CN104535544B - A kind of device that heavy metal-polluted soil is detected using LIBS - Google Patents

Abstract

The invention discloses a device for detecting heavy metals in soil by using laser-induced breakdown spectroscopy, which includes a control system and a hand-held working head. The control system includes an industrial computer, a gas supply module connected to the industrial computer, a spectrometer, a laser The power supply module and the KVM virtual machine are installed on the industrial computer; the present invention adopts the space constraint technology and the multi-channel signal acquisition mode in the plasma excitation and collection part, and supplemented by the method of coaxial blowing to disperse the suspended fine dust, while using the acoustic signal sensor Monitor the sound signal when the plasma is excited, and normalize the spectral signal; use this system for comparative spectral testing, the detection stability of spectral data and the intensity of spectral line signals have been significantly improved, which can meet the on-site detection of heavy metal pollution in soil need.

Description

A device for detecting heavy metals in soil using laser-induced breakdown spectroscopy

technical field

The invention belongs to the technical field of laser-induced plasma spectrum analysis, and in particular relates to a device for detecting heavy metals in soil by using laser-induced breakdown spectrum.

Background technique

Soil is an important part of the ecological environment, but with the rapid development of social economy and the rapid advancement of urbanization, the degree of heavy metal pollution in soil is increasing, and the polluted area is expanding year by year. According to the "National Soil Pollution According to the State Survey Bulletin, the content of heavy metals such as cadmium, mercury, arsenic, copper, lead, chromium, zinc, and nickel in my country's surface soil has increased significantly compared with the "Seventh Five-Year Plan" period. Heavy metals are changeable in soil and difficult to degrade. They are absorbed and enriched by plant roots, and enter the human body through the food chain, which will cause serious harm to human health.

The current methods for detecting heavy metals in soil mainly include graphite furnace atomic absorption spectrophotometry, flame atomic absorption spectrophotometry, and electrochemical methods. There is a risk of secondary pollution during collection, transportation and processing, which cannot meet the on-site survey of heavy metal pollution in soil and emergency monitoring of heavy metal pollution accidents.

Laser-Induced Breakdown Spectroscopy ((Laser-Induced Breakdown Spectroscopy, LIBS)) detection technology is a rapid elemental analysis technology developed in the past two decades. The material composition and composition content of the sample can be determined by detecting and analyzing the atomic or ion spectral lines radiated during the plasma cooling process. LIBS technology has the characteristics of no need for sample pretreatment and simultaneous detection of multiple elements, which can effectively make up for the shortcomings of traditional methods that cannot be monitored rapidly in the field and in real time.

Contents of the invention

The purpose of the present invention is to provide a method and device for detecting heavy metals in soil by laser-induced breakdown spectroscopy, which solves the problems of time-consuming, expensive and labor-intensive detection of soil heavy metals in existing technical equipment and cannot be detected on-site, and realizes rapid and on-site detection of heavy metal pollution in soil. , Quantitative detection.

The technical scheme adopted in the present invention is:

A device for detecting heavy metals in soil using laser-induced breakdown spectroscopy, characterized in that it includes a control system and a hand-held working head, and the control system includes an industrial computer and a gas supply module connected to the industrial computer, a spectrometer, a laser A power supply module, a KVM virtual machine is installed on the industrial computer; the handheld working head includes a working head main body base and a handle, a laser head is installed on the working head main body base, the laser power supply module supplies power for the laser head, and the working head main body base The front and rear ends of the seat are respectively provided with a plasma excitation and collection unit and a transmission cable tube; the plasma excitation and collection unit includes a rotating mechanism fixed at the front end of the base of the main body of the working head, and the front end of the rotating mechanism is adjusted Bolts and disc springs are installed with a hollow cavity, a lens seat is installed in the rotating mechanism, a focusing lens is fixed on the lens seat, air guide holes are opened on the side wall of the hollow cavity, and a plasma excitation hole is fixed at the center of the front end of the hollow cavity. Confinement cavity, both sides of the plasma excitation confinement cavity are equipped with equi-stereo signal coupling optical components, an acoustic signal sensor is fixed on the plasma excitation confinement cavity, and the front surface of the plasma excitation confinement cavity contacts the sample to be detected; the adjustment bolt and The disc spring adopts a 120° evenly divided triangular support point adjustment mechanism, that is, the adjusting bolts and disc springs are evenly distributed on the circumference at an angle of 120°, and the two-dimensional angle of the plasma excitation confinement cavity can be achieved by adjusting the tightness of the adjusting bolts and disc springs The purpose of adjustment is to realize that the position of the laser excitation point emitted by the laser head is coaxial with the focal point of the focusing lens; while the relative position of the laser focusing point to the surface of the sample to be tested is adjusted to drive the lens seat and the focusing lens to move back and forth along the optical axis to achieve fine Adjustment; said isostereo signal coupling optical assembly is connected with signal transmission optical fiber, the signal transmission optical fiber is connected with the spectrometer of the control system through the transmission cable tube, the acoustic signal sensor is connected with the industrial control electromechanical, and the air supply module is connected with the air guide hole through the air supply pipe .

The device for detecting heavy metals in soil by using laser-induced breakdown spectroscopy is characterized in that: the control system is installed in a wheeled cabinet.

The device for detecting heavy metals in soil by using laser-induced breakdown spectroscopy is characterized in that: the plasma excitation confinement cavity adopts a hemispherical hollow structure, and the hemispherical radius is between 5mm and 8mm.

The advantages of the present invention are:

In the plasma excitation and collection part, the present invention adopts space constraint technology and multi-channel signal acquisition mode, supplemented by coaxial air blowing to disperse the suspended fine dust method, and meanwhile uses an acoustic signal sensor to monitor the sound signal when the plasma is excited. The signal is normalized and processed; using this system for comparative spectral testing, the detection stability of spectral data and the signal intensity of spectral lines are significantly improved, which can meet the needs of on-site detection of soil heavy metal pollution.

Description of drawings

Fig. 1 is a system block diagram of the present invention.

Fig. 2 is a structural schematic diagram of the hand-held working head of the present invention.

Fig. 3 is a schematic structural diagram of the plasma excitation and collection unit of the present invention.

Figure 4 is a comparison chart of laser-induced soil plasma spectral data.

Figure 5 is a comparison chart of laser-induced soil plasma spectral data.

detailed description

As shown in Figure 1, a device for detecting heavy metals in soil using laser-induced breakdown spectroscopy includes a control system, a hand-held working head F, and the control system includes an industrial computer A and a gas supply module B connected to the industrial computer A, The spectrometer C, the laser power supply module D, and the KVM virtual machine E are installed on the industrial computer A.

As shown in Figure 2, the hand-held working head F includes a working head main body base 3 and a handle 4. The laser head 1 is installed on the working head main body base 3. The laser power supply module D supplies power to the laser head 1. The working head main body base The front and rear ends of 3 are respectively provided with plasma excitation and collection unit 2 and transmission cable tube 5 .

As shown in Figure 3, the plasma excitation and collection unit 2 includes a rotating mechanism 209 fixed on the front end of the base of the working head body. The front end of the rotating mechanism 209 is equipped with a hollow cavity 201 through an adjusting bolt 210 and a disc spring 211. 209 is provided with a lens seat 208, on which a focusing lens 207 is fixed, and an air guide hole 206 is opened on the side wall of the hollow cavity body 201, and a plasma excitation confinement cavity 202 is fixed at the center of the front end of the hollow cavity body 201. Both sides of the volume excitation confinement cavity 202 are equipped with equi-stereo signal coupling optical components 203, the plasma excitation confinement cavity 202 is fixed with an acoustic signal sensor 205, and the front surface of the plasma excitation confinement cavity 202 is in contact with the sample to be tested; the adjustment bolt 210 and the plate The spring 211 adopts a 120° evenly divided triangular support point adjustment mechanism, that is, the adjusting bolt 210 and the disc spring 211 are evenly distributed on the circumference at an angle of 120°, and the plasma excitation confinement cavity can be achieved by adjusting the tightness of the adjusting bolt 210 and the disc spring 211. 202 for the purpose of adjusting the two-dimensional angle, thereby realizing that the position of the laser excitation point emitted by the laser head 1 is coaxial with the focal point of the focusing lens 207; The focus lens 207 moves back and forth along the optical axis to realize fine adjustment; the stereo signal coupling optical assembly 203 is connected with a signal transmission fiber 204, and the signal transmission fiber 204 is connected with the spectrometer C of the control system through the transmission cable tube 5, and the acoustic signal sensor 205 is connected with the industrial control system. The machine A is electrically connected, and the air supply module B is connected to the air guide hole 206 through the air supply pipe.

The control system is installed in a wheeled cabinet. The plasma excitation confinement cavity 202 adopts a hemispherical hollow structure, and the radius of the hemisphere is between 5mm and 8mm.

Principle of the present invention is as follows:

When using LIBS to detect the soil matrix, due to the complex physical and chemical properties of the soil, the soil particle size, humidity, compactness, and surface smoothness vary greatly, and the uniformity is poor, resulting in relatively poor sensitivity and signal stability of LIBS signal detection. . In order to solve these problems, the present invention designs the plasma excitation and collection unit 2, which is mainly embodied in the plasma excitation confinement cavity 202, the stereo signal coupling optical assembly 203, the signal transmission optical fiber 204, the acoustic signal sensor 205, and the air guide hole 206. , lens holder 208 and rotating mechanism 209. The plasma excitation and confinement cavity 202 is designed based on the confinement of the plasma expansion by the echo of the laser-induced shock wave. The purpose is to improve the detection sensitivity of the system. The excitation and confinement cavity adopts a hemispherical hollow structure, and its parameters can be selected from a radius of 5mm to 8mm. , the specific selection is closely related to the laser energy provided by the system laser. The three-dimensional signal coupling optical component 203 and the signal transmission optical fiber 204 constitute the LIBS signal coupling transmission channel. In order to collect LIBS signals relatively comprehensively, a 360-degree range multi-channel receiving method is adopted, and an all-in-one optical fiber transmission is applied to realize multi-directional signal reception and improve The strength of the LIBS signal received by the spectrometer. A large number of experimental studies have shown that under certain system parameters, the laser-induced acoustic signals of samples with different substrate properties (such as humidity, particle size, compactness, and surface smoothness) are significantly different, and each laser pulse There is a high correlation between the generated LIBS signal change and the acoustic signal change. Based on this, the acoustic signal sensor 205 is installed on the plasma excitation confinement cavity 202, and the LIBS signal is normalized by using the signal collected by the acoustic signal sensor 205. In order to improve the stability of the system to collect LIBS signals. In the process of laser-induced plasma generation of spectral signals, suspended particles generated by laser ablation will accumulate in the plasma excitation confinement cavity 202, and the suspended particles will absorb and scatter the energy of subsequent pulsed lasers, which will have a serious impact on the stability of subsequent LIBS signals , and the optical components in the plasma excitation and collection unit 2 will be polluted for a long time, so the hollow cavity 201 has an air guide hole 206, and the gas supply module provides the plasma excitation confinement cavity 202 with the same optical axis when working. Directional airflow to blow away suspended particles, thereby improving the stability of LIBS detection signals. For detection substances with different characteristics, the position of the laser focal point to obtain the best detection signal relative to the surface of the detection sample is different, so the lens seat 208 and the rotating mechanism 209 in the structure realize that the rotation of the rotating mechanism 209 becomes the lens seat 208 along the light. The translational movement of the axis is used to fine-tune the focus of the focusing lens 207 relative to the surface of the detection sample. This adjustment mechanism can easily realize the fine-tuning of the position of the laser focus point, and its adjustment accuracy is 0.02mm.

Example

Using the device of the above invention, taking the detection of lead element (Pb) in soil as an example, the method and device for detecting heavy metals in soil are described.

Test sample: The example uses a standard soil sample (GBW07408), and based on the standard soil sample, chemical reagents are added to analyze pure lead nitrate (Pb(NO ₃ ) ₂ ) to configure the soil sample. The content of Pb in the configured sample is shown in Table 1. System parameters: pulse laser energy 100mJ, repetition rate 1Hz, gas flow rate 10mL/s, plasma excitation confinement cavity radius 6.4mm. Detection data: Each spectral data is the cumulative average of 15 laser pulses, and 20 spectral data are collected for each soil sample.

Table 1

Sample serial number 0 (standard soil sample) 1 2 3 4 5 6 7 Concentration (ppm) 21.8 52.4 82.1 112.8 142.5 173.3 202.2 233.6

At first the actual spectral data acquisition of the inventive device was compared, as shown in Figure 4, it can be seen that the inventive device has significantly improved compared to the non-invented device in terms of spectral intensity and detection sensitivity, compared with the calculated Pb element characteristic line (405.781mm ) intensities with relative standard deviations (RSDs) of 5.7% and 14.2%, respectively.

Secondly, the spectral detection is performed on the Pb concentration gradient sample, and the actual measured spectral data and the acoustic signal data detected by the acoustic signal sensor 205 are normalized and processed, and the results are shown in FIG. 5 . The detection limit of the Pb element of the inventive device is 13.5ppm, which is far lower than the primary standard value of 35ppm in the soil environmental quality standard.

Claims (3)

1. A device that utilizes laser-induced breakdown spectroscopy to detect heavy metals in soil, characterized in that: it includes a control system, a hand-held work head, and the control system includes an industrial computer and a gas supply module and a spectrometer that are connected to the industrial computer control , a laser power supply module, a KVM virtual machine is installed on the industrial computer; the hand-held working head includes a working head main body base and a handle, and a laser head is installed on the working head main body base, and the laser power supply module supplies power to the laser head, and the working head The front and rear ends of the main body base are respectively provided with a plasma excitation and collection unit and a transmission cable tube; the plasma excitation and collection unit includes a rotation mechanism fixed at the front end of the main body base of the working head, and the front end of the rotation mechanism A hollow cavity is installed by adjusting bolts and disc springs, a lens seat is installed in the rotating mechanism, a focusing lens is fixed on the lens seat, air guide holes are opened on the side wall of the hollow cavity, and a plasma is fixed at the center of the front end of the hollow cavity. A volume excitation confinement cavity, plasma signal coupling optical components are arranged on both sides of the plasma excitation confinement cavity, an acoustic signal sensor is fixed on the plasma excitation confinement cavity, and the front surface of the plasma excitation confinement cavity contacts the sample to be detected; the adjustment The bolts and disc springs adopt a 120° evenly divided triangular support point adjustment mechanism, that is, the adjusting bolts and disc springs are evenly distributed at an angle of 120° on the circumference. The purpose of adjusting the dimensional angle, so that the position of the laser excitation point emitted by the laser head is coaxial with the focus of the focus lens; and the relative position of the laser focus point to the surface of the sample to be tested drives the lens holder and the focus lens to move back and forth along the optical axis by adjusting the rotating mechanism Realize fine adjustment; said plasma signal coupling optical component is connected with signal transmission optical fiber, the signal transmission optical fiber is connected with the spectrometer of the control system through the transmission cable tube, the acoustic signal sensor is connected with the industrial control electromechanical, the gas supply module is connected with the guide through the gas supply pipe Air hole connection. 2. The device for detecting heavy metals in soil by laser-induced breakdown spectroscopy according to claim 1, wherein the control system is installed in a wheeled cabinet. 3. The device for detecting heavy metals in soil by laser-induced breakdown spectroscopy according to claim 1, characterized in that: the plasma excitation confinement cavity adopts a hemispherical hollow structure, and the hemispherical radius is between 5mm and 8mm.