Still the Panacea for Trace Metals Analysis?
Gerhard A. Meyer The Dow Chemical Company Michigan Applied Science and Technology Labs 1602 Building Midland, Mich. 48674
Of all the various plasma devices used for elemental analysis during the past 25 years, the inductively coupled plasma (ICP) has had the most significant impact on the field of atomic spectroscopy. Other plasmas, such as the direct-current plasma (DCP), the microwave-induced plasma (MIP), and afterglow discharges, have been found to be useful for spectrochemical analysis; however, the ICP appears to be the primary source used for assay and tracelevel metals analysis. Plasmas sustained at radio frequencies (rf) have been a familiar tool used by physicists and engineers since the turn of the century. In the early 1900s, the plasmas could only be sustained at reduced pressures; however, by the 1940s it was possible to sustain plasmas at atmospheric pressure using induction heating techniques. The closest ancestor to the modern ICP first appeared in the early 1960s with the stabilization of an rf plasma operated at atmospheric pressure in an open-ended tube with flowing argon. This device displayed certain advantages for spectrochemical analysis, although it was originally designed for growing highly purified crystals. After 1965, the development of the ICP as an analytical tool for atomic spectroscopy progressed rapidly. Scientists in Great Britain and the United States realized independently the advantages of the ICP for the spectrochemical analysis of liquid samples. Numerous accounts appeared on applications and hardware, including descriptions 0003-2700/87/A359-1345/$01.50/0 © 1987 American Chemical Society
INSTRUMENTATION showing bulky rf generators and impedance matching circuits coupled with elaborate gas delivery manifolds. Plasmas generated with this equipment typically required 1-15 kW of rf power to be sustained. Better detection limits were obtained eventually, and improved operating conditions together with wavelength tables unique to argon ICPs enhanced the usefulness of this source for spectrochemical analysis. These developments led to the availability of the first commercial ICP in the early 1970s. Today there are ap-
proximately 30 different manufacturers of ICP instruments and more than 6000 installed units. The maturity of the ICP technology can best be assessed using H. A. Laitinen's scale of t h e common phases through which an analytical method passes during its life—conception, design and construction, demonstration of instrument, evaluation of figures of merit, general acceptance, improved understanding, and senescence. The development of the ICP is currently at the "general acceptance" and "im-
Spectrometer
Emission zone
and readout Lens
Induction coil
Torch
Sample transport
Figure 1. Schematic diagram of a typical inductively coupled plasma-optical emission spectroscopy instrument featuring parts of the instrument most important to the user. ANALYTICAL CHEMISTRY, VOL. 59, NO. 23, DECEMBER 1, 1987 · 1345 A
proved understanding" steps. Extensive research on the mecha nisms of operation of an ICP has result ed in the acceptance of several theoret ical models, or simulations. Such simu lations have aided analytical chemists in anticipating problems with one or another sample. A "sub-step" might therefore be added after Laitinen's "general acceptance" step: the predic tion of results based on artificial intelli gence. With the enormous database generated from diagnostic measure ments on analytical ICPs, such artifi cial intelligence is currently aiding sci entists in refining the process of mea suring the emission signal from the plasma and in improving instrument design and performance. Considerations for selecting an ICP instrument
The task of selecting the right ICP in strument involves a keen understand ing of the targeted applications and a critical eye toward the different capa bilities offered by the manufacturers. Based on a detailed review of the ana lytical requirement, the buyer is ad vised to prepare a checklist that in cludes the following: type of sample to be analyzed, variety of matrices ex pected, analytical figures of merit re quired, range of concentration re quired, operator experience, size con straints (bench vs. floor space), instrument location (plant vs. lab), se quential or simultaneous analysis, speed of analysis, general problem solv ing or routine analysis, desired vs. needed automation, budget (including operation and service), anticipated fu ture goals or purchases, adaptation to research projects, and flexibility. The ICP instrument is comprised of four fundamental parts (Figure 1): ICP
Computer controlled scanning monochromator
source, optics and spectrometer, sam ple introduction, and data analysis. A schematic of a typical atomic emission instrument is presented in Figure 2. New developments in each part need to be appreciated before judging the mer its of the instrument as a whole. This necessitates "hands-on" experience by the prospective user to ensure that there is a match between the instru ment and the analytical requirement. Optimum use of the instrument hinges on a broad understanding of its capa bilities and on knowing about different technologies that can be used in con junction with the ICP. The following is a fairly complete summary of the important develop ments in ICP technology during the past several years. Some of these devel opments are specific and have gained limited popularity, whereas others are propelling the technology into the next decade. ICP sources
The ICP is as practical to use as it is aesthetic to observe. Figure 3 is a pho tograph of an argon ICP sustained in a demountable torch with 1% yttrium as the sample solution taken at 1/500 of a second. It clearly illustrates the princi pal analyte emission zones typical for this source. The red emission of the yttrium oxide ion in the upper part of the photograph highlights the flow pat tern of the hot gases as they exit from the torch. The blue region in the center of the plasma is the zone where optical emission from the analyte ion is mea sured. The pink color inside the coil region is attributable to atomic emis sion of yttrium. Historically, the ICP has been com posed of a "torch," which contains the hot plasma, and some means to sustain
Grating.
Tailflame Sample plume Induction coil
Polychromatot
ICP torch
Data lines
s
}
α.
ε Data out
Figure 2. Diagram of an ICP emission system detailing the combination of the se quential and simultaneous wavelength detectors in one complete unit. 1346 A · ANALYTICAL CHEMISTRY, VOL. 59, NO. 23, DECEMBER 1, 1987
the plasma inside the torch. Radio frequency power of approximately 0.5-5 kW is transferred to the plasma using a coil wound around the torch. This coil inductively couples the ener gy to the plasma. The rf coil, replacing the classical electrode of an arc/spark discharge, does not come into direct contact with the plasma. This elimi nates the problem of elemental con tamination in the source from the elec trode. This coil, once energized with kilowatt rf power, induces an electro magnetic field within the torch. This field inductively heats the formed plas ma to temperatures exceeding 5000 K. The gas that sustains the plasma is ini tially made electrically conductive by Telsa sparks before a self-sustained plasma results. The entire process of forming and stabilizing an ICP takes several milliseconds. The torch typically consists of a se ries of annular tubes made of quartz or other high-temperature-resistant ma terial. The various tubes making up the torch carry gases of different flow ve locities through the rf coil region. In this region the gas is rapidly heated and subsequently ionized. (The ICP is said to be only 1% ionized.) The outer stream flows at a high rate (5-15 L/ min) and serves to sustain the plasma. It also carries away the heat that is dis sipated by the plasma to the inside walls of the torch. A centrally located gas stream flowing at a low rate (0.3-1.5 L/min) carries a sample aerosol up through the existing plasma. After pen etrating the hot core of the plasma, the aerosol is desolvated, dissociated, at omized, and excited. Upon passing through and out of the plasma, the var ious ionic and atomic species relax to their ground states, emitting charac teristic radiation. This emission ap pears in a well-defined region along the central axis of the plasma. A third flow of gas, typically used to optimally con trol the position of the plasma inside the torch, becomes important when aerosols of different matrices (e.g., or ganic solvents) are introduced into the plasma. This torch configuration ap peared in the first publication on atmos pheric rf plasmas and has since domi nated the design. Formation of a symmetric plasma in side the torch is critically dependent on the concentricity of the tubes making up the torch. Quartz torches are artworks of the laboratory glassblower and require tedious attention during fusion of the various tubes to ensure proper operation. Damage to any sur face of the torch leads to costly repair or its disposal. To facilitate easier re search of torch parameters, numerous demountable ICP torch designs have been suggested. Various elaborate de signs featuring metal or plastic mounts for fixing the tubes of the torch have
been reported. Replacement of the central aerosol introduction tube with a new one or with one made of a refractory material (i.e., boron nitride or alumina) is made possible with a demountable design. The design and function of the ICP source has remained relatively unchanged since it first appeared in commercial instruments. Although torches today are unique to each manufacturer, they all have the same outside diameter of 20 mm. One torch developed in Great Britain has an outside diameter of 25 mm and can be operated at higher power levels than the smaller-diameter torch. Argon was the gas of choice for the 15-year period before commercialization and is still popular because of its proven analytical performance. Since 1979, however, attention to the cost of operating an ICP system has promoted the redesign of the torch, and, to a large extent, also the complete torch/rf generator instrument. Leading the way toward more economical operation of the ICP was the suggestion of replacing the higher flow gas with one of lower cost, namely nitrogen or air, yet still sustaining the plasma with argon. Originally implemented for the larger-diameter torch, this arrangement was ignored because of the higher power levels it required, which were not typically available to the common user. Gas consumption rates of the earlier designs were close to 30 L/min. Concerted research efforts and better generator and torch design have allowed areas of the world where argon is expensive to routinely operate with nitrogen- or air-cooled argon plasmas. Under operating conditions specific to molecular gas-cooled argon plasmas, analytical performance has reportedly exceeded that of the argon ICP. Torches with low consumption (1 L/ min) have alternate means of dissipating the extreme heat from the plasma. This process is accomplished by using a water-cooled torch or by directing forced air at the torch. Other new torch designs exhibit lower gas consumption through torch miniaturization. These "mini-torches" are 30-40% smaller in overall diameter than the typical 20-mm torches and require correspondingly less power to operate. Streamlining the inner torch surfaces and creating constrictions in the torch design to increase swirl flow velocity for the same gas flow rate have been successful in many cases at reducing the overall gas consumption by 50%. The replacement of argon (the plasma-sustaining gas) by other gases has also been achieved. Analytical ICPs sustained totally in pure diatomic gases or air exhibit higher kinetic temperatures and are thought to be more efficient at sample decomposition.
cific detectors for various chromatographic techniques, and helium is also the preferred gas for sustaining tiny plasmas at 50-100 W of microwave energy. Helium has a higher excitation potential than argon (18 vs. 14 eV, respectively), which makes it a more efficient exciter for elements such as the halogens that exhibit poor detection limits in argon. A helium ICP generated at atmospheric pressure with an rf generator in a torch similar in outside dimensions to the argon torch presently exists. Plasma rf generators
Figure 3. Photograph of an argon inductively coupled plasma sustained at 1 kW in a demountable quartz torch with a boron nitride aerosol introduction tube. An yttrium solution is aspirated to highlight the atomic emission (pink), ionic emission (blue), and oxide emission (red) zones of the plasma. The plasma was photographed at 1/500th s shutter speed.
These types of plasmas can be sustained at low-kilowatt power levels with 40.68-MHz generators facilitating conversion from an argon system. Unlike the argon plasma, which can tolerate only tens of micrograms of aerosol per minute, molecular gas ICPs need up to several hundred micrograms of sample per minute to achieve relative equivalence in detection limits. Helium microwave plasmas have been used extensively as element-spe-
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Improvements in rf generator and impedance matching circuits necessary to maintain high source stability have accompanied the advances made with the ICP torch. Compact, rack-mountable models have replaced the bulky floor model rf circuit design of the 1960s and 1970s. This has been achieved in part by improvements in the power-dissipation ability of solid-state devices. Although most kilowatt-size rf generators still use vacuum tubes, generators operating below 2 kW are available with transistor power amplifiers. Smaller package size and computer control of the ICP is being facilitated by transistor-based rf generators. Automatic ignition of the ICP, which when performed manually used to be the nemesis of even experienced users, has become a reality. The reduction of electrical interferences in data transfer lines through operation at lower control voltages and currents has facilitated measurements of signals at the picoampere level. Fiber optics have also been introduced for translating the image of the plasma to the entrance slit of a spectrometer. Two types of circuit design are currently used in commercial rf generators. These are the crystal-controlled, multistage rf amplifiers and the freerunning rf generators. With the first type, a crystal sets a fundamental frequency and some multiple of that frequency is tuned with a resonance circuit. This frequency drives the rest of the rf generator. Because frequency is fixed, changes in the impedance along the rf transmission line results in an impedance mismatch and affects power transfer. This result occurs when the plasma is disturbed from its equilibrium, as by the introduction of sample or by a change in gas flow rate. The mismatch is then mechanically retuned using various vacuum and airgap capacitors. Although this impedance matching is automatic with most systems, it does require moving parts and extra sensing circuitry. The alternative to the "characteristic impedance" matching method is the use of a free-running generator. With this design the load coil is an integral.
component in the tank circuit of a selfresonating oscillator. Impedance changes brought about by perturbations to the plasma are tuned by shifting the frequency. In this case the frequency shifts, because the other components of the tank circuit are held constant. There are no moving parts in this design; maintenance is minimal, and the overall size is reduced by nearly 50%. Currently, generator frequencies of 27.12 and 40.68 MHz, respectively, are available with crystal-controlled and free-running systems sold in the United States; frequencies ranging from 13.56 MHz to nearly 70 MHz are available elsewhere around the world. Spectrometers The optics and spectrometer of the early ICP commercial systems (normally 0.75-1.5 m focal length) were typically borrowed from established arc/spark systems concurrently available and retrofitted with the newer ICP source. Manufacturers emphasized the different spectrometer designs and did not explore the development of the ICP. In the past 10 years, however, improvements have been made on both the design of ICP sources as well as novel optical spectrometers. The size of the ICP-optical spectrometer system was formerly one indicator of instrument performance. Direct-reading systems capable of analyzing nearly 50 elements at one time were expensive and space-consuming. Spectrometers enclosed in temperaturecontrolled housings were mounted on bulky frames and required extensive computer power for data handling. The ICP source was typically mounted on one end of the spectrometer on a translation table that permitted manual positioning of the ICP for optimum signal observation. Before the production and general availability of holographic or interferographic gratings, spectral resolution necessary for the ICP (0.01 nm) was achieved through focal lengths of 1 m or more. The advent of better optical configurations using holographic gratings with 3600 grooves/mm, more efficient methods for slewing the grating from one end of the spectrum to the other, and capitalizing on the échelle configuration suddenly took the bulky instrument design of the 1970s into the more practical, bench-mountable systems of the 1980s. Reliability and performance of a truly sequential monochromator under computer control, not available until the late 1970s, finally stood up to customers' specifications. Optical designs went from sophisticated means of precise movement of the grating to fixed gratings with slit and photomultiplier combinations moving along the focal plane of the spectrometer. The
combination of the sequential and simultaneous capability in one and the same spectrometer was another significant advance in spectrometer design. Formerly these were separate instruments often focused on a common ICP. This design offered the ultimate flexibility in speed of analysis and wavelength selection not realized with older systems. The échelle optical configuration, formerly established as a successful matchup with the DCP, was also introduced as a computer-automated ICP spectrometer. Earlier models required manual adjustment of the échelle grating and prism for wavelength selection. Scanning of the emission line was instrumentally awkward. To improve stability and reliability, the grating and prism were left fixed and an exit slit/ photomultiplier combination was instead slewed across the focal plane of the spectrometer. This increased the speed of analysis radically and brought the échelle monochromator into the arena of high-resolution, computercontrolled spectrometers for ICP spectrochemical analysis. The combination of the spectrometer and a linear silicon photodiode array was also investigated. Replacing the photomultiplier tube with an array was expected to yield multichannel capability for a single-channel instrument. Improvements in the sensitivity of the device at wavelengths below 300 nm through cooling and better manufacturing made it a prime candidate for a multiwavelength detector for optical emission spectrometry. The problem of limited wavelength coverage when used in a high-dispersion monochromator limited the utility of these detectors. This was solved in one case by dissecting the signal from the ICP into wavelength regions of interest before being dispersed by the grating using masks specified by the analyst. The variously dispersed regions were then recombined onto the photodiode array. Unwanted spectral emissions from interfering species in the ICP could then be extracted, permitting the analyst to concentrate on the elements being determined. Another group of solid-state detectors with low noise characteristics is the family of detectors called charge injection or charge-coupled devices, CID and CCD, respectively. When used together with an échelle spectrometer, the two-dimensional variety of this detector matched well with the two-dimensional focal plane of the spectrometer. The primary advantage of these detectors over conventional photodiode arrays is the ability to treat individual groups of diodes as randomly accessible, independent detectors. This is not possible with the linear photodiode array. Although some of the
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devices themselves are available commercially, they have not found a firm foothold in a commercial instrument primarily because there is no concerted research showing justification for its use. The increased interest in miniaturization combined with multiwavelength detection has led to application of Fourier transform (FT) techniques to the UV-visible portion of the ICP emission spectrum. A Michaelson interferometer, a form of multiplex detection, has been shown to produce high-resolution spectra from an ICP. These devices have a spectral range from UV to near-IR wavelengths. One problem encountered thus far is the detection of low-intensity emission lines in the presence of more intense lines. The analysis of trace amounts of one element in the presence of another, more dominant species is especially difficult. The combination of a conventional optical monochromator with an F T device, using the former as a prefilter for the latter, has also been suggested. The main advantage of an F T detector for the ICP is the extremely fast scanning speed achievable and wide wavelength range with the interferometer. This is possible with the very small distance the mirror must move inside the interferometer for one complete scan. The use of atomic fluorescence as the tool for determining elemental concentration emerged in 1981 as one of two detection schemes combining high sensitivity with compact size. The ICP source is operated under conditions that promote the formation of atomic species. This includes sustaining the plasma under 1 kW of power and using high flow rates for the central gas stream carrying the aerosol through the plasma. Hollow cathode lamps, mounted in a circular pattern about the ICP, excite the atomic species in the cool tail-flame region of the plasma with resonant energy, while interference filters with photomultiplier tubes measure the characteristic fluorescence emission. This method offers relative freedom from matrix spectral interference compared with classical emission methods, greater confidence in elemental determination, and partsper-billion detection limits. The technique is simultaneous for up to 12 elements because each element has its own excitation source and detector. Certain elements not exhibiting atomic fluorescence in the UV-visible spectrum (e.g., boron) cannot be determined by this technique. The most recent detector to be interfaced with the ICP for the determination of metals in sample solutions has been the mass spectrometer (MS). First reported as a viable detector for metals analysis by plasmas as early as
1975, a quadrupole MS coupled to an ICP was not commercially available until 1984. Currently, manufacturers in Great Britain, North America, and Japan produce such a system. The ini tial challenge was the coupling of a high-temperature atmospheric pres sure plasma with the vacuum require ments of a quadrupole mass spectrom eter. This was accomplished by inter facing the mass spectrometer to the ICP through a series of differentially pumped sampling openings. The diam eter of the holes in these sampling cones is 0.5-1.5 mm. One such orifice is placed directly in the plasma at the ap propriate sampling location. Ionized atoms from the sample aerosol as well as all other species present in the plas ma enter into a differentially pumped intermediate chamber. There a small fraction of the analyte ions, together with other background species, enter the low-pressure region and are accel erated with a series of lenses through the quadrupole mass filter. The ICP is an efficient ion source, and when cou pled to such a quadrupole MS it offers a less complex background spectrum, determination of more than 70 ele ments in one sample, and detection limits down to tens of parts per trillion in solution for most elements. Isotopic analysis for geologic purposes is now greatly simplified. Isotopic dilution methods facilitate highly accurate con centration determination of elements with more than one stable isotope in a single step. ICP-MS also offers compa rable detection limits for elements tra ditionally determined by hydride gen eration methods. It can also operate in negative ion mode to determine halo gens at sub-parts-per-million concen trations. This technique is preferred for rare-earth elements and exhibits some limitations for lighter-weight ele ments with masses
