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- Herausgeber: Wiley-VCH GmbH
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
This practical guide shows how to efficiently use the method for a variety of applications. Numerous examples as well as ideas for problem solving and troubleshooting are presented. The new edition is updated with the latest developments in methods and applications.
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Seitenzahl: 512
Veröffentlichungsjahr: 2021
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Table of Contents
Cover
Title Page
Copyright
Foreword
Preface
1 An Overview
1.1 Features of ICP-OES
1.2 Inductively Coupled Plasma Optical Emission Spectrometry – the Name Describes the Technique
1.3 Distribution of ICP-OES
1.4 Related Techniques for Elemental Analysis
1.5 Terms
2 Plasma
2.1 The Spectrometric Plasma
2.2 Excitation to Emit Electromagnetic Radiation (Light)
2.3 Excitation Unit
2.4 Sample Introduction System
3 Optics and Detector of the Spectrometer
3.1 Basic Principles of Optics
3.2 Detectors
3.3 Types of Emission Spectrometer Mounts
4 Method Development
4.1 Wavelength Selection
4.2 Processing and Correction Techniques
4.3 Non-spectral Interference
4.4 Optimization
4.5 Validation
5 Routine Analysis
5.1 Preparation
5.2 Calibration
5.3 Quality Assurance
5.4 Software and Data Processing
6 Troubleshooting and Maintenance
7 Applications
7.1 General Notes
7.2 Comments on Selected Elements
7.3 Selected Applications
8 Procurement of Equipment and Preparation of the Laboratory
8.1 Which Atomic Spectrometric Technique Is the Most Suitable?
8.2 Which ICP Emission Spectrometer Is the Most Suitable?
8.3 Preparation of the Laboratory
References
Index
End User License Agreement
List of Tables
Chapter 2
Table 2.1 Characteristic data for the noble gases.
Table 2.2 Ionization energy for selected elements and fraction of the ionized...
Table 2.3 Selected excitation energies for atoms.
Table 2.4 Selected excitation energies for ions.
Table 2.5 Influence of the sample temperature on the recovery rate.
Table 2.6 Measured detection limits with the modified nebulizer chamber for t...
Chapter 3
Table 3.1 Line width of selected lines.
Chapter 4
Table 4.1 Calculated concentrations of Ag at 243 nm in the presence of an int...
Table 4.2 Calculated intensity of Ag at 243 nm in the presence of an interfer...
Table 4.3 Improved detection limits when processing several lines of the same...
Table 4.4 Typical ranges of reproducibility from three repeats (using PerkinE...
Table 4.5 Decrease in limits of detection in ICP-OES over the course of the t...
Chapter 6
Table 6.1 Diagnostic aids according to Mermet [350].
Chapter 7
Table 7.1 Specific aspects of selected elements analyzed by ICP-OES.
Table 7.2 Comparison of the legal limits in drinking water in accordance with...
Table 7.3 Analytical lines for environmental samples.
Table 7.4 Analytical lines for samples of biological origin.
Table 7.5 Analytical lines for geological samples.
Table 7.6 Analytical lines for steel and iron-based alloys.
Table 7.7 Analytical lines for selected nonferrous metals and their alloys.
Table 7.8 Analytical lines for noble metals and their alloys.
Table 7.9 Analytical lines for ceramics used as a catalyst base.
Table 7.10 Analytical lines for samples from different production processes.
Table 7.11 Analytical lines for eluates of toxicologically relevant elements ...
Table 7.12 Analytical lines for wear metals, contaminants, and additives in o...
List of Illustrations
Chapter 1
Figure 1.1 Use of ICP-OES in different application areas in Germany. Similar...
Figure 1.2 Schematic representation of a three-electrode direct current plas...
Chapter 2
Figure 2.1 Temperature distribution in the argon plasma [±140 K] maintained ...
Figure 2.2 A schematic drawing of a plasma torch. The torch has an extended ...
Figure 2.3 The drawing shows a tulip-shaped torch. In this torch model, the ...
Figure 2.4 As the distance between the injector and the bottom of the induct...
Figure 2.5 Rinse-out behavior of boron after aspiration of a solution contai...
Figure 2.6 Energy level diagram of Mg. The energy rises from bottom to top f...
Figure 2.7 Influence of temperature on the distribution of species (atoms, i...
Figure 2.8 The intensity distribution of the excitation temperaturefollows t...
Figure 2.9 Spectroscopic zones in the plasma. Typically, the normal analytic...
Figure 2.10 Schematic representation of a plasma into which a stock solution...
Figure 2.11 Impact of the cooling of the plasma due to introduction of matte...
Figure 2.12 Influence of the RF power on the plasma robustness expressed as ...
Figure 2.13 Influence of the nebulizer gas flow on the plasma robustness exp...
Figure 2.14 Influence of the RF power on the intensities of selected emissio...
Figure 2.15 Influence of the nebulizer gas flow rate on the intensities of s...
Figure 2.16 Influence of the plasma gas flow rate on the intensities of sele...
Figure 2.17 Influence of the auxiliary gas flow rate on the intensities of s...
Figure 2.18 Influence of the viewing height on the intensity of the emission...
Figure 2.19 Directions for plasma viewing: axial and radial.
Figure 2.20 A mirror on the other side of the optics reflects the light from...
Figure 2.21 Spectra illustrating the increase in the intensity of emission s...
Figure 2.22 The plasma is viewed axially by arranging the transfer optics to...
Figure 2.23 Example of transfer optics that allow both radial and axial view...
Figure 2.24 When using a dichroic mirror in transfer optics, the UV light fr...
Figure 2.25 When turning the knob below, it turns the induction coil above f...
Figure 2.26 The alkali effect refers to the observation that the calculated ...
Figure 2.27 Influence of the radial viewing height on the recovery of K (1 m...
Figure 2.28 Schematic diagram of a frequency-stabilized RF generator.
Figure 2.29 Solid-state generatorwith induction coil.
Figure 2.30 The flat plate technology induction coil produces a homogeneous ...
Figure 2.31 Schematic diagram of the sample introduction system for a vertic...
Figure 2.32 Photograph of a sample introduction system equipped with a cross...
Figure 2.33 The sample is supplied to the concentric nebulizer via a tube (f...
Figure 2.34 Concentric nebulizer for low sample consumption. It is metal fr...
Figure 2.35 Schematic diagram of a membrane desolvator.
Figure 2.36 Nebulizer for low sample consumption with heated nebulizer chamb...
Figure 2.37 Cross-section of the cross-flow nebulizer. Details of the posit...
Figure 2.38 V-groove nebulizer. Details of the V opening and the exact posit...
Figure 2.39 Cross section of a ConeSpray nebulizer. The sample enters via th...
Figure 2.40 In the flow blurring nebulizer, turbulent mixing occurs between ...
Figure 2.41 Comparison of droplet size distributions of flow blurring (left)...
Figure 2.42 Schematic diagram of an ultrasonic nebulizer.
Figure 2.43 In this photograph, the heart of an ultrasonic nebulizer is show...
Figure 2.44 As the temperature of the nebulizer chamber rises, the analyte s...
Figure 2.45 A thermostatted nebulizer chamber keeps the temperature of the i...
Figure 2.46 A cooled cyclonic nebulizer chamber. The cooling liquid is pumpe...
Figure 2.47 The formation of comparatively large drops on a very smooth surf...
Figure 2.48 Schematic diagram of a baffled cyclonic nebulizer chamber. A cro...
Figure 2.49 Cyclonic nebulizer chamberwith a concentric nebulizer. The conne...
Figure 2.50 Three of the four types of cyclonic nebulizer chambers: the left...
Figure 2.51 A constant slope of the waste tube is important to avoid signal ...
Figure 2.52 Influence of the pump speed of a peristaltic pump on the intensi...
Figure 2.53 Photo of a piston pump design to be used with ICP instruments....
Figure 2.54 The argon humidifier helps prevent clogging of the nebulizer jet...
Figure 2.55 Use of a modified cyclonic nebulizer chamberfor hydride generati...
Figure 2.56 Another modification of the cyclonic nebulizer chamber for conve...
Figure 2.57 Long-term stability (over one hour) when continuously aspirating...
Figure 2.58 Cross section of an oven for electrothermal vaporization (ETV) w...
Figure 2.59 Temperature program for an ETV furnace (axis on the right), whic...
Figure 2.60 A laser ablation unit consists of the laser (in this example a f...
Figure 2.61 Craters formed by laser shots for a repeated analysis for 20 ele...
Chapter 3
Figure 3.1 Idealized emission line where the full width at half height is hi...
Figure 3.2 Test for the optical resolution for the Cd–As interferenceat 228....
Figure 3.3 Improvement in the resolution of ICP emission spectrometers since...
Figure 3.4 The detector receives emitted light not only due to the analyte p...
Figure 3.5 Diffraction at a grating. The incident ray strikes the grating at...
Figure 3.6 Reflection efficiency of the various optical orders at an optical...
Figure 3.7 Efficiency of the light diffraction at a mechanically ruled grati...
Figure 3.8 On the left (a), a schematic diagram of a “double” Paschen–Runge ...
Figure 3.9 In the Czerny–Turner mount shown here, the radiation emitted by t...
Figure 3.10 In this Echelle mount, the light of the entry slit (top right) r...
Figure 3.11 In this variation of an Echelle optical mount, both a prism and ...
Figure 3.12 In an Echellogram, the wavelengths are positioned in bands of fe...
Figure 3.13 Combination of Echelle and Littrow features in one optical mount...
Figure 3.14 One technical solution for interfacing the torch with the optics...
Figure 3.15 The horizontal plasma strikes against the interface, which is po...
Figure 3.16 Absorption coefficient of oxygen as a function of the wavelength...
Figure 3.17 An alternative to purging the optics with fresh gas is to pump t...
Figure 3.18 Comparison of the wait time to reach full transparency in the va...
Figure 3.19 Prior to electronic detectors, spectra were recorded with a phot...
Figure 3.20 Schematic diagram of a photomultiplier tube. Light falls from th...
Figure 3.21 A subarray is a group of adjacent pixels (in this example, the p...
Figure 3.22 Comparison of the quantum efficiencies (QEs) of different detect...
Figure 3.23 The dark current of a CTD detector depends on its temperature. A...
Figure 3.24 Photograph of a charge injection device detector.
Figure 3.25 Schematic representation of the readout process of a CID detecto...
Figure 3.26 Because of the readout characteristics of the CID, each signal i...
Figure 3.27 Photograph of a charge-coupled device, which has been optimized ...
Figure 3.28 Cross-section of an SCD (segmented charge-coupled device detecto...
Figure 3.29 The registers of A, B, and C of a CCD are arranged one over anot...
Figure 3.30 The working range of a solid-state detector (here the example SC...
Figure 3.31 Comparison of overhead time for signal processing for a CID dete...
Chapter 4
Figure 4.1 By changing the excitation conditions, the intensity ratio betwee...
Figure 4.2 The use of a spectrometer with superior resolution makes possible...
Figure 4.3 Spectrum of the plasma between 175 and 475 nm. The horizontal dou...
Figure 4.4 Example of OH bands in the wavelength range of 309.193–309.497 nm...
Figure 4.5 Molecular bands in the wavelength range around 230 nm when aspira...
Figure 4.6 If there is contamination in the blank, as is shown for the examp...
Figure 4.7 Decreasing signals for repeated measurements of the blank solutio...
Figure 4.8 A background structure should not be interpreted as an analyte si...
Figure 4.9 If emission lines of the analyte and the interferent are found at...
Figure 4.10 In order to determine whether an interfering peak affects the re...
Figure 4.11 Spectrum of Ag and Ni at 243 nm with a concentration of 1 mg/L e...
Figure 4.12 Processing of the spectrum shown in Figure 4.11 by the peak heig...
Figure 4.13 Processing of the spectrum shown in Figure 4.11 by area processi...
Figure 4.14 Because of the relatively large pixel width in comparison with t...
Figure 4.15 The background of a plasma driven by a 40 MHz generator increase...
Figure 4.16 The intensity of the background decreases by cooling of the plas...
Figure 4.17 The plasma background decreases as less power is coupled into th...
Figure 4.18 Increase in background at 396.152 nm caused by introducing a 100...
Figure 4.19 The background of the Pb line at 220.353 nm is increased by an A...
Figure 4.20 The line calculated with the aid of the background correction po...
Figure 4.21 At a distance of about twice the FWHH from a peak, the wing has ...
Figure 4.22 The dynamic automatic background correction analyses the spectru...
Figure 4.23 Options for setting background correction points for a curved ba...
Figure 4.24 When using a nonlinear fit of the background, several background...
Figure 4.25 When setting background correction points, the scale must be set...
Figure 4.26 When setting the background correction points, nearby peaks shou...
Figure 4.27 If, for safety's sake, two background correction points are set ...
Figure 4.28 The background correction points should be as near as possible t...
Figure 4.29 If there is no space for a background correction point on one si...
Figure 4.30 Dependence of the detection limit on the number of pixels per pe...
Figure 4.31 Processing of a doublet peak for Tl at 190 nm. When incorporatin...
Figure 4.32 While the distance of the background correction points was gradu...
Figure 4.33 The calculated detection limits depend heavily on the correction...
Figure 4.34 The example of nonlinear background correction for Tl at 190 nm ...
Figure 4.35 Single-element spectra of the components are taken as part of th...
Figure 4.36 The background spectrum is subtracted from the single-element mo...
Figure 4.37 The net single-element spectra are rescaled until the spectrum c...
Figure 4.38 The quality of spectral interference correctionwith multivariate...
Figure 4.39 The optimum concentration for single-element solutions to be use...
Figure 4.40 In the hotter regions of the plasma, radiation is emitted (spect...
Figure 4.41 Spectra used for the MSF method development. The example is give...
Figure 4.42 Example of a check spectrum, which can be displayed during multi...
Figure 4.43 A negative control peak can have various causes: (i) an interfer...
Figure 4.44 The control peak is almost invisible. One can merely guess at it...
Figure 4.45 Subtraction of one spectrum from another. At the top (a), the sp...
Figure 4.46 Effects of spectral and non-spectral interference on the slope o...
Figure 4.47 When using an internal standard, the norm temperatures of the li...
Figure 4.48 In order to determine the concentration by the technique of anal...
Figure 4.49 Rough indication of the relative importance of typical analysis ...
Figure 4.50 Check of the linearity of Sn at 189 nm with axial viewing. The g...
Figure 4.51 The linearity test may yield two variations from the straight li...
Chapter 5
Figure 5.1 Example of standard operating procedures (SOP).
Figure 5.2 Experimental determination of the delay time. The example shown h...
Figure 5.3 The rinse-out behavior shown for the example of Zn. This element ...
Figure 5.4 The concentration of the most concentrated calibration solution s...
Figure 5.5 A measured nonlinear concentration–intensity relationship (indic...
Figure 5.6 The concentrations of the calibration solutions are fixed so they...
Figure 5.7 A measurement protocol of a calibration procedure with comments. ...
Figure 5.8 Example of a mean QC control chart. (CU: upper control limit, WU:...
Chapter 7
Figure 7.1 The elements on the right side of the periodic table (primarily i...
Figure 7.2 Cl has some very sensitive emission lines in the low vacuum-UVin ...
Figure 7.3 The photograph shows a cooled nebulizer chamber (center right) fo...
Figure 7.4 The two Echellograms of the plasma emission in the spectral range...
Chapter 8
Figure 8.1 What we want… …and what we can have.
Figure 8.2 A general assessment of which element should be analyzed preferen...
Guide
Cover Page
Table of Contents
Begin Reading
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ICP Emission Spectrometry
A Practical Guide
Joachim Nölte
Second Edition
Author
Joachim Nölte Analytik Support Rietweg 5 8260 Stein am Rhein Switzerland
All books published by WILEY-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.
Library of Congress Card No.:applied for
British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library.
Bibliographic information published by the Deutsche NationalbibliothekThe Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at <http://dnb.d-nb.de>.
© 2021 WILEY-VCH GmbH, Boschstr. 12, 69469 Weinheim, Germany
All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law.
Print ISBN: 978-3-527-34657-8ePDF ISBN: 978-3-527-82362-8ePub ISBN: 978-3-527-82364-2oBook ISBN: 978-3-527-82363-5
Cover Design SCHULZ Grafik-Design, Fußgönheim, Germany
Foreword
A long time has passed since Stanley Greenfield and coworkers at Albright & Wilson, and Vemer Fassel and colleagues Richard Wendt, George Dickinson, and Dick Knisely at Iowa State University devised the spectroanalytical inductively coupled plasma in the 1960s. For almost a decade, they worked exhaustively for recognition of ICP spectrometry among hesitant manufacturers and potential users. With others, Paul Boumans from Eindhoven and Jacques Robin and Jean-Michel Mermet in Lyon joined an enlarging community of researchers generating convincing evidence of the attributes of ICP emission spectroscopy. Fortunately, their efforts were fruitful by the mid-1970s and blossomed in the 1980s. Today, ICP emission and mass spectroscopy are mainstay techniques in the analytical chemistry laboratory, field stations, and testing facilities. The numbers of instruments, users, and applications continue to grow, and no competitive spectroscopic technique has been proved as truly universal. The ICP source has been a very bright “star” of twentieth century analytical chemistry, and the ICP will continue to fulfill this role in the foreseeable decades as the need for ultrasensitive, matrix-free, and ultra-microanalyses intensifies.
With tens of thousands of ICP instruments found throughout the world in laboratories for education and training purposes, for routine environmental applications, and for sophisticated isotopic and/or speciation research in human nutrition and environmental forensics, a practical monograph is needed to address instrument basics, design and operation, software features, and applications. Much has been written about ICP spectrometry, and numerous excellent chapters, monographs, and texts now compete for a featured place on the ICP library bookshelf. This book provides a unique resource, since it is intended as a tutorial for novices.
Joachim Nö1te has extensive practical ICP experience. For more than a decade Dr. Nölte worked as an ICP application chemist at Perkin-Elmer in Überlingen, Germany, where he provided customer support, developed ICP methods, and conducted training courses. Recently, through his own business enterprise, Analytik Support, he has been offering ICP consulting services and training programs. This experience with early and contemporary ICP emission systems stimulated his desire to write a practice guide that links theory with everyday ICP applications.
This know-how shows up in the eight chapters of this textbook suited for the ICP beginner. Written in a conversational tone and without many equations or detailed theoretical development, the book qualitatively describes the instrument, method development, routine analysis, troubleshooting and maintenance, and brief application examples. General equipment procurement and site preparation considerations also are described succinctly. Distributed through the text are informative side boxes highlighting practical tips for new users, frequently asked questions with reasonable answers, and complementary theory coupled to practice. This material is appropriately illustrated and documented, and it should be a useful resource for anyone interested in carrying out ICP emission spectrometry.
28 October 2002
Ramon M. Barnes
University Research Institute
for Analytical Chemistry
Amherst, Massachusetts
Preface
For two decades (1981–2000), I worked with ICP emission spectrometry in various functions (mainly as PhD student and as an application specialist for an instrument manufacturer) focusing on a variety of applications. The absence of a practically orientated book on this working horse in the inorganic analytical laboratory inspired me to write such a book myself. In the following two decades, there were a number of changes in the instruments from various manufacturers: The trend of using a vertically aligned torch and thus plasma for many ICP instruments, the option to observe the plasma radially and axially in one instrument, the use of sold-state devices, be it for detectors or in RF generators. Also, there were new developments in sample introduction. Last but not least, the extended ways of peak and background processing demonstrate attractive influences on lowering detection limits.
In short, nearly two decades after the first edition of this book was published, it seemed logical to include the various new aspects in a second revised edition, which you hold in your hands. Hopefully, this book will give you valuable insights about the basic concepts as well as on applications of this multielement analytical technique. I wish you successful transformation in your work practice!
1An Overview
ICP emission spectrometry (ICP-OES) is one of the most important techniques of instrumental elemental analysis. It can be used for the determination of approximately 70 elements in a variety of matrices. Thanks to its versatility and productivity it is used in many different applications, and nowadays it carries the basic workload in many routine laboratories.
This book gives an introduction to the basic principles of ICP emission spectrometry and provides some background information as well as practical hints to the user. This knowledge should enable the reader to appreciate the possibilities and limitations of this analytical technique in order to use it in an optimal way.
Throughout the text, you will find complementary information, which is indicated by a frame around the text. Symbols indicate the type of information given:
Practical tips
Additional information
Complementary theory
1.1 Features of ICP-OES
The heart of an ICP emission spectrometer is the plasma, an extremely hot “gas” with a temperature of several thousand Kelvin. It is so hot that atoms and mainly ions are formed from the sample to be analyzed. The very high temperature in the plasma destroys the sample completely, so the analytical result is usually not influenced by the nature of the chemical bond of the element to be determined (absence of chemical interference). In the plasma, atoms and ions are excited to emit electromagnetic radiation (light). The emitted light is spectrally resolved with the aid of diffractive optics, and the emitted quantity of light (its intensity) is measured with a detector. In ICP-OES, the wavelengths are used for the identification of the elements while the intensities serve for the determination of their concentrations.
Since all elements are excited to emit light in the plasma simultaneously, they can be determined simultaneously or very rapidly one after another. Consequently, the analytical results for a sample can be obtained after a short analysis time. The time needed for the determination depends on the instrument used and is of the order of a few minutes. The fact that all the elemental concentrations are determined in one analytical sequence and not by measuring one series of samples for one element, another series for another element, and so on usually makes the technique attractive with respect to speed.
Samples analyzed are normally liquids, occasionally solids, and (quite rarely) gases. For the determination of an element, no specific equipment (such as the lamp used in atomic absorption spectrometry) is needed. As a rule, one only needs a calibration solution of the element to be analyzed and a little time for method development. Hence, an existing analytical method can easily be extended to include another element. This makes ICP emission spectrometry very flexible.
ICP-OES has a very large working range, typically up to six orders of magnitude. Depending on the element and the analytical line, concentrations in the range from less than μg/L up to g/L can be determined. Time-consuming dilution steps are therefore rarely needed, which considerably increases the analysis throughput.
Particularly in environmental analysis, the working ranges for many elements correspond to the concentrations normally found in the samples, and this is one of the reasons why this technique is widely used in environmental applications; about half of all users of ICP-OES use it in these or related areas.
Because of the widespread use of this technique in environmental applications, there are a number of standards and regulations that apply. The most important of these are ISO 11885 [1] and EPA Method 200.7 [2]. Moreover, ICP-OES is used in a variety of other applications, such as metallurgy and the elemental analysis of organic substances.
Plasma was first described as an excitation source for atomic spectroscopy in the mid-1960s [3–6], and the first instrument appeared in research laboratories a decade later. After a further 10 years the technique was commercialized [7–9]. At first slowly, but then at an increasing rate, ICP emission spectrometers were introduced into routine laboratories. During the same period, the instruments were refined to make them more user friendly [10]. Since the early 1990s, ICP-OES has become the “workhorse” in the modern analytical laboratory [11, 12]. These years also brought a number of significant improvements, most importantly the use of solid-state detectors [13].
1.2 Inductively Coupled Plasma Optical Emission Spectrometry – the Name Describes the Technique
As a rule, the technique is referred to as ICP or ICP-OES. The latter is the abbreviation for inductively coupled plasma optical emission spectrometry. The complete name describes or implies the analytic features of this technique: “Plasma” describes an ionized gas at very high temperatures. The energy necessary to sustain the plasma is transferred electromagnetically via an induction coil. This method of energy transfer is found in the first part of the name of the technique: “Inductively coupled plasma.”
The sample to be analyzed is introduced into this hot gas. As a rule, all chemical bonds are dissociated at the temperature of the plasma, so that the analysis is independent of the chemical composition of the sample. The atoms and ions are excited in the plasma to emit electromagnetic radiation (“light”), which mainly appears in the ultraviolet and visible spectral range. The emission of light occurs as discrete lines, which are separated according to their wavelength by diffractive optics and utilized for identification and quantification.
Spectrometry is a technique for quantification that uses the emission or absorption of light from a sample. Its goal is the determination of concentrations and differs from qualitative analysis by spectra, which is commonly referred to as spectroscopy [14].
As a rule, in ICP emission spectrometry there is a linear relationship between intensity and concentration over more than 4–6 decades. This intensity concentration function depends on a number of parameters, some of which are unknown. Hence, there is a need for empirical proportionality factors. Consequently, in ICP emission spectrometry, these factors have to be determined before the analysis (calibration). One assumes that the slope of the calibration functions does not change between standards and the samples. It is an important prerequisite to ensure good accuracy of the analytical results to prove that this is actually the case. Instrument performances as well as method development have a large influence on this, which can be challenging at times.
Since all atoms and ions emit light simultaneously, ICP-OES is a typical representative of a sample-orientated multielement technique. This means that the results for the elements in one sample are measured in one step, unlike the element-orientated mode of operation where all samples are examined for one element. After all the samples have been analyzed for the first element, they are then measured in a new series for the next element. A typical representative of the element-orientated mode of operation is classical atomic absorption spectrometry. The advantages of the sample-orientated mode of operation for routine analysis are obvious, since the sample is characterized very quickly.
ICP, ICP-OES, ICP-AES, ICP/AES, ICP emission spectrometry, ICP ES: What is the correct name for this technique?
The variety of names for this technique reminds one of the tower of Babel. Which version should one follow? We will try to throw some light on this while attempting to trace the origin of the terms used:
Let us start with “ICP,” the abbreviation for “inductively coupled plasma.” This is widely accepted. Everyone agrees to use this abbreviation, at least in written communications.
However, the abbreviation “ICP” alone is no longer sufficient as a clear identification of the technique since a similar technique, ICP-MS (ICP mass spectrometry), exists. To distinguish these from each other, it is recommended to add “OES” or “MS” to the abbreviation “ICP” in order to clearly specify which technique is meant. “ICP” should rather be understood as a generic term for both techniques.
The abbreviation “OES” is the short form for “optical emission spectrometry” and has been around for many decades. Originally, it was used in connection with excitation by spark or glow discharge long before inductively coupled plasma was used analytically. Since plasma only represents another excitation source, it makes sense to stay with the abbreviation “OES.”
Sometimes one finds the name “atomic emission spectrometry” or “AES.” Typically, this version is used by users and manufacturers who in many cases have worked with “atomic absorption spectrometry” (AAS) before. The use of the term “atomic” is plausible to some extent since ICP-OES as well as AAS and ICP-MS are categorized under the group name “atomic spectrometry.” However, the reference to “atoms” is misleading in a way since most particles in the plasma are ions (Table 2.2).
Complications could also arise from the fact that the abbreviation “AES” stands for “Auger Electron Spectroscopy.” Since this is a completely different technique, it is not likely that there would be confusion. However, in order to be sure, it seems wise not to use the abbreviation “AES” for “emission spectrometry.”
The light can be diffracted only by optical means. Therefore, it is redundant to include the term “optical.” The term “optical emission” is a tautology similar to “cold ice” or “wet water.” For this reason, ISO 12235 [15] suggests dropping the “O” (and “A”) completely. The author follows this logic. This leads to the fact that in addition to the abbreviations “OES” and “AES” there would be yet another form “ES.” The likelihood of confusion between these terms increases, so throughout this book “OES” will be used.
In a few rare cases, a slash “/” is used to connect “ICP” and “OES.” However, to the author's knowledge there is no international norm or regulation that suggests the use of a slash. For the sake of clarity, the use of a slash should therefore cease.
1.3 Distribution of ICP-OES
The first applications of ICP-OES were in metallurgy; however, environmental analysis was the driving force leading to its widespread use in routine laboratories. In addition, the technique is used for a variety of other areas of element determination, as shown in Figure 1.1.
With respect to geographical distribution, nearly half of all the instruments in operation worldwide are located in North America. Germany is the next biggest market, with more than 10%, while Japan and the Netherlands have fewer than 10% of the installed instruments. Australia, China, Britain, and France have shares in the small percentage range.
1.4 Related Techniques for Elemental Analysis
Atomic absorption spectrometry (AAS) [16, 17] was the standard analytical technique for elemental analysis until a few years ago. Classical AAS is a single-element technique. AAS uses the absorption of light by atoms that originate from the sample and are in resonance with the light emitted by a specific element lamp. The greater the number of the atoms in the light beam, the higher the absorption, and this is used to measure the concentration. As a rule, the working range is about two orders of magnitude. Flames, furnaces (mainly made of graphite), or quartz cuvettes with chemical reduction reactions (hydride and cold vapor techniques) as sampling systems serve as an atomizing source. Furnaces and chemical reactions typically yield very good limits of detection. The furnace will typically modify the sample, so in a way it serves also for sample preparation, which is a great advantage of this specific technique. This is especially important in clinical applications.
Figure 1.1 Use of ICP-OES in different application areas in Germany. Similar patterns can be found in most other countries of the world.
High-resolution continuum source AAS (HR-CS AAS) uses a xenon lamp rather than a multitude of single element lamps [18]. The use of a xenon lamp allows the determination of all elements in a sample simultaneously. Thus, the risk of spectral interferences is increased. Using high-resolution optics compensates for this effect.
Atomic fluorescence spectrometry (AFS) makes use of the fluorescence that is emitted by atoms in all directions after excitation [19]. Because the detection can be done off-axis from the light beam used to excite the atoms, very low background emission is typical for AFS, unlike AAS and OES. In principle, because of this fact, very low limits of detection are achievable. There are only a few commercial AFS instruments in use, and these are mostly used for the analysis of hydride-forming elements and Hg. The possibility of using fluorescence with an ICP as an excitation source [20] has not been pursued further.
As in ICP-OES, plasma is used in ICP mass spectrometry (ICP-MS). Here, the ions formed in the plasma are used for quantification. The separation of the ions is carried out (fast) sequentially with a quadrupole (resolution 1 amu) or sector field (resolution of 300–10 000 amu with a typical instrument setting of 4000 amu) or simultaneously using the time-of-flight principle (TOF). The great advantage of ICP-MS consists in its detection power, which is especially high for the high-resolution instruments (if they are operated in a low resolution mode). A greater concentration of dissolved substances in the sample solution may cause clogging of the interface between the plasma and the high-vacuum section of the mass spectrometer. Therefore, the excellent detecting power cannot always be converted to overall better limits of quantification in the sample because in many cases, the samples must be very highly diluted prior to aspiration into an ICP-MS instrument.
An alternative to high-resolution ICP mass spectrometers to eliminate interferences is the use of reaction or collision cells. These transform either interfering ions or shift the analyte ion to a compound ion that is free from interferences.
In a direct current plasma (DCP) [21], the sample is introduced into a direct current arc. There it is excited to emit light. The region of the arc besides the electric current sustaining the plasma is viewed for quantification. A three-electrode plasma has a cathode and two anodes (Figure 1.2). The sample aerosol is introduced via an injector between the two anodes. The analysis with DCP is very susceptible to excitation interference, particularly by easily ionizable elements. In addition, molecular bands will often interfere [22].
A microwave-induced plasma (MIP) [24] uses He as the plasma gas. This enables much higher excitation temperatures to be obtained, so that nonmetals are excited particularly well. The MIP is highly subject to matrix influences, even water. Therefore, it is preferably used for the analysis of gases. The combination with electrothermal vaporization works very well [25], as does the quantification of hydrides. MIP is an ideal detector for gas chromatography [26–28].
In the year 2011, an instrument based on microwave plasma using nitrogen as operating gas was commercially introduced. It uses an air-cooled magnetron similar to the ones used in kitchen microwave ovens. The magnetron produces a magnetic field, which produces a rotational symmetric plasma, similar to an ICP. This plasma reaches a temperature of about 5000 K [29]. Like in ICP-OES, a torch is used. The applications are similar to flame atomic absorption spectrometry [30].
Figure 1.2 Schematic representation of a three-electrode direct current plasma.
Source: Adapted from [23].
The plasma temperature of a capacitively coupled microwave plasma (CMP) [31] is very low (below 5000 K) [32]. Therefore, excitation interference is quite pronounced. It has been replaced by ICP-OES.
Glow discharge optical emission spectrometry (GDOES) uses the light emitted from a glow discharge formed between a hollow cathode and a sample anode in a reduced-pressure argon atmosphere [33]. Argon cations are formed, which are accelerated in the direction of the negatively charged solid sample. When they hit the surface, atoms from the surface are released and excited. The light from these atoms is used for quantification. GDOES is a technique for surface analysis of electrically conductive materials. Since the composition of the surface has a great influence, a correction must be applied, but this is only successful if all components are known [34].
In spark optical emission spectrometry (SOES) [35–37] part of the material from a metal sample is vaporized with an electric spark. It is further atomized and ionized. The emission during the excitation that takes place is used for quantification. SOES is a fast method to check the composition of metals [38]. Particularly compact instruments are in use as mobile spectrometers.
Laser-induced plasma spectrometry ([LIPS], also laser-induced breakdown spectroscopy [LIBS]) [39], is a relatively new technique of solid sampling [40]. The irradiation of a solid sample with a laser [41, 42] will cause an immediate transfer of the material into the plasma phase. The radiation emitted from this plasma is used directly for quantification. Since the duration of such an emission signal is very short [43], array spectrometers are typically used in LIPS. Lasers operating in the UV range are preferred since the absorption of lower wavelengths is better than that of wavelengths in the visible or infrared region [44]. Both the radiation of the laser to the sample for the generation of the plasma and the radiation emitted can be transferred by fiber optics. Therefore, this technique is particularly attractive for the direct online analysis of unapproachable sample material, such as that in nuclear plants [45] or the Mars exploration [46].
The irradiation of a sample with X-rays triggers the fluorescence of the atoms and ions. Here the lower electron shells (K and L) are excited. The fluorescence emitted is used for qualitative and quantitative analysis in X-ray fluorescence (XRF) [47]. In wavelength-dispersive XRF, several dispersive crystals are used to cover the frequency range. In energy-dispersive instruments, the separation is carried out in the detector, which converts the different energies of radiation into electric current or voltage. XRF is used successfully for the analysis of solid samples, especially for large number of samples with small matrix composition changes. The determination of light elements is problematical in XRF. Mutual interference of the elements requires calibration with well matrix-matched standards and advanced processing of intermediate results.
In total-reflection X-ray fluorescencespectrometry (TXRF) [48], a liquid sample is placed on a quartz plate and dried. This plate is then placed in the beam of the X-ray at a very flat angle so that it is totally reflected. Directly above and normal to this primary beam, a detector (Si [Li]) measures the fluorescence. TXRF is particularly suitable for determining elements with an atomic number of >11 in very low concentrations in small volumes.
1.5 Terms
The element to be determined is called the analyte. Its concentration can be in different ranges. If its concentration in the sample is at least 10%, then it is a main component. If its concentration is between 10% and 0.01% it is a minor component. A component with a lower concentration is called a trace. The accuracy and reproducibility normally depend on the magnitude of the concentration to be analyzed. As a rule, only very small deviations are tolerated for main and minor components (typically up to about 1%), while in trace analysis greater tolerances (up to 10% and sometimes even more) are accepted.
The tolerances consist of a statistical part, the reproducibility of the measurement, which is usually given as standard deviation of the measurement (e.g. intensity in c/s) or in concentration units of the transformed result (e.g. concentration in mg/kg). The reproducibility is frequently also termed the precision, although the term is sometimes used in a different context encompassing accuracy and reproducibility. Furthermore, error tolerances include the deviation from the true value, which is referred to as the accuracy. It can be determined only on synthetic samples, since true values for real samples cannot be known. However, some well-characterized samples, which were examined by a number of renowned laboratories and evaluated following strict statistical rules, the standard reference materials (SRMs), exist. The concentration values given in the certificates of these SRMs are generally accepted as “true” values. Quite frequently, the deviations of the measurements from those of the SRMs are used for the determination of the accuracy.
In many cases, the analysis is influenced by the sample components. The sum of all the components of a sample is called the matrix. The matrix components may cause an analytical error. This is called interference. Since analytical errors are not only linked to the measurement with the ICP instrument itself but are also caused by the complete analytical process, which includes sampling, storage, pretreatment, and measurement, to name the most important ones, the overall error of the analysis is greater than the measurement tolerances. This concept leads to the idea of the uncertainty. The determination of this quantity is quite complex, so that it is very rarely carried out.
The sensitivity is the slope of the calibration function, or expressed mathematically, the first derivative of the calibration function [49].
Although not permitted in any standard, the concentration descriptor “parts per million” (ppm) still is quite widespread. Its main disadvantage is that this “unit” does not unambiguously describe whether the concentration refers to particles, mass, or volume. Therefore, units such as “mg/L” or “mg/kg” are to be preferred.
2Plasma
Plasma is an ionized gas. It is sometimes referred to as the “fourth” state of aggregation besides the states of aggregation, solid, liquid, and gas [50]. The states of aggregation can be differentiated by their order at the molecular or atomic level. A phase change is always associated with a qualitative change. As the temperature rises – and particularly at every phase change – the mobility of the particles increases and the order decreases. Plasma therefore exhibits the greatest amount of “disorder” (entropy); independent motion of electrons and ions characterizes this state.
People refer to the plasma “burning.” How can a noble gas like argon burn?
When argon gas forms a plasma it changes its state of aggregation. This is clearly a physical event and is unlike a chemical reaction in which a gas burns in the presence of oxygen. The “burning” terminology is nevertheless quite widespread – one speaks about a “burning” plasma in a “torch.” Although not quite correct, this terminology has become established and its meaning is correctly understood in the context of inductively-coupled plasma optical emission spectrometry (ICP-OES).
2.1 The Spectrometric Plasma
To sustain the plasma, argon is used in most cases as the operating gas. Positively charged argon ions and negatively charged electrons move independently. The motion of the charged particles (Ar+ and e−) follows the acceleration, which is driven by an alternating electromagnetic field. Neutral particles become charged by collisions with the loaded particles and are then accelerated [51].
The transfer of energy in the inductively coupled plasma by the alternating field with an induction coil functions similarly to that in an electric transformer. In this analogy, the primary coil would be the induction coil; the secondary coil contains a current that corresponds to the core of the plasma itself. Typically, the operating frequency is 27 or 40 MHz. A homogeneous magnetic field is important for the optimal coupling efficiency to the plasma.
The plasma is shaped like a toroid. This form results from the torch geometry, the gas flows, and the energy transfer (given by induction coil form, RF power, and excitation frequency). This “reversed heart form” is especially suitable to “inject” a gas flow that contains the solution nebulized as an aerosol into the plasma. The carrier gas loaded with the aerosol then forms the so-called analyte channel. The mean dwell time of the sample in the plasma is of the order of a few milliseconds.
Figure 2.1 Temperature distribution in the argon plasma [±140 K] maintained with an RF power of 1200 W.
Source: Plasma from PerkinElmer Instruments and Temperature from Sperling [52].
The temperature of the plasma is not uniform (Figure 2.1). It is hottest in the ring-shaped zone inside the induction coil to which the energy of the coil is coupled. Here the plasma reaches temperatures of about 10 000 K. The aerosol-loaded carrier gas is introduced into the center of the plasma ring. From this ring, the aerosol obtains energy. As a result, a temperature gradient exists from the outside to the inside. The carrier gas, which forms the analyte channel, takes the energy and reaches its temperature maximum just after the plasma ring. After the sample has passed through this segment of the plasma, no more energy is supplied to it, so the energy is radiated and the temperature decreases.
Why do I have to turn on the exhaust before igniting the plasma?
Nitric oxides and ozone are formed from the nitrogen and oxygen of the air entering into the plasma. Turbulence enhances this effect, which produces small amounts of these harmful gases, which must be removed from the ambient air in order to avoid health hazards affecting laboratory personnel.
In addition, the exhaust also carries away part of the waste heat.
2.1.1 The Operating Gas
2.1.1.1 Argon
In commercial inductively-coupled plasma (ICP) instruments, argon is mainly used. As a noble gas with a relatively high atomic number, its electron shell can be easily polarized. Hence, electrons can be more easily released in comparison with noble gases of lower atomic number (Table 2.1) and molecular gases. In order to ionize molecular gases such as nitrogen, much more energy is necessary. In the early days of ICP, some instruments worked with plasmas using molecular gases such as nitrogen or air. As the cost of the operating gases (e.g. air) decreases, the cost of constructing the RF generator clearly increases. There appears to be no evidence that the analytical performance is better with these systems [53]. Therefore, this concept was not pursued in commercial instruments. Argon is the most common of the “higher” noble gases and thus relatively inexpensive.
Table 2.1 Characteristic data for the noble gases.
Source: From [54–56].
Element
Atomic number
Ionization energy (eV)
Concentration in the atmosphere (ppm)
Typical cost of a 50 L 200 bar bottle (€)
He
2
24.59
5.2
500
Ne
10
21.47
18.2
6000
Ar
18
15.68
9 340
70
Kr
36
13.93
1.1
24 000
Xe
54
12.08
0.09
200 000
Other countries – other customs
During training courses for participants from the former Union of the Socialist Soviet Republics (USSR) and its successor states, I noticed that the instrument operators were almost without exception strong men, while the laboratory management lay in the hands of women. On asking for the reason, I got the surprising answer: “Men are strong enough to carry the heavy gas bottles.”
Apart from any sports aspect, one should avoid handling heavy argon gas bottles as much as possible because there is an inherent accident risk involved. A central gas supply station with automatic switchover if the gas bottle (bundle) becomes empty should be part of the standard ICP laboratory. If the laboratory has a regularly high consumption of argon, liquefied gas is the preferred option, as typically not only are the costs lower but the quality is also better. A delivery service, which supplies gas at user-specific intervals, is the most convenient way of obtaining the necessary operating gas for the ICP instrument [57].
2.1.1.2 Addition of Air or Oxygen
Small amounts of air or oxygen may destabilize the argon plasma to the extent that the precision deteriorates dramatically or the plasma may even extinguish. On the contrary, extremely small amounts may be beneficial to avoid clogging of the injector due to carbon buildup when aspirating organic material. Also, the pyrolysis of organic molecules will produce C2 fragments, which will cause an extensive pattern of molecular emission bands mainly in the visible region (Swan bands), which may severely interfere with the analysis. The addition of a small volume percent of air or oxygen to the nebulizer gas will avoid the clogging of the injector due to soot and the molecular bands will be greatly reduced so they will not interfere with the analysis. More details can be found in Section 7.3.7.
2.1.2 Plasma Torch
The coupling of the energy to the plasma is carried out with an induction coil. The primary circuit consists of the electrical current in the induction coil, while the secondary current consists of argon ions and electrons in the plasma core.
In order to separate both electrical currents, an isolator is required. Typically, quartz is used as material for a tube between both currents. This is the outer tube of three concentric tubes making up the ICP torch. Recently, ceramics have also been used for this purpose. These are more expensive but last a lot longer than quartz tubes.
Because of the skin effect, the charges are found on the outside in a high-frequency alternating field. As a result, the secondary current, which forms the plasma, extends in the direction of the induction coil. The protection of the induction coil is the second function of the outer tube, which shields it from the plasma.
However, the tube must be cooled in order to prevent its melting. This is achieved by a strong argon gas flow, which is directed tangentially to the inner surface of the tube. Since other tubes are included in the construction of the plasma torch, this one is referred to as the outer tube[58] (Figure 2.2). Its diameter is typically 20 mm.
Figure 2.2 A schematic drawing of a plasma torch. The torch has an extended outer tube. The short torch variation ends just behind the induction coil.
Source: Based on drawing by Perkin Elmer.
In many cases, this outer tube ends directly above the induction coil. Some torches have an extended outer tube with one or more slits cut into it pointing in the direction of the optics. The advantage of this extension is to keep ambient air away from the plasma [59, 60]. This clearly leads to a lower rate of formation of harmful gases (ozone and nitric oxides). The analytical advantage is that in the absence of nitric oxides, their molecular spectra do not appear. Otherwise, they appear as a strongly structured background in the wavelength region around 230 nm.
When should I replace the quartz torch?
If the inner surface of the outer tube becomes rough, then the coolant cannot cool it satisfactorily any longer. In the worst case, the outer tube can melt. It is recommended to replace the torch before this happens because a melted outer tube can also damage the induction coil, causing even greater harm.
The gas for cooling the outer tube is called the outer gas or coolant. This gas also maintains the plasma, so it is frequently also referred to as plasma gas. The coolant or plasma gas flows tangentially into the torch along the outer tube to cool it as efficiently as possible. The plasma gas flows are between 10 and 20 L/min with a typical flow of 12–15 L/min.
The sample is carried as an aerosol after nebulization in a central tube until just before it enters the plasma. This tube is called the injector (tube) or the inner tube. The inner gas is also referred to as the carrier gas or nebulizer gas. Typical carrier gas flows range from 0.6 to 1 L/min, possibly 0.3–2 L/min. The speed of the carrier gas flow has an influence on the residence time of the aerosol in the plasma. The longer the dwell time of the sample aerosol in the plasma, the more energy can be absorbed from the plasma, leading to a higher excitation temperature. Consequently, as low a carrier gas flow as possible is preferred. However, there is a limit to reducing the flow. A minimum momentum is necessary for the aerosol to penetrate into the plasma. Therefore, extremely small gas flows cannot be realized. The inside diameter of the injector is typically between 0.8 and 2 mm.
The injector is frequently made from quartz as is the case with the other parts of the torch. However, if hydrofluoric acids are in the measuring solution, HF-resistant materials must be used. Frequently, aluminum oxide is utilized.
Another tube is located between the two tubes: the intermediate tube. This has a typical diameter of 16 mm and serves two purposes: it forces the coolant gas to flow tangentially along the outer tube until shortly before the plasma; it also offers the possibility of introducing another gas flow: the intermediate or auxiliary gas flow. Its task is to push the plasma away from the injector tip if necessary. This is important for solutions with a high concentration of dissolved matter (particularly critical just below the solubility limit of the compound). If the injector tip gets too hot, then the solution dries up and solid particles are deposited at this point. In the course of time, the tip of the injector clogs. In addition, if organic solvents are introduced into the plasma, an injector tip that is too hot is unfavorable because the solvents can pyrolyze and form carbon deposits, which also may clog the injector tip. As a rule, the auxiliary gas flow is in the range between 0 and 2 L/min.
At increasing distances from the region where they meet, the gas flows become progressively better mixed [61].
In some torch designs, the intermediate tube is widened shortly before the region where the plasma burns. This is the so-called tulip-shaped torch (Figure 2.3) [62]. The widened intermediate tube forces the coolant immediately before the plasma to an accelerated flow, which results in an even more intensive cooling.
Figure 2.3 The drawing shows a tulip-shaped torch. In this torch model, the middle tube is widened immediately before the plasma. The purpose is to accelerate the coolant flow so that it cools the outer tube more efficiently.
Source: Glass Expansion.
Figure 2.4 As the distance between the injector and the bottom of the induction coil increases, so does the sensitivity using radial as well as axial observation.
Reproduced by kind permission of PerkinElmer, Inc.
It is important that all three tubes are orientated concentrically to each other and that the torch is installed centrally in the induction coil. The distance of the injector and the intermediate tube to the induction coil is also important. In any case, the manufacturer's recommendations should be followed.
As the distance between injector and induction coil increases, the sensitivity of the analytical lines also rises (Figure 2.4). However, stability decreases at the same time. At a certain distance further away, the plasma will not ignite.
Besides the typical dimensions of the torch and the gas consumption associated with it, a torch with smaller dimensions and a gas consumption of about 30% less [63–65] was used in some instruments, several years ago. Since the gas consumption is a relatively large factor in the running costs, reduction of argon use has become an issue also during recent times. Especially for the plasma gas, some manufactures report, for example, that by changing the design of the induction coil (Section 2.3.2) the consumption of plasma gas, even for difficult matrices such as soil, has been reduced to 8 L/min [66].
