Sensitive determination of Ag, Bi, Cd, Hg, Pb, Tl, and Zn by inductively coupled plasma optical emission spectrometry combined with the microplasma-assisted vapor generation.

A technique of vapor generation assisted by a microplasma was used for sample introduction into inductively coupled plasma optical emission spectrometry (ICP OES). Replacing a pneumatic nebulizer with a novel microplasma device improved the sensitivities of analytical signals for Ag, Bi, Cd, Pb, Tl, and Zn (by 2-13 times), as well as a concomitant reduction in their detection limits (DLs). Moreover, an outstanding improvement (30-fold) was achieved for Hg. The factors contributing to the boosted signal intensities were higher analyte fluxes and less water vapor produced by the microplasma system. The DLs of Ag, Bi, Cd, Hg, Pb, Tl, and Zn in microplasma-ICP OES were 0.4, 4, 0.06, 0.2, 2, 5, and 0.2 µg L-1, respectively, and the measurement precision was within the range of 0.7-2.4% (it was significantly improved as compared to that achievable with pneumatic nebulization). The proposed microplasma-assisted vapor generation eliminates the use of toxic reductants, e.g., sodium tetrahydridoborate, and it is characterized by higher resistance to matrix effects from transition metal ions (as compared to chemical vapor generation (CVG) and photochemical vapor generation (PVG)). To validate the trueness of the SAGD-ICP OES method, certified reference materials of lobster hepatopancreas (TORT-2), cormorant tissue (MODAS-4) as well as spiked tap water and seawater samples were analyzed to determine levels Cd and Hg. The standard additions method was used for calibration in both cases. Recoveries of the analytes in the case of the analysis of TORT-2 and MODAS-4 samples as well as the spiked tap water and seawater was within the range of 98-113%, which indicated that the developed sample introduction system can be successfully used for very sensitive determinations of selected hazardous elements in environmental samples.

Determination of Ag, Bi, Cd, Hg, Pb, Tl, and Zn by inductively coupled plasma mass spectrometry combined with vapor generation assisted by solution anode glow discharge - A preliminary study.

A new technique of vapor generation assisted by a microplasma was proposed for an inductively coupled plasma mass spectrometry (ICP MS). It was found that, by replacing a traditional pneumatic nebulizer with a microplasma (solution anode glow discharge, SAGD), analytical signals of Ag, Bi, Cd, Hg, Pb, Tl, and Zn were improved 8, 4, 13, 13, 9, 10, and 7 times, respectively. The main factor contributing to boosted analytical signal intensities was the higher analyte flux produced by the novel microplasma system. The measurement precision in SAGD-ICP MS was comparable to that achievable for ICP MS (with pneumatic nebulization), and it did not exceed 2%. The detection limits of Ag, Bi, Cd, Hg, Pb, Tl, and Zn in SAGD-ICP MS were 5, 2, 6, 5, 4, 10, and 20 ng L-1, respectively. The analytical performance of this method may be further improved if the observed memory effects could be minimized. To validate the trueness of the novel method, certified reference materials of lobster hepatopancreas (TORT-2), cormorant tissue (MODAS-4), and wastewater (ERM CA-713) were analyzed to determine traces of Cd, Hg, and Pb. Recoveries of certified values for these analytes were ranged from 91 to 111%, which indicated that the studied microplasma system in combination with ICP MS can be successfully used for very sensitive determinations of selected hazardous elements in environmental samples.

Response surface methodology assisted development of a simplified sample preparation procedure for the multielement (Ba, Ca, Cu, Fe, K, Mg, Mn, Na, Sr and Zn) analysis of different coffee brews by means of inductively coupled plasma optical emission spectrometry.

The Box-Behnken response surface design together with the individual desirability functions were used to develop the new and greenish sample preparation procedure of coffee brews prior to their multielement (Ba, Ca, Cu, Fe, K, Mg, Mn, Na, Sr and Zn) analysis by inductively coupled plasma optical emission spectrometry (ICP OES). The developed procedure required only 2-fold dilution of the samples with a 1.8 mol L-1 HNO3 solution and then, the sonication of the resulting samples solutions for 8 min at room temperature. The proposed method was precise (0.6-7.5% as RSD), true (relative errors changing from -5.2% to +4.6%) and guaranteed the limits of detection (LODs) of the studied elements between 0.1 and 5 ng g-1. Finally, this simplified ICP OES based method was applied for the multielement analysis of brews of different Arabica coffees as well as those prepared with seldomly reported devices, i.e., dripper, slow dripper, French press, aeropress and syphon.

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.

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.

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.

Study and reduction of matrix effects in flowing liquid anode - Atmospheric pressure glow discharge - Optical emission spectrometry.

The effect of different interfering elements, i.e., Na, K, Mg, Ca, Al, Cu, Fe, Mn, Ni, and Zn, on the analytical performance of flowing liquid anode atmospheric pressure glow discharge optical emission spectrometry (FLA-APGD OES) was extensively studied. In the presence of interfering ions, the emission from analytes was suppressed by ∼10% in the case of Hg and Tl, ∼20% for Cd and Ag, and up to ∼80% for Zn and Pb. This study revealed that interfering elements did not affect the atomization/excitation conditions, and the reason for the observed decrease in analytical response was the impaired efficiency of analytes transport from liquid to plasma phase. To reduce matrix effects, the use of different masking agents capable of complexing the interfering ions, e.g., organic acids, crown ethers, chelating agents, and other compounds, was investigated. FLA-APGD appeared to be quite susceptible to the presence of masking agents and only their small amounts could be added (limiting the effectiveness of this approach). Despite this, it was possible to significantly reduce the matrix effects originating from transition metals and alkali/alkaline earth metals. Based on the results presented herein, different sample treatment procedures, aimed at the minimization of matrix effects in microplasma excitation sources, can be developed. As an example, a method for the determination of trace amounts of Zn and Pb in the Fe-rich matrix has been shown. By the use of the selected masking agents, the recovery of Zn and Pb was improved 5.6-fold (from 10.4 to 57.8%) and 2.8-fold (from 13.6 to 38.0%), respectively. Despite the presence of Fe-rich matrix, the detection limits of Zn and Pb were quite low (0.1 and 0.8 µg L-1, respectively) and they were apparently lower than offered by ICP OES.

Hanging drop cathode-atmospheric pressure glow discharge as a new method of sample introduction for inductively coupled plasma-optical emission spectrometry.

This work reports the use of hanging drop cathode-atmospheric pressure glow discharge (HDC-APGD) as a new method of sample introduction for inductively coupled plasma-optical emission spectrometry (ICP-OES). The developed arrangement was characterized by a low sample uptake (0.56 mL min-1) and the fact that the entire sample solution volume was consumed by the discharge. This resulted in a very high transport efficiency of analytes from the sample solution into the ICP torch (usually > 80%). Under the optimal operating conditions of HDC-APGD, intensities of emission lines of studied elements were, on average, 2 times higher as compared to those obtained with conventional pneumatic nebulization (PN). Moreover, in the case of I and Y, the observed signal enhancements were even higher, i.e., 6.2 and 6.1 times, respectively. It was also shown that in the case of B and some elements that are known to form different volatile species (Ag, Bi, Cd, Hg, Os, Pb, and Se), the presence of low molecular weight organic compounds in the sample solution, i.e., CH3OH, C2H5OH, HCOOH, CH3COOH, or HCHO, resulted in the additional enhancement of their signals. It was especially evident in the case of Hg for which a 8.6-fold signal enhancement in the presence of HCOOH was noticed. The system presented herein was distinguished from other competitive APGD-type discharges because it could be successfully used for the determination of a vast group of elements, including alkali metals, alkaline earth metals, transition metals, and non-metals.

Enhancement of emission from indium in flowing liquid anode atmospheric pressure glow discharge using organic media.

The effect of addition of low molecular weight organic compounds, i.e. acids, salts and alcohols, on the emission measured from In with flowing liquid anode atmospheric pressure glow discharge optical emission spectrometry (FLA-APGD-OES) was studied. It was revealed that a small addition of CH3OH or C2H5OH (0.5%, v/v) enhanced the intensity of In 451.1 nm atomic line over 30-times. The observed boosting of emission was caused both not only by promotion of the analyte transport efficiency from the liquid sample to the plasma phase (increased from 19% to 35%) but also due to some other factors, presumably the improvement of atomization/excitation conditions. The developed FLA-APGD-OES method offered a very low detection limit of In which was 0.21 µg L-1 (using a miniaturized spectrometer) or even 0.030 µg L-1 (using a high-resolution spectrometer). Furthermore, it was demonstrated that addition of alcohols significantly reduced the matrix effects from several interfering species, e.g. Ag, Cu, Pb and Zn, and only in case of Na, the anti-interference effect was observed. The intensity improvement which resulted from the addition of alcohols, was observed for In only and it did not occur for other elements such as Ag, Cd, Hg, Pb, Tl and Zn.

In-situ generation of Ag, Cd, Hg, In, Pb, Tl and Zn volatile species by flowing liquid anode atmospheric pressure glow discharge operated in gaseous jet mode - Evaluation of excitation processes and analytical performance.

The use of flowing liquid anode atmospheric pressure glow discharge (FLA-APGD) operated with the aid of a gaseous jet as an efficient and novel excitation source for optical emission spectrometry (OES) was evaluated in details. Although about 50 elements have been tested by introducing respective standard solutions into a discharge system, only emission lines of Ag, Cd, Hg, In, Pb, Tl and Zn have been identified. In this arrangement, the surface of solution was bombarded by electrons resulting in generation of volatile species of mentioned elements. After penetrating the plasma, they were excited and extraordinary atomic emission was observed for these elements. It was found that the use of the Ar or He jet provided beneficial excitation conditions, while reduction of the discharge gap led to suppression of the intensity of interfering diatomic molecular bands like NO, OH and N2. Under optimal operating conditions of the discharge system, detection limits (DLs) assessed for Ag, Cd, Hg, In, Pb, Tl and Zn were respectively 0.001, 0.006, 0.16, 0.093, 0.076, 0.007 and 0.018 µg L-1, and they were on average by 10-times better as referred to earlier FLA-APGD constructions operated without the gaseous jet. In addition, the discharge was characterized by greater stability and precision of measurements (< 2%). Dynamic ranges of calibration curves covered between 3.5 (Cd, In, Zn) and over 5 orders of magnitude (Ag, Tl). The biggest weakness of the developed system turned out to be high susceptibility to matrix interferences. It was found that presence of foreign ions (at concentrations of 1-10 mg L-1) suppressed emission from studied elements on average by 20-60%. To clarify the reason for extremely high sensitivity of emission lines of Ag, Cd, Hg In, Pb, Tl and Zn, efficiency of transport of these elements from the liquid phase to the plasma was determined. It was established that from 5% to 70% of analytes presented in sample solutions were released, whereas evaporation of water was merely 5-15%. Since the FLA-APGD plasma was enriched with analytes atoms, it was concluded that such processes like electrospray formation or solution evaporation do not make a major contribution to transport of analytes from the liquid phase to the discharge, and that it was mainly due to plasma-induced volatile species generation processes.

Development and optimization of simplified method of fast sequential HR-CS-FAAS analysis of apple juices on the content of Ca, Fe, K, Mg, Mn and Na with the aid of response surface methodology.

A new method for fast sequential measurements of concentrations of Ca, Fe, K, Mg, Mn and Na in apple juices by high resolution-continuum source-flame atomic absorption spectrometry (HR-CS-FAAS) was developed using response surface methodology (RSM). By combination of Box-Behnken response surface design and desirability functions, compromise conditions of simplified sample preparation of apple juices, including 2-fold dilution with water and acidification with HNO3 to 0.8 mol L-1, were established and verified that provided reliable results. The method was applied for analysis of 20 different samples of commercially sold apple juices. Precision (as relative standard deviation for 3 parallel samples, RSD) and accuracy (as relative error in relation to reference values obtained with prior wet digestion of samples) were very good, i.e. from 0.5% to 6% and from -5 to + 6%, respectively.

Sensitive Determination of Cd in Small-Volume Samples by Miniaturized Liquid Drop Anode Atmospheric Pressure Glow Discharge Optical Emission Spectrometry.

A novel liquid drop anode (LDA) direct current atmospheric pressure glow discharge (dc-APGD) system was applied for direct determination of Cd in liquid microsamples (50 µL) by optical emission spectrometry (OES). The microdischarge was generated in open-to-air atmosphere between a solid pin type tungsten cathode and a liquid drop placed on a graphite disk anode. The arrangement of the graphite disk placed on a PTFE chip platform as well as the solid pin type cathode was simple and robust. The limit of detection (LOD) of Cd for the developed LDA-APGD-OES method was 0.20-0.40 µg L-1, while precision (as the relative standard deviation for the repeated measurements) was within 2-5%. By using the liquid drop of 50 µL, the linearity range of 1-1000 µg L-1 was achieved. The effect of addition of the low-molecular weight (LMW) organic compounds, easily ionized elements (EIEs), i.e., Ca, K, Mg, and Na, as well as the foreign ions (Al, Cu, Fe, Mn, Zn) to the solution on the in situ atomization and excitation processes occurred during operation of the LDA-APGD system, and the response of Cd was studied. Validation of the proposed method was demonstrated by analysis of Lobster hepatopancreas (TORT-2), pig kidney (ERM-BB186), and groundwater (ERM-CA615) certified reference materials (CRMs) and recoveries of Cd from water samples spiked with 25 µg L-1 of Cd. Very good agreement between the found and certified values of Cd in the CRMs (the recoveries were within the range of 96.3-99.6%) indicated trueness of the method and its reliability for determination of traces of Cd. In the case of the spiked water samples, the recoveries obtained were in the range from 95.2 to 99.5%.

Flowing Liquid Anode Atmospheric Pressure Glow Discharge as an Excitation Source for Optical Emission Spectrometry with the Improved Detectability of Ag, Cd, Hg, Pb, Tl, and Zn.

A novel atmospheric pressure glow discharge generated in contact with a flowing liquid anode (FLA-APGD) was developed as the efficient excitation source for the optical emission spectrometry (OES) detection. Differences in the appearance and the electrical characteristic of the FLA-APGD and a conventional system operated with a flowing liquid cathode (FLC-APGD) were studied in detail and discussed. Under the optimal operating conditions for the FLA-APGD, the emission from the analytes (Ag, Cd, Hg, Pb, Tl, and Zn) was from 20 to 120 times higher as compared to the FLC-APGD. Limits of detections (LODs) established with a novel FLA-APGD system were on average 20 times better than those obtained for the FLC-APGD. A further improvement of the LODs was achieved by reducing the background shift interferences and, as a result, the LODs for Ag, Cd, Hg, Pb, Tl, and Zn were 0.004, 0.040, 0.70, 1.7, 0.035, and 0.45 µg L(-1), respectively. The precision of the FLA-APGD-OES method was evaluated to be within 2-5% (as the relative standard deviation of the repeated measurements). The method found its application in the determination of the content of Ag, Cd, Hg, Pb, Tl, and Zn in a certified reference material (CRM) of Lobster hepatopancreas (TORT-2), four brass samples as well as mineral water and tea leaves samples spiked with the analytes. In the case of brass samples, a reference method, i.e., inductively coupled plasma optical emission spectrometry (ICP-OES) was used. A good agreement between the results obtained with FLA-APGD-OES and the certified values for the CRM TORT-2 as well as the reference values obtained with ICP-OES for the brass samples was revealed, indicating the good accuracy of the proposed method. The recoveries obtained for the spiked samples of mineral water and tea leaves were within the range of 97.5-102%.

The influence of stabilizers on the production of gold nanoparticles by direct current atmospheric pressure glow microdischarge generated in contact with liquid flowing cathode.

Gold nanoparticles (Au NPs) were prepared by direct current atmospheric pressure glow microdischarge (dc-µAPGD) generated between a miniature argon flow microjet and a flowing liquid cathode. The applied discharge system was operated in a continuous flow liquid mode. The influence of various stabilizers added to the solution of the liquid cathode, i.e., gelatin (GEL), polyvinylpyrrolidone (PVP), or polyvinyl alcohol (PVA), as well as the concentration of the Au precursor (chloroauric acid, HAuCl4) in the solution on the production growth of Au NPs was investigated. Changes in the intensity of the localized surface plasmon resonance (LSPR) band in UV/Vis absorption spectra of solutions treated by dc-µAPGD and their color were observed. The position and the intensity of the LSPR band indicated that relatively small nanoparticles were formed in solutions containing GEL as a capping agent. In these conditions, the maximum of the absorption LSPR band was at 531, 534, and 535 nm, respectively, for 50, 100, and 200 mg L-1 of Au. Additionally, scanning electron microscopy (SEM) and dynamic light scattering (DLS) were used to analyze the structure and the morphology of obtained Au NPs. The shape of Au NPs was spherical and uniform. Their mean size was ca. 27, 73, and 92 nm, while the polydispersity index was 0.296, 0.348, and 0.456 for Au present in the solution of the flowing liquid cathode at a concentration of 50, 100, and 200 mg L-1, respectively. The production rate of synthesized Au NPs depended on the precursor concentration with mean values of 2.9, 3.5, and 5.7 mg h-1, respectively.

On the coupling of hydride generation with atmospheric pressure glow discharge in contact with the flowing liquid cathode for the determination of arsenic, antimony and selenium with optical emission spectrometry.

The miniaturized atmospheric pressure glow discharge (APGD) sustained between a liquid flowing cathode and a He nozzle jet anode was combined with hydride generation (HG) to improve the performance of the determination of As, Sb and Se with optical emission spectrometry (OES). As(III), Sb(III) and Se(IV) species were converted into volatile hydrides in the reaction with NaBH4 and right after that they were delivered to the near-anode region of APGD through the nozzle. The transport efficiency of As, Sb and Se to the discharge was several times higher, while intensities of atomic emission lines of As, Sb and Se were improved 3 orders of magnitude (as compared to intensities acquired for the near-cathode region in a APGD system with a typical introduction of analytes through sputtering of the flowing liquid cathode). The effect of the concentration of NaBH4 and HCl in a sample solution, the discharge current, the flow rate of He carrier/jet-supporting and He shielding gases on the emission yield coming from As, Sb, Se, He and H atomic lines and OH and N2 band heads as well as the electron number density was thoroughly studied. Under compromised conditions, limits of detection (3σ criterion) of As, Sb and Se were respectively 4.2, 1.2 and 3.1 µg L(-1). Usefulness of the method was confirmed by the analysis of Sniadecki and Marchlewski highly mineralized spring waters (Kudowa Zdroj, Poland) on the content of As, Sb and Se. Recoveries of elements added to these spring waters were within 90.3-103.7% proving good accuracy of the HG-APGD-OES method.

The improvement of the analytical performance of direct current atmospheric pressure glow discharge generated in contact with the small-sized liquid cathode after the addition of non-ionic surfactants to electrolyte solutions.

A low power direct current atmospheric glow discharge sustained in the open to air atmosphere in contact with a small-sized flowing liquid cathode was used as an excitation source in optical emission spectrometry. The composition of electrolyte solutions served as the liquid cathode was modified by the addition of non-ionic surfactants, namely Triton x-45, Triton x-100, Triton x-405 and Triton x-705. The effect of the concentration of each surfactant was thoroughly studied on the emission characteristic of molecular bands identified in spectra, atomic emission lines of 16 metals studied and the background level. It was found that the presence of both heavy surfactants results in a significant increase in the net intensity of analytical lines of metals and a notable reduction of the intensity of bands of diatomic molecules and the background. In conditions considered to be a compromise for all metals, selected figures of merit for this excitation source combined with the optical emission spectrometry detection were determined. Limits of detection for all metals were within the range of 0.0003-0.05 mg L(-1), the precision was better than 6%, while calibration curves were linear over 2 orders of the magnitude of the concentration or more, e.g., for K, Li, Mg, Na and Rb. The discharge system with the liquid cathode modified by the addition of the surfactant found its application in the determination of Ca, Cu, Fe, K, Mg, Mn, Na and Zn in selected environmental samples, i.e., waters, soils and spruce needles, with the quite good precision and the accuracy comparable to that for measurements with flame atomic absorption spectrometry (FAAS) and flame atomic emission spectrometry (FAES).