ACKNOWLEDGMENT The authors thank V. I. Birss and R. J. Parker for their help with experimental portions of this work. Appreciation is expressed for samples of Nafion from E. I. du Pont de Nemours 8 2 co.
LITERATURE CITED (1) M. Lopez, B. Kipling, and H. L. Yeager. Anal. Chem., 48, 1120 (1976). (2) F. Heifferich, “Ion Exchange”, McGraw-HIII, New York, N.Y., 1962, Chapter 6. (3) Y. Marcus in “Ion Exchange and Solvent Extraction”, Vol. 4, J. A. Marinsky and Y. Marcus, Ed., Marcel Dekker, Inc., New York, N.Y., 1973. (4) J. M. Mackie and P. Meares, Proc. R. SOC. London, Ser. A, 232, 498 (1955). (5) R . Fernandez-Prini and M. Phillip, J. Phys. Chem., 80, 2041 (1976). (6) M. F. Hoover and G. B. Butler, J. Polym. Scl., Part C, 45, 1 (1974). (7) A. Eisenberg, Macromolecules. 3, 147 (1970). (8) E. P. Otocka, J. Macromol. Sci., Rev., Macromol. Chem., 5, 275 (1971). (9) C. L. Marx, D. F. Caulfield, and S. L. Cooper, Macromolecules, 6, 344 (1973).
A. Eisenberg, J. Polym. Scl., Part C, 45, 99 (1974). W. J. MacKnight, W. P. Taggart, and R. S. Stein, J. Polym. Sci., Part C,45, 113 (1974). R. Longworth in “Ionic Polymers”, L. Holliday, Ed., Halsted Press (John Wiley), New York, N.Y., 1975. S. C. Yeo and A. Eisenberg, Polym. Prepr., Am. Chem. SOC.,Div. Polym. Chem., 16, 104 (1975). S. C. Yeo and A. Eisenberg, J. Appl. Polym. Sci., in press. H. L. Yeager, J. D. Fedyk, and R. J. Parker, J. Phys. Chem., 77, 2407 (1973\. - -I. %
(16) T. G. Kaufman and E. F. Leonard, AlChE J., 14, 421 (1968). (17) F. Helfferich, “Ion Exchange”, McGraw-Hill, New York, N.Y., 1962, Chapter 8. (18) J. A. Pople and M. Gordon, J. Am. Chem. SOC., 89,2517 (1967). (19) J. X. Khym, “Analytical Ion-Exchange Procedures In Chemistry and Biology”, Prentice-Hall, Englewood Cliffs, N.J., 1974. (20) D. J. Vaughan, Du Pont Innovation, 4 (3), 10 (1973). (21) W. J. Blaedel, T. J. Haupert, and M. A. Evenson, Anal. Chem.. 41, 583 (1969).
RECEIVEDfor review August 12,1976. Accepted January 13, 1977. This work was supported by the National Research Council of Canada and the University of Calgary.
Multielement Detection Limits and Sample Nebulization Efficiencies of an Improved Ultrasonic Nebulizer and a Conventional Pneumatic Nebulizer in Inductively Coupled Plasma-Atomic Emission Spectrometry K. W. Olson, W. J. Haas, Jr., and V. A. Fassel’ Ames Laboratory-ERDA and Department of Chemistry, Iowa State University, Ames, Iowa 500 1 1
Two important aspects of the analytical performance of a newly developed ultrasonic nebulizer and a specially designed pneumatic nebulizer have been compared for use In lnductlvely coupled plasma-atomic emission spectroscopy (ICP-AES). The ultrasonic nebulizer, when comblned wlth a conventional aerosol desolvatlon apparatus, provlded an order of magnitude or more improvement In simultaneous muitleiement detection limits as compared to those obtained when the pneumatic nebuilzer was used either wlth or wlthout desolvation. Appllcation of a novel method for dlred measurement d the overall efflclency of nebulizatlonto the two systems showed that an approxlmately tenfold greater rate of sample delivery to the plasma torch was prlmarliy responslble for the superior detectlon limits afforded by the ultrasonic nebuilter. A unique feature of the ultrasonic nebulizer described Is the protectlon agalnst chemical attack which Is achieved by completely enclosing the transducer in an acoustically coupled borosllicate glass cylinder. Direct sample introduction, convenient sample change, and rapid cieanout are other Important characteristlcs of the system which make it an attractlve alternate to pneumatic nebulizer systems.
Several recent reviews (1-4) have provided adequate documentation that radiofrequency, inductively coupled argon plasmas are superior excitation sources for the simultaneous multielement determination of the metals and metalloids a t major, minor, trace, and ultratrace concentration levels. The most commonly used approach for introducing sample material into the plasma is based on the injection of aerosols generated by pneumatic (5-7) or ultrasonic (8-12) nebulization of aqueous or organic (13)solutions. Desolvation of the 632
ANALYTICAL CHEMISTRY, VOL. 49, NO. 4, APRIL 1977
sample aerosol prior t o its injection has also been employed by some workers (6,9-12,14). Most analytical applications of the plasma have employed aerosol carrier-gas flows of approximately one liter per minute. Because the aerosol carrier-gas is also the nebulizing gas in pneumatic nebulizers, the required low flow rate imposes rather severe restrictions on the dimensions of the nebulizer orifices. Special pneumatic nebulizers that operate successfully a t low flow rates have been developed (14-1 7), but their efficiency of nebulization is low, and they have a tendency to clog. Low carrier-gas flows into the plasma have also been achieved through the combination of conventional pneumatic nebulizers with stream splitters (12, 18).However, this approach has not been well accepted for ultratrace determinations because of analyte losses associated with the stream splitter, and it is not suitable when sample volumes are limited. Ultrasonic nebulizers possess a number of attractive characteristics (19). First, the rate of aerosol production a t the transducer does not depend on the carrier-gas flow as it does in pneumatic nebulizers. Thus the aerosol production rate and the carrier-gas flow rate may be varied independently to optimize analytical performance of the plasma system. Second, ultrasonic nebulizers can produce aerosols of greater number density and of more uniform particle size than pneumatic nebulizers. Third, the mean size of the particles produced by an ultrasonic nebulizer is frequency-dependent (20);smaller particles can be produced by increasing the ultrasonic frequency employed. The advantage gained is that smaller particles are more efficiently transported and are more rapidly desolvated and atomized in the excitation cell. Two types of ultrasonic nebulizers have been used previously with inductively coupled plasmas. In continuous feed
Table I. Instrumental Facilities Component Description Manufacturer Plasma generator: Model MN-2500-E Plasma-Therm, Inc. Kresson, N.J. 08053 Plasma torch: All quartz Ames Laboratory construction (1) Pneumatic nebulizer: Teflon and glass Ames Laboratory construction (15) Ultrasonic nebulizer: Figure 1 Model CPMT, Channel No. 5800 lead zirconate titanate transducer with Corning No. 7740 glass protective coating/acoustic transformer Channel Products, Inc. Chagrin Falls, Ohio 44022 Ultrasonic power supply: Model 7050 Tomorrow Enterprises Portsmouth, Ohio 45662 Desolvation system: Modified Margoshes-Veillon ( I 7) type
Polychromater: Model QVAC-127 Applied Research Laboratories Sunland, Calif. 91040
Data acquisition system: Ames Laboratory design utilizing the following in sequence: Model 18000 remotely programmable current-to-voltage converters Keithley Instruments Cleveland, Ohio 44139 2-pole active low-pass filters Ames Laboratory construction 12 bit A/D converter/multiplexer Model 721, Zeltex Inc. Concord, Calif. 94518 PDP-8/e minicomputer and Decwriter 11 1/0terminal Digital Equipment Corp. Maynard, Mass. 01754 ultrasonic nebulizers (20,19, 21), the analyte solution is continuously pumped onto the transducer surface or transfer plate. These nebulizers are preferred for routine analyses because rapid sample interchange can be achieved and because the sample cleanout time required t o avoid memory effects is acceptably short. Solutions of high salt content or of high acidity or alkalinity have been found, however, to attack the transducer, even when various protective coatings were employed. In batch type ultrasonic nebulizers (8,9,20), the ultrasonic energy is coupled t o a known initial volume of analyte solution, either directly or through a n inert liquid or solid interface, thus circumventing the corrosion problem.
Operating conditions
1100-W forward power;
