AC needle-to-needle bare electrode discharge with nebulized sample injection for elemental analysis

Abstract

In this study, atmospheric pressure AC needle-to-needle bare electrode discharge coupled with a nebulized sample injection was developed for elemental analysis (Ni, Cu, Cd, and Pb). The effects of various parameters—including carrier gas composition, nebulizer gas flow rate, organic additives, solution pH, discharge power, and discharge gap—on the discharge mode and analytical performance were systematically investigated. Oxygen was identified as the optimal carrier gas due to its low background noise in the optical emission spectrum and favorable plasma properties. Importantly, the transition state between streamer and glow-like discharge was found to be most favourable for elemental analysis. Under optimized conditions (2.2 L min⁻¹ nebulizer gas flow rate, 2% methanol additive, 3 mm discharge gap, and 15 kV applied voltage), the device achieved sensitive limits of detection (LODs) for Ni (0.38 mg L⁻¹), Cu (0.05 mg L⁻¹), Cd (0.09 mg L⁻¹), and Pb (0.24 mg L⁻¹), with relative standard deviations (RSDs) ≤6.1% (n = 10), demonstrating its potential for portable and rapid elemental analysis applications.

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Introduction

Elemental analysis is a fundamental technique for identifying and quantifying the elements in a sample, and has been widely applied in deep space exploration,¹ geological prospecting,² environmental monitoring,³ and food safety assessment.⁴ Common methods for elemental analysis include inductively coupled plasma – atomic emission spectrometry (ICP-AES), inductively coupled plasma – mass spectrometry (ICP-MS), atomic absorption spectrometry, and atomic fluorescence spectrometry.^5–7 Among them, ICP-AES/MS has the advantages of more detectable elements, higher detection accuracy, and a lower detection limit.^7,8 However, its applicability is limited to laboratory settings due to its high discharge power and high consumption of noble gases, such as argon.⁹ In efforts to develop a portable, rapid, and in situ detection technique, atmospheric pressure discharge plasma (APDP) was introduced into elemental analysis.^5,10–12 The basic principle of APDP for elemental analysis is similar to that of ICP-AES, i.e. the target elements are atomized by the high temperature and particle bombardment of plasma, and the generated atoms are excited or ionized predominantly under the action of energetic electrons.¹³ Then, the photons emitted from excited atoms are detected by an emission spectrometer, so that the target elements are analyzed qualitatively and/or quantitatively. Importantly, APDP offers the advantages of a miniature device, low energy consumption, simple operation, and the capability to operate in ambient air, making it highly promising for portable, rapid, and in situ elemental analysis.^5,12

Since samples are commonly dissolved in acid prior to elemental analysis, most APDP electrode structures are designed as the gas–liquid discharges.^14–18 For example, Cserfalvi et al. first introduced the electrolyte cathode discharge (ELCAD) for elemental detection.¹⁴ Moreover, Webb et al. developed a solution cathode glow discharge (SCGD) device, which showed better analytical performance than ELCAD.¹⁵ However, the stability of gas–liquid discharge is more difficult to control than that of gas discharge, because some factors cause the deformation of the liquid surface, such as shockwaves or water vaporization.¹⁹ As a result, the nebulized sample injection was introduced into APDPs in recent years, which is inspired by ICP-AES.^20–24 More importantly, nebulized sample injection can efficiently increase the contact area between plasma and target elements, and further improve the sensitivity of analysis.²⁵ In 2016, Cai et al. employed a pneumatic micro-nebulizer to provide the nebulized sample, and generated a dielectric barrier discharge (DBD) at the nebulizer nozzle to atomize metals in the spray.²⁰ Then, Li et al. employed a similar design with a miniature electro-thermal vaporizer instead of a pneumatic nebulizer for detecting 11 kinds of elements.²¹

Regarding the choice of plasma source, both bare electrode discharge and DBD are employed for elemental analysis with nebulized sample injection.^20–24 Among them, needle-to-needle bare electrode discharge is one of the most classic and simplest configurations, but its discharge stability and analytical performance are variable due to its multi-mode transitions of streamer, glow-like, arc, and spark.^26–28 In this study, we achieved needle-to-needle bare electrode glow-like discharge at atmospheric pressure by connecting a group of resistors and capacitors in the discharge circuit, and employed it for detecting Ni, Cu, Cd and Pb with nebulized sample injection. Discharge images, waveforms of voltage and current, optical emission spectra (OES), gas temperature, and electron density were investigated to understand the relationship between discharge mode and analytical performance. In addition, the effects of gas composition, nebulizer gas flow rate, solution pH value, organic additives, discharge power, and discharge gap on discharge mode and analytical performance were systematically studied.

Experimental

The experimental setup of AC needle-to-needle bare electrode discharge with nebulized sample injection for elemental analysis is illustrated schematically in Fig. 1. It consists of a needle-to-needle bare electrode structure, a high-voltage AC power supply, a nebulized sample injection system, an electrical measurement system, and an optical detection system. Two tungsten needle-shaped electrodes (0.4 mm curvature radius and 40° cone angle of the tip) were mounted on a polytetrafluoroethylene (PTFE) holder, as the high-voltage electrode and ground electrode, respectively. The gap between two needle electrodes can be mechanically adjusted within a 0–20 mm range, and discharge gaps of 2, 3, and 4 mm were used in this study. The AC power supply (CTP-2000K, Nanjing Suman Electronic Co. Ltd) provides a sinusoidal AC high-voltage in the range of 0–40 kV, and the driving frequency was kept as 8 kHz in this study. A group of resistors and capacitors was connected in series between the AC power supply and high-voltage electrode to limit the increase in discharge current and regulate the electric charge of each discharge.^27,29 The resistance and capacitance were set to 3 kΩ and 75 pF, respectively. A quartz pneumatic nebulizer (WNA-II, Beijing Puxi Standard Technology Co. Ltd) was mounted above the discharge region to introduce the nebulized sample for elemental analysis. The nebulizer has optimal nebulization efficiency with a gas flow rate in the range of 2–5 L min⁻¹, and the corresponding liquid injection rate is about 2.5–4 mL min⁻¹. Air, argon and oxygen were used as carrier gases for the nebulizer. Argon and oxygen were supplied by gas cylinders, while air was artificially synthesized by mixing nitrogen and oxygen at a volume ratio of 4 : 1. The flow rate of carrier gas was controlled by using a mass flow controller (S49 32/MT, Beijing Horiba Metron Instruments Co. Ltd). Besides, a glass dish was placed under the needle electrodes to collect the waste sample from the nebulizer.

Fig. 1 Schematic diagram of AC needle-to-needle bare electrode discharge with nebulized sample injection for elemental analysis.

For the electrical measurement system, a high-voltage probe (Tektronix P6015A, 1000×, 75 MHz) and a current probe (Pearson 4100, 35 MHz) were employed to measure the discharge voltage and the discharge current, respectively. And the waveforms of voltage and current were displayed and recorded using an oscilloscope (Tektronix MDO3034, 350 MHz). The discharge power can be calculated from discharge voltage and discharge current using the following equation:

P = f × ∫P(t)dt = f × ∫U(t)I(t)dt, (1)

where f, P(t), U(t), and I(t) are the driving frequency, the instantaneous power, the instantaneous voltage, and the instantaneous current, respectively. For the optical detection system, the light emitted from plasma was collected via an optical fiber positioned in front of the discharge region. The collected light was dispersed by using a grating monochromator (Andor SR-750i), and subsequently was detected by a CCD (Newton DU940P-BV), and the resulting signals were recorded on a computer.

In this study, Ni, Cu, Cd and Pb were selected as the target elements, and the aqueous sample was prepared by dissolving Ni(NO₃)₂·6H₂O, Cu(NO₃)₂·3H₂O, Cd(NO₃)₂·4H₂O, and Pb(NO₃)₂ in ultrapure water. The pH of the sample solution was adjusted by adding nitric acid, and measured using a pH meter (Sartorius PB-10, resolution: 0.01 pH). Additionally, methanol and formic acid were added into the solutions with a volume fraction of 2% to improve the analytical performance. Among them, Ni(NO₃)₂·6H₂O (analytical reagent grade) was purchased from Macklin Biochemical Technology Co. Ltd. Cu(NO₃)₂·3H₂O, Pb(NO₃)₂, nitric acid, methanol, and formic acid (all analytical reagent grade) were purchased from Tianjin Damao Chemical Reagent Factory. Cd(NO₃)₂·4H₂O (analytical reagent grade) was purchased from Sinopharm Chemical Reagent Co., Ltd. High-purity nitrogen, argon, and oxygen (99.999%) were supplied in gas cylinders by Dalian Guangming Special Gas Products Co. Ltd.

Results and discussion

Comparison of carrier gas composition

Gas composition is one of the key factors in APDP generation, which can significantly influence the electron density, electron energy, and active species in plasma.^30–32 In this study, the ambient gas of needle-to-needle discharge was maintained as air, while the carrier gas introduced through the nebulizer was varied among air, argon, and oxygen. Since the carrier gas is sprayed from the nebulizer to the discharge region and participates in plasma generation, its type affects both plasma characteristics and analytical performance.

The OES of needle-to-needle discharge with air, argon and oxygen as the carrier gas of nebulizer are shown in Fig. 2(a–c), respectively. The experimental conditions were set to 3 mm discharge gap, 15 kV applied voltage, and 2.4 L min⁻¹ gas flow rate of the nebulizer. The sample solution contained 50 mg per L Ni²⁺, without added nitric acid or organic additives. As shown in the figure, the OES of three carrier gases are all mainly composed of the atomic spectral lines of Ni (3d⁹4p³Fo 4 → 3d⁹4s³D₃, 341.5 nm), H_α (3 → 2, 656.3 nm), and O (3p⁵P → 3s⁵So 2, 777–778 nm), the bands of OH (A²Σ → X²Π), and the second positive bands of N₂ (C³Π_u → B³Π_g). Additionally, the OES of argon also exhibit spectral lines of Ar (4p → 4s, 696–795 nm), as shown in Fig. 2(b). The main physicochemical processes of these active species are listed in Table 1. Reactions R1–R9 occur for all three carrier gases, while reactions R10 and R11 only occur when argon is used. Fig. 2(a) presents higher emission intensities of the bands of N₂ (C³Π_u → B³Π_g), indicating that reaction R9 occurs more frequently when air is used as the carrier gas. Similarly, higher emission intensities of Ar (4p → 4s), and O (3p → 3s) are observed when argon and oxygen are used, respectively, because reaction R10 and reaction R5 are the dominant reactions, respectively. As shown in Fig. 2(a and b), the emission intensities of H_α and OH (A²Σ → X²Π) are close with air and argon as the carrier gases. However, with oxygen as the carrier gas, the intensity of OH (A²Σ → X²Π) is lower, as shown in Fig. 2(c). This is because OH (A²Σ) is produced via reaction R8 and reaction R11 in air and argon, respectively, but these reactions rarely occur in oxygen.

Fig. 2 OES of needle-to-needle discharge with different carrier gases: (a) air; (b) argon; (c) oxygen; (d) comparison in the range of 339–344 nm.

Table 1 Key physicochemical processes of dominant active species in plasma (* stands for the excited state of the particle)

No.	Reactions
R1	Ni^2+ + 2e^− → Ni,^33
R2	Ni^2+ + 2H → Ni + 2H^+,^33
R3	Ni + e^− → Ni* + e^−,^13
R4	H2O + e^− → H* + OH + e^−,^29
R5	O2 + e^− → O* + O + e^−,^29
R6	H2O + e^− → O* + H2 + e^−,^29
R7	H2O + e^− → OH* + H + e^−,^33
R8	H2O + N2* → OH* + H + N2,^34
R9	N2 + e^− → N2* + e^−,^33
R10	Ar + e^− → Ar* + e^−,^34
R11	Ar* + H2O → Ar + OH* + H,^34

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Fig. 2(d) shows a magnified view of Fig. 2(a–c) with the wavelength from 339 nm to 344 nm, highlighting the peak of Ni (4p → 4s) under the three carrier gases. The emission intensity of Ni (4p → 4s) depends on the reaction rates of R1–R3. The nearby peaks constitute the background of the Ni signal, which is mainly sourced by the bands of N₂ (C³Π_u → B³Π_g) and OH (A²Σ → X²Π), according to our previous study.³³ Since the fluctuation amplitudes of these bands are generally positively correlated with their emission intensities, the intensity of the Ni background can be used to evaluate the noise level. We compared the signal-to-background ratio (SBR) of Ni (4p → 4s) among different carrier gases, as shown in Fig. 3(a). The Ni signal was estimated by integrating the emission intensity between 341.38 nm and 341.55 nm, while the background was estimated by integrating the emission intensity of blank solution over the same range. The SBR of Ni was then calculated by dividing the intensity of its signal by that of the background. Notably, the highest SBR is obtained when oxygen is selected as the carrier gas, despite the emission intensity of Ni (4p → 4s) being the lowest in this case. This is attributed to the substantially weaker background for oxygen compared with air and argon, depending on the weakest intensities of both OH (A²Σ → X²Π) and N₂ (C³Π_u → B³Π_g). Moreover, the error bars of the Ni background (SDs of ten measurements) reflect the noise levels. Among the three carrier gases, higher background intensities correspond to larger error bars, which is consistent with the assumption that noise is positively correlated with the background. As a result, the SBR can be regarded as a positive indicator of nickel detection sensitivity.

Fig. 3 Comparison among air, argon and oxygen as the carrier gases: (a) signal intensity, background intensity and SBR of Ni; (b) rotational temperature, vibrational temperature, and electron density of plasma (error bars are the standard deviations (SDs) of three measurements, except for the background intensity, based on ten measurements).

Plasma temperature and electron density (n_e) are important plasma parameters influenced on the reaction rates of reactions in Table 1. As a result, rotational temperature (T_rot), vibrational temperature (T_vib), and n_e of needle-to-needle discharge under the different carrier gas were compared, as shown in Fig. 3(b). Among them, T_rot and T_vib were determined by “Specair” software, and n_e was calculated with the Stark broadening of H_α. The detailed methods and the best-fitted spectra for calculating T_rot, T_vib, and n_e are shown in Fig. S1 and S2 in the SI, respectively. Based on the approximate equality between gas temperature (T_g) and T_rot of N₂ (C³Π_u) in atmospheric pressure plasma,³⁵ it is found that there is a similar T_g among three carrier gases, by comparing the T_rot in Fig. 3(b). On the other hand, it can also be observed that oxygen has the highest T_vib, which suggests that the electrons may have relatively higher energy compared with air and argon. This inference is consistent with Ullah et al.,³⁶ who reported higher electron temperature in the presence of oxygen and explained this phenomenon. Finally, n_e in oxygen is similar to that in argon, and higher than that in air. Both the higher electrical energy and n_e of oxygen may be another reason for it being beneficial for elemental analysis. As a result, oxygen is selected as the optimal carrier gas of the nebulizer, and is kept for the following study.

Effect of the gas flow rate of the nebulizer

When the nebulized sample injection is employed in elemental analysis, the carrier gas flow rate of the nebulizer is a critical parameter for analytical performance, which is discussed in this study. For a pneumatic nebulizer, the nebulizer would achieve optimal efficiency only within a specific range of gas flow rate. Besides, the nebulized sample volume and its residence time in the discharge region depend on the gas flow rate of the nebulizer, and these two factors have significant impacts on the analytical performance. The variation of Ni signals, backgrounds, and SBR with the gas flow rate of the nebulizer is shown in Fig. 4(a), with the experimental conditions of 3 mm discharge gap, and 15 kV applied voltage. The Ni signal and Ni background were estimated using the same method as in Fig. 3(a), by integrating the emission intensity within the 341.38 nm to 341.55 nm range for the 50 mg per L Ni²⁺ solution and the blank solution, respectively. It is found that the Ni signal increases first when the gas flow rate of the nebulizer increases from 1.8 L min⁻¹ to 2.2 L min⁻¹, and then reduces with the continual increase in the gas flow rate. The Ni background is enhanced when the gas flow rate increases from 1.8 L min⁻¹ to 2.0 L min⁻¹, and then remains almost constant with the continual increase in the gas flow rate. As a result, there is a highest SBR of Ni, which is about 9.6, with a gas flow rate of 2.2 L min⁻¹.

Fig. 4 Optimization of experimental conditions for needle-to-needle discharge with nebulized sample injection: (a) variation of Ni signals, noises, and SBR with the gas glow rate of the nebulizer; (b) variation of the Ni signal with an organic additive in an aqueous sample; (c) variation of Ni signals, noises, and SBR with discharge power; (d) variation of Ni SBR with the discharge gap distance (error bars are the SDs of three measurements).

Droplet formation in a pneumatic nebulizer follows the Bernoulli principle: a high-velocity gas stream generates a low-pressure zone, aspirating the liquid into the nebulizer.^25,37 With the increase in the gas flow rate, the pressure difference generated by gas flow increases, resulting in more aqueous sample drawn into the nebulizer. Then the liquid is broken into the droplets under the action of high-speed gas flow. Under ideal conditions, the droplet size becomes smaller with the increase in the gas flow rate. When the gas flow rate increases from 1.8 L min⁻¹ to 2.2 L min⁻¹, the contact area between plasma and the nebulized sample increases owing to more nebulized sample and smaller droplet size, resulting in the increasing SBR of Ni. When the gas flow rate is higher than 2.2 L min⁻¹, not only does the residence time of the nebulized sample in the plasma decrease, but some of the nebulized sample is also blown outside the plasma region because of the higher speed of gas flow. Besides, a higher gas flow rate can also promote the heat dissipation of the plasma region, and the lower temperature will reduce the rate of reactions R1 and R2.³³ Therefore, the SBR of Ni decreases with the increase in gas flow rate beyond 2.2 L min⁻¹, even though the nebulized amount of the sample increases and the size of the droplet decreases continually.

Effect of the organic additive in the aqueous sample

Fig. 4(b) shows the intensities of the Ni signal after adding methanol or formic acid in the aqueous sample, which is an effective method to improve the detection sensitivity for elemental analysis.^5,10 It is found that the corresponding Ni signal improves by a factor of 2.6 and 2.3, for methanol and formic acid addition in the aqueous sample, respectively, under the experimental conditions of 2.2 L min⁻¹ gas flow rate of nebulizer, 3 mm discharge gap, and 15 kV applied voltage. There are three main reasons for the effect of the organic additive: (1) organic additives such as methanol have lower surface tension and viscosity compared to pure water, which can generate finer droplets and enter the plasma more efficiently through the nebulizer;³⁸ (2) the organic additive is pyrolyzed in plasma to generate C⁺ ions, which have strong charge transfer ability, promoting reaction R1;³⁹ and (3) H atoms are also generated by organic additive pyrolysis, which can promote reaction R2.²⁰

Additionally, the effect of the pH value of the aqueous sample on the intensity of the Ni signal is also illustrated in Fig. 4(b). Because the aqueous samples are typically prepared by dissolving solid in acid for analyzing solid samples, the pH value is an important factor for elemental analysis. It can be seen from Fig. 4(b) that the intensity of the Ni signal remains relatively constant with the variation of the pH value, no matter whether the sample contains methanol or formic acid, or is without any organic. The relative standard deviations (RSDs) of three curves are 3.8%, 4.3% and 5.2%, respectively, which are within the acceptable measurement error range. Therefore, in contrast to conventional APDP devices that require adjustment of the sample to a specific pH,^11,18,39 this device presents less dependence on pH, and enables reliable elemental analysis across a relatively broad pH range.

Effect of discharge power

The effect of discharge power on nickel detection is shown in Fig. 4(c), with a gas flow rate of the nebulizer of 2.2 L min⁻¹, a discharge gap of 3 mm, and 2% methanol in sample solution. The horizontal axis in Fig. 4(c), representing the discharge power, was adjusted by varying the applied voltage from 9 kV to 21 kV in 3 kV steps, and was calculated using eqn (1). Although both the Ni signal and Ni background are enhanced with increasing discharge power, the SBR of Ni initially increases and then saturates. In the range from 7.5 W to 14.6 W, the Ni signal grows more rapidly than the background, resulting in an improved SBR. However, beyond 14.6 W, the SBR gradually declines due to the slower growth of the signal relative to the background. The highest SBR, of about 24.9, is acquired at a discharge power of 14.6 W. In this case, the corresponding applied voltage is 15 kV, which is selected as the optimal value for Ni detection.

The main reason for the SBR's change is that the needle-to-needle bare electrode discharge presents various discharge modes with increasing discharge power. Fig. 5 shows the waveforms of voltage and current, as well as discharge images, varying with discharge power. As shown in Fig. 5(a), with a discharge power of 7.5 W, the discharge morphology presents several purple and filamentary channels, and the discharge current presents a higher current peak (about 4 A) and a short duration time (less than 0.1 µs), which both indicates that the discharge in this case is the streamer mode.²⁷ Due to the short duration of streamer discharge, there is insufficient time for reaction R3 to occur following reactions R1 and R2, and thus almost no Ni signal can be detected.

Fig. 5 Discharge modes of needle-to-needle discharge varying with discharge power: (a) streamer mode; (b) mixed mode of streamer and glow; (c) glow-like mode; (d) arc mode.

When the discharge power gradually increases to 14.6 W (Fig. 5(b)), two different discharge modes are observed during the positive and negative half-cycles of voltage. In the positive half-cycle, the discharge remains as the streamer mode, which has the similar current waveform to Fig. 5(a). In the negative half-cycle, the current peak decreases to about 0.2 A and the duration time increases to about 3.5 µs, indicating that the discharge transitions to the glow-like mode.²⁷ The difference in discharge modes between the positive and negative half-cycles may be attributed to minor machining-induced variations in the curvature radius and cone angle of the high-voltage electrode and ground electrode, because it can be also observed that the positive half-cycle exhibits glow-like discharge, while the negative half-cycle displays streamer discharge. Moreover, the discharge morphology further supports the coexistence of streamer and glow-like modes, which presents a bright and wider channel in the middle, and the filamentary channels around it. The coexistence of streamer and glow-like modes show that the discharge is transitioning from streamer to glow-like. In this case, the discharge duration is sufficient for the sequential reactions R1–R3; meanwhile the energy remains moderate, preventing the significant promotion of reactions R7 and R9, which would increase the background of Ni.

When the discharge power increases to 17.2 W, it is found from Fig. 5(c) that the positive half-cycle discharge transitions from streamer to glow-like, though the current peak (about 0.6 A) is higher than it is in the negative half-cycle. Meanwhile, the filamentary channels around the main discharge disappear. The emission intensity of the Ni background increases significantly, leading to the decline of the SBR.

With the discharge power of 22.3 W (Fig. 5(d)), a new current peak (about 0.08 A) appears following each glow-like discharge current, and the discharge channels slide downward along the direction of the airflow, resembling a glide arc discharge. Through the current waveform and discharge morphology, the discharge is identified as the arc mode.²⁷ In the arc mode, the emission intensity of the Ni signal remains almost constant, while the emission intensity of the Ni background further increases, causing a continuous decline of the SBR.

Effect of discharge gap distance

Not only discharge power but discharge gap distance can also influence the discharge mode of needle-to-needle bare electrode discharge. The effect of discharge gap on the SBR of Ni is shown in Fig. 4(d). When the discharge gap is 2 mm, the discharge initiates at an applied voltage of 7 kV, and the corresponding discharge power is 6.2 W. In this case, the SBR is about 14.5, which is significantly higher than that acquired at the beginning of discharge with a 3 mm gap. This is mainly because the short discharge gap distance makes it difficult to remain a pure streamer mode. The discharge transitions to the mixed mode of streamer and glow-like immediately after gas breakdown. Then the discharge transitions to the glow-like mode, and the SBR reaches its maximum value of 19.2 at a discharge power of 9.5 W. When the discharge power increases to 19.3 W, the discharge transitions to arc mode, which occurs earlier than the mode transition under 3 mm gap. Fig. 6(a) shows the current waveform and discharge image of needle-to-needle discharge with 14.3 W discharge power and 2 mm discharge gap, supporting that the discharge is in the glow-like mode in this case.

Fig. 6 Discharge modes of needle-to-needle discharge at different discharge gap distances: (a) glow-like mode observed at 2 mm gap distance; (b) mixed mode of streamer and arc observed at 4 mm gap distance.

At 4 mm gap distance, the increased distance prevents glow-like mode formation. As shown in Fig. 4(d), when the discharge power is ≤12.1 W, the discharge remains in the streamer mode. With the power increasing to 16.3 W, the discharge directly transitions to the mixed mode of streamer and arc, which is further confirmed by the current waveform and discharge image in Fig. 6(b). With a further increase in discharge power to 22.9 W, the discharge transitions to the arc mode. The highest SBR, about 9.7, is acquired at a discharge power of 19.1 W. By comparing the highest SBR acquired under the three discharge gaps, it is found that 3 mm is optimal for nickel detection.

Analytical performance

According to the above discussion, it is found that the AC needle-to-needle bare electrode discharge with nebulized sample injection is beneficial for elemental analysis under the conditions of 2.2 L min⁻¹ nebulizer gas flow rate, 2% methanol additive, 3 mm discharge gap, and 15 kV applied voltage. To verify the applicability of the above conditions to other elements, their effects on the Cu SBR were examined, with the results provided in Fig. S3 in SI. Under the optimal conditions, we evaluated the analytical performance of this device by using the calibration curves for Ni, Cu, Cd, and Pb, as well as their limits of detection (LODs). The calibration curve for each element is plotted by fitting a series of concentrations (0, 2, 5, 10, 15, 20, and 25 mg L⁻¹) versus their emission intensities,⁷ as shown in Fig. S4 in the SI. Then the LOD for each element is calculated by using the following equation:where s is the standard deviation of 10 measurements of the emission intensities in blank solution, and k is the slope of its calibration curve. The corresponding results are given in Table 2. For a self-consistent comparison under the same AES system, the LODs of ELCAD obtained in our previous work are listed. Compared to previous work, the LODs for Cu, Cd, and Pb in this work are improved by factors of 2.1, 7.6, and 4.1, respectively, which indicates that nebulized sample injection provides a higher detection sensitivity for elemental analysis. Table 2 also lists representative LODs via SCGD from recent studies, demonstrating that while the performance of this device is competitive, it could be further optimized due to the gap with the current state of the art. Additionally, we evaluated the stability of the device by determining the RSDs of emission intensities of Ni, Cu, Cd, and Pb in 10 times of measurement. The measurement results and corresponding RSDs of four elements with a concentration of 10 mg L⁻¹ are shown in Fig. S5 in the SI. The RSDs of four elements are in the range of 3.0–6.1%, indicating good precision and repeatability of this device for elemental analysis.

Table 2 Analytical performance of AC needle-to-needle bare electrode discharge with nebulized sample injection for detecting Ni, Cu, Cd, and Pb

Elements	Wavelength (nm)	Calibration curves	R ^2	s	LOD (mg L^−1)
					This work	ELCAD (our previous work^40)	SCGD (from references)
Ni	341.5	I = 4.54c + 7.92	0.992	0.568	0.38	—	0.70 (ref. 41)
Cu	324.8	I = 28.92c + 12.59	0.998	0.496	0.05	0.109	0.008 (ref. 42)
Cd	228.8	I = 11.79c + 2.07	0.994	0.354	0.09	0.687	0.002 (ref. 42)
Pb	405.8	I = 16.76c + 71.79	0.987	1.32	0.24	0.969	0.010 (ref. 42)

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Conclusions

In summary, we developed atmospheric pressure AC needle-to-needle bare electrode discharge with nebulized sample injection for detecting Ni, Cu, Cd, and Pb. Systematic investigations revealed that both the carrier gas composition of the nebulizer and the discharge mode play critical roles in determining the plasma behavior and analytical sensitivity. Among the investigated gases, oxygen was identified as the optimal carrier gas due to its ability to reduce the background noise intensity of elemental spectral lines, and it also supports higher vibrational temperature and electron density. Furthermore, we achieved several discharge modes by connecting a group of resistors and capacitors in the discharge circuit, and adjusting the discharge power and discharge gap, of which the transitioning state between streamer and glow-like discharge was demonstrated to be most favourable for elemental analysis. Additionally, the carrier gas flow rate of the nebulizer, and the organic additive in sample solution were also optimized. Under the optimal conditions (2.2 L min⁻¹ nebulizer gas flow rate, 2% methanol additive, 3 mm discharge gap, and 15 kV applied voltage), the device exhibited sensitive and repeatable detection of Ni, Cu, Cd, and Pb, with LODs in the range of 0.05–0.38 mg L⁻¹, and RSDs in the range of 3.0–6.1%.

Author contributions

H. Yuan: investigation, methodology, writing – original draft; S.-Q. Li: investigation; J.-P. Liang: conceptualization, writing – review & editing; Z.-L. Cui: writing – review & editing; D.-Z. Yang: funding acquisition, resources, supervision; R. S. Rawat: supervision.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5ja00315f.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (12305279, 52307163, and 52077026), the Liaoning Provincial Science and Technology Program Joint Program (2023JH2/101800036), the China Postdoctoral Science Foundation (2022M710590), and the Fundamental Research Funds for the Central Universities (DUT23YG227).

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