Polarization LIBS-LIF system for trace element detection

Abstract

The invention belongs to the technical field of spectrum detection, and relates to a polarization LIBS-LIF system for trace element detection, the system comprises a laser emission unit, a first light path structure, an optical parametric oscillator, a second light path structure, a displacement platform, a polarization acquisition module and an analysis control module; the laser emitting unit is used for emitting a first laser beam to the first light path structure; the first light path structure is used for converting the received first laser beam into circularly polarized light and emitting the circularly polarized light into a sample on the displacement platform; the optical parametric oscillator is used for emitting a second laser beam into the second light path structure; the second light path structure is used for focusing the second laser beam and then emitting the second laser beam into the same position of the circularly polarized light to the sample so as to excite the to-be-detected element in the plasma; the polarization acquisition module is used for acquiring plasma spectral signals, converting the plasma spectral signals into electric signals and transmitting the electric signals to the analysis control module. The plasma excitation efficiency can be improved, the detection limit can be reduced, and the stability of the detection result can be improved.

Description

Technical Field

[0001] The present application belongs to the field of spectral detection technology, and more specifically, relates to a polarization LIBS-LIF system for trace element detection. Background Art

[0002] Laser-induced breakdown spectroscopy (LIBS) uses ultrashort pulse lasers to focus on the sample surface, ablate and generate high-temperature plasma, and analyze the plasma emission spectrum to determine the element type and content of the sample to be tested. Laser-induced fluorescence assisted laser-induced breakdown spectroscopy (LIBS-LIF) is based on LIBS, using a second laser beam (resonant laser) to further excite specific elements in the plasma. The lower-level atoms absorb energy and are excited to the upper energy level, and then transition downward to produce fluorescence, achieving the effect of spectral enhancement. LIBS-LIF technology has broad application prospects in environmental monitoring, biomedicine, food testing and other fields.

[0003] Although LIBS-LIF technology has advantages such as eliminating spectral interference, rapid detection, and high sensitivity, it still faces some challenges in practical applications, such as poor stability of spectral signals, and the sensitivity of detecting trace elements in complex matrices is still difficult to meet the requirements. The excitation laser of the traditional LIBS-LIF system is linearly polarized light, which affects the excitation efficiency and characteristics of the plasma. Laser-induced plasma is a very complex transient process, which is easily disturbed by external factors and produces large fluctuations, including uneven distribution of plasma particle number density, temperature, and morphology changes, resulting in poor spectral stability, which poses a challenge to the precise quantification of trace elements in samples by LIBS-LIF. At the same time, when conventional linearly polarized light interacts with matter, electrons cannot be continuously accelerated due to the fixed direction of the electric field. The low electron kinetic energy leads to insufficient density of the plasma, and the number of ground state particles in the plasma is relatively small. Therefore, the particles excited by the resonant laser are limited, which limits the further enhancement of the spectral intensity, making the detection of low-concentration trace elements more difficult. In addition, for some heavy metal elements that cannot be metabolized by the human body, such as lead, even extremely low doses of exposure will accumulate in the human body, causing irreversible health damage to the human body. Therefore, a trace element detection device is needed to solve the above problems. Summary of the invention

[0004] In view of the defects of the prior art, the purpose of this application is to provide a polarization LIBS-LIF system for trace element detection, aiming to solve the problem that the existing LIBS-LIF system is easily disturbed by external factors during the laser-induced plasma process and produces large fluctuations, and the plasma particle number density distribution is uneven, and temperature and morphology changes lead to poor spectral stability, thereby affecting the accurate quantitative detection of trace elements.

[0005] To achieve the above-mentioned purpose, the present application provides a polarization LIBS-LI F system for trace element detection, the system comprising: a laser emitting unit, a first optical path structure, an optical parametric oscillator, a second optical path structure, a displacement platform, a polarization acquisition module and an analysis control module; the laser emitting unit is used to emit a first laser beam to the first optical path structure; the first optical path structure is used to convert the received first laser beam from linearly polarized light to circularly polarized light, and make the circularly polarized light incident on the sample on the displacement platform to excite plasma; the optical parametric oscillator is used to emit a second laser beam to the second optical path structure; the second optical path structure is used to focus the second laser beam and then make it incident on the same position where the circularly polarized light is incident on the sample, so as to excite the element to be measured in the plasma; the polarization acquisition module is used to collect the plasma spectrum signal of the element to be measured that is excited, and convert the plasma spectrum signal into an electrical signal and transmit it to the analysis control module for analysis.

[0006] Through the above technical scheme conceived by the present application, compared with the prior art, since the LIBS-LIF detection system provided by the present application can realize polarization excitation and polarization resolution when detecting trace elements, it is possible to improve the excitation efficiency and the stability of the detection results by changing the polarization state of the excitation laser.

[0007] Furthermore, the first optical path structure includes a laser wavelength reflector, a polarization optical element and a first laser focusing lens, wherein: the angle between the laser wavelength reflector and the first laser beam is an acute angle, which is used to make the first laser beam vertically enter the polarization optical element after reflection; the polarization optical element and the first laser focusing lens are coaxial and parallelly arranged, which are used to convert the received first laser beam into circularly polarized light and then vertically inject it into the sample.

[0008] Furthermore, the second optical path structure includes a second laser focusing lens, which is perpendicular to the propagation direction of the second laser beam and is used to focus the second laser beam and emit it into the same position as the circularly polarized light is emitted to the sample.

[0009] Furthermore, the polarization acquisition module includes a third optical path structure, a spectrometer and an ICCD. The third optical path structure includes a first focusing lens, a polarizer and a second focusing lens that are coaxially and parallel to each other. The first focusing lens is used to focus the plasma spectrum signal and transmit it to the polarizer. The polarizer is used to suppress background noise in the plasma spectrum signal and then transmit the plasma spectrum signal to the second focusing lens. The second focusing lens is used to focus the plasma spectrum signal and then transmit it to the spectrometer and ICCD in sequence via optical fiber.

[0010] Furthermore, the analysis control module includes a timing controller, which is respectively connected to the laser emission unit and the optical parametric oscillator, and is used to control the laser pulse emission delay between the first laser beam and the second laser beam; the timing controller is also connected to the ICCD, and is used to control the acquisition delay of the ICCD.

[0011] Furthermore, the analysis control module also includes a computer, which is connected to the optical parametric oscillator and the ICCD respectively, and is used to control the output wavelength of the optical parametric oscillator and to analyze the plasma spectrum signal collected by the ICCD and display the spectrum image.

[0012] Furthermore, the laser pulse emission delay between the first laser beam and the second laser beam is at least 10 μs.

[0013] Furthermore, the wavelength range of the optical parametric oscillator is 251.51 nm-251.71 nm.

[0014] Furthermore, the wavelength of the optical parametric oscillator is 251.61 nm.

[0015] Furthermore, the polarizer can be rotated to a specific angle to remove the spectral background of the element to be measured.

[0016] In general, the above technical solutions conceived by this application have the following technical advantages compared with the prior art:

[0017] 1. The polarization LIBS-LIF system for trace element detection provided in the present application improves the plasma excitation efficiency and the stability of the detection results by changing the laser polarization state; uses a polarization acquisition module to suppress the background signal in the plasma excitation spectrum signal, further improves the signal-to-background ratio of the detection signal, and achieves high-sensitivity detection of characteristic spectral lines of trace elements, further improving the accuracy of the LIBS-LIF system in quantitative analysis.

[0018] 2. The polarization LIBS-LIF system for trace element detection provided in the present application has a first optical path structure that uses only a reflector, a polarization optical element and two focusing lenses to achieve the effect of changing the polarization state of the laser, without the need for other complex optical path systems. The overall polarization LIBS-LIF system has a simple and compact structure, which is easy to adjust and maintain. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 is a schematic structural diagram of a polarization LIBS-LIF system provided in an embodiment of the present application;

[0020] Figure 2 It is a comparison diagram of the characteristic spectrum peak of Si I 288.16nm in microalloyed steel detected by the conventional detection system provided in the embodiment of the present application in the linear polarization state and the polarization LIBS-LIF system of the present application in the circular polarization state;

[0021] Figure 3 It is a comparison chart of the fluctuations measured one hundred times by the conventional detection system provided in the embodiment of the present application in the linear polarization state and by the polarization LIBS-LIF system of the present application in the circular polarization state.

[0022] Throughout the drawings, the same reference numerals are used to denote the same elements or structures, wherein:

[0023] 1-laser emitting unit, 2-optical parametric oscillator, 3-optical path structure, 4-displacement platform, 5-polarization acquisition module, 6-spectrometer, 7-ICCD, 8-timing controller, 9-computer, 10-laser wavelength reflector, 11-polarization optical element, 12-first laser focusing lens, 13-second laser focusing lens, 14-first focusing lens, 15-polarizer, 16-second focusing lens, 17-optical fiber. DETAILED DESCRIPTION

[0024] In order to make the purpose, technical solution and advantages of the present application more clearly understood, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0025] The embodiment of the present application provides a polarization LIBS-LIF system for trace element detection, which includes a laser emitting unit 1, a first optical path structure, an optical parametric oscillator 2, a second optical path structure, a displacement platform 4, a polarization acquisition module 5 and an analysis control module. Among them, the laser emitting unit 1 is used to emit a first laser beam to the first optical path structure; the first optical path structure is used to convert the received first laser beam from linearly polarized light to circularly polarized light, and make the circularly polarized light enter the sample on the displacement platform 4 to excite plasma; the optical parametric oscillator 2 is used to emit a second laser beam to the second optical path structure; the second optical path structure is used to focus the second laser beam and then inject it into the same position of the sample as the circularly polarized light to excite the element to be measured in the plasma; the polarization acquisition module is used to collect the plasma spectrum signal of the element to be measured that is excited, and convert the plasma spectrum signal into an electrical signal and transmit it to the analysis control module.

[0026] Specifically, the aforementioned laser emitting unit 1 is a Nd:YAG laser, which is used to generate high-energy, stable and repeatable laser pulses to ablate the sample and generate plasma on the sample surface. The displacement platform 4 is used to place the sample to be tested and analyzed, and can drive the sample to move in any direction at a constant speed. The polarization acquisition module 5 is used to collect spectra and convert optical signals into electrical signals. The analysis control module is used to control the coordinated operation of the laser and the polarization acquisition module, and to process and analyze the electrical signals, draw the corresponding spectrum diagram and display the results.

[0027] More specifically, the first laser beam generated by the Nd:YAG laser is converted from linearly polarized light to circularly polarized light after passing through the first optical path structure; the second laser beam generated by the optical parametric oscillator 2 is incident on the same point on the sample surface as the circularly polarized light after passing through the second optical path structure, and the transmission directions of the two laser beams are perpendicular to each other.

[0028] The aforementioned first optical path structure includes a laser wavelength reflector 10, a polarization optical element 11 and a first laser focusing lens 12, wherein: the angle between the laser wavelength reflector 10 and the first laser beam is an acute angle, which is used to make the first laser beam vertically enter the polarization optical element 11 after reflection; the polarization optical element 11 and the first laser focusing lens 12 are coaxially and parallelly arranged, which are used to convert the received first laser beam into circularly polarized light and then vertically enter the sample.

[0029] Specifically, the angle between the laser wavelength reflector 10 and the first laser beam is 45 degrees, and the angle between the laser wavelength reflector 10 and the polarization optical element 11 is also 45 degrees. The polarization optical element 11 uses a quarter wave plate with a working wavelength of 532nm, so that the fast axis of the quarter wave plate is rotated in the horizontal plane to an angle of 45 degrees with the polarization direction of the first laser beam. When the linearly polarized light reflected by the laser wavelength reflector 10 passes through the quarter wave plate, the polarization component undergoes phase delay, and the linear polarization state can be converted into a circular polarization state.

[0030] The second optical path structure comprises a second laser focusing lens 13, which is perpendicular to the propagation direction of the second laser beam and is used to focus the second laser beam and emit it into the same position as the circularly polarized light is emitted to the sample.

[0031] The aforementioned polarization acquisition module includes a third optical path structure, a spectrometer 6 and an ICCD 7. The third optical path structure includes a first focusing lens 14, a polarizer 15 and a second focusing lens 16 which are coaxially and parallel to each other. The first focusing lens 14 is used to focus the plasma spectrum signal and transmit it to the polarizer 15. The polarizer 15 is used to suppress the background noise in the plasma spectrum signal and then transmit the plasma spectrum signal to the second focusing lens 16. The second focusing lens 16 focuses the plasma spectrum signal and then transmits it to the spectrometer 16 and ICCD 7 in sequence via the optical fiber 17.

[0032] The aforementioned analysis control module includes a timing controller 8, which is connected to the laser emission unit 1 and the optical parametric oscillator 2 respectively, and is used to control the laser pulse emission delay between the first laser beam and the second laser beam; the timing controller 8 is also connected to the ICCD 7, and is used to control the acquisition delay of the ICCD 7. The analysis control module also includes a computer 9, which is connected to the optical parametric oscillator 2 and the ICCD 7 respectively, and is used to control the output wavelength of the optical parametric oscillator 2, and is also used to analyze the plasma spectrum signal collected by the ICCD 7 and display the spectrum image.

[0033] When the polarization LIBS-LIF system for trace element detection provided by this embodiment is used for actual detection, the Nd:YAG laser 1 is first controlled to generate a first laser beam, and the first laser beam is focused on the surface of the sample to be tested through the laser wavelength reflector 10, the polarization optical element 11 and the first laser focusing lens 12 in sequence, and excites and generates plasma; after a delay time, the optical parametric oscillator 2 is controlled to generate a second laser beam, and the second laser beam is focused on the same position on the surface of the sample to be tested where the first laser beam is incident through the second laser focusing lens 13, so as to selectively excite the atoms of the element to be tested in the plasma. After excitation, the plasma spectrum is incident in parallel to the polarizer 15 through the first focusing lens 14. The polarizer 15 is rotated to select the angle with the best background removal effect of the target element spectrum. The plasma spectrum after reducing the background signal interference enters the second focusing lens 16 for focusing and is transmitted to the spectrometer 6 and ICCD7 through the optical fiber 17 for spectrum collection and electrical signal conversion in turn, and the electrical signal is transmitted to the computer 9 for processing and analysis. The computer 9 performs qualitative analysis on the sample according to the measured spectral wavelength and quantitative analysis according to the emission light intensity to determine the content of trace elements.

[0034] In order to reflect the best detection results that can be achieved by the polarization LIBS-LIF system for trace element detection provided in the above embodiment, the silicon element in the microalloyed steel is detected and analyzed in detail using a traditional detection system and the polarization LIBS-LIF system provided in this embodiment.

[0035] When using the polarization LIBS-LIF system provided in this embodiment for detection and analysis, it is first necessary to adjust the working parameters of the Nd:YAG laser, the optical parametric oscillator 2, the ICCD 7 and the timing controller 8 so as to collect the best spectrum effect. The specific working parameters are: the wavelength of the Nd:YAG laser is 532nm, the laser single pulse energy is 50mJ, and the frequency is 10Hz; the wavelength of the optical parametric oscillator 2 is 251.61nm, the laser single pulse energy is 1mJ, and the frequency is 10Hz; the timing controller 8 sets the time interval between two pulsed lasers to 10μs; the acquisition gate width of ICCD7 is 10ns, the delay is 10μs, and 100 spectra are collected.

[0036] Then, the required plasma spectrum is detected and collected, such as Figure 2 As shown, it is a comparison diagram of the spectral intensity of the traditional detection system using a linearly polarized laser beam for excitation detection and the system provided in the present application using a circularly polarized laser beam for excitation detection, which illustrates that the circularly polarized excitation state has the effects of spectral enhancement, lowering the detection limit, and improving the detection sensitivity. The horizontal axis is the wavelength of the spectrum and the vertical axis is the intensity.

[0037] Figure 3 This is a comparison chart of the fluctuations of the traditional detection system in the linear polarization state and the polarization LIBS-LIF system of the present application measured one hundred times in the circular polarization state. The horizontal axis is the number of measurements and the vertical axis is the spectral intensity of each measurement, indicating that the spectrum detected in the circular polarization state has a smaller fluctuation and is more stable.

[0038] The detection results are shown in Table 1. Compared with the traditional detection system using a linearly polarized laser beam for excitation detection, the intensity of the characteristic spectral line of Si with a wavelength of 288.16 nm obtained by the circularly polarized laser beam for excitation detection provided by the present application is increased by 1.2 times, the relative standard deviation RSD is reduced by 6.28%, and the signal-to-noise ratio SNR is increased by 1.14 times.

[0039] Table 1

[0040]

[0041] Through the above technical scheme conceived by the present application, compared with the prior art, since the LIBS-LIF detection system provided by the present application can realize polarization excitation and polarization resolution when detecting trace elements, it is possible to improve the excitation efficiency and the stability of the detection results by changing the polarization state of the excitation laser.

[0042] It should be understood that expressions such as "include" and "may include" used in this application indicate the existence of the disclosed functions, operations or constituent elements, and do not limit one or more additional functions, operations and constituent elements. In this application, terms such as "include" and / or "have" may be interpreted as indicating specific characteristics, numbers, operations, constituent elements, components or combinations thereof, but may not be interpreted as excluding the existence or possibility of adding one or more other characteristics, numbers, operations, constituent elements, components or combinations thereof.

[0043] It should be understood that the terms "center", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "inside", "outside", "clockwise", "counterclockwise", "axial", "radial", "circumferential" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present application.

[0044] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of this application, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0045] In this application, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be an indirect connection through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in this application can be understood according to specific circumstances.

[0046] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present application shall be included in the scope of protection of the present application.

Claims

1. A polarization LIBS-LIF system for trace element detection, characterized in that: The system comprises: a laser emitting unit (1), a first optical path structure, an optical parametric oscillator (2), a second optical path structure, a displacement platform (4), a polarization acquisition module (5) and an analysis control module; the laser emitting unit (1) is used to emit a first laser beam to the first optical path structure; the first optical path structure is used to convert the received first laser beam from linearly polarized light to circularly polarized light, and make the circularly polarized light incident on a sample on the displacement platform (4) to excite plasma; the optical parametric oscillator (2) is used to emit a second laser beam to the second optical path structure; the second optical path structure is used to focus the second laser beam and make it incident on the same position where the circularly polarized light is incident on the sample to excite the element to be measured in the plasma; the polarization acquisition module (5) is used to collect a plasma spectrum signal of the element to be measured that is excited, and convert the plasma spectrum signal into an electrical signal and transmit it to the analysis control module for analysis.

2. A polarization LIBS-LIF system for trace element detection according to claim 1, characterized in that: The first optical path structure comprises a laser wavelength reflector (10), a polarization optical element (11) and a first laser focusing lens (12), wherein: the angle between the laser wavelength reflector (10) and the first laser beam is an acute angle, so that the first laser beam is reflected and vertically incident on the polarization optical element (11); the polarization optical element (11) and the first laser focusing lens (12) are coaxially and parallelly arranged, so that the received first laser beam is converted into circularly polarized light and then incident vertically on the sample.

3. A polarization LIBS-LIF system for trace element detection according to claim 1, characterized in that: The second optical path structure comprises a second laser focusing lens (13), which is perpendicular to the propagation direction of the second laser beam and is used to focus the second laser beam and emit it into the same position as the circularly polarized light is emitted to the sample.

4. A polarization LIBS-LIF system for trace element detection according to claim 1, characterized in that: The polarization acquisition module (5) comprises a third optical path structure, a spectrometer (6) and an ICCD (7); the third optical path structure comprises a first focusing lens (14), a polarizer (15) and a second focusing lens (16) which are coaxially and parallel to each other; the first focusing lens (14) is used to focus the plasma spectrum signal and transmit it to the polarizer (15); the polarizer (15) is used to suppress background noise in the plasma spectrum signal and then transmit the plasma spectrum signal to the second focusing lens (16); the second focusing lens (16) is used to focus the plasma spectrum signal and then transmit it to the spectrometer (6) and the ICCD (7) in sequence via an optical fiber (17).

5. A polarization LIBS-LIF system for trace element detection according to claim 4, characterized in that: The analysis control module comprises a timing controller (8), which is respectively connected to the laser emission unit (1) and the optical parametric oscillator (2) and is used to control the laser pulse emission delay between the first laser beam and the second laser beam; the timing controller (8) is also connected to the ICCD (7) and is used to control the acquisition delay of the ICCD (7).

6. A polarization LIBS-LIF system for trace element detection according to claim 5, characterized in that: The analysis and control module further comprises a computer (9), which is connected to the optical parametric oscillator (2) and the ICCD (7) respectively, and is used to control the output wavelength of the optical parametric oscillator (2), and is also used to analyze the plasma spectrum signal collected by the ICCD (7) and display the spectrum image.

7. A polarization LIBS-LIF system for trace element detection according to claim 5, characterized in that: The laser pulse emission delay between the first laser beam and the second laser beam is at least 10 μs.

8. A polarization LIBS-LIF system for trace element detection according to claim 1, characterized in that: The wavelength range of the optical parametric oscillator (2) is 251.51 nm-251.71 nm.

9. A polarization LIBS-LIF system for trace element detection according to claim 8, characterized in that: The wavelength of the optical parametric oscillator (2) is 251.61 nm.

10. A polarization LIBS-LIF system for trace element detection according to claim 4, characterized in that: The polarizer (15) can be rotated to a specific angle to remove the spectrum background of the element to be measured.

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