Geoanalysis using plasma spectrochemistry – milestones and future prospects

Highlights of plasma spectrochemistry in geoanalysis are reviewed. The techniques are evaluated in terms of recent instrumental developments, calibration strategies, spectral and matrix interferences and analytical performance. While acid decomposition results in solutions containing low salt contents, this decomposition strategy is inappropriate for numerous sample types due to poor recoveries. On the other hand, alkali fusions result in total decomposition, but solutions containing high salt contents constrain the accuracy due to interference effects in the inductively coupled plasma (ICP), the sample introduction system, and in the quadrupole mass spectrometer interface. Therefore, practical limits of determination are evaluated in terms of salt tolerances. It is concluded that ICP-atomic emission spectrometry (AES) is employed mainly for the accurate determination of the major and minor elements and the more abundant trace elements. On the other hand, ICP-mass spectrometry (MS) is used mainly for the determination of trace elements and together with the possibility of obtaining some isotopic information, it profoundly enhances the capability for solving geochemical problems. Several methods of direct solid sample introduction are described. These include direct current (DC) arc emission spectroscopy (DC-AES), slurry nebulization (SN), spark ablation (SA), laser ablation (LA) and glow discharges (GD). These devices allow direct solid analysis of bulk samples, single minerals and inclusions.     

Inorganic trace analysis by mass spectrometry

Mass spectrometric methods for the trace analysis of inorganic materials with their ability to provide a very sensitive multielemental analysis have been established for the determination of trace and ultratrace elements in high-purity materials (metals, semiconductors and insulators), in different technical samples (e.g. alloys, pure chemicals, ceramics, thin films, ion-implanted semiconductors), in environmental samples (waters, soils, biological and medical materials) and geological samples. Whereas such techniques as spark source mass spectrometry (SSMS), laser ionization mass spectrometry (LIMS), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), glow discharge mass spectrometry (GDMS), secondary ion mass spectrometry (SIMS) and inductively coupled plasma mass spectrometry (ICP-MS) have multielemental capability, other methods such as thermal ionization mass spectrometry (TIMS), accelerator mass spectrometry (AMS) and resonance ionization mass spectrometry (RIMS) have been used for sensitive mono- or oligoelemental ultratrace analysis (and precise determination of isotopic ratios) in solid samples. The limits of detection for chemical elements using these mass spectrometric techniques are in the low ng g⁻¹ concentration range. The quantification of the analytical results of mass spectrometric methods is sometimes difficult due to a lack of matrix-fitted multielement standard reference materials (SRMs) for many solid samples. Therefore, owing to the simple quantification procedure of the aqueous solution, inductively coupled plasma mass spectrometry (ICP-MS) is being increasingly used for the characterization of solid samples after sample dissolution. ICP-MS is often combined with special sample introduction equipment (e.g. flow injection, hydride generation, high performance liquid chromatography (HPLC) or electrothermal vaporization) or an off-line matrix separation and enrichment of trace impurities (especially for characterization of high-purity materials and environmental samples) is used in order to improve the detection limits of trace elements. Furthermore, the determination of chemical elements in the trace and ultratrace concentration range is often difficult and can be disturbed through mass interferences of analyte ions by molecular ions at the same nominal mass. By applying double-focusing sector field mass spectrometry at the required mass resolution—by the mass spectrometric separation of molecular ions from the analyte ions—it is often possible to overcome these interference problems. Commercial instrumental equipment, the capability (detection limits, accuracy, precision) and the analytical application fields of mass spectrometric methods for the determination of trace and ultratrace elements and for surface analysis are discussed.     

Applications of inductively coupled plasma mass spectrometry and laser ablation inductively coupled plasma mass spectrometry in materials science

Inductively coupled plasma mass spectrometry (ICP-MS) and laser ablation ICP-MS (LA-ICP-MS) have been applied as the most important inorganic mass spectrometric techniques having multielemental capability for the characterization of solid samples in materials science. ICP-MS is used for the sensitive determination of trace and ultratrace elements in digested solutions of solid samples or of process chemicals (ultrapure water, acids and organic solutions) for the semiconductor industry with detection limits down to sub-picogram per liter levels. Whereas ICP-MS on solid samples (e.g. high-purity ceramics) sometimes requires time-consuming sample preparation for its application in materials science, and the risk of contamination is a serious drawback, a fast, direct determination of trace elements in solid materials without any sample preparation by LA-ICP-MS is possible. The detection limits for the direct analysis of solid samples by LA-ICP-MS have been determined for many elements down to the nanogram per gram range. A deterioration of detection limits was observed for elements where interferences with polyatomic ions occur. The inherent interference problem can often be solved by applying a double-focusing sector field mass spectrometer at higher mass resolution or by collision-induced reactions of polyatomic ions with a collision gas using an ICP-MS fitted with collision cell. The main problem of LA-ICP-MS is quantification if no suitable standard reference materials with a similar matrix composition are available. The calibration problem in LA-ICP-MS can be solved using on-line solution-based calibration, and different procedures, such as external calibration and standard addition, have been discussed with respect to their application in materials science. The application of isotope dilution in solution-based calibration for trace metal determination in small amounts of noble metals has been developed as a new calibration strategy. This review discusses new analytical developments and possible applications of ICP-MS and LA-ICP-MS for the quantitative determination of trace elements and in surface analysis for materials science.     

Determination of trace Ag, Au, Ge, Pb, Sn and Te by microwave plasma torch atomic emission spectrometry coupled with an electrothermal vaporization sample introduction system

An electrothermal vaporization (ETV) sample introduction device tantalum filament was combined with microwave plasma torch atomic emission spectrometry (MPT-AES) for determination of several trace elements. Some operating parameters of the system were optimized. The effects of easily ionized elements (EIEs) on the emission intensities of the tested elements were studied in detail. It was revealed that there was no interference resulting from small amount of sample matrix; while with the existence of large amount of sample matrix, the method of standard addition could be used to determine trace elements in samples. So, no modifier was required in this method. The results indicated that ETV-MPT-AES not only has the advantage of micro sample consumption (a volume of 3 mul for each injection), but also offers high sensitivities for the determination of Ag, Au, Ge, Pb, Sn and Te as compared with those obtained with pneumatic nebulization (PN) MPT-AES.     

Electrothermal vaporization in inductively coupled plasma atomic emission spectrometry for direct multielement analysis of food samples with slurry sampling

Slurry sampling followed by electrothermal vaporization (ETV) was used as sample introduction technique in inductively coupled plasma atomic emission spectrometry (ICP-AES) for the direct determination of trace elements in food samples. A polytetrafluoroethylene (PTFE) emulsion was used as a fluorinating reagent to promote vaporization and the transportation of analytes. The main factors affecting the analytical signals were investigated in detail. Under optimum operating conditions, the detection limits (DL) for this method varied from 1.8 (Cu) to 215 ng/mL (Zn), while the relative standard deviations (RSD) were in the range 2.6% (Cu)-7.2% (Zn). The proposed method was successfully applied to the direct determination of trace amounts of V, Cu, Cr, Fe, Zn, and La in rice without any chemical pretreatment. The precision was evaluated by analyzing a standard reference material (tea leaves, GBW 07605) and comparing the results from this method with results obtained by pneumatic nebulization (PN) ICP-AES after the wet-chemical decomposition of the same sample.From Zhurnal Analiticheskoi Khimii, Vol. 60, No. 3, 2005, pp. 286–290.Original English Text Copyright © 2005 by Chen.This article was submitted by the author in English.     

Bestimmung von Elementspuren im ng- und pg-Bereich durch optische Emissionsspektrometrie mit He-MIP-Anregung nach elektrolytischer Abtrennung im Graphitrohr und nach-folgender elektrothermischer Atomisierung

This article deals with the determination of traces of elements in the ng and pg range by emission spectroscopy with a He-MIP excitation after electrolytic preconcentration in a graphit tube followed by electrothermal atomization. A multi-stage combined procedure is described for the sensitive and reliable determination of trace elements in high-purity metals. Electrolytically depositable elements such as noble metals, copper, as well as bismuth, cadmium, iron, cobalt, zinc, and others are preconcentrated from acidic solutions with concentrations ? 0.05 ng ml^?1 after the decomposition of the sample if the matrix elements are not deposited. The electrolyte is cycled through a small cylindrical cathode of pure graphite on the inner wall of which the elements are deposited. The graphite tube is coupled directly to the quartz capillary of a microwave induced helium plasma (MIP). After electrothermal atomisation the trace elements are determined by emission spectrometry. Different types of MIP excitation sources are investigated. The MIP with the TM₀₁₀ microwave resonator shows optimal properties. For the determination of the trace elements in niobium and beryllium detection limits near 1 ng g^?1 and relative standard deviations between 6.5 and 15% are obtained.     

Direct liquid ablation: a new calibration strategy for laser ablation-ICP-MS microanalysis of solids and liquids

Trace elements in microliter quantities of aqueous solutions were analysed by direct liquid ablation using an 193 nm excimer with an inductively coupled plasma mass spectrometer (ICP-MS). Fractionation resulting from splashing and evaporation can be minimised by covering the liquid surface with a thin plastic film, through which a 20 μm hole is drilled with the laser. Particle-size distribution and oxide formation in the plasma resulting from the direct liquid ablation are similar to those generated by solid ablation. The ICP-MS response in cps/ppm is approximately 100 × higher for the direct ablation, but is proportional to the response from solid ablation, within an accuracy < 15% for most trace elements in NIST 610 and NIST 612 glass standards. A matrix load up to 2.5 wt.-% NaCl in the solution does not affect the proportionality of trace element responses. Thus, direct liquid ablation is not only suited for analysing small volumes of complex aqueous solutions (e.g., the quantitative microanalysis of fluid inclusions in minerals), but also provides a new approach for calibrating laser ablation ICP-MS microanalysis of solids. Received: 2 December 1996 / Revised: 3 March 1997 / Accepted: March 1997     

Determination of additives in PVC material by UV laser ablation inductively coupled plasma atomic emission spectrometry

UV laser ablation inductively coupled plasma atomic emission spectrometry (LA-ICP-AES) has been applied to the direct determination of additives in solid poly(vinyl chloride) materials. A Nd:YAG laser, operating at its fourth harmonic (266 nm), was used with a beam masking device, in the most reproducible conditions, to introduce solid particles into the plasma torch of a simultaneous ICP-AES system. Emphasis was placed on both precision and accuracy in the analysis of PVC materials by LA-ICP-AES. A series of six in-house PVC reference materials was prepared by incorporating several additives in increasing concentrations. Three alternative methods were evaluated to certify the amount of incorporated elements: ICP-AES with sample dissolution, NAA and XRF. Satisfactory results and good agreement were obtained for seven elements (Al, Ca, Cd, Mg, Sb, Sn and Ti) among the ten incorporated. Sample homogeneity appeared to be satisfactory, and calibration graphs obtained by LA-ICP-AES for several elements are presented. Finally, the performance of the technique in terms of repeatability (1.6-5%), reproducibility (2–5%), and limits of detection was investigated.     

Use of tandem calibration for analyzing steels and alloys by inductively coupled plasma atomic emission spectrometry

A method is proposed for plotting calibration graphs by reference analyte solutions (tandem calibration) for the analysis of steels and alloys by inductively coupled plasma atomic emission spectrometry (ICP-AES) with spark ablation. The use of spectral lines of analytes and an internal standard with similar values of sums of ionization and excitation energy ensures an excellent repeatability and accuracy of the results of analysis using tandem calibration. A nebulizer chamber for the simultaneous introduction of solutions and solid sample aerosols into inductively coupled plasma (ICP) is designed. The optimal parameters of the introduction of sample aerosols and a method of plasma observation are chosen; the requirements for the choice of the analytical lines of analytes are proposed.     

Single-step microwave digestion with HNO(3) alone for determination of trace elements in coal by ICP spectrometry

A microwave digestion method with HNO₃ alone was conducted at a temperature as high as 250 °C for determination of 19 trace elements (Li, Be, V, Cr, Mn, Co, Ni, Cu, Zn, Ga, As, Se, Rb, Sr, Cd, Cs, Ba, Hg, and Pb) in coal jointly by inductively coupled plasma optical emission spectrometry (ICP-OES), inductively coupled plasma mass spectrometry (ICP-MS), and flow injection ICP-MS (FI-ICP-MS). The validity of determination was assessed by using three standard coals, SRM 1632c, BCR 180, and SARM 19. It was found that the high-temperature digestion led to an extensive decomposition of the organic matrix and clay in coal, and no dissolved and solid carbon remained in the final solution after evaporation. Good recoveries were observed for all trace elements in three coals, with the exception of V, Rb, and Cs in high-ash SARM 19. Additionally, FI-ICP-MS combined with the present digestion without evaporation pretreatment was proved to be a rapid and efficient approach for determination of ultra-trace elements such as Se, Cd, and Hg in coal.