CN101196471A - Quantitative detection system and detection method of soil heavy metal pollution - Google Patents

Abstract

The invention discloses a soil heavy metal pollution quantification testing system and the testing method, which is used to conduct quantification monitoring to medium and light heavy metal polluted soil. The quantification testing system of the invention comprises a laser, a beam splitter, a collecting optics lens, an optical fiber detector, optical fiber, a spectrograph, a laser dynamometer instrument and a computer. The quantification testing system of the invention normalizes the laser induction and plasma spectrum strength value of the trace element in soil standard sample into the strength value of injecting laser, and then utilized the linear relation between the normalized value and the element content to build calibrating plot, and finally utilizes the calibrating plot and the same normalization method to conduct quantitative detecting to the heavy metal element in the unknown soil sample.

Description

Quantitative detection system and detection method of soil heavy metal pollution

technical field

The invention relates to a plasma spectrum quantitative analysis technology for substances, and specifically refers to a soil heavy metal pollution quantitative detection system and detection method, which are used for quantitative monitoring of moderately and lightly heavy metal polluted soil.

Background technique

Heavy metals are "a class of pollutants" in the environment. With the rapid development of social economy, the degree of pollution and the area of pollution are increasing year by year. Once the soil is polluted by heavy metals, it will be enriched and magnified through the food chain and ultimately affect human health. Therefore, both domestic and foreign countries are very concerned about the monitoring and treatment of heavy metal pollution in soil.

There are various types of soil heavy metal elements, and their background values in the environment and the critical values stipulated in the "China Soil Environmental Quality Standards" are different. It is very difficult to analyze soil heavy metals at the same time using traditional methods, and usually go to the polluted site first Collect soil samples, and then return to the laboratory for chemical analysis using atomic absorption spectroscopy, atomic fluorescence spectroscopy, plasma-atomic emission spectroscopy or mass spectrometry, and there are differences in different heavy metal analysis methods. It also has a certain impact on the health of the experimenters. Laser-induced plasma spectroscopy is a chemical element analysis technology that has developed rapidly in recent years. It has the advantages of real-time, rapid, on-site detection and simultaneous detection of multiple elements, and is very suitable for monitoring heavy metal pollution in soil.

The working mechanism of laser-induced plasma spectroscopy technology is: the high-energy pulsed laser beam converges on the surface of the sample, and a megawatt-level photon energy density (unit square centimeter) is obtained at the convergence point. Ionization”, the ionized free electrons collide violently with the atoms under the acceleration of the laser electric field and ionize it to generate more free electrons, these free electrons continue to participate in the impact ionization process, thus a similar “chain reaction” occurs "Cascade ionization", when the electron density increases to a certain level, dissociation occurs to form a high-temperature and high-energy plasma, accompanied by a crisp shock wave sound and a strong flash. The duration of the laser pulse is usually below 10ns. After the laser pulse disappears, the plasma gradually cools down and radiates the atomic spectrum of various elements at the same time. These atomic spectral lines are the "fingerprints" of the elements contained in the substance. The wavelength position corresponds to the type of element, and the signal intensity corresponds to the content of the element. This is the basis for the qualitative and quantitative analysis of the composition of material elements by light-induced plasma spectroscopy.

At present, the qualitative analysis capability of laser-induced plasma spectroscopy has been recognized all over the world, but due to the influence of "non-linear effects" in quantitative analysis, it has been difficult to find a general quantitative method. The so-called "non-linear effect" refers to the fact that the signal intensity of the atomic spectral line of a specific element radiated by the laser-induced plasma is not only determined by the content of the element in the plasma, but also affected by many other factors, resulting in this There is a non-linear relationship between the intensity of the atomic spectral line of each element and the corresponding content. Among the influencing factors that lead to nonlinearity, the most important is the "self-absorption" effect of atomic spectral lines, that is, atomic radiation of a specific wavelength undergoes "self-absorption" and attenuates when passing through high-density plasma. "The extent of the influence is difficult to quantify, which leads to the nonlinear relationship between the intensity of the atomic spectral line of the element and the corresponding content is also difficult to quantify.

A large number of experimental studies have shown that for trace elements with a content of less than 300ppm in the sample, the "self-absorption" effect of atomic spectral radiation has little effect, and a good linear relationship can be maintained between the element atomic spectral line intensity and the corresponding content. In the "Soil Environmental Quality Standards" promulgated by the Chinese government, soil heavy metal pollution is divided into three levels, the first level pollution is the lightest; the third level pollution is the heaviest; the second level pollution is divided into A, B, and C levels, of which C level pollution heaviest. Quantitative monitoring of heavy metal pollution below 300ppm, compared with quality standard values of different levels, can well evaluate the primary and secondary pollution levels of most heavy metal elements, that is to say, it can accurately quantify medium and light pollution below 300ppm Degree.

In addition, when using laser-induced plasma spectroscopy for quantitative analysis, due to the small size of the sampling point (usually less than 1mm), the inhomogeneity of the measured sample matrix can easily lead to large dispersion of measurement results. In order to improve The precision and accuracy of quantitative analysis usually requires repeated measurements, and gives ideal results based on the statistical analysis of a large amount of data, but each measurement process often introduces a large measurement deviation due to the instability of laser pulse energy , it is necessary to take necessary measures to eliminate this effect. The usual method is to normalize the intensity value of the atomic spectral line of a specific element at a specific wavelength. The selection of the normalization standard directly determines the precision and accuracy of the quantification. Usually The normalization standard is selected as the signal intensity of a certain atomic line of the main element (element with high content) in the sample. The starting point of this choice is that the atomic line intensity of the trace element and the atomic spectrum of the main element The relative relationship between the line intensities can maintain a good consistency in different measurement processes, but in fact, due to the inhomogeneity of the sample matrix, the content of the main elements also changes in each measurement process, and this change It cannot be quantified at all, so that the intensity of the atomic spectral line of the main element is also changing and cannot be quantified; therefore, when a variable and unquantifiable quantity is used as a standard value for normalization, the normalization result must also have a certain degree of The randomness will have an inevitable negative impact on the final statistical results.

Contents of the invention

The purpose of the present invention is to establish a soil heavy metal pollution quantitative detection system and its detection method, to eliminate the influence of the jitter of laser pulse energy on the measurement accuracy during each laser excitation process, and to realize the detection of medium and light heavy metal pollution below 300ppm Soil for accurate quantitative detection.

As shown in accompanying drawing 1, the soil heavy metal pollution quantitative detection system of the present invention comprises: laser 1, beam splitter 2, converging optical lens 3, sample 4, optical fiber probe 5, optical fiber 6, spectrometer 7, laser power meter 8, computer 9. The working principle of the system is: the laser beam emitted by the laser 1 first passes through the beam splitter 2, 10% of the laser pulse energy is reflected by the beam splitter 2 to the probe of the laser power meter 8, and the other 90% of the laser pulse energy passes through the splitter The beam mirror 2 is then converged on the surface of the sample 4 to induce plasma through the converging mirror 3, and the laser power meter 8 measures 10% of the laser pulse energy and transmits the energy value to the computer 9 through the interface circuit as a normalized standard value. The atomic spectrum radiated by the plasma is collected by the optical fiber probe 5, coupled to the incident slit of the spectrometer 7 through the optical fiber 6, and the spectral analysis data is transmitted to the computer 9 through the interface circuit, and the computer 9 is equipped with spectral data and laser power data acquisition and processing software The data acquisition and processing software realizes the acquisition and processing of spectral data and laser pulse energy data, and the processing results are compared with the quantitative calibration curve of specific elements in the computer, so as to realize the quantitative detection of specific elements in the tested sample.

The quantitative detection method of the soil heavy metal pollution quantitative detection system of the present invention comprises the following steps:

1) Establish a quantitative calibration curve

The laser beam emitted by the laser 1 first passes through the beam splitter 2, 10% of the laser pulse energy is reflected by the beam splitter 2 to the probe of the laser power meter 8, and the other 90% of the laser pulse energy passes through the beam splitter, and then passes through the beam splitter. The converging mirror 3 converges on the surface of the specific model standard sample 4 to induce plasma; the laser power meter 8 measures 10% of the laser pulse energy and transmits the energy value to the computer 9 through the interface circuit for storage as a normalized standard value; the laser induces plasma The atomic spectrum radiated by the body is collected by the optical fiber probe 5, coupled to the incident slit of the spectrometer 7 through the optical fiber 6, and the spectral analysis data is transmitted to the computer 9 for storage through the interface circuit; The laser energy value measured by the power meter 8 is used as a normalized standard value to normalize the spectral line intensity value. Repeat the above steps for different specific models of standard samples, and combine the measured data and the content value of the element into a (x, y) coordinate (y represents the normalized atomic spectral line intensity value, x represents the element content value) , and fit a calibration curve for the analyte.

2) Measure the normalized atomic spectral intensity value of the analyte in the unknown sample

The laser beam emitted by the laser 1 first passes through the beam splitter 2, wherein 10% of the laser pulse energy is reflected by the beam splitter 2 to the probe of the laser power meter 8, and the other 90% of the laser pulse energy passes through the beam splitter 2, and then Converging on the surface of the unknown sample 4 via the converging mirror 3 to induce plasma; the laser power meter 8 measures 10% of the laser pulse energy and transmits the energy value to the computer 9 through the interface circuit for storage as a normalized standard value; the laser induces plasma The radiated atomic spectrum is collected by the optical fiber probe 5, coupled to the incident slit of the spectrometer 7 through the optical fiber 6, and the spectral analysis data is transmitted to the computer 9 for storage through the interface circuit; , normalize the atomic line intensity value of the same wavelength of the element to be measured in the unknown sample, and obtain the normalized measured value y ₀ of the atomic line intensity value.

3) Obtain the quantitative data of the elements to be measured

According to the normalized atomic spectral line intensity value (y=y ₀ ) of the unknown sample obtained in step 2) and the quantitative calibration curve obtained in step 1), the content value of the element x(x =x ₀ ), so as to realize the quantitative detection of specific elements.

The advantages of the present invention are:

(1) The quantitative detection method of the measurement system is only aimed at trace elements below 300ppm, which can effectively avoid the nonlinear relationship between the spectral line intensity value and element content caused by the "self-absorption" effect of atomic spectral lines.

(2) The measurement system normalizes the intensity values of the plasma lines of various elements to the intensity value of the incident laser, which completely eliminates the influence of the jitter of the laser pulse energy on the measurement accuracy during each laser excitation process, and improves the quantification. measurement accuracy.

Description of drawings

Fig. 1 is the detection system structural representation of the present invention; Wherein:

1 - laser;

2—beam splitter;

3—converging optical lens;

4 - the sample to be tested;

5——optical fiber probe;

6 - optical fiber;

7 - spectrometer;

8—laser power meter;

9 - Computer.

Detailed ways

A preferred embodiment of the present invention is provided below according to FIG. 1 .

The system for quantitative detection includes the following parts:

1) Laser 1

The beam emitted by the laser 1 is used to excite the sample to generate a plasma spectrum, and at the same time generate the normalized standard value required for the quantification process. Nd:YAG Q-switched pulse laser is selected, the working wavelength is 1064nm, the pulse width is 3-5ns, the maximum pulse energy is 200mJ, the pulse energy stability is ±2%, the maximum pulse repetition frequency is 20Hz, and the laser beam divergence angle is less than 1mrad.

2) beam splitter 2

The beam splitter 2 is used to split the laser beam emitted by the laser. The beam splitter is placed at 245°, and the beam splitter 2 is coated with a reflective film on the incident side of the beam, so that 10% of the pulse energy of the incident beam is reflected to the probe of the laser power meter, and the other 90% of the pulse energy is transmitted to excite the sample to form plasma.

3) Converging optical lens 3

The converging optical lens 3 is used to condense the laser beam, so that the laser energy density on the surface of the sample reaches the breakdown threshold, thereby inducing plasma. The lens diameter is 20mm and the focal length is 200mm.

4) Fiber optic probe 5

The optical fiber probe 5 is used to collect the atomic spectral energy radiated by the plasma and couple it into the optical fiber 6 . The optical fiber probe has a caliber of 30mm, a focal length of 52mm, and a transmission wavelength range of 200-1100nm.

5) Spectrometer 7

The spectrometer 7 is used for spectral analysis of the laser-induced plasma spectrum collected by the optical fiber probe 5 . The spectrometer 7 adopts an echelle grating, and the response wavelength of the spectrometer 7 is 200-850 nm. The incident slit of the spectrometer 7 is replaced by an ultraviolet transmission fiber with a core diameter of 50 μm, and the spectral resolution is 0.04-0.2 nm. The spectrometer detector adopts an enhanced CCD device, and the spectral signal is connected to the computer through the PCI interface card after its A/D conversion.

6) Laser power meter 8

The laser power meter 8 is used to measure the energy value of the laser pulse incident on the probe to obtain the normalized standard value required for the quantification process. The spectral response range is 0.2-1.65μm, the measurement power range is 1mW-10kW, and the measurement accuracy is ±0.5%.

7) Computer 9

The computer 9 is equipped with spectral data and laser power data acquisition and processing software. The spectrometer is connected to the computer through the PCI interface card, the laser power meter is connected to the computer through the RS232 interface, and the data acquisition and processing software realizes the collection of spectral data and laser pulse energy data, as well as the realization of the quantification process, and the elements to be measured in the standard sample can be obtained The quantitative calibration curve can realize the quantitative detection of analyte elements in unknown samples.

Claims (2)

1. soil heavy metal pollution quantification detecting system, it comprises laser instrument, converging optical lens, fibre-optical probe, optical fiber, spectrometer and computing machine, it is characterized in that: it also has beam splitter (2) and laser powermeter (8), the laser beam that laser instrument (1) is launched is at first by beam splitter (2), wherein 10% pulsed laser energy is reflexed on the probe of laser powermeter (8) by beam splitter (2), other 90% pulsed laser energy sees through beam splitter (2), be focused at tested sample (4) surface via convergent mirror (3) again and bring out the generation plasma, laser powermeter (8) is measured 10% pulsed laser energy and energy value is passed to computing machine (9) as the normalization standard value by interface circuit, the atomic spectrum that laser induced plasma gives off is collected by fibre-optical probe (5), be coupled in the entrance slit of spectrometer (7) by optical fiber (6), spectral analysis data is passed to computing machine (9) by interface circuit, spectroscopic data and laser power data acquisition process software are housed in the computing machine (9), data acquisition and processing (DAP) software is realized the collection and the processing of spectroscopic data and pulsed laser energy data, the spectral intensity values that records is normalized to the intensity level of incident laser.

2. soil heavy metal pollution quantification detection method, it is characterized in that: it comprises the steps:

A. set up the quantification calibration curve, a series of specific model master samples are measured, the content value of survey data and this element is formed (an x, y) coordinate, y represents the atomic spectral line intensity level after the normalization, x representative element content value, and simulate a calibration curve at element to be measured;

B. measure the normalization atomic spectrum intensity level of element to be measured in the tested sample (4);

C. the normalized atomic spectral line intensity level of element to be measured in the tested sample that records according to step B (4) from the quantification calibration curve, obtains the corresponding content value of this element, realizes the quantification of element-specific is detected.

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