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

Advances in atmospheric pressure plasma-based optical emission spectrometry for the analysis of heavy metals

Over the past decade, miniaturized optical emission spectrometry (OES) systems utilizing atmospheric pressure plasmas (APPs) as radiation sources have exhibited impressive capabilities in trace heavy metal analysis. As the core of the analytical system, APPs sources possess unique properties such as compact size, light weight, low energy requirement, ease of fabrication, and relatively low manufacturing cost. This critical review focuses on recent progress of APP-based OES systems employed for the determination of heavy metals. Influences of technical details including the sample introduction manner, the sampling volume, the sample flow rate, the pH of the solutions on the plasma stability and the intensity of analytical signals are comprehensively discussed. Furthermore, the review emphasizes the analytical challenges faced by these techniques and highlights the opportunities for further development in the field of heavy metal detection.

Highly sensitive analysis of trace germanium derived from the efficient electrosynthesis and spectral introduction of GeH4 on foam electrode

BACKGROUND

The electrochemical hydride generation technology, which uses electrolysis instead of chemical reagents to generate reducing species to achieve gaseous transformation and sample introduction of the tested elements, has received widespread attention in the field of atomic spectroscopy due to its simple, economical, and green characteristics. However, limited by the effective area of the electrode, the introduction efficiency and spectral signal of most elements (e.g., germanium) in practical applications are lower than traditional chemical hydride generation.

RESULTS

In this paper, an efficient electrochemical hydride generation (EHG) method based on metal foam electrode for μg L-1 level germanium was constructed. Systematic electrochemical and spectral tests showed that the low charge transfer resistance and the high electrochemical activity of nickel-based foam electrodes jointly promoted the efficient electroreduction of Ge(IV). Besides, the porous network structure of the metal foam material improves the contact probability of reactants while reducing the gas-evolution effect caused by bubble accumulation. Interestingly, adequate reaction sites are crucial for the conversion of germanium, but large foam electrodes are not always compatible with analytical performance. After coupling atomic fluorescence spectroscopy, this new electrolysis method has been proven to be suitable for efficient conversion and quantitative detection of Ge over a wide concentration range (5-150 μg L-1).

SIGNIFICANCE

Our proposal to improve the electrosynthesis efficiency of germanane (GeH4) by using metal foam electrode is extremely effective for the detection of trace or ultra-trace germanium. The exploration of electrode material, structure, and especially effective area will also provide ideas for the establishment of highly sensitive analysis methods in the future.

Stripping Voltammetry with Nanomaterials-based Electrode in the Environmental Analysis of Trace Concentrations of Tin

A method for the voltammetric determination of tin using a multiwall carbon nanotubes/spherical glassy carbon (CNTs/SGC) electrode is described. The new procedure is based on the adsorptive accumulation of the Sn(II)-cupferron complex on a CNTs/SGC electrode modified with a lead film, followed by electrochemical reduction of the adsorbed species. The optimal experimental conditions include the use of 0.10 mol L-1 acetate buffer (pH 5.7), 4.0×10-4 M cupferron and 1.0×10-4 M Pb(II). The peak current is proportional to the concentration of Sn(II) over the range of 1.0×10-9 -1.0×10-7 M and the detection limit is 3.1×10-10 M for a 95 s accumulation time. The proposed method was used to determine tin in real samples and certified reference materials.

Electrochemical Methods for the Analysis of Trace Tin Concentrations-Review

Tin determination allows for the monitoring of pollution and assessment of the impact of human activities on the environment. The determination of tin in the environment is crucial for the protection of human health and ecosystems, and for maintaining sustainability. Tin can be released into the environment from various sources, such as industry, transportation, and electronic waste. The concentration of tin in the environment can be determined by different analytical methods, depending on the form of tin present and the purpose of the analysis. The choice of an appropriate method depends on the type of sample, concentration levels, and the available instrumentation. In this paper, we have carried out a literature review of electrochemical methods for the determination of tin. Electrochemical methods of analysis such as polarography, voltammetry, and potentiometry can be used for the determination of tin in various environmental samples, as well as in metal alloys. The detection limits and linearity ranges obtained for the determination of tin by different electrochemical techniques are collected and presented. The influence of the choice of base electrolyte and working electrode on signals is also presented. Practical applications of the developed tin determination methods in analyzing real samples are also summarized.

Rapid detection of ultratrace urinary arsenic by direct sampling microplasma vaporization based on silicon nitride

At present, immediate monitoring urinary arsenic is still a challenge for treating arsenic poisoning patients. Thus, a fast, reliable and accurate analytical approach is indispensable to monitor ultratrace arsenic in urine sample for health warning. In this work, a silicon nitride (SN) rod was first integrally utilized as a sample carrier for ≤50 μL urinary aliquot, an electric heater for removing water and ashing sample as well as a high voltage electrode for dielectric barrier discharge vaporization (DBDV). The direct analytical method of arsenic in urine without sample digestion was thus developed using atomic fluorescence spectrometer (AFS) as a model detector. After 4 V electrically heating the SN rod for 60 s, urine sample was dehydrated and ashed outside; then, DBD was exerted under 0.8 A with 0.8 L/min H₂ + Ar (1:9, v:v) for 20 s to vaporize arsenic analyte from the SN rod. After optimization, 0.014 μg/L arsenic detection limit (LOD) was reached with favorable analytical precision (RSD <5%) and accuracy (91-110% recoveries) for real sample analysis. As a result, the whole analysis process only consumes <3 min to exclude complicated sample preparation; furthermore, the designed DBDV system only occupies 25 W and <2 kg, which renders a miniature sampling component to hyphenate with a miniature detector to detect arsenic. Thus, this direct sampling DBDV method extremely fulfills the fast, sensitive and precise detection of ultratrace arsenic in urine sample.

Fast and simple analysis of the content of Zn, Mg, Ca, Na, and K in selected beverages widely consumed by athletes by flowing liquid cathode atmospheric pressure glow discharge optical emission spectrometry

An atmospheric pressure glow discharge (APGD) system, generated between a flowing liquid cathode (FLC) and a gas (He) jet anode, was applied for the determination of Zn, Mg, Ca, Na, and K in selected beverages commonly chosen by athletes (namely Coca-Cola Zero, energy and vitamin drinks, pre-workout, branched-chain amino acids, almond drink, and whey protein) by optical emission spectrometry (OES). In some cases (i.e., Coca-Cola, energy drink, and almond drink), sugared and sugar-free versions of the beverages were analyzed with the purpose of establishing the impact of added sugar on the analyte signal intensities. The analysis was performed after a simplified sample preparation procedure, which involved only their dilution and acidification with HNO₃ to a concentration of 0.2 mol L^-1. To determine the most suitable conditions for performing the analysis, optimization of the crucial operating parameters and sample dilution was carried out. Under the compromise conditions, the instrumental detection limits (DLs) were established and found to be 21, 0.91, 20, 0.062, and 0.14 μg L^-1 for Zn, Mg, Ca, Na, and K, respectively. Due to the relatively low detection limits, the analyte content could be determined for a fairly high dilution, being concurrently the same for all analytes, which further simplified the whole procedure. It was found that the vast majority of samples could be determined using external calibration with simple standard solutions. The standard addition technique used for calibration was only required for the determination of Mg in three samples. The analysis results were consistent (in the majority of cases the recovery values were in the range of 88-111%) with the values obtained for the reference method (inductively coupled plasma optical emission spectrometry, ICP-OES), which proved the reliability of the results obtained from the developed FLC-APGD-OES system.

Array Point Discharge as Enhanced Tandem Excitation Source for Miniaturized Optical Emission Spectrometer

A new compact tandem excitation source was designed and constructed by using an array point discharge (ArrPD) microplasma for a miniaturized optical emission spectrometer through coupling a hydride generation (HG) unit as a sample introduction device. Three pairs of point discharges were arranged in sequence in a narrow discharge chamber to construct the ArrPD microplasma, for improved excitation capability owing to the serial excitation. Besides, the discharge plasma region was greatly enlarged, therefore, more gaseous analytes could be intercepted to enter into the microplasma for sufficient excitation, for improved excitation efficiency and OES signal. To better understand the effectiveness of the proposed ArrPD source, a new instrument for simultaneous detection of atomic emission and absorption spectral responses was also proposed, designed, and constructed to reveal the excitation and enhancement process in the discharge chamber. Under the optimized conditions, the limits of detection (LODs) of As, Ge, Hg, Pb, Sb, Se, and Sn were 0.7, 0.4, 0.05, 0.7, 0.3, 2, and 0.08 μg L^-1, respectively, and the relative standard deviations (RSDs) were all less than 4%. Compared with a commonly used single point discharge microplasma source, the analytical sensitivities of these seven elements were improved by 3-6-fold. Certified Reference Materials (CRMs) were successfully analyzed with this miniaturized spectrometer, which features low power, compactness, portability, and high detectability, and is thereby a great prospect in the field of elemental analytical chemistry.

Multi-element Simultaneous sensitization of solution cathode glow discharge atomic emission spectrometry by using portable semiconductor anode refrigeration

In this study, a modified solution cathode glow discharge atomic emission spectrometry (SCGD-AES) was used to detect metal elements in electroplating sewage. The SCGD-AES device was equipped with a portable semiconductor anode refrigeration unit, which was built independently. The red-heat effect of tungsten electrode was alleviated by adding the portable refrigeration unit, thus improving thermal stability with the withstand voltage from 1040 V to 1140 V. Compared with the devices without semiconductor refrigeration, the chromium was excited more favorable when the discharge voltage increased, and the limit of detection (LOD) decreased by 8.5 times. Furthermore, the LODs of Zn, Cd, Ni, Cu and Pb decreased by 1.8-3.2 times, respectively, which realized the detection of elements in electroplating sewage and showed high performance in the field of trace elements analysis. Furthermore, the accuracy of the method was verified by stream sediment reference material (GBW07312), and the results were consistent with the certified values. The recoveries of elements added to industrial sewage and seawater range were from 90.5 to 98.7%, demonstrating good accuracy of the proposed method.

Photochemical vapor generation for germanium: synergistic effect from cobalt/chloride ions and air-liquid interfaces

Photochemical vapor generation (PVG) of germanium (Ge) was first reported in this work. The synergistic effect from cobalt/chloride ions and air-liquid interfaces was found for the PVG of Ge. No obvious signal response was observed from the standard solution of Ge in 10% (v/v) formic acids (FAs) under UV irradiation. The addition of 300 mg L^-1 of Co²⁺ and 30 mmol L^-1 of Cl^- resulted in enhanced photochemical reduction for Ge, and the introduction of air-liquid interfaces proceeding and succeeding the sample solution caused another 4.6 folds of enhancement in signal response of Ge. Under the selected condition, the limit of detection (LOD, 3σ, n = 11) was obtained to be 0.008 ng mL^-1 with inductively coupled plasma mass spectrometry (ICP MS) measurement. A good precision, expressed as a relative standard deviation (RSD, n = 7) of 2.0%, was found from replicated measurements of 2 ng mL^-1 of Ge. The generation efficiency was found to be no better than 9 ± 2%. The PVG mechanism of Ge was investigated in this work. The new finding is useful for understanding the principle of PVG, and further exploring the analytical and environmental application of PVG.

Microdischarge in Flame as a Source-in-Source for Boosted Excitation of Optical Emission of Chromium

A compact tandem excitation source-in-source was designed by arranging a point discharge (PD) ignited in argon/hydrogen (Ar/H₂) flame and utilized for boosted excitation for the optical emission of chromium. Through a tungsten coil (W-coil) electrothermal vaporizer (ETV) located right under the tandem source without any interface for sample introduction, a miniaturized optical emission spectrometer was realized. Because the discharge gaseous atmosphere of PD was activated in the flame, the energy consumption of PD for breaking down discharge gas and maintenance of plasma was greatly saved. In addition, the flame could partially atomize or keep the atomized state of analyte atoms through its reducing environment. Therefore, the excitation capability of the tandem source was greatly improved, owing to the synergistic effect of PD microplasma and Ar/H₂ flame. In addition, part of the analyte was atomized/excited on the W-coil, and thereby, dry, pure, and activated analyte species were released from the W-coil and swept into the tandem source for atomization/excitation. Through the collective effect of W-coil ETV, Ar/H₂ flame, and PD microplasma, analytical sensitivity for Cr was greatly enhanced. Under the optimized conditions, with 10 μL sample solution, a limit of detection of 1.5 μg L^-1 and a relative standard deviation of 3.6% (20 μg L^-1, n = 5) were achieved. Its accuracy was demonstrated by successful analysis of several certified reference materials. Owing to the advantages including high sensitivity, compactness, and cost effectiveness, it is promising to facilitate the miniaturized spectrometer for more elements and potential field analytical chemistry.

Direct analysis of wines from the province of Lower Silesia (Poland) by microplasma source optical emission spectrometry

New microplasma source optical emission spectrometry (OES) for the determination of Na, K, Mg, Ca, and Zn in wine was developed. As the microplasma source, a solution anode glow discharge (SAGD) or a solution cathode glow discharge (SCGD) were employed. The diluted samples solutions (0.5-2%) were directly analyzed (no acid digestion required) and the detection limits of Na, K, Mg, Ca, and Zn were 0.015, 0.03, 3, 12, and 0.1 µg L^-1, respectively. The developed method was used for the analysis of wine samples from the province of Lower Silesia (Poland). It was found that 1) red wines were characterized by a higher content of K and Mg, 2) it was possible to discriminate between Regent and Pinot Noir grape varieties (both red) by the concentrations of K and Ca, 3) the concentration of Na in the analyzed wines was lower than that found in wines from other European countries.

Three-Dimensional Printed Dual-Mode Chemical Vapor Generation Point Discharge Optical Emission Spectrometer for Field Speciation Analyses of Mercury and Inorganic Selenium

Due to the large size and high energy consumption of instruments, field elemental speciation analysis is still challenging so far. In this work, a portable and compact system device (230 mm length × 38 mm width × 84 mm height) was fabricated by using three-dimensional (3D) printing technology for the field speciation analyses of mercury and inorganic selenium. The device comprises a cold vapor generator, photochemical vapor generator, and miniaturized point discharge optical emission spectrometer (μPD-OES). For mercury, inorganic mercury (IHg) was selectively reduced to Hg⁰ by cold vapor generation, whereas the reductions of both IHg and methylmercury (MeHg) were obtained by photochemical vapor generation (PVG) in the presence of formic acid. For selenium, Se(IV) and total inorganic selenium were converted to their volatile species by PVG in the presence and the absence of nano-TiO₂, respectively. The generated volatile species were consequently detected by μPD-OES. Limits of detection of MeHg, IHg, Se(IV), and Se(VI) were 0.1, 0.1, 5.2, and 3.5 μg L^-1, respectively. Precision expressed as the relative standard deviations (n = 11) were better than 4.5%. The accuracy and practicality of the proposed method were evaluated by the analyses of Certified Reference Materials (DORM-4, DOLT-5, and GBW(E)080395) and several environmental water samples with satisfactory recoveries (95-103%). This work confirms that 3D printing has great potential to fabricate a simple, miniaturized, easy-to-operate, and low gas and power consuming atomic spectrometer for field elemental speciation analysis.

Five years of innovations in development of glow discharges generated in contact with liquids for spectrochemical elemental analysis by optical emission spectrometry

The newest achievements in the field of glow microdischarges generated in contact with a flowing liquid cathode (FLC) and a flowing liquid anode (FLA), used as the excitation sources for optical emission spectrometry (OES), were summarized herein. The design of recently reported discharge systems was compared and comprehensively discussed. A lot of effort was devoted to evaluate the effect of selected operating parameters, i.e., discharge voltage and current, sample flow rate, sample pH, jet-supporting gas flow rate, and discharge gap, on the microplasma stability and the intensity of measurable analytical signals. Furthermore, the influence of chemical modifiers, i.e., organic acids, alcohols, and surfactants, aimed at improving the sensitivity and reducing matrix effects, was referred to as well. Finally, the analytical performance and the application of these promising excitation sources for the elemental analysis of different-matrix samples were presented.

A miniaturized UV-LED array chip-based photochemical vapor generator coupled with a point discharge optical emission spectrometer for the determination of trace selenium

Ultraviolet light emitting diode array chip-based photochemical vapor generation was combined with hollow electrode point discharge to establish a miniaturized optical emission spectrometer for efficient vapor generation and excitation of selenium.

On the coupling of hydride generation (HG) with flowing liquid anode atmospheric pressure glow discharge (FLA-APGD) for determination of traces of As, Bi, Hg, Sb and Se by optical emission spectrometry (OES)

A novel atmospheric pressure glow discharge (APGD) microplasma system, sustained between a miniaturized flowing liquid anode (FLA) and a He jet nozzle cathode, was combined with a hydride generation (HG) technique to improve the determination performance of As, Bi, Hg, Sb, and Se with the aid of optical emission spectrometry (OES). The discharge current, the He flow rate, and the concentrations of HCl and NaBH₄ were considered to affect both the HG reaction and the excitation conditions in the discharge, thus they were thoroughly studied. Under the optimized conditions, the detections limits (LODs), assessed on the basis of the 3σ criterion, reached 1.7, 0.85, 0.04, 0.51, and 2.9 μg L^-1 for As, Bi, Hg, Sb, and Se, respectively. The HG and transport efficiency for these elements was evaluated to be 88-100%, which is notably better, as compared to their transport efficiency in the conventional FLA-APGD system, without the HG technique. This yielded an improvement of the LODs achievable in this system and, simultaneously, enabled to determine As, Sb, and Se at a level, which is unobtainable with the use of the FLA-APGD system alone. The proposed methodology was then successfully applied for a quantitative determination of the examined elements in wastewater (ERM-CA713) and spiked water samples. The recoveries of the elements added to these waters (at the maximum acceptable levels in drinking water set by the U.S. Environmental Protection Agency) ranged between 81 and 104%, confirming the excellent accuracy, usefulness, and reliability of the developed HG-FLA-APGD technique.

Online, Continuous, and Interference-Free Monitoring of Trace Heavy Metals in Water Using Plasma Spectroscopy Driven by Actively Modulated Pulsed Power

This work presents the development of an online continuous heavy metals monitoring system using optical emission spectroscopy of plasma in water. The plasmas were driven by actively modulated pulsed power (AMPP) to control the plasma and its emission behavior in solutions with a wide range of conductivity. The AMPP quantified in situ the solutions' conductivity and modulated in real time the pulse width based on the conductivity. We demonstrated the online monitoring of the metallic elements. The results show that multiple metallic elements, namely Pb and Zn, can be independently and simultaneously detected with less than a 10% variation in the corresponding optical emission lines in solutions with a wide range of conductivity. An alert system was integrated to demonstrate the capability of an instant warning via e-mail once metallic elements were detected. Finally, we demonstrated that this system was robust even with the existence of several interferences and able to perform online continuous monitoring for days. We believe the system using plasma spectroscopy with AMPP for online monitoring of metals in water will have a significant impact on the fields of environmental monitoring and protection.

Photochemical Vapor Generation of Tellurium: Synergistic Effect from Ferric Ion and Nano-TiO2

Photochemical vapor generation (PVG) is emerging as a promising analytical tool for Te determination, thanks to its efficient matrix separation, and simple and green procedure. However, the low PVG generation efficiency of Te is the bottleneck for its wide application in environmental samples containing trace Te. Herein, we reported a high efficient PVG for Te determination by synergistic effect of ferric ion and nano-TiO₂. The analytical sensitivity was enhanced approximately 15-fold for Te(IV) in the presence of both ferric ions and nano-TiO₂, comparing to conventional PVG. Besides, the use of nano-TiO₂ can provide Te(VI) and Te(IV) an equal and high PVG efficiency in the presence of ferric ions, owned to the high photocatalytic performance of TiO₂ under short-wavelength UV irradiation (254 and 185 nm). Under the optimized experimental conditions, a detection limit of 1.0 ng L^-1 was obtained. The precision of replicate measurements was 2.3% (RSD, n = 7) at 0.5 μg L^-1 for Te(IV). The methodology was validated by successful determination of Te in surface waters and two standard reference sediment samples. To our best knowledge, this is the first report of the synergistic enhancement of transitional metal ions and nano-TiO₂ in PVG, which possesses potential for highly sensitive determination of vapor-forming elements.