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Microwave Induced Plasma Analytical Spectrometry
By Krzysztof J. Jankowski, Edward Reszke The Royal Society of Chemistry
Copyright © 2011 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-1-84973-214-7
CHAPTER 1
An Introduction to Microwave Plasma Spectrometries
1.1 Introduction
Plasmas generated by the interaction of electromagnetic fields with gases such as argon or helium were introduced in the middle of the 20th century as very promising media for atomic excitation. Microwave plasmas (MWPs) are included in this new generation of spectrochemical sources, which in the last 30 years have considerably widened the possibilities of trace analysis and speciation studies. MWPs operating at a GHz frequency have been especially used as emission sources for optical emission spectrometry (OES) and, later, also as ionization sources for mass spectrometry (MS). The microwave energy is coupled to the gas stream passing through the torch with an external cavity or antenna.
Generally, two groups of MWPs are mentioned according to the method of power transmission to the plasma gas, as well as the plasma shape and its position against the plasma torch. A flame-like plasma formed at the tip of the electrode, first developed by Cobine and Wilbur in 1951, is commonly known as a capacitively coupled microwave plasma (CMP). A CMP torch is equipped with an inner conductor that forms a capacitance against the ground and transfers the microwave energy into the plasma gas through its tip. In the second type, the plasma is produced through the inductive transfer of energy from standing waves in a suitable resonator and sustained in a quartz or ceramic tube which is located within a resonant cavity. This electrodeless system is commonly referred to as a microwave induced plasma (MIP) and is the most successful and commonly used type of microwave discharge. The first application to spectrochemical analysis with the use of MIPs was published by Broida and coworkers in 1952. The microwave energy is coupled either by an electrical field (E coupling) or a magnetic field (H coupling) to the working gas. However, such a classification has no strict scientific foundation. A more advanced one will be given in Chapter 3.
1.1.1 Historical Background
Historically, the development of CMP and MIP was in parallel, owing to some essential differences in technical design and operation of these two microwave approaches. One of these differences is related to the operating frequency. CMPs can operate over a wide range of frequencies and a great number of publications is devoted to radiofrequency plasmas. An individual approach can be relatively easily tuned to different frequencies. Resonant cavities for sustaining a MIP are dedicated to one operating frequency, mostly 2.45 GHz, and sometimes one kind of plasma gas. A more detailed comparison of CMP and MIP will be given below.
In the 1960s, analytical applications of the CMP were focused on the elemental analysis of solutions and as a result a commercial CMP instrument was developed by Murayama et al. at the Hitachi Central Research Centre in 1968. In the next decade, two commercial spectrometers were explored [Hitachi 300 UHF Plasma Scan and the Applied Research Laboratories (ARL) Model 31000]. However, these devices did not compare favourably with the inductively coupled plasma (ICP) method due to severe inter-element effects.
In 1985, a renaissance of CMPs appeared thanks to Jin and others, who developed the so-called the microwave plasma torch (MPT). The plasma operation was significantly improved and it offered a much better analytical performance for the introduction of aqueous aerosols. In the 1990s, various sample introduction methods as well as spectroscopic techniques based on the MPT were successfully introduced. Finally, in 1999 a commercialized MPT-OES instrument (JXY-1010 MPT) was introduced in China. Other designs used for microwave-powered CMPs over time have included the "torch à injection axiale" (TIA).
Until the mid-1970s, the MIP discharge was obtained almost exclusively in gases under reduced pressure. Because of this, this period of MIP technique development is often recognized as the low-pressure MIP era. A number of resonant cavities were constructed and examined at a wide pressure range (1–760 Torr), including foreshortened cavities and a tapered rectangular cavity. When low-pressure discharges were used, analyte introduction was performed predominantly by gas chromatography, electrothermal vaporization or chemical vapour generation, owing to the difficulty in sustaining the plasma and its relatively low loading immunity. These difficulties caused part of the analytical spectrometry community to approach (and still approach) the MIP technique with reserve. On the other hand, the focusing of this early work on applications in which small amounts of sample were delivered to the plasma in the gas phase led to the fast development of the MIP technique with regard to its application to gas chromatographic detection. The first successful analytical application of the MIP method seems to have been the analysis of nitrogen isotopes by Broida and Chapman in 1958. McCormack et al. developed the first element-selective gas chromatography (GC) detector based on MIP emission spectrometry in 1965. The most successful commercial microwave plasma detector (MPD) used in GC systems was introduced by Quimby and Sullivan in 1990.
A breakthrough in the development of the MIP technique was the designing by Beenakker in 1976 of the TM010 resonator, in which the plasma discharge could be obtained under atmospheric pressure. This is often recognized as the end of the low-pressure MIP era and the beginning of the atmospheric pressure MIP era. However, the essential improvement introduced by Beenakker was not the sustaining of the plasma at atmospheric pressure (some successful studies were reported previously), but the introduction of symmetrical coupling and improved transfer of electrical energy that allowed plasma stability. At the same time, in 1975, Moisan et al. introduced a different microwave structure with symmetrical coupling based on surface wave propagation, called the "surfatron".
In next years, various types of resonators were devised, permitting the achievement of high discharge stability and excitation efficiency. Improved versions of Beenakker's cavity, surfatrons and strip-line sources allowed a stable discharge to be obtained over a wide range of plasma gas flow rates and microwave power levels. Designs permitting the use of moderate and high power were worked out to increase the discharge energy. Finally, the vertically positioned, aerosol-cooled MIP system based on the TE101 integrated cavity for OES was proposed by Jankowski et al.
Microwave plasma analytical spectrometry (MWP-AS) has gained new importance for trace analysis since 1981, when Douglas and Frech applied the plasma as an ionization source for MS. In 1990, Okamoto developed a surface-wave-excited non-resonant cavity for MIP. This high-power source was applied firstly for OES and secondly, and more importantly, for MS. In 1994, a nitrogen MIP-MS spectrometer (Hitachi P-6000) appeared on the market. An annular-shaped helium plasma was obtained by Okamoto in 1999 for improving the analytical characteristics of non-metals. Recently, this type of plasma geometry, obtained in a transverse electromagnetic mode (TEM mode) resonant cavity at a helium flow rate below 3 L min-1 and at a low power level, has been reported by Jankowski et al. Even more recently, a special three-phase microwave power source, applying non-stationary fields which are rotating around the plasma axis causing a flat planar plasma geometry with a triangular-shaped annular centre, was announced by the same group as a promising method for analytical spectrometry.
In the last decade, a number of interesting fields for applying MWP-AS have appeared. In 1995 the particle-sizing instrument (Yokogawa, PT1000) based on a helium MIP was manufactured (see Chapter 12). It offers the possibility of the determination of both the chemical composition as well as the size and basic physical structure of micro- and nanoparticles. The second hot topic seems to be the design and application of microplasma sources in miniaturized analytical systems. In 2000, Engel et al. proposed an MIP-based microstrip device, taking advantage of the ability to form so-called cold plasmas using microwaves.
The development of MWP instrumentation and analytical applications has been regularly reviewed and some references can be recommended. The milestones in MIP and CMP development are summarized in Table 1.1.
1.1.2 The Present Status of Microwave Plasma Spectrometry
MWPs have evolved considerably over the past 15 years as an excitation source for OES and as an ionization source for MS. At present, MWPs can be produced under a large variety of operating conditions, and the devices that are now available allow the production of stable and reproducible plasmas. Several common forms of this plasma source exist, including the low- and high-power MIP, the CMP, the surface-wave plasma and the MPT. On the other hand, this variety of designs causes a "hotchpotch" and breeds reservation to this technique among many analytical spectroscopists. There is a need to give a comprehensive theory and practice of the field for a better understanding of the basic science of MWP-based techniques.
Summarizing their present status in terms of spectroanalytical applications, MWPs have found a consistent need in plasma spectrometry. A specific excitation mechanism permitting the determination of many metals and non-metals with good sensitivity and low running costs for the apparatus are advantages of this technique. MWPs have found openings in various fields of chemical analysis. Gas chromatography microwave induced plasma optical emission spectrometry (GC-MIP-OES) holds a prominent position as the hyphenated technique for speciation and metallomics. Other spectacular applications, such as continuous emission monitoring, particle sizing, microanalytics and soft fragmentation of molecules, as well as tandem sources, continue to receive the attention of spectroscopists. There is no doubt that an increasing interest is observed in the benefits which can be gained from the use of the MWP method.
With regard to commercial acceptance of the technique, progress can be observed in the past 15 years. Five instruments have appeared on the market and some others have been announced as a preserial. However, there is still a lack of apparatus used worldwide, excluding MPDs. The appearance of such a spectrometer would undoubtedly cause rapid development of this technique. Unfortunately, despite many obvious assets of MWPs, until now no adequate interest has been shown by analysts or by apparatus manufacturers. The authors are aware of this, since they themselves participated in the design and commercialization of an MIP-OES (Analab, MIP 750 MV), produced in the 1980s.
1.2 Energy Flow between Microwave Plasma and Analyte
1.2.1 Microwave Power Absorption by the Plasma
There are several ways by which microwave energy can be absorbed by a plasma. In general, the energy is transmitted to the plasma via the microwave electric field, which accelerates the electrons until they have sufficient energy to cause further ionization in a chain reaction. The electrons are the only ones to follow the oscillations of the electric field. On the other hand, an electron is able to gain energy from the field only if its ordered oscillatory motion is changed by collision with a plasma gas atom. The electric field may be applied within a resonant cavity or microwaves may be guided along the plasma column. In oscillating fields the electron gains energy from the field and, in addition, receives further energy from the field as a result of elastic collisions. However, the electron can also dissipate its energy through a great number of elastic and inelastic collisions with neutral and ionic species, causing excitation and ionization processes. At low discharge pressures (up to 50 Torr), a heavy particle will experience approximately 107 collisions before being excited. At atmospheric pressure the collision frequency is so high that an increase of microwave power input is required to ensure sufficient ionization of the plasma gas. The electron acceleration and collision energy exchange mechanisms in high-frequency fields have been discussed in detail by Brown.
The mean power absorbed by electron from the field is given as follows:
P = (e2E2max/2mv) (v2/v2 + ω2) (1.1)
where e and m are the charge and mass of the electron, respectively, Emax is the maximum field amplitude (strength), v is the collision frequency between the electron and the gaseous atoms and ω is the field frequency. As one can deduce from eqn (1.1), only very limited energy can be absorbed directly by the plasma gas ionized species, considering their considerable mass and the brief time period before the field reverses polarity.
A distinctive feature of a MWP is that free electrons gain kinetic energy from the microwave all over the discharge. The whole discharge remains in ionizing mode, i.e. free electrons are underpopulated almost everywhere. This means that plasma heating occurs more homogeneously, and no plasma decay will take place. On the other hand, the presence of the microwave electric field induces plasma filamentation and makes the power transfer to the discharge limited by the skin effect. This causes both plasma inhomogenity and a radial gradient of the gas temperature; as a consequence there is limited penetration of the sample into the plasma. The plasma inhomogenity varies inversely as the thermal conductivity of the gas and is more pronounced in argon then in helium.
1.2.2 Plasma–Sample Interaction
When the sample is introduced to the plasma in the form of a wet aerosol, an individual droplet at first undergoes desolvation at high temperature. The resulting microscopic salt particles (called "dry aerosol") explode and vaporize into a gas of individual molecules ("molecular vapour") that are then dissociated into atoms (atomization). These processes, which occur very fast in a peripheral plasma zone, are exactly the same as those taking place in flames and other plasma sources.
However, for conventional MIP sources the plasma is sustained inside the torch, forming a ?lament located at the axis surrounded by a low-temperature zone. In this case, the sample may partially travel around the outside of the discharge, where it experiences a lower temperature. As a result, the sample is not homogeneously mixed with the plasma. This may lead to more severe matrix effects and poorer plasma stability. The degree of lateral diffusion of the aerosol into the central plasma zone depends on the plasma operating parameters, including plasma gas properties, as well as sample composition. Moreover, the aerosol passing around the discharge interferes with the transfer of the energy and sustainment of the discharge. To alleviate these problems, in modern MIP sources some improvement has been made concerning microwave coupling, cavity and torch construction and application of a higher power level. The other possibility is to arrange the sample introduction through the central channel of the plasma, similarly to the ICP method. Previously, this was realized with the use of tangential torches and, recently, by a more advanced way in the MPT, Okamoto cavity and TEM-based MWP sources.
The other important factor that is largely responsible for the efficiency of the above-mentioned processes is the residence time of the analyte particles in the plasma. This depends on the plasma height and linear velocity of the gas stream and varies considerably for different MWPs. For annular-shaped microwave plasmas, mentioned above, it is similar to that for ICP spectrometry (approximately 2–3 ms). However, for filament-type MIPs it could be comparatively long (about 10 ms) due to the much lower gas flow rates used. This is beneficial for the sample–plasma interaction and compensates in part the above discussed limitations of the lateral diffusion of the aerosol into the plasma.
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Excerpted from Microwave Induced Plasma Analytical Spectrometry by Krzysztof J. Jankowski, Edward Reszke. Copyright © 2011 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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