Multielemental self-absorption reduction in laser-induced breakdown spectroscopy by using microwave-assisted excitation

Yun Tang; Jiaming Li; Zhongqi Hao; Shisong Tang; Zhihao Zhu; Lianbo Guo; Xiangyou Li; Xiaoyan Zeng; Jun Duan; Yongfeng Lu

doi:10.1364/OE.26.012121

1. Introduction

Laser-induced breakdown spectroscopy (LIBS) is a promising spectrochemical analysis technique for elemental determination. In LIBS, a pulsed laser ablates samples to generate plasmas. The plasma emission light is collected and analyzed to deduce the elemental information [1,2]. Due to its attractive characteristics, such as remote sensing capability, rapid response, no or simple sample preparation, in situ analyses, and simultaneous multielemental detection, LIBS has shown great potential in material science [3], environmental monitoring [4], agricultural production [5], the metallurgical industry [6], and biomedicine [7], etc.

However, as a result of self-absorption, matrix effects, and intensity fluctuations, the analytical accuracy and precision of LIBS are still unsatisfactory [8]. Among these problems, self-absorption is the most serious one, especially in high concentration elements, because it directly causes peak intensity reduction, even self-reversal [9]. To reduce the influence of the self-absorption effect, some researchers have established mathematical models to correct the self-absorbed spectra [10–14], while others have optimized experimental parameters and conditions to weaken the self-absorption effect [15–19]. All of these solutions only weaken the influence of the self-absorption effect to a certain extent due to the complexity of laser-target interaction, the fast temporal evolution of the plasma, and the spatial inhomogeneity of the plasma plume. To prevent self-absorption in the open air, we proposed an approach using laser-stimulated absorption LIBS (LSA-LIBS) in our previous work [20]. However, LSA-LIBS can reduce self-absorption of only one element at a time due to its high wavelength selectivity; and the use of wavelength-tunable lasers is complex and costly.

In addition, use of the microwave has proven to be feasible to enhance LIBS spectra [21–27]; and Jan Viljanen et al. [24] found that microwave-assisted LIBS could also reduce the self-reversal effect of the copper (Cu) element. However, multielemental self-absorption within a wide spectral range by using microwave-assisted excitation in LIBS (MAE-LIBS) has not been reported. In this work, a simple and low-cost setup was proposed to simultaneously reduce multielemental self-absorption using MAE-LIBS in the open air. Multielemental self-absorption reduction of sodium (Na), potassium (K), aluminum (Al), silicon (Si), and calcium (Ca) in potassium feldspar using MAE-LIBS was investigated within a wide spectral range of 200 to 900 nm. Furthermore, the mechanisms of self-absorption reduction in MAE-LIBS were also investigated.

2. Experimental

2.1 Experimental setup

The schematic diagram of the MAE-LIBS setup used in this study is shown in Fig. 1. A Q-switched Nd:YAG laser (Beamtech Optronics, Vlite 200, pulse duration of 8 ns, flattened Gaussian beam) operating at 1064 nm and 2 Hz was used for plasma generation. Laser energy fixed at 60 mJ/pulse was measured by an energy meter (National Institute of Metrology, China) to ensure the stability of energy. The laser beam was reflected by a mirror, through a 45° ultraviolet (UV)-enhanced, aluminum-plated pierced mirror (>80% reflectivity), and focused by a quartz lens (focal length of 100 mm) onto the sample surface. The samples were placed on an XY motion stage so that their positions could be changed to provide a fresh surface for each laser pulse. The laser-induced plasma emission was reflected through the pierced mirror to a quartz lens (focal length, 100 mm) focused on a fiber (diameter of 200 μm, length of 2 m) and transmitted to an eight-channel fiber spectrometer (Avantes, AvaSpec-2048-USB2-RM, spectral range from 198 to 1057 nm) equipped with a charge-coupled device (CCD) detector. A digital delay generator (Stanford Research Systems, DG645, USA) was adopted to trigger the laser and control the integration time delays of the CCD. In this work, the integration time was 2 ms.

Fig. 1 Schematic diagram of the experimental setup.

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The continuous microwave was generated using a magnetron at a frequency of 2.45 GHz. The microwave generator output power was from 2 W to 198 W (step length of 2 W). A flexible coaxial cable (50 ohms) was used to guide the microwave radiation to a microwave radiator. The microwave radiator configuration is shown in Fig. 1. It consists of two needle-shaped copper conductors with a radius of 0.45 mm. One of them is an oscillator, which is fed by a standard connector for radio-frequency (RF) connection, and the other is welded to the inner wall of the rigid coaxial cable. The distance between the two needles is 2 mm.

2.2 Samples

A potassium feldspar powder sample (GBW03116) was used in this study. The certified concentrations of sodium oxide (Na₂O), potassium oxide (K₂O), aluminum oxide (Al₂O₃), silicon dioxide (SiO₂), and calcium oxide (CaO) in the sample are listed in Table 1. To obtain a uniform surface for laser ablation, all of the samples were pressed into pellets with a diameter of 40 mm under a pressure of 20 MPa.

Table 1. The certified concentration of potassium feldspar sample (wt.%)

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3. Results and discussion

3.1 Dependence of the self-absorption effect on the position of the microwave radiator

The coupling efficiency of microwave power to laser plasma depends on the position of the microwave radiator [24, 28, 29]. Therefore, the position of the microwave radiator needs to be optimized. Two special positions of the microwave radiator, positions A and B, were compared to find a better position for the self-absorption effect reduction, as shown in Fig. 2.

Fig. 2 Schematic presentation of two different microwave radiator locations related to the laser plasma and the sample in measurements: the two needles of a microwave radiator located about 2 mm above the sample surface and (a) 0.5 mm horizontally away from the ablation spot and (b) a 5 mm horizontal pass away from the ablation spot.

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Figure 3 shows the spectra of K I 766.5 and 769.9 nm of two different microwave radiator locations. Laser energy of a 60 mJ per pulse, the delay time of 10 μs, the integration time of 2 ms, and a microwave power of 160 W was used. Obviously, the serious self-reversal phenomenon of the two lines was observed in traditional LIBS (black line). After adding the microwave radiator, the self-reversal phenomenon disappeared at both positions A (blue line) and B (red line). Furthermore, compared with a microwave radiator at position A, the spectra with a microwave radiator at position B exhibited sharper peaks and better profiles, which means that position B is the best place for self-absorption reduction.

Fig. 3 Emission spectra of K I 766.5 and 769.9 nm with different microwave radiator positions.

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3.2 Dependence of the self-absorption effect on microwave power

Several works have reported that spectral signals can be enhanced by increasing the microwave power [30]. To investigate the dependence of the self-absorption effect on microwave power, the laser-induced plasma spectra of K I 766.5 nm in potash feldspar at microwave powers of 0, 80, 120, 160, and 180 W were compared. The self-absorption effect resulted in a reduction of peak height and the growth of spectral line widths [31]. Generally, the full width at half maximum (FWHM) was employed to evaluate the degree of the self-absorption effect. If the self-absorption effect became severe, the FWHM would be broadened as a result, and vice versa [32]. Figure 4 shows FWHMs of K I 766.5 nm with different microwave powers (from 0 to 180W) under a series of delay time (from 2 to 30 μs) at 60 mJ laser energy. As shown in Fig. 4, the FWHMs showed small values at a short delay time, because the self-absorption was weak at the plasma early stage in LIBS [19, 31]. In MAE-LIBS, the FWHMs decayed faster at longer delay time than at short delay time. It indicated that the effectiveness of the microwave for plasma was increasing as delay time increases. Figure 4 also shows that the FWHMs under MAE-LIBS were all less than those under conventional LIBS (with a microwave power of 0 W), which indicates that the self-absorption effect can be effectively reduced with MAE-LIBS. In addition, the spectra at 160 W microwave power exhibited the least FWHMs among all kinds of microwave powers, which meant that microwave power at 160 W was used for the later measurements. The results showed that the method of microwave-assisted excitation has great potential for relieving the self-absorption effect in LIBS.

Fig. 4 FWHMs of K I 766.5 nm as a function of microwave power under different delay time (from 2 to 30 μs) at a laser energy of 60 mJ per pulse.

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3.3 Multielemental self-absorption reduction by MAE-LIBS

Based on the results of the parameter optimization previously discussed, multielemental self-absorption reduction of Na, K, Al, Si, and Ca in potassium feldspar using MAE-LIBS was studied. The spectra of traditional LIBS (black lines) and MAE-LIBS (red lines) under the same conditions are shown in Fig. 5. The laser power was 60 mJ, the delay time was 16 μs, and the integration time was 2 ms. Figure 5 depicts the potassium feldspar spectra from 200 to 900 nm. Enlarged spectra of Na, K, Al, Si, and Ca elements are shown in Fig. 6. The distinct enhancement effect of Na, K, Al, Si, and Ca emission lines can be observed from the comparison.

Fig. 5 LIBS and MAE-LIBS spectra from 200 nm to 900 nm in potassium feldspar pellets.

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Fig. 6 LIBS and MAE-LIBS spectra of (a) Na (b), K (c), Al and Si and (d) Ca in potassium feldspar pellets.

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In some cases, self-absorption appeared as a self-reversal. In most cases, the self-absorption appeared as a height reduction, which is not well recognized from the shape of the spectrum [33, 34]. As depicted in Figs. 6(a) and 6(b), using conventional LIBS, the spectral lines of Na I (589.0 and 589.6 nm) and K I (766.5 and 769.9 nm) in potassium feldspar pellets seriously suffered from the self-reversal effect. With microwave excitation, their self-reversal disappeared simultaneously. Sharp peaks were observed for all four spectral lines in the MAE-LIBS spectra. It indicates that MAE has a great potential for relieving the self-reversal phenomenon in LIBS. On the other hand, Figs. 6(a) to 6(d) depict the LIBS and MAE-LIBS spectral profiles of other spectral lines in potassium feldspar pellets. Although these lines have no obvious self-reversal phenomenon as in conventional LIBS, their spectral intensities are slightly increased in MAE-LIBS. In most cases, the self-absorption appeared as a height reduction, which is not well recognized from the shape of the spectrum.

3.4 Evaluation of multielemental self-absorption reduction in MAE-LIBS

The FWHM was employed to evaluate the degree of the self-absorption effect. The results of FWHMs of Na, K, Al, Si, and Ca spectra in LIBS and MAE-LIBS are shown in Table 2. Distinguished FWHM reduction for different spectral lines is also shown in Table 2. Compared with LIBS, MAE-LIBS spectra had FWHM reductions of about 43%, 43%, 53%, and 47% for Na I 589.0 nm, Na I 589.6 nm, K I 766.5 nm, and K I 769.9 nm, respectively. It also had a little reduction for the FWHMs of weak self-absorption spectral lines. The reduction for the FWHM of Ca I 422.7 nm was more pronounced than the Ca II 393.4 and Ca II 396.8 nm. The results demonstrate that the effective reduction of the self-absorption can be achieved by MAE-LIBS.

Table 2. The FWHMs of LIBS and MAE-LIBS.

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Theoretically, the self-absorption coefficient ( $S A$ ) is usually used to evaluate the self-absorption effect ( $S A$ is equal to one if there is no self-absorption, while it decreases to zero if the self-absorption becomes serious); and the $S A$ can be expressed as [35]:

S A = Δ λ \frac{1 - e^{- K / Δ λ_{0}}}{K},

where

Δ λ_{0}

is the true FWHM of a spectral line, there is a negative correlation between

S A

and

K

, which means the higher

K

corresponds to the more serious self-absorption effect. And

K

can be calculated as [35]:

K = 2 \frac{e^{2}}{m c^{2}} n_{i} f λ_{0}^{2} l,

where

e

and

m

are the charge (statcoulomb) and the mass (g) of the electron,

f

is the oscillator strength (dimensionless) of the transition,

c

is the speed of light (cm/s),

n_{i}

is the number densities (cm⁻³),

λ_{0}

is the central wavelength (cm) of the transition, and

l

is the absorption path length (cm).

Under local thermodynamical equilibrium (LTE), the distribution of particles in the energy levels obey the Boltzmann equation:

n_{i} = \frac{N g_{i}}{U (T)} e^{- \frac{E_{i}}{k T}}

where

N

is total particle number which is proportional to elemental concentration (C),

U (T)

is a partition function, and

k

is Boltzmann constant.

The oscillator strength is proportional to the transition probability, according to the Ladenburg formula:

f = \frac{g_{k}}{g_{i}} \frac{m c}{8 π^{2} e^{2}} λ_{0}^{2} A_{k i},

where

g_{k}

and

g_{i}

are the statistical weight of the upper and lower levels, respectively.

Using Eqs. (2) to (4), the K can be expressed as:

K = \frac{1}{4 π^{2} c} \frac{N g_{k}}{U (T)} e^{- \frac{E_{i}}{k T}} λ_{0}^{4} A_{k i} l .

Obviously, $K$ is proportional to $N$ , $g_{k}$ , $λ_{0}$ , and $A_{k i}$ , while it is inversely proportional to $E_{i}$ . For $N$ proportional to elemental concentration (C), Eq. (5) can be modified as:

K = A K^{'},

where A is a constant,

K^{'} = \frac{C g_{k}}{U (T)} e^{- \frac{E_{i}}{k T}} λ_{0}^{4} A_{k i},

the higher

K^{'}

corresponds to the more serious self-absorption effect.

The spectroscopic parameters of the spectral lines and calculated value of $K^{'}$ are listed in Table 3.

Table 3. Spectroscopic parameters of the spectral lines and calculated value of $K^{'}$ .

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As shown in Table 3, there was a positive correlation between FWHM reductions of MAE-LIBS spectra and $K^{'}$ values, which indicated that the more serious self-absorption effect corresponded to the higher FWHM reductions of MAE-LIBS spectra. For example, Na I 589.0 and 589.6 nm are resonant lines ( $E_{i}$ = 0), while Na I 818.3 and 819.5 nm are nonresonant lines. The self-absorption of Na I 589.0 and 589.6 nm are more serious than that of Na I 818.3 and 819.5 nm. Therefore, as can be seen from Table 2, the self-absorption reduction of Na I 589.0 and 589.6 nm was more obvious than that of Na I 818.3 and 819.5 nm by using MAE-LIBS.

In this case, K I 766.5 and 769.9 nm and K I 404.4 and 404.7 nm are all resonant lines; but the central wavelength and transition probability for K I 766.5 and 769.9 nm are higher than for K I 404.4 and 404.7 nm. Therefore, as can be seen from Table 2, the self-absorption reduction of K I 766.5 and 769.9 nm is more obvious than that of K I 404.4 and 404.7 nm by using MAE-LIBS. This conclusion can apply to the elements of Al, Si, and Ca as well; however, that is not verified here. The conclusion indicates that MAE-LIBS can effectively reduce both serious and weak self-absorption of spectral lines.

3.5 Mechanism of multielemental self-absorption reduction in MAE-LIBS

A simple description of the self-absorption mechanism in MAE-LIBS is depicted in Fig. 7. Under ideal conditions, plasma in LIBS is optically thin; and the self-absorption can be negligible. Technically, in traditional LIBS (Fig. 7(a) without microwave), plasma is often optically thick, where the temperature in the plasma center is much higher than the periphery. Therefore, there are massive ground-state atoms at the plasma periphery [20]. Interior atoms in the excited state transit down to the ground state and emit photons. However, when the emission passes through the colder regions outside, the photons are absorbed by the same type of emitting atoms. As a result, the spectral lines observed in a spectrometer are weakened, even with dips at the centers of the spectral lines.

Fig. 7 A simple description of the self-absorption reduction mechanism in MAE-LIBS.

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As shown in Fig. 7(b), the microwave energy was coupled into laser-induced plasma by near-field radiation to reduce the self-absorption effects. The two needles of the microwave radiator covered the plasma completely. If the plasma electron density of the laser-induced plasma was above the order of 10¹¹ cm⁻³ (7x10¹⁰ cm⁻³ for a radiation at 2.45 GHz) [21], the microwave did not interact with the laser-induced plasma; and it was like a mirror for the microwave radiation. Initially, the electron density of laser-induced plasma was high, about 10¹⁷-10¹⁹ cm⁻³; however, it decreased below the critical electron density when the plasma relaxed and at its periphery. Therefore, the plasma periphery was then no longer a mirror for the microwave radiation and could be coupled to the electromagnetic field. The electrons in the plasma periphery absorbed the microwave radiation and were then accelerated to provide kinetic energy to excite the atoms by multiple collisions [21]. In this situation, ground-state atoms at the plasma periphery absorbed microwaves and transited up to excited states. The number of atoms in the ground state, which is responsible for self-absorption, decreased drastically. The photons emitted from the plasma interior were not absorbed. Accordingly, the self-absorption effect was effectively reduced. As a result of the microwave energy was transferred to electrons, not to a specific atom or molecule, utilizing a microwave can stimulate almost all species of ground-state atoms instantaneously. Therefore, MAE has no wavelength selectivity, which makes it possible to simultaneously reduce self-absorption of multielements. It should be pointed out that the lifetime of ionic lines was usually only a few microseconds. When the electron number density decreased to the critical electron density, the microwave can couple into laser-induced plasmas, while most ionic lines were disappeared. It means that most ionic lines are less affected by the microwave, except for a few ionic lines with long lifetimes.

4. Conclusions

In summary, to reduce multielemental self-absorption within a wide spectral range from 200 to 900 nm in LIBS, MAE-LIBS was investigated under ambient conditions. The spectral lines of high-concentration K and Na elements in potassium feldspar samples detected by different methods of LIBS and MAE-LIBS were compared. Obvious self-absorption phenomena were observed in conventional LIBS but not in MAE-LIBS. The results showed that the FWHMs were reduced by about 43%, 43%, 53%, and 47% for Na I 589.0 nm, Na I 589.6 nm, K I 766.5 nm, and K I 769.9 nm in the potassium feldspar samples, respectively. MAE-LIBS also had an enhancement to the spectral peak intensity and a little reduction to the FWHMs of the weak self-absorption spectral lines. The self-absorption reduction by MAE-LIBS depended on the extent of self-absorption of the spectral lines which was determined by many parameters, such as the elemental concentration, the central wavelength, the statistical weight of upper level, the transition probability, and the energy level. The results confirmed that MAE-LIBS is capable of reducing the self-absorption effect for multiple elements simultaneously. Moreover, microwave generators are compact and economical. This work shows great potential and feasibility for MAE to improve LIBS analyses.

Funding

This research was financially supported by National Natural Science Foundation of China (61575073, and 51429501).

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Element	Line (nm)	FWHM (Å)		FWHM Reduction
Element	Line (nm)	LIBS	MAE-LIBS	FWHM Reduction
Na(I)	589.0	4.15	2.36	43%
	589.6	3.12	1.77	43%
	818.3	2.40	2.36	2%
	819.5	2.65	2.48	7%
K(I)	404.4	1.52	1.48	3%
	404.7	1.15	1.15	0%
	766.5	8.29	3.94	53%
	769.9	6.21	3.30	47%
Al(I)	394.4	1.28	1.19	7%
Al(I)	396.2	1.32	1.22	8%
Si(I)	288.2	1.14	1.06	7%
Ca(I)	422.7	1.12	0.92	18%
Ca(II)	393.4	1.05	1.04	1%
Ca(II)	396.8	1.18	1.16	2%

Element	Line (nm)	Transition Probability A_ki(s⁻¹)	Lower level energy E_i (eV)	Upper level energy E_k (eV)	Upper Statistical weight g_k	Partition function U(T) T = 5300K	C (wt.%)	$K^{'}$ (x10¹⁷)
Na(I)	589.0	6.16x10⁷	0	2.104	4	2.09	3.69	523
	589.6	6.14x10⁷	0	2.102	2	2.09	3.69	262
	818.3	4.29x10⁷	2.102	3.617	4	2.09	3.69	14
	819.5	5.14x10⁷	2.104	3.617	6	2.09	3.69	24
K(I)	404.4	1.16x10⁶	0	3.065	4	2.31	9.6	5
	404.7	1.07x10⁶	0	3.063	2	2.31	9.6	2
	766.5	3.80x10⁷	0	1.617	4	2.31	9.6	2180
	769.9	3.75x10⁷	0	1.610	2	2.31	9.6	1095
Al(I)	394.4	4.99x10⁷	0	3.143	2	5.89	18.63	76
Al(I)	396.2	9.85x10⁷	0.014	3.143	2	5.89	18.63	149
Si(I)	288.2	2.17x10⁸	0.781	5.082	3	9.56	66.26	56
Ca(I)	422.7	2.18x10⁸	0	2.933	3	1.23	0.76	129
Ca(II)	393.4	1.47x10⁸	0	3.151	4	2.26	0.76	47
Ca(II)	396.8	1.40x10⁸	0	3.123	2	2.26	0.76	23

Element	Line (nm)	FWHM (Å)		FWHM Reduction
Element	Line (nm)	LIBS	MAE-LIBS	FWHM Reduction
Na(I)	589.0	4.15	2.36	43%
	589.6	3.12	1.77	43%
	818.3	2.40	2.36	2%
	819.5	2.65	2.48	7%
K(I)	404.4	1.52	1.48	3%
	404.7	1.15	1.15	0%
	766.5	8.29	3.94	53%
	769.9	6.21	3.30	47%
Al(I)	394.4	1.28	1.19	7%
Al(I)	396.2	1.32	1.22	8%
Si(I)	288.2	1.14	1.06	7%
Ca(I)	422.7	1.12	0.92	18%
Ca(II)	393.4	1.05	1.04	1%
Ca(II)	396.8	1.18	1.16	2%

Element	Line (nm)	Transition Probability A_ki(s⁻¹)	Lower level energy E_i (eV)	Upper level energy E_k (eV)	Upper Statistical weight g_k	Partition function U(T) T = 5300K	C (wt.%)	$K^{'}$ (x10¹⁷)
Na(I)	589.0	6.16x10⁷	0	2.104	4	2.09	3.69	523
	589.6	6.14x10⁷	0	2.102	2	2.09	3.69	262
	818.3	4.29x10⁷	2.102	3.617	4	2.09	3.69	14
	819.5	5.14x10⁷	2.104	3.617	6	2.09	3.69	24
K(I)	404.4	1.16x10⁶	0	3.065	4	2.31	9.6	5
	404.7	1.07x10⁶	0	3.063	2	2.31	9.6	2
	766.5	3.80x10⁷	0	1.617	4	2.31	9.6	2180
	769.9	3.75x10⁷	0	1.610	2	2.31	9.6	1095
Al(I)	394.4	4.99x10⁷	0	3.143	2	5.89	18.63	76
Al(I)	396.2	9.85x10⁷	0.014	3.143	2	5.89	18.63	149
Si(I)	288.2	2.17x10⁸	0.781	5.082	3	9.56	66.26	56
Ca(I)	422.7	2.18x10⁸	0	2.933	3	1.23	0.76	129
Ca(II)	393.4	1.47x10⁸	0	3.151	4	2.26	0.76	47
Ca(II)	396.8	1.40x10⁸	0	3.123	2	2.26	0.76	23

Multielemental self-absorption reduction in laser-induced breakdown spectroscopy by using microwave-assisted excitation

Abstract

1. Introduction

2. Experimental

2.1 Experimental setup

2.2 Samples

3. Results and discussion

3.1 Dependence of the self-absorption effect on the position of the microwave radiator

3.2 Dependence of the self-absorption effect on microwave power

3.3 Multielemental self-absorption reduction by MAE-LIBS

3.4 Evaluation of multielemental self-absorption reduction in MAE-LIBS

3.5 Mechanism of multielemental self-absorption reduction in MAE-LIBS

4. Conclusions

Funding

References and links

Cited By

Figures (7)

Tables (3)

Equations (6)

Optics Express