Laser induced breakdown spectroscopy (LIBS) is used to measure the emission spectrum of the plasma formed by the focused intense laser beam.A typical LIBS system consists of a laser, a spectrometer with wide wavelength coverage and high sensitivity, and a detector with fast response and time sampling.After the above instruments are connected with the computer, the data can be processed quickly.Therefore, LIBS technology as a spectral analysis technology is easy to operate.The instruments in the laser-induced breakdown spectroscopy testing system are mainly divided into four parts: laser source, beam transmission system, light splitting system, signal receiving and acquisition system.For the instruments used in this paper, this chapter describes the related principles of the main instruments involved and the performance of each system.At the end of this chapter, sample preparation and reagent selection are briefly introduced.
The light source is to provide enough energy to excite the sample, so it must have enough output energy and stability.Nd: YAG and excimer pulse lasers are commonly used in laser-induced breakdown spectroscopy experiments. Generally, the pulse width is less than 20ns and the pulse energy fluctuation is small.The output wavelength of Nd: YAG laser is 1064nm, the pulse width is 10ns, and the power density of focusing point is more than 1GWcm-2 after focusing by lens.Excimer laser and Nd: YAG laser’s double frequency 532nm and triple frequency 355nm ‚ are often used as excitation sources in LIBS devices, and the output laser is located in the ultraviolet and visible range.
The main laser source used in this experiment is the Nd: YAG laser produced by quantul company in the United States, and the model is VIVRANTB35511.After nonlinear interaction with the second harmonic crystal, 532 nm output can be obtained.The repetition rate of laser pulse is 10Hz and the pulse width is about 8-10ns.When the laser output is 532nm, the maximum energy is 280mJ.
The beam transmission system is mainly composed of a prism and two focusing lenses. Nd: YAG laser output 1o64nm laser through the second harmonic crystal, the output 532nm laser pulse. Before the laser beam is focused by a lens with a focal length of 100 mm ‚ diameter of 25 mm and vertically incident on the sample surface, the laser beam should be guided to the focusing lens through a prism according to the experimental optical path. The focusing lens can move back and forth along the direction of the laser beam.
The task of the spectrometer is to separate the monochromatic light incident into monochromatic light through the built-in grating. According to the different requirements of the test, spectrometer and double grating monochromator are used in the spectroscopic system. The spectral analysis system consists of two parts: monochromator or spectrometer as a spectroscopic component, photomultiplier tube, or charge-coupled device(CCD) as detection instrument. Because it can obtain a large detection range of spectrum at one time, and used together with Intensified CCD/ICCD (ICCD), it has become a classic combination of laser-induced breakdown spectroscopy detection systems and is widely used.
The task of the spectrometer is to divide the light, that is, to decompose the compound light with multiple wavelengths.Through the decomposition, the intensity distribution of different wavelengths is arranged with the wavelength as the coordinate.Spectrometer is the basic equipment to study the absorption and emission of light and the interaction between light and matter.Modern spectrometers have made great progress in spectral recording and spectroscopic thinking, which depends on the development of detection devices and computer technology. The combination of spectral recording and processing has achieved a high degree of automation.
Table 1 Indicators of main parameters of raster for spectrometer
|Raster Model||Scratch Density||Blaze Wavelength||Optimal wavelength range||Resolution (at 500nm)|
As shown in Figure 1, diffraction grating G is used in the spectrometer. The incident slit S1 is located on the focal plane of the concave mirror M2. The incident light passing through the slit S1 is reflected by the mirror M2 and then projected onto the mirror M2. After being reflected by the mirror M2, the incident light is projected onto the grating G as a parallel beam. The grating g disperses the incident light into many parallel monochromatic lights and shoots them onto the concave mirror M3, and M3 converges these monochromatic lights.M4 is a plane mirror which makes the light beam turn. The exit slit S2 is located on the focal plane of M3.When the grating g rotates around its rotation center, different wavelengths of outgoing beams can be obtained at the exit slit. A photodetector is placed at the exit slit to receive the outgoing beam.
The grating spectrometer decomposes the compound light with multiple wavelengths, and the intensity distribution is arranged according to the wavelength after decomposition. Therefore, a complete spectral information can be obtained at the exit of the monochromator at the same time. In the experiment, by setting the software of the spectrometer, a suitable grating is selected according to the requirements of the experiment to obtain a certain spectral measurement range and resolution. At the exit of the spectrometer, the CCD and other detection instruments collect the spectrum at one time.
The number of imaging pixels of the CCD array used for visible and ultraviolet spectrum detection in the laboratory is 1340 × 400, and the size of each pixel is 20μm×20μm. When the selected notch of blazed grating is 150g/mm, the spectral width range measured by Spectro pro-500i spectrometer (F = 500mm) is related to the grating density and CCD width, which is expressed in M. M=DX(1)
Where D is the reciprocal of the linear dispersion and X is the imaging width of the array.The reciprocal of the linear dispersion, in nm / mm, is calculated and multiplied by the imaging width of the array to get the detection range M.
The line resolution is proportional to the focal length and scattering rate of the focusing imaging system. For atomic emission spectroscopy, a narrow slit is usually used in qualitative analysis, which can improve the resolution and make the adjacent spectral lines separated clearly.In addition, if the background emission is too strong, the slit width should be appropriately reduced.In general, the slit width should be as large as possible without reducing the absorbance.
The linear dispersion Dl is the distance between two spectral lines with wavelength difference of △λ in the imaging plane of the spectrometer, and its unit is mm/nm.Linear dispersion is defined as Dl=dl/dλ=f‘m/(d cosβ ) (2)
Therefore, the linear dispersion D1 is proportional to the focal length and the dispersion of the focusing imaging system.The reciprocal of linear dispersion, dy / dl, is in nm/mm. The smaller the value is, the larger the dispersion is M=dcosβ/(mf‘) X=(1/150)/500×20μm×1340≈357.3nm
The spectrometer collects electromagnetic radiation in a wide range, in order to get the maximum number of spectral lines of analytical elements.The response range of spectrometer is from 170nm (far ultraviolet) to 1100nm (near infrared), which is also the wavelength response range of CCD. All elements have characteristic spectral lines in this range.Resolution is also an important factor in the selection of spectrometer. High resolution can distinguish two closely connected spectral lines, reduce interference and increase selectivity.The resolution of spectrometer is more important when the analysis sample contains many elements.
2 Double grating monochromator
In order to get higher resolution, most of the spectra were collected by using double grating monochromator （JOBIN YVON HRD1, Division d’instruments SA, notch density 1200g/mm).Double grating monochromator is a combination of two monochromators. The exit grating of the first monochromator is the entrance slit of the second monochromator. There are two ways to combine two monochromators, that is, dispersion addition or dispersion subtraction.The dispersion and resolution are improved simultaneously. The dispersion subtraction type can effectively eliminate the interference of stray light because the two gratings used as dispersion rotate in the opposite direction, but the dispersion rate and resolution are the same as that of a single monochromator. The Figure 2 of the additive double grating monochromator is as follows.
The signal receiving system consists of detector and data acquisition device.Similarly, according to different experimental requirements and different instruments used in the spectroscopic system, the signal receiving system is also different.The signal receiving system is composed of charge coupled detector (CCD) and its controller (ST133).When the spectrometer is a double grating monochromator, the signal receiving system of this experiment is a combination of photomultiplier and sampling averager, which can complete a small range of fine detection and meet the requirements of spectral characteristics analysis.
1 Charge coupled detector
Charge coupled device (CCD) is a kind of device which expresses the amount of light by the amount of charge and transfers the amount of charge by coupling.The basic unit of charge coupled detector is MOS capacitor, which is commonly known as metal insulator semiconductor capacitor.CCD has wide spectral response and high quantum efficiency.The common response wavelength range is 400nm-1000ns, and the peak value of the response curve is 500nm-700nm.Under normal working conditions, CCD detector pixels are exposed at the same time, which has a wide dynamic response range and ideal response characteristics.
2 Photomultiplier tube
Photomultiplier tube (PMT) is a kind of photoelectric converter that can convert weak optical signals into measurable electrical signals. It has high sensitivity and ultrafast time response. The PMT used in this experiment is made by the Hamamatsu company. Its model is R376, the spectral range is 160nm-850nm, and the current gain is 5.3×105. the high voltage of the model R376 photomultiplier tube can be adjusted in the range of 0-1000v. Fig. 3 is the physical diagram of a typical photomultiplier tube.
Using photomultiplier tube, the optical signal is transformed into an electrical signal and then amplified. Finally, the information of the optical signal is recorded through the detection of the electrical signal.Photomultiplier tube (PMT) is a kind of vacuum photoemission detector to detect weak light signal.The secondary electron emission of the middle electrode of the photocell is used to amplify the photocurrent, and the magnification is as high as 103-108. Therefore, it has the advantages of high signal-to-noise ratio, high sensitivity, good linear photoelectric characteristics, good frequency characteristics, stable operation, long life, and so on. It is one of the most commonly used photodetectors in laser spectrum research. The main sources of noise in photomultiplier are dark current, shot noise of photocurrent, thermal noise of load resistance and background noise of light incidence. Fig. 5 is the spectral response curve of the photomultiplier tube. The wavelength response range of the photomultiplier tube is ultraviolet and visible, and the quantum efficiency is high between 300-700nm.
The main characteristic parameters of a photomultiplier tube are as follows
Integral sensitivity: the sensitivity of the photomultiplier tube refers to the sensitivity RA of the anode, which is related to the sensitivity RK of photocathode, RA = GRK. When the incident light energy is too high, the light will lead to the linear deterioration of the measurement, reduce the service life, and even cause the electrode to burn. Therefore, the incident light flux must be strictly controlled to ensure that the anode sensitivity of R376 is 80A / lm and the cathode sensitivity is 150μA / lm without strong light irradiation under pressure.
Magnification: the ratio of anode signal current iA to cathode signal current iK.The current gain of R376 is 5.3 x 105.
Photoelectric characteristics: it is the relationship between the anode current IA and the luminous flux φreceived by the photocathode. For the photomultiplier tube with good performance, the linear deviation is less than 3% in the range of luminous flux 10-10—10-4.
Time characteristics: refers to the photoelectron from the photocathode to the anode time, called the photomultiplier tube transit time.The transit time of R376 is 60ns.
Stability: refers to the change of anode current with working time.Generally there are two processes, the initial “build-up time” process, the current changes rapidly with time, generally ranging from a few minutes to dozens of minutes.Then it goes into slow change and tends to be stable.Therefore, accurate experiments must be carried out in a stable state.
After the sample is excited by laser, the spectral information of plasma is generated, and the monochromatic light signal is received by detector after being separated by monochromator.When the monochromator is scanned, the spectral information of different wavelengths can be recorded.The measurement methods include DC measurement, AC measurement and pulse measurement, which determines the type of measuring instruments used.
Boxcar integrator is a special instrument for recovering weak signal waveform which is annihilated by noise.As early as the early 1950s, Dawson, a British neurologist, put forward the concept of boxcar. In 1955, Holcomb put forward the sampling principle, which revealed the rules that must be followed in the process of sampling to fully characterize the original signal and reproduce the original signal;In 1962, Klein, Lab. of the University of California, laureates, realized it with electronic technology, which is called boxcar integrator. Its basic concept is that sampling and integral averaging are carried out at the same time, and the waveform is slowly recovered and recorded by moving the sampling gate.The signal average integration technology is used to sample and average the signal at one time and accumulate synchronously.The signal enhancement depends on the sampling number n, while the random noise only increases by N1/2.
The sampling integrator consists of a gate control circuit and a gate integral circuit, and its core is a sampling averager.In the spectrum measurement, the sampling integral is applied to the spectrum detection system. A pulse synchronized with the signal is used to trigger the gating circuit of the sampling integrator. The sampling point is controlled by an adjustable time delay, and the sampling time is controlled by the pulse width TG. The signal with noise is integrated.
According to the control mode and test requirements of the sampling averager, the sampling integrator can be divided into two working modes: fixed-point and scanning.Usually, there are two working modes in the same instrument.The fixed-point working mode is that the measured signal is sent to the sampling gate after pre amplification.The trigger signal (synchronized with the measured signal) triggers the time base generator.By comparing the time base signal with the time delay voltage, a fixed time delay trigger pulse is obtained to trigger the gate width generator, which generates a sampling pulse with a fixed time delay and a certain pulse width, and then drives the sampling gate.A certain instantaneous value of the input signal is sampled through the sampling gate, and the integrator is used to accumulate and average.After M times of accumulation, the signal-to-noise ratio is improved as follows: SNIR= √m..