measurement system for high-sensitivity libs analysis using iccd camera in labview environment

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Page 1: Measurement system for high-sensitivity LIBS analysis using ICCD camera in LabVIEW environment

This content has been downloaded from IOPscience. Please scroll down to see the full text.

Download details:

IP Address: 130.60.206.42

This content was downloaded on 01/07/2014 at 13:26

Please note that terms and conditions apply.

Measurement system for high-sensitivity LIBS analysis using ICCD camera in LabVIEW

environment

View the table of contents for this issue, or go to the journal homepage for more

2014 JINST 9 P06010

(http://iopscience.iop.org/1748-0221/9/06/P06010)

Home Search Collections Journals About Contact us My IOPscience

Page 2: Measurement system for high-sensitivity LIBS analysis using ICCD camera in LabVIEW environment

2014 JINST 9 P06010

PUBLISHED BY IOP PUBLISHING FOR SISSA MEDIALAB

RECEIVED: March 19, 2014ACCEPTED: May 2, 2014

PUBLISHED: June 9, 2014

Measurement system for high-sensitivity LIBSanalysis using ICCD camera in LabVIEW environment

S.M. Zaytsev,1 A.M. Popov, N.B. Zorov and T.A. Labutin

Lomonosov Moscow State University,Leninskie gory, 1, bld. 3, Moscow, Russia

E-mail: [email protected]

ABSTRACT: A measurement system based on ultrafast (up to 10 ns time resolution) intensifiedCCD detector “Nanogate-2V” (Nanoscan, Russia) was developed for high-sensitivity analysis byLaser-Induced Breakdown Spectrometry (LIBS). LabVIEW environment provided a high level ofcompatibility with variety of electronic instruments and an easy development of user interface,while Visual Studio environment was used for creation of LabVIEW compatible dll library withthe use of “Nanogate-2V” SDK. The program for camera management and laser-induced plasmaspectra registration was created with the use of Call Library Node in LabVIEW. An algorithm ofintegration of the second device ADC “PCI-9812” (ADLINK) to the measurement system wasproposed and successfully implemented. This allowed simultaneous registration of emission andacoustic signals under laser ablation. The measured resolving power of spectrometer-ICCD systemwas equal to 12000 at 632 nm. An electron density of laser plasma was estimated with the use ofH-α Balmer line. Steel spectra obtained at different delays were used for selection of the optimalconditions for manganese analytical signal registration. The feature of accumulation of spectrafrom several laser pulses was shown. The accumulation allowed reliable observation of silversignal at 328.07 nm in the LIBS spectra of soil (CAg = 4.5 ppm). Finally, the correlation betweenacoustic and emission signals of plasma was found. Thus, technical possibilities of the developedLIBS system were demonstrated both for plasma diagnostics and analytical measurements.

KEYWORDS: Plasma generation (laser-produced, RF, x ray-produced); Plasma diagnostics - in-terferometry, spectroscopy and imaging; Spectrometers; Detector control systems (detector andexperiment monitoring and slow-control systems, architecture, hardware, algorithms, databases)

1Corresponding author.

c© 2014 IOP Publishing Ltd and Sissa Medialab srl doi:10.1088/1748-0221/9/06/P06010

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Contents

1 Introduction 1

2 Experimental setup 2

3 Software development 53.1 Software for ICCD camera “Nanogate-2V” 53.2 Integration of ADC “PCI-9812” 93.3 Data preprocessing program 11

4 Hardware adjustment 12

5 Applications 14

6 Conclusions 17

1 Introduction

Laser-Induced Breakdown Spectroscopy (LIBS) is a kind of atomic emission spectroscopy where afocused high-power laser radiation evaporates a sample with production of luminous plasma, whichis both the atomization and excitation source. The growing number of monographs published in thelast 15 years [1–4] demonstrates the potential of the method. The main advantages of LIBS are thehigh throughput, multi-element (from hydrogen to uranium), and quasi non-destructive analysis.Moreover, remote and local analysis along with the possibility of miniaturization became avail-able due to progress in fundamentals of LIBS and advancement in laser and detector technology.Numerous recent works aimed to develop mobile or portable LIBS systems for a wide range oftasks. Whitehouse et al. developed the mobile laser probe with a long optical fiber (75 m) for thecontrol of copper content in 316H austenitic stainless steel superheater bifurcation tubing duringthe routine nuclear reactor outage program [5], as well as stand-off LIBS spectrometer for analysisof contamination of the baskets for recycling of nuclear plants [6]. Anglos presented the LIBS sys-tem for the field analysis and dating of archeology objects [7]. The possibilities of stand-off LIBSfor remote identification of explosives were demonstrated by Laserna et al. [8]; the inline controlsystem of steels manufacturing is described in the book of Noll [4]. Marwan Technology s.r.l. incooperation with the Laboratory of Applied Laser Spectroscopy in Pisa developed a mobile devicefor LIBS to perform calibration-free analysis [9].

To improve the efficiency of LIBS analysis, a high level of automation of process control andcompatibility of apparatus with existing industry standards are needed. Measurement system forLIBS should combine the laser and detector control, program for data acquisition, and algorithmsfor data processing. Nowadays, the ICCD arrays (charge-coupled device with the image intensifier)

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are widely used in LIBS as detectors. They increase weak signals and provide a registration of emis-sion spectra of non-stationary and short-lived laser-induced plasma with ns time resolution, whichensures the achievement of maximum of the signal-to-background and signal-to-noise ratio. TheICCD cameras are easily combined with both Czerny-Turner and echelle spectrographs, which arethe most prevalent in LIBS. The cross-dispersed echelle design has the advantage of a broad-bandspectrum (range 200–900 nm) to be recorded with high resolution (∼0.05 nm) by one measurement(single laser shot). The LIBS laboratory setup utilizing echelle spectrograph, equipped with ICCDsystem (Andor Mechelle ME5000), was developed by Unnikrishnan et al. [10]. It was calibratedby sensitivity and by wavelength with the use of certified lamps. Unfortunately, the sensitivityof the system does not look attractive because the difference between spectrum of blank soil andhighly-contaminated one (400 ppm of copper) was negligible.

Myers et al. [11] successfully used the LabVIEW software environment for integration ofthe AvaSoft R© spectra software for Avantes spectrometer and Kigre laser software. Three com-bined Stellarnet Czerny-Turner spectrometers covered the wide spectral range (200–800 nm) wereused for spectral measurements, but the spectral resolution of such system was relatively low(about 0.2 nm–0.4 nm). Thus, only qualitative and semi-quantitative analysis of main componentswas possible.

General procedure of processing spectra in LIBS was presented by Sobron and co-workers [12]. They cover different methods for background calculation and its removal [13, 14],normalization of analytical signal, excluding faults from the series of measurements, averagingthe spectra from several laser pulses, and deconvolution of the spectra. The main shortcomings ofLIBS technique are the matrix effect and pulse-to-pulse fluctuations of analytical signal. Differentnormalization techniques with sophisticated signal processing may be useful for reducing of theseeffects [15], for example, the applying of acoustic signal from the plasma to compensate shot-to-shot fluctuations and the decrease of the emission signal during the drilling through the glaze [16].Obviously, the possibility of an implementation of new devices should be given in the main soft-ware/hardware platform. For the development of this platform LabVIEW software environmentlooks preferable, because it provides easy access to a variety of features for easy creation of graph-ical user interface and allows the combination of several instruments in one project. Moreover,nowadays many manufacturers of ADC, cameras, lasers and other devices provide their productswith LabVIEW libraries (named also “LabVIEW driver”), which can be easily integrated into theexisting program. LabVIEW environment is the widely used both in science and industry for de-vice management and data acquisition [17–19]. Programs in LabVIEW environment are namedvirtual instruments (VIs). The advantages of LabVIEW allow users to create a subVIs and easilyadd them to the main project.

Thus, the aim of the present work was the development of high sensitivity platform for LIBSto provide an easy integration of several devices to one measurement system in the LabVIEWsoftware environment. To provide the highest sensitivity of our LIBS setup we used Czerny-Turnerspectrometer because its high sensitivity.

2 Experimental setup

We used a commercially available devices and accessories for the assembling of our laboratorysetup for LIBS, which is described below. The scheme of the experimental setup is shown in

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Figure 1. The scheme of the experimental setup.

figure 1. Q-switched Nd:YAG lamp pumped laser (1) (Lotis Tii LS-2134UTF, Belarus, for detailssee website [20]) was used for producing plasma. Laser can produce fundamental, second, third,and fourth harmonics (1064 nm up to 270 mJ/pulse, 532 nm up to 170 mJ/pulse, 355 nm up to60 mJ/pulse and 266 nm up to 40 mJ/pulse, respectively) with the typical pulse width τ0.5 6 8 ns.A special optical attenuator (3) (Lotis Tii) was applied for laser energy adjustment. A PM100Dpower and energy meter with ES220C pyroelectric sensor (Thorlabs) was used for laser energymeasurements. A laser radiation was directed to an achromatic air-spaced doublet (6) (Thorlabs)with F = 150 mm by a dielectric mirror (4) with the high reflection coefficient (R>99%) and a set ofright-angle UV fused-silica prisms (5). A laser beam (d66.3 mm) was focused perpendicularly tothe sample, positioning by linear mechanical stage (8), to ablate a material and produce plasma (7).We varied position of the focusing doublet (6) over the sample surface to change spot size and laserfluence. The emission from the central part of laser-induced plasma was projected by UV fusedsilica two-lens condenser (9) with the decrease of an image 2:1 onto the slit (25 µm) of the 32 cmCzerny-Turner spectrometer “HR 320” (10) (ISA Instruments, U.S.A.), equipped with a diffractiongrating with 1800 grooves/mm and blaze angle 10◦. The center of the laser plume, lenses, and slitwere aligned coaxially. Spectra was registered by ICCD camera “Nanogate-2V” (11) (Nanoscan,Russia, table 1a) with PCI controller (12). The sensitivity curve of the photocathode is shown infigure 2. The Hg, Ga, and Na plasma lamps were used for wavelength calibration of the spectro-meter. We also registered acoustic signal produced during laser ablation by a condenser micro-phone (13). An analogue signal from microphone was amplified (14) and, thereafter, was digitizedby ADC “PCI-9812” (ADLINK) (15) (table 1b). A Q-switch sync out signal from laser powersupply (2) triggered both ICCD camera (11) and ADLINK’s ADC card (15). The special softwarein LabVIEW environment was developed for management of the camera and ADC “PCI-9812” anddata acquisition to PC (16). Special LabVIEW VI was developed for the spectrum pre-processing.

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Table 1. Devices specifications.

a) Nanogate-2V Value

Spectral range, nm 210 – 850Diameter of intensifier, mm 18Spatial resolution of intensifier > 32 line pairs/mmGate pulse duration 10 ns – 20 µs, step 10 nsDelay of registration 40 ns – 1300 µs, step 5 nsJitter, ns 0.2Gain of intensifier, V 501 – 1011, step 1Phosphor afterglow, ms 7CCD Sony ICX285ALNumber of effective pixels 1390 (H)×1040 (V)CCD exposure period 3 µs – 502 sPixel size, µ 6.45Maximum frame rate, Hz 7A/D resolution, bit 12Interface PCISupported OS Windows XP, 32-bit, no PAE

b) PCI-9812 Value

No. of channels 4Input range, V ±1; ±5A/D sampling rate, MHz Up to 20A/D resolution, bit 12Maximum frame rate 7 HzInterface PCI

Figure 2. The spectral response of the “Nanogate-2V”photocathode.

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Figure 3. The scheme of “Nanogate-2V” integration in LabVIEW environment.

3 Software development

Camera proprietary software allowed a registration of a single-shot image (*.tiff or *.bmp format)only which did not meet requirements of LIBS experiment. We developed the software in Lab-VIEW environment and created fully functional LIBS system, which also combined additionalmeasuring device (ADC “PCI-9812”) for an advanced research of laser ablation.

3.1 Software for ICCD camera “Nanogate-2V”

The scheme of suggested solution for integration of “Nanogate-2V” in LabVIEW environment isshown in figure 3. The driver library vslib3.dll is not compatible with LabVIEW, because passingof complicated C structures by pointer (or even by pointer to pointer) is required to call the majorityof library functions (e.g. struct VS ERROR DATA, manufacturer SDK [21]). In such a case, theuse of LabVIEW clusters (the LabVIEW counterpart to C structures) does not work well, because itbecomes very difficult to gain access to the actual memory structures of a dll as LabVIEW managesthe memory like a high-level programming language. Illegal passing of clusters leads to memorycorruption, and results in a LabVIEW crash. Moreover, a number of functions such as interfaceinitialization, error check, etc. must be called even for simple operations (e.g. setting of any cameraparameter). Calling a lot of auxiliary functions for one operation complicates G language blockdiagram greatly. Hence, we created a fully LabVIEW compatible shell vslib3 new.dll in VisualStudio, where native functions of vslib3.dll were combined to provide single function for eachcamera operation (table 2). Also, two functions for images conversion into spectra were added intothe library (table 2, lines 15–16). We created a program (*.vi) with user-friendly graphic interfaceapplying new library in LabVIEW for camera management and spectra registration (figure 4).

The flowchart of the program is shown in figure 5. A camera initialization includes bothassignment of ID number for the device and driver initialization. Then a user should specify datapaths, parameters of the camera operation (CCD exposure period, delay, gate, etc., see table 2,line 2–11) and the number of receiving frames (for a 12-bit A/D camera mode). Pressing the

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Figure 4. Screenshot of the front panel of the main VI: tab “Camera” for “Nanogate-2V” management andspectra registration.

“Apply” button configures the camera with defined parameters. The “Start” button turns on theoperation mode of the camera. During the operation mode, camera works independently from PCwith external (or internal) trigger and an image is rewritten in the own camera memory buffer. In a12-bit mode, the certain number of function “InputFrame” (table 2, line 12) calls are performed towrite each frame in a separate 16-bit gray .tiff file (the number of input frame is displayed). Afterthis procedure, the image intensifier is disabled automatically, and the conversion of the imagefiles to the .txt file is performed by the simple algorithm of vertical binning. The first columnof the .txt file represents the number of pixel, and the other ones contain relative intensities ofspectra. After conversion, camera waits for the new operation cycle. In an 8-bit mode, the function“InputFrame8bit” is called continuously. A current image from the camera and a correspondingspectrum are displayed due to two times less data stream than in the case of 16-bit image, anddata files are rewritten for each pulse. This mode is commonly used for online adjustments ofspectrograph and camera (choosing the suitable spectral range, gain, exposure period etc.). Thebutton “Stop adjustment” interrupts the calling of “InputFrame8bit”, and switches the camera towaiting mode. For quantitative measurements, we commonly used the 12-bit mode to obtain thespectra with the maximal intensity resolution.

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Figure 5. Flowchart of the program for camera management and spectra registration.

We presented in figure 6a an image of the laser-induced plasma spectrum of a high-purity Fe(Sigma Aldrich, 99.98%) in the wavelength range ∼403 nm (center of the image) as an example ofspectral data acquiring. The corresponding spectrum after vertical binning of the image is presentedin coordinates CCD pixel No.–Intensity in figure 6b.

Two functions (SetWorkMode and SetAmplify) of “Nanogate-2V” should be discussed in de-tails since they influence the output signal greatly. The first function has options “enable”, “disable”and “adjustment” work mode of intensifier (table 2, line 6). When “adjustment” mode is active, thetrigger signals with the high repetition rate (770 Hz) starts the intensifier. This is harmful regime

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Figure 6. a) Image of the laser-induced plasma spectrum of a pure Fe in the range 403 nm obtained bycamera “Nanogate-2V”. b) LIBS spectrum obtained by vertical binning of the image.

Table 2. LabVIEW compatible functions of the library vslib3 new.dll for “Nanogate-2V”.

Function Description1 SystemInit Loading the driver to PC memory and creating an environment for work

(“system”)2 SetVoltage Setting a gain of the image intensifier3 SetCameraExpos Setting CCD exposure period4 SetAmplify Setting amplification of analog signal from CCD (0–1023)5 SetEOMExpos Setting a gate6 SetWorkMode Setting a work mode of image intensifier (enable, disable or adjustment)7 SetSyncSource Setting an external or internal synchronization8 SetDelayExpos Setting a delay of registration9 SetFlowLoss Setting a gate in adjustment mode10 SetBinning Setting horizontal and vertical binning of the CCD (up to 8x)11 FramePrepare Setting a bit mode of ADC (8 or 12);

setting a horizontal and vertical position and a size of the input window (image)12 InputFrame Checking the data readiness for transfer, data transfer from internal memory

to PC memory, and writing a 16-bit gray .tiff file with spectrum image13 InputFrame8bit The same as InputFrame, except for an 8-bit gray .bmp image as output file14 SystemDestroy Release of interfaces of the “system”, destroying it and PC memory release15 ConvertTiff Conversion of a 16-bit gray .tiff image into a spectrum by vertical binning16 ConvertBmp Conversion of an 8-bit gray .bmp image into a spectrum by vertical binning

for intensifier (manufacturer does not recommend more than 2 min of its use). Therefore, we de-signed a regime with work mode “enable” and 8-bit mode of image registration for adjustment. Thesecond function manages of both an onchip preamplifier of ADC and a correction of digital imagecontrast by setting the “Amplify” level. The value of this option can be varied in the range 0-1023(table 2, line 4). The preamplifier can be useful when a signal from a charge amplifier of CCD isnot enough to cover the dynamic range of an integrated ADC or when an autocontrast is necessaryfor image visualization. The last one interferes with spectra registration extremely. Therefore, it

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Figure 7. a) Number of distinct values of pixel intensity vs. “Amplify” value b) Maximal observed pixelintensity vs. “Amplify” value.

was necessary to determine the border between amplifying of signal and autocontrast correction.For this, we obtained the LIBS spectra of pure Fe with certain gain (900 V), gate (500 ns) and delay(5 µs) at different “Amplify” level. Under these conditions, we observed both a CCD blooming(i.e. actually the maximum charge of a CCD cell) and dark areas without emission lines (minimalcharge). At a zero level of “Amplify”, the number of distinct values of pixel intensity was ∼1500(212 = 4096 was expected for 12-bit color gamut, figure 7a) and a pixel maximal intensity was∼28000 (instead of 216− 1 = 65535 for 16-bit tiff representation, figure 7b). Therefore, we actu-ally had 11-bit mode (210 < 1500 < 211). The number of distinct values of pixel intensity and itsmaximal value were increased linearly up to “Amplify” = 127. It meant that until “Amplify” = 127function “SetAmplify” provided an amplifying of a signal only, and the maximal achieved valueof pixel intensity restricted by blooming was ∼54500 (i.e. 83% of the theoretical maximum of16-bit representation). The further increasing of “Amplify” was meaningless because the imagewas distorted by autocontrast.

3.2 Integration of ADC “PCI-9812”

The next step of development of our platform was integration of additional ADC card formultisignal measurements. We synchronized “PCI-9812” and “Nanogate-2V” by the same triggerfor simultaneous start. ADLINK’s SDK included a set of virtual instruments (VIs) based onfunctions from library PCI-Dask.dll for the development of the software in LabVIEW. This VIsallowed controlling of an ADC card and obtaining datasets (amplitude-time) for each channel.Therefore, we tried to run two independent VIs for ADC card and for camera simultaneously. Un-fortunately, the “PCI-9812” driver function AI AsyncCheck [22], which was used in ADLINK’sVI for checking data availability, blocked PCI bus for all time between input trigger pulses and,consequently, data transfer from camera. To avoid this limitation we integrated a module for“PCI-9812” channels scan into the function “InputFrame” of the library vslib3 new.dll accordingto the scheme in figure 8. The first step was the channels scan of ADC card and data transfer toPC, and the second one was the data transfer from internal data buffer of “Nanogate-2V” controllerto PC. It should be noted, that the camera configured by calling the function SetSyncSourceworked independently from PC and stored data in the own buffer. Thus, we avoided the loss of

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Figure 8. Flowchart of “PCI-9812” integration to the function “InputFrame” (table 2).

Figure 9. Screenshot of the front panel of the main VI. Tab for “PCI-9812” management and data acquisi-tion.

data. Function “AI AsyncCheck” checked the completeness of data acquisition by “PCI-9812”after simultaneous triggering of an A/D conversion and an image registration by the Q-switch,and then the consequence of “Nanogate-2V” functions transferred the data from camera bufferto PC. In this configuration “AI AyncCheck” released PCI bus before the next trigger pulse,and the remaining time was enough for data transfer from camera. Finally, we used the updatedlibrary vslib3 new.dll and the library PCI-Dask.dll in one VI. The front panel (tab of main VI) for“PCI-9812” management and data acquisition is shown in figure 9.

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Figure 10. VI front panel for spectral data preprocessing.

3.3 Data preprocessing program

Besides devices management and data acquisition another important point for successful LIBSmeasurements is spectrum preprocessing. We have created a special VI (see its front panel infigure 10), which can perform the following operations:

• removal of instrumental CCD background;

• peaks search (No. of pixel) as the local maximum in spectra (the weak peaks can be excludedby the user defined criterion);

• linear calibration of the spectral region by assigning several revealed peaks with thecertain wavelength (manual) or with the use of predefined values of the intercept and slope(automatic);

• spectra correction using spectral response curve for photocathode;

• normalization of a spectrum to background value (minimal intensity of the spectrum withina spectral range);

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Figure 11. a) The calibration curve of the spectrometer (λ , nm = 71.6 ± 0.3 + (6.500 ± 0.005)*10−2*SC,R2 = 0.99999, SC — spectrometer counts). b) Calculated and experimental reciprocal linear dispersion(RLD) of the spectrograph.

• removal of a spectrum background;

• averaging of spectra, calculation of a standard deviation (SD) and a relative standarddeviation;

• baseline removal by the iterative algorithm described by Torres et al. [14].

The advantage of the program consists in batch processing of the files with spectral data for autocalibration mode. The result of program execution is the .txt files with the same structure as in-put files. The mean values of corrected intensity, SD and relative SD of signal are added to fi-nal columns.

4 Hardware adjustment

First of all, we placed the camera in the focal plane of the spectrograph to obtain the finest verticaland narrow lines on the image spectrum of mercury lamp. Then we calibrated our spectrometer-ICCD system with the use of mercury and gallium lamps (figure 11a). The first point on the plotcorresponds to the Hg I 253.6521 nm line and the last one corresponds to the Ga I 639.6561 nmline. High linearity of the calibration curve was obtained (R2=0.99999). The maximum uncertaintyof wavelength position in the center of the CCD with the use of obtained calibration curve wasapproximately 0.6 nm at 460 nm.

He-Ne laser (λ = 632.8 nm) was used to determine the experimental value of resolvingpower of our system. We neglected the Doppler width of the laser line due to its small value(∼0.0023 nm at T = 300 K [23]) and equated the laser line width to instrumental function. Thespectral profile was in a good agreement with Lorentz function (figure 12) with the width (FWHM)of 4.63±0.03 pixels.

The experimental reciprocal linear dispersion (RLD) of the system was equal to0.01117 ± 0.00003 nm/pix (figure 13b) at 655 nm with the use of stainless steel spectrum (sampleC2, BAM, Germany). Actually it was the slope of calibration function. Instrumental function ofthe spectrograph was the result of convolution of rectangular slit function and several diffraction

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Figure 12. The spectrum of the He-Ne laser line.

functions, including the slit, grating, collimator and focusing mirror diffraction. Spectral slit widthfor the different wavelength depends on RLD only, which theoretical value is:

dl=

d cosϕ

F

where d is the grating spacing, ϕ is the angle of diffracted beam with respect to the normal of thegrating and F is the focal length of the focusing mirror of the spectrograph. The angle ϕ can becalculated from the following equations:{

ϕ−ψ = constd (sinϕ + sinψ) = mλ

where ψ is the angle of incidence of light to the grating, const is the constant of the Czerny-Turnerspectrograph — the angle between the incident beam and diffracted one (in our case 24◦), m is thediffraction order. We assumed that the diffraction parts of instrumental function at 632.8 nm and655 nm were equal to each other, and we calculated the FWHM for He-Ne laser line in nm as:(

dl

)λ=632.8(

dl

)λ=655

×(

d pix

)λ=655

×FWHM(pix)λ=632.8 =

(dλ

d pix

)λ=632.8

×FWHM(pix)λ=632.8

The instrumental spectral width of the line 632.8 nm and R at this wavelength were equal to0.053 nm and 12000, respectively. We calculated the RLD in nm/mm of our spectrograph for theused grating and m = 1 (working order diffraction) and compared it with experimental values, ob-tained for certain spectral region (figure 11b). The experimental dispersion at λ = λn was found as:(

dl

)λ=λn

=∆λn

∆l

where ∆λn was the observed width of the spectral range n (for example, 15.51 nm at λn = 655 nm,λn — the center wavelength of the range, see figure 12b) and ∆l was the width of the spectra image

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Figure 13. a) Temporal evolution of LIBS spectra of steel C2 in the range 403 nm. b) Decrease of the widthof H-α Balmer line in LIBS spectra of steel C2 in air.

in the focal plane of the spectrograph. The value of ∆l was 13.63± 0.03 mm, which corresponds tothe diagonal size of our CCD 17.02 mm in the focal plane. The real effective size of CCD diagonalis 11.197 mm, since the image was decreased by focon in 1.52 times.

5 Applications

The developed software/hardware platform can be used for laboratory LIBS analysis of variety ofobjects. Since detailed description of each application needs own paper, we just considered fewbrief examples of our platform possibilities for LIBS.

It is well known that a registration of laser plasma emission should be delayed for LIBS mea-surements due to high initial intensity of continuous background, which does not allow observationof emission of ion and atomic lines [1]. The selection of optimal delay and gate is also needed forthe best signal to noise ratio (SNR) as well as for the suppression of spectral interferences since theintensity of the lines depends on experimental conditions in plasma. Evolution of spectra of high-alloy steel sample C2 obtained by our LIBS platform (without removal of spectrum background) ispresented in figure 13. All spectra were measured at gate 0.3 µs, gain 700 V (figure 13a) or 800 V(figure 13b) and then averaged over 5 laser shots. Fluence was ∼10 GW/cm2 for second harmonic(532 nm). The spectra in figure 13, a were used to optimize registration parameters for determina-tion of Mn in steels (the best SNR=110 was observed at delay 1.5 µs, gate 2 µs). We also used theplatform for plasma diagnostics (electron density Ne was estimated as (ws/(4.63*10−12))3/2 [24],where ws = wL−winstr, and the parameter 4.63*10−12 was retrieved from Stark profiles of the lineH I 656.3 nm at T = 104 K and Ne = 1022 m−3 obtained by Gigosos and Cardenoso [25]). Theparameter wL was found as FWHM of Lorentz fit of hydrogen line (figure 13b). Electron densitywas equal to 1.8*1017 cm−3 at the delay 2 µs.

Besides temporal selection for spectra registration, the great advantage of an ICCD detectoris sufficient enhancement of the emission signal. It can be useful for quantitative determination oftrace elements in solid sample (several ppm or less). The enhancement of the Ag signal in LIBSspectra of rock sample (CAg ∼17 ppm) in ∼3 times is shown in figure 15, a with the increase of thegain value from 850 V to 950 V. Moreover, we realized a special mode for accumulation of signal

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Figure 14. The timing diagram of CCD signal accumulation mode. Emission signal collected from 5 lasershots.

Figure 15. a) Enhancement of the Ag signal in LIBS spectra of rock (CAg ∼17 ppm) with the increasingof the gain. b) LIBS spectra of standard soil sample BAM U110 (CAg=4.51 ppm) at different registrationparameters: black and blue — averaged over 10 single laser shots (i.e. 10 measurements); red — averagedover 2 measurements with accumulation of each one from 5 laser shots.

from consequent laser pulses directly on CCD. In practice, we set an external synchronization ofthe CCD, its exposure period equal to sum of several periods of laser repetition and intensifierphosphor afterglow (figure 14). The intensifier and CCD started simultaneously with the first laserpulse. Then the CCD accumulated light until the end of the exposure period while image intensifieris fired several times (number of pulses). For example, we measured a single spectrum from 5 laserpulses at the laser repetition rate 5 Hz and CCD exposure period 820 ms.

The possibility of signal accumulation from several laser pulses gives the significant enhance-ment of the signal to noise ratio. The spectra averaged over 10 laser shots (third harmonic 355 nmof Nd:YAG laser, fluence ∼3 GW/cm2, gate 5.5 µs, gain 950 V and delay 1.5 µs or 6.5 µs) areshown in figure 15b. For comparison, the spectra accumulated from 5 laser shots and averagedover 2 measurements (i.e. equal to 10 single laser shots) are presented (red). Everyone can see, thatthe appropriate delay allows avoiding overlap of Ag analytical signal with interfering signals, and

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Figure 16. a) LIBS spectrum of the Al-Mg-Li-Cu alloy from single laser pulse. b) Oscillogram of simulta-neously measured acoustic signal.

Figure 17. Decrease of emission signal of copper and acoustic signal under the ablation of Al-Mg-Li-Cualloy.

accumulation of spectra increases its intensity significantly. These spectra demonstrate the highpotential of our system to improve the sensitivity of LIBS.

Finally, the possibility of using of the acoustic signal for emission signal correction under laserablation is demonstrated. We performed 60 laser pulses at a single point of the Al-Mg-Li-Cu alloysample with simultaneous registration of emission spectra and oscillograms of the acoustic signal(figure 16a and b, respectively). Third harmonic of the laser radiation was used for ablation (initialfluence ∼15 GW/cm2). Temporal parameters of spectra registration were 1 µs for both delay andgate, and gain was 750 V. The sampling rate of the acoustic signal registration was set to 1 MHz.The decrease of peak signal of Cu I 282.44 nm line was observed with the growth of the laser pulsenumber (figure 17). We calculated the acoustic signal as the difference between two extremesin oscillogram (figure 16b, red points). The acoustic signal also decreased with the laser pulseNo. therefore, acoustic signal can be used for correction of analytical emission one due their highcorrelation (figure 17).

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6 Conclusions

Software/hardware platform used ultrafast ICCD detector “Nanogate-2V” and LabVIEW environ-ment was developed for the LIBS analysis of trace elements. Combination of functions for cameraand ADC “PCI-9812” in one dll library compatible with LabVIEW allowed simultaneous mea-surements of emission and acoustic signal from laser-induced plasma. The theoretical and experi-mentally measured values of resolution power of the system, which is the principal characteristicof spectral measurements, were in a good agreement. Several key points of the LIBS system havebeen achieved, such as user-friendly interface, fast visualization of spectrum, its preprocessing andaccumulation signal from consequent laser pulses for high-sensitivity analysis. The performanceof the above platform was tested by electron density measurements with H-α Balmer line, in-vestigation of emission spectra evolution for optimization of manganese determination and silverrevealing in soils at extremely low level. These results give us confidence to provide precise andsensitive analytical measurements with the developed LIBS system.

Acknowledgments

The work was financially supported by the Russian Foundation for Basic Research (RFBR grantNo. 14-03-31227-mol a).

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