Microwave-induced Plasma Atomic Emission Spectrometry Research Articles

Determination of trimethyl‐lead in rainwater and road dust by capillary GC MIP‐AE spectrometry after in situ ethylation and extraction

AbstractA rapid and sensitive method for the determination of trimethyl‐lead in water and road dust is described. It is based on in situ ethylation of ionic methyl‐lead by sodium tetraethylborate and the extraction of the compound formed into hexane. The extract is gas‐chromatographed and the lead species determined by microwave‐induced plasma atomic emission spectrometry (MIP‐AES). The reaction conditions are optimized and the method is applied to the analysis of artificial rainwater and road dust in the framework of an international round‐robin exercise.

First investigations for the development of a microwave-induced plasma atomic emission spectrometry system to determine trace metals in gases

Up to now the direct analysis of reactive gases has not been investigated sufficiently. A continuously running sampling system has been developed for the direct determination of the iron concentration in hydrogen chloride. Atomic emission spectra were generated by using a medium power microwave induced plasma with a rectangular type cavity. Different parameters were optimized, e.g., the design of discharge tubes, gas flow, forward power and volume of sample gas. In order to characterize the plasma, the excitation and gas temperatures have been determined. The calibration was carried out by means of a certified gas reference sample containing pentacarbonyliron in argon. The detection limit was found to be 0.25 µg l–1 of iron in argon (T= 25 °C) with a reproducibility of 6%(n= 10).

Direct solid sample analysis in a moderate-power argon microwave-induced plasma with spark generation

A spark–Ar microwave-induced plasma (MIP) atomic emission spectrometry system has been developed for the direct analysis of solid samples. Particles of the sample are generated with a spark and swept into a moderate-power Ar MIP connected to a sequential spectrometer. Low-Alloy Steel and Aluminium Alloy Standard Reference Materials were analysed by the proposed system and the detection limits for most elements of interest were around 10 µg g–1 or less, which are comparable to a spark–Ar inductively coupled plasma (ICP) system. Precisions were in the range 3–11%, which are 2–3 times higher than those of the spark–ICP system. Hence spark–MIP AES is a viable alternative technique to spark–ICP-AES.

Characterizing the factors that influence oxygen selectivity in gas chromatography–microwave-induced plasma atomic emission spectrometry

Factors affecting oxygen-selective detection with gas chromatography–microwave-induced plasma atomic emission spectrometry are described. The instrumental factors include helium support gas flow, microwave power and viewing position in the plasma; position affects oxygen selectivity the most. When viewed laterally, oxygen selectivity is 4850: 1 over carbon compared with 155:1 for the axial viewing geometry. Reduction of the residual oxygen in the plasma support gas is crucial to improved oxygen selectivity, but excessive purification and concomitant reduction of hydrogen in the plasma gas results in negative responses and broad peaks. Adding 0.03% hydrogen to the plasma gas restores oxygen selectivity. Several analyte species with varied carbon and hydrogen composition and halogenation show no significant differences in oxygen sensitivity, indicating a lack of compound-specific effects in the system. Emission for recombination products of oxygen was lowest at the point in the plasma where selectivity was highest.

Determination of oxygen-containing additives in gasoline by gas chromatography–microwave-induced plasma atomic emission spectrometry

The development and characterization of a method for the determination of oxygen-containing (oxygenated) species in a complex mixture of hydrocarbons is described. A microwave-induced plasma (MIP) atomic emission detector is coupled to a gas chromatograph, and atomic oxygen emission is monitored as an indication of oxygenated compounds in the gas chromatograph effluent. On-line helium purification and the use of a tangential-flow torch provide conditions ideal for oxygen-selective analysis without make-up gases or extensive background correction. Furthermore, large amounts of solvent or sample do not extinguish the plasma so samples can be analysed directly without dilution, addition of reagent gases or solvent venting procedures. Both axial and lateral viewing positions were investigated for analytical use. Detection limits for oxygen were 1 ng s–1 for both axial and lateral positions but sensitivity was higher in the axial position. However, selectivity over carbon in the lateral position was 4850 : 1 compared with 155 : 1 in the axial position. The gas chromatography MIP method was applied, using lateral viewing, to the analysis of alcohols in National Institute of Standards and Technology (NIST) reference fuel standards and to commercial gasoline samples collected from Columbia, SC, USA and Greensboro, NC, USA. Experimental results of oxygen-selective analysis of NIST reference fuel are in good agreement with the certified values. Results obtained for the determination of oxygenated compounds in gasoline samples further demonstrate the usefulness of the method for the analysis of a complex mixture of hydrocarbons. Most spike recoveries were within 10% of the expected values. Examples of oxygen-selective chromatograms are presented to demonstrate the usefulness and selectivity of the method.

Continuous flow and flow injection halogen generation for chloride, bromide and iodide determinations by microwave induced plasma atomic emission spectroscopy

Continuous flow generation of molecular, volatile halogens (I 2, Br 2, Cl 2), based on chemical oxidation, is a very useful sample introduction technique in microwave induced plasma atomic emission spectrometry (MIP-AES), which provides improved transport efficiency for sample to plasma, thus enhancing the sensitivity of the determinations. Optimum conditions for halogen generation/MIP detection (chemical oxidant system, type and size of gas-liquid separator, influence of halide concentration and MIP instrumental parameters) are discussed with an aim to improving both the intensity of the emission signal, and its stabilization time for I −1, Br −1 and Cl −1 determinations. For continuous halogen generation, the influence on the MIP emission signal (and its stabilization time) of (i) the size and form of the gas-liquid separator, and (ii) the atomic weight and concentration of the halogen itself has been investigated in detail. Even for optimum conditions, iodine generation/transport to the plasma is rather slow (i.e. 4 min for signal stabilization). The use of flow injection analysis (FIA) however, allows an increase of the attainable sample throughput by up to 60 samples/h, but at the expense of a slight loss of sensitivity (20–30%) compared with continuous flow generation. The influence on the MIP signal that was observed with FIA parameters (such as the injected sample volume and flow rates of the different reagents) is discussed.

Evaluation of a Demountable Tangential Flow Torch for Microwave-Induced Plasma Atomic Emission Spectrometry

A demountable torch for the microwave-induced plasma that utilizes a tangential flow pattern is described. This new torch is compared to a conventional all-quartz design in a series of comparative studies. Diagnostic studies consist of measurements of electron number densities, excitation temperatures, and ionization temperatures of an argon plasma generated in each torch. Hydrodynamic flow measurements, vertical emission profiles, and limits of detection provide further contrast between the characteristics of the demountable design and conventional all-quartz torch. The compatibility of the new torch design with supercritical CO, flows is demonstrated where the MIP is coupled with a packed-column supercritical fluid chromatograph for the purpose of element-specific detection. The physical advantages of the demountable torch over the all-quartz design are discussed.

Study of a Low-Powered He Microwave-Induced Plasma for Determination of P, Cl, Br, and I by Atomic Emission Spectrometry

Aqueous sample solutions were nebulized by an ultrasonic nebulizer and the solvent was removed by a desolvation apparatus before introduction of the aerosol into the microwave-induced plasma (MIP). The desolvation system consists of a heating tube and a condenser associated with a concentrated H2SO4 desiccator. The water vapor was removed almost completely with the desolvation system. The detectability of microwave-induced plasma atomic emission spectrometry was therefore improved. The detection limits for P, Cl, Br, and I were 0.0045, 0.12, 0.23. and 0.06 μg/ml, respectively. Based on a study of the emission spectra of P, Cl, Br, and I in the range of 200 to 600 nm in He MIP, the optimum analytical lines for determination of corresponding elements were chosen.

Comparative study of argon and helium plasmas in a TM 010 cavity and a surfatron and their use for hydride generation microwave-induced plasma atomic emission spectrometry

Argon and helium microwave-induced plasmas (MIPs) operated in a TM 010 Beenakker resonator and in a surfatron were investigated for the determination of hydride-forming elements. The MIPs in both devices were optimized with elemental mercury vapour. The analytical figures of merit were determined for As, Sb and Se in the case of hydride generation followed by trapping in a hot graphite furnace and subsequent evaporization of the analyte into the plasma. The long-term stability of the plasmas obtained in both devices was found to be very good. With the surfatron the absolute detection limits for the determination of As, Sb and Se (0.1–0.2 ng) were half those with the TM 010 cavity.

Determination of lead by electrothermal vaporization microwave-induced plasma atomic emission spectrometry after flow-through electrolytic deposition in a graphite tube packed with reticulated vitreous carbon

A simple method is described for the determination of lead in salt solutions by microwave-induced plasma atomic emission spectrometry (MIP-AES). Traces of Pb were preconcentrated electrochemically in a flow system incorporating a flow-through cell with a graphite tube packed with reticulated vitreous carbon serving as the working electrode. After washing the cell with water the tube was placed into the graphite furnace coupled to the capillary of an argon MIP operated in a surfatron. After the tube had been dried the plasma was ignited and the evaporation temperature was applied to the tube. The evaporated deposit was transported by argon gas into the plasma for excitation. A detection limit of 0.2 ng ml–1 of Pb in 1 ml of preconcentrated sample solution was achieved.

Measurement of trace elements in ceramic and quartz by laser ablation microwave-induced plasma atomic emission spectrometry. Plenary lecture

Laser ablation into a microwave induced plasma and atomic emission spectrometry of the plasma was applied to the direct determination of trace elements (magnesium, aluminium, silicon and iron) in a high-temperature superconducting ceramic (YBa2Cu3O7) and the determination of low sodium concentrations in high-purity natural and synthetic quartz used for the production of optical fibres. The emission spectra were measured by time-gated detection, normalized by internal standardization and calibrated with relevant spectra of standard reference samples (aluminium, copper and borax glass).

Electrothermal vaporization for sample introduction in microwave-induced plasma atomic absorption spectrometry

An electrothermal vaporization device has been designed for use in sample introduction and this has been coupled with microwave-induced plasma atomic absorption spectrometry (ET-MIP-AAS). Various vaporization chambers were tested to minimize the turbulence and the dead volume. A series of experimental parameters were examined and optimized. Some influences, such as concomitant element and easily ionized element (EIE) effects, which are commonly encountered in microwave-induced plasma atomic emission spectrometry were investigated. The analytical figures of merit for ET-MIP-AAS are discussed. The characteristic concentrations obtained in this work were found to be at the ng ml–1 or higher level. The precision for the determination of Ca, Ag and Cu was shown to be in the range 5.6–7.8%. The recommended method was also used to analyse real samples, and satisfactory results were obtained.

Different sample introduction systems for the multaneous determination of As, Sb and Se by microwave-induced plasma atomic emission spectrometry

The determination of hydride-forming elements using different sample introduction procedure into microwave-induced plasmas (MIPs) has been studied. For the determination of As, Sb and Se analyte introduction was accomplished with a pneumatic concentric glass nebulizer,a graphite furnace, or with thydride generation followed by cold-trapping or hot-trapping in a graphite furnace. The detection limits obtained with different types of low power MIPs (toroidal, 1 or 3 filament MIPs) operated in a TM 010 cavity according to Beenakker also were investigated. The contruction of the flow system used for preconcentration and the effects of reagent concentration, gas flow rates as with MIP-atomic emission spectrometry (AES) were investigated. The trapping of the hydrides followed by their vaporization showed substantial advantages over the other introduction systems investigated, especially with respect to power of detection. Further, mutual interferences being a big problem in atomic absorption spectrometry are widely absent in MIP-AES. Under compromise operating conditions the detection limits for As, Sb and Se are 0.4; 0.35; 0.25 ng ml −1 respectively, while the sample volume can be varied from 0.05 ml up to several millilitres.

A New Method for Feedback Stabilization of a Microwave Power Supply

A new method has been developed for the feedback stabilization of a microwave power supply, with the intent to reduce emission noise in microwave-induced plasma atomic emission spectrometry (MIP-AES). The control method would be applicable to any microwave generator whose output power is affected by the input ac line voltage. In the feedback process, the effective line voltage is adjusted by means of a controllable inductance placed in series with a variable transformer at the input of the generator. In turn, this inductance is controlled by selecting the dc current in the primary winding of an auxiliary transformer, so the inductance in its secondary winding is altered. The long-term stability of the feedback-controlled system is verified and the range of stabilization is examined. Noise in the power output of the microwave generator is shown to be reduced at frequencies below 1.5 Hz. Although stable microwave power generation does not necessarily result in stable emission from a microwave-induced plasma source, an emission-based feedback signal is shown to improve the stability.

Sensitive speciation analysis of lead ion environmental waters by capillary gas chromatography microwave-induced plasma atomic emission spectrometry

Optimization of microwave-induced plasma atomic emission detection for the speciation of organolead compounds in water samples is described. The absolute detection limits achieved vary from 0.02 to 0.1 pg (as Pb) and are dependent on the volatility of the species determined. Two procedures for the separation and enrichment of organolead in water samples were evaluated. Ionic organolead species were extracted as their diethyldithiocarbamate complexes either with one portion of hexane at pH 8 or twice with pentane at pH 9, with preconcentration in the latter instance by rotary evaporation of the solvent. The detection limits obtained were down to 2 and 0.1 ng l −1, respectively for 100-ml samples. Propylation and butylation were compared for derivatization of organolead species in the extracts. Examples of results for rain and tap water analyses are given.

Analyte volatilization procedure for continuous-flow determination of bromine by atmospheric pressure helium microwave-induced plasma atomic emission spectrometry

A simple method to generate a continuous flow of volatile bromine by oxidation of aqueous bromide is described for the determination of low concentrations of bromine by atmospheric pressure helium microwave-induced plasma atomic emission spectrometry. The bromine atom emission lines at 447.78 and 734.86 nm and the bromine ion emission line at 470.49 nm were selected as the analytical lines of interest. Of various oxidation reactions investigated, an oxidizing solution of 100 mmol dm–3 of potassium persulfate in 5.0 mol dm–3 sulfuric acid is the most appropriate for the generation of elemental bromine. The gaseous bromine is separated from the solution in a simple gas–liquid separator, dried with a desiccant of anhydrous calcium chloride and swept into the helium stream of a microwave-induced plasma for analysis. The best attainable detection limits (3σ criterion) for bromine at 447.78, 470.49 and 734.86 nm were found to be 29.5, 7.46 and 18.4 ng ml–1, respectively, with corresponding background equivalent concentrations for bromine of 5.05, 1.72 and 2.42 µg ml–1. Typical analytical calibration graphs obtained under the optimized experimental conditions are rectilinear over approximately three orders of magnitude of concentration.

Microwave-induced plasma as an element-specific detector for speciation studies at the trace level. Invited lecture

The microwave-induced plasma (MIP) is a promising excitation source for atomic spectrometry. In 1976 a major breakthrough was made by Beenakker, who introduced a cylindrical TM010 cavity capable of operating at atmospheric pressure for argon and helium plasmas. The helium-sustained MIP is particularly attractive because both non-metals and metals can be detected with good sensitivity. The characteristics of the MIP as an atomization and excitation source are briefly discussed and some advantages (multi-element detection of non-metals and metals) and limitations (only small amounts of sample can be tolerated) are highlighted. The MIP is efficient in the excitation of elements but the limited thermal energy available leads to difficulties with sample desolvation and atomization. In order to overcome the problems several techniques have been investigated to introduce volatilized samples into the MIP. Coupling with gas chromatography (GC), electrothermal vaporization, chemical vapour generation and other techniques is also discussed. The primary focus of work in this area has dealt with the utilization of the MIP as an element-specific detector for GC. With MIP atomic emission spectrometry it is possible to investigate chromatographic processes and to determine empirical molecular formulae of materials eluted from a GC column. Combination of the MIP with separation techniques for speciation of organic and organometallic compounds even at trace concentrations is the most important application for many disciplines, such as medicine and toxicology. Some of the useful features of MIP detection are exemplified by the determination of specific organomercury compounds at trace levels in environmental samples.

Determination of Total Iodine in Brines and Seawaters by Atmospheric-Pressure Helium Microwave-Induced Plasma Atomic Emission Spectrometry with Continuous-Flow Gas-Phase Sample Introduction

Determination of low concentrations of iodine is of great importance in environmental and geochemical analyses. Therefore, the analytical techniques for the determination of iodine in environmental and geochemical samples need to be sensitive. Among the many possible methods, at present, neutron activation analysis is probably the most sensitive technique. On the other hand, in general, although atomic spectrometric techniques are excellent methods for metal determinations, the determination of nonmetals such as iodine by atomic emission spectrometry (AES) as well as atomic absorption spectrometry (AAS) has been quite limited. However, analytical plasmas such as the inductively coupled plasma (ICP) and microwave-induced plasma (MIP) do show promise in the area of nonmetal determinations. The microwave-induced plasma, in particular, has received considerable attention as an excitation source for atomic emission spectrometry. Very recently, the present authors have described the continuous-flow determination of trace iodine by atmospheric-pressure helium MIP-AES using generation of volatile iodine from iodide and reported good detection limits of 2.3 and 3.2 ng/mL for iodine at 183.04 and 206.16 nm, respectively, with linear dynamic ranges of approximately four orders of magnitude in concentration. Of various oxidation reactions examined, an oxidizing solution of 1.0 mM sodium nitrite in 5.0 M sulfuric acid is the most favorable for the generation of elemental iodine from iodide. The aim of this communication is to report the application of atmospheric-pressure helium MIP-AES combined with continuous-flow gas-phase sample introduction to the determination of low concentrations of total iodine (i.e., iodide plus iodate) in brines and coastal seawaters.

Evaluation of a Tantalum-Tip Electrothermal Vaporization Sample Introduction Device for Microwave-Induced Plasma Mass Spectrometry and Atomic Emission Spectrometry

An integrated electrothermal vaporization/microwave-induced plasma device has been shown to be a viable source for atomic emission and mass spectrometry. The tantalum-tipped electrothermal vaporizer ensured that no analyte condensation occurred during transport to the plasma and facilitated the formation of an annular helium plasma. Detection limits obtained with atomic emission spectrometry were 0.6, 21, and 2400 pg for Ag, Pb, and I, respectively, and those obtained with mass spectrometry were 0.03, 0.09, 0.75, and 1.5 pg for Ag, Cd, Pb, and Br, respectively. Detection limits were blank limited due to contaminants in the materials used in construction of the furnace. The current study was limited to high-volatility elements.

Some observations on the effects of easily ionized elements in a microwave-induced plasma

The characteristics of the effects of easily ionized elements (EIEs) in a microwave-induced plasma (MIP) were investigated in detail. Aqueous samples were introduced from an ultrasonic nebulizer and the solvent was removed by a desolvation system. To understand the EIE effects in nebulization and desolvation processes, either a single or a double cup nebulization system was used. To elucidate further the EIE behavior in atomization and excitation processes, the effects of EIEs on emission intensities and absorbances of some elements were studied by microwave-induced plasma-atomic emission spectrometry (MIP-AES) and microwave-induced plasma-atomic absorption spectrometry (MIP-AAS). The experimental results demonstrate that in comparision with EIE effects on nebulization and desolvation, the EIE effects occurring in the plasma play a more important role in the overall EIE effect. Under certain conditions the EIE effects on excitation are more important and more complex than those on atomization.