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Abstract
This paper studies the analysis of Na element concentration in NaCl aqueous solution using laser-induced breakdown spectroscopy (LIBS). The NaCl solution is transformed to a thin water film. The water film can provide a stable liquid surface, and overcome the disadvantage that laser focusing position cannot be fixed due to liquid level fluctuation (when nanosecond laser is used as the excitation light source, there is serious liquid splash phenomenon, which affects the signal stability). And, femtosecond pulse laser is used to excite the water film to produce the plasma, avoiding liquid splashing. The measured emission lines are Na (I) at 589.0 nm and 589.6 nm. The calibration curves of sodium are plotted by measuring different concentrations of NaCl solution. The linear correlation coefficients of Na (I) lines at 589.0 nm and 589.6 nm are 0.9928 and 0.9914, respectively. In addition, the relative standard deviation is also calculated; its range is from 1.5% to 4.5%. The results indicate that the combination of femtosecond laser and water film can significantly improve the signal stability for liquid analysis in LIBS.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Sodium (Na) is an indispensable trace element in human body. An appropriate amount of Na is beneficial to human health. The main functions of Na are as follows: 1) main positively charged ion in extracellular fluid, which participates in water metabolism and regulate water and osmotic pressure in body; 2) ensuring acid balance in body; 3) a major component of pancreatic juice, bile, sweat, and tears; 4) affecting muscle movement, cardiovascular function, and energy metabolism; 5) maintaining blood pressure [1]. However, excessive Na will damage the human body including hypertension, cardiovascular and cerebrovascular diseases, and endocrine disorders. Therefore, controlling the intake of Na is very important to human health. In China, drinking water mainly comes from river or groundwater. Unexpectedly, in recent years, the use of bleach is becoming more and more in industrial production, an ocean of untreated bleach is discharged into river or groundwater with industrial water. Bleach is rich in Na ion, which affects drinking water quality. Once people drink the water which rich in Na ion for a long-time, this will endanger the health of human body. So that, how to detect the Na element in water scientifically and effectively has become a hot-spot for environmental protection departments and researchers.
At present, the quantitative detection methods of elements in liquid mainly include atomic absorption spectrometry [2–4], inductively coupled plasma mass spectrometry [5, 6], inductively coupled plasma atomic emission spectrometry [7–9], graphite furnace atomic absorption spectrometry [10], and X-ray fluorescence spectroscopy [11]. Although these methods are excellent for analyzing trace elements in liquid samples, the shortcomings (such as instrument complexity, high cost, and complicated analysis process) severely limit their widespread application. Emerging laser-induced breakdown spectroscopy (LIBS) as an effective technique for detecting material compositions has the advantages of fast, real-time, micro-damage, and detection of samples in any physical states (solid, liquid, gas and aerosol) [12–17]. And the technique has its ability to achieve simultaneous in-situ analysis of multiple elements. Based on these advantages, LIBS has been widely used in water pollution [18, 19], heavy metal pollution of soil [20, 21], industry [22–24], food security [25], artwork identification [26, 27], explosive composition analysis [28], and so on.
In the detection of liquid sample, there are two main types of laser focusing. One is that, the laser is focused inside the liquid sample. In this way, the light collection device is complicated, and the liquid environment will absorb more laser energy and plasma emission, resulting in a reduction in the detection capability. Another is that, the laser is focused on the surface of the liquid sample. The influence of the liquid pressure on the expansion of plasma is very weaker. A longer plasma lifetime can be obtained. And, laser energy and plasma emission will not be absorbed by the liquid in the surrounding environment, leading to a stronger spectral line. Therefore, this type of laser focusing is widely used by researchers. When the laser is focused on the liquid surface, there are different ways of sample injection: 1) stationary liquid surface; 2) horizontal flowing liquid surface; 3) vertical flowing liquid column. Cremers et al. focused laser pulse inside a stationary liquid sample to generate a plasma [29], their results showed that the collected atomic spectral lines of metallic elements was very weaker due to the liquid absorption and the scattering of particles or bubbles inside the liquid. St-Onge et al. used LIBS to analyze liquid formulations, and compared the laser focusing at the liquid inside, stationary liquid surface, and horizontally flowing liquid surface [30], their results indicated that the signal on the liquid sample surface was better than that inside the liquid. However, when the laser pulse is focused on the liquid surface, it is easy to generate a large amount of liquid vaporization splashes, and water surge waves, resulting in a reduction in the repeatability of the experiment. To avoid these drawbacks, researchers have introduced a liquid jet method. Yaroshchyk et al. performed a LIBS analysis of Na, Mg, Al, Ca, and other metal elements in the motor oil [31], they found that vertical flowing liquids had less splashes and lower detection limits than horizontal stationary liquid surface. Skocovska et al. designed a nozzle to control jet velocity [32], their results showed that the relative standard deviation was reduced from 200% to 30%. Thereafter, the liquid jet method has been widely used in the detection of liquid samples by LIBS. However, the liquid jet method or vertical droplets can still contaminate the lens due to water splashing or vapor. The optical system will be so disturbed that analytical sensitivity and repeatability is reduced. And, the absorption of optical emission by the liquid around the laser focusing is also the reason of weak spectral signal [31, 33, 34]. Liquid splashing and poor signal stability still exist in application of LIBS for detecting liquid samples. In recent years, researchers have turned their attention to femtosecond pulse lasers with shorter time scales [35–40]. Golik et al. studied femtosecond LIBS of liquid sample, and identified trace chemical elements in liquid sample [41], confirming the feasibility of quantitative analysis of liquid sample by femtosecond laser. Femtosecond laser has better pulse stability and short pulse duration, can provide sufficient output power. The plasma produced by femtosecond laser is smaller on the spatial scale [42]. Moreover, the magnitudes of mass density, temperature, pressure, and velocity for femtosecond laser-induced plasma (LIP) are lower compared with nanosecond LIP [43]. The plasma plume produced by femtosecond laser shows a narrower angular distribution compared to a broader angular distribution in nanosecond LIP [44]. And, the influence of ambient gas pressure on femtosecond LIP is weaker than that on nanosecond LIP [45–47]. Moreover, the shock wave from femtosecond LIP is the much weaker than that from nanosecond LIP [48, 49]. Therefore, the liquid splashing effect in femtosecond LIP may be weaker compared with nanosecond LIP. The signal stability and repeatability may be improved by femtosecond laser. In recent years, some new methods based on the nanosecond LIBS was used to detect liquid samples, such as surface assisted LIBS [50], surface enhanced LIBS (SEN-LIBS) [51–53], LIBS combined with laser-induced fluorescence (LIBS-LIF) [54], and chemical replacement combined with surface-enhanced LIBS (CR-SENLIBS) [55]. These methods realize the transformation from liquid sample to solid sample by drying the analysis sample on the surface of metal matrix, which is effective to improve the detection stability and sensitivity by overcoming the liquid sample splashing. However, these methods require a series of complicated sample preparation processes, leading to the online analysis of liquid samples cannot be realized.
In this work, we used femtosecond LIBS to detect NaCl aqueous solution. To improve the stability of the liquid surface, a water film system was used as the target. The calibration curve between Na element concentration and spectral intensity was obtained, and linear correlation coefficient R2, relative standard deviation (RSD), and limit of detection (LoD) were discussed. Finally, the R2, RSD and LoD obtained in the experiment were compared with those obtained by other methods based on LIBS, indicating that the combination of femtosecond laser and water film has better application prospect in improving the signal stability of liquid samples detected by LIBS.
2. Experimental detail
2.1 LIBS system
The experimental setup for femtosecond laser-induced water film plasma spectroscopy is presented in Fig. 1(a). The excitation source was a one-box ultrafast Ti:sapphire amplifier (Coherent, Libra) with a wavelength of 800 nm. The output pulse energy was 1.6 mJ, the pulse width was 50 fs. The repetition frequency of the femtosecond laser was adjusted to 2 Hz by setting the synchronization and delay generator. The laser beam was reflected by two mirrors and was focused by a focusing lens with a focal length of 100 mm onto NaCl water film. The optical emission from the femtosecond LIP was collected with another lens (BK7, focal length = 75 mm) placed at an angle of 45° to the laser beam, and was transmitted through a fiber to a spectrometer (Spectra Pro 500i, Princeton Instruments) with a grating of 1200 grooves/mm, the resolution is 0.04 nm. The dispersed light was detected by using an ICCD (PI-MAX4, Princeton Instruments) with 1024×1024 pixels. The ICCD was triggered by the synchronization and delay generator of femtosecond laser system. In order to improve the signal-to-background ratio of the spectrum, we set the ICCD gate width to 5.0 μs and the delay time to 0.6 μs. The experiment was performed in air, the ambient temperature was 22 oC, and the relative humidity is 40%. Each spectral data was an average of 20 laser shots. The aqueous solution was prepared by adding a certain amount of NaCl solute to deionized water, the corresponding eight NaCl aqueous solutions with different Na concentrations were listed in Table 1.
Fig. 1. (a) Experiment setup for femtosecond laser-induced water film plasma spectroscopy (M, mirror; I, iris; L, lens; ICCD, intensified CCD). (b) Water film. (c) Sketch detail diagram of water film forming device.
Table 1. Na concentration in NaCl aqueous solution.
2.2 Water film
We customized a water film forming device using 3D printing technology, Fig. 1(c) displays the sketch diagram of the device. The design idea was on the basis of the liquid jet method [56], we placed two aluminum wires with a diameter of 0.2 mm on both sides of the water outlet, and the distance between the two aluminum wires was 4 mm. By adjusting the water flow velocity to 40 mL/min, the liquid sample formed a free-flowing, stable and continuous water film under the gravity and surface tension, as shown in Fig. 1(b). The sampling method of the water film could stabilize the position of liquid target, and minimize the absorption of optical signal by the liquid. The combination of the femtosecond laser and the water film can improve the signal stability as much as possible.
3. Results and discussion
In LIBS of liquid sample, as laser energy exceeds breakdown threshold, optical breakdown will occur in laser focused area. A high-temperature and high-pressure plasma is generated, and emits some characteristic lines. In the experiment, we measured a series of optical breakdown spectra for the eight aqueous solutions with different Na concentrations. The obtained Na (I) lines with different Na concentrations are presented in Fig. 2. In the wavelength range from 586 nm to 596 nm, Na (I) 589.0 nm and Na (I) 589.6 nm have stronger signals. Therefore, the two spectral lines were selected as the analytical lines of the Na element. As shown in Fig. 2, the intensity of the spectral line decreases significantly with the decrease of the Na element concentration. Near 0.01 μg/mL concentration, the spectral lines can still be observed, but the signal-to-background ratio is smaller.
Fig. 2. LIBS emission spectra of NaCl solutions with different Na concentrations. Laser energy is 1.6 mJ.
The quantitative relationship between the element concentration and the spectral peak intensity can be obtained by linear fitting. The calibration curves of Na (I) 589.0 nm and Na (I) 589.6 nm on different NaCl aqueous solution are shown in Fig. 3. The linear correlation coefficients (R2) of Na (I) 589.0 nm and Na (I) 589.6 nm are 0.9928 and 0.9914, respectively. The R2 reflects the fitting correlation between independent variable and dependent variable. If the R2 is closer to 1, and it indicates that there is a better correlation between predicted result and actual measurement. By the 3σ criterion, the LoDs of Na (I) 589.0 nm and Na (I) 589.6 nm are 0.043 and 0.071 μg/mL, respectively.
Fig. 3. Calibration curves of Na (I) 589.0 nm (a) and Na (I) 589.6 nm (b) for NaCl aqueous solution.
The relative standard deviation (RSD) is crucial for comparing the uncertainty between different measurements. The RSD can descript the precision and repeatability in analytical field. The stability and repeatability in LIBS is very important for wide analytic applications [57]. The RSD can be calculated as follows:
where, σ represents the standard deviation of the background intensity, n represents the number of measurements, ${x_i}$ is expressed as the spectral intensity value of the i-th measurement, $\bar{x}$ is the average value of the multiple measurements. Figure 4 presents the RSDs of Na (I) lines for NaCl solutions with different Na concentrations. It can be seen that the obtained RSD fluctuates between 1.5% and 4.5% as the concentration increases. The RSD can reflect the fluctuation of the characteristic spectral lines measured by LIBS. So the lower the RSD value is, the more stable the measured spectrum is. The result indicates that the repeatability obtained by using the method in current work is acceptable.According to previously published reports, there are several disadvantages when using LIBS technique as analytical technique in liquid sample, including severe liquid splashing, low signal intensity, and poor signal stability. As mentioned above, nanosecond lasers are widely used for the quantitative analysis of liquid sample. Although nanosecond light source has been rapidly developed, it is undeniable that it has some shortcomings for liquid sample: 1) fluctuation in laser pulse and large focused spot cause low accuracy and stability of spectral measurement; 2) nanosecond laser has longer pulse width and is easy to generate plasma shielding; 3) If measuring more complex sample, the characteristic spectral lines of each element will cause serious interference, which will greatly reduce the repeatability of the quantitative analysis of the measured sample [58]. In addition, the energy of shock wave and cavitation bubble generated by nanosecond laser is relatively high, resulting in serious liquid splashing of the sample and reducing detection stability. The shock wave and cavitation bubble are physical phenomena accompanied by laser breakdown of liquid substance [59]. During the interaction between laser and liquid sample, the high-temperature and high-pressure plasma generated by laser will expand outward. After a certain delay time, the inertial ions will follow free electrons, which is the origin of the shock wave. Another phenomenon is that the cavitation bubble is macroscopic and occurs in soft tissues or fluids. The liquid in laser focused area is first vaporized and becomes saturated vapor. Gradually expand outward, and eventually collapse after several contractions and rebounds [60]. If liquid walls are presented around the bubbles, liquid splashing will occur [61]. Vogel et al. compared the energy of shock waves and cavitation bubbles generated by nanosecond and femtosecond lasers [62], their conclusions was that the energy of shock waves and cavitation bubbles with femtosecond laser was much lower than that with nanosecond laser. This is the reason that femtosecond lasers can produce weaker liquid splashing. In this work, the sampling method of water film is equivalent to supply a stable liquid surface, which solves the problem that the position of laser focusing on the liquid surface is not stable. Therefore, using a focused femtosecond laser to excite the water film can reduce liquid splashing and effectively improve the stability of LIBS detection.
To further confirm the feasibility of the method used in the experiment, we compared different methods of analyzing liquid samples based on LIBS, the R2, RSD, and LoD obtained from these methods are listed in Table 2. These methods have obtained high linear correlation coefficients and relative standard deviations, and have a celebrated effect for improving the stability of liquid sample detection. However, these methods are either complicated in experimental design or tedious pretreatment of samples. In the experiment, femtosecond laser is used to induce the NaCl water film, which greatly simplifies the experiment. Simultaneously, a higher R2 is obtained. Moreover, the RSD of the spectral lines is generally in the range from 5% to 10%, but the RSD from the experiment is in the range from 1.5% to 4.5%. Therefore, combining femtosecond pulse laser and water film can significantly improve the signal stability of LIBS in liquid sample detection.
Table 2. Comparison of R2, RSD, and LoD obtained from different methods based on LIBS.
4. Conclusions
In this paper, the quantitative analysis of Na element concentration in NaCl aqueous solution was performed based on LIBS technique. By changing two experimental conditions, the stability of liquid sample detection was improved in LIBS. Firstly, changing the excitation source, the commonly used nanosecond laser was replaced by femtosecond laser. The energy of the generated shock wave and cavitation bubble were weak, which reduced the liquid splashing. Secondly, based on the liquid jet, the water film was used to provide a relatively stable liquid surface for laser excitation, which solved the disadvantage that the liquid surface fluctuation could not determine the position of laser focusing. Combining two experimental conditions performed the measurements of NaCl aqueous solutions with eight Na concentrations. The calibration curves of Na element were plotted, the R2 of Na (I) 589.0 nm and Na (I) 589.6 nm were higher than 0.99, and the RSD was in the range from 1.5% to 4.5%. According to the 3σ criterion, the LoDs of Na (I) were calculated to be 0.043 and 0.071 μg/mL, respectively. In addition, we compared the results obtained by different LIBS analysis methods in liquid samples, indicating that using femtosecond pulse laser to induce water film could get higher R2 and RSD, and the experiment had simple and easy operation. Therefore, the method of combing femtosecond pulse laser and water film can significantly improve the detection stability and accuracy of LIBS technique for liquid sample analysis.
Funding
National Key Research and Development Program of China (2019YFA0307701); National Natural Science Foundation of China (11674124, 11674128, 11974138); Education Department of Jilin Province (JJKH20200937KJ).
Disclosures
The authors declare no conflicts of interest.
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