Calibration-free elemental analysis combined with high repetition rate laser-ablation spark-induced breakdown spectroscopy

Spectrochimica Acta Part B 161 (2019) 105711

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Calibration-free elemental analysis combined with high repetition rate laserablation spark-induced breakdown spectroscopy

T

⁎

Juan Kang, Yuqi Chen, Runhua Li

School of Physics and Optoelectronics, South China University of Technology, Guangzhou 510641, China

A B S T R A C T

Calibration-free (CF) method combined with high repetition rate laser-ablation spark-induced breakdown spectroscopy (HRR LA-SIBS) was first utilized to realize elemental analysis of alloy samples. A compact fiber laser operated at 30 kHz pulse repetition rate was used to ablate the sample and the spectra were recorded with a compact fiber spectrometer in non-gated signal recording mode. Three standard aluminum alloy samples were analyzed to evaluate the performance of this technique. Median filtering method was applied to reduce the contribution of the continuum background to the intensity of the atomic lines. The averaged electron density was determined to be (2.36–2.49) × 1017 cm−3 according to the Stark broadening of four ionic lines. 11,800 K averaged plasma temperature was estimated from the Saha-Boltzmann plots. The plasma could be verified to be in a state close to local thermodynamic equilibrium. The analytical error was < 0.5% for the major element and < 35% for minor elements with > 0.1% concentrations. It was demonstrated that CF method combined with HRR LA-SIBS was possible to realize reliable quantitative elemental analysis for aluminum alloy samples.

1. Introduction In the past decades, laser-induced breakdown spectroscopy (LIBS) has gained much more attention in the field of analytical atomic spectroscopy. In LIBS technique, the samples can be analyzed directly without requiring any sample preparation or only requiring simple sample preparation. For this reason, LIBS is able to realize fast, in-situ or remote elemental analysis. However, limited by low excitation efficiency for the atomic levels in laser-induced plasma and the influence of the continuum background of the plasma to sensitive signal detection, the analytical sensitivity of LIBS is relatively low in comparison with conventional techniques of analytical atomic spectroscopy. Researchers in LIBS community have developed different methods to enhance the atomic emission of the laser-induced plasma, such as double-pulse excitation [1–3], spatial confinement [4–6], magnetic field confinement [7,8], spark discharge assistance [9–13], microwave assistance [14,15] and so on. More signal enhancement techniques in LIBS can be found in a recent review article written by Li et al. [16] Under the assistance of these methods, 1–2 orders improvement factor on the analytical sensitivity for different elements can be usually achieved if compared with single pulse LIBS. Laser-ablation spark-induced breakdown spectroscopy (LA-SIBS) can be considered as a similar technique with spark discharge assisted LIBS (SD-LIBS) [9]. Generally, if the spark discharge is used to enhance the emission of the laser-induced plasma which has already been deeply breakdown by the high power laser, it can be termed as SD-LIBS. But if

⁎

the laser power is relatively low, the sample is ablated by laser and deeper breakdown is completed by the spark discharge, it can be better termed as LA-SIBS. When pulsed laser with high repetition rate is used as the ablation laser source and the spark discharge is also operated under the same repetition rate, this can be called as high repetition rate laser-ablation spark-induced breakdown spectroscopy (HRR LA-SIBS). In order to improve the analytical performance of laser-ablation based analytical atomic spectroscopy, such as LIBS, we have developed HRR LA-SIBS technique and realized fast and sensitive elemental analysis [17]. In that work, the laser was operated at 1 kHz pulse repetition rate and a compact fiber spectrometer was used to record the plasma emission spectra in non-gated signal recording mode. 14.0 ppm and 9.9 ppm limit of detection (LOD) have been determined for magnesium and copper in aluminum alloys. This technique has also been used to analyze lead and aluminum in copper alloys and the LODs were determined to be 15.5 and 1.9 ppm, respectively [18]. LA-SIBS based on femtosecond laser operated at 1 kHz pulse repetition rate has also been developed with potential advantages of better lateral resolution and better analytical reproducibility [19]. Under the operation of high pulse repetition rate, signal duty cycle can be improved significantly in comparison with that operated at low pulse repetition rate, such as 1–20 Hz; thus the signal can be processed with a lock-in amplifier. Sensitive trace elements analysis has been successfully demonstrated by Kang et al. with HRR LA-SIBS combined with lock-in signal detection [20]. In our previous works, matrix-matched standard samples were required for quantitative elemental analysis. To expand the applications

Corresponding author. E-mail address: [email protected] (R. Li).

https://doi.org/10.1016/j.sab.2019.105711 Received 13 July 2019; Received in revised form 5 September 2019; Accepted 30 September 2019 Available online 21 October 2019 0584-8547/ © 2019 Elsevier B.V. All rights reserved.

Spectrochimica Acta Part B 161 (2019) 105711

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of HRR LA-SIBS, the possibility of quantitative analysis without requiring standard samples by this technique is necessary to be investigated. In 1999, calibration-free laser-induced breakdown spectroscopy (CF-LIBS) was first proposed by Ciucci et al. to realize quantitative elemental analysis without requiring standard samples [21]. CF method was based on the Boltzmann distribution of the population on different energy levels in an equilibrium state. In laser-induced plasma, local thermodynamic equilibrium (LTE) is possible within a specific time window, thus CF-LIBS can be used to realize quantitative or semiquantitative elemental analysis for different samples. Tognoni et al. wrote a review article on the state of the art of CF-LIBS in 2010 [22]. CF-LIBS has been successfully applied to realize quantitative elemental analysis of different samples, including alloys [23,24], archaeological artifacts [25,26], thin films [27], aerosols [28], vegetables and foods [29,30], rocks and so on [31,32]. Double pulse CF-LIBS has also been developed to analyze the elements in soil, steel and Ge/Si alloys with better analytical sensitivity [33–35]. If the laser-induced plasma is not optically thin, self-absorption will happen for strong resonant lines. To eliminate the influence of the selfabsorption to the accuracy of the analytical results of CF-LIBS, selfabsorption should be corrected with adequate methods. Bulajic et al. proposed a procedure for correcting self-absorption in CF-LIBS based on Curve-of-Growth (COG) method [36]; Sun et al. proposed an internal reference method to correct self-absorption effect in CF-LIBS [37]; Praher et al. proposed a fast iteration procedure to correct self-absorption while analyzing oxide materials with CF-LIBS [38]. Recently, Li et al. proposed a new method to correct the self-absorption based on black-body radiation reference [39]. More accurate analytical results have been reported after taking these correction procedures. More contributions have been made by different researchers to further improve the reliability of CF-LIBS. For example, Dong et al. applied genetic algorithm (GA) method to determine more accurate plasma temperature in CF-LIBS [40]. Lednev and Pershin introduced a plasma stoichiometry correction method to reduce the influence of non-stoichiometric ablation on the analytical results [41]. In this work, we'll investigate the possibility of using CF method in HRR LA-SIBS technique to realize quantitative elemental analysis of aluminum alloy samples for the first time. In order to evaluate the accuracy of the quantitative analysis of this technique, three standard aluminum alloy samples with certified elemental concentrations have been analyzed. Spectra were recorded with a fiber spectrometer in nongated signal recording mode and median filtering method was applied to remove the weak continuous background from the recorded spectra. The analyzed results obtained with this technique were compared with the certified concentrations of the elements and some useful discussions on using CF method in HRR LA-SIBS were made.

ln

3 3 II II I ⎛ I jh λ ⎞ ⎛ Ej + Eion ⎞ ⎛ 2(2πme ) 2 (kB T ) 2 ⎞ + ln ⎛ Cs F ⎞ = − + ln ⎜ ⎟ ⎜ ⎟ ⎜ k T ⎟ I 3 ⎜ A II g II ⎟ N h U B e ⎝ s (T ) ⎠ ⎝ ⎠ ⎝ ⎠ ⎝ jh j ⎠

(2) Where Eion is the ionization energy, me is the mass of the electron, Ne is the electron density, h is the Planck constant. Defining the coordinates in the Saha-Boltzmann plane:

I λ ⎧ ln ⎜⎛ ki ⎟⎞ for neutral lines ⎪ ⎪ ⎝ Aki gk ⎠ y= 3 3 ⎨ ⎛ I jh λ ⎞ 2(2πme ) 2 (kB T ) 2 ⎞ ⎪ ln ⎜ − ln ⎜⎛ ⎟ for ionic lines ⎟ 3 Ne h ⎪ ⎝ Ajh gj ⎠ ⎝ ⎠ ⎩

(3)

Ek for neutral lines x=⎧ Ej + Eion for ionic lines ⎨ ⎩

(4)

And defining.

m = −k

1

BT

, qs = ln

( ) (5). Cs F Us (T )

A linear equation between x and y can be obtained:

y = mx + qs

(6)

The electron density Ne can be obtained independently from measurements of Stark broadening. The spectroscopic parameters Aki, gk, Ek, Eion and Us(T) can be found from the NIST database and other possible references. From the Saha-Boltzmann plot, the concentration of the emitting species Cs can be determined by the intercept qs. The slope of the Saha-Boltzmann plot reflects the plasma temperature T. Since the sum of all the species concentration should be 1, the experimental factor F can be determined using the normalization relation:

∑ Cs = 1 s

(7)

Finally, the concentration of each species in the sample can be calculated according to.

Cs =

Us (T ) qs F

(8)

3. Experimental The experimental setup is shown in Fig. 1, which is similar with that reported in our previous work [44]. Briefly, an acousto-optically Qswitched fiber laser was used as the ablation laser source. Center wavelength, repetition rate and pulse width of the laser were 1064 nm, 30 kHz and 100 ns, respectively. Pulse energy of the laser was about

2. Calibration free method The basics of CF-LIBS method can be summarized here. Supposing sample ablation is stoichiometric, the laser-induced plasma is in a state close to LTE, and the plasma is optically thin, then for atomic state I, the line intensity Iki has the following relation [21,42,43]:

II λ EI CI F ⎞ ln ⎛⎜ kiI I ⎟⎞ = − k + ln ⎛⎜ Is ⎟ kB T ⎝ Us (T ) ⎠ ⎝ Aki gk ⎠

(1)

Where i and k indicate the lower and upper level of the transition, λ is the wavelength of the transition, Aki is the transition probability, gk is the degeneracy of the upper level, kB is the Boltzmann constant, Cs is the relative concentration of the emitting species, F is an experimental factor, and Us(T) is the partition function of the species s at the plasma temperature T. For the ionic state II,

Fig. 1. Schematic diagram of the experimental setup of HRR LA-SIBS in this work. 2

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0.6 mJ. The laser beam was focused on the sample surface with a spherical lens L1 (f = 60 mm) to ablate the sample and produce a plasma. The estimated maximum laser fluence on the sample surface was about 7.6 J/cm2. The sample was mounted on a two-dimensional moving stage which was kept moving at a speed of 3 mm/s while running the experiments. The spark discharge unit consisted of a direct current (DC) high voltage power supply (3 kV, 200 mA), two resistors R1, R2 (R1 = 16.7 kΩ, R2 = 2 Ω), a 5 nF capacitor C and a tungsten needle (Dia. = 2 mm). The tungsten needle was used as the anode, which was placed horizontally at a 45° angle to the sample surface. The sample plate was selected as the cathode of the spark discharge. The distance between the needle tip and sample surface was about 1.0 mm. The voltage of the DC power supply was set in the range of 1800–2200 V. In each cycle, once laser-induced plasma was produced by the ablation laser, spark discharge would be ignited immediately and enhanced optical emissions could be observed. The spark was imaged on the fiber entrance of a multichannel compact fiber spectrometer (Avantes, AvaSpec-ULS2048–3-USB2) with two quartz lenses L2 and L3. The diameters of L2 and L3 were 40 mm and the focal lengths of L2 and L3 were 100 mm and 150 mm, respectively. Each charge-coupled device (CCD) linear array in this fiber spectrometer has 2048 pixels and its spectral response has been calibrated using standard lamps (Avantes, AvaLight-DH-BAL-CAL). The spectral resolution of this fiber spectrometer was better than 0.15 nm within the wavelength range of 200–550 nm. Because the repetition rate of laser pulse was 30 kHz, gated signal detection was impossible, thus the CCD was operated in non-gated signal recording mode in the experiments. Further data processing for CF HRR LA-SIBS was carried out on a laptop computer. In order to monitor the temporal profiles of the plasma emission at different wavelengths, a 50 cm monochromator coupled with a photomultiplier tube (PMT) (Hamamatsu, CR 114) was used to analyze the spectra and the signal was recorded with a 200 MHz oscilloscope (Rigol, DS1202CA). Temporal profile of the discharge voltage was also monitored and recorded with this oscilloscope. Three standard aluminum alloy samples purchased from Institute of Standard Materials in Fushun Factory of Aluminum (in China) were analyzed with this technique. The elemental concentrations of these standard samples are listed in Table 1.

Fig. 2. Temporal profiles of magnesium emission at 383.83 nm and background emission of the plasma at 382.00 nm.

plasma emission recorded at 383.83 nm and 382.00 nm. The discharge voltage was 2 kV and the sample No.1 was analyzed in this experiment. The selected 383.83 nm and 382.00 nm correspond to analytical line of magnesium and a wavelength close to this analytical line without showing any observable atomic emissions. It can be seen that the continuum background at 382.0 nm is relatively weak and the persistence time of the magnesium emission at 383.83 nm was about 6 μs. Therefore, in non-gated signal recording mode, even the total recording time is 10 ms or even longer, the effective integration time for CCD will be < 6 μs in each cycle. 4.2. Background subtraction In HRR LA-SIBS, CCD has to be worked in non-gated signal recording mode. Thus weak continuum background will be recorded by the fiber spectrometer. In order to have net intensities of the atomic emissions, this continuum background was first subtracted using median filtering method. Fig. 3(a) shows the original spectrum recorded by the fiber spectrometer (black line) and the continuum background calculated by median filtering method with 300 data points median filter window (red line). The sample No.1 was analyzed and the high voltage of the spark discharge was 2 kV. The integration time of the spectrometer was

4. Results and discussion 4.1. Temporal profiles of plasma emission In our previous work [44], the characteristics of the spark discharge, such as discharge voltage, current and electrical pulse energy deposited into the laser-induced plasma in fiber laser-based HRR LA-SIBS have been studied. Both discharge voltage and current oscillated in time domain and the persistence time was about 3 μs. Under 2.2 kV, and R2 = 2 Ω, the electrical pulse energy deposited into the plasma was estimated to be 2.1 mJ. Both atomic emission signal intensity and signal-to-background ratio have also been investigated under different output voltages of the DC power supply. Maximum voltage of the DC power supply was 2.2 kV in order to control the erosion of the tungsten needle. The temporal profiles of the plasma emission were observed at different wavelengths. Fig. 2 shows typical temporal profiles of the Table 1 List of the concentrations (% wt.) of all elements in three standard aluminum alloy samples. Sample No.

Al

Mg

Cu

Mn

Cr

Zn

1 2 3

98.639 98.843 99.062

1.00 0.81 0.60

0.19 0.15 0.10

0.010 0.051 0.099

0.15 0.10 0.05

0.011 0.046 0.089

Fig. 3. Recorded spectra before and after background subtraction. (a) Original spectrum (black line) and the background calculated by median filtering method (red line). (b) The spectrum after background subtraction. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 3

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10 ms and the final spectrum was an averaged result of 500 repeated measurements. Fig. 3(b) shows the spectrum after this processing. After the background subtraction, the baseline of the spectrum regresses to zero; however, the shape of each spectral line has not been changed. This spectral data processing is helpful to have more accurate net line intensities of the atomic emissions.

results determined with CF method agree well with the certified values. For the major element aluminum, the relative errors are < 0.5%. For minor elements with the concentrations between 0.1% and 1% (Mg, Cu, Cr in the sample No.1–2 and Mg, Cu in the sample No.3), the relative errors are < 35%. Under current experimental condition, without zinc lines can be identified for all three samples thus zinc concentration could not be determined with CF method. Fortunately, zinc concentration is quite low, thus it will not bring obvious measurement error for other elements in the studied samples.

4.3. Estimation of electron density Neglecting other line broadening contributions, the relationship between the electron density and Stark broadening width of an atomic line can be simplified as.

Ne ≈

Δλ1/2 × 1016 2ω

4.6. Discussion Usually, self-absorption effect should be considered in CF-LIBS method. Fortunately, self-absorption effect in fiber laser based HRR LASIBS is not obvious. Because the laser pulse energy is quite low thus the sample mass ablated by each laser pulse is very low. Under the re-excitation by the spark discharge, higher plasma temperature can be achieved; the plasma size will also expanded. Therefore, the plasma will become to optically thin. This has been demonstrated experimentally in a similar experiment carried out by Xiong et al. [47] In their work, a fiber laser with low pulse energy was used as ablation laser and the ablated sample was re-excited by a second laser pulse from the direction parallel with the sample surface, the self-absorption effect has been observed to be reduced obviously. Similarly, in an orthogonal fs/ns double pulse LIBS study carried out by Santagata et al., the self-absorption effect has also not been observed [48]. On the other hand, the concentrations of the minor elements in the samples are quite low, thus the self-absorption of these elements will be very weak. In order to reduce possible self-absorption effect for aluminum lines, the strong resonant lines of aluminum, such as 394.40, 396.15, 308.22 and 308.28 nm were discarded in CF calculation. More safely, a few strong resonant lines of the minor elements were also discarded in CF calculation. According to the time-resolved studies on the characteristics of laser-induced plasma, during the evolution of the plasma, there exists a time window the plasma is in a state close to LTE [49,50]. Therefore it's better to collect spectra data of LIBS within a specific gate width after an adequate time delay. However, the spectrometer with this capbility is usually expensive and it's not suitable for portable instruments. For this reason, in most cases, CCD is used as photon detector in fiber spectrometer and the minimum gate width is usually 1 ms or 2 ms. Grifoni et al. compared the plasma parameters evaluated from timeintegrated spectra recorded with CCD and time-resolved spectra recorded with different spectrometer, similar results have been obtained [51]. More examples can be found in literatures where using CCD as photon detector in CF-LIBS studies or laser-ablation fast pulse discharge plasma spectroscopy (LA-FPDPS) [35,52–54]. This is understandable, since the plasma emission intensity in LIBS decays to almost zero after a few microseconds, thus the effective signal integration time for the CCD is only a few microseconds, not the set minimum integration time of the CCD, e.g. 1 ms or 2 ms. In current work, under the re-excitation of the spark discharge, the persistence time of the atomic emission is about 6 μs. Although 300 pulses of the atomic emission can be recorded within 10 ms under 30 kHz repetition rate, in each cycle, the effective integration time is only about 6 μs. Considering signal decay, major contribution to the observed line intensity is coming from the time window of 0.4–4.0 μs, the spectra should reflect the plasma characteristics within this time window when the plasma is in a state close to LTE, and the calculated electron temperature and electron density should be an averaged one within this period. It's worth to mention that, the contribution of the weak continuum background appears at the early time of the laser-ablation and spark discharge has already been reduced by median filtering method. Therefore, the contribution of the plasma emission at early time, in which the plasma was departure from LTE, has been reduced after this background subtraction. The McWhirter criterion is a necessary but not sufficient condition

(9)

Where ω is Stark-broadening parameter, Δλ1/ = Δλobserved − Δλinstrumental is the full width at half maximum (FWHM) of a studied line. Δλinstrumental is the instrument broadening which is found to be 0.11 nm by measuring the broadening of several emission lines of a low-pressure mercury lamp. The electron density was estimated from the Stark broadening of four ionic lines including Al II 281.62 nm, Al II 466.30 nm, Mg II 280.27 nm and Mg II 293.65 nm. The Stark-broadening parameters of these four ionic lines are listed in Table 2 [45,46] and the Lorentzian fitting plots of the four transitions are shown in Fig. 4. The averaged value of electron density estimated from these four transitions was (2.46 ± 0.6) × 1017 cm−3, (2.36 ± 0.5) × 1017 cm−3, and (2.49 ± 0.6) × 1017 cm−3 for sample No. 1, No. 2 and No. 3, respectively. 2

4.4. Saha-Boltzmann plot In order to have Saha-Boltzmann plots, adequate transition lines of different elements should be selected from the recorded spectra. Spectral lines with almost no self-absorption and those can be clearly assigned to certain element have been selected in this work. Table 3 lists the selected spectral lines and their corresponding transition parameters. The Saha-Boltzmann plots of different elements for sample No. 1–3 are shown in Fig. 5. The plasma temperature was first calculated from the slope of the Saha-Boltzmann plot for each element and then an average was taken in order to have better results. The averaged plasma temperature was finally determined to be 11,800 ± 800 K according to the experimental results of this work. According to the McWhirter criterion, a necessary requirement for LTE is

Ne ≥ 1.6 × 1012T1/2∆E 3

(10)

Where ΔE is the largest value of energy of the corresponding electronic transition. The lowest electron density for LTE is calculated to be about 3.39 × 1016 cm−3. Hence the electron density determined in Section 4.3 is obviously greater than this one. 4.5. Concentration determination with CF method Concentrations of different elements in sample No.1–3 have been determined using CF method and the results are listed in Table 4. The Table 2 The Stark broadening parameters of different transitions. Species

λ (nm)

ω (10−3 nm)

Al II

281.62 466.30 280.27 293.65

2.12 6.03 4.40 3.60

Mg II

4

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Fig. 4. Lorentzian fit of four lines for sample No.1. (a) Al II 281.62 nm; (b) Al II 466.30 nm; (c) Mg II 280.27 nm; (d) Mg II 293.65 nm.

than the plasma dimension which as about 1.5–2.0 mm. Together with the already satisfied McWhirter criterion, it can be believed that the plasma generated on aluminum surface in current HRR LA-SIBS is close to LTE. More experimental results are able to support this confirmation. For example, The plasma was demonstrated to be close to LTE in LAFPDPS and in electric arc discharge by different research groups [54,56]. Therefore, CF method is possible combined with HRR LA-SIBS to realize quantitative elemental analysis. According to our previous experimental observation, in HRR LASIBS, the sample was mainly ablated by the laser radiation; and the contribution of spark discharge to the sample ablation was negligible

to ensure LTE for laser-induced transient plasma, because the McWhirter criterion is derived from a station plasma. Critoforetti et al. proposed other two criteria to help determine whether a transient plasma is close to LTE [55]. One criterion is, in LTE, the thermodynamic relation time τrel is much shorter than the expansion time τexpof a plasma; another criterion is, in LTE, the diffusion length λ of an element is much smaller than the plasma dimension. We have estimated τrel and λ according to eqs. 18 and 21 in reference 55. The τrelwas calculate to be 0.8–4.5 ns for different atoms in the plasma, much shorter than the plasma expansion time which is about one or more than one microsecond; the λ was calculated to be about 0.04–0.09 mm, much smaller Table 3 List of the selected spectral lines and their corresponding transition parameters. Species

λ (nm)

A × 108 (s−1)

gup

Eup (ev)

Species

λ (nm)

A × 108 (s−1)

gup

Eup (ev)

Al I Al I Al I Al I Al I Al I Al I Al I Al II Al II Al II Mg I Mg I Mg I Mg I Mg I Mg I Mg I Mg I Mg II Mg II Mg II Mg II Mg II Mg II Cu I

256.7983 257.5094 265.2475 266.0386 305.0072 305.7144 306.429 309.271 281.6185 358.6557 466.3046 285.2126 333.6674 382.9355 383.2304 517.2684 518.3604 383.8292 516.7321 279.5528 280.2705 292.8633 293.651 310.4715 448.1126 324.7537

0.192 0.360 0.142 0.284 0.321 0.750 0.892 0.729 3.57 2.35 0.58 4.91 0.17 0.899 1.21 0.337 0.561 1.61 0.113 2.60 2.57 1.15 2.30 0.797 2.33 1.395

4 6 2 2 6 6 2 6 1 9 3 3 3 3 5 3 3 7 3 4 2 2 2 8 6 4

4.83 4.83 4.67 4.67 7.67 7.67 7.65 4.02 11.82 15.30 13.26 4.35 6.43 5.95 5.95 5.11 5.11 5.95 5.11 4.43 4.42 8.65 8.65 12.86 11.63 3.82

Cu I Cu I Cu II Cu II Cu II Cu II Cu II Cu II Cu II Cr I Cr I Cr I Cr II Cr II Cr II Cr II Cr II Mn I Mn I Mn II Mn II Mn II Mn II Mn II Mn II

327.3954 330.7945 213.5981 217.941 224.2618 224.7003 203.5854 211.731 221.0268 425.4336 427.4715 428.9717 313.2053 312.4973 267.1803 340.8757 342.2732 403.0753 403.4483 255.8606 257.6105 263.8167 293.3055 293.9308 294.9205

1.376 2.216 5.59 2.15 2.50 3.30 3.70 7.20 1.53 0.315 0.307 0.316 0.815 0.819 1.00 0.956 1.39 0.17 0.158 2.00 2.80 2.711 2.04 1.98 1.96

2 12 9 5 7 5 3 9 5 9 7 5 10 8 4 6 4 8 4 9 9 7 3 5 7

3.79 8.82 8.52 8.66 8.78 8.23 9.06 14.34 8.86 2.91 2.90 2.89 6.44 6.42 6.15 6.11 6.08 3.08 3.07 8.26 4.81 8.12 5.40 5.39 5.38

5

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Fig. 5. The Saha-Boltzmann plots for different samples, x and y have been defined in Eq. (4) and Eq. (3). (a) Sample No.1; (b) sample No.2; (c) sample No.3.

5. Conclusion

Table 4 Quantitative analytical results of the elements in three standard aluminum alloy samples using CF HRR LA-SIBS. Sample No.

Element

Certified concentration (%)

Measured concentration (%)

1

Al Mg Cu Cr Al Mg Cu Cr Al Mg Cu Cr Mn

98.639 1 0.19 0.15 98.843 0.81 0.15 0.10 99.062 0.60 0.10 0.05 0.099

98.29 1.35 0.16 0.20 98.72 1.07 0.13 0.08 99.21 0.48 0.13 0.10 0.08

2

3

The possibility of using CF method combined with HRR LA-SIBS on quantitative elemental analysis of aluminum alloy samples was investigated in this work. The contribution of the continuum background to atomic line intensity could be reduced using median filtering method. Averaged electron density and plasma temperature have been estimated to be (2.36–2.49) × 1017 cm−3 and 11,800 K and the plasma was verified to be close to LTE. Due to low mass ablation and re-excitation by the spark discharge, the plasma could be considered optically thin. It was successfully demonstrated that CF method combined with HRR LA-SIBS could be used to realize reliable quantitative elemental analysis of aluminum alloys samples. Fiber laser based HRR LASIBS system is compact, cost-effective and convenient for operation. Once combined with CF method, convenient quantitative elemental analysis with HRR LA-SIBS for different alloy samples will be possible without requiring matrix-matched standard samples. This is much helpful to extend the applications of HRR LA-SIBS in the field of analytical atomic spectroscopy.

[17]. In current work, the electrical energy deposited into the laser induced plasma during each cycle was estimated to be < 2.1 mJ [44], lower than 6.14 mJ in the reference 17. Thus the mass ablation by spark discharge in current work should also be negligible. However, because the laser pulse width was ~100 ns, thermal effect should be considered in the mechanism of laser-ablation by nanosecond laser. If the work function for different elements is different, non-stoichiometric ablation will happen. Lednev and Pershin have established a method to correct this non-stoichiometric ablation effect after considering selective evaporation of different components in the samples. This correction method has been demonstrated to be useful for different alloys [41,57]. In this work, the concentrations of aluminum are higher than 98.6% in the studied samples, non-stoichiometric ablation effect may not obvious. Therefore we have not taken this correction in this work. If different samples are analyzed, it's better to take this correction in order to reduce the analytical error due to non-stoichiometric ablation. This work demonstrated that CF method could be used in HRR LASIBS to realize reliable quantitative elemental analysis for aluminum alloy samples. In comparison with conventional LIBS technique, in fiber laser based HRR LA-SIBS, the continuum background is very weak; this gives a chance of recording plasma emission with non-intensified CCD in non-gated signal recording mode. Thus only a compact fiber spectrometer is able to complete spectra data recording in HRR LA-SIBS. Fiber laser has the advantages of compactness, robustness and cost-effective, and the spark discharge unit is also very simple. Therefore, it's possible to build a portable HRR LA-SIBS system to realize elemental analysis of different alloy samples. When CF method is applied, matrixmatched standard samples will not be required anymore. Better reliability on the analytical results can be guaranteed by correcting possible self-absorption and non-stoichiometrical ablation effects using the established correction methods. Thus CF HRR LA-SIBS can be a good replacement of CF-LIBS on the elemental analysis of different solid samples, especially alloy samples.

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