Vaporization, electrothermal

Electrothermal atomization (ETA) for use with atomic absorption (AA) has proved to be a very sensitive technique for trace element analysis over the last three decades. However, the possibility of using the atomization/heating device for electrothermal vaporization (ETV) sample introduction into an ICP mass spectrometer was identified in the late 1980s. The ETV sampling process relies on the basic principle that a carbon furnace or metal filament can be used to thermally separate the analytes 

FIGURE 17.7 Analyte and blank spectral scans of (a) Co, (b) Cu, (c) Cd, and (d) Pb in NASS-4 open-ocean seawater certified reference material, using flow injection coupled to ICP-MS. (From S. N. Willie, Y. lida, and J. W. McLaren, Atomic Spectroscopy, 19[3], 67,1998.) 

Analytical Results for NASS-4 Open-Ocean Seawater Certified Reference Material, Using FI-ICP-MS Methodology 

Source From S. N. Willie, Y. lida, and J. W. McLaren, Atomic Spectroscopy, 19(3) 67,1998. 

Over the past 20 years, ETV sampling for ICP-MS has mainly been used for the analysis of complex matrices including geological materials, biological fluids, seawater, and coal slurries, which have proved difficult or impossible by conventional nebulization. By removal of the matrix components, the potential for severe spectral and matrix-induced interferences is dramatically reduced. Even though ETV-ICP-MS was initially applied to the analysis of very small sample volumes, the advent of low-flow nebulizers has limited its use for this type of work. 

Thermal evaporation of the analyte elements from the sample has long been used in atomic spectrometry. For instance, it had been applied by PreuE in 1940 [170], who evaporated volatile elements from a geological sample in a tube furnace and transported the released vapors into an arc source. In addition, it was used in so-called double arc systems, where selective volatilization was also used in direct solids analysis. Electrothermal vaporization became particularly important with the work of L vov et al. [171] and Massmann in Dortmund [172], who introduced elec-trothermally heated sytems for the determination of trace elements in dry solution residues by atomic absorption spectrometry of the vapor cloud. Since then, the idea has regularly been taken up for several reasons. 

Firstly, an analyte can be released from a solution residue and thus be brought into an atom reservoir, a radiation or an ion source free of solvent. This is particularly useful for the case of sources operated at low power and gas consumption, 

Practical Guide to ICP-MS A Tutorial for Beginners, Second Edition 

FIGURE 17.8 A graphite furnace ETV sampling device for ICP-MS, showing the two distinct steps of sample pretreatment (a) and vaporization (b) into the plasma (copyright 2003-2007, all rights reserved, PerkinEhner Inc.). 

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Electrothermal vaporization can be used for 5-100 )iL sample solution volumes or for small amounts of some solids. A graphite furnace similar to those used for graphite-furnace atomic absorption spectrometry can be used to vaporize the sample. Other devices including boats, ribbons, rods, and filaments, also can be used. The chosen device is heated in a series of steps to temperatures as high as 3000 K to produce a dry vapor and an aerosol, which are transported into the center of the plasma. A transient signal is produced due to matrix and element-dependent volatilization, so the detection system must be capable of time resolution better than 0.25 s. Concentration detection limits are typically 1-2 orders of magnitude better than those obtained via nebulization. Mass detection limits are typically in the range of tens of pg to ng, with a precision of 10% to 15%. 

Moens L, Verreft P, Boonen S, Vanhaecke F and Dams R (1995) Solid sampling electrothermal vaporization for sample introduction in inductively coupled plasma atomic emission spectrometry and inductively coupled plasma mass spectrometry. Spectrochim Acta 508 463-475. Mooijman KA, In t Veld PH, Hoekstra JA, Heisterkamp SH, Havelaar AH, Notermans SHW, Roberts D, Griepink B, Maier E (1992) Development of Microbiological Reference Materials. European Commission Report EUR 14375 EN, Community Bureau of Reference, Brussels. 

ScHiFFER U, Krivan V (i999) A graphite furnace electrothermal vaporization system for inductively coupled plasma atomic emission spectrometry. Anal Chem 70 482-490. 

With solid sampling-electrothermal vaporization-inductively coupled atomic emission spectrometry (SS-ETV-ICP-AES), Cu in two environmental CRMs was determined using a third CRM with similar matrix as calibrant. Comparison with a reference solution showed good agreement (Verrept et al. 1993). 

The concept of selectivity and specificity has been applied to characterize interferences appearing in two different ICP-MS techniques (Horn [2000]). Classical ICP-MS with pneumatic nebulization and ETV-ICP-MS are compared for the determination of traces of zinc in sea-water. Whereas spectral interferences decrease using the ETV device, nonspectral interferences increase significantly (Bjorn et al. [1998]). A quantitative comparison of the both analytical procedures, here called PN (pneumatic nebulization) and ETV (electrothermal vaporization, Sturgeon and Lam [1999]) is possible by means the specificity as a function of the Zn concentration (Horn [2000]). The spectral interferences on the four zinc isotopes are listed in Table 7.4. 

Bjorn E, Freeh W, Hoffmann E, Liidke C (1998) Investigation and quantification of spectroscopic interferences from polyatomic species in inductively coupled plasma mass spectrometry using electrothermal vaporization or pneumatic nebulization for sample introduction. Spectrochim Acta 53B 1766... 

Nebulization is inefficient and therefore not appropriate for very small liquid samples. Introducing samples into the plasma in liquid form reduces the potential sensitivity because the analyte flux is limited by the amount of solvent that the plasma will tolerate. To circumvent these problems a variety of thermal and electrothermal vaporization devices have been investigated. Two basic approaches are in use. The first involves indirect vaporization of the sample in an electrothermal vaporizer, e.g. a carbon rod or tube furnace or heated metal filament as commonly used in atomic absorption spectrometry [7-9], The second involves inserting the sample into the base of the... 

SAMPLE INTRODUCTION INTO ICP-AES ORICP-MS BY ELECTROTHERMAL VAPORIZATION... 

Although electrothermal vaporization has been widely accepted as an extension of atomic absorption, its use in inductively coupled plasma spectroscopy is fairly recent. In this technique the requirement for the vaporizer is somewhat different—the electrothermal vaporizer does not have to double as the atom cell. In fact, it is only needed to effect efficient and reproducible sample transfer from the rod, or a similar device, into the plasma. 

Gunn et al. [44] described the apphcation of a graphite-filament electrothermal vaporization apparatus as a sample introduction system for optical emission spectroscopy with an inductively coupled argon plasma source. Good detection levels were reported for the elements, and details of the interfacing requirements between the ICP and the graphite filament were explored. 

Fig. 5.17 Gas flow arrangement preferred for linking an electrothermal vaporizer to ICP/MS.

To optimize the applicability of the electrothermal vaporization technique, the most critical requirement is the design of the sample transport mechanism. The sample must be fully vaporized without any decomposition, after desolvation and matrix degradation, and transferred into the plasma. Condensation on the vessel walls or tubing must be avoided and the flow must be slow enough for elements to be atomized efficiently in the plasma itself. A commercial electrothermal vaporizer should provide flexibility and allow the necessary sample pretreatment to introduce a clean sample into the plasma. Several commercial systems are now available, primarily for the newer technique of inductively coupled plasma mass spectroscopy. These are often extremely expensive, so home built or cheaper systems may initially seem attractive. However, the cost of any software and hardware interfacing to couple to the existing instrument should not be underestimated. 

Electrothermal vaporization has many appHcations relating to ICP analysis the abihty to vaporize solvents and to handle small soHd or viscous samples are particularly important. In addition, the measurement of the isotopic ratios of lead in various ores provides a unique identification of the ore and its source. Now that commercial systems are available, the technique will undoubtedly become popular. These techniques seem to have found favour, particularly in Japan. 

Volume 1 consists of chapters covering the development. Instrumentation, and results of a wide range of materials, including background correction lasers, inductively coupled-mass sp>ectroscopy plasmas, electrothermal vaporizers, sample introduction, and Fourier transform atomic spectrocopy. 

ICP-AES has been used to analyse a wide variety of sample types. It is usually necessary to present the sample in the form of a liquid, although solids can be analysed directly in the form of slurries, by direct insertion, laser ablation or electrothermal vaporization (see Chapter 7). 

Because of its capability for rapid multielement analysis, ICP-MS is particularly suited to sample introduction methods which give rise to transient signals. For example, electrothermal vaporization, flow injection and chromatographic methods can be interfaced and many elements monitored in a single run (see Chapter 7). 

Various efficient devices have been utilized for sample introduction into an inductive plasma source, for example the application of several nebulizers, hyphenated techniques, hydride generation, laser ablation and electrothermal vaporization. The role of the solution introduction system in an inductively coupled plasma source is to convert the liquid sample into a suitable form (e.g.,... 

Electrothermal Vaporization Coupled to an Inductively Coupled Plasma Ion Source... 

Viera et al.59 used the isotope dilution technique as a calibration procedure for the determination of As, Ge, Hg, Pb, Se and Sn in coal slurries using chemical vapour generation combined with electrothermal vaporization ICP-MS (CVG-ETV-ICP-MS). 

Apart from multi-element analysis employed for large scale studies, single element analysis (e.g., especially of toxic elements such as Cd, Hg, Tl or Pb) is performed in environmental science for special applications. For example, Hg and Tl have been determined in environmental samples by slurry sampling using electrothermal vaporization (ETV) ICP-MS. If potassium permanganate is employed as a modifier in ETV at optimized pyrolysis temperatures of 300 °C for Hg and 500 °C for Tl, detection limits of 0.18p,gg 1 (Hg) and 0.07p,gg 1 (Tl) are obtained.58... 

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