Anal. Chem. 1990, 62, 2745-2749
2745
Conversion of an Ultrasonic Humidifier to a Continuous-Type Ultrasonic Nebulizer for Atomic Spectrometry Robert
H.Clifford and Akbar Montaser*
Department of Chemistry, George Washington University, Washington, D.C. 20052 S c o t t P.Dolan and S t e p h e n G . C a p a r Division of Contaminants Chemistry, Food and Drug Administration, Washington, D.C. 20204 INTRODUCTION The most commonly used solution nebulizers (1)in atomic spectrometry are: (a) pneumatic nebulizers (PNs), (b) ultrasonic nebulizers (USNs), and (c) glass frit nebulizers (GFNs). Most P N s are extremely inefficient because the majority of test solution (98-99%) is directed to the drain. Glass frit nebulizers are highly efficient a t low uptake rate (50-150 pL/min), but the most glaring disadvantage of the current GFNs is the reduction in aerosol production as the result of repeated usage ( 2 , 3 ) . For USNs (4-IO),efficiency of aerosol production is improved by a factor of approximately 10 compared to PNs if the test solution is not highly viscous. However, the present commercial USNs (11-13) are quite expensive (approximately $12 000-$17 000) compared to PNs and GFNs ($100-$800). Conversion of ultrasonic humidifiers to low-cost ultrasonic nebulizers for plasma spectrometry has been described in two recent reports (14, 15). In principle, these nebulizers are similar in design in that a transmitting bath was used to transfer the ultrasonic radiation to the test solution t o be nebulized ( 4 , 5 , 8, 9). Such devices are referred to as geyser-type ultrasonic nebulizers. Because the nebulizers (14,15) were operated in the batch-type sampling mode, long-term precisions were not satisfactory due to gradual consumption of the test solution in the USN. Sample changeover was also time consuming. In this report, we describe simple conversion of an inexpensive (approximately $50), commercial ultrasonic humidifier to a continuous-type ultrasonic nebulizer suitable for analytical atomic spectrometry. The total cost of the proposed system used in the batch or continuous mode is less than $800. The new USN was compared to a commonly used P N with respect to atomic emission detection limits, precision of the analyte signal, signal-to-background ratios (S/B), percent relative standard deviation (% RSD) of background, droplet size, and noise power spectra (NPS). Detection limits obtained with the new system were also compared to results reported for commercial USNs (11-13). EXPERIMENTAL SECTION The Geyser-Type Ultrasonic Nebulizer. Figure 1shows the major electrical components of an ultrasonic humidifier unit (Model HM-310, Holmes Products Corp., Milford, MA). The electronic portion of the humidifier consists of a 120-V outlet, a fan motor, a step-down transformer (from 120 to 48 V), a power supply, a piezoelectric crystal (transducer), and a float switch for monitoring the water level in the humidifier tank (not shown in Figure 1). To convert the ultrasonic humidifier to an USN, the fan motor and water tank were removed and the float switch was shorted. Figure 2 shows the major components of the USN. The nebulizer consisted of (a) control electronics, (b) an acrylic base, (c) a metal plate for supporting the transducer, (d) a piezoelectric crystal, (e) an acrylic coolant system, (f) a Mylar sheet for separating the test solution from the transmitting bath, (g) a Teflon sample cell, and (h) a dual-tube glass spray chamber. The box insert in Figure 2 shows that the Teflon sample cell contains three threaded orifices (l/., in., 28 threads/inch) for introducing and
* To whom correspondence should be addressed. 0003-2700/90/0362-2745$02.50/0
removing the test solution. A dual-head peristaltic pump (Rabbit-type, Rainin Instruments Co., Inc., Woburn, MA) was used to pump in test solution continuously and to maintain a constant solution level simultaneously. A second pump (Model MPC-1A1, Fluorocarbon Co., Anaheim, CA) was used to drain the sample cell when a new test solution was to be analyzed. These pumps were used because they were available in our laboratories, otherwise a less expensive four-channel pump (Model V34042, Markson Science, Inc., Phoenix, AZ) used in conjunction with a variable-steptransformer (Model V34105, Markson Science, Inc.) is also suitable for the cited operations. Ultrasonic waves were propagated from a 1.7-MHz transducer through the coolant water, Mylar sheet, and test solution to form the aerosol. The transducer was surrounded by a rubber gasket (not shown) and held in place by a metal plate. The metal plate was then mounted on an acrylic base, which held the nebulization unit vertically. To improve nebulization stability, the transducer was cooled with water at room temperature. Coolant water was circulated in and out of the acrylic cell (14 mm i.d. and 17 mm in length) through two threaded orifices (1/4 in., 28 threads/inch) with a peristaltic pump (Model Minipulse 2, Gilson Medical Electronics, Middleton, WI) operated at a rate of 3 mL/min. Again, this pump was used only for the reason of availability in our laboratories. Only doubly deionized, degassed water was used to cool the transducer, otherwise small bubbles could be formed in the transmitting bath, thereby producing an unstable signal. An acrylic material was used to construct the cell so that formation of air bubbles in the cell could be observed. The sample cell had an i.d. of 12 mm, was 32 mm long, and protruded 10 mm into the spray chamber. The dual-tube spray chamber was 15 cm long with inner and outer tubes having a 21 mm i.d. and 25 mm o.d., respectively. The inner tube was placed 30 mm above the bottom of the spray chamber. To produce the most dense aerosol, the solution level was maintained at approximately 8 mm above the sample cell. For this purpose, a straight Teflon tube (1mm 0.d.) was inserted into the drain orifice such that the tip of the Teflon tube was located 8 mm above the sample cell. The total volume of the test solution required to fill the sample cell at the optimum level was 9 mL. This volume may be reduced by fabricating a smaller sample cell. Procedures for Sample Delivery to the USN. The sample cell was filled to the optimum level with test solution by using the first channel of the peristaltic pump, while excess solution above the constant-level drain was removed by the second channel of the same pump. This process continued until another test solution was to be analyzed. The sample uptake tube was then removed from the test solution with the pump still on, and the main drain system was engaged until the sample cell was empty. The sample cell was then flooded with doubly deionized water with the main drain pump still engaged. After the cleanup process, the drain pump was disengaged and test solution was introduced into the sample cell via the peristaltic pump. At the present time, the toal time required for a complete sample change is approximately 6 min. In principle, reduction of the sample cell size and/or use of higher speed pumps should reduce this time significantly. The Desolvation Unit. Aerosol exiting the spray chamber was desolvated with a 40 cm long heating chamber wrapped with heating tape powered with a variac instrument. Chamber temperature was monitored with an RTD probe attached to a temperature controller (Model Dyna Sense 2157, Cole Palmer Instrument Co., Chicago, IL). This controller was not used to maintain the temperature because temperature fluctuations of 0 1990 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 24, DECEMBER 15, 1990
Table I. Experimental Facilities and Operating Conditions 2.5-kW, 27.12-MHz crystal-controlled generator (Henry Electronics, Los Angeles, CA) with auto power control. The automatic matching network is described elsewhere (16). Extended tangential flow torch with side arm (Applied Research Laboratories, Valencia, CA). Ar ICP torches See Table I1 for operating conditions. A 3.5-turn, shielded load coil was used (17). See text. For detection limit studies, a multielement solution of the elements (10pg/mL for sample introduction system PN and 1 pg/mL for USN) shown in Table V was prepared in 1%nitric acid solution. For studies involving the noise power spectra, the nebulizers were operated wet (deionized water) or dry. 1-m focal length direct reader in a Paschen-Runge mounting (Model 3580, Applied Research spectrometer Laboratories, Valencia, CA) with a 1080 groove/" grating, and 21- and 20-pm entrance and exit slit widths, respectively. Slit height was 10 mm. A 1:l image of the plasma was formed on the entrance slit. detection system for NPS measurements The sequential spectrometer (Model 3580, Applied Research Laboratories, Valencia, CA) was used, 21- and 20-pm entrance and exit slit widths, respectively. Slit height was 10 mm. A 1:l image of the plasma was formed on the entrance slit. Current output from the photomultiplier (Type R106 UH, Hamamatsu Corp., Bridgewater, NJ), operated at the same voltage for all measurements, was amplified by a linear current-to-voltageconverter (Model 427, Keithley Instrument, Inc., Cleveland, OH). The data acquisition system consisted of a Labmaster ADC (Tecmar, Inc., Cleveland, OH) installed on an IBM-PC-AT microcomputer. ASYSTANT+ (Asyst Software Technologies, Inc., Rochester, NY) was used to acquire the noise power spectra; see refs 17 and 18. radiofrequency generator
I
;:1
TOP VElW OF SAMPLE CELL IPLE
I
CONSTANIT LEVEL DRAIN
lNLtI
I
7 cm
48vI I
I
STEP D O W N TRANSFORMER
-t
- 1 4 c m
e&/
TUBE
DRAIN OUTLET
GLASS SPRAY CHAMBER TEFLON DRAIN TUBE TEFLON SAMPLE CELL
SAMPLE UPTAKE OR DRAIN
CONSTANT LEVEL DRAIN
7 cm
W FLOAT SWITCH
t
\
1
TRANSDUCER
CON~ROL ELECTRONICS
Flgure 1. Major electrical components of the commercial ultrasonic humidifier.
f 5 OC were noted, thus causing poor precision. Two 40-cm condensers (Graham and Allihn type) maintained at 0 "C (Model Coolflow 33, Neslab Instruments, Inc., Portsmouth, NH) were used to condense the water vapor. The Graham condenser was placed after the heating chamber to remove most of the water vapor. Because a large amount of wet aerosol exited the Graham condenser, the use of a second condenser was essential. Pneumatic Nebulizer and Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) Instrumentation. A concentric glass P N (Type TR-30-A3, J. Meinhard Associates, Santa Ana, CA) with an ARL conical spray chamber (Applied Research Laboratories, Valencia, CA) was also used in these studies. A peristaltic pump (Model Minipulse 2, Gilson Medical Electronics, Middleton, WI) was used to deliver test solutions to the nebulizer. A mass flow controller (Model 8200, Matheson Gas Products, East Rutherford, NJ) was used to control the injector gas flow. The ICP-AES spectrometer (Model 3580, Applied Research Laboratory, Valencia, CA) and the operating conditions are listed in Tables I and 11, respectively. Aerosol Particle Counting System. The techniques (1-3) used to estimate droplet diameter include measurement with
MYLAR SHEET ACRYLIC COOLANT CELL
L : ! -
i
WATER
a
PIEZOELECTRIC CRYSTAL TRANSDUCER HOLDER ACRYLIC BASE CONTROL ELECTRONIC
Flgure 2. Components of the geyser-type ultrasonic nebulizer fabricated from an ultrasonic humidifier.
Table 11. Plasma Operating Conditions for Ar ICP-AES Studies forward power, W reflected power, W observation height, mm outer gas flow rate, L/min intermediate gas flow rate, L/min injector gas flow rate, L/min Meinhard geyser-type uptake rate, mL/min desolvation unit for the geyser-type USN heating chamber, "C condensers, "C
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