the air a t these cities was determined by use of R A C gas samplers with two sampling tubes in each box and a common membrane prefilter for both sampling tubes. The particulate lead collected on the membrane prefilters was not determined for this study. Two replicate organic lead samples were collected in each 24-hour sampling period at each of the six cities. The organic lead analyses for each city (Table IV) indicate that the average level of organic lead a t these cities is about 0.2 pg/m3. Although this value is close to the detection limit of the method, it appears to be relatively constant for the six cities sampled. Five of the samples listed in Table IV showed organic leas values greater than 0.5 pg/m3; however, two replicate values for each of these samples almost invariably did not agree, a n indication of error in the collection or determination of these samples. This number of questionable samples was not unexpected, considering that this study was the first field evaluation of the method. Samples for which replicate values differed from their average by more than twice the standard deviation (34 relative standard de-
viation a t 0.4 pg/m3) were not included in calculations of the average organic lead value for each city. I n order to determine the total airborne lead and percentage lead in the particulate form, organic lead was sampled with bubbler boxes side-by-side and concomitantly with collection of suspended particulate matter on glass fiber filters using a Hi-Vol sampler. The results of this study are given in Table V ; although almost all ( 8 9 z ) of the lead found in the atmosphere in this study was in the form of particulate matter. Although the organic lead levels were found to be comparable in all cities, the particulate lead concentrations varied by as much as a factor of 3 from city to city. This suggests that organic lead in urban air approaches a steady state. RECEIVED for review August 8, 1972. Accepted November 6, 1972. Mention of product or company name does not constitide endorsement by the Environmental Protection Agency.
Investigations into the Use of a Pulse Ultrasonic Nebulizer-Burner System for Atomic Absorption Spectrometry N. E. Korte and J. L. Moyers Atmospheric Analysis Laboratory, University of Arizona, Tucson, Ariz. 85721
M. B. Denton’ Department of Chemistry, Unicersity of Arizona, Tucson, Ariz. 85721 An integral pulse ultrasonic nebulizer-burner system is described which com bines the desirable characteristics of flame atomization with an ultrasonic nebulizer system capable of reproducibly nebulizing small volumes of solution. Data are presented comparing this sytem with a conventional pneumatic slot burner system, showing improved sensitivity and a reduction in the required sample volume.
ATOMICABSORPTION SPECTROMETRY has been widely accepted as a sensitive and practical analytical technique for trace metals. Most commercial systems employ indirect pneumatic nebulization into a long-path flame. Such pneumatic systems generate a rather large distribution of drop sizes (1-3). Generally, the larger droplets are selectively removed by baffling systems within the burner before the aerosol reaches the flame. Eliminating the larger droplets reduces light scattering problems such as those observed in total consumption burners and has been shown to reduce certain types of interferences ( 4 ) . However, such systems d o not make effiAuthor to whom correspondence should be addressed. (1) J. Stupar and J. B. Dawson, Appl. Opt., 7, 1351 (1968). (2) John A. Dean and William J. Carnes, AXAL.CHEM.,34, 192
(1962). (3) John A. Dean and Theodore C. Rains, Ed., “Flame Emission and Atomic Absorption Spectrometry-Volume I, Theory,” Marcel Dekker, New York, N.Y., 1969. (4) William B. Barnett, ANAL.CHEM.,44, 695 (1972). 530
ANALYTICAL CHEMISTRY, VOL. 45, NO. 3, MARCH 1973
cient use of the sample since only 3 to 12% of the solution aerosol is actually introduced into the flame (3,5). Through the use of high frequency ultrasonic nebulization, an aerosol can be generated which is composed of only small droplets which can be efficiently introduced into the flame with negligible loss of sample. A previously described ultrasonic nebulizer-burner system ( 6 ) showed an average improvement of more than an order of magnitude in sensitivity; however, this type of system requires 20 to 50 milliliters of sample solution. This requirement is a severe limitation in analyses where sufficient quantities of sample are not available. A number of flame-related “sampling boat” (7, 8) and flameless techniques-filaments (9-11), ribbons (22, 13), and (5) R. Herrmann and C. T. J. Alkemade, “Chemical Analysis by Flame Photometry,” Wiley-Interscience, New York, N.Y., 1963.
(6) M. B. Denton and H. V. Malmstadt, ANAL.CHEM.,44, 241 (1972). (7) H . L. Kahn, G. E. Peterson, and J. E. Schallis, At. Absorption New’slett, 7 (2), 35 (1968). (8) J. D. Kerber and F. J. Fernandez, !bid.,10 (3), 78 (1971). (9) R. G. Anderson, I. S. Maines, and T. S. West, Atial. Chim. Acra, 51, 355 (1970). (10) R. Woodriff. B. R. Culver, and K. W. Olson. Appl. Spectrosc., ‘ 25, 328 (1971).’ (11) R. Woodriff and D. Shrader, ANAL.CHEM., 43, 1918 (1971). (12) J. Y . Hwang, P. A. Ullucci, S. B. Smith, and A. L. Malenfant, ibid., p 1319. (13) H. M. Donega and T. E. Burgess, ibid., 42, 1521 (1970) ~~
LAMP
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graphite rods (14, 1 3 , etc.-have been investigated. While such nonflame techniques have shown excellent results in many cases, various interference problems can limit their use in varying real world samples (16). In certain examples, the elimination or reduction of interferences requires a complicated optical system and temperature programming (17). The purpose of this study was to determine if an integral burner-pulse ultrasonic nebulizer system could be used t o reproducibly generate atomic vapor suitable for analysis by atomic absorption and to evaluate the performance of such a system. This device would allow analysis of smaller samples than conventional pneumatic nebulizer-burner systems while retaining the basic characteristics of time-proven flame atomization,
1:
Block diagram of experimental
ESTERLINE ANGUS
n
EXPERIMENTAL
Apparatus. The measurement system employed was a Varian Techtron Model AA-5 (Walnut Creek, Calif. 94598) atomic absorption spectrophotometer system equipped with an Esterline Angus Model 1101 (Indianapolis, Ind. 46224) strip chart recorder. This system allows scale expansion up to 0.1 absorbance unit for full scale deflection and has a response time of less than 0.2 second. The specific hollow cathode tubes used for this study were Varian Techtron HCNCd, HCN-Mn, HCN-Hg, HCN-Ni, and HCN-Ag. Improved flow settings when operating the pulse ultrasonic system at low flow rates were achieved through connecting Dwyer (Michigan City, Ind. 46360) series VF flow gauges in series with the flow gauges on the AA-5. Eppendorf microsyringes (Brinkmann Instruments, Westbury, N.Y. 11590) were used to manipulate the small solution volumes. A Varian Techtron 10-cm air acetylene burner Model 02100036-00 equipped with a pneumatic nebulizer was compared t o the described pulse ultrasonic nebulizer-burner system (Figure 1). The ultrasonic system consists of a BC-191 highpower radio frequency source which drives the piezoelectric transducer located in the base of the burner. The radio frequency source is switched on by a precision timer conMatousek and B. J . Stevens, Cliri. Chem., 17,363 (1971). (15) M. D. Amos, P. A. Bennett, K. G. Brodie, P. M. Y.Lung. and J. P. Matousek, ANAL.CHEM., 43, 211 (1971). (16) J . F. Alder and T. S. West, Aiiul. Clrim. Acta, 51, 365 (1970). (17) G. Baudin, M.Chaput, and L. Feve, Specfroclzim Acta, Purt B, 26, 425 (1971). (13) J. P.
Oxidizer
Teflon Solution CUP
G l a s s Coated Piezoelectric Transducer
3 MHr
RF
Figure 2. Detailed view of burner-nebulizer assembly structed from a Signetics (Sunnyvale, Calif. 94086) NE-555V monolithic timing circuit wired as a monostable multivibrator. Switch selectable resistors and a 100-kohm ten-turn precision potentiometer vary the time constant in the circuit, allowing the choice of nebulization times from 0.4 to 8 seconds. The radio frequency power source is composed of an army surplus BC-191 transmitter (Farnsworth Television and Radio Corporation, Fort Wayne, Ind.) and a TU-5B tuning unit which are powered by an RA-34J power supply (Radio Receptor Company, Philadelphia, Pa.). This R F source is capable of delivering up to 200 watts of power at the operating freANALYTICAL CHEMISTRY, VOL. 45, NO. 3, MARCH 1973
531
Table I. Comparison of Sensitivity between a Commercial Pneumatic Nebulizer-Burner System and the Described Integral Pulse Ultrasonic Nebulizer-Burner System Observed Analyte required Analyte required Observed for 1% for 1 % absorpManufacuturer’s sensitivity for sensitivity for tion with pulse literature AA-5 pneumatic pulse ultrasonic absorptiona with sensitivity 10-cm burner, system, 2 0 0 4 pneumatic ultrasonic system, Element Line, A (191, PPm ppm sample, ppm nebulizer, pg 2 0 0 4 sample, pg 0.07 0.003 3280.7 0.035 0.07 0.015 2288 0.025 0,009 0.025 0.0018 0.014 2.2 0.8 5 0.16 2536.5 5 2320 0.07 0.1 0.0076 0.1 0.038 2794.8 0.03 0.04 0.025 0.04 0.005 a Based on 1 ml required for analysis with pneumatic nebulizer.
L
Figure 3. Typical reproducibility data. Recorder tracings of 0.45 ppm Mn with 5 times scale expansion (0-0.2 A) quency of 3 MHz. The frequency of this system can be varied over a range sufficient to obtain the transducer resonance frequency required for maximum production of aerosol. The one-inch piezoelectric transducer is composed of C 600 barium titanate fabricated with a coaxial electrode geometry available from Denco Research (Tucson, Ariz. 85715) as part number 1012. The surface exposed to analyte solutions has been coated with glass to provide the required inert chemical properties. The transducer has a resonance frequency of 1 MHz and was operated overtone at 3 MHz (18). A detailed view of the nebulizer-burner assembly is shown in Figure 2. The combination sample cup and nebulizer is constructed of Teflon (Du Pont) and was made to press fit into the bottom of the burner. The burner was constructed with a 5-cm path length composed of 33 holes 0.813-mm diameter, spaced 1.52 mm, and with a head thickness of 1.27 cm. Vertical slits are incorporated at both ends of the burner to ensure that all of the hollow cathode radiation reaching the monochromator travels through the flame. These slits were spaced 4 mm apart. A horizontal cut-off placed 3 m m from the burner head was used to prevent hollow cathode radiation from traveling through the central flame cones when working very low in the flame. This is necessary since the support gases are not burning in the flame cones and aerosol present in the cones would cause light scattering. As long as the hori-
zontal cut-off is the same height as the flame cones, no scattering is observed with the pulsed ultrasonic burner for deionized water at any of the wavelengths studied. In contrast, at the same height above the Varian 10-cm air acetylene burner at 2795 A , the scattering signal is slightly greater than 1 absorption. While this does not mean that scattering is a problem in determining M n with the Varian burner because maximum sensitivity is achieved higher in the flame where scattering is not a problem, it does illustrate that the smaller droplets produced by the ultrasonic nebulizer are desolvated much more rapidly and in these studies eliminated solvent scattering even in the lowest areas of the flame. Procedure. The proper frequency of excitation for a particular transducer is initially determined by operating the transducer outside of the burner. The frequency is adjusted to provide maximum generation of fog without generation of large “splash” droplets. Production of the very large “splash” droplets must be avoided to prevent burner port clogging and to ensure reproducible aerosol generation. Once the frequency is determined for a particular transducer, no further adjustment is required. Typically, a 0.2-ml or smaller sample is then injected into the sample cup and onto the face of the transducer. The assembly is then inserted into the bottom of the burner. The time constant for the timer is set for a nebulization time of 1.5 seconds. The timer is then activated, and the resulting peak height gives the absorbance.
(18) M. B. Denton, Ph.D. thesis, University of Illinois at UrbanaChampaign, Urbana, Ill., 1972. (19) “Hollow Cathode Lamp Data,” Varian Techtron, Walnut Creek, Calif., 1970. 532
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RESULTS AND DISCUSSION
Table I shows a comparison between the performance observed with the commercial pneumatic and integral pulse ultrasonic nebulizer-burner systems for a representative group of elements. The data demonstrate that in every case sensitivity was as good as or better than the commercial pneumatic nebulizer-burner system even though the pulse ultrasonic nebulizer-burner system has a light path only one-half as long. An average improvement exceeding fourfold was obtained with the ultrasonic system compared with that observed with the pneumatic nebulizer-burner. With the conventional burner system, a minimum of approximately 0.75 ml is needed to perform an accurate reproducible determination. Furthermore, convenience and the attainment of a steady state by the signal often necessitates the use of up to 2 ml of sample per element. It is important to realize then that in order to determine a number of elements in a particular sample, dilution of the sample is often necessary. The pulsed ultrasonic nebulizer-burner system, however, enables convenient and reproducible analyses with as little as 25 p1 of sample, thus eliminating or reducing dilution requirements. This technique was developed as a means of conveniently performing multielement analyses on single small samples routinely examined in this laboratory. Versatility is added in
Figure 4. Effect of sample volume on absorbance. Results shown for 0.9 ppm Mn indicate that with a sample volume larger than 200 p1 no appreciable increase in sensitivity is obtained
second
0
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A
1.5 second8
0
2 second8
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100
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250
300
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Figure 5. Effect of pulse time on absorbance. Results shown for 200-pl samples of 0.9 ppm Mn indicate that pulse times larger than 1.5 seconds result in no increase in sensitivity
0.01
1
0
Nebulization
that if a particular element is apt to be present in very low concentrations, a 200-4 sample can be used for maximum sensitivity. On the other hand, for elements expected to be present in substantial amounts, a 2 5 4 sample can be used, thus allowing much more efficient use of the total sample. Figure 3 presents typical reproducibility data with the 20O-pl sample volume. The average per cent difference from the mean for a typical set of data is less than 2z. The per cent difference between the largest and smallest response for a particular sample is typically better than 6z. No signal is observed for pure solvent, even when the radiation from the hollow cathode passes very low in the flame. This indicates the rapid desolvation of the small drops produced by ultrasonic nebulization. Studies were undertaken into the effect of sample volume and pulse time. The data sho-ed that both sensitivity and reproducibility are dependent on the amount of solution in-
2
/
3
Time in Second8
troduced upon the transducer. As shown by Figure 4, maximum sensitivity is reached a t 200 p1. This is the point at which the entire crystal is just covered with solution. As volume increases further, more and more liquid remains in the sample cup, indicating that the pulse time is now too short to nebulize the entire sample. At longer pulse times, although more sample is nebulized for the larger volumes, sensitivity does not appreciably increase, indicating that with a sample volume of 200 pl and a pulse time of 1.5 seconds, optimum aerosol density is achieved for this particular burner configuration. With volumes larger than 350-400 pl, large splash drops begin to be produced hhich can clog the burner ports. Figure 5 shows a plot of pulse time us. absorbance for a 200-pl sample. Within 1.5 seconds, the sample is essentially 100% nebulized, and larger pulse times have no further effect. At shorter pulse times, an optimum aerosol density is not achieved and sensitivity decreases. Figures 3 and 5 demonstrate that ANALYTICAL CHEMISTRY, VOL. 45, NO. 3, MARCH 1973
533
with the present system use of a 2 0 0 4 sample volume and a pulse time of 1.5 seconds provides optimum absorbance for a given solution concentration. However, absolute sensitivity for a given weight of a n element decreases with sample size. Manganese is cited as a typical example of this behavior. Sensitivity dropped from 0.025 ppm at 200 pl down to 0.06 ppm for a 50-p1 sample volume. Thus, sensitivity was decreased by a factor of two, while sample size was decreased by a factor of four. Reproducibility also suffered somewhat; however, the per cent deviation for any number of readings never exceeded 10% and was typically 6 to 8%. This represents a substantial improvement in the volume of sample normally required for flame spectrometric techniques. The loss in reproducibility at smaller sample volumes can be attributed primarily to the manipulation of the very small volumes. Second, the piezoelectric crystal can produce nodal points, depending upon the exact frequency applied. With very small samples, liquid falling on these portions of the crystal is not efficiently nebulized and erratic data can result. These points may be eliminated either by slightly changing the frequency of the radio frequency power generator or by simply not applying sample to effected areas. Either procedure eliminates the problem, yielding reproducible data. Cross-contamination from one sample to the next has not been observed; nor has solution been observed collecting on the inside of the burner. This is attributed to the geometry of the burner and to the reduced likelihood of very small droplets produced by ultrasonic nebulization at 3 MHz being impacted on a surface. These droplets possess so little inertia that they
are easily swept away from stationary boundary layers (existing next to the burner walls) by the flowing gas stream. CONCLUSIONS
Use of a high-power pulse ultrasonic nebulizer incorporated into the base of a low flow rate long-path multihole burner has proved to be a n efficient and reproducible system for converting small samples into aerosol and delivering virtually the entire sample into the flame. A 25- to 200-p1 sample injected upon the piezoelectric transducer is converted to fog in 1.5 seconds and swept by the support gases into the flame. The increase in sensitivity with this system is attributed to the production of a dense aerosol composed of uniform small droplets by the nebulizer. This results in an increase in the efficiency of production of atomic vapor. Knowledge gained from this study indicates that additional sensitivity should be possible by lengthening the burner path length or by a multipass optical system. ACKNOWLEDGMENT
The authors wish to thank Denco Research (Tucson, Ariz. 85715) for providing the radio frequency power unit and machine shop facilities and Dale Mack for assistance with electronic systems. RECEIVED for review August 21, 1972. Accepted October 20 1972. This work was supported in part through funds provided by the Arizona Mining Association to the University of Arizona Atmospheric Analysis Laboratory.
Rapid and Selective Pyrocatechol Violet Method for Tin Homer B. Corbin M & T Chemicals Inc., Subsidiary of American Can Co., Rahway Research Laboratory, P.O.Box 1104, Rahway, N.J. 07065 A spectrophotometric pyrocatechol violet method has been developed that is more rapid and selective than previous methods. Conventional methods are used to prepare a sulfuric acid solution. Either directly or after an iodide extraction separation, the absorbance of the tin(lV)-PCV complex is measured at the 660-nm maximum in a sulfuric and citric acid solution. The method has been applied to the direct measurement of tin in metals and after acid oxidation and separation to the determination of 0.01 to 1 part per million tin in biological samples. For different types of samples, the detection limit varied from 0.026 to 0.076 pg Sn. Relative standard deviations of 0.55% to 1.4% were obtained in the 1 to 10 pg Sn range on samples receiving various treatments. THISANALYTICAL LABORATORY has been involved in the determination of tin in a great variety of materials and over a range from decigram to microgram quantities. In recent years, the requirements have been increasingly in the direction of more analyses and lower concentrations. Major areas involved are the determination of residues of organotin compounds used as pesticides and the evaluation of organotin stabilizers for plastic food-wrap materials. The need for measurement at levels below 0.1 part per million in materials difficult to destroy, such as oils, composite food materials, and some cosmetic preparations containing highly reactive materials, prompted the evaluation of 534
ANALYTICAL CHEMISTRY, VOL. 45, NO. 3, MARCH 1973
other more sensitive methods to serve as a complement to the dithiol methd which is used when the Sn easily available is about 5 pg or more. Recently a fluorometric method for Sn has been described ( 1 ) . This showed a detection limit of 0.007 microgram of Sn at a concentration of 0.3 part per billion in solution. This method, whether for pure tin or tin separated by extraction of the iodide, involves several steps including evaporations, fuming, and transfers. The reaction requires close control of temperature. The two most widely used of the more sensitive colorimetric reagents are phenyl fluorone (2-4) and pyrocatechol violet (5-7). Methods using these reagents have required separation of tin from a large number of interferences to obtain the required degree of selectivity. Because the reagent-tin complex is water soluble, the pyrocatechol violet-tin reaction was studied in an effort to develop a method reliably sensitive t o sub-microgram quantities of tin with less manipulation necessary than published methods. (1) T. D. Filer, ANAL.CHEM.,43, 1753 (1971). (2) C. L. Luke, ibid., 28, 1276 (1956). (3) R. L. Bennett and H. A. Smith, ibid., 31, 1441 (1959). (4) J. D. Smith, A m l y s t (Lotidoti),95, 347 (1970). (5) W. J. Ross and J. C. White, ANAL.CHEM., 33, 421 (1961). (6) E. J. Newman and P. D. Jones, Amlyst (Londo)?).91,406 (1966). (7) Analytical Methods Committee, ibid., 92, 320 (1967).
