Ultrasonic Sprayer for Atomic Emission and Absorption Spectrochemistry C . D. West1 Beckman Instruments, Inc., 2500 Harbor Blud., Fullerton, Calif. 92634
ULTRASONIC WAVES directed onto the surface of a liquid will, under certain conditions, expel a “fog” of the solution into the surrounding atmosphere. For applications such as atomic emission and absorption, the spray thus produced, and the method of producing it, have several advantages over the commonly used pneumatic systems. These advantages include: an atomization rate independent of any gas flow rate; very small droplet sizes which, for emission and absorption analysis, indicate that the fog can be transported relatively long distances without extensive condensation losses ; also, because of the small droplet sizes, the fog can be more efficiently used by the excitation source, increasing sensitivity and reducing interferences due to the longer residence time of dry aerosol in the flame or plasma; the particle size distribution is narrower than that produced by pneumatic atomizers; and, there is no capillary to clog. In spite of these advantages, use of ultrasonic atomization has not been widespread, perhaps because of the inconvenience of sample changing and clean-up with previously used systems. Indeed, designing a convenient system is not a simple task, because of the stringent requirements of both the ultrasonic sprayer and the analytical technique used. Workers who have successfully used ultrasonic atomization systems for spectrochemical analysis include Gerken et al. ( I ) , Dunken et at. (2), Wendt and Fassel (3,4), West and Hume (5), and Hoare and Mostyn (6). A somewhat different sort of ultrasonic atomization was made possible by the recent development of a nebulizer which totally nebulizes solution introduced at a fixed drop rate (7). In all cases known to this writer, ultrasonic spraying has compared favorably with pneumatic spraying. The ultrasonic atomizing system described here has the dual advantages of high sensitivity and high efficiency. Because it does not depend on a rapidly moving gas stream to form the droplets, the often necessarily-low flow rates may be used. The rate of sample consumption is low (less than 1 ml/min) and a large percentage of the atomized sample enters the torch chamber. The ultrasonic generator used was a Macrosonics Corp. Model 250-FF aerosol generator, with a Type A-2000 transducer. This unit operates at 300 kHz, delivering 12 W/cm2 of power to the active face. The transducer itself is cylindrical in shape, 6.15 cm in diameter by 3.35 cm high overall. The diameter of the (circular) active face is 3.5 cm. Present address, Department of Chemistry, Occidental College, Los Angeles, Calif. 90041
(1) D. B. Gerkin, L. M. Ivantsov, and B. I. Kostin, Ind. Lab., 28, 1550 (1962). (2) H. Dunken, G. Pforr, W. Mikkeleit, and K. Geller, 2.G e m . 1963, 196. (3) R. H. Wendt and V. A. Fassel, ANAL.CHEM., 37,920 (1965). (4) Ibid., 38, 337 (1966). (5) C. D. West and D. Hume, Ibid., 36, 412 (1964). (6) H. C. Hoare and R. A. Mostyn, Ibid., 39, 1153 (1967). (7) W. J. Kirsten and G. 0. B. Bertillsson, Ibid., 38, 648 (1966).
Figure 1 Ultrasonic sprayer apparatus
EXPERIMENTAL
Figure 1 shows the cell designed for spectrochemical research and analysis. The requirement of water cooling was met by constructing a water bath 10 X 6 X 4 inches high, using a 1/4-inch Lucite sheet, to contain the cooling water. It is not possible to allow the sample itself to provide the cooling function as this condition would require sample volumes too large to be practical. The sample was therefore placed in a separate cell, A , 1.5 inch in diameter X 1 inch high. This cell was then placed in the water bath 2 inches above the transducer. Experience demonstrated the necessity of placing the cell at a reproducible position in space above the transducer. The cross-hatched area ( B ) is an aluminum support bracket constructed for this purpose. The transducer, C, is placed against the back of this bracket and held firmly by spring clamps. Above the center of the transducer, a hole which is the exact size of the cell, is drilled into a 1/4-inchaluminum plate to a distance of lis inch. The final inch is drilled to a diameter slightly smaller than the cell in outside diameter. This arrangement allows simple and reproducible positioning of the cell above the transducer. The cell was constructed of polyethylene because this material attenuates the sound waves less than other readily available materials. It was found that neither steel, Teflon (polytetrafluoroethylene), nor glass is suitable because of the very strong attenuation of the ultrasonic energy by these materials. It should be mentioned here that air is one of the worst offenders in this regard, and a trapped air bubble beneath the cell will prevent the formation of fog in the sample. No difficulty with this problem was experienced in the design described, because air bubbles are quickly flushed away by the turbulent water. For operating convenience, it is highly desirable to have a system which permits rapid interchange of samples. This interchange was accomplished by producing cells with a lip whose internal edge is cut at a 45-degree angle. A Lucite (polymethyl methacrylate) tube (glass is satisfactory for organic solvents) of the same inside diameter as the cell was VOL 40, NO. 1, JANUARY 1968
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made with a rounded bottom edge, so that a gas-tight fit was obtained by pressing the tube firmly into the lip of the cell. To make the changing of samples still more rapid and convenient, a Knu-Vise No. V-400, E, was fitted with a clamp to hold the Lucite tube, and placed on an aluminum block behind the water bath (see Figure 1). To change samples, the lever is simply pushed back, raising the tube; the polyethylene cell is lifted out, and is replaced by a new cell. By pulling the lever forward again a gas-tight seal is produced. Sample changing takes only a few seconds. and reproducible geometry is maintained. For easy rinsing, the tube is fitted with a screw cap, F, (also machined from Lucite). Three holes are drilled into this cap: two holes lj4 inch in diameter for introduction of the carrier gas and for addition of the solution, and one hole inch in diameter for the removal of the gas and atomized sample. The carrier gas enters the Lucite tube through a polyethylene funnel, G, as shown in Figure 1. This funnel allows rapid sweeping of the mist uniformly around its sides, and permits the unatomized liquid and condensed liquid to return rapidly to the cell. (While the apparatus is in operation, the ultrasonic waves produce a fountain of unatomized liquid. Any design must allow this fountain to be produced, as the desired mist appears to come from all portions of the fountain.) In order to prevent sample liquid from flowing back into the cell and contaminating future samples, the liquid trap, H , was placed just above the fog and carrier-gas outlet.
Condensed liquid flows down the inner walls of a 2-oz polyethylene bottle and collects until it is released by the pinch clamp. Teflon is recommended for the transfer tubing beyond this point as a nonwetting characteristic of this material causes less condensation to occur. RESULTS AND DISCUSSION
The apparatus described will produce a satisfactorily constant rate of fog from a 10-ml initial sample volume for some eight to 10 minutes. In one experiment, magnesium (at the 10-ppm level in water) was sprayed into a radiofrequency plasma emission source (3, and the net magnesium signal was made to give a full-scale signal on a 10-inch recorder. Without making any instrumental adjustments, and periodically adding solution as required, the signal held constant within 1 % for 30 minutes. This demonstrates that the spray rate can be quite constant, and that no significant concentration of magnesium occurred in the sample cell. The apparatus is easily constructed from readily available material, allows rapid interchange and reproducible positioning of sample cells, can be easily cleaned, and is not easily clogged by a liquid. RECEIVED for review August 8, 1967. Accepted November 6, 1967.
A Chemically Selective Polarographic Detector for Gas Chromatography R . P. Van Duyne’ and David A. Aikens Department of Chemistry, Rensselaer Polytechnic Institute, Troy, N . Y . 12181
A NUMBER OF DETECTORS are currently used in gas chromatographic analysis, but few of these exhibit significant selectivity with respect to the chemical properties of sample components. The thermal conductivity detector is almost totally nonselective because it responds to the thermal conductivity of the sample. The flame ionization detector has a small degree of selectivity in that it does not respond to the common carrier gases, water and ammonia. The electron capture detector achieves the highest degree of selectivity of the detectors in common use. The basis of electron capture detection is the electron affinity of the species being detected and it thus exhibits selectivity for compounds containing electronegative elements or electron-withdrawing groups. These three detectors belong to a class of gas chromatographic detectors known as internal detectors. The internal detector is located within the chromatographic oven and hence its response is closely related to the operating conditions of the chromatograph. For example, the thermal conductivity detector peak area response decreases with increasing carrier gas flow rate and the peak width is a function of the column temperature. The external detector, located outside the heated zone of the chromatograph and operated at room temperature, has the advantage that its reponse is essentially independent of the operating conditions of the gas chromatograph. External 1 Present address, Department of Chemistry, University of North Carolina, Chapel Hill, N. C .
(1) R. S . Juvet, Jr., ANAL.CHEM., 38, 565 (1966). (2) D. M. Coulson, J. Gas Chroniatog., 3, 134 (1965).
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ANALYTICAL CHEMISTRY
detectors reported in the literature seem to be directed at the problem of achieving chemical selectivity in gas chromatographic detection. The flame photometric detector of Juvet (I) achieves selectivity through measurement of the emission intensity at an appropriate wavelength. Coulson’s ( 2 ) electrolytic conductivity detector achieves selectivity by responding only to components which can be combusted catalytically to gases such as HC1 and SO2 which give highly conducting solutions in water. Polarography also exhibits a high degree of selectivity toward chemical structure. This work is concerned with exploitation of the selective response of polarography as a gas chromatographic detector with a high degree of chemical selectivity. The limit of detection of conventional dc polarography is approximately 5 X lO-5M for organic compounds and 1 pl of an organic liquid whose density is approximately 1 gram/ml produces a concentration 1000 times this limit in a volume of 1 ml. Thus polarography is a suitable technique for measuring the small amounts of material commonly encountered in gas chromatographic analysis. Higher chemical selectivity and a lower limit of detection can be obtained by using catalytic polarographic waves as the basis of detection. The cyclic regeneration of the catalyst causes the sensitivity with respect to the catalyst to be several orders of magnitude greater than in direct polarography and high selectivity is achieved because only compounds with specific chemical properties can catalyze a given electrode reaction. In this work the nickel(I1)-pyridinc system has been used to characterize polarographic detection. A number of other volatile compounds also exhibit catalytic ability suggesting
