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considerable structure is added by the spectral instability of the laser. Finally we show the spectrum measured by using the phase-conjugate feedback self-scanping dye laser in Figure 2c. Greatly improved resolution is achieved with highly symmetric line shapes. The fwhm of the Na transitions are 6.7 GHz and the Ar transition measures 5.7 GHz. This is very nearly what we expected considering the =5 GHz line width of the laser, Part d of Figure 2 shows the fringes produced by the monitor etalon. The fringes are 14.2 GHz (0.016 nm) ai8art, being essentially linear in wavelength over this brief interval. Figure 2, through the spectrum and in particular the etalon fringes, shows for the first time that this selfscanning laser can be useful as a high-resolutionspectroscopic source. The narrow bandwidth is maintained across a relatively broad range, and scans are continuous and monotonic on this high-resolution scale. The spectral region shown in Figure 2 is only a small portion of the total scan possible with the dye laser as described. A scan from 585 to 610 nm is possible a t 5-GHz resolution corresponding to more than 4000 separate resolution elements. Scans over other regions of the visible spectrum should be possible using different gain media and mirror reflectivities. The wavelength limits of the scan range determined by a given dye/mirror combination can also be modified. A scan can be stopped at any time and restarted at any wavelength, within the tuning range, by inserting a coarse tuning element in the dye laser for a few seconds. In addition, we have been able to reduce and adjust the tuning range by inserting a broad band filter in the dye cavity. The scan rate of this laser can be adjusted over a broad range, from several seconds to 10 ms per resolution element. The slow scan rates are important when using long time constants for signal-to-noise improvements. Higher scan rates might be used with signal averaging
where the etalon fringes could provide spectra registration of the signal or a wavemeter could be used rather than an etalon to provide absolute wavelength measurement during a scan, although at greater expense. We feel this high-resolution self-scanning dye laser system has many possible applications in molecular and atomic spectroscopy. this source could be particularly useful for solid-state and gas-phase spectroscopy where high-resolution measurements are useful. The repetitive, passive scanning feature of this system might prove useful for low-resolution spectra as well. Future improvements may include further bandwidth reduction, flexible control of scan ranges, and bandwidth reduction without scanning.
Registry No. R6G, 989-38-8; BaTiOB,12047-27-7;Na, 744023-5. LITERATURE CITED (1) (2) (3) (4) (5)
Feinberg, J. Opt. Lett. 1982 7 , 488-488. Whltten, W. B.; Ramsey, J. M. Opt. Lett. 1984, 9 , 44-48. Yariv, A. I€€€ J . Quantum €/ecfron. 1978, 74, 850-680. Giuiiano, C. R. Phys. Today 1881, 31, April, 27-35. Travis, J. C.; Turk 0. C.; Oreen, R. 6. Anal. Chern. 1982, 5 4 , 1006A1018A.
J. M. Ramsey* W. B. Whitten Analytical Chemistry Division Oak Ridge National Laboratory Oak Ridge, Tennessee 37831 RECEIVED for review March 26,1984. Resubmitted August 21, 1984. Accepted August 21,1984. Research sponsored by the Office of Energy Research, US.Department of Energy, under Contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc.
Segregated Sampling and Excitation with a Dual Inductively Coupled Plasma Sir: In recent years increasing attention has been placed on the development of atomic discharge systems which segregate the sampling from the excitation step. The sampling step includes those processes which create an atomic and/or ionic reservoir from the analyte of interest. The excitation step involves the coupling of additional energy to the resulting reservoir for spectrochemical observation. In practice this segregation may be in time and/or in space. Also, it is unlikely that absolute or complete separation of the two processes is possible. For example, we used a high-voltage spark as a sampling device to create a long-lived toroidal-shaped reservoir of species sampled from a metallic electrode surface. Once formed, this material was inductively reexcited with an intense pulse of radio frequency energy (1). The inductive field was established with a coupling coil located circumferentially about the spark interelectrode axis. When compared with direct spark emission, spectra resulting from the reexcitation process (1)exhibit narrow line widths, (2) are simplified with preferential population of lower level resonant transitions, and (3) are free from background continuum interferences. Recent results (2,3) indicate excellent detection limits for trace elements in aluminum matrices with 3-4 decades of linearity in analytical working curves. These properties are a direct result of segregating the sampling and excitation steps in order to allow independent control and optimization of each process. 0003-2700/84/0356-2961$0 1.50/0
In the present communication a new dual inductively coupled plasma device is introduced which embodies several aspects of segregated sampling and excitation spectroscopy. This device is designed to eventually allow direct introduction of powdered (e.g., particulate) samples into an ICP and to facilitate fundamental studies. Several approaches to independent sample introduction for the ICP have been reported. These include electrothermal vaporization (4),introduction of spark-sampled aerosol (5-7), and transport of laser-ablated material (8,9) to the ICP. For each experiment enhanced detection limits and long linear working curves were reported. In addition, research has been published which describes both the dropping of powders directly into an ICP (10)as well as conventional aerosol injection of powders into an ICP for analysis (11,12). The direct introduction of powders and refractory materials avoids expensive and time-consuming dissolution normally associated with aqueous nebulization of these sample types. It was noted (11)that energy transfer was insufficient to completely vaporize large particles before they excited the plasma zone. Reasonable detection limits were observed, but continuum and other background levels were not specified. C m o et al. (13) recently reported a simple device for direct injection of coal fly ash samples into a conventional high-power ICP. Detection limits of 1-7 ng were reported for several elements in NBS coal fly ash (No. 16333a) samples. Possible 0 1984 American Chemlcai Society
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ANALYTICAL CHEMISTRY, VOL. 56. NO. 14, DECEMBER 1984
r Flgure 1.
s
Plctorlal representation of the dual ICP system
limitationa due to large particle size were discussed. The dual ICP is shown conceptually in Figure 1. This figure incorporates several operational possibilities. The function of the lower coil (A) is to desolvate, vaporize, and atomize the injected sample. The function of the upper coil (B) is to reexcite the resulting atomic reservoir with controlled introduction of radio frequency energy. Although speculative, extrapolations from our spark-sampled/rf reexication work (I) would indicate the possibility of improvement in signalto-background levels, due in part to a reduction in continuum levels. This factor may be especially critical for the introduction of solid particles.
EXPERIMENTAL SECTION Several operational modes were envisioned. The fmt involved simultaneous nse of two independent ICP generators and impedance-matching networks. Experiments utilizing this approach demonstrated intense coupling between the two radio frequency coils and resultant electromagnetic fields. This interaction was so severe that creation of a stable dual-dischargesystem seemed unlikely. A second envisioned operational mode involved minimizing this interaction by alternately energizing the sampling and excitation coils at a high-repetition rate. This has not been attempted in view of our current approach which is to drive simultaneously two plasma dischargeswith radio frequency energy supplied by a single generator. The generatorlimpedance-matching network combination, designated in Figure 1 to power the sampling coil, is a Plasma Therm Model HFP 2500D 27-MHz generator. This system is equipped with APCS-1 automatic power control and AMNS automatic impedance-matchingnetworks. A generator forward power of 1.0 kW was used. The matching network associated with the excitation coil has been described elsewhere (14). The plasma torch design is based 00 a low-power, low-argonconsumption assembly first reported by Hieftje (15). The dual ICP torch is illustrated in Figure 2. Dimensions for the lower portion of the torch are detailed in ref 13. Dimensions for the upper portion of the torch include a 0.5-mm clearance between the inner and outer quartz tubes, for coolant flow. Inlets C,, C,, and A, are constricted to a 1 mm inner diameter. A distance of 10 cm was maintained between the sampling and excitation coils. Argon flow rates include (C,) coolant, = (C,)coolant, = (A2) auxiliary, = 5 L/min., (A,) auxiliary, = 0.1 L/min, and (SI) aerosol, = 0.5 L/min. The spectrometer/monochromator is a Model 310 SMC, Minuteman Laboratories, Inc., Acton, MA, 1-M focal length asymmetric Czemey-Turner with a 2400 g/mm d e d grating and first-order dispersion of 4.2 A/mm. RESULTS AND DISCUSSION The dual I C P is shown in full operation in Figure 3. Radio frequency power is directed only to the lower (sampling) im-
Flgure 2. Torch assembly used for dual ICP OpBraIion.
3. Photograph of a dual ICP in operation. Both discharges are supported by radio frequency energy delivered to the lower (sampling) coil only. Successful igniiion and operation is dependent on selection of argon flow rates and upon careful adjustment of both matching networks. -re
pedance-matching network and plasma coil. This mode of operation is highly dependent on argon gas flow characteristics and on the tuned condition of both impedance-matching networks. Three distinct modes of operation are possible for various adjustments of the matching networks when power is delivered to the lower coil. The first mode results in formation of a single-plasma discharge in the sampling coil zone. This condition prevails when the lower matching network is tuned for minimum VSWR conditions while the upper network is severely mismatched. A long, thin,and deformed plasma tail plume extends upward toward the excitation coil. As the
ANALYTICAL CHEMISTRY. VOL. 56. NO. 14. DECEMBER 1984 2983
A
modest conditions in the excitation zone. Several technical problems are currently receiving attention. Ignition of the plasma is complex and requires simplification. Radio frequency VSWR is high. Reflected power levels of 100 W are typical a t this stage of development, and systematic reduction of reflected power to acceptable levels is necessary. Finally, a t the current state of developmentthe plasma torch design is being optimized to avoid intermittent meltdown. Results typical of preliminary spectral studies are shown in Figure 4. Emission from a 1ppm aqueous solution of iron (in 2% "03 is okrved simultaneously in the sampling (A) and excitation (B) zones. Qualitatively, a significant increase in signal-bbackground ratio is evident. These observations were taken under conditions equivalent to those described for Figure 3 except that a torch with a 2 cm long quartz upper extension was employed so that both observation zones were viewed through equivalent quartz pathways. Current efforts are to refine the instrument in order to improve upon the aforementioned limitations. Ultimate assessment of analytical utility of the discharge system will depend upon these refinements and upon the ability to deal with real (powdered) samples. A sample introduction system similar to that reported in ref 13 w i l l be implemented for initial experiments. Fundamental ramifications of segregation of the sampling and the excitation processes are important outgrowths of this work.
Sampling
Flgue 4. Iron emission observed simultaneously In samplng (A) and exdtatim (6)zones. Spectroswpic emulsion = SA- 1 . Spectmmeter entrance slit = 100 pm X 2 mm. Exposure = 15 s. Aspiration rate = 1.2 W m l n . Both observations were taken in optimal zone (10 mm above each load coil and 3-4 mm off the central discharge axis).
ACKNOWLEDGMENT Gifts-in-kind in the form of electronic instrumentation and a commercial ICP system from the Baird Carp.(Bedford, MA) are sincerely appreciated.
Other experimental details are Included in the text. upper network is manually readjusted to approach a resonant condition, the tail plume extension is constricted and is magnetically centered within the upper portion of the torch assembly. In contrast, a second mode of operation allows formation of a single plasma in the excitation coil zone, with radio frequency energy channeled to the lower coil. This mode requires that the excitation coil resonant matching circuit he tuned to resonance and that the lower network is mismatched. Finally, the third operational mode is formation of the dual-plasma discharge. When all conditions are properly adjusted, initiation of dual ICP operation is accompanied by rapid plasma formation which oscillates between the two plasma coil regions. During thisperiod the matching network are manually fme-tamed to achieve stable operation. It appears that operation is characterized by continuous plasma formation and quenching, a t a high but as yet undetermined repetition rate, with the plasma alternating between the two operational zones. Visually, however, simultaneous dual ICP operation appears to be continuous. Under the present conditions it appears as if the (1 kW) radio frequency energy is shared about equally between the sampling and excitation plasmas. the relative energy concentrated in each plasma is ultimately controllable hy the intercoil spacing, by individual coil confimrations. and hv matching network settinm including termination (indicated by a ;lummy load in figure 1). Analytically it may be desirable to establish a higher power discharge in the sampling zone in order to create an effective atomic reservoir. Reexcitation could then occur under mnre
LITERATURE CITED (1) W m n , D. M.: Saim. M. A. Anal. M e m . 1980. 52. 748. (2) Coleman. D. M.; Sahr. M. A,, submilled fw publicalkm h Appl. Specmlbc . (3) Sainz. M. A.. m.D. Dksarlalbn. Wayne smte Unlvershy. Debon. MI. 1982. V. E.; Knisaiey. R. N. Anal. M e m . 1974. 46. (4) Nixon. 0. E.: Fa-I.
'
210. (5) Waners. R. L.. Jr.. p p e r presented at Insttut hlr Spkbochemle und angewandte Spkboskople. 13 April. 1978. Datmurd. Germany. (6) Beaty. J. S.. JarrelCAsh Mvlsbn, Allled Corp.. Waltham. MA. personal cammunlcatbns. (7) Marks. J. Y.: Fwnwah, D. E.: Yungk. R. E. SpeClmcMm. Acta. part8 1 9 8 3 . 3 a ~ .107. (8) Carr. J. W.: Horllck, 0. SpeCmhLn. Acta. part 8 1982. 378, 1. (9) Ishlzuka. T.; Uwamlno. Y . Specmhhn. Acta. Part B 1983. 388. 519. (10) Reed, T. B. J . App/. phyp. 1961, 32. 2534. (11) Hoare. H. C.; Moslym. R. A. Anal. M e m . 1987. 39. 1153. (12) Dagnall. R. M.: SmW, D. J.: West. T. S. Anal. C % h . Acta 1971. 54, 39i. (13) Ng. K. C.: Zererghi. M.: C a w O . J. A. Anal. Chem. 1984, 58. 417. (14) Allen. G. M.; weman, D. M. AMI. c m . A& 1984, 158, 267. 1151 ReLBAvBBII. R.: Henle. G. M.: Anderson. H.: Kaisar. H.: MBwln(10. 8. APPI. k p c m c 19b2,3a. 627.
G. Mark Allen David M. Coleman'
Department of Chemistry Wayne State University Detroit, Michigan 48202 RECEIVED for review November 8, 1983. Resubmitted .July 26,19&1. Accepted July 26.1984. Portions of this work were funded by the National Science Foundation under Grant CHE-80-16148.
