1, characterized in that: The LIBS method samples the central area of ​​the sample.

10. The method for detecting heavy metals in wheat by LIBS based on circular hole constrained ultrasonic extraction according to claim 1, characterized in that: The calibration curve is a linear relationship between the characteristic spectral line intensity of Cd and the Cd element concentration, and the detection limit is 0.0816 ± 0.08 mg / kg.

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