2327
Anal. Chem. 1984, 56,2327-2329 (45)
Cheng, H. N.; Ellingsen, S. J. J . Chem. Inf. Comput. Scl. 1983, 23, 1.-~.7 -- m-a.
(46) Morbnl, T.; Iwasakl, H. Macromolecules 1978, 7 1 , 1251-1259. (47) Ebdon, J. R.; Kandel, S. H.; Morgan, K. J. J . folym. Sci., folym. Chem. Ed. 1979, 17, 2763-2790. (48) Suggate, J. R. Makromol. Chem. 1979, 180, 679-691. (49) Elgert, K.-F.; Stutzel, B. Polymer 1975, 18, 758-761. (50) Cheng, H. N., unpublished data. (51) Zambelll, A.; Qattl, 5. Macromolecules 1978, 7 1 , 485-489. (52) Schilling, F. C.; Tonelll. A. E. Macromolecules 1980, 73, 270-275. (53) Reference 24, p 33.
(54)
Mlrabella, F. M., Jr. Polymer
1977, 18, 925-929.
RECEIVED for review May 1, 1984. Accepted June 26, 1984. Presented at the American Chemical Society Delaware Division Analytical Topical Group Meeting, Feb 22, 1983. Interested readers may write to the authors for a program listing Of PSPEC* This is Research Center Contribution Number 1764.
Modified Ultrasonic Nebulizer for Inductively Coupled Argon Plasma Atomic Emission Spectrometry P.D. Goulden* and D. H.J. Anthony National Water Research Institute, Canada Centre for Inland Waters, Burlington, Ontario, Canada L7R 4A6
Stable and high efficiency operation of an uttrasonlc nebulizer Ls achieved when both the sample and the cooling water are properly coupled to the transducer. I n the horizontal configuration this is achieved by ensuring that no air bubbles can remain trapped in the cooling water beneath the transducer and that the sample wets the upper surface of the nebulizer.
Ultrasonic nebulizers used for sample introduction in analysis by inductively coupled argon plasma (ICAP) spectrometry are typified by those described by Olsen et al. (I) and by Taylor and Floyd (2) in which the ultrasonic transducer is mounted with its face in a vertical (I) or horizontal (2) position. Although having several advantages over conventional pneumatic nebulizers, these ultrasonic systems do not seem to have gained general acceptance in ICAP analysis. Our experience has been that the equipment as described (and purchased) has problems with both long- and short-term stability. We found that while the nebulizers would produce copious quantities of fine aerosol, on many occasions, within a few minutes or perhaps the next day, the amount of aerosol would be drastically reduced for no reason apparent to us. Discussions with other workers in the field suggest that that was a typical experience. We, therefore, developed a pneumatic nebulizer/heated spray chamber technique (3) for routine analysis of freshwater samples. This technique, which also involves a 10-fold preconcentration step, has proven to be a reliable and reproducible way to determine trace elements in freshwater. However, the physical properties of the dried aerosol change as the total solids content of the sample solution increases so that at high total solids levels the amount of sample reaching the plasma is drastically reduced. This makes the technique unsuitable for such samples as seawater and digested sediment and fish samples. We find the ultrasonic nebulizer to be much less subject to this type of matrix effect. This is presumably because of the different particle size distributions in the aerosol from the two nebulizers, although we have made no particle size measurements. Hence, if the ultrasonic nebulizer could be made to operate in a stable and reproducible manner, it would broaden the range of environmental sample types that could be conveniently analyzed by ICAP. In the present work some causes of instability in the operation of the ultrasonic nebulizer have been identified. We find that minor modifications in the construction and oper0003-2700/84/0356-2327$01.50/0
ation of the commonly used equipment give a system suitable for routine analysis of all of our environmental samples. Compared to the heated spray chamber, the short-term (Le., within 1 day) stability is better and the long-term (i.e., week to week) reproducibility is at least as good. The nebulizer produces a large volume of aerosol, and the heated chamber-condenser system used with the pneumatic nebulizer would not properly desolvate it. Desolvation equipment with a greater capacity was therefore built to handle this large volume of aerosol.
EXPERIMENTAL SECTION Apparatus. Figure 1shows the construction of the transducer assembly. The transducer is a piezoelectric disc type (Channel Products Inc., Model CPMT) operating at a nominal frequency of 1.4 MHz. The transducer, 26.5-mm diameter, is bonded to a glass disc, 27.0-mm diameter. The glass tube to hold the transducer is standard wall borosilicate glass, 28-mm 0.d. The end of this tube is expanded and ground with a “glass-joint”taper so that when the transducer is inserted in it there is a lip 2 mm high around it. The glass disc is bonded to the tube with acoustic epoxy cement (”Eccobond 55”). The rf power for the transducer is generated with a frequency synthesizer/function generator (Hewlett-Packard,Model 3325A) amplified with an rf amplifier (ENI, Model 2100L). Reflected power is monitored with an rf directional wattmeter (Bird “Thruline”, Model 43). The sample is pumped with a peristaltic pump (Gilson, Minipuls). Delivery of the sample to the nebulizer is made via Teflon standard wall tubing (size,AWG No. 22). This delivers the sample to the outer edge of the glass disc inside the lip. The precise position is not important; it makes no difference whether or not the end of the tube touches the glass. The desolvation equipment is shown in Figure 2. The wet aerosol is dried in an annular chamber heated with a quartzhalogen lamp (CGE Model DYH) at ita center. The power to the lamp is supplied through a variable transformer. The resulting water vapor is removed in the condenser in tandem with the drying chamber. The ICAP system is as previously described (3). Reagents. The reagent was 0.01% (by volume) solution of a surfactant (Rohm and Haas, Triton X-405). Procedure. The sample flow rate is 3.0 mL/min-l; the carrier gas flow rate is 850 mL/min-l. A sample is pumped for 2.5 min; between each sample is interspersed a 1-min wash of 0.2% “OS. Integration times are 50 s “on-peak”and 50 s “off-peak”. These are the same conditions as those previously described (3). The power applied to the transducer is 40-W forward power with less than 1-W reflected power. The heating lamp is operated at 250 W (70 V). The plasma operating conditions are the same as those previously described (3). 0 1984 American Chemical Society
2328
ANALYTICAL CHEMISTRY, VOL. 56,
NO. 13, NOVEMBER 1984
h
Flgure 1. Transducer assembly: a, transducer; b, No. 3 rubber stopper: c, “0”rings; d, 28-mm-0.d. borosilicate tubing: e, coollng water in; f, 38-430 screw cap; g, sillcone rubber seal; h, cooling water Out.
0
35 mm 0.D
\
180mm
8mm 0.D
Flgure 2. Drying-desolvatlon apparatus.
Each day, before samples are analyzed, the surfactant wash solution is pumped over the transfucer surface for 10 min.
RESULTS AND DISCUSSION Method Optimization. Transducer Operating Stability. For routine use in ICAP analysis, the most important requirement of the ultrasonic nebulizer is that it produce continuously a large and stable amount of aerosol over the analytical period and that the rate and stability of that production be reproducible from day to day. This ability is influenced by the electromechanical stability of the transducer and is related to the efficiency of coupling between the transducer and the medium adjacent to each face, i.e., sample on one side, cooling water on the other. Stable operation of the transducer is achieved only when both faces are uniformly covered with liquid. Instability, as shown by erratic fluctuations of forward and reflected power in the rf circuit, produces variations in aerosol production and in ICAP response.
30
40
50
60
POWER (watts)
Flgure 3. ICAP response at various transducer power levels and sample flow rates. (Figures on each curve are flow rates in mL.min-’.)
If the transducer is completely immersed in water under “high-flow”conditions so that the temperature is controlled and no bubbles of air can collect on the swfaces, it will operate for many days with no variation in the forward and reflected power levels. If air bubbles are allowed to collect on either of the surfaces, the power levels fluctuate as the air bubbles grow and then break free. In the vertical configuration used by Olsen et al. (I), the cooling water side of the transducer can be kept free from bubbles but it is impossible to maintain a uniform water film on the sample side. In the horizontal configuration used by Taylor and Floyd (2),the sample side can be uniformly wetted but there are problems with air bubbles collecting at the bottom surface of the transducer. These bubbles arise both from air bubbles brought in with the cooling water and from dissolved air released through the cavitation produced by the transducer motion. This problem is overcome in the present system by passing a large volume of cooling water (about 0.5 L/min-l) through the narrow space between the rubber stopper and the transducer. The all-glass construction makes it possible to adjust the thickness of this space and monitor how effective it is in preventing the collection of bubbles. The ground surface of the glass disc on the transducer as supplied (by Channel Products Inc.) is rough enough that, if it is clean, water will wet it satisfactorily. However, after use, the surface becomes contaminated with grease, from either the equipment or the samples, and it is no longer completely wetted. This results in erratic operation of the transducer. If the glass surface is washed with a surfactant solution, the surface remains wettable for some considerable time, depending on the samples being analyzed. The samples we analyze are mostly clean water from the Great Lakes. With these samples it is sufficient (and convenient) to wash the transducer glass surface each day as part of the start-up procedure. The Triton X-405 surfactant was at hand and proved satisfactory. We have not investigated other surfactants. The sample flow rate and the power applied to the transducer must also be balanced in order to keep a water film on top of the nebulizer. For any particular sample rate there is a power minimum below which a standing wave is formed and no aerosol is produced. As the power is increased, aerosol begins to be formed; as the power is increased further, a point is reached at which the sample is thrown off the surface so violently that dry spots occur. At this point the transducer operation becomes unstable and the ICAP response decreases
rj,& T
20
Anal. Chem. 1984, 56,2329-2335
Table 1. Detection Limits with Ultrasonic Nebulizer detection limit,
detection limit,
element
pg.L-1
element
/.kg.L-'
A1
5.1 0.091 0.55 0.42 0.91 0.35
Mn Mo Ni Pb V Zn
0.042 0.50 0.46
Cd co Cr
cu
Fe
0.94 0.35 0.12
because the dry spots are alternately formed and then covered with sample. The relationship between power and ICAP response for various sample flow rates is shown in Figure 3. The ICAP response shown is the reading in the manganese channel. The responses for other metals were similar. It is seen that at each flow rate there is an optimum power level and that the optimum response increases with sample flow rate up to about 7 ml.min-l. We attribute the flattening of response at the higher flow rate to the inability of the desolvation system to transport the aerosol. The sample flow rate and power levels of 3.0 ml-min-l and 40 W, respectively, were chosen as representing an acceptable ICAP response with extended transducer life.
DRYING AND DESOLVATION A large amount of aerosol is produced by the nebulizer. Under the "routine analysis" conditions (3 ml-rnin-l, 40 W) approximately 0.5 mL.min-l of water leaves the nebulizer chamber and is collected via the condenser system. From measurements similar to those previously described (3), we estimated that the equivalent of 0.3 ml-min-' of sample reaches the plasma under these conditions. We believe that the largest potential loss of aerosol is by agglomeration of the wet particles. The small volume nebulizer chamber and the annular drying chamber permit the rapid removal and drying
2329
of the particles to minimize this agglomeration. The condenser provides fast and efficient condensing of the water vapor with a minimum of dead volume. The volumes of the nebulizer chamber, drying chamber, and condenser are 26, 25, and 35 mL, respectively. These small dead volumes also give fast sample washout characteristics;e.g., when a sample containing 1mg-L-' of manganese is followed by a 0.2% HNO, wash, the ICAP signal falls to 1% of the sample reading within 30 s of the changeover.
SENSITIVITY AND DETECTION LIMITS The detection limits of the system were determined in a manner similar to that previously described (3). The detection limits observed for a number of elements in water from Lake Ontario as is, i.e., with no preconcentration,are shown in Table I. These are about 6 times better than those obtained with the pneumatic nebulizer/heated spray chamber system. The improvement is caused by a 3-fold increase in sensitivity and a 4-fold reduction in the variance; i.e., the standard deviation of replicate analyses decreased by a factor of 2. This lower variance obtained with the ultrasonic nebulizer, as compared to the pneumatic nebulizer/heated spray chamber system, is a measure of the greater stability of the nebulizer within the analytical period of a day. The long term stabilities of the two systems are about the same. Analyses carried out on the same standards over a period of 3 months give "millivolt readings" that do not vary by more than 10% for either of the systems. LITERATURE CITED (1) Olsen, K. w.; Haas, W. J.; Fassel, V. A. Anal. Chem. 1977, 49, 632-637. (2) Taylor, C. E.; Floyd, T. L. Appl. Specfrosc. 1981, 35, 408-413. (3) Goulden, P. D.; Anthony, D. H. J. Anal. Chem. 1982, 5 4 , 1678-1681.
RECEIVED for review March 7,1984. Accepted June 25, 1984.
Inductively Coupled Argon Plasma Atomic Emission Spectrometry with an Externally Cooled Torch Peter A. M. Ripson, Liesbeth B. M. Jansen, and Leo de Galan* Laboratorium voor Analytische Scheikunde, Technische Hogeschool, Jaffalaan 9,2628 B X Delft, The Netherlands An evaluation is presented of a torch for inductlvely coupled plasma atomic emlsslon spectrometry that is coded externally by either air or water. The total argon consumption Is 1 Umln and the air-cooled torch requires substafitlaliy less rf power than elther the water-cooled torch or a conventional ICP. The optimization study shows that only the Incident power, the argon flow rates, and the tip diameter of the sample Introduction tube are critical parameters. When run under compromise conditions, the externally cooled ICP Is easy to operate and accepts hlghly satted aqueous solutions as well as organic solvents. All designs tested show excellent anaiytkal dynamic range, precision, and stablilty. However, in terms of detection power and matrix Interferences the air-cooled torch with an outer tube diameter of 16 mm yields superior performance, fully Comparable to that of a conventional ICP torch.
In a previous publication we have described a torch for inductively coupled plasma (ICP) atomic emission spectrom0003-2700/84/0358-2329$0 1.50/0
etry that uses a total argon consumption of only 1L/min by virtue of external cooling with pressurized air (I). The analytical data provided were promising but did not allow a complete assessment of the analytical performance of the torch. Since then a similar torch utilizing external cooling with water has been designed. Because a liquid is a much more efficient coolant medium than a gas, the use of water cooIant would enlarge the range of practical operating conditions (2). In a later publication an analysis of the power requirements of both externally cooled torches (3) demonstrated that the air-cooled torch runs on less than 400 W of incident power, whereas the water-cooled torch requires about 800 W. In order to compare the two designs with existing ICP torches, an evaluation of their analytical performance is necessary. Nearly all investigators who have constructed a low consumption ICP either by modified internal or by external cooling (4-20) indicate that the detection limits are roughly comparable to those of a conventional ICP. If matrix interferences are reported, they occasionally show less promising results (5-7,I.l). Other analytical properties are only rarely @ 1984 American Chemlcal Society
