Table 111. Nebulizer/Air-Acetylene System: Interference and Precision Data Calibration curve gradient (arbitrarj Metal
Lead
Silver
Cadmium
Zinc
Copper
hiatrix
Aqueous Aluminum Blood Oil Salt Aqueous Aluminum Oil Salt Aqueous Aluminum Oil Salt Aqueous Aluminum Oil Salt Serum Aqueous Oil
Serum
units)
1.oo 0.83 0.98 1.53
1.oo 1.oo 0.86 2.61
-
1.01 1.oo 0.72 3.22
1.oo 1.oo 0.72
RSD (”)
1.1 6.4
1.1 1.1 8.1 1.2 3.7
1.2 5.3 1.5 2.4
1.3 4.2
0.7
5.36 1.02
4.4 0.8 8.2
1.01
1.7
1.oo
2.96
1.oo
1.2
1.3 2.2
This study was confined to five elements, and others may not show the same freedom from interferences. Interference studies with a number of other elements are in progress, and results will be presented in due course. Elements which form compounds that volatilize at temperatures well below 1800 K are most likely to be substantially interference-free. This group should include As, Au, Bi, Hg, Sb, Se, Sn, and T1. Elements (such as Ba, Co, Cr, Mn, and Ni) which form less volatile compounds will not necessarily perform as satisfactorily, though the cup/nitrous oxideacetylene flame system may still yield better results than other microatomization techniques.
LITERATURE CITED (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18)
where a conventional nebulizer-burner system is suitable, it remains the method of choice, except perhaps for analyses in an organic phase where suitable standards are unavailable.
(19)
D. A. Segar and J. G. Gonzalez, Anal. Chim. Acta, 58, 7 (1972). F. J. Fernandez and D. C. Manning, At. Absorpt. Newsl.,,lO, 65 (1971). W. M. Barnard and M. J. Fishman. At. Absorpt. Newsl., 12, 118 (1973). M. A. Evenson and D. D. Pendergast, Clin. Chem., 20, 163 (1974). C. W. Fuller, Anal. Chim. Acta, 62, 442 (1972). F. Dolingek and J. &par, Analyst(London),98, 841 (1973). M. Glenn, J. Savory, L. Hart, T. Glenn, and J. Winefordner, Anal. Chim. Acta, 57, 263 (1971). J. P. Matouhk and B. J. Stevens, Clin. Chem., 17, 363 (1971). B. J. Stevens, Clin. Chem., 18, 1379 (1972). T. Takeuchi. M. Yanagisawa, and M. Suzuki, Talanta, 19, 465 (1972). J. Y. Hwang, P. A. Ulucci, S. B. Smith, and A. L. Malenfant, Anal. Chem., 43, 1319 (1971). J. E. Cantle and T. S. West, Talanta, 20, 459 (1973). F. J. Fernandez, At. Absorpt. News/., 12, 70 (1973). F. J. Fernandez and H. L. Kahn, At. Absorpt. Newsl., 10, 1 (1971) G. Heinemann. 2. Klin. Chem. Win. Biochem., 5 , 197 (1973). H. T. Delves, Ana/yst(London),95, 431 (1970). D. G. Mitchell, K. M. Aldous, and F. J. Ryan, paper presented at International Symposium on Environmental Health Aspects of Lead, Amsterdam, October (1972). A. F. Ward, D. G. Mitchell. M. Kahl, and K. M. Aldous, Clin. Chem., 20, 1199(1974). K. M. Aldous. D. G. Mitchell, and F. J. Ryan, Anal. Chem., 45, 1990 (1973).
RECEIVED for review March 7, 1975. Accepted May 12, 1975.
Utilization of Ultrasonic Nebulization in Atomic Absorption Spectrometry-A Study of Parameters Haleem J. Issaq’*2 and Lawrence P. Morgenthaler3 Department of Chemistry, Georgetown University, Washington, D . C . 20007
The aim of the present study is the use of ultrasonic nebulization in atomic absorption. The nebulizer and desolvation system consisted of an “Ultramist” nebulizer connected to a temperature controlled heater in series with a condenser and burner head. Parameters which affect heated chamber systems were studied. The temperature of the chamber was established as a critical parameter. It was found that there exists an optimum chamber temperature which might vary from element to element, depending on the melting point and vapor pressure of the analyte. Our system was the first one to use a temperature-controlled heater, with fast response to changes in thermal load, and which holds the temperature constant to within f10 ‘C. The desolvation efficiency was found to be a function of the chamber’s temperature, air and sample flow rate, and the temperature of
Author to whom correspondence should be addressed. Present address, Litton Bionetics, Frederick Cancer Research Center, P.O. Box B, Frederick, Md. 21701. Present address, Fisher Scientific, Waltham, Mass. 02134.
the cooling water circulating in the condenser. The proposed system, at optimum conditions of temperature and flow rate, has a sample efficiency of 86%, and a desolvation efficiency of 72 YO.
The use of atomic absorption for the analysis of metals is widely used and well accepted. Certainly, no analytical technique has witnessed a more rapid growth than atomic absorption spectrometry ( 1 ) . Basically, the technique involves a means of introducing the sample to be analyzed into the flame where the metal ions are converted into the atomic vapor state. For most elements, this atomic vapor exists in the ground state and can absorb resonant radiation of appropriate wavelengths. T h e critical parameter in atomic absorption is the concentration of free analyte atoms in the flame. The concentration of these atoms must be proportional to their concentration in the sample, and they must be uniformly distributed in the flames. ANALYTICAL CHEMISTRY, VOL. 47, NO. 9, AUGUST 1975
1661
A number of different techniques can be used for introducing the sample into the flame. Most commonly, the technique consists of aspirating the bulk liquid sample to form an aerosol with the flame oxidizing agent. The aerosol is then mixed with a fuel gas and passed through a burner into the flame. The aerosol droplets must rapidly undergo a complex series of changes on entering the flame as they will be in the reaction zone of the flame for less than a millisecond ( 2 ) . The solvent of the droplet must be evaporated (or decomposed), and the resulting dry, amorphous agglomerates are then dissociated and atomized. Any phenomenon that inhibits any process associated with the ultimate concentration of free atoms in the flame will, of course, decrease the sensitivity of the analysis. Mavrodineanu and Boiteux ( 3 ) described in great detail the development of means of introducing samples into the flame. These techniques will not be discussed here. The discussion will be confined to the pneumatic and ultrasonic nebulizers. Virtually all modern flame atomic absorption work uses the pneumatic neubulizer ( 3 ) . In such a nebulizer, a significant quantity of sample is lost through the drain, and only about 10% reaches the flame ( 4 ) including some relatively large droplets which are not desolvated in the area under observation within the flame ( 5 ) .It is clear that this pneumatic system has a tenfold potential improvement, in terms of delivery of aerosol to flame, by simply not rejecting 90% of the sample. Heiftje and Malmstadt (6) showed that atomic excitation of droplets introduced into the flame does not occur until desolvation is complete and that the time required for desolvation is a strong function of droplet diameter. Spitz and Uny ( 7 ) reported that the time necessary for complete evaporation of solvent varies with the square root of the initial diameter of the droplet. Therefore, there is a need for developing a new nebulization technique which produces uniform fine droplets. This was accomplished by using an ultrasonic nebulizer which was originally developed for therapeutical purposes. In ultrasonic nebulization, the surface of the liquid is exposed to ultrasounds generated with an oscillating piezoelectric crystal. Under the proper conditions, a very fine dense aerosol is produced. The aerosol particle size and its density are independently controlled, unlike pneumatic nebulization where particle size can only be reduced a t the expense of density because the air flow must be increased. In ultrasonic nebulization, the aerosol density can be varied simply by adjusting the air flow past the liquid surface (8). The size of the aerosol particle can be varied by changing the frequency of the ultrasound; the higher the frequency, the smaller are the droplets produced (9, 10). The mechanism of droplet formation in ultrasonic systems is described in the literature (7,8-12) and will not be discussed. Bisa et al. (13) derived a relation between the mean diameter of the droplets, the frequency of the ultrasound, and the physical properties of the solution. The use of ultrasonic nebulization in atomic absorption and flame emission was first reported by Dunken et al. in 1963 (14) and 1964 (15, 16) and by Hume and West (17)in 1964. Fassel and Wendt (18, 19) used an ultrasonic nebulization system similar in design to that of West and Hume. In 1966, Kirsten and Bertilsson (20) developed a new ultrasonic nebulizer which has a vibrating surface similar in design to the one reported by Herzog, Norlander, and Engstrom (21). The rate of nebulization was 0.3 ml/min and they obtained a twofold improvement in sensitivity for sodium and calcium. 1662
ANALYTICAL CHEMISTRY, VOL. 47, NO. 9, AUGUST 1975
Hoare and Mostyn ( 2 2 ) , and later Hoare, Mostyn, and Newland (12) used Hume and West’s type nebulizer. They (12) modified the transducer head assembly to reduce power losses. In their experiment, the efficiency of the ultrasonic nebulizer was 5 times that of the pneumatic; however, the sample volume consumed per unit time was larger for the pneumatic by about 1.5 times. When performance of the two atomizers was tested, there was no significant difference in the absorbance for different elements by the two systems. When comparing the total volume consumed in each system, the pneumatic nebulizer used about eight times that of the ultrasonic nebulizer, and the volume lost is more than twenty times larger in the pneumatic than in the ultrasonic nebulizer. Van Rensberg and Zeeman (23) designed their ultrasonic nebulizer by what they call a “spherical arrester”. In their study, the highest rate of nebulization did not give improved sensitivity. This was attributed to losses in flame energy which is used ‘In evaporating the sample. A similar behavior was confirmed by Spitz and Uny ( 7 ) and by Woodriff and Stone (24). Other researchers (25-30) used ultrasonic nebulization and reported improved sensitivities. The purpose of this work is the utilization of ultrasonic nebulization in atomic absorption in such a way that it will increase the efficiency of the nebulizer and to desolvate the aerosol before it enters the burner. Another purpose of this study is to investigate parameters that affect burner systems with heated chamber and to comment on these parameters, such as drop size; desolvation efficiency and its effect on the analytical sensitivity; sample flow rate and air (oxidant) velocity and their effect on desolvation efficiency; melting point; and vapor pressure of analyte and its relation to the temperature of the heated chamber.
EXPERIMENTAL Apparatus. A Jarrell-Ash Y-m Ebert monochromator with a 100-pm entrance and 150-pm exit slit was used. High voltage for the R106 photomultiplier tube was supplied by a McKee-Pederson MP-1030 power supply. The read-out electronics consisted of a Brower Laboratories Model 131 lock-in amplifier with mechanical chopper operating a t 55 Hz. All measurements were made with a 3-sec time constant on the lock-in amplifier. The output of the amplifier was connected to a 10-mV full scale Leeds and Northup recorder and to a Perkin-Elmer DCR-2 digital readout. The DCR-2 was used in the transmittance function operated in the eight average mode, Le., averaging eight successive data points. The hollow cathode lamps were commercially available Westinghouse lamps powered by Heath (No. EUW-15) Universal power supply through a 5000-ohm, 20-watt resistor. All the lamps used were multi-element lamps with neon as filler gas. The ultrasonic nebulizer was an “Ultramist 111” (presently available from Biologics, Inc., West Jordan, Utah). This unit operates a t 800 KHz at an acoustic power output of 15 to 20 watts. The maximum aerosol output is 1.1 ml/min with a droplet diameter range of 1.5 to 10 pm for dilute aqueous solutions (as reported by the manufacturer). In this unit, the ultrasonic transducer, Figure 1 is mounted in the base of a shaped aluminum cup. The sample is placed in a polycarbonate cup, having a maximum volume of 15 ml, which fits within the transducer mount. Water is used between the transducer and sample cup as an acoustic coupling medium. A molded polycarbonate cap screws onto the aluminum base and holds the sample cup in place. A rubber “0” ring between the cap and sample cup ensures that the system will be airtight. The desolvation unit, shown in Figure 2, is made of quartz and consists of three parts; the heating chamber ( A ) , the condenser (B), and the mixing chamber (C). All three parts form one unit which is connected on one side to the nebulizer and on the other t o the burner head. The heating chamber is 18 cm long and has an internal diameter of 1 cm. The chamber is heated by a temperaturecontrolled heater, which will be described later. The tube of the heating chamber is bent twice a t about 30’ to prevent any droplets from running to the heated area because, if they do, there is a rapid pressure increase in the system which changes flow rate of gases, composition of gas mixture (water vapor), and causes the
gas in
0
A k
A aerosol out Y
Rubber “0”ring Solution to be analyzed
.----:
I
I
Coupling water Transducer
Figure 1. Transducer head sample assembly
1
.15V
Figure 3. Circuit diagram of temperature controller
0.25 cm
Figure 2. Desolvation Unit
flame to pulse, thus increasing the noise. The condenser is 15 cm long and 0.5-cm i.d. and is inclined a t about 30” as an aid to the removal of the droplets of the solvent to the drain, thus preventing the accumulation of condensate in the tube. If these droplets remain in the tube, they might pick up sample particles, which as a result will decrease the amount of sample that reaches the flame. This decreases the efficiency of the nebulizer. The drain (D) is 0.25-cm i.d. and is connected to a water trap to maintain pressure inside the system. Fuel is introduced through a side arm (E), 0.25cm i.d., located 2 cm above the drain. The mixing chamber ( C ) is 8 cm long and 1-cm i.d. The spikes, which are made of quartz, are about 0.4 cm long. They extend over 2.5 cm of the mixing tube. The spikes induce turbulence to ensure mixing of aerosol sample with oxidant and fuel. The mixing chamber is plated on both inside and outside with platinum, and connected to circuit ground, to prevent accumulation of static charge. The outside was plated to make electrical connection with the inside through the edge of the tube. The heating element used consisted of 2.5-m chrome1 wire, 28-gauge, 4.1 ohmift, wrapped around the heating chamber, and was connected to the temperature controller. The burner mount and head were standard Techtron components, 10-cm slot air-acetylene burner head. Fuel and oxidant flows were determined by J.T. Baker Rotameter gauge No. 568278. The temperature controller, shown in Figure 3, was designed in this laboratory. In brief, the operation of the controller can be described as follows: a bias potential is applied to the temperature sensing alumel-chrome1 thermocouple, which is located 11 cm inside the heating chamber in the gas stream, in such a way that the sum of the bias and thermocouple potentials will be zero a t the desired temperature. Any difference between these two potentials is amplified by the pA741 integrated circuit operational amplifier. The output of this amplifier is connected to the input of the PA436 triac trigger circuit. The PA436 produces an output pulse during each half-cycle of the 60-Hz line voltage. The point in time a t which this pulse is produced during the half-cycle is determined by the input voltage. The triac is a bidirectional switch wired in series with the heater. The switch turns “off” each time the line voltage crosses zero and will not turn on again until a proper trigger pulse is applied to its gate. Power will flow to the heater only during the time the triac is “on”. If the thermocouple temperature drops below the temperature set by the bias voltage, an error signal is produced which is proportional to the temperature discrepancy. This causes the trigger pulses to occur earlier in each half-cycle, thus applying power to the heater. The greater the temperature discrepancy, the greater the amount of power developed in the heater. When the thermocouple temperature is equal to, or greater than, the temperature set by the bias voltage, no trigger pulses are produced and no
Values of electrical components: R1 = 1 kR, R2 = 240 kR. R3 = 0.1 kR. R4 10 kR, R5 = 10 kR, Re = 10 kR, R, = 200 kR, Re = 0 . 0 9 1 kR. RQ = 7.5 kR, R l o = 500 kR, R l r = 20 kR, RIP = 20 kR, R13 = 1 . 1 kR, R11 = 0.620 kR, R75 = 1 kR, Rye = 12 kR, R17 = 5 kR, and R18 = 0.100 kR. C1 = 0.22 pF, C? = 0.1 pF, C3 = 0.1 wF, C4 = 1000 pF, and C5 = 1000 pF. H means heater power is developed in the heater. When used in conjunction with a 340-watt element, the unit will control the temperature at the sensing point to within & l o “ C over the range of 100-750 “C when sample is flowing. This was checked by monitoring the output voltage of the operational amplifier pA741. The temperatures reported are as defined by the thermocouple inside the heating chamber. The thermocouple, as mentioned earlier, was located 11 cm inside the heating chamber in the gas stream and maintained throughout this work. Reagents. All samples, unless otherwise stated, were prepared from Baker analytical grade reagents. Deionized water was used for preparation of solutions. A stock solution of the element to be analyzed was prepared and then diluted to the required concentration. Stock solutions, unless otherwise stated, were prepared from the chloride salts of copper, calcium, nickel, cadmium, iron, manganese, and from seleneous acid and magnesium metal, dissolved in dilute hydrochloric acid. All solutions and subsequent dilutions were acidified (0.l-l.ON) with hydrochloric acid. Salts were dried a t 110 “C.
RESULTS AND DISCUSSION Desolvation Unit. The heating chamber temperature was evaluated by nebulizing a solution of 2 kg/ml copper for twenty minutes a t a sample flow rate of approximately 0.75 ml/min (air flow rate 0.5 l./min). The solvent that condensed and flowed into the drain was collected and its volume measured. The solvent remaining in the sample cup was also measured. The difference between the total volume nebulized and the condensate collected in the drain was the amount of solvent which reached the flame. The volume of the sample left in the condenser was negligible. Efficiency of condenser is defined as the proportionality between the solvent volume collected in the drain to the solution volume nebulized. The efficiency of the desolvation unit is a compromise among the temperature of the coolant in the condenser, the apparent temperature of the heated chamber, and the flow rate of the carrier gas. Table I shows that the efficiency of the desolvation unit, in terms of solvent collected in the drain, is a function of the temperature of the heated chamber. In Figure 4, it is shown that the fraction of solvent that condenses and is collected in the drain is a function of the apparent chamber temperature. I t is surprising t o see from the graph that, a t 100 “C, 60% of the solvent was collected in the drain. While it was not measured, a great portion of the solvent is probably in the form of sample drops that condensed in the desolvation unit as the vaporization efficiency is very low a t this temANALYTICAL CHEMISTRY, VOL. 47, NO. 9, AUGUST 1975
1663
Table I. Efficiency of Condenser as a Function of Heating Chamber Temperature ?;
Solvent removed, ?
1600
0.549 0.650 0.341
0.323 0.438 0.329
41 33 4
% decrease = A 3 5 0 0 - AssOo/ A 3 6 O 0 X 100.
Table VI. Effect of Chamber Temperature on the Absorbance of Different Compounds of Cadmium and Copper ConcenCompound
CdC12
CdBrz CdI, CdS04 Cu(N0312 CUC1,
CuBr,
trarion
0.4 0.4 0.4 0.4 1.4 1.4 1.4
m.p.
568 567 387 1000 dis. at 170 620 498
b.p.
960 863 796
... ...
993 ,
..
%50°c
0.270 0.252 0.240 0.318 0.391 0.407 0.275
0.351 0.352 0.331 0.327 0.450 0.432 0.491
ment nebulized and the temperature of the chamber. T o demonstrate these relations, different salts of cadmium and copper were nebulized a t 650 and 350 "C and their absorbances measured. These results, which show that a relation exists between the melting and the boiling point of the element nebulized and the temperature of the chamber, are listed in Table VI. The following interpretation is offered. When the sample passes through the heater, water (solvent) evaporates; if the remaining salt particles melt or vaporize at the applied temperature, one of two processes take place. If the sample salt particles melt and have no appreciable vapor pressure, they will condense in the heating chamber and stick to its walls. If the sample salt particles vaporize a t the applied temperature, part of the salt, while passing through the condenser, condenses and collects in the drain with the solvent, the other part is carried with the air stream to the burner. When iron chloride (bp 285') was nebulized at 650 "C, one could see the brownish (FeC13) solution flowing from the drain with a corresponding decrease in the signal. As the melting point of calcium chloride is so high, there is only a 4% loss, which is less than for either copper or selenium. The 4% calcium can just be lost in the condenser. If a solid particle touches the wet walls, it will end up in the drain. The efficiency of the condenser as a function of condenser water temperature was evaluated by measuring the abANALYTICAL CHEMISTRY, VOL. 47, NO. 9, AUGUST 1975 * 1665
Table VII. Efficiency of Condenser as Function of Condenser Cooling Water Temperature
Table VIII. Effect of Air Flow Rate on Signal
Cooling water temp,
23 4
OC
Absorbance
Std dev
Re1 std dev, ',
0.628 0.698
0.009 0.029
1.4 4.2
sorbance of approximately 2 wg/ml copper. Sample nebulization rate was 0.75 ml/min and the heater temperature was regulated at 350 O C . The temperature of the cooling water a t the entrance of the condenser was 23 O C . The flow rate of the coolant was approximately 400 ml/min. Immediately before the condenser inlet, a 2.4-m length, 0.6-cm 0.d. coiled copper tubing was immersed in an icesalt batch so that the exit temperature of the coolant was about 4 "C when the flow rate was the same as in the absence of the cooling coil. Again the same copper sample as used previously was aspirated through the system and its absorbance measured. In both cases, the standard deviation of a series of eight measurements was calculated. The increase in relative standard deviation is possibly due to fluctuation of the cooling water temperature, which we could not hold constant at 4 O C . Results are given in Table VII. Effect of Air Flow Rate on the Signal. To determine the effect of air flow rate on the analytical sensitivity, a copper solution of approximately 1 pg/ml was nebulized. The sample aspiration rate was measured a t each air flow rate. The fuel to air ratio was held constant throughout all measurements. In all cases, the heating chamber temperature was 350 O C , and the results are the averages of eight readings. Table VI11 shows that the signal increases with air flow rate, reaches a maximum, then decreases. This is true even though sample aspiration rate increases proportionately to air flow rate. This behavior was reported before by Van Rensberg and Zeeman (23), who passed the aerosol through a heated chamber although they did not use a condenser. They attributed the decrease in signal a t high aspiration rates to losses in flame energy when the larger quantities of sample were vaporized in the flame. This behavior was also reported by Kirsten and Bertilsson (20), and by Woodriff and Stone (24) who reported a higher sensitivity for an air flow rate of 3.6 l./min than for one of 7.2 l./min. They did not speculate as to why this is so. In the system under discussion, the optimum air flow rate was found to be approximately 0.5 l./min. Above this air flow rate, the atomic absorption signals decrease. This is attributed to lack of efficient condensation of solvent a t high flow rates. Table I1 shows that, at high flow rate, 70% of the solvent reaches the flame while, a t the optimum flow rate (0.5 l,/min), 28% of the solvent vapor reaches the flame, This means that at high flow rates, the flame gases are diluted by excessive solvent vapor (33), while, at optimum flow rate, the analyte density within the flame has increased. Also, introducing 70% of the solvent vapor into the flame means that too much flame energy is used in dissociating the solvent and the remaining reduction steps are inefficient. Another reason is that a t the optimum flow rate (0.5 l./ min), the analyte has a greater residence time in the flame than a t higher flow rate, say 1.1 l./min. Increased residence time of the sample in the flame increases the degree of atomization, thus increasing sensitivity. Lang ( 8 ) , in his study of ultrasonic nebulization of liquids using a focusing system, reported that, a t high flow rates, the droplet sizes are less uniform than at low flow rates; he attributes this non-uniformity to collision and ag1666
ANALYTICAL CHEMISTRY, VOL. 47, NO. 9, AUGUST 1975
Air flow rate,
Sample aspiration
ml/min
rate, m l i m i n
365 415 470 570 672 870 1140
0.7 0.72 0.75 0.85 0.95 1.1 1.1
Signa1,A
Std dev
0.186 0.248 0.293 0.247 0.201 0.194 0.173
0.007 0.005 0.006 0.005 0.006 0.004 0.003
glomeration of the droplets after leaving the liquid surface. His position finds support in the fact that particle size increases with aspiration rate, which in ultrasonic nebulization is proportional to air flow rate. Non-uniform droplets would affect the sensitivity of the system, because larger droplets require longer residence time in the heater tube to vaporize, and longer residence time in the flame to atomize, thus decreasing the sensitivity and increasing the noise. Ultrasonic Nebulizer Parameters and Their Effect on the Signal. Dunken et al. (16) reported that the amount of spray produced is a function of sample height in the cup. This was later supported by Van Rensburg and Zeeman (23) who reported that in order to obtain reproducible results, it is essential to have the surface of the solution in the sample cup a t exactly the same height for each determination. These researchers used a focusing system. Hoare et al. (12) also used a focusing system and found that the quantity of mist produced did, in fact, tend to decrease as the level in the sample container fell. The effect was small and would not be detectable during a normal absorbance measurement of thirty seconds. Spitz and Uny (7), who used a non-focusing system (similar to one we have used), contradicted the above results and showed that the quantity of aerosol produced is constant with time and is independent of the height of the solution above the bottom of the cup. This was supported by Mermet and Robin ( 2 5 ) ,who reported that the height of the liquid above the bottom of the cup does not influence the output during the period of time longer than that required for a test (the time period is not specified). Our results agree with those of the latter workers (7,251. Starting with 15 ml of copper solution in the sample cup and aspirating these samples at rates of 0.8 ml/min and 1.0 ml/min, .the copper absorption signal remained constant for about five minutes and then began to increase slowly. The increase in the signal is probably due to an increase in the temperature of the coupling water ( 2 5 ) ,i.e., the water between the sample cup and the transducer. The temperature was measured and found to increase approximately 1 O C per minute. Another factor which might play a role is the shape of the cup. All the preceding systems (7, 12, 16, 23, 25) used a cylindrical cup, while, in the present system, a cup which has the shape of a frustrum of a cone was used. This we believe is a factor which must be taken into consideration. Support for this idea was received from Mercer (34), who reported that marked differences in the shape of the cup could alter the efficiency of nebulization. The increase in signal with time is due to increasing solute concentration due to evaporation of solvent from the liquid in the cup. The explanation given is that the air leaving the nebulizer is saturated with water vapor, at the temperature of the cup solution, only a small part of which comes from the aerosol droplets. Since the input air is seldom saturated even at room temperature, and the volume of solution in the cup is small, the solute concentration can increase significantly in a short time.
Mermet and Robin (25) reported that the amount of spray produced seems to depend on four parameters: Height between membrane (bottom of cup) and transducer; generator power; carrier gas flow rate; and temperature of the transmitting liquid. In our case, the first two are constant, the third one could be changed to meet optimum condition and then kept constant during measurements. The fourth parameter varies by approximately 1 OC/min and is not observed to have an appreciable effect for the first five minutes. All measurements reported in this study were made after the first and before the fifth minute. When absorption measurements were made during this time period, no statistical trend in values could be detected. The measurements were made after the first minute for two reasons: 30 seconds were allowed for warm-up of the ultrasonic nebulizer; 20 seconds were allowed for the heater to regain equilibrium after a thermal load is applied to it. As mentioned in the Experimental section, when the sample passes through the heater tube, the thermocouple temperature drops below the temperature set by the bias voltage. This causes the trigger pulses to occur, thus applying power to the heater. During this period in which the heated chamber regains equilibrium, which requires approximately 20 seconds, the signal is noisy due to temperature oscillations in the heated chamber. For example, a copper solution was aspirated a t a sample flow rate of 0.75 ml/min and the chamber was maintained a t approximately 350 "C. Measurements of absorption was made at 20-second intervals beginning a t the second minute after nebulization began. The measured values were, in sequence, 56.1, 55.5, 56.2, 55.8, 55.9, 56.4%. After this sample had been aspirated for seven minutes, the value had increased to 62.8% and, after ten minutes, to 65.2% absorption.
CONCLUSIONS The aim of the present study was to further explore the potential of ultrasonic nebulization in atomic absorption, with emphasis on investigation of parameters that affect heated chamber systems. The system consisted of an Ultramist nebulizer connected to a temperature controlled heater followed by a condenser and a Techtron burner. This study investigated the parameters that affect burner systems with heated chamber. Our study revealed that the temperature of the heated chamber is the most critical one. At optimum conditions of temperature and flow rate, the efficiency of the system in terms of sample reaching the flame is 86%, which is extremely high for a chamber-condenser-burner system ( 3 5 ) . I t was also established that there exists an optimum temperature a t which the chamber should be operated. The optimum temperature changes from element to element, depending on the melting point and vapor pressure of the
compounds nebulized. I t is important to hold the chamber temperature constant to keep the noise level down. Our system is the first one to use a temperature controlled heater with a fast response to changes in thermal loads.
ACKNOWLEDGMENT The authors express their thanks to W. Craig, for his help in building the temperature controller, E. Morris for building the desolvation unit, both of Georgetown University; one of us (H.J.I.) would like to express his sincere gratitude to T. C. Rains for reading the thesis, from which this article is taken, and for his constructive criticism and helpful comments.
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RECEIVEDfor review August 6, 1974. Accepted April 11, 1975. Taken from the Ph.D. Thesis submitted to Georgetown University by Haleem J. Issaq, June 1972.
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