Anal. Chem. 1983, 55,2175-2179
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Carbon Furnace for Sample Introduction into a Metastable Nitrogen Plasma John T. McCafffrey and R. G . Michel* Department of Clhemistt-y, University of Connecticut, Storrs, Connecticut 06268
Introduction of solutions into a low-pressiure metastable nltrogen plasma (MNP) has been accomplished by uslng an electrically heated carbon furnace. Slmplex optimization of elght factors which were postulated to aflect the Intensity of atomic emisslon slgnals in the plasma reslulted in a detectlon limit of 3 pg L-' (30 pg) for the determination of chromlum In aqueous solution. A 'spectral interference, indicatlve of cyanogen formation caussd by the reactlon between nitrogen and the carbon furnace, only decreased the sensltlvlty of the determlnation of chromlum and this only occurred after 500 firings of the furnace.
The metastable nitrogen plasma (MNP) appears to have the potential for high sensitivity in the detection of atomic species (1-7) by atomic emission spectrometry. Many of the stable elements of the periodic table have been established as detectable by this method (1, 2) while nearly two-thirds of the elements of the periodic table have been estimated to be detectable (1). To prevent matrix interferences and sample loading effects, predrying of the sample 11srequired prior to its introduction into the plasma. The introduction of solutions into the M N P using electrically heated tantalum boats ( 3 , 4 ) and tungsten filaments (5) has been demonstrated. Here is reported the use of a carbon furnace as a mleans of introducing a sample into a low-pressure MNP. There are many potential advantages of using a carbon furnace instead of a metal filament or metal boat atomizer. A carbon furnace will allow higher maxinnum temperatures, allowing for the optimal atomization of relatively nonvolatile metals. Other advantages are similar to when carbon furnaces are used for atomic absorption. For example, metal atomizers may form metal nitrides, making them unsuitable for a nitrogen atmosphere (8) and generally metal atomizers give poorer absolute atomic absorption detect ion limits (9) compared to the carbon furnace. Finally, the reducing atmosphere provided by carbon is sometimes advantageous for the production of metal atoms f,rom the oxide prior to a$omization. The carbon furnace has not so far been used in conjunction with the MNP. 1 t has been suggested (5) that its use would lead to excessive quenching of the metastable nitrogen species and to the production of cyanogen bands. However, various studies of the prioduction of cyanogen from the interaction of molecular nitrogen or metastable nitrogen with heated carbon (10-12) gave contradictory results concerning the efficiency of the interactions. This suggested to us that perhaps the graphite furnace may be analytically useful for samplie introduction into the MNP. A vacuum chamber to test the analytical utility of graphite furnace sample introduction into the low-pressure MNP was designed and tested. Because of the large number of possibly critical and interdependent variables inherent in a combined plasma/furnace technique, a simplex optimization was undertaken for eight factors which were postulated to affect the intensity and detection limits of chromium atomic emission signals. Subsequent univariate studies of the effect of nitrogen flow rate, chambler pressure, atomization voltage, and mi-
crowave power further defined the optimum region for analysis and provided some information as to how critical are the factors governing the sensitivity of the instrument.
EXPERIMENTAL SECTION The MNP vacuum chamber was designed to allow atomic fluorescence, abnorption, and emission measurements in the plasma. A vertical cross-sectionalview of the chamber is shown in Figure 1. The chamber used a standard CRA 63 furnace (Varian Techtron, Palo Alto, CA) as the sample introduction device and consisted of a metal cylinder closed at each end and to which flanges (Huntington Mechanical Laboratories, Inc., Mountain View, CA) were vacuum welded. The various flanges allowed for O-ring seals with three quartz windows (Spectrosil B, 1/4 in. thick; Thermal American Fused Quartz, Montville, Nd). Two quartz windows facing each other allowed absorption or emission measurements and were each 3l/, in. in diameter while a third window had its optical axis at 90" to that of the other windows in order to allow fluorescence measurements. A 28.25-mm internal diameter stainless steel bellows (EdwardsHigh Vacuum Inc., Grand Island, NY, Model 08-C110-02-442)connected the chamber to the vacuum line. The detachable baseplate used a 3/s in. 0.d. quick couple adapter (Huntington Mechanical Laboratories, Inc., Mountain View, CA, Model S-38-KM) for the quartz tube ring injector feedthrough. Additionally,an electrical feedthrough (CeramasealInc., New Lebanon Center, NY, Model 808B2805-1) was provided for the furnace which was welded to the baseplate. The original cooling system of the furnace was used without modification, with the water inlets brazed into and through the baseplate. The sample introduction porthole had an O-ring seal grooved onto the angled chamber wall. Two pegs on the chamber near the upper corners of the porthole held a faceplate in position and allowed for simple removal of the fnceplate for sample introduction. Nitrogen Flow Control. Laboratory grade nitrogen was passed through a gas filter and a liquid nitrogen trap prior i o introduction into the flow tube. Normal nitrogen flows of 5 t o 10 cm3/s were used during measurement, while a standby flow rate of less than 5 cm3/min was used in order to maintain the flow tube free of contamination (discussed later). An onloff flow switch and then a rough and fine control valve provided the necessary nitrogen flow control. The measured flow into the vacuum chamber was experimentally corrected to correspond to the true flow by using water displacement measurements. This correction was necessary since the flowmeter that was used was calibrated for acetylene gas (specific gravity 0.907) at 14.7 psia. A pirani vacuum gauge (Model 275112, Granville Philips, Boulder, CO) measured the pressure in the vacuum chamber. An approximately 1-L ballast tank was used to dampen fluctuations in the nitrogen flow rate. Evacuation and venting of the chamber was controlled by several solenoid operated valves. Venting with nitrogen was used in preference to venting t o air as a precaution against contamination. A solenoid controller provided for evacuating, venting, and isolating the chamber from the vacuum pumps. The pumping speed was continuously variable from 0 to the maximum afforded by two vacuum pumps (W. M. Welch ManufacturingCo.; Chicago, IL; Model 1400) in parallel (maximim free air displacement of approximately 50 L/min). This allowed the pressure to be varied independently of the flow rate. MNP Flow Tube. The flow tube consisted of 8 mm 0.d. qua&, as it passed through the microwave cavity and through the quick couple adapter, at which point it was joined to 6 mm 0.d. quariz
0003-270D/83/0355-2176$01.50/0 1983 American Cheinical Society
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Microwave
Figure 1. Cross-sectional view of the MNP vacuum chamber: (A) CRA 63 furnace; (B) carbon cup furnace; (C) furnace electrodes; (a)quartz ring injector; (E) sample introduction porthole; (F) faceplate pegs. The plasma is viewed through a quartz window and approximately 1.5 cm above the quartz ring injector. The positions of the second and third quartz window (optical axis perpendicular to the plane of the paper) are indicated by the dashed circle.
which formed the ring injector. The ring injector (1in. outside diameter) had four evenly spaced holes directed inward (about 60' to the horizontal ring axis) near the top of the ring. The nitrogen afterglow traveled about 15 cm from the microwave cavity to the ring injector. The ring injector sat flush with the top of the CRA 63 furnace body, and therefore the atom cloud was entrained into the plasma close to where the plasma entered the chamber. The ring injector helped to concentrate the plasma a t the point of sample introduction and was based entirely on the design of Capelle and Sutton (1). The plasma was viewed from the side and approximately 1.5 cm above the ring injector. Baffles around the furnace were used to prevent furnace blackbody radiation from directly entering the monochromator. Na and Niemczyk (3) found that the intensity of the analyte signal was very dependent on the condition of the flow tube walls, and the intensity of the Lewis-Rayleigh afterglow was seen to diminish rapidly with time. Prior to the use of the liquid nitrogen trap, this problem also occurred in our system and was probably due to quenching by contaminants. A whitish deposit appeared on the inside walls of the quartz flow tube at the position of the microwave cavity. The deposit was removable with hydrofluoric acid washing but the tube performance was still degraded relative to a new tube. However, it was found that the use of a liquid nitrogen trap along with maintaining both a very low nitrogen flow through the system (less than 5 cm3/min) and a low pressure (150-200 mtorr) at all times when the system was not in use gave reproducible analyte atomic emission intensities for a t least 3-4 weeks of heavy use (150-200 h). Even under these conditions the tube eventually showed degraded atomic emission intensities but hydrofluoric acid washing followed by maintaining the tube with a low nitrogen flow and low pressure overnight served to restore the tube performance for a further 3-4 weeks. Only 8-12 h of continuous duty with the system was possible, as after that period the system would begin to show irreproducible behavior, presumably due to the large amount of water collected by the liquid nitrogen trap. After this period, removing the liquid nitrogen trap and placing the system a t a low pressure and low nitrogen flow for half an hour to an hour restored the system pepformance. It is possible that a purer grade of nitrogen gas would solve this problem. Microwave power was supplied by a 2450-MHz Raytheon PGMlO generator. The microwave cavity wave Evenson) was tuned to provide under 2 W reflected power. The cavity required forced air cooling to prevent rapid deterioration of the flow tube. Optical Components. Atomic emission from the plasma was monitored with a 0.2-m monochromator (Jobin Yvon DH2OA;
linear dispersion, 2 nm/mm) with a spectral band-pass of 0.5 nm and incorporating sectored wheel wavelength modulation as described previously (13,14). Wavelength modulation at a frequency of 70 Hz and an angle of incidence of the light beam on the wheel of 50' was used for background correction. The 50" angle of incidence corresponded to a wavelength modulation interval of approximately 1.6 nm. A silica lens (Esco Products, Oak Ridge, NJ, 2 in. diameter, biconvex, focal length of 50 mm) was used to collect and focus the light on the monochromator entrance slit. The output from the photomultiplier tube (EMI-Gencom Inc., Plainview, NY, Model 9893QB/350) was detected by a lock-in amplifier (Princeton Applied Research, Princeton, NJ, Model 5204) using a lOOK load resistor, and 0.1 s time constant. The output signal was recorded on a chart recorder, and therefore the signal rise times were limited by the response time of the chart recorder. However, while the absolute signal response may have been influenced by the recorder response time, the general trends observed will still be qualitatively valid. The wavelength was set with a chromium hollow cathode lamp. Ideally a higher resolution monochromator may be necessary to discriminate against possible spectral interferences in this atomic emission technique. This will be considered for future work. Experimental Design-Simplex Optimization. With chromium (425.4 nm) as the test element, a modified Nelder and Mead simplex optimization (16) was carried out by using the following eight factors (conditions of the best vertex are given in parentheses) which were postulated to control the performance of the instrument: (1) furnace ashing time (10 s); (2) furnace ashing voltage (1.9 V); (3) atomization voltage (8.0 V), atomization time (not optimized) held a t 7 s; (4) final nitrogen pressure, measured at the chamber (13 torr); ( 5 ) nitrogen flow (8.6 cm?/s); (6) forward microwave power (60 W); ( 7 ) nitrogen pressure a t chamber before initiation of nitrogen flow and microwave discharge (650 mtorr); (8) time after reaching final pressure before atomization (90 s). The last factor was included in the optimization because others have stated that ignition of a microwave induced plasma (MIP) immediately prior to the analysis can adversely affect precision (16,17). Bache et al. (It?), who achieved acceptable precision by using gas chromatography (GC) for sample introduction into a microwave plasma, ignited the MIP after the solvent peak from the GC effluent had passed. During the work reported here the MNP exhibited both poorer precision and a poorer detection limit when the atomization cycle was begun immediately after reaching the final pressure. When the time after reaching the final pressure before atomization was a t the optimized 90-s level, the precision of the analysis was approximately &lo%. The furnace drying cycle was performed a t atmospheric pressure. The drying time of 50 s and the drying voltage were not optimized, but gave reproducible drying of the 1O-wL sample without spattering. The procedure for analysis of each sample was as follows. First, with the chamber at atmospheric pressure and a low flow rate of nitrogen, a 1O-pL sample of the metal solution was placed in the Varian cup furnace with an Eppendorf pipet (Model 4700). The chamber design allowed pipetting the sample into the furnace through the sample porthole. Secondly, the faceplate was placed over the porthole and the sample was dried for 50 s. Once the sample was dry, the nitrogen flow was switched off, and the chamber was evacuated until the pressure specified by factor 7 was reached. At this point, the microwave discharge was ignited with a Tesla coil, and once ignited, the nitrogen flow was initiated. Once the final nitrogen pressure was reached (factor 4 above), the discharge and the chamber pressure were allowed to stabilize for the time specified by factor 8 above. The ashing/atomization cycle using the CRA furnace controller was then begun, and after atomization a second furnace firing was used t o remove any remaining analyte from the furnace. The discharge was extinguished, the chamber was vented, and the cycle could begin again. The sample turn-around time was 6 min. The detection limits were based on the noise being equivalent to 3 times the standard deviation of the peak height of the chromium signal a t a concentration within a factor of 10 of the detection limlt. Blank measurements were made by using 10 ILL of 0.04 nitric acid and blank signals were subtracted from all
ANALYTICAL CHEMISTRY, VOL. 55, NO. 13, NOVEMBER 1983 401
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CHROMIUM CONCENTRATION (,ug/ml)
Aqueous solution calibration curve for chromium at 425.4 nm based on peak height measurements.
Flgure 2.
chromium signals. Background correction was achieved automatically by the wavelength modulation system (see above) and hence it was not necessary to manually subtract background signals. Under the above conditions carbon furnace's each lasted at least 500 firings. The reduced pressure and low oxygen content of the nitrogen atmosphere in the MNP chamber ,account for the long furnace lifetime. Solutions. Chromium stock solutions (1000 pg/mL) were prepared from Analytical Reagent grade chromium nitrate (J.T'. Baker Chemical Co., Phillipsburg, NJ). Dilutions of the stock solution were macle by using 0.04 M nitric acid prepared froni deionized distilled water.
8.0 9.0 100 ATOMIZATION VOLTAGE (volts)
7.0
Flgure 3. Change in peak helght (0)and detection limit (0)with the carbon furnace atomization voltage for chromium at 425.4 nm.
RESULTS AND DISCUSSION Linearity of Chromium Calibration Curves. As shown in Figure 2, the aqueous solution calibration curve for chromium was linear over a range of about 4 orders of magnitude. Each point on the graph (corresponds to tlhe average of three trials. A least-squares fitting of the points from 0.05 to 100 pg/mL gave a slope of 0.997 and a correlation coefficient of 0.9999. These refiults show that carbon furnace and Ta filament sample intiroduction into the MNlP give comparable calibration curves (3). The linear range of these curves is probably a function of metastable population as discussed by Dodge and Allen (5). Background Radiation. Blackbody radiation from the carbon cup was the major source of background at the chromium 425.4-nm Iline. The maximum blackbody radiation signal occurred somewhat after the analyte signal. Measurements of the furnace background radiation at the chromium 425.4-nm line were made with and without the plasma flowing above the carbon Furnace. The background radiation did not increase when active nitrogen was present rather than molecular nitrogcin (microwave power switched off). The peak-to-peak fluctuations due to the furnace radiation were 5 to 10 times that of the plasma itself, and therefore the background radiation contributed by the furnace was the limiting noise source. More extensive baffling in the chamber may further reduce the light reflected from the furnace, possibly leading tlo improvements in the detection limit with a view eventually Lo a situation limited only by the low plasma background. Cyanogen Bainds. Cyanogen spectra have been found when carbon is heated in the presence of nitrogen (19). The rate of cyanogen product,ion is quite low, however, and no interaction seems to occur below 1400 OC (12). However, the reaction seems to increase with the number of centers at which nitrogen adsorption can take place, and Cullis et. al. (12), studying the formation off cyanogen from the interaction of molecular nitrogen with heated carbon, piroposed a reaction mechanism consistent with the large-scale destruction of the graphite structure (i.e., transformation from lustrous gray to
0
50 75 100 MICROWAVE POWER (watts)
25
125
Change in peak helght (0)and detection limit (0)with microwave power for chromium at 425.4 nm.
Figure 4.
sooty carbon). This can be compared to a common observation made by experimenters using graphite furnace atomic absorption spectrometry. That is, when pyrolytic tubes age with use, a fine powdery material forms on the surface of the graphite. Rapid heating of the tube can cause this material to be released from the graphite. The fact that this material is a fine powder could increase the likelihood of reaction with the nitrogen plasma. Occasionally in the work reported here, after approximately 500 firings of the furnace, an extremely large nonatomic signal appeared in the time period just after the chromium signal was observed. This nonatomic signal was not discriminated against by the wavelength modulation which indicated that it was finely structured witliin the 1.6-nm wavelength modulation interval. Further, the normally yellow afterglow became lilac, which was indicative of the cyanogen reaction flame (20). Point by point spectra (sequential furnace firings and wavelength changes) of the region showed some of the spectral features indicative of cyanogen, but the spectra were inconclusive because the lilac flame was not reproducible and disappeared after several trials. The rapid appearance and disappearance of this signal seemed to indicate a sporadic breakdown of the carbon surface resulting in occasionally favorable conditions for cyanogen formation. Since the analytical signal was time resolved from the large background signal, i t was still possible to determine chromium, though with reduced sensitivity. However, the easiest approach was to replace the carbon furnace when these problems arose (after approximately 500 firings).
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Figure 5. Change in peak height (0) and detection limit (0)with nitrogen pressure (measured at the vacuum chamber) for chromium at 425.4 nm. Nitrogen flow rate was heid at 8.6 cm3/s.
401
NITROGEN
FLOW RATE (crn’/sec)
Figure 8. Change in peak height (0) and detection limit (0)with nitrogen flow rate for chromium at 425.4 nm. Nitrogen pressure was held at 13 torr.
A Sample Transport Model. The effects of atomization voltage (Figure 3), microwave power (Figure 4), nitrogen pressure (Figure 5 ) , and nitrogen flow (Figure 6) on the chromium atomic emission detection limit and peak height were investigated. For each of these univariate studies the remaining variables were held a t the conditions of the best simplex vertex. The optimum level of each variable, which resulted from the univariate searches, was close to the optimum level found by the simplex optimization. This indicated that it was not necessary to account for any interdependency between variables by further iteration of the univariate searches and that the univariate plots (Figures 3-6) probably do represent how critical each variable was in the region of the optimum set of analytical conditions. A simple model is presented here for the process of sample transport between the furnace and plasma. The relationships between the chromium peak height and the above four parameters are interpreted below in terms of the model, but a more detailed model and more experiments are certainly required to fully characterize the sample transport process. The transfer of the sample from the furnace to the plasma is probably governed by the rate of vaporization of the metal species and the rate of their movement between the furnace and the observation region in the plasma. An inert gas flow was not used here to entrain and transfer the vaporized sample to the plasma although many previous authors have used such gas flows ( 1 , 3 , 4 , 7). It is postulated here that the vaporized metal species are transferred initially by the rapid expansion
of the gas inside the carbon cup followed by diffusion into the plasma gas flow. The expansion driven transfer assumes that the vaporization of atomic species takes place during the initial phase of the atomization cycle when the temperature of the carbon cup is rising rapidly (21). This is likely to be the case for the rather slowly heated Varian CRA 63 furnace used here although with very rapidly heated furnaces, for example, Chakrabarti’s and L’vov’s capacitively heated devices (21,22), this gas expansion could be over before the atomic species are vaporized. Transfer could then be by simple diffusion from the hot environment of the furnace. Atomization Voltage. Although the absolute value of the atomization voltage has no significance here, the results of the optimization of atomization voltage, shown in Figure 3, indicated in a preliminary way that the rate of vaporization of the metal species was the dominant factor in the instrument described here. This is because the peak height (Figure 3) increased with atomization voltage, probably a direct result of the fact that the heating rate of the Varian furnace increases with atomization voltage. If the rate of movement of the metal species was significantly slower than the vaporization rate, then little or no increase in peak height would have been seen. The limiting response time of the recorder that was used does not allow useful interpretation of peak width and area effects. Further, it is not possible to provide a detailed interpretation of the curves in Figure 3 because of the influence of several other variables. For example, the change in heating rate with atomization voltage was not known because we did not have the equipment to measure it. However, the peak in the plot of atomization voltage vs. detection limit was caused by a combination of the inception of the plateau in the peak height with a continued increase in furnace blackbody radiation at high atomization voltages. Microwave Power. As can be seen by Figure 4, the relationship between the peak height and microwave power exhibited a rapid increase in peak height until approximately 70 W followed by a plateau at hjgher power levels. A parallel change was seen in detection limit. These results were similar to those of Na and Niemczyk for the relationship between microwave power and cadmium emission signal intensity in the M N P (3). Microwave power was kept a t 80 W in subsequent univariate searches. Pressure and Flow Rate. As can be seen from Figure 5 , the atomic emission peak height decreased steadily throughout the pressure range from 12 to 30 torr when the nitrogen flow rate was held constant at 8.6 cm3/s (a pressure of 12 torr was the minimum possible in the system with a nitrogen flow of 8.6 cm3/s). The changes in detection limit were due almost entirely to the changes in the peak height of the analyte signal, as the background remained almost constant. One possible explanation is that the reduced signal with pressure is a result of a reduced number of metastable species available for excitation of the atoms. Indeed, it is known that the number of metastable species is a function of pressure. In our instrument the intensity of the nitrogen afterglow at the 11,7 band (580.4 nm) of the nitrogen first positive system rose steadily with decreased pressure in the range 12-30 torr. This was consistent with the results of Kurzweg and Broida (23), who found a similar trend, and with the results shown in Figure 5 where the maximum atomic emission peak signal obtainable with our system rose in a similar way. However, the approximate factor of 40 decrease in the atomic emission signal with increased pressure contrasts with only a factor of 2 decrease in nitrogen afterglow intensity found by ourselves and the factor of 6 decrease found in ref 23 over the same pressure range. One possible explanation is that increased pressure decreases the rate of diffusion of sample during some part of the transfer from the furnace to the plasma. This is
Anal. Chem. 1983, 55, 2179-2184
consistent with the simple model of the transfer process described earlier. As can be seen in Figure 6, the nitrogen flow rate exhibited a plateau in detection limit above a flow of 6 cm3/s at a pressure of 13 torr. The changes in detection limit were due to changes in the peak height of the analyte signal, as the background remained almost constant. Wle were able to verify the trends of Kuirzweg and Broida (23), and found a 2-fold change in intensiity of the 11,7 band of the nitrogen first positive system over a flow range from 4 to 9 cm3/s. The analyte atomic emission peak height changed by a factor of 3 over thiei same flow range. Therefore, it appears that the change in emission signal with flow rate is due primarily to a change in metastable concentration. This rules out the hypothesis that increased flow rate would increase physical entrainment into the edges of the plasma gas flow due to the close proximity of furnace and plasma. If this had been happening during our measurements, then the increase in atomic emission signal with flow rate would probably have been greater than the factor of 3 that was found.
ACKNOWLEDGMENT The authors express their appreciation to John E. Gammerino in the Chemistry Department Instrument Shop and Stanley Manter and Ted Swols a t the Iristitute of Material Science for construction of the M N P chamber and to A1 Brown and Ward Cornell in the technical services glass shop for construction of the quartz ring injectors. Registry No. Nitrogen, 7727-37-9;carbon, 7440-44-0.
LITERATURE CITED (1) Capelle, G. A,; Sutton, D. G. Rev. Scl. Instrum. 1978, 49 1124-1 129.
(a),
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(2) Alien, R.; Barish, E. Nlnth Annual Meeting of the Federation of Analytical Chemistry and Applied Spectroscopy Societies, Philadelphia, PA, Sept 19-24, 1982; Paper Number 104. (3) Na, H. C.; Nlomczyk, T. M. Anal. Chem. 1982, 5 4 , 1839-1843. (4) Melzer, J. E., Jordan, J. L.; Sutton, D. G. Anal. Chem. 1980, 5 2 , 348-349. (5) Dodge, W. B., 111; Allen, R. 0.Anal. Chem. 1981, 53, 1279-1286. (6) D'Siiva, A. P., Rice, G. W.; Fassel, V. A. Appl. Spectrosc. 1980, 3 4 , 578-584. (7) Capelle, G. A.; Sutton, D. G. Appl. Phys. Lett. 1977, 30 (E),407-409. (8) Fuller, C. W . "Electrothermal Atomization for Atomic Absorption Spectroscopy"; The Chemical Society: London, 1977; p 52. (9) Fuller, C. W. "Electrothermal Atomization for Atomic Absorption Spectrometry"; The Chemlcal Society: London, 1977; p 21. (10) Zinman, W. G. J . A m . Chem. SOC. 1960, 8 2 , 1262-1263. (11) Goldstein, H. W. J . Phys. Chem. 1964, 6 8 , 39-42. (12) Cullis, C. F.; Yates, J. G. Trans. Faraday SOC. 1984, 6 0 , 141-148. (13) Michel, R. G., Sneddon, J.; Hunter, J. K.; Ottaway, J. M.; Fell, G. S. Analyst (London) 1981, 706, 288-298. (14) McCaffrey, J. T.; Mlchel, R. G. Anal. Chem. 1983, 5 5 , 488-492. (15) Deming, S. M.; Morgan, S. L. Anal. Chem. 1973, 4 5 , 278A-283A. (16) Zander, A. T.; Wllllams, R. K.; Hieftje, G. M. Anal. Chem. 1977, 49, 2372-2374. (17) McLean, W. FI.; Stanton, D. L.; Penketh, G. E. Analyst (London) 1973, 9 8 , 432-442. (18) Bache, C. A.; Lisk, D. J. Anal. Chem. 1985, 3 7 , 1477-1480. (19) Hutton, R. C.; Ottaway, J. M.; Epstein, M. S.;Ralns, T. C. Analyst (London) 1977, 702, 658-663. (20) Savadatti, M. I.; Brolda, H. P. Bull. A m . Phys. SOC. 1965, 70, 1210. (21) Chakrabarti, C. L.; Hamed, H. A.; Wan, C. C.; Li, W. C.; Bertels, P. 6.; Gregolre, D. C.; Lee, S. Anal. Chem. 7980, 5 2 , 167-176. (22) L'vov, B. V. Specfrochlm. Acta, P a r t s 1978, 3 3 8 , 153-193. (23) Kurzweg, U. H.; Broida, H. P. J . Mol. Spectrosc. 1959, 3 , 388-404.
RECEIVED for review March 29, 1983. Accepted August 15, 1983. Acknowledgment is made to the Research Corp. for support of this research. This paper was presented in part a t FACSS IX, Philadelphia] PA, on Sept 20, 1982.
Sampling and Determination of Boron in the Atmosphere Thomas R. Fogg* and Robert A. Duce Center for Atmospheric Chemistry Stuldies, Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island 02882 James L. Fasching Department of Chemistry, University of Rhode Island, Kingston, Rhode Island 02881
Atmospherlc boron has been separated Into gaseous and partlculata components simultaneously for the flrst tlme by passing air through a simple impregnalted filter sampllng system. A Nuclepore filter for particulate boron collection preceded KOH Impregnated Whatman 41 fllters for gaseous boron collection. The filters were contained in a single 47mm filter holder. Boron was determlned by vlslble spectrophotometry with 2,4-dlnltro-l,8-naphthalensrdlol (DNNDO) and brllllant green. Results obtained during field use at a variety of sampllng sltee around the United States reveal that the gaseous fractlon Is the major (>96%) phiase of atmospheric boron and suggest that much of the gaseolus fraction of boron may exist as B(OH),. The ease, precision, and low detection limit of the DNNDO method for boron ainalysis are Ideally suited for use wlth Impregnated fllter collection of boron, and the DNNDO methlod allows short tlme scale (daily) varlatlons of atmospheric bioron to be measured.
Boron has been reported by several authors to be a trace constituent of the atmosphere. Published concentrations af
ambient atmospheric boron range from 20 to 450 ng/m3 (measured with dry ice-acetone traps (1) or "atmospheric moisture condensers" (2)) to 40 000 ng/m3 (using bubblers containing aqueous mannitol solutions (3)). In addition to the rather wide range in reported concentrations, the relative contributions of the gaseous and particulate fractions to the total atmospheric boron were not assessed by these collection techniques. Beginning with Gast and Thompson (41, most investigators have assumed that the volatile atmospheric boron species is boric acid. For example, Savenko (5) calculated, on the basis of available thermodynamic data, an equilibrium atmospheric concentration assuming evaporation of B(OH)3from seawater. His calculated concentration, 18 ng/m3, agrees almost exactly with the concentration of 19 ng/m3 calculated from laboratory seawater/B(OH), equilibrium studies conducted by Nishimura and Tanaka ( I ) . The large discrepancy in reported conceintrations of atmospheric boron, along with the paucity of information on itt3 chemical and physical form, makes assessment of the atmospheric geochemical cycle of boron difficult. The cold trap and bubbler techniques previously used for total ambient boron sampling have drawbacks, primarily with
0003-270~0/83/0355-2179$01.50/0 0 1983 American Chemical Society
