Shenghong Hu

Signal enhancement in laser ablation ICP-MS by addition of nitrogen in the central channel gas

The effects of adding nitrogen to the central gas flow (Ar + He) of an Ar plasma in laser ablation inductively coupled plasma mass spectrometry are presented. The optimum central gas flow rate was found to be negatively correlated with the N2 gas flow rate. The addition of 5–10 ml min−1nitrogen to the central channel gas in LA-ICP-MS increases the sensitivity for most of the 65 investigated elements by a factor of 2–3. The degree of enhancement depends, to some extent, on the 1st ionization energy. Another important advantage of N2 mixed gas plasma for LA-ICP-MS is that the oxide ratios (ThO+/Th+) are significantly reduced (one order of magnitude). The hydride ratio (ArH+/Ar+) is also reduced up to a factor of 3, whereas the doubly charged ion ratio (Ca2+/Ca+) is increased. The background signals at masses 29, 31, 42, 51, 52 and 55 are significantly increased due to the nitrogen based polyatomic interferences. Compared to the spatial profiles of the ion distributions in the normal mode (without nitrogen), the addition of 5 ml min−1nitrogen leads to significant wider axial profiles and more uniform distribution of ions with different physical and chemical properties. Our results also show that the makeup gas flow (central channel gas) rate has a significant effect on the ion distribution of elements with different physical and chemical properties. A very consistent increase of argon signal by the addition of nitrogen (5 ml min−1) corroborates better energy transfer effect of nitrogen in the plasma.

Geochemistry of lower crustal xenoliths from Neogene Hannuoba basalt, North China craton: implications for petrogenesis and lower crustal composition

Thirty granulite and pyroxenite xenoliths from the Neogene Hannuoba basalt of the North China craton have been analyzed for major and trace element compositions. The granulites range in composition from mafic to felsic with SiO2 = 45.7 to 73.0% and also contain metasediments. The compositions of mafic and intermediate granulites can be explained by fractional crystallization of a magma chamber in the lower crust. The magmatic granulite xenoliths are interpreted as product of basaltic underplating and subsequent fractional crystallization at the base of the crust. Thermobarometric studies and correlation of calculated P-wave velocities with regional seismic refraction results suggest that the upper part of the lower crust, which accounts for two thirds of the entire lower crust in the North China craton at a 24- to 38-km depth, is dominated by intermediate and felsic compositions. Only the lowermost crust (38–42 km) has a mafic composition. This is also supported by the abundance of intermediate and felsic granulite xenoliths, which account for 45% of the granulite population collected. The calculated bulk lower crust in the Hannuoba area has an intermediate composition with SiO2 = 58%. Because other parts of North China also show a similar velocity structure, with the high-velocity layer confined to the lowermost 3 to 5 km of crust, the results from the Hannuoba area are considered to be representative of the reactivated North China craton as a whole.

A local aerosol extraction strategy for the determination of the aerosol composition in laser ablation inductively coupled plasma mass spectrometry

Aerosol transport efficiency in UV-ns LA-ICP-MS of less than 100% requires a representative aerosol composition for precise and accurate quantitative analysis. Therefore, aerosol expansion related changes in the composition of aerosols generated using a 193 nm excimer laser were studied within the ablation cell using an in-cell aerosol extraction strategy. The gas flow pattern within the ablation cell in the proposed local aerosol extraction was modelled using computational fluid dynamics techniques. Compared to commonly applied ablation cell geometry, the peak height of a single laser shot was increased by a factor of 13.5, the signal width was reduced by a factor of 12 and the washout time of the sample cell was consequently shortened to approx. 2 s, thereby almost eliminating processes of aerosol recirculation within the cell. The selective extraction of aerosol from different positions of the expanding laser plume was realized by subsequently changing the sampling distance between the ablation site and the gas outlet nozzle tip. The results show a similar distribution of siderophile elements (P, Cr, Mn, Fe, Co, Ni, Ga, Ge, Mo, W, Au), chalcophile elements (Cu, Zn, As, Se, Rh, Ag, Cd, In, Sn, Sb, Te, Pt, Tl, Pb, Bi) and some of lithophile elements (Li, B, Na, Mg, Si, K, V, Rb, Ba, U) within the expanding plume. In contrast, lithophile elements Be, Al, Ca, Sc, Y, Zr, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta and Th were significantly depleted towards increasing sampling distances from the ablation site. Furthermore, the plume composition depends strongly on the ambient gas used within the ablation cell. Evaluating various physical properties of individual elements it becomes clear that no single parameter dependence exists. However, elements with oxide melting points higher than 1500 °C tend to be depleted towards cooler regions of the expanding aerosol. The proposed local aerosol extraction strategy is suitable for the identification of position dependent and, therefore, an indirect indicator for particle size dependent elemental composition of 193 nm laser generated aerosols under He atmosphere, which could not be studied using particle separation devices. In contrast to the ablation in helium, the changes of the aerosol composition in argon were less variable amongst different elements when sampling at different distances from the ablation crater. However, Zn and Cd intensities increased when sampling further away from the ablation crater. Results indicate that aerosol expansion within the ablation in UV-ns laser ablation can be a significant source of non-stoichiometric sampling, especially induced by aerosol deposition on the sample surface.

Elemental determination of microsamples by liquid film dielectric barrier discharge atomic emission spectrometry.

In this study, a new liquid-film dielectric barrier discharge (LFDBD) atomic emission source was developed for microsample elemental determination. It consists of a copper electrode, a tungsten wire electrode, and a piece of glass slide between them, which serves as the dielectric barrier as well as the sample plate. The sample solution with 1 mol L(-1) nitric acid, when deposited onto the surface of the glass slide, forms a thin liquid film. The plasma is generated between the tip of the tungsten wire electrode and the liquid film surface when alternating-current (ac) high voltage (peak voltage ~3.7 kV, frequency ~30 kHz) is applied on the electrodes. Qualitative and quantitative determinations of metal ions in the sample solution were achieved by atomic emission measurements in the plasma and were demonstrated in this study with elements Na, K, Cu, Zn, and Cd. Detection limits were in the range from 0.6 ng (7 μg L(-1)) for Na to 6 ng (79 μg L(-1)) for Zn. Repeatability, expressed as relative standard deviation from seven repetitive analyses of samples with analyte concentrations at 1 mg L(-1), varied from 2.1% to 4.4%. Compared with other liquid discharge systems that operate at atmospheric pressure, the current system offers several advantages: First, it eliminates the use of a sample flow system (e.g., syringe or peristaltic pump); instead, a small aliquot of sample is directly pipetted onto the glass slide for analysis. Second, it is a microanalysis system and requires sample volume ≤80 μL, a benefit when a limited amount of sample is available. Third, because the sample is applied in aliquot, there is no washout time, and the analysis can be easily extended to sample array for high-throughput analysis. The proposed LFDBD is promising for in-field elemental determination because of its simplicity, cost effectiveness, low power supply, and no inert gas requirement.

Plasma Jet Desorption Atomization-Atomic Fluorescence Spectrometry and Its Application to Mercury Speciation by Coupling with Thin Layer Chromatography

A novel plasma jet desorption atomization (PJDA) source was developed for atomic fluorescence spectrometry (AFS) and coupled on line with thin layer chromatography (TLC) for mercury speciation. An argon dielectric barrier discharge plasma jet, which is generated inside a 300 μm quartz capillary, interacts directly with the sample being analyzed and is found to desorb and atomize surface mercury species rapidly. The effectiveness of this PJDA surface sampling technique was demonstrated by measuring AFS signals of inorganic Hg(2+), methylmercury (MeHg), and phenylmercury (PhHg) deposited directly on TLC plate. The detection limits of the proposed PJDA-AFS method for inorganic Hg(2+), MeHg, and PhHg were 0.51, 0.29, and 0.34 pg, respectively, and repeatability was 4.7%, 2.2%, and 4.3% for 10 pg Hg(2+), MeHg, and PhHg. The proposed PJDA-AFS was also successfully coupled to TLC for mercury speciation. Under optimized conditions, the measurements of mercury dithizonate (Hg-D), methylmercury dithizonate (MeHg-D), and phenylmercury dithizonate (PhHg-D) could be achieved within 3 min with detection limits as low as 8.7 pg. The combination of TLC with PJDA-AFS provides a simple, cost-effective, relatively high-throughput way for mercury speciation.

Flowing and Nonflowing Liquid Electrode Discharge Microplasma for Metal Ion Detection by Optical Emission Spectrometry

Abstract Liquid electrode discharge microplasma offers a simple, portable, and small platform for fast spectrochemical determination of metal ions in solution samples with lower consumption of consumables (e.g., working gases, electric current, and cooling water). Various liquid electrode discharge devices for metal ions determination with optical emission spectrometry have been developed in the past 20 years. We aim here to provide an overview of this field and to show the similarities and differences, and strengths and weaknesses of different flowing liquid electrode discharge systems and nonflowing liquid electrode discharge systems. We also discuss sample flow rates in flowing systems, required sample volume in nonflowing systems, and the two systems’ applications to real complex samples in this review.

Suppression of interferences for direct determination of arsenic in geological samples by inductively coupled plasma mass spectrometry

We have developed a method for direct determination of arsenic in geological samples using ICP-MS by reduction of interferences, without preconcentration, separation and use of the hydride generation technique. Concentrations of HNO3 have a significant effect on the arsenic signal. This type of interference cannot be corrected by internal standards (Rh and In) because the signal suppression due to HNO3 is apparently dependent on the first ionization potential of elements. Addition of 4% (v/v) ethanol to 1–10% (v/v) HNO3 was found to be an excellent method for reducing this type of matrix effect from 30–40% to less than 5% for high first ionization potential elements 75As (9.81 eV), 82Se (9.75 eV), and 126Te (9.01 eV). Direct determination of arsenic in geological samples by ICP-MS is often complicated by the presence of Nd2+, Sm2+and Eu2+ interferences, in addition to the well known interference of ArCl+, and the high first ionization potential of As (9.81 eV) also results in relatively low analytical sensitivity in ICP-MS. It is shown that both problems can be overcome by a combination of a 4% ethanol matrix modifier with nebulizer gas flow rate adjustment. For example, the interference from doubly charged ions of a rare earth element (Ce2+) is reduced by a factor of 30 with the addition of 4% ethanol at a nebulizer gas flow rate of 1.00 l min−1 and rf power of 1350 W, while the signal intensity of As is similar in both solutions. A nebulizer gas flow rate of 0.94 l min−1, an rf power of 1350 W and 4% ethanol modifier were chosen in practical sample analysis. Under these conditions, the interference of doubly charged ions of the rare earth element (Ce2+) was reduced by a factor of 6.5 and the signal intensity of As was improved by a factor of 3 relative to that in 3% (v/v) HNO3 solution at the corresponding optimum nebulizer gas flow rate of 0.98 l min−1 and rf power of 1350 W. The arsenic equivalent concentration caused by ArCl+ interference was reduced by a factor of 10 under our given experimental conditions in the presence of 4% (v/v) ethanol. The developed method was applied to the direct determination of arsenic in a series of international geological reference materials. Most of the results were found to be in reasonable agreement with the reported values in the literature, particularly for those having recommended values. This simple method shows a great potential for the direct determination of arsenic in geological and environmental samples.

Non-chromatographic determination of inorganic and total mercury by atomic absorption spectrometry based on a dielectric barrier discharge atomizer

A novel, fast and simple non-chromatographic approach for determination of inorganic and total mercury has been developed by cold vapor atomic absorption spectrometry based on dielectric barrier discharge (DBD) atomizer. The determination of inorganic mercury and total mercury can be achieved in a fast sequential fashion (1 min sample−1). In the proposed method, with 0.01% (m/v) NaBH4 used as reductant, inorganic mercury is reduced to elemental mercury, whereas the methylmercury forms an intermediate volatile methylmercury hydride (CH3HgH). A low temperature DBD atomizer was employed for the atomization of CH3HgH. Only inorganic mercury can be measured in the absence of the DBD plasma. However, in addition to inorganic mercury, CH3HgH can be atomized and total mercury is determined in the presence of the plasma. The methylmercury concentration can then be obtained from the difference. The effects of several experimental parameters, such as NaBH4 concentration, HCl concentration, discharge gas and gas flow rate on the analytical performance were investigated. The method provided good reproducibility (<3% RSD) and the detection limits of Hg2+ and CH3Hg+ were found to be 0.35 ng mL−1 and 0.54 ng mL−1, respectively. The proposed method was validated by the analysis of a certified reference material (tuna fish). In addition, it was successfully applied to the analysis of fish samples. The method is excellent for mercury speciation measurements as it provides short analysis time and good reproducibility.

Significant sensitivity improvement of alternating current driven-liquid discharge by using formic acid medium for optical determination of elements.

A method has been developed to improve the performance of alternating-current electrolyte atmospheric liquid discharge (ac-EALD) optical emission spectrometry for the determination of elements. Significant enhancement of emission intensity was achieved by adding organic substance into the nitric acid electrolyte solutions. Under the optimized conditions, 3% (v/v) formic acid in nitric acid (pH 1.0) produced 13 times enhancement for Ag and 7% (v/v) formic acid resulted in 17 times enhancement for Cd. The emission of Pb was even enhanced 78 times in the presence of 3% formic acid. In addition, the signal stability was also improved compared with that in the absence of organic substances. Repeatability was 0.8% for 0.1 mg L(-1) Ag, 0.7% for 0.2 mg L(-1) Cd and 2.6% for 1 mg L(-1) Pb standard solutions (n=5). The limits of detection of Ag, Cd and Pb were 1, 17 and 45 μg L(-1), respectively. The accuracy of the method was demonstrated by determination of elements in simulated natural water samples (GBW(E)080402 and GBW(E)080399).

Dielectric Barrier Discharge for High Efficiency Plasma-Chemical Vapor Generation of Cadmium

A novel approach for Cd vapor generation was developed on the basis of a plasma-assisted chemical process. The generated Cd vapor was subsequently measured by atomic fluorescence spectrometry. Dissolved Cd species were readily converted into volatile species by reaction with hydrogen in a coaxial thin-film dielectric barrier discharge (DBD) plasma reactor. Both atomic and molecular Cd species were produced when a solution containing Cd(2+) was exposed to hydrogen-containing DBD plasma. Fast and efficient vapor generation of Cd was achieved simply in plain (neutral) water medium. Optimal conditions for the DBD-plasma Cd vapor generator were identified. The performance of this thin-film DBD plasma-chemical vapor generation (CVG) was evaluated through comparison with that arising from the conventional HCl-KBH4 system. The vapor generation efficiency of the proposed method (He-DBD) was found to be superior to the conventional CVG approach. Under the optimized conditions, the detection limits of Cd were found to be from 0.03 ng mL(-1) (Ar-DBD) to 0.008 ng mL(-1) (He-DBD) with a heated quartz tube atomizer (QTA); good repeatability (relative standard deviation (RSD) = 1.4%, n = 5) was obtained for a 1 ng mL(-1) standard. The new thin-film DBD plasma-CVG provides several additional advantages including simple setup, easy coupling with flow injection, low power consumption (≤18 W), cost-effectiveness, and long operation lifetime. The accuracy of the proposed method was validated through analysis of cadmium in reference material of simulated natural water sample GBW(E)080402 and rice reference material GBW10045. The concentration of cadmium determined by the present method agreed well with the reference values.