Solution Cathode Glow Discharge Research Articles

Effects of solvent composition on ionization and fragmentation within the solution-cathode glow discharge

Recently, solution-electrode glow discharges (SEGDs) have shown great utility as ionization sources for mass spectrometry (MS). The solution composition of SEGD electrodes is pivotal to their performance as it influences analyte-ion formation. The performance of electrospray ionization is heavily dictated by spray-solution composition, which can alter ionization efficiency and pathways. While SEGDs produce Taylor cones similar to electrospray ionization, the influence of solution-electrode composition on molecular-ion formation has not been studied. Here, we examine how additives to an atmospheric-pressure solution-cathode glow discharge (SCGD) influence molecular ionization and fragmentation. The impact of several additives to the acidic solution of an SCGD ionization source was evaluated based on mass-spectrometric performance. Addition of methanol increased molecular- and fragment-ion signals for peptide angiotensin II. This effect is likely due to improved desolvation and a greater interaction of analyte molecules with glow-discharge species. Several high-boiling-point reagents were tested to examine changes in the ion signal, the average charge state, and the degree of fragmentation. Overall, these additives inhibited fragmentation but significantly lowered intact molecular-ion signals. Interestingly, loss of fragment ions trended with the boiling point of the reagent used. We hypothesize that analyte molecules become trapped in droplets produced at the solution-cathode surface. These droplets do not fully desolvate before escaping the discharge region, sparing analyte molecules from fragmentation. For low volatility additives, droplets do not desolvate, even as they enter the heated MS, which yields a loss in molecular ions. The changing composition of the SCGD solution alters analytical performance, but also provides insight into analyte ionization and fragmentation processes.

Modulation of the solution-cathode glow-discharge and solution-anode glow discharge using a rotating magnetic field

The effects of an external magnetic field on the solution-cathode glow-discharge (SCGD) and solution-anode glow-discharge (SAGD) are investigated. The SCGD is atmospheric-pressure glow discharge sustained between a metal pin and a liquid cathode electrode in the ambient atmosphere, and it is often used for trace elemental analysis by atomic emission spectroscopy. Here, the SCGD is modified to allow an external permanent magnetic field to be applied, either in a static orientation or as a rotating field, as a means of stabilizing the SCGD plasma and modulating atomic emission from the discharge. The effect of the external magnetic field on the physical structure, electrical characteristics, and spectroscopic response of the SCGD and SAGD are investigated. A rotating external magnetic field was found to change both SAGD and SCGD structure and spatial emission pattern. Analytical figures of merit are examined, and a lock-in amplifier is used to discriminate analytical atomic emission from background emission, improving limits of detection.

The solution-cathode glow discharge in slow motion: characterization of glow discharge filament structure and droplet ejection using a rectangular capillary

A solution-cathode glow discharge (SCGD) using a novel rectangular-shaped cathode capillary is used to study aspects of the plasma-liquid interface. High-speed video of the plasma-liquid interface captured simultaneously with low-angle laser scattering from droplets near the plasma-liquid interface are studied in concert to evaluate potential mechanisms related to surface-plasma interaction. Frame-by-frame analysis of high-speed video allows estimation of droplet number density, translational speed, and rate of ejection. The data are evaluated to provide insight into potential mechanisms of analyte transport that are of importance for the use of SCGD in analytical atomic spectrometry.

Evaluation of solution-cathode glow discharge atomic emission spectrometry for the analysis of nanoparticle containing solutions

The Solution Cathode Glow Discharge (SCGD) is a novel atmospheric-pressure glow discharge plasma sustained in the ambient atmosphere that is an appealing alternative to the inductively-coupled plasma as a source for atomic emission spectrometry. Simple, low power, and inexpensive, the SCGD is an attractive source for continuous environmental monitoring applications such as the quantitation of metallic nanoparticle solutions by atomic emission spectrometry. Metallic nanoparticles (NP) with diameters from 5 nm-150 nm were directly analyzed by SCGD-AES and found to exhibit lower, and size-dependent, elemental sensitivity when compared to dissolved free-ion standard solutions. Acid digestion with matrix-matching was shown to be an effective approach to achieve accurate quantitation. The origin of these morphological matrix effects was studied by investigating experimental parameters such as discharge power, solution flow rate, and influence of added surfactants. Examination of the spatial distribution of atomic emission between cathode and anode showed a shift in peak atomic emission towards the anode of the SCGD for some NPs as compared to free-ion solutions. A novel nested capillary design was used to introduce NP into the SCGD without dissolution within the acidic solvent and showed difference in sensitivity from free-ion solutions to be a NP morphological effect. Correlation of NP boiling point with difference in sensitivity between free-ion and NP solutions supports the conclusion that delayed vaporization of nanoparticles is the source of the morphological matrix effect, due primarily to lower rate of vaporization within the SCGD by lower gas temperatures and short residence time. Analysis of NP of different chemical form, and alloyed nanoparticles composed of more than one element, showed correlation with boiling point. Implications of these results on the possible mechanisms by which material is transferred from the liquid cathode into the plasma are considered.

Ultrasensitive determination of mercury by atmospheric pressure glow discharge optical emission spectroscopy coupled with solution cathode glow discharge microplasma

The ultrasensitive determination of Hg was achieved by atmospheric pressure glow discharge optical emission spectroscopy coupled with solution cathode glow discharge microplasma.

Solution anode glow discharge optical emission spectrometry: Volatile hydride introduction from the gas jet nozzle cathode for ultrasensitive determination of lead

An ultrasensitive method for the determination of Pb was developed by coupling solution anode glow discharge–optical emission spectrometry (SAGD-OES) with hydride generation (HG). Compared to solution cathode glow discharge, the introduction of analytes yielded via HG from the discharge cathode into the microplasma was demonstrated to be easily performed by SAGD in which the gas jet nozzle served as cathode and further enhanced sensitivity for Pb determination was achieved. The susceptibility of SAGD-OES to the matrix-induced interferences in the analysis of real samples was significantly improved owing to the coupling of HG. After a thorough optimization of the HG-SAGD-OES system parameters, the developed system achieved Pb detection limit of 0.061 ng mL−1, with the corresponding relative standard deviation being <2.2% at analyte concentrations of 50 ng mL−1. The potential application of this method was validated by successfully analyzing three certified reference materials (CRMs: GBW07311, GBW07312, and GBW07601a (GSH-1)) and human blood samples.

Determination of gallium and indium by solution cathode glow discharge as an excitation source for atomic emission spectrometry

In this paper, gallium (Ga) and indium (In) in water samples were determined by atomic emission spectrometry (AES) with solution cathode glow discharge (SCGD) as an excitation source. The optimal working conditions for the detection of Ga and In by proposed method are 660 V discharge voltage, 3.0 mL min−1 solution flow rate and pH = 1.0 HNO3 as supporting electrolyte. Under these conditions, it is found that there are no obvious interferences from most of the foreign ions when they exist in 20-fold excess over the In. But for Ga, the interferences from foreign ions are severe. The linear correlation coefficient (R2) is better than 0.99 with a range of 1–40 μg mL−1 for Ga and 0.1–30 μg mL−1 for In. The limits of detection (LODs) for Ga and In are 0.10 and 0.016 μg mL−1, respectively. The relative standard deviations (RSDs) are 2.9% for Ga and 3.9% for In, and the power is below 55 W. To validate the utility and accuracy of the proposed system, two water samples spiked with various concentration (0.3–20 μg mL−1) of Ga and In are directly determined, and the measured results are only in good agreement with spiked values of In. After diluted 10 times for water samples, the spiked values (0.3–20 μg mL−1) of Ga agree well with the measured value, suggesting that the matrix can interfere with the determination of Ga. The satisfactory recoveries between 90% and 109% of In and Ga are achieved.

Sensitivity improvement of solution cathode glow discharge-optical emission spectrometry by external magnetic field for optical determination of elements

A method has been developed to improve elemental determination by solution cathode glow discharge (SCGD) optical emission spectrometry. The introduction of a 0.65 T external magnetic field to the plasma discharge cell considerably enhanced the metal signal. Compared with the field-free case under the same voltage condition (400 V), the net intensities of the atomic emission lines of Cd, Cu, Pb, and Zn are enhanced by a factor of 2.9, 2.3, 2.1, and 3.2 in the presence of the external magnetic field, respectively. Moreover, the external magnetic field improved signal stability and extended the discharge voltage range from 300–420 V to 340–510 V. When the voltage was 490 V with a 0.65 T magnetic field, the emission intensity can be further enhanced by a factor of 4.2, 3.7, 3.1, and 5.0 for Cd, Cu, Pb, and Zn, respectively. The established limits of detections (LODs) are several times better as those obtained without a magnetic field at 400 V. The accuracy of the proposed method was demonstrated by the determination of elements in the standard water samples.

Imaging studies of emission and laser scattering from a solution-cathode glow discharge

Imaging experiments were used to explore solution-to-plasma transport and plasma structure in a solution-cathode glow discharge.

Ultra-sensitive determination of antimony valence by solution cathode glow discharge optical emission spectrometry coupled with hydride generation

An ultra-sensitive solution cathode glow discharge-optical emission spectrometry (SCGD-OES) method coupled with hydride generation (HG) for the determination of antimony valence is described.

Classification of bottled mineral waters using solution cathode glow discharge optical emission spectroscopy and chemometrics methods

Water is the most important substance for daily life and contains minerals which play an important role in nutrition. In this study, seven kinds of bottled water in Chinese market were analysed and identified by the solution-cathode glow discharge optical emission spectroscopy (SCGD-OES) combined with chemometrics methods. The score matrix was obtained by principal component analysis (PCA), and the back propagation artificial neural network (BP-ANN) model was established to identify the kind of the brands of the bottled water based on their spectral properties. The average classification accuracy based on BP-ANN is 99.74%, support vector machine (SVM) model and linear discrimination analysis (LDA) model were also employed to classify the kind of the brands of the bottled water based on the PCA analysis results, and the correct identification rate based on SVM model and LDA model are 99.50% and 98.00%, respectively. It is confirmed that SCGD-OES combined with chemometrics methods is promising for automatic real time, reliable and robust measurement, and can be integrated into a simplified form for non-specialist users.

Spatially Resolved Characteristics of Solution Cathode Glow Discharge Source Coupled with an Interference Filter Wheel as Spectral Discrimination Device

Solution cathode glow discharge-atomic emission spectrometry (SCGD-AES) has attracted wide attention globally in recent years. The technology has the advantages of on-line detection of various metal elements, small volume, and convenience. In this work, the original spectrometer based on SCGD-AES technology was replaced by filter-wheel, photomultiplier tube (PMT) and picoammeter, which was named FPP spectral discrimination device. Analytical performance of the device with narrow-band monochromator (0.08 nm) was compared to its performance with wide-bandpass filters (10 nm) with optimized spatial. The relative standard deviation of SCGD-FPP for Na, K, Ca, Li, Sr, Rb, and Cs ranged from 0.2 to 0.8%, comparable to the 0.6 to 1.4% with the monochromator. With optimized spatial resolution, the detection limits ranged from 0.15 to 350 μg/L, and the optimal plasma spectral collected points of Na, K, Ca, Li, Sr, Rb, and Cs at 0.8 mm, 0.8 mm, 0.6 mm, 0.4 mm, 0.2 mm, 0.6 mm, and 0.6 mm, respectively. The deviation analysis of the samples containing Na, K, Ca, and Li were analyzed under optimal conditions. The results show that a SCGD-FPP source with spatial resolution improved the detection performance.

Ultratrace Pb determination in seawater by solution-cathode glow discharge-atomic emission spectrometry coupled with hydride generation

A novel method for the determination of ultratrace Pb in seawater by solution-cathode glow discharge atomic emission spectrometry (SCGD-AES) coupled with hydride generation (HG) has been developed. In the SCGD process, the hollow Ti tube, which can introduce gas instead of a solid tungsten rod is used as a discharge anode and combined with hydride generator. HG technology can not only separate Pb from a seawater matrix and reduce the matrix interference on detection, but can also significantly improve the transfer and atomization efficiencies for SCGD-AES. The optimum working conditions for HG and SCGD-AES for quantitative analysis of Pb were determined as 1% H2C2O4 as the masking agent and addition of 3% formic acid to increase Pb volatilization. The limit of detection (LOD) for Pb was decreased by two orders of magnitude in comparison to the SCGD-AES process alone. Under optimal conditions, the LOD for Pb was 0.17 μg L−1, and the relative standard deviation was 2.1% (n = 11) for 100 μg L−1 Pb in artificial seawater. The proposed quantification method was verified using a seawater reference material (BWQ7001-2016), and the obtained results agreed well with certified values. Finally, the approach was used to quantitatively analyse Pb in seawater, and the results agreed well with those obtained by inductively coupled plasma mass spectrometry. The recoveries of standard addition were between 96.0% and 103.0%.

Battery-operated portable high-throughput solution cathode glow discharge optical emission spectrometry for environmental metal detection

In this study, miniaturized optical emission spectrometry (OES) based on solution cathode glow discharge (SCGD) was developed for the determination of metal elements in samples.

Determination of gallium in water samples by atomic emission spectrometry based on solution cathode glow discharge

A novel and rapid method was established for gallium determination by a home-made solution cathode glow discharger coupled with a portable fiber-optic spectrometer. Plasma was generated by the discharger and the emission line at 417 nm for gallium was observed with a portable fiber-optic spectrometer, whose spectral resolution was 1.1 nm based on the peak width of half height using a 10 mg/L gallium standard solution. The analytical conditions and instrumental parameters were optimized. The results showed acidity turned to be significantly influential for the emission intensity of the same sample. Therefore, acidity control of the testing solution was crucial. Under the optimized conditions, the limit of detection (LOD) for gallium was 0.47 mg/L calculated by 3SDblank/S. The stability was evaluated by the seven replicates of 10 mg/L gallium standard solution, and the relative standard deviation (RSD) was 2.1%. This method was applied to the analysis of gallium for river and waste water available in Beijing. The recoveries in the range of 92.0%–113.1% were acquired while the RSD values were below 7% (n = 5), which showed this method was feasible for gallium determination of real water samples. Moreover, only 1% HNO3 is demanded while the whole instrument is portable and works under ordinary atmospheric pressure so that the field test demand can be satisfied.

Comparison of the Plasma Temperature and Electron Number Density of the Pulsed Electrolyte Cathode Atmospheric Pressure Discharge and the Direct Current Solution Cathode Glow Discharge

A novel-pulsed electrolyte cathode atmospheric pressure discharge (pulsed-ECAD) plasma source driven by an alternating current (AC) power supply coupled with a high-voltage diode was generated under normal atmospheric pressure between a metal electrode and a small-sized flowing liquid cathode. The spatial distributions of the excitation, vibrational, and rotational plasma temperatures of the pulsed-ECAD were investigated. The electron excitation temperature of H Texc(H), vibrational temperature of N2 Tvib(N2), and rotational temperature of OH Trot(OH) were from 4900 ± 36 to 6800 ± 108 K, from 4600 ± 86 to 5800 ± 100 K, and from 1050 ± 20 to 1140 ± 10 K, respectively. The temperature characteristics of the dc solution cathode glow discharge (dc-SCGD) were also studied for the comparison with the pulsed-ECAD. The effects of operating parameters, including the discharge voltage and discharge frequency, on the plasma temperatures were investigated. The electron number densities determined in the discharge system and dc-SCGD were 3.8–18.9 × 1014 cm–3 and 2.6 × 1014 to 17.2 × 1014 cm–3, respectively.

Analytical Characterization of a Solution Cathode Glow Discharge with an Interference Filter Wheel for Spectral Discrimination

ABSTRACTSolution cathode glow discharge–atomic emission spectrometry (SCGD–AES) has attracted increasing attention in recent years in water and environmental analysis. In this article, the traditional spectrometer was replaced by an interference filter wheel for spectral discrimination using SCGD–AES. The aforementioned interference filters selected the spectral lines of Na, K, Ca, Li, Sr, Rb, and Cs. These spectral signals were transferred to weak currents using a photomultiplier tube and amplified by a picoammeter. The solution flow rate, discharge current and slit width were optimized individually. The analytical performance of SCGD coupled with an interference filter wheel was studied for atomic emission (SCGD–IFW–AES). The results show that the limits of detection for Na, K, Ca, Li, Sr, Rb, and Cs were 0.16, 0.36, 107, 0.53, 330, 1.93, and 13.58 µg/L, respectively. The relative standard deviations of the spectral intensities for the seven elements were 0.58, 0.94, 0.69, 0.70, 0.63, 0.67, and 0.65%, which indicated that the system operated stably. Matrix effect experiments were performed to study the effect of different concentrations of Na on the spectral intensities of K and Ca. Based on these measurements, the concentration of K in physiological saline was determined to be 0.534 mg/L by SCGD–IFW–AES.

Flow Injection Photochemical Vapor Generation Coupled with Miniaturized Solution-Cathode Glow Discharge Atomic Emission Spectrometry for Determination and Speciation Analysis of Mercury.

A novel, compact, and green method was developed for the determination and speciation analysis of mercury, based on flow injection photochemical vapor generation (PVG) coupled with miniaturized solution cathode glow discharge-atomic emission spectroscopy (SCGD-AES). The SCGD was generated between a miniature hollow titanium tube and a solution emerging from a glass capillary. Cold mercury vapor (Hg(0)) was generated by PVG and subsequently delivered to the SCGD for excitation, and finally the emission signals were recorded by a miniaturized spectrograph. The detection limits (DLs) of Hg(II) and methylmercury (MeHg) were both determined to be 0.2 μg L-1. Moreover, mercury speciation analysis could also be performed by using different wavelengths and powers from the UV lamp and irradiation times. Both Hg(II) and MeHg can be converted to Hg(0) for the determination of total mercury (T-Hg) with 8 W/254 nm UV lamp and 60 s irradiation time; while only Hg(II) can be reduced to Hg(0) and determined selectively with 4 W/365 nm UV lamp and 20 s irradiation time. Then, the concentration of MeHg can be calculated by subtracting the Hg(II) from the T-Hg. Because of its similar sensitivity and DL at 8 W/254 nm, the simpler and less toxic Hg(II) was used successfully as a primary standard for the quantification of T-Hg. The novel PVG-SCGD-AES system provides not only a 365-fold improvement in the DL for Hg(II) but also a nonchromatographic method for the speciation analysis of mercury. After validating its accuracy, this method was successfully used for mercury speciation analysis of water and biological samples.

Atmospheric-pressure solution-cathode glow discharge: A versatile ion source for atomic and molecular mass spectrometry

An atmospheric-pressure solution-cathode glow discharge (SCGD) has been evaluated as an ion source for atomic, molecular, and ambient desorption/ionization mass spectrometry. The SCGD consists of a direct-current plasma, supported in the ambient air in the absence of gas flows, and sustained upon the surface of a flowing liquid cathode. Analytes introduced in the flowing liquid, as an ambient gas, or as a solid held near the plasma are vaporized and ionized by interactions within or near the discharge. Introduction of acidic solutions containing metal salts produced bare elemental ions as well as H2O, OH− and NO3− adducts. Detection limits for these elemental species ranged from 0.1 to 4 ppb, working curves spanned more than 4 orders of linear dynamic range, and precision varied between 5 and 16% relative standard deviation. Small organic molecules were also efficiently ionized from solution, and both the intact molecular ion and fragments were observed in the resulting SCGD mass spectra. Fragmentation of molecular species was found to be tunable; high discharge currents led to harder ionization, while low discharge currents produced stronger molecular-ion signals. Ambient gases and solids, desorbed by the plasma from a glass probe, were also readily ionized by the SCGD. Indeed, strong analyte signals were obtained from solid samples placed at least 2 cm from the plasma. These findings indicate that the SCGD might be useful also for ambient desorption/ionization mass spectrometry. Combined with earlier results that showed the SCGD is useful for ionization of labile biomolecules, the results here indicate that the SCGD is a highly versatile ion source capable of providing both elemental and molecular mass-spectral information.

Ultratrace Determination of Tin, Germanium, and Selenium by Hydride Generation Coupled with a Novel Solution-Cathode Glow Discharge-Atomic Emission Spectrometry Method.

We herein describe a novel method of hydride generation (HG) coupled to a newly designed atmospheric pressure solution-cathode glow discharge (SCGD) spectrometric technique for the ultratrace determination of tin, germanium, and selenium. In this novel SCGD process, gas introduction was permitted using a hollow titanium tube as both the anode and sampling port. In these experiments, the analytes were converted into volatile hydrides upon passing through the hydride generator, and were introduced into the near-anode region of the SCGD system, where they were detected directly by atomic emission spectrometry (AES). A significant improvement in both selectivity and sensitivity was achieved, which was reflected in an improvement in the detection limits (DLs) by 3 orders of magnitude, in addition to successful valence analysis of Se without the requirement for chromatographic separation. In the absence of a strict sample pretreatment process and with a reduction in electrolyte consumption, the detection limits of Sn, Ge, and Se were determined to be 0.8, 0.5, and 0.2 μg·L-1. Moreover, our HG-SCGD-AES system demonstrated excellent repeatability (<3% peak height relative standard deviation) and more than 2 orders of linear dynamic range. The optimal operating conditions are outlined herein, and the analytical performance of the system is evaluated as described. Furthermore, our method was applied for the analysis of Sn, Ge, and Se in both environmental and biological samples, and the obtained results were in good agreement with reference values.