Multiplexed electrothermal vaporization sample introduction system for inductively coupled plasma spectrometry

A multiplexed electrothermal vaporization (ETV) system for sample introduction into an inductively coupled plasma was designed in an effort to increase sample turn-around time. Tungsten filaments (300 W), originally designed for overhead projectors, were chosen for use as ETVs to avoid the high power requirements associated with other ETV devices, e.g. graphite furnaces (2–3 kW). In short, we have multiplexed the thermal stages have been multiplexed such that a vaporization event can take place every 20 s. This represents a significant increase in the throughput typically associated with ETV-ICPMS, which is normally approximately 20–30 samples/h. Evaluated with respect to common figure of merit criteria, the performance of the multiplexed ETV system is similar to that seen with conventional graphite furnace ETV systems. However, several mass spectral interferences can be introduced by the presence of W into the plasma, which hinder the analysis of certain metals (Hg, Mo, etc.). Thus, other low power vaporizers (e.g. Re, Ta) should be considered for use in future systems.     

Simultaneous electrothermal vaporization and nebulizer sample introduction system for inductively coupled plasma mass spectrometry

The novel analytical application of the combination of an inline electrothermal vaporization (ETV) and nebulization source for inductively coupled plasma mass spectrometry (ICP-MS) has been studied. Wet plasma conditions are sustained during ETV introduction by 200 mL/min gas flow through the nebulizer, which is merged with the ETV transport line at the torch. The use of a wet plasma with ETV introduction avoided the need to change power settings and torch positions that normally accompany a change from wet to dry plasma operating conditions. This inline-ETV source is shown to have good detection limits for a variety of elements in both HNO₃ and HCl matrices. Using the inline-ETV source, improved limits of detection (LOD) were obtained for elements typically suppressed by polyatomic interferences using a nebulizer. Specifically, improved LODs for ⁵¹V and ⁵³Cr suffering from Cl interferences (⁵¹ClO⁺ and ⁵³ClO⁺ respectively) in a 1% HCl matrix were obtained using the inline-ETV source. LODs were improved by factors of 65 and 22 for ⁵¹V and ⁵³Cr, respectively, using the inline-ETV source compared to a conventional concentric glass nebulizer. For elements without polyatomic interferences, LODs from the inline-ETV were comparable to conventional dry plasma ETV-ICP time-of-flight mass spectrometry results. Lastly, the inline-ETV source offers a simple means of changing from nebulizer introduction to inline-ETV introduction without extinguishing the plasma. This permits, for example, the use of the time-resolved ETV-ICP-MS signals to distinguish between an analyte ion and polyatomic isobar.     

Electrothermal vaporization and laser ablation sample introduction for flame and plasma spectrometric analysis of solid and solution samples

High-temperature vaporization-ablation sources provide vapor and/or dry aerosol that can be transported to an analytical observation volume where revaporization of particulates and atomization/ionization/excitation of the vapor can take place. In the previous three decades, graphite-arc vaporization, electrothermal vaporization and laser ablation techniques have been combined with flame and inductively coupled plasma sources in the author's laboratories. These works constitute the basis for a more general review and evaluation of the concepts and results documented in related literature.     

Microliter sample introduction into an inductively-coupled plasma by electrothermal carbon cup vaporization

An atomic emission spectrometric system is described for quantifying trace elements in microvolume samples. The system involves vaporizing the sample by electrothermal carbon cup vaporization followed by the atomization and excitation of the vapor cloud in an inductively coupled plasma (i.c.p.). The detection limits for 21 elements in 10-μl samples are at ng ml^-1 and sub-ng ml^-1 levels with linear dynamic ranges of over four orders of magnitude. Carbon cups coated with pyrolytic graphite are overcoated with tantalum carbide. These cups have resulted in improved detection levels (performances) for Al, As, Bi, Co, Cu and Sn relative to those not containing tantalum. However, cups not treated with tantalum are superior for Au, Cd, Ge, Hg, K, Li, Mg, Mn, Rb and Zn. Comparisons between the two types of carbon cups are presented and discussed. Results also are compared with literature values available for other electrothermal vaporization systems.     

Efficiency of sample introduction into inductively coupled plasma by graphite furnace electrothermal vaporization

A laboratory constructed graphite furnace electrothermal vaporizer (GF-ETV) was used for studying transport efficiencies. This device enables collection of the vaporization products that exit the central sampling hole of the horizontal graphite tube. For determination of the transport efficiency between the GF-ETV and the ICP-torch three methods were tested: (1) deposition of the aerosol particles and the vapour of certain elements by mixing the vaporization product with supersaturated steam and subsequent condensation (direct method); (2) dissolution of the deposited sample fraction from the interface components (indirect method); and (3) calculation from line intensities when applying GF-ETV and pneumatic nebulization sample introduction methods using mercury as a reference element. The latter, `mercury reference method' required 100% transport efficiency for mercury with the ETV, which could be approximated with the use of argon as carrier gas (without halocarbon addition). With a 200 cm³/min flow rate of internal argon in the graphite tube, the transport efficiency was between 67 and 76% for medium volatility elements (Cu, Mn and Mg) and between 32 and 38% for volatile elements (Cd and Zn). By adding carbon tetrachloride vapour to the internal argon flow, the transport efficiency increased to 67–73% for the five elements studied.     

Comparison of tungsten coil electrothermal vaporization and thermospray sample introduction methods for flame furnace atomic absorption spectrometry

Flame furnace atomic absorption spectrometry (FF-AAS) is a newly developed flame atomic absorption spectrometric technique based on arranging a flame furnace onto the top of the flame burner head. In this fundamental investigation, 25 elements were carefully tested by using either thermospray FF-AAS or tungsten coil electrothermal vaporization FF-AAS, of which 15 volatile and semi-volatile elements (Cd, Tl, Ag, Pb, Zn, Hg, Cu, Sb, Bi, Te, In, As, Se, Sn and Au) exhibited better limits of detection compared to those by conventional FAAS; however, non-volatile or refractory elements (Fe, Co, Ni, Cr, Mn, Pd, Pt, Al, Be and V) showed inferior sensitivities by the proposed methods.     

Peak broadening from an electrothermal vaporization sample introduction source into an inductively coupled plasma

Signal broadening using electrothermal vaporization with inductively coupled mass spectrometry (ETV-ICPMS) occurs at a rate much faster than would be predicted by simple longitudinal diffusion. A Monte Carlo simulation that focused on particle motion within the transport tubing was created to elucidate the causes of this dispersion within ETV-ICPMS. Several parameters, including the diffusion coefficient, tube diameter, transport tube length, and flow rate were varied to discern their role in signal broadening. Using typical instrumental parameters, the parabolic flow profile generated by laminar flow of the carrier gas was shown to be the primary cause of dispersion. Manipulating the aforementioned variables to lessen the effects of laminar flow led to a decrease in dispersion. Conversely, increasing the role of laminar flow promoted broadening. The broadening processes should be applicable to any transient introduction system where material must be transported to a detection system. Due to the difference in the rate of broadening expected for particles of different sizes, the simulation was used to calculate the average size of particles generated in the ETV using different mass amounts of sample. No change in particle size (∼1 nm) was seen for mass amounts ranging from 10–10 000 pg, which suggests that the particle number is increased with increasing sample mass rather than the average particle size. Using this method of determining particle size, it might be possible to further evaluate the mechanisms of physical ‘carrier’ action.     

Plasma behavior during electrothermal vaporization sample introduction in inductively coupled plasma atomic emission spectrometry

Electrothermal vaporization (ETV) sample introduction in inductively coupled plasma atomic emission spectrometry suffers from severe matrix effects. In the present study, the differences between wet and dry plasma conditions are studied. In addition, the influence of the sample composition was investigated. An inductively coupled plasma optical emission spectrometer, with detection based on charge transfer, allowed the simultaneous measurement of ionic and atomic emission line intensities during the transient signal. Mg and Cr were the test elements. The ion-to-atom line ratio increases at higher power settings, but the changes were larger when a nebulizer was used for sample introduction than with ETV sample introduction. The decrease of ion-to-atom line ratios at increasing observation height was more pronounced when ETV was used, due to the absence of water vapor. The gas flow rate showed a stronger influence for nebulization than for ETV. In the presence of a calcium matrix, lower ion-to-atom line ratios were observed, but the ratio did not change significantly within the transient emission signal. Similar line ratios were observed for different amounts of calcium matrix. The values of ion-to-atom line ratios for Mg and Cr indicate that the plasma ionization and thermal characteristics are not modified due to the presence of the calcium matrix.     

电热蒸发进样电感耦合等离子体原子发射光谱（ETV－ICP－AES）联用技术研究

本文采用国产部件组装了一套ＥＴＶ－ＩＣＰ－ＡＥＳ仪器体系，对装置的连接及操作参数进行优化。深入系统地考察了分析物的蒸发过程和传输过程，提出了难熔元素的蒸发和传输机理。研究了ＥＴＶ－ＩＣＰ－ＡＥＳ中基体效应，提出了以聚四氟乙烯为氟化剂，氟化辅助ＥＴＶ－ＩＣＰ－ＡＥＳ测定难熔元素的新方法，应用于环境和生物标样中痕量元素分析，获得满意结果。     

Slurry sample introduction with fluorinating electrothermal vaporization for the direct ICP-AES determination of boron in plant leaves

Summary The slurry sample introduction with fluorinating electrothermal vaporization has been applied to ICP-AES for the determination of boron in plant leaves. The main working parameters for ETV-ICP-AES, such as r.f. power, gas flow rate, drying temperature and atomization temperature have been studied. The concentration of the fluorinating agent polytetrafluoro-ethylene was examined for the boron determination. The matrix concentrations of Na, K, Ca and Mg up to 5 mg/ml do not interfere with the fluorinating vaporization of boron. Several plant samples were analysed with the standard addition method. A good agreement of analytical results between this method and a spectrophotometric method was obtained. The determined value of boron in NBS SRM 1573 tomato leaves coincides with literature values.     

Studies on transport phenomena in electrothermal vaporization sample introduction applied to inductively coupled plasma for optical emission and mass spectrometry

In the present work electrothermal vaporization (ETV) was used in both inductively coupled plasma mass spectrometry (ICP-MS) and optical emission spectrometry (OES) for sample introduction of solution samples. The effect of (Pd + Mg)-nitrate modifier and CaCl₂ matrix/modifier of variable amounts were studied on ETV-ICP-MS signals of Cr, Cu, Fe, Mn and Pb and on ETV-ICP-OES signals of Ag, Cd, Co, Cu, Fe, Ga, Mn and Zn. With the use of matrix-free standard solutions the analytical curves were bent to the signal axes (as expected from earlier studies), which was observed in the 20–800 pg mass range by ICP-MS and in the 1–50 ng mass range by ICP-OES detection. The degree of curvature was, however, different with the use of single element and multi-element standards. When applying the noted chemical modifiers (aerosol carriers) in microgram amounts, linear analytical curves were found in the nearly two orders of magnitude mass ranges. Changes of the CaCl₂ matrix concentration (loaded amount of 2–10 μg Ca) resulted in less than 5% changes in MS signals of 5 elements (each below 1 ng) and OES signals of 22 analytes (each below 15 ng). Exceptions were Pb (ICP-MS) and Cd (ICP-OES), where the sensitivity increase by Pd + Mg modifier was much larger compared to other elements studied. The general conclusions suggest that quantitative analysis with the use of ETV sample introduction requires matrix matching or matrix replacement by appropriate chemical modifier to the specific concentration ranges of analytes. This is a similar requirement to that claimed also by the most commonly used pneumatic nebulization of solutions, if samples with high matrix concentration are concerned.     

Direct determination of mercury in solid algal cells by direct-current argon-plasma emission spectrometry with sample introduction by electrothermal vaporization

A study on the use of electrothermal vaporization for introduction of solid samples into a direct-current plasma is presented. The solid samples investigated were milligram masses of an alga exposed to various mercury solutions. The results are independent of the algal mass in the 0.6–9 mg range, and give acceptable calibration curves up to 20 mg of mercury absorbed on 5-mg masses of the alga; the relative standard deviation was 7.4% for nine successive samples of 100g of mercury absorbed by a mass of approximately 3 mg of algal cells. Various mercury compounds gave more than one peak, with different appearance times. Addition of sulphur-containing algal cells or cysteine modified the mercury signal, making it the same (for the compounds tested) irrespective of the mercury compound originally present.     

A macrokinetic theory of sample vaporization in electrothermal atomic absorption spectrometry

A macrokinetic theory of the vaporization of a substance based on a consideration of the discrete distribution of a sample and of surface diffusion in the condensed phase has been developed. An analysis of mathematical models describing the vaporization process for the cases of a sample distributed in a porous bulk and over the vaporizer surface revealed several regions. The vaporization rate in these regions depends in different ways on the vaporization rate constant and the diffusion coefficient of the sample in the condensed phase. This offers an opportunity of explaining the appearance of breaks and inflections in the Arrhenius plots as well as of finding the relationship between the activation energies of vaporization in the different regions. The theoretical conclusions are in good agreement with the available and recently obtained data. The macrokinetic theory provided a better understanding of the mechanism of atomization for many elements in the graphite furnace. The macrokinetic theory offers an explanation for many aspects of electrothermal atomic absorption spectrometry associated with the effects of the furnace material and the wall structure, as well as of sample distribution in the furnace on the process of vaporization and, in particular, on the signal shape, sensitivity and matrix interferences.     

Determination of lead and nickel in animal bone by microwave-induced plasma atomic emission spectrometry with sample introduction by electrothermal vaporization

Sulphuric/nitric acid digestion of the bone precedes introduction of the diluted digest into a helium microwave-induced plasma via a tantalum electrothermal vaporizer. Atomic emission is measured at the Ni I 352.454-nm and Pb I 405.781-nm lines. Detection limits (aqueous solution) are 1.1 × 10^?10 g Ni and 8 × 10^?11 g Pb. The technique was applied to the analysis of IAEA H-5 animal bone reference material.     

Determination of trace rare earth elements in natural water by electrothermal vaporization ICP-MS with pivaloyltrifluoroacetone as chemical modifier

A novel method based on gaseous compounds introduction into the plasma as their pivaloyltrifluoroacetone (PTA) chelates was developed for electrothermal vaporization inductively coupled plasma mass spectrometry determination of trace rare earth elements (REEs) in natural water. The factors affecting formation of the chelates and their vaporization behaviors were studied in detail. Under the optimized conditions, the limits of detection (3σ) of REEs for this method were from 0.20 to 0.91 ng L⁻¹, the relative standard deviations for 0.1 μg L⁻¹ REEs were 2.5–9.1% (n = 9), and the linear ranges of calibration curve for REEs covered at least three orders of magnitude. The proposed method had been applied to the determination of trace REEs in water samples of Yangtze river, Han Shui river and East lake (Wuhan), and the recoveries for the spiked water samples were 93–105%.     

Quantitation methods using laser ablation ICP-MS

Laser Ablation ICP-MS offers the capability of direct analysis of conductive and non-conductive samples. However the quantitation capabilities of LA-ICP-MS are often questioned. In order to qualify the term quantitation, different methods, appropriate for laser ablation, are discussed. Measurements on different sample matrices were performed both semiquantitatively and quantitatively using standards with the same matrix. Additionally, the use of an internal standard, a minor isotope of the matrix element or a known concentration of a trace or a minor element can be selected as an aid to quantitation. In cases where there is no certified element concentration available, a user value for the LA-ICP-MS sensitivity can be chosen. These different methods of quantitation are compared and the rationale behind each approach is discussed.     

Direct determination of metals in milligram masses and microlitre volumes by direct-current argon-plasma emission spectrometry with sample introduction by electrothermal vaporization

The development of an electrothermal vaporization and direct-current argon-plasma emission spectrometric system which allowed the determination of metals in microlitre volumes and milligram masses is described. For the five metals investigated, the concentration detection limits were comparable to those for conventional pneumatic nebulization of solutions, and the mass detection limits were superior. The use of an ashing stage was found to reduce enhancement by sodium in the determination of copper and manganese by the plasma method. The system was shown to give accurate results for complex biological, nutritional, water and geological samples.     

Investigation and quantification of spectroscopic interferences from polyatomic species in inductively coupled plasma mass spectrometry using electrothermal vaporization or pneumatic nebulization for sample introduction

A new method for quantification of spectral interferences based on analyte isotope ratio measurements in the presence of various concentrations of a specific matrix is presented. Within the method, a tolerance level is used, defined as the matrix concentration at which the ratio between analyte isotopes with and without interferences is altered by 10% compared to a pure water reference standard, normalized with respect to the analyte concentration in the solutions. This can be used to estimate the lowest analyte concentration which can be determined with a defined accuracy in the presence of a known concentration of a specific matrix. Regarding spectroscopic interference effects, comparative results for sample introduction into the ICP–MS by electrothermal vaporization, ETV, and nebulization are presented for common matrix — (Ca, Na, K, Cl, P, O) and analyte (Cr, Ni, Cu, As, Se) elements. With the exception of the spectral overlap of ³¹P₂⁺ on ⁶²Ni⁺, spectroscopic interferences were reduced by 1–2.5 orders of magnitude when using ETV for sample introduction. Reasons for the increase in the spectral interference of ³¹P₂⁺ on ⁶²Ni⁺ are discussed. For sample introduction by nebulization, it was found that spectral interferences from CaO⁺ on ⁵⁸Ni⁺ and ⁶⁰Ni⁺ were reduced in the presence of phosphate.