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Matrix Assisted Laser Desorption Ionization

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The generation of laser desorption / ionization (LDI) method can be traced back to the late 1960s.Like SIMS-MSI imaging, LDI-MSI needs to focus the laser on the sample and focus the spot to the diffraction limit to achieve the best spatial resolution.Early laser desorption / ionization mass spectrometers focused pulsed laser on thin film samples in vacuum.There are two ways to realize it: transmission focusing (laser focusing on the back of the sample) or reflection focusing (laser focusing on the front of the sample).The LDI-TOF mass spectrometer was first designed by Fenner et al. In 1966.The instrument uses a 694 nm ruby laser to focus on thin metal foil or glass to sputter materials.The focal spot size is 50μm. The resolution of the mass spectrum is 30. Because of the thermal effect of laser sputtering, the spot on the sample is enlarged.

LDI-MSI research began in the 1970s. Hillenkamp and his colleagues improved Fenner’s design, applied lasers to the field of mass spectrometry imaging, and developed Laser Microprobe Mass Spectrometer (LMMS) and Laser Microprobe Mass Analyzer (LAMMA).They doubled the frequency of a 694 nm ruby laser to generate a 347 nm ultraviolet (UV) laser, focused it on a solid sample (without matrix) on a glass slide through a lens, and the glass slide was also used as a vacuum window of a mass spectrometer.Using different mass analyzers (such as TOF and FTICR) to collect ion signals, ions with mass m / Z less than 200 Da can be detected.The system uses a lens with high numerical aperture, which is similar to the design of a microscope system. The diameter of the laser focus spot is up to 500 nm.

In the late 1970s, the design of hillenkamp et al. Was later developed into a commercial instrument – laser microprobe mass analyzer, Lamma 500 (Leybold Heraeus).Because the sample is required to be thick, then the Lamma 1000 was introduced. The light source used is the third harmonic generation (355 nm) or fourth harmonic generation (266 nm) of the 1064 nm Nd: YAG infrared laser. It is focused by a 50 times lens with a numerical aperture of 0.6, and the imaging resolution can reach 2.5μm。The emergence and development of Lamma 1000 has laid a good foundation for the development of MALDI imaging method.This method has been used in the analysis of inorganic and biological samples, such as single bacterial cell analysis, and can detect small molecules of M / Z 150.Since then, Fourier transform ion cyclotron resonance (FTICR) mass spectrometer and laser desorption ion probe instrument have been developed.The traditional reflection focusing method is used to get 5 ~ 10μIt can be used for the analysis of mouse embryonic fibroblasts and extracellular biological samples.In 1985, it was reported that LDI mass spectral imaging could obtain mass spectral imaging with a mass range of about 1000 DA and a lateral resolution of about 1μm, similar to that of TOF SIMS.

In the mid-1980s, the emergence of MALDI expanded the application of LDI in mass spectrometry.In 1997, Caprioli’s team focused a 337 NM NITROGEN LASER AT ~ 25m to analyze samples with either a matrix of 2,5-dihydroxybenzoic acid (DHB) or a cyanide-4-hydroxycinnamic acid (CHCA)2,5-dihydroxybenzoic acid, by associating Maldi mass spectrometry with sample scanning, mass spectrometry imaging of multiple biological samples has been realized. After that, the quality range of MSI detection methods based on laser becomes wider and wider. Coupled with the continuous development and improvement of mass spectrometry imaging technology and instruments, MALDI-MSI is widely used in the field of mass spectrometry imaging.At the same time, due to the development of tandem mass spectrometry and proteomics, mass spectrometry imaging has become an effective method to analyze biomolecules.It can be used to study the recognition and spatial localization of elements, metabolites, lipids, drugs, even peptides and proteins in biological tissues.The current MALDI-MSI instrument can detect the mass spectrum with molecular mass up to 30 kDa and image the mass spectrum of proteins with molecular mass more than 20 kDa (the lateral resolution is 5μm）。The lateral resolution of commercial MALDI instruments is increasing from 50μM direction 10μM continuous improvement.

With the development of ambient and atmospheric ionization methods, a variety of new MSI methods have been developed.Most of these imaging methods use pulsed laser to desorb / ionize the sample.For example, MALDI can carry out experiments under normal pressure;Matrix free MS imaging can be realized by using infrared laser or short pulse laser;New matrix materials and conditions are used to produce highly charged ions;Laser sputtering combined with ionization or chemical ionization after electrospray.These ambient / atmospheric laser ionization methods provide new ideas for direct and rapid tissue mass spectrometry imaging.

All examples of MSI methods described above are in microprobe mode, i.e. the mass spectra of each point on the sample surface are collected sequentially.Another method is microscope mode MSI. In this mode, the ions generated in the sample area and the spatial information of the corresponding collection area are detected simultaneously, as shown in Figure 2.4.The original design of microscope mode MALDI-MSI is based on TOF-SIMS mass spectrometry.The mass spectrometer was combined with nitrogen laser at 337 nm, equipped with electrostatic analyzer, dual microchannel plate, fluorescent screen detector assembly and CCD camera.The diameter of the spot focused on the sample is 200μm. The light intensity energy density is 20 MJ / cm2.Because the detector can not detect all the ions fast enough, it can only collect the ion signals of interest.When the laser irradiates the sample, the ions from the sample are amplified on the detector to realize mass spectrometry imaging.The system can get a lateral resolution of 4μM peptide and protein samples.

Using mid infrared laser in microscope mode MSI can realize mass spectrometry imaging below the diffraction limit of laser.For example, peptide imaging with a lateral resolution of 4μm can be achieved using a 2.94μm wavelength Er: Yag Laser. Microscope mode mass spectrometry imaging can be integrated with electronics-related technologies, but it needs expensive professional instruments. However, when the microscope mode is applied to SIMS, the ion beam can be focused by a method similar to focusing the beam.With the rapid development of new ionization and sampling methods, environmental LDI-MSI has gradually become a new field.This method generally uses infrared laser, because the infrared wavelength is long, which makes it at a disadvantage in the lateral resolution.However, if better optical configuration such as transmission focusing and high numerical aperture objective is adopted, the infrared laser imaging performance can be closer to the requirements of high resolution MSI.

The improvement of lateral resolution in mass spectrometry imaging experiment is realized by reducing the spot size of laser beam.The minimum spot diameter D of a focused laser beam can be expressed as
(d=λ/2nsinθ=λ/2NA  (1-1) )

Among λ Is the laser wavelength, n is the refractive index of the medium andθ is the half angle of the beam leaving the lens.nsin is known as the numerical aperture (NA) of a lens. In high-quality optical systems, the number can reach about 1.5.According to the above idea, the coaxial objective lens can be used in MALDI to reduce the laser spot size to 1μm or less.However, this kind of coaxial objective lens is usually large and must work at a short distance, which requires the lens to be very close to the sample and is not suitable for ion detection after laser desorption and sputtering.It is difficult to realize the desorption and ionization of sample molecules with traditional high numerical aperture objective because the lens itself blocks the flight path of ions from the sample.This problem is usually solved by using a lens with a central hole.This kind of imaging mass spectrometer is reported in the literature. It uses a central hole objective with numerical aperture of 0.6, inner diameter of 6 mm and working distance of 16 mm.The objective is placed in a vacuum chamber above the sample, and the focused laser is guided to the sample.The desorbed and ionized ions pass through the central hole of the objective lens and accelerate into the flight tube of the mass spectrometer.The lateral resolution of the instrument can reach 0.6μm. The step size is 0.25μm。Controlling the deposition, sublimation and recrystallization of the matrix can achieve high spatial resolution, such as the lateral resolution less than 10μm. Mass spectrum imaging of protein with mass up to 27 kDa andMass spectral imaging of biological samples with lateral resolution of 2μm.

For a specific laser wavelength, the diffraction limit corresponding to this wavelength has been given by the above formula 2-1.However, in the experiment, many parameters can not meet the requirements of diffraction limit, such as desorption laser has a certain divergence angle from the laser, spherical and aspherical lens system processing and installation errors, which increase the difficulty of obtaining diffraction limit spot.In addition, in addition to optimizing the processing error and assembly of the lens surface, the focusing effect can also be optimized by beam shaping.In practical experiments, the diameter D of the focused laser beam can be expressed a D(f)=(4fλM^2)/πD(0)   (1-2)

Among λ is the laser wavelength, f is the focal length of the lens, M2 is the beam quality factor, and D (0) is the unfocused laser beam diameter.With the improvement of beam quality, the beam quality factor decreases.For an ideal Gaussian beam, the beam quality factor M2 is 1.When low power CW laser works in transverse oscillation mode, M2 can be as low as 1.1, while high power multimode laser M2 can be more than 10.It can be seen from the above formula that the laser wavelength, the focal length of the lens and the M2 factor have a great influence on the final focusing of the light source.Among these three factors, the laser with shorter wavelength, especially the light source with wavelength less than 150nm, is more difficult to produce.Recently, Ilya Kuznetsov et al. Achieved a mass spectrum imaging with a transverse resolution of 75 nm by using a 46.9 nm EUV light source.

At present, the reflection focusing scheme is mostly used in mass spectrometry imaging, that is, the laser spot is focused on the front of the sample.In 1997, this reflective focusing design geometry was reported.This design is usually realized by Schwarzschild objective lens.Schwarzschild objective is a group of mirrors. A convex lens with a smaller diameter is paired with a concave auxiliary lens with a larger diameter and a central aperture.The achromatic objective has a long working distance, which can make the spatial resolution less than 10μm。So as to achieve better laser desorption ionization.For example, a FTICR mass spectrometer can obtain a spatial resolution of 1μm by focusing its laser pulses through a Schwarzschild objective.In this system, the sample surface is analyzed by desorption / ionization with 337 nm laser (nitrogen laser). For some applications, 213 nm laser is used for post ionization (5-fold frequency of Nd: YAG).Similarly, t is also possible to obtain transverse resolution less than 1μm by using the line scan mode in the Schwarzschild microscopic system.In this system, a 524 nm Nd: YLF or Nd: YAG Frequency Doubled 532 nm laser is used to desorb / ionize the sample, and a 800 nm Ti: sapphire femtosecond laser is used to post ionize the sample. The mass spectrum image of rhodamine dye deposited on the copper grid is obtained, and its transverse resolution is less than 2μm。

In LDI-MSI, besides reflective focusing scheme, transmission focusing scheme is also used.Like laser microprobe, transmission focusing needs larger numerical aperture, so it is necessary to redesign maldi-msi instrument.Thinner tissue sections must also be used because the laser must penetrate the sample to desorb the matrix and analyte on the surface of the sample.Lens focusing is different from reflection focusing, which tends to desorb and ionize larger particles, which may reduce ion efficiency.Nevertheless, the maldi-msi instrument with transmission design has been developed.A Commercial MALDI-TOF Mass Spectrometer uses a 355nm laser to image 5μm thick tissue sections and individual mammalian cells through a 100x microscope objective.In the transmission focusing mode, the minimum laser diameter corresponding to mass spectrum imaging is 1μm. By oversampling, the step size is as low as 0.5μm. For example, mass spectrum imaging of biological tissue with laser Facula of 2μm and step length of 2μm has been reported,The ND: YLF LASER is focused to 1 m and the scanning step is 2.5μm.It is worth noting that the transmission focusing scheme is not suitable for mass spectrometry imaging with VUV light source, because the interaction depth between VUV light and sample is very shallow, less than 100 nm.

MALDI-MSI is now widely used in LDI-MSI.People try to improve the quality of laser beam in MALDI system by transmitting laser through fiber, but this method is not suitable for high power pulsed laser.It is a good choice to use pinhole aperture in MALDI-MSI.A good way is to place pinholes in the path of laser propagation.Commercial MALDI-MSI instruments can perform mass spectrometry imaging with 5 m lateral resolution through 25μm ceramic pinholes.In MALDI-MSI, Gaussian beams can also be focused through pinholes and aspheric lenses to achieve high spatial resolution.In order to improve the lateral resolution of MALDI-MSI, besides optimizing the laser focusing scheme, there are also oversampling methods.

The oversampling method does not need large-scale modification of MALSI-MSI instrument, so the transverse resolution can be smaller than the focal spot diameter of laser beam.In the oversampling, the scanning step is set lower than the laser beam diameter to obtain higher spatial resolution mass spectrometry imaging.A 100μm ~ 200μm laser spot mass spectral imaging system can obtain 40μm transverse resolution by setting the step length of 25μm to scan samples. A recent paper reported that 5M resolution mass spectral imaging can be achieved by over sampling the laser beam diameter of 20.However, oversampling takes a long time to collect data, and it is easy to cause signal loss.

Another method is sample stretching.The tissue sections were deposited on glass beads embedded in paraffin film and stretched evenly in two dimensions.In order to maintain the stability of the paraffin film, the paraffin film is connected to the glass slide.The stretching method has no diffusion problem and can increase the spatial resolution and extract analytes more effectively.

Laser Induced Breakdown Spectroscopy (LIBS)

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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.

1 spectrometer
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 ModelScratch DensityBlaze WavelengthOptimal wavelength rangeResolution (at 500nm)
1-015-300150g/mm300nm200-500nm13nm/mm
1-015-500150g/mm500nm330-950nm13nm/mm
1-030-1300g/mm1μm650-1800nm6.5nm/mm
1-060-1.6600g/mm1.6μm1-2.4μm3.2nm/mm
1-120-3001200g/mm300nm200-500nm1.5nm/mm
1-120-HVIS1200g/mmholographic450-1400nm1.5nm/mm

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 cos⁡β )  (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..

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Pretium lorem primis senectus habitasse lectus scelerisque donec ultricies tortor adipiscing fusce morbi volutpat pellentesque consectetur risus molestie curae malesuada. Dignissim lacus convallis massa mauris enim mattis magnis senectus montes mollis taciti phasellus accumsan bibendum semper blandit suspendisse faucibus nibh metus lobortis morbi cras magna vivamus per risus fermentum. Dapibus imperdiet praesent magnis parturient.

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