CN112945936B - Liquid sample spectral measurement method and device based on laser plasma self-constraint - Google Patents

Abstract

The invention provides a method and device for measuring the spectrum of a liquid sample based on laser plasma self-constraint, including forming a jet flow from the liquid sample; symmetrically distributed multi-beams of equal energy laser pulses are focused radially on the liquid sample jet along the jet flow column for burning Eclipse to form a laser self-confined plasma with a spatially symmetrical distribution; collect and measure the spectrum of the laser self-confined plasma. The invention uses multiple laser pulses to simultaneously act on the symmetrically distributed plasma formed by the liquid jet, which can also suppress the sputtering of the liquid, reduce sample loss in the measurement process, avoid contamination of optical devices, and maintain the consistency of measurement conditions. The method can significantly improve the signal intensity of the laser-induced breakdown spectrum of elements in the liquid and the stability of multiple measurements, thereby achieving the purpose of improving the accuracy and sensitivity of the measurement.

Description

Method and device for spectral measurement of liquid samples based on laser plasma self-containment

technical field

The invention relates to the technical field of laser spectroscopy, in particular to a method and device for measuring laser-induced breakdown spectroscopy of liquid samples based on laser plasma self-constraint.

Background technique

Heavy metal pollution in water is one of the important problems facing environmental governance. The increasingly serious heavy metal pollution in water has seriously threatened the ecological environment and human life and health. Commonly used detection methods for heavy metals in water include inductively coupled plasma spectrometry (ICP), inductively coupled plasma mass spectrometry (ICP-MS), atomic fluorescence spectrometry (AFS), atomic absorption spectrometry (AAS) and X X-ray fluorescence spectrometry (XRF), etc. When these methods are used to analyze the content of heavy metal elements in water, they need to use chemical reagents to pretreat the samples, and the operation process is complicated. On the one hand, there are great safety hazards to the environment and instrument operators and maintenance personnel. Therefore, it is urgent to develop a rapid, real-time, multi-element detection analysis technology.

Laser-induced breakdown spectroscopy (LIBS) technology is a technique that focuses high-energy laser pulses on the surface of a sample to generate laser-induced plasma, and conducts qualitative and quantitative analysis of samples by measuring the line spectrum emitted by the plasma. Due to the advantages of fast, real-time, and no need for sample preparation, this technique has been widely used in the detection of samples with different forms. However, when LIBS is directly used for liquid detection, due to the high-energy laser pulse acting on the liquid surface, serious sputtering will occur on the liquid surface, contaminating the optical components, resulting in a decrease in the energy of the laser pulse acting on the liquid; at the same time, The shock wave generated by the laser action will lead to violent fluctuations in the liquid level, which will cause a large change in the laser power density coupled to the liquid, which will also cause the generated laser plasma to be very unstable. These factors will lead to the use of laser-induced breakdown spectroscopy. When the technology measures liquid samples, the intensity of the spectral signal is low and the stability is poor. It is impossible to obtain high-precision and high-sensitivity quantitative measurement results, and it is even impossible to achieve long-term continuous measurement. The short cooling time of liquid plasma will also lead to low detection signals and low detection sensitivity. This greatly limits the application of LIBS technology in the field of liquid sample detection and analysis.

Therefore, how to provide a liquid sample laser-induced shock based on laser plasma self-constraint that avoids liquid sputtering, changes in laser power density caused by liquid surface fluctuations, and poor spectral signal stability when the pulsed laser directly acts on the liquid sample for measurement. The measurement method and device of the transmission spectrum are problems urgently needed to be solved by those skilled in the art.

Contents of the invention

In view of this, the present invention provides a liquid sample spectral measurement method and device based on laser plasma self-constraint, which can avoid the liquid splashing and liquid surface fluctuations caused by the pulsed laser directly acting on the liquid sample measurement. There are technical difficulties such as changes in laser power density, poor spectral signal stability, uneven spatial distribution of laser plasma, and rapid plasma cooling.

In order to achieve the above object, the present invention adopts the following technical solutions:

One aspect of the present invention provides a method for measuring laser-induced breakdown spectroscopy of liquid samples based on laser plasma self-constraint, comprising the following steps: the liquid sample forms a jet; symmetrically distributed multi-beams of equal-energy laser pulses focus radially along the jet flow column Ablation is performed on the liquid sample jet to form a laser self-confined plasma with a spatially symmetrical distribution; the spectrum of the laser self-confined plasma is collected and measured.

Preferably, the schlieren imaging is performed on the laser self-constrained plasma expansion process formed by multiple equal-energy laser pulses.

Preferably, the liquid sample is drawn out from the sample pool and a jet flow is generated to fall back into the sample pool to form a circulating liquid flow.

Another aspect of the present invention provides a liquid sample laser-induced breakdown spectroscopy measurement device based on laser plasma self-constraint, including a laser pulse emission device, a laser transmission and modulation optical path, a liquid jet device, and a spectral analysis device; wherein,

The liquid jet device is used to make the liquid sample form a jet;

The laser pulse emitting device includes a nanosecond pulse laser as a plasma excitation light source, and generates a pulsed laser beam;

The laser transmission and modulation optical path is used to split the pulsed laser beam into multiple beams of equal energy laser pulses that are symmetrically distributed relative to the jet, and focus on the liquid sample jet along the radial direction of the jet column for ablation to form a spatially symmetrical distribution. Laser self-confinement plasma;

The spectrum analysis device is used to collect and measure the spectrum of the laser self-confined plasma.

Preferably, it also includes a plasma expansion transient shadow imaging device, which is used to perform schlieren imaging on the laser self-constrained plasma expansion process formed by multiple beams of equal-energy laser pulses, including: a visible light pulse laser, an optical filter and a CCD camera;

The light emitted by the visible light pulse laser passes through the laser self-confined plasma region of the liquid jet, and is collected by a CCD camera through a filter for schlieren imaging, and the CCD camera is connected to a computer.

Preferably, the liquid jet device includes a peristaltic pump, a damper and a metal capillary; the peristaltic pump draws the liquid sample from the sample pool, passes through the silicone tube and the damper into the metal capillary, and forms a constant flow rate at the metal capillary nozzle. , a liquid jet with a diameter of hundreds of microns.

Preferably, the laser transmission and modulation optical path includes a number of laser beam splitters, a number of reflection mirrors and a number of focusing lenses arranged along the propagation direction of the pulsed laser beam generated by the nanosecond pulsed laser; wherein,

The plurality of laser beam splitters are used to split the pulsed laser beam into multiple beams of equal energy laser pulses;

The reflector is used to change the propagation direction of multiple beams of equal-energy laser pulses, so that the multiple beams of equal-energy laser pulses are distributed radially symmetrically along the jet flow column;

The focusing lenses are respectively located on the optical paths of multiple equal-energy laser pulses, and focus the equal-energy laser pulses on the liquid sample jet for ablation.

Preferably, the laser beam splitter adopts a polarization beam splitter; the propagation direction of the pulsed laser beam generated by the nanosecond pulse laser is sequentially provided with a half-wave plate and a polarization beam splitter one, and the polarization beam splitter one divides the pulsed laser beam into Optical path 1 and optical path 2, and optical path 1 and optical path 2 propagate perpendicularly to each other;

The optical path one is provided with a half-wave plate and a polarizing beam splitter two along the propagation direction, and the polarizing beam splitting plate two divides the optical path one into an optical path three and an optical path four; the optical path four is focused on the liquid sample jet through a focusing lens, and the Optical path three is focused on the liquid sample jet through two mirrors and a focusing lens in turn, and the focused optical path three and optical path four are perpendicular to each other in the direction of the liquid sample jet section;

The second optical path is provided with a half-wave plate and a third polarization beam splitter along the propagation direction, and the second optical path is divided into an optical path five and an optical path six by the third polarization beam splitter; the sixth optical path is focused on the liquid sample jet through a focusing lens, and the optical path Five is focused on the liquid sample jet through two mirrors and a focusing lens in turn, and the focused optical paths five and six are perpendicular to each other in the direction of the liquid sample jet cross section.

Preferably, the spectrum analysis device includes a spectrum collection lens group, an optical fiber, and a spectrometer; the spectrum collection lens group of the spectrometer collects the spectrum of the laser self-confinement plasma, and transmits it to the spectrometer through an optical fiber; the spectrometer is connected to a computer.

Preferably, a digital time-delay signal generator is also included, the digital time-delay signal generator is connected to the pulse laser, visible light pulse laser and CCD camera, and simultaneously controls the triggering of the pulse laser, visible light pulse laser and CCD camera.

It can be seen through the above-mentioned technical solution that compared with the prior art, the beneficial effects of the present invention include:

The invention utilizes four beams of laser pulses to excite the liquid jet to form a symmetrically distributed plasma, and forms a self-constrained plasma through the spatial interaction of four plasma shock waves, thereby producing a more uniform and longer-lasting stable laser in the central region plasma. This method can achieve the purpose of improving the signal intensity of laser-induced breakdown spectroscopy measurement of elements in the liquid and the stability of multiple measurements, thereby improving the accuracy and sensitivity of the measurement, which is very useful for the field detection of sewage by laser-induced breakdown spectroscopy and industrial production. Applications such as real-time analysis of elements in liquids such as process monitoring and salt lake resource investigation are very important.

Description of drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings that need to be used in the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description are only It is an embodiment of the present invention. For those of ordinary skill in the art, other drawings can also be obtained according to the provided drawings without creative work;

Fig. 1 is a schematic diagram of an optical path for spectral measurement of a liquid sample based on laser plasma self-containment provided by an embodiment of the present invention;

Fig. 2 is a schematic diagram of a liquid sample laser-induced breakdown spectroscopy measurement device based on laser plasma self-constraint provided by an embodiment of the present invention;

Fig. 3 is a transient shadow imaging diagram of plasma expansion under the action of a single-sided laser pulse provided by an embodiment of the present invention;

Fig. 4 is a transient shadow imaging diagram of plasma impulse expansion under the simultaneous action of double pulses provided by the embodiment of the present invention;

Fig. 5 is a calibration curve of Cr element content measured by a single-sided laser pulse provided by an embodiment of the present invention;

Fig. 6 is a calibration curve of Cr element content measured by double-sided laser pulse provided by the embodiment of the present invention.

Detailed ways

The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only some, not all, embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without making creative efforts belong to the protection scope of the present invention.

A self-contained liquid sample spectral measurement method based on laser plasma 20 disclosed in this embodiment includes the following steps: the liquid sample forms a jet; symmetrically distributed multi-beams of equal energy laser pulses focus on the liquid radially along the jet flow column Ablation is performed on the sample jet to form a laser self-confined plasma 20 with a spatially symmetrical distribution; the spectrum of the laser self-confined plasma 20 is collected and measured. Since the plasma 20 is spatially confined by multiple laser pulses, this method effectively improves the stability of the experimental measurement.

In a specific embodiment, schlieren imaging is performed on the expansion process of the laser self-confined plasma 20 formed by multiple beams of equal energy laser pulses, so as to observe the shape of the liquid jet and the plasma 20 to determine the effect of the four laser pulses on the Spatial Constraint Effects on Liquid Jets.

In a specific embodiment, the liquid sample is drawn out from the sample pool 21 and a jet flow is generated and falls back to the sample pool 21 to form a circulating liquid flow. This method changes the liquid from static to dynamic, and the dynamic liquid jet is constantly refreshed, so that the laser can focus on the refreshed liquid jet every time, and also reduce the fluctuation and sputtering of the liquid, thereby improving the repeatability of experimental measurements and stability.

Another aspect of the present invention provides a liquid sample laser-induced breakdown spectroscopy measurement device based on laser plasma 20 self-constraint, including a laser pulse emission device, a laser transmission and modulation optical path, a liquid jet device, and a spectral analysis device; wherein,

The liquid jet device is used to make the liquid sample form a jet;

The laser pulse emitting device includes a nanosecond pulse laser 1 as a plasma 20 excitation light source, and generates a pulsed laser beam 2;

The laser transmission and modulation optical path is used to split the pulsed laser beam 2 into multiple beams of equal energy laser pulses that are symmetrically distributed relative to the jet, and focus on the liquid sample jet along the radial direction of the jet column for ablation to form a spatially symmetrical laser pulse Self-confinement plasma 20;

The spectrum analysis device is used to collect and measure the spectrum of the laser self-confined plasma 20 .

In this embodiment, the nanosecond pulse laser 1 may be a Q-switched nanosecond pulse laser 1 .

In a specific embodiment, it also includes a plasma 20 expansion transient shadow imaging device, which is used to perform schlieren imaging on the laser self-constrained plasma 20 expansion process formed by multiple beams of equal energy laser pulses, including: a visible light pulse laser 8 , optical filter 15 and CCD camera 14;

The shadow imaging light 13 emitted by the visible light pulse laser 8 passes through the laser self-confined plasma 20 region of the liquid jet, and is collected by the CCD camera 14 through the filter 15 for schlieren imaging, and the CCD camera 14 is connected to the computer 12 .

In this embodiment, the visible light pulse laser 8 is connected with the digital delay signal generator 11, and the light emitted by the visible light pulse laser 8 is collected by the CCD camera 14 through the plasma 20 region and the optical filter 15 of the liquid jet, and the multi-beam laser is formed Schlieren imaging during the plasma expansion process.

In a specific embodiment, the liquid jet device comprises a peristaltic pump 16, a damper 19 and a metal capillary 18; the peristaltic pump 16 extracts the liquid sample from the sample pool 21, enters the metal capillary 18 through the silicone tube 17 and the damper 19, and A liquid jet with a constant velocity and a diameter of hundreds of microns is formed at the nozzle of the metal capillary 18 . The peristaltic pump 16 draws the liquid sample out of the sample cell 21, passes through the silicone tube 17 and the damper 19 and enters the metal capillary 18, and forms a liquid jet with a constant flow rate and a diameter of hundreds of microns at the nozzle of the metal capillary 18.

In a specific embodiment, the laser transmission and modulation optical path includes several laser beam splitters, several mirrors 5 and several focusing lenses 6 arranged along the propagating direction of the pulsed laser beam 2 generated by the nanosecond pulsed laser 1; wherein,

Several laser beam splitters are used to split the pulsed laser beam 2 into multiple equal-energy laser pulses;

The reflector 5 is used to change the propagation direction of multiple beams of equal energy laser pulses, so that the multiple beams of equal energy laser pulses are distributed radially symmetrically along the jet stream column;

Focusing lenses 6 are respectively located on the optical paths of multiple equal-energy laser pulses, and focus the equal-energy laser pulses on the liquid sample jet for ablation.

In a specific embodiment, the laser beam splitter adopts a polarization beam splitter; the direction of propagation of the pulsed laser beam 2 generated by the nanosecond pulse laser 1 is arranged with a half-wave plate 3, a polarization beam splitter-41 in sequence, and a polarization beam-splitter-41 divides the pulse Laser beam 2 is divided into optical path one and optical path two, and optical path one and optical path two propagate perpendicularly to each other;

The first optical path is provided with a half-wave plate 3 and a second polarization splitter 42 along the propagation direction, and the second polarization splitter 42 divides the first optical path into an optical path three and an optical path four; the fourth optical path is focused on the liquid sample jet through the focusing lens 6, and the third optical path passes through The two reflectors 5 and the focusing lens 6 are focused on the liquid sample jet, and the focused optical paths three and four are perpendicular to each other in the direction of the liquid sample jet section;

The second optical path is provided with a half-wave plate 3 and a third polarizing beam splitter 43 along the propagation direction. The third polarizing beam splitter 43 divides the optical path into two optical paths five and six; the sixth optical path is focused on the liquid sample jet through the focusing lens 6, and the fifth optical path passes through two A reflecting mirror 5 and a focusing lens 6 focus on the liquid sample jet, and the focused optical paths five and six are perpendicular to each other in the direction of the liquid sample jet cross-section.

In this embodiment, the nanosecond pulse laser 1 is connected to a digital delay signal generator 11, and the digital delay signal generator 11 gives an external trigger signal to the nanosecond pulse laser 1 to control the operation of the pulse laser. The pulsed laser beam 2 emitted by the nanosecond pulsed laser 1 is divided into four laser beams with equal energy through the splitting optical path composed of the half-wave plate 3 and the polarizing beam splitter, and then the laser pulses are separated from the four laser beams by the reflector 5 and the focusing lens 6 respectively. The direction is focused on the liquid jet, forming a spatially symmetrically distributed self-confined plasma 20 .

In a specific embodiment, the spectral analysis device includes a spectrum collection lens group 7, an optical fiber 9, and a spectrometer 10; the spectrometer 10 spectrum collection lens group 7 collects the spectrum of the laser self-confinement plasma 20, and propagates to the spectrometer 10 through an optical fiber 9; 10 is connected to a computer 12.

In the present embodiment, the spectrum of laser plasma 20 enters spectrometer 10 through spectrum collection lens and optical fiber 9, and spectrometer 10 is connected with digital delay signal generator 11 and computer 12, is used for controlling the delay time that spectrometer 10 collects and spectrometer The data is stored and processed, and the characteristic spectrum is analyzed by the spectrometer 10 and the computer 12 .

In a specific embodiment, the digital time-delay signal generator 11 is connected to the pulsed laser, the visible light pulsed laser 8 and the CCD camera 14 , and simultaneously controls the triggering of the pulsed laser, the visible light pulsed laser 8 and the CCD camera 14 .

The concrete realization principle of this embodiment is:

The liquid sample is pumped from the sample cell 21 to the metal capillary 18 through the silicone tube 17 in the peristaltic pump 16, and a liquid jet with a constant flow rate is formed at the nozzle of the metal capillary 18; the laser pulse emitted by the nanosecond pulse laser 1 passes through the laser beam splitter and then splits into Four beams of laser light with equal energy are symmetrically focused on the liquid jet from four directions through the focusing lens 6, so that plasma is formed on the four sides of the liquid column. Four symmetrically distributed plasma shock waves will check and balance each other to realize the spatial self-constraint effect of plasma expansion, thereby forming a plasma that is more stable and lasts longer than traditional single-sided laser beam excitation. The plasma emission spectrum is collected by the spectrum collection lens group 7 and enters the optical fiber 9, and is transmitted to the spectrometer 10 for analysis and measurement. In addition, a plasma plume transient imaging device composed of a visible light pulse laser 8 and a CCD camera 14 is also installed in the device, which can perform schlieren imaging on the plasma expansion process formed by four laser beams.

Practical example 1: Using symmetrically distributed laser beams to improve the uniformity and stability of plasma

Fig. 3 is the plasma shock wave generated when a single beam pulse acts on the liquid at an arbitrary moment, and Fig. 4 is the plasma shock wave generated when two 1064nm nanosecond laser pulses act on the liquid at an arbitrary moment. It can be seen that when a single beam of nanosecond pulsed laser acts on the liquid, the shock wave on the side of the incident direction of the laser is very strong, and at the same time the plasma shock wave will also expand to the side of the liquid. The shock wave on the incident side of the laser is significantly reduced. When using the liquid sample laser-induced breakdown spectroscopy measurement device and method based on laser plasma self-confinement, due to the mutual extrusion of two plasma shock waves, compared with a single beam of pulsed plasma, a larger and more Stable plasma region, and the mutual extrusion of plasma also constrains the dynamic behavior of the liquid and reduces sputtering; the extrusion of two beams of plasma can also form a local transient high-pressure environment near the target sample, so that the plasma temperature and density By maintaining a higher state and delaying the cooling time of the plasma, the plasma produced by this method is more stable.

Practical example 2: Using symmetrically distributed laser beams to improve the detection sensitivity and measurement stability of elements in liquids

When using laser-induced breakdown spectroscopy to detect liquids, a pulsed laser with a repetition rate of 10 Hz is used in this experiment. When the 10 Hz pulsed laser acts on the liquid, the spectral signal intensity will be low and stable due to the fluctuation and sputtering of the liquid. However, the accuracy and sensitivity of the measurement results are poor, and the quantitative detection requirements for metal elements in aqueous solutions cannot be met. Many methods have been proposed to avoid these problems, such as the liquid jet method, which uses a peristaltic pump to change the liquid from static to dynamic liquid jet. can focus on the refreshed liquid jet every time, and also reduce the fluctuation and sputtering of the liquid, thereby improving the repeatability and stability of the experimental measurement. In this experiment, four laser beams act on the liquid jet at the same time to measure the Quantitative detection of metal elements, due to the space constraints formed by the four laser pulses on the plasma, this method effectively improves the stability of the experimental measurement, and will greatly help to improve the accuracy and sensitivity of the experimental results. In this experiment, the combination of liquid jet and four laser pulses is used to quantitatively detect the self-configured sodium chromate solution, and at the same time, shadow imaging is performed on the liquid plasma to observe the expansion process of the plasma shock wave.

In this experiment, 532nm pulsed laser is used as the imaging shadow light source, and the triggering of 1064nm Nd:YAG pulsed laser, 532nm pulsed laser and CCD camera is simultaneously controlled by DG-645 digital delay signal generator. Fig. 5 is the calibration curve of measuring the Cr element content in the liquid using a single-sided laser pulse, and Fig. 6 is the measurement of the Cr element content in the liquid when using a laser-induced breakdown spectroscopy measurement device and method based on laser plasma self-constraint. Measured calibration curve. It can be seen from Fig. 5 and Fig. 6 that the correlation coefficient of the calibration curve measured by the symmetrically distributed laser beam proposed by this method reaches 0.993, which is significantly improved compared with the correlation coefficient 0.985 obtained by unilateral laser pulse excitation. The accuracy of quantitative measurement results can be improved, and the relative standard deviation (RSD) of the spectral line signal is also significantly reduced, and under the condition of equal total laser energy, the signal intensity of the double-sided laser pulse is compared with that of the single-sided laser pulse. There is an improvement of 40%. Combining the improvement of these two aspects, the detection limit of elements in the liquid that can be obtained by using this method is lower than that of the conventional single-sided laser beam directly interacting with the liquid nozzle, which shows that the proposed method can be effectively Improve the intensity and stability of spectral signals, and obtain higher detection sensitivity.

The method and device for spectral measurement of liquid samples based on laser plasma self-constraint provided by the present invention have been introduced in detail above. In this paper, specific examples are used to illustrate the principle and implementation of the present invention. The description of the above examples is only used To help understand the method of the present invention and its core idea; at the same time, for those of ordinary skill in the art, according to the idea of the present invention, there will be changes in the specific implementation and scope of application. In summary, the content of this specification does not It should be understood as a limitation of the present invention.

The above description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the present invention will not be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims (10)

1. A liquid sample laser-induced breakdown spectroscopy measurement method based on laser plasma self-confinement is characterized by comprising the following steps: the liquid sample forms a jet; a plurality of symmetrically distributed laser pulses with equal energy are focused on the liquid sample jet along the radial direction of the jet flow column for ablation to form laser self-confinement plasma which is spatially and symmetrically distributed; and collecting and measuring the spectrum of the laser self-confinement plasma.

2. The laser-induced breakdown spectroscopy measurement method for the liquid sample based on the laser plasma self-confinement according to claim 1, wherein the schlieren imaging is performed on the laser self-confinement plasma expansion process formed by a plurality of equal-energy laser pulses.

3. The laser-induced breakdown spectroscopy measurement method and apparatus for liquid samples based on laser plasma self-confinement according to claim 1, wherein the liquid sample is drawn from the sample cell and the jet stream is generated and falls back to the sample cell to form a circulating liquid stream.

4. A liquid sample laser-induced breakdown spectroscopy measuring device based on laser plasma self-confinement is characterized by comprising a laser pulse emitting device, a laser transmission and modulation light path, a liquid jet device and a spectral analysis device; wherein,

the liquid jet device is used for enabling the liquid sample to form a jet;

the laser pulse emitting device comprises a nanosecond pulse laser serving as a plasma excitation light source and generates a pulse laser beam;

the laser transmission and modulation light path is used for splitting the pulse laser beam into a plurality of equal-energy laser pulses which are symmetrically distributed relative to the jet flow, and the laser pulses are radially focused on the liquid sample jet flow along the jet flow column to be ablated so as to form laser self-confinement plasma which is symmetrically distributed in space;

the spectrum analysis device is used for collecting and measuring the spectrum of the laser self-confinement plasma.

5. The laser-induced breakdown spectroscopy measurement device for the liquid sample based on laser plasma self-confinement according to claim 4, further comprising a plasma expansion transient shadow imaging device for performing schlieren imaging on the laser self-confinement plasma expansion process formed by a plurality of laser pulses with equal energy, comprising: a visible light pulse laser, a light filter and a CCD camera;

the light emitted by the visible light pulse laser passes through the laser self-confinement plasma area of the liquid jet and is collected by a CCD camera through an optical filter for schlieren imaging, and the CCD camera is connected to a computer.

6. The laser-plasma-self-confinement-based liquid sample laser-induced breakdown spectroscopy measurement device according to claim 4, wherein the liquid jet device comprises a peristaltic pump, a damper and a metal capillary; the peristaltic pump pumps a liquid sample out of the sample cell, the liquid sample enters the metal capillary through the silicone tube and the damper, and liquid jet flow with constant flow rate and hundred-micron-order diameter is formed at a spout of the metal capillary.

7. The laser plasma self-confinement based liquid sample laser-induced breakdown spectroscopy measurement device according to claim 4, wherein the laser transmission and modulation optical path comprises a plurality of laser beam splitters, a plurality of reflectors and a plurality of focusing lenses arranged along the propagation direction of the pulse laser beam generated by the nanosecond pulse laser; wherein,

the laser beam splitters are used for splitting the pulse laser beam into a plurality of equal-energy laser pulses;

the reflector is used for changing the propagation direction of the multiple equal-energy laser pulses to ensure that the multiple equal-energy laser pulses are symmetrically distributed along the radial direction of the jet flow column;

the focusing lenses are respectively positioned on a plurality of equal-energy laser pulse light paths and focus the equal-energy laser pulses on the liquid sample jet flow for ablation.

8. The laser-plasma-self-confinement-based liquid sample laser-induced breakdown spectroscopy measurement device according to claim 7, wherein the laser beam splitter adopts a polarization beam splitter; the nanosecond pulse laser comprises a nanosecond pulse laser, a half-wave plate and a first polarization beam splitter are sequentially arranged in the propagation direction of a pulse laser beam generated by the nanosecond pulse laser, the first polarization beam splitter divides the pulse laser beam into a first light path and a second light path, and the first light path and the second light path are vertically propagated;

the light path I is provided with a half-wave plate and a polarization beam splitter II along the transmission direction, and the polarization beam splitter II divides the light path I into a light path III and a light path IV; the light path four is focused on the liquid sample jet flow through the focusing lens, the light path three is focused on the liquid sample jet flow through the two reflectors and the focusing lens in sequence, and the focused light path three and the focused light path four are perpendicular to each other in the cross section direction of the liquid sample jet flow;

the second light path is provided with a half-wave plate and a third polarization beam splitter along the transmission direction, and the third polarization beam splitter divides the light path into a fifth light path and a sixth light path; the light path six is focused on the liquid sample jet flow through the focusing lens, the light path five is focused on the liquid sample jet flow through the two reflectors and the focusing lens in sequence, and the focused light path five and the focused light path six are perpendicular to each other in the cross section direction of the liquid sample jet flow.

9. The laser plasma self-confinement based liquid sample laser-induced breakdown spectroscopy device of claim 1, wherein the spectral analysis device comprises a spectrum collection lens group, an optical fiber and a spectrometer; the spectrometer spectrum collection lens group collects the spectrum of the laser self-confinement plasma and transmits the spectrum to the spectrometer through the optical fiber; the spectrometer is connected to a computer.

10. The laser plasma self-confinement based liquid sample laser-induced breakdown spectroscopy measurement device according to claim 5, further comprising a digital time-delay signal generator, wherein the digital time-delay signal generator is connected with the pulse laser, the visible light pulse laser and the CCD camera, and controls the triggering of the pulse laser, the visible light pulse laser and the CCD camera.

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