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For aqueous-solution-based elemental analysis, we used a thin liquid sheet (μm-scale thickness) in laser-induced breakdown spectroscopy with nanosecond laser pulses. Laser-induced plasma is emitted by focusing a pulsed Nd:YAG laser (1064 nm) on a 5- to 80-μm-thick liquid sheet in air. To optimize the conditions for detecting elements, we studied how the signal-to-background ratio (SBR) for Hα Balmer and Na-neutral emission lines depends on the liquid-sheet thickness. The SBR of the Hα Balmer and Na-neutral lines was maximized for a sheet thickness of ~20 μm at the laser energy of 100 mJ. The hydrodynamics of liquid flow induced by the laser pulse was analyzed by laser flash shadowgraph imaging. Time-resolved observation of the hydrodynamics and plasma emission suggests that the dependence of the SBR on the liquid-sheet thickness is correlated with the volume of flowing liquid that interacts with the laser pulses.
© 2014 Optical Society of America
1. Introduction
Laser-induced breakdown spectroscopy (LIBS) is an attractive technique for the rapid determination of the elemental composition of gas, solid, and liquid samples and offers advantages for multi-elemental, noninvasive, and remote analysis with little sample preparation [1, 2]. Despite the wide application of LIBS to elemental analysis of gas and solid samples, it is difficult to use this method with liquid samples because splashes and ripples that form on the liquid prevent efficient detection of plasma emission in the ablation [3–5]. Moreover, laser optics contaminated by splashes compromise the reliability of LIBS. To overcome these difficulties, researchers have tried several approaches: single-pulse and double-pulse ablation in laminar flows with sheath or carrier gases [6–11], liquid jets [12–16], liquid droplets [17, 18], aerosols [19–21], and the use of paper substrates [22]. The most sensitive analysis method of these is the laminar-flow technique with the sheath or carrier gas. The liquid-jet technique, however, is more sensitive than the liquid-droplet or the aerosol technique and requires a simpler device than that required for the laminar-flow technique with sheath or carrier gas [12–16]. Thus, LIBS using liquid jets would be the most cost-effective technique that is suitable for remote on-line analysis.
Several groups have reported various liquid-jet techniques that improve LIBS sensitivity. Samek et al. prepared a 0.1-mm-thick liquid film using a special jet nozzle that is often employed in dye laser systems and detected Na in water with a detection limit of 80 ppb in a conventional single-pulse LIBS measurement [12]. The result shows that the liquid-jet technique can provide high sensitivity. Comparing an LIBS experiment with a thick 1.0 mm jet, a thin 0.3 mm jet, and mist, Kumar et al. observed that the thick jet leads to better performance and showed that the signal is greater for a thick jet, although the detection limit was the same for the thick and thin jets [14]. Furthermore, Yaroshchyk et al. optimized the optical conditions of a 1.0-mm-thick liquid jet and observed that the atomic emission was strongly correlated with both laser pulse energy and focal position [15]. Additionally, they noted that the atomic emission intensity is related to the light-emitting volume of the liquid sample.
We consider that the liquid–laser interaction volume wherein the laser and the liquid interact is an important factor for detecting sensitivity because ablation in thicker liquid targets leads to very low emission intensity. This is essentially due to the splash of the liquid in the ablation, where the laser energy is consumed by the formation of liquid droplets instead of by the formation of high-density plasma. Therefore, it is essential to find the conditions that suppress splashing. For example, one way to mitigate splashing is to use thin liquid jets to minimize droplet formation at the plasma emission point. However, LIBS with a liquid jet less than 0.1 mm thick has yet to be reported.
Several publications discuss the fluid dynamics of ultrathin liquid jets with very flat surfaces, which is employed a colliding-type jet nozzle [23, 24]. The slit-type jet nozzle is the simplest nozzle to obtain optically flat liquid sheets, which can also be formed by the double-razor-blade nozzle that was developed for dye laser systems [25]. In the present study, we investigate the optimal jet thickness using micrometer-thick liquid jets with the slit-type jet nozzle for single-pulse LIBS measurements by high-sensitivity elemental analysis of a liquid sample. The dependence of detection efficiency on jet thickness is discussed based on time-resolved observation of the hydrodynamics and breakdown-plasma emission of the liquid jet, which were investigated by laser flash shadowgraph imaging.
2. Experimental procedure
2.1 Liquid-sheet formation
The system for forming liquid sheets is shown schematically in Fig. 1. The liquid recirculation system consists of a reservoir, a pump, and a slit-type nozzle equipped with XYZ motion and a rotation stage. The liquid jet of the sample solution was formed by a trapezoidal-shaped grooved nozzle tip composed of stainless steel with a nozzle exit of 0.6 mm × 0.3 mm (Alignment Systems Co., Type L), similar to a double-razor-blade nozzle [24, 25]. The sample solution, which could be distilled water or an aqueous solution of NaCl, was continuously circulated by a peristaltic pump (Cole-Parmer, Masterflex 7553-70, 7518-10) through silicon tubes and was pumped from a 200-ml reservoir. The circuit also contained a pulse dampener (Cole-Parmer, Masterflex EW-07596-20) and a flow meter (HORIBA STEC, LM05ZZWN).
2.2 Setup for plasma spectroscopy and image acquisition
Figure 2 shows a schematic of the LIBS setup, which comprises a laser for generating the breakdown plasma, the liquid sheet of the sample solution, and the detection system for analyzing the plasma emission. The breakdown plasma was generated by 6 ns fundamental pulses of a Q-switched Nd:YAG (1064 nm, Continuum, Powerlite-8000). The laser beam was incident normally onto the liquid sheet and was focused by a 100-mm-focal-length plano-convex lens. The focus of the Nd:YAG laser was at the center of the liquid sheet. For this study, the laser energy was fixed at 100 mJ.
Fig. 2 Experimental setup for laser flash shadowgraph imaging and laser-induced emission spectroscopy.
To detect the optical emission spectra, the plasma emission generated by the laser was collected at 90° from the plasma and imaged into an optical fiber by a pair of 100-mm-focal-length plano-convex lenses. The optical fiber guided the emission into a spectrometer (Acton Research, Spectra Pro 2300i) equipped with a 300-grooves/mm grating and a 20-μm entrance slit. Time-resolved plasma emission spectra were recorded by an intensified charge-coupled-device camera (ICCD, Princeton Instruments, PI-MAX3: 1024iRB), which was triggered with given time delay with respect to the laser pulse. A fast photodiode was used to adjust the delay time between the onset of plasma generation and the beginning of data acquisition.
The method of laser flash shadowgraph imaging was employed to analyze the hydrodynamics of the liquid sheet. The setup was similar to that in [26], Nguyen et al.. Briefly, pump and probe (Q-switched Nd:YAG, 532 nm; Continuum, Surelite II) pulses were synchronized with a proper delay time determined by a delay generator (Stanford Research Systems, DG535). The delay generator was triggered by the pump laser, and the gate width of the ICCD camera (Hamamatsu, C2925-0, C6558) was set to 40 ns. Focusing the ablation laser onto the liquid surface created bright plasma at the surface. To avoid interference of the plasma emission with the shadowgraph imaging laser, we used a 532 nm band-pass filter to observe only the probe laser light. By controlling the delay time between the pump laser pulse and the ICCD camera trigger, the temporal evolution of the plasma emission was also captured without the probe laser and the band-pass filter.
3. Results and discussion
3.1 Thickness of liquid sheet
Figure 3 shows front- and side-view photographs of the stable liquid sheet of water flowing at 130 mL/min that was used for this experiment. The liquid surface tension causes a thin fluid sheet to form between two thick cylindrical rims. The thick cylindrical rims collide, producing another thin sheet in the plane orthogonal to the first link. The length and width of the sheet increase with increasing flow rate, but the length-to-width ratio remains constant.
Fig. 3 Front- and side-view photographs of liquid sheet of water with flow rate of 130 mL/min. r: the distance from the nozzle exit.
The liquid-sheet jet produced by the trapezoid-shaped nozzle has excellent optical properties, so that the thickness of the ultrathin jet can be measured using interference fringes [24, 25]. When collimated light of wavelength λ is incident on the jet, the light reflected from the front surface of the jet interferes with that reflected from the back surface of the jet. As a result, the reflected spectrum has a sine-wave structure, where the intensity periodically varies as a function of λ. Based on Snell’s law, the thickness d of the jet is
where λ1 and λ2 indicate the wavelengths of neighboring peaks in the sine-wave structure, n is the index of refraction of the liquid sample, and θ is the angle of incidence of the collimated light.We focused white light on the jet and collected reflection spectra with a spectrometer (StellarNet Inc. EPP2000). A typical interference pattern collected from a water jet at 20 mm from the nozzle exit is shown in Fig. 4(a). Using Eq. (1) with θ = 45°, λ1 = 581.11 nm, and λ2 = 568.33 nm, we obtained a thickness d = 11.2 μm. The index of refraction of water was taken from [27], Lide. The thickness measured along the centerline of the liquid sheet varied from 5 to 40 μm for distilled water, as shown in Fig. 4(b). The liquid-sheet thickness d was inversely proportional to the distance r from the nozzle exit, which is similar to the case of a liquid sheet formed by two impinging jets as reported by Taylor [23]. The interference spectrum was difficult to observe for r < 5 mm from the nozzle exit, where the liquid sheet was too thick. The dashed line shows an extrapolation that uses d∝1/r. Furthermore, the horizontal thickness of the liquid sheet was approximately uniform. The thickness of the liquid sheet remained constant for flow rates ranging from 90 to 140 mL/min. For flow rates greater than 145 mL/min, the liquid sheet broke up before the first link, making it unusable for LIBS measurements because it generated too much mist.
Fig. 4 (a) Typical interference pattern for water-sheet jet at 20 mm from nozzle exit. (b) Thickness variations in liquid-sheet jet along centerline for distilled water.
3.2 LIBS emissions as a function of sheet thickness
LIBS spectra were acquired from the ablation of thin liquid sheets in air as a function of time between laser pulse irradiation and ICCD detection. For all measurements, the ICCD gate width was 1 μs. Figure 5 shows typical LIBS emission lines (Hβ Balmer line at 486.134 nm) from distilled water in air. The high wing at the short wavelength side of the lines is most likely due to the tail of the Hγ Balmer line at 434.04 nm. Figure 5(a) shows the Hβ lines for jet thickness of 7, 17, and 75 μm and for a delay time of 1.0 μs, and Fig. 5(b) shows those for a delay time of 5.0 μs. The Hβ Balmer line at 1.0 μs delay was spectrally broadened by the Stark effect and was characteristically asymmetric in jet thickness [28]. The broadening was drastically reduced for the delay time of 5.0 μs. At 1.0 μs delay, the measured line intensity was low for thinner jet thickness and increased with increasing thickness, but the intensity decreased as the jet thickness was 75 μm [see Fig. 5(a)] whereas, at 5.0 μs delay, the emission intensity of the Hβ Balmer line increased in the order 75 μm < 7 μm < 17 μm [Fig. 5(b)]. If the emission intensities were simply proportional to the liquid–laser interaction volume, then the emission intensity would be enhanced upon increasing the jet thickness (7 μm < 17 μm < 75 μm). However, the observed emission intensity as a function of liquid-sheet thickness is not consistent with this interpretation. This result suggests that, for LIBS measurements that use aliquid sheet of micrometer thickness, the intensity of atomic emission is not determined exclusively by the interaction volume.
Fig. 5 Measured Hβ line profiles for time delays of (a) 1.0 μs and (b) 5.0 μs following laser-induced breakdown for liquid-sheet thicknesses of 75, 17, and 7 μm.
Stark-broadened line widths can be used to estimate the electron density in plasmas. For hydrogen and hydrogenic ions, the Stark effect is linear to the first approximation, and the expression connecting the electron density Ne in cm−3 with the wavelength shift ∆λFWHM [full width at half maximum (FWHM)] in Å is given by Griem [29] as
where e is the electron charge, α1/2 is the fractional half width of the reduced Stark profile, and α1/2 is a function of the electron density and temperature, i.e., α1/2 = α1/2 (Ne, T), which is tabulated in Table 3.a of [29], Griem for the Hβ line. For an electron density of ~1017 cm−3, the electron temperature is ~10000 K and the fractional width α1/2 = 0.0851 Å. The FWHM of Hβ was obtained after correcting the measured line width for instrumental broadening: Δλtotal = Δλobserved + Δλinstrument. In our case, Δλinstrument was 0.128 nm as determined by measuring the FWHM of the Hg lines emitted by a standard high-pressure Hg lamp. Furthermore, we determined plasma temperatures in the range 9000–11000 K from Boltzmann plots by measuring the three hydrogen lines of the Balmer series (Hα: 656.27 nm, Hβ, Hγ). Figure 6 shows the temporal evolution of the electron density for liquid-sheet thicknesses of 75, 17, and 7 μm. For each jet thickness, the electron density decreased with increasing delay time. This tendency explains the reduced spectral broadening due to the Stark effect with increasing time delay [Figs. 5(a) and 5(b)], because the broadening is proportional to the electron density of the ablation plasma. By comparing the electron density for the different liquid-sheet thicknesses, we noticed that the electron density for the 75-μm-thick liquid sheet decayed faster than that for the 7- and 17-µm-thick liquid sheets, although the time decay for the 7- and 17-µm-thick liquid sheets is essentially the same. This result suggests that the plasma from a thicker liquid sheet is easier to quench than that from a thinner liquid sheet.Fig. 6 Electron number densities determined from full width at half maximum of Hβ profiles, following laser-induced breakdown for jet thicknesses of 75, 17, and 7 μm.
To optimize the stability and sensitivity of the LIBS method, we examined the signal-to-background ratio (SBR) for different liquid-sheet thicknesses. Figure 7 shows how the SBR of the emission from excited H and Na neutrals depends on the liquid-sheet thickness for thicknesses in the range 5–10 µm. The emission from excited H neutrals (the Hα line) was measured by ablating distilled water, and that from excited Na neutrals (588.99 nm) was measured by ablating an aqueous solution of 1 ppm NaCl. The gate width of the ICCD camera was set to 1.0 μs for Hα and to 20 µs for Na. The results for both H and Na are similar to thosefor the SBR: they increase with liquid- sheet thickness. An optimal thickness seems to be approximately 20 μm in our experiments, but the SBR decreased when the liquid-sheet thickness exceeded 25 μm. For increasing liquid-sheet thickness, the splash increased because the liquid–laser interaction volume increased. This effect may have shortened the lifetime of the laser-produced plasma.
Fig. 7 Signal-to-background ratio as a function of liquid-sheet thickness for distilled water and 1 ppm Na aqueous solution.
3.3 Shadowgraph imaging of flowing liquid
Figures 8(a)–8(c) show the dynamics of liquid ablation due to laser breakdown for liquid-sheet thicknesses d = 80, 20, and 7 μm. The central line is the image of the liquid sheet when observed from the side. No difference appears between the liquid-sheet thicknesses in Figs. 8(a)–8(c) because the jet is composed of a thin fluid sheet bounded by a relatively thick cylindrical rim, as shown in Fig. 3. The laser propagation direction is from left to the right in the image. At a delay time of 0.5 µs, a shock wave that was induced by laser-induced breakdown of the air and/or liquid occurs in the direction of laser irradiation, as shown by the white lines in Fig. 8(a). Here we define the direction of the incoming laser beam to be forward and the opposite direction to be backward. The thin end of the shock wave is toward the forward direction and the broad end is toward the backward direction. Simultaneously, the splash from the liquid sheet starts to spread. The spread of the splash depends strongly on the liquid-sheet thickness. The splash spreads forward and backward for d = 80 μm, whereas it spreads only forward for d = 20 and 7 μm. Additionally, the liquid-sheet surface on the side on which the laser is incident fluctuated for d = 80 μm, but remained flat for d = 20 and 7 μm. A thinner liquid sheet thus avoided splashing during the plasma onset. This indicates that it is possible to avoid contaminating the laser optics.
Fig. 8 Time-resolved shadowgraph images of liquid jet flow, resulting from a 100-mJ laser pulse in air, as a function of liquid-sheet thickness d. (a) d = 80 μm, (b) d = 20 μm, (c) d = 7 μm. The laser propagation direction is from left to right in the image. Horizontal bars at the left top in the figures indicate 1 mm.
For high-power laser heating of absorbing liquids, explosive vaporization, which is characterized by superheating and cavitation, is the dominant mechanism of ablation in the early stages [30–32]. The plasma expands at velocities well above the speed of sound, which results in the emission of a strong shockwave. Shortly after the end of the laser pulse, the plasma recombines, forming a volume of superheated water vapor, which then grows as a gas bubble. Ultrafine liquid droplets are produced by a phase explosion of superheated liquid that spontaneously decomposes into a mixture of saturated vapor and saturated liquid. Thus, our experiment shows that growth of the cavity and collapse of the bubble occur approximately at the same time, thereby creating the splash because the liquid volume is very small.
The spread of the splash can determined whether the interaction of the pulsed laser with the liquid sheet causes ablation at the liquid surface or breakdown in the air. The thicker liquid flow from the bulk liquid surface terminates the ablation process. For a thick liquid sheet, ablation at the liquid surface and breakdown in the air occur simultaneously; therefore, a large quantity of droplets is generated in both directions because of cavity growth and collapse. However, for a thin liquid sheet, bubbles do not become sufficiently large and the breakdown in air is dominant. Air breakdown occurs in front of the focal point of the lens, as described in the next section. As a result, for the thin liquid sheet, the splash spreads from the liquid in the forward direction because of recoil momentum of the strong shock wave that in turn results from air breakdown.
For our experimental conditions, the average velocity of the shock wave, which propagates perpendicular to the laser beam in the ambient air, was calculated by a detailed analysis of the time-resolved shadowgraph image to be 3.0 km/s in the backward direction. This value is close to the velocity of laser ablation of the water jet over several hundred micrometers, which shows a weak dependence on the water-jet diameter [33]. In the early stage of our experiment, the average velocity of the shock wave in the backward direction does not depend on the liquid-sheet thickness [see Figs. 8(a)–(c)]. This result suggests that the shock wave does not affect the optical emission intensities.
3.4 Dynamics of plasma emission
Figure 9(a) shows the dynamics of the laser-induced plasma in the absence of the liquid sheet, and Figs. 9(b)–9(d) show the dynamics when the liquid sheet is present and for liquid-sheet thicknesses d = 80, 20, and 7 μm. The horizontal lines indicate the centerline of the incident laser beam and vertical lines in Figs. 9(b)–9(d) indicate the position of the liquid sheet. Figure 9(a) suggests that the air spark, which is bimodal in appearance, initiates slightly before the focal point of the lens. When the laser power exceeds the optical breakdown threshold, plasma forms in the focal volume. As the laser power increases during the pulse, the optical breakdown threshold is exceeded further upstream of the focal volume. Therefore, the region of highest emission is located in the direction of the incoming laser beam because the optical energy is absorbed by the plasma that is generated before the beam waist [34–37].
Fig. 9 Images showing dynamics of emission from a laser-induced spark for a range of time delays and for a 100-mJ incident laser pulse (a) in air (without liquid sheet) and at a thicknesses of (b) d = 80 μm, (c) d = 20 μm, (d) d = 7 μm. The laser propagation direction is from left to right in the image. Horizontal lines indicate the centerline of the incident laser beam and vertical lines in panels (b)–(d) indicate the position of the liquid-sheet flow. Horizontal bars at the top left in the figures indicate 1 mm.
As shown in Figs. 9(b)–9(d), when the liquid sheet was at the center of the air spark, the appearance of the spark changed from bimodal to unimodal, and a single strong plasma emission line was observed only on the side of the liquid sheet where the laser was incident because of plasma shuttering by the water. The liquid sheet used in these experiments was so thin that laser-induced ablation occurred not only in the liquid but also in the surrounding atmosphere. Comparing Figs. 9(a)–9(d) reveals that plasma emission in the liquid sheet was brighter than that in the air. The breakdown threshold in an aerosol (mist or small liquid droplets) is known to be two orders of magnitude lower than that in air [38]. At the beginning of laser ablation in the liquid sheet, the ablation plasma causes the liquid to evaporate and produces mist or small droplets; therefore, only limited splashing occurs afterwards. The presence of the mist or small droplets thus leads to intense plasma emission.
The emission volume of the laser breakdown plasma depends on the jet thickness. For d = 20 and 7 μm, the plasma emission propagated along the centerline of the incident laser as a function of delay time [see Figs. 9(c) and 9(d)] whereas, for d = 80 μm, the emission away from the central axis was quenched near the liquid surface [Fig. 9(b)] because a large splash was generated and the liquid surface was deformed. Furthermore, the lifetime of the plasma emission for d = 80 μm was shorter than that for d = 20 and 7 μm. This tendency is consistent with the dependence of electron density on delay time, as shown in Fig. 6.
The ideal intensity of the plasma emission declines with decreasing thickness because the liquid–laser interaction volume decreases. Thus, to apply LIBS to a thin liquid sheet, the ideal intensity of the plasma emission is directly proportional to the liquid thickness. Yaroshchyk et al. reported that a large liquid–laser interaction volume causes greater emission intensity; that is, the variation in volume is consistent with the variation in emission intensity [15]. Realistically, quenching by splashing and fluctuations in the liquid sheet should be considered when observing plasma emission. As shown in Figs. 7 and 9, this effect is serious above d = 20 μm and can be neglected below d = 20 μm under the present conditions. Accounting for the quenching effect due to splashing and liquid-sheet fluctuation, the dependence of the SBRs on the liquid thickness can be qualitatively explained. On comparing the dynamics of the plasma emission shown in Figs. 9(b)–9(d), we conclude that, for a liquid-sheet thickness of d = 80 μm, the plasma emission in the vicinity of the liquid-sheet surface was quenched by a combination of splashing and fluctuation of the liquid surface, which resulted in the low SBRs shown in Fig. 7. The emission volume at d = 20 μm was larger than that at d = 7 μm, which is consistent with the dependence of the SBR on emission volume, as shown in Fig. 7.
4. Conclusions
We studied the breakdown induced by the interaction of a laser pulse with distilled water and with a Na aqueous solution using ultrathin (5 to 80 μm) liquid sheets in air. Both H and Na follow similar trends, and the SBR is an increasing function of jet thickness d up to d = 20 μm, but the SBR reduces for liquid sheets thicker than 25 μm. For our experimental conditions, the optimum SBR occurred for a liquid-sheet thickness of approximately 20 μm. The dynamics of liquid flow and plasma emission behavior during plasma expansion were investigated by the techniques of laser flash shadowgraph imaging and laser-induced emission spectroscopy. The time-resolved observation of hydrodynamics and optical emission suggest that the detection sensitivity was affected by splashing from the liquid, which depended on the liquid–laser interaction volume. Using the appropriate liquid-sheet thickness, the liquid LIBS technique would be attractive for detecting elements in aqueous solution and could be extended to a wide range of industrial applications of atomic emission spectroscopy.
Acknowledgments
The authors gratefully acknowledge the support of M. Toshimitsu and K. Tanabe for the experimental work. The present study includes the result of the work entitled “Development of laser remote analysis for next-generation nuclear fuel and applied study by MOX sample” entrusted to Japan Atomic Energy Agency by the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT). This work was partly supported by JSPS KAKENHI Grant No. 24560068.
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