Instrumentation QUARTZ TUBE
Inductively
INDUCTION COIL
Coupled Plasmas Velmer A. Fassel and Richard N. Kniseley
ARGON GAS
Ames Laboratory, USAEC and Department of Chemistry
Figure 1 . Inductively c o u p l e d p l a s m a m o d e l
Iowa State University, Ames, Iowa 50010
In the accompanying REPORT (1), we discussed the analytical perfor mance of a promising new analytical system for the simultaneous determi nation of trace elements at the frac tional microgram to nanogram level. This system is based on the observa tion of atomic emission spectra when samples in the form of an. aerosol, thermally generated vapor, or powder are injected into an inductively cou pled plasma atomization and excita tion source. In this article we discuss the formation and stabilization of these plasmas and their unique prop erties and characteristics that make them very promising atomization-excitation sources. Formation and Stabilization of Inductively Coupled Plasma To understand the nature of these plasmas, it is essential to remember three simple facts: by definition, plas mas are gases in which a significant fraction of their atoms or molecules is ionized; that being so, magnetic fields may readily interact with plasmas; and one of these interactions is an in ductive coupling of time-varying mag netic fields with the plasma, analogous
to the inductive heating of a metal cyl inder. Referring to Figure 1, suppose we take a quartz tube approximately 2.5 cm diameter and place it inside a coil connected to a high-frequency genera tor operating typically in the 4-50MHz range at generator output levels of 2-5 kW. When the generator power is turned on, nothing happens—Ar is a nonconductor. To form a plasma it is necessary to plant a "seed" of elec trons in the coil space; the simplest way to do this is to "tickle" the tube with a Tesla coil. Given the proper flow pattern of Ar inside the tube, the plasma is formed spontaneously after a few popping flashes at the open end of the tube. Let us now examine the course of events leading to the formation of the plasma. The high-frequency currents flowing in the induction coil generate oscillating magnetic fields whose lines of force are axially oriented inside the quartz tube and follow elliptical closed paths outside the coil as shown sche matically in Figure 2. The induced axial magnetic fields, in turn, induce the seed of electrons and ions to flow in closed annular paths inside the
quartz tube space. This electron flow— the eddy current—is analogous in be havior to the current flow in a shortcircuited secondary of a transformer. If we recall that the induced magnetic fields are time-varying in their direc tion and strength, then we can appre ciate the fact that the electrons are ac celerated on each half cycle. The ac celerated electrons (and ions) meet re sistance to their flow, Joule or ohmic heating is a natural consequence, and additional ionization occurs. The steps just discussed lead to the almost in stantaneous formation of a plasma of extended dimensions whose unique properties and characteristics make it a very promising excitation source. Thermal Isolation of Plasma The plasma formed in this way at tains a gas temperature in the 900010,000°Κ range in the region of maxi mum eddy current flow (2). At these temperatures it is desirable to provide some thermal isolation of the plasma to prevent overheating of the quartz containment cylinder. This isolation is achieved by Reed's vortex stabiliza tion technique (3, 4), which utilizes a flow of argon that is introduced tan-
ANALYTICAL CHEMISTRY, VOL. 46, NO. 13, NOVEMBER 1974 ·
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gentially in the manner shown in Figure 3. The complete assembly of concentric quartz tubes and argon flow patterns for sustaining the plasma after it is formed is also shown in the figure. The tangential flow of argon, which is typically in the 10 l./min range for the apparatus shown, streams upward, cooling the inside walls of the outermost quartz tube and centering the plasma radially in the tube. The plasma itself is anchored near the exit end of the concentric tube arrangement. In addition to the vortex stabilization flow of Ar, there is another lower velocity flow of approximately 1 l./min that transports the sample to the plasma either as an aerosol, a powder, or a thermally generated vapor. The total Ar flow required is therefore ~ 1 1 1 . / min. Thus, the operating cost of these plasmas, exclusive of electrical power, is approximately 50% lower than for a nitrous oxide-acetylene flame. Sample Injection into Plasma
If these plasmas are to be used as efficient free-atom generators, the sample should be effectively injected into the plasma and remain in the interior high-temperature environment as long as possible. This physical condition has been difficult or impossible to attain in many plasma systems suggested for analytical purposes, and the ICP presents a problem as well (5). In the ICP the gases are heated internal-
the flow velocity of the carrier gas that injects the sample into the plasma. Thus, the degree to which the annular or toroidal shape is developed can be controlled by the frequency of the primary current generator and the flow velocity of the carrier gas stream. At ~30 MHz, a carrier gas flow of ~ 1 1 . / min assures effective injection of sample into the plasma, if properly designed injection orifices are used. The entire torch configuration now in common use in our laboratories is shown in Figure 5. Although efficient sample injection is achieved, the analyte does not experience the hottest part of the plasma. As the sample particles travel upstream in a rather narrow axial channel, they avoid the high-temperature doughnut, i.e., the region of maximum eddy current flow shown schematically by crosshatching in Figure 3. According to our preliminary temperature measurements, the gas temperature in the axial channel of the eddy current flow region is ~7000°K. This Figure 2. Magnetic fields temperature is a factor of two higher and eddy currents genthan those achieved in the hottest erated by induction coil commonly used combustion flames. Typical residence times of the sample in the plasma before the observation height is reached are about 2.5 msec. The combination of high temperatures and relatively long particle-plasma inly, causing them to be accelerated in a teraction times should lead to comdirection perpendicular to the exterior plete solute vaporization and a high, if surface of the plasma. The sample injection process must therefore overcome the expansion thrust of the hot gases without causing the plasma to collapse. Figure 3. Complete plasma configuraThe skin-depth effect of induction tion heating may be turned to good advantage in solving this problem. If the plasma itself is generated at lower frequencies (~5 MHz), a teardropshaped plasma, as shown on the left portion of Figure 4, tends to be formed. Sample material that approaches the plasma tends to follow a rather disconcerting path around the outside surface. As shown in highly schematic form on the right-hand portion of Figure 4, an increase in the oscillating frequency of the power source causes the eddy current to flow more closely to the outer or skin portions of the plasma. This results from the wellknown skin-depth effect, i.e., the depth at which the eddy current declines to 1/e of its maximum value near the surface is proportional to the square root of the current frequency. In this way, an incipient annular plasma shape is developed. Viewed from the bottom, the plasma has the appearance of a doughnut. Since the hole possesses a somewhat lower temperature than the doughnut, it offers less resistance to the injection of sample material. The annular shape can be further developed by optimizing
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not total, degree of atomization of the analyte species in the core of the plasma. Once the free atoms are formed, they occur in a chemically inert environment, as opposed to the violently reactive surroundings in combustion flames. Thus, their lifetime, on the average, should be longer than in flames. Because the quenching cross section of Ar is far less than for the molecular species found in combustion flames, these plasmas should provide a more favorable environment for attaining improved atomic fluorescence power yields. The plasma system described above possesses other unique advantages. First, after the free atoms are formed, they flow upstream in a narrow cylindrical radiating channel. The optical aperture or viewing field of conventional spectrometers can be readily filled by this narrow radiating channel. In this way, the radiating free atoms are used most effectively. Second, at the normal height of observation, the central axial channel containing the relatively high number density of analyte free atoms has a rather uniform temperature profile. The number density of free atoms in the hot argon sheath surrounding the axial channel is far lower. Under these conditions, the analyte free atoms tend to behave as an optically thin emitting source. If a large range of emission intensities can be accommodated linearly by the measurement system, linear analytical calibration curves covering five orders
of magnitude change in concentration can be readily achieved. Thus, the variable dilutions often needed in flame spectroscopic techniques are essentially eliminated. Other advantages of these plasmas are worthy of note. No electrodes are used. Therefore, contamination from the self-electrodes normally used in other plasmas is eliminated. Because the plasma operates on nonexplosive noble gases, the systems can be used in locations where combustibles are not allowed. Description of Spectra Emitted The plasma has the overall appearance of a very intense, brilliant white, nontransparent core and a flame-like tail. The plasma core, which resides inside and extends a few millimeters above the induction coil region, emits an intense continuum in addition to a rather fully developed spectrum of neutral argon. The continuum presumably arises from ion recombination processes and bremsstrahlen emission. Because of the high continuum emission, radiation from the plasma core has little analytical utility. The core fades into a second recognizable zone of the plasma which extends from approximately 1 to 3 cm above the induction coil. This zone is also bright but slightly transparent. In the mid-to-upper regions of this zone, the continuum emission is sharply reduced by several orders of magnitude from the core emission. Structural
Figure 5. Plasma tube dimensions
background consists of Ar lines, the OH band emission between 260-325 nm, and weak band emission from NO, NH, CN, and C 2 . In general, the highest signal/noise ratios for analyte species are observed in the second zone. The tail flame or third zone of the plasma is barely visible when distilled water is nebulized but assumes typical flame colors when analytes are added to the plasma. The axial passage of the sample aerosol and its decomposition products through the plasma is clearly visible. The above description of the emission properties of the ICP highlights another one of its distinctive advantages, namely, solute vaporization and atomization occur in the high-temperature environment of the core of the plasma, which otherwise has little utility as an emitting source for analytical purposes. The free atoms released in the core may then be observed upstream in temperature environments ranging downward from 7000°K to typical combustion flame temperatures. Typical Experimental Facilities
Figure 4. Sample particle paths for several plasma shapes 1162 A · ANALYTICAL CHEMISTRY, VOL. 46, NO. 13, NOVEMBER
1974
Because of the relatively simple spectra emitted above the plasma core, low-cost, table-model spectrometers provide acceptable spectral resolution. A typical system for the sequential determination of all of the metals and metalloids is shown in Figure 6. A complete description of this
