Table VIII. Recovery Experiments on Acetic Acid Soil Extracts Element __~__ __.____K" Mg" Mnh Znb Soil Ca5 Cub 1 A 0.34 0.02 0.12 0.06 1.59 0.61 B 1.34 1.05 1.11 1.06 2.61 1.66 C 100% 103% 99% 100% 102% 105% 2 A 1.74 0.01 0.14 0.03 1.50 0.13 B 2.72 1.00 1.14 1.04 2.51 1.17 C 98% 99% 100% 101% 101% 104% 3 A 1.22 0.00 0.13 0.05 1.35 0.40 I3 2.26 1.02 1.14 1.06 2.33 1.45 C 1047< 102% 101% 101% 98% 105% 4 A 1.10 0.01 0.10 0.09 1.52 0.80 B 2.08 0.98 1.10 1.10 2.50 1.87 C 98% 97% 100% 101% 98% 107% 5 A 1.31 0.02 0.11 0.09 1.40 0.92 B 2.32 1.02 1.08 1.10 2.85 1.97 C 101% 100% 97% 101% 95% 105% 6 A 3.13 0.05 0.16 0.50 1.48 2.34 B 4.13 1.10 1 . 1 1 1.48 2.52 2.30 C 100% 105% 95% 98% 104z 96% 7 A 3.10 0.00 0.26 0.75 1.99 0.27 B 4.08 0.98 1.18 1.63 3.03 1.27 C 98% 98% 92% 98% 104% 100x A. Initial concentration (pg/ml) found in suitably diluted extract. B. Concentration (pg/ml) found in suitably diluted extract after addition of 1 pgjml of each analyte element. C. Percentage recovery of the added element. Using extract diluted 100-fold. * Using extract diluted 2-fold. CONCLUSION The analytical results show that the atomic fluorescence and atomic absorption determinations on each soil extract were identical within experimental error. The former, however, may be obtained virtually simultaneously, resulting in considerable saving of time when many samples are t o be analyzed. Although the sample preparation time is identical for both techniques, the determination of the six analyte
elements in large numbers of samples with the multichannel atomic fluorescence spectrometer takes only one eighth of the time required using the single channel atomic absorption spectrophotometer when the latter is used with the longest integration time (10 seconds). This must be compared with the 16-second measuring cycle in which the atomic fluorescence spectrophotometer yields a result for each of the six channels. Additionally, time is saved and less operating skill is required because of the use of interference filters in the atomic fluorescence instrument. It is not necessary to change sources repetitively and to set the required wavelength accurately for a particular analyte element, and no possibility of monochromator drift arises during the course of analysis of a large number of samples. The attained sensitivity and precision of the atomic fluorescence determinations is sufficient to permit the determination of the six elements examined in all common soil samples. With some samples, however, preconcentration of copper before analysis might be necessary. The results of the recovery experiments conducted indicate that atomic fluorescence measurements made for the analyte elements on the ammonium acetate and acetic acid extracts are free from interference effects from extraneous ions. The multielement analysis of soil extracts in a single extract a t two dilutions by atomic fluorescence spectrophotometry is accurate and precise, and offers the potential of considerable time saving in routine determinations. ACKNOWLEDGMENT We are grateful to Technicon Instrument Corp. for the loan of the atomic fluorescence spectrophotometer used in this work. We would like to thank also the Macaulay Institute for Soil Research, Aberdeen, Scotland, for the provision of soil samples. RECEIVED for review May 24, 1971. Accepted August 16, 1971. Financial support was provided t o one of US (R. W.) by the Technicon Instrument Corp.
Atomic Fluorescence Spectrometry with Continuous Nebulization into a Platinum Furnace M. S. Black, T. H. Glenn, M. P. Bratzel,' a n d J. D. WinefordneP Department of' Chemistrj., Unicersit)*o f Florida, Gainescille, Fla. 32601 Atomic vapor of several metals (Cd, Zn, Cu, Hg, and Fe) was produced by continuous nebulization of an aqueous sample through a simple platinum tube furnace, and atomic fluorescence was excited by means of line radiation from electrodeless discharge lamps. Two furnace designs were evaluated and compared with respect to limits of detection, ranges of linearity of analytical curves, and interferences. MOST ATOMIC FLUORESCENCE spectrometric studies have employed flames produced by nebulizer-burners t o generate an atcmic vapor of the analyte. Flames are rather inefficient
Present address. Department of Chemistry, Carleton University, Ottawa. Canada. Author to whom reprint requests should be sent.
atomizers for many elements and also contain significant concentrations of efficient quenchers of excited atoms, such as CO?, CO, Nz, etc. ( I ) . Therefore, nonflame cells have been utilized in analytical atomic fluorescence spectrometry. Nonflame cells were first used by King ( 2 ) for fundamental studies and by L'vov (3) for analytical atomic absorption spectrometry. However, the first nonflame cell used in atomic fluorescence spectrometry, a graphite tube furnace, was described in 1968 by Massmann ( 4 ) . Since then, West (1) J. Winefordner, V. Svoboda, and L. Cline, CRC. Crit. Reu. A t i d . Clieni., 1, 233 (1970). (2) A. King, Astroplzys. J . , 28, 300 (1908). (3) B. L'vov, Spectrochim. Acta, 17, 761 (1961). (4) H.Massmann, ibid., 23B, 215 (1968).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
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p1
m-
FILAMENT
I-t
I
1
STRAP9 TO AC POWER
t DESIGN
I:
SAMPLE AEROSOL FROM NEBWZER
W U
FURNACE FILAMENT SYSTEM
SYSTEM
Figure 2. Block diagram of experimental system SPOT-WELDED STRAPS CONCENTRIC PI FILAMENT
3-RADIAL FORTS FOR AEROSQ STRAPS TO AC POWER
c3
STEEL JET
SAMPLE AEROSOL ? FROM NEBULIZER
DESIGN R
:
FURNACE FILAMENT SYSTEM
Figure 1. Schematic diagram of furnace filament systems and Williams (5-7)and Amos et al. (8) have described graphite filament atomizers and Bratzel, Dagnall, and Winefordner (9) have described a platinum loop atomizer for atomic fluorescence spectrometry. All nonflame atomizers are operated in a n inert gas atmosphere (generally Ar) t o minimize air entrainment which would decrease the free-atom fraction, decrease the fluorescence quantum yield, and decrease the lifetime of the heating element. Some problems associated with the previous nonflame cell atomizers for atomic fluorescence spectrometry have included complexity of design, poor reproducibility of measurements, and poor accuracy due t o appreciable matrix interferences. I n a n effort to overcome some of these problems, the present investigation was undertaken. This study utilizes the more convenient and reliable method of continuous nebulization of sample solution (as opposed to discrete sampling with a hypodermic syringe) through a platinum tube furnace. An aerosol of salt particles, produced by a n efficient nebulizer, was carried into the compact, electrically-heated platinum furnace (two designs were evaluated) where atomization was affected. The resulting analyte atomic vapor was excited with line radiation from the appropriate electrodeless discharge tube, and the fluorescence signal was measured. EXPERIMENTAL
Platinum Furnace-Design I. The first design of the platinum furnace (see Figure 1) consisted of a hairpin shaped platinum strip which was bent into a cylindrical shape (I/*in. diameter by 1/2-in. high-the actual dimensions are not critical, however) and it was held in place by two water-cooled stainless steel mounting blocks through which the heating current was supplied. A stainless steel cover with an opening (5) T. West and X. Williams, .4/7u/. Chirn. A c f a , 45, 27 (1969). (6) R. C. Anderson, I. S . Maines, and T, S . West, ibid.. 51, 355 (1970). (7) J. F. Alder and T. S . West, ibid., p 365. (8) M. D. Amos, P. A. Bennett, K. G. Brodie, P. W. Y . Lung, and J. P. Matousek ANAL.CHEM., 43, 211 (1971). (9) M. Bratzel, R. Dagnall, and J. Winefordner, A n d . Chirn. Acru, 48, 197 (1969). 1770
for the filament was fastened t o the base of the blocks. A 7/16-in.i.d. alumina tube was placed over the opening so that it surrounded the filament. The aerosol of salt particles from the nebulizer entered the furnace axially through a 8-mm 0.d. quartz tube fitted directly below the cylindrical platinurn filament. The furnace housing was continually flushed with Ar introduced through a n opening in the base of the furnace. Current for the filament was provided by a variable ac power supply (0-lOOV, 0-200A). In the initial studies, a tantalum filament was used. Tantalum was chosen because of its high melting point of 2995 "C and its resistivity t o most reagents at high temperatures. However, the tantalum tube filament oxidized readily and crumbled after heating for a short time, even though argon was continually flowing through the system. It was felt that tantalum could not be practically used as the filament material with this system, and so platinum, (melting point of 1775 "C) was chosen for the filament material, particularly because of its stability at high temperatures and its resistance to oxidation. Platinum Furnace-Design 11. The second design of the platinum furnace (see Figure 1) consisted of two vertical platinum cylinders (Engelhard Industries, Carteret, N.J.). One cylinder of 1-in. length and 5/16-in.diameter was mounted concentric t o the other which was 1.5-in. length and 7/16-in. diameter. The inner and outer cylinders were attached by spot-welding straps between them at the top. Aerosol from the nebulizer system was sprayed through a stainless steel jet prior to introduction into the annulus formed between the cylinders. The jet was machined from a 5/16-in.diameter stainless steel rod and spot-welded to the bottom of the inner platinum cylinder. Aerosol entered the bottom of the stainless jet and flowed axially t o three equally spaced '/*-in. radial holes just below the platinum-stainless steel weld, Two platinum straps extending from the outer cylinder were attached to the stainless steel mounting blocks (same blocks as described previously for the first furnace design); also the stainless steel cover was placed over the platinum furnace and attached t o the base, but the alumina tube was not used. Current, from the same variable ac power supply used for the Design I furnace, was passed through the stainless mounting blocks connected to the outer cylinder t o the grounded inner cylinder. The furnace had about ten times more heated surface area than Design I and many times greater residence time for aerosol particles within the heated surfaces of the furnace. These factors should result in improved volatilization of particles and atomization of analyte. General Experimental System. The experimental setup was similar t o that previously described by Mansfield, Winefordner, and Veillon (IO),with the exception of the nebulizer system, platinum furnace, and the dc electrometer. A schematic diagram of the experimental setup is given in Figure 2. A Czerny-Turner grating monochromator (No. 4-8400, (10) J. Mansfield, J. Winefordner, and C. Veillon, ANAL. CHEM., 37, 1049 (1965).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
American Instrument Co., Silver Spring, Md.) equipped with a grating blazed at 300 nm, a n adjustable slit mechanism (No. D42-61041, American Instrument Co., Silver Spring, Md.), and a n entrance field lens (Aminco No. A83-61041) was used in the study. A light box assembly which was painted flat black o n the inside surrounded the platinum furnace and the monochromator entrance slit. Quartz lenses (Commercial G1 quartz type, Esco Products, Oak Ridge, N.J.) with 2411. diameter and 7-cm focal length were used. One lens was placed between the platinum furnace and the monochromator entrance slit (14 cm from each so that a 1 :1 image was produced). The second lens was positioned 12 cm from the furnace and 10 cm from the source. The image of the source was focused just beyond the top of the furnace t o assure illumination of the entire region across the top of the platinum furnace outlet. The optical bench components were similar t o those described by Winefordner and Staab (11). Sources used in this study were electrodeless discharge tubes prepared as described by Zacha et a!. (I2). The lamps were powered by a medical diathermy unit (PGM-1041, The Raytheon Company, Waltham, Mass.) equipped with a reflected power meter (No. 725-3, Microwave Devices Inc. Farmington, Conn.), provided up t o 120 watts at 2450 MHz. The “A” antenna with a reflector (No. 2254-500261, The Raytheon Company, Waltham, Mass.) was used to couple the microwave field t o the tubes. A suitable mercury electrodeless discharge tube was not available, and therefore mercury Spectroline quartz pencil lamp (Edmund Scientific Co., Barrington, N.J.) was used. The system used for the nebulization of aqueous samples was the same as used by Fallgatter (13) and quite similar to the one described by Veillon and Margoshes (14). A dc electrometer, described by O’Haver and Winefordner (15), and a 1-mA recording galvanometer (Rectilinear Milliammeter, Texas Instruments, Inc., Houston, Texas) were used t o amplify and record the signal from a1 P28 R C A multiplier phototube. The phototube voltage was 800 volts. Solutions and Methodology. Stock solutions of the elements Cd, Zn, Hg, Cu, and F e were prepared from analytical reagent grade chemicals. Cadmium, zinc, mercury, and iron solutions were prepared from their metal chlorides, and the copper solution was prepared from copper oxide. The salts were first dissolved in approximately 5 ml of 0.1M HCl and then diluted with de-ionized water. Solutions of lower concentrations were prepared by successive dilution with deionized water. Atomic fluorescence signals were measured in the following manner. The heated chamber of the nebulizer system was allowed t o reach a steady state temperature of 120 “C. The argon flow rates through the nebulizer and furnace were adjusted to their optimum values. Current was gradually applied t o the platinum tube furnace until the desired temperature was reached. De-ionized water was first aspirated into the system to remove any residual metal vapor from the nebulizer. The sample solution was then aspirated into the argon stream which carried the aerosol of salt particles into the furnace. Fluorescence measurements were recorded for 30-sec intervals (an additional 30-sec delay preceded the measurement). The steady state peak height occurred during this time. After each sample measurement, pure solvent was aspirated into the system, and a background reading was obtained. The relative fluorescence signal was (11) J. Winefordner and R. Staab, ANAL. CHEM., 36, 165 (1964). (12) K . Zacha, M. Bratzel, Jr., J. Winefordner, and J. Mansfield, ihitl., 40, 1733 (1968). (1 3) K . Fallgatter, 1’h.D. Dissertation, University of Florida, 1970. (14) C . Veillon and M. Margoshes, Specrrochitn. Actu, 23B, 553 ( 1968). (15) T. O’Haver and J. Winefordner, J . Chen7. Eclrrc., 46, 241 (1969).
Table I. Optimum Operating Conditions Including Lamp Parameters and Monochromator Slit Widths MonoMicrowave Microwave chromator coupling power, slit Metal Line, mm device watts width, mm 228,8 Cadmium A-Oa loo 3.0 213.9 Zinc A-0‘ 100 2.0 324,8 Ab 70 3.5 Copper 253.7 Pen lamp NAd 3.5 Mercury Iron 248,3 A-V@ 120 2.0 “A” antenna with quartz jacket, “A” antenna. “A” antenna with quartz vacuum jacket. Not applicable.
obtained by subtracting the background signal from the total signal due t o the sample solution. Atomic emission signals were negligible in this study. Argon flow rates were monitored with rotameters (No. 4-15-2, Ace Instrument Inc., Vineland, N.J.). Two-stage regulators were used to control the pressure at the tanks. The flowmeters were calibrated with a wet test meter (Precision Scientific Co., Chicago, Ill.). Platinum filament temperature measurements were made with a calibrated Pyro optical pyrometer (The Pyrometer Instrument Co., Bergenfield, N.J.). The precision of the measurement was approximately i50 OK. Optimization of Experimental Conditions. Optimum operating conditions (maximum signal-to-noise, S / N ratio) for the electrodeless discharge tubes were those previously described by Zacha et al. (12) with the exception that the zinc and copper tubes were operated at higher powers. A summary of these conditions is given in Table I. Fluorescence signals obtained with Design I and I1 of the platinum furnace were studied with respect t o argon flow rates through the nebulizer and furnace, monochromator slit width, and height of measurement above the tip of the furnace. The optimum flow rates of argon were taken as those producing the largest signal-to-noise (S/N) ratio for each analyte. Optimum A r flow rates of 1.6 l./min and 2.4 l./min were found for the nebulizer system with Design I and Design I1 furnaces. These flow rates produced solution uptake rates of 2.4 mlimin and 4.8 mlimin, respectively. The efficiency of nebulization was about 2 0 z for both flow rates. The optimum sheathing gas flow rate was 0.8 l./min for both designs. At higher A r flow rates, S/N decreased sharply because of cooling of the platinum filament. The optimum height of measurement was 1.5 cm above the top of the Design I furnace and 2.0 cm above the top of the Design I1 furnace. The optimum monochromator slit widths for Cu, Zn, Fe, and H g were determined by making fluorescence measurements at a height of 1.5 cm above either furnace top; these data are given in Table I. F o r the elements studied, there was only a small change (less than 5 7 3 in S/N with variation of 5 @ zin either the slit width o r in the measurement height for either furnace design. RESULTS AND DISCUSSION Scatter Profiles. In addition to the fluorescence signal produced with either furnace design, a background signal was present as a result of scatter of excitation radiation off unvaporized water droplets and solute particles. To evaluate the effect of scatter on the measurement of fluorescence signals, scatter signals were measured as a function of height above the top of the Design I platinum tube furnace (approximate temperature of 1600 “C)at the fluorescence wavelength of cadmium a t 228.8 nm (Cd was chosen because scatter
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
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Table 11. Standard Deviations of Measurement and Upper Concentration Limits of Analytical Curves Obtained with Design I and I1 Furnaces
Element Cadmium
Filament temperature, "C 1350 1600 1100 1350 1600 1100 1350
Line, mm 228.8
Upper concentration limit" 10-1 10-1 10-1
Furnace type Std dev,b Design I 3.3 Design I 3.3 Design I1 2.4 Zinc 213.9 Design I 10-1 1.7 Design I 10-1 1.7 Design 11 10' 2.9 Mercury 253.7 Design I 102 3.9 Design I 1600 102 3.9 Design 11 I100 10' 1.2 Copper 324.8 Design I 10' 1350 1.3 Design 1 102 1600 1.3 Design 11 102 1100 1.8 1350 Iron 248.3 Design I 103 2.8 Design I 1600 102 2.8 Design I1 102 1100 2.2 Highest concentration corresponding to the linear portion of the analytical curve. Analytical curve is linear from limit of detection (see Table 111) to the upper concentration limit. Measured at concentration 10-fold below upper concentration limit.
5.01
4.0
1 0.5
I
I .o
I
I
I .5
2 .o
I
2.5
Height of Measurement, cm
Figure 3. Variation of scatter signal at 228.8 nm with height of measurement a.
Water being nebulized
b. Aqueous solution of 1000 pg/ml AIC13 being nebulized
should be more severe at short wavelengths). The scatter profiles obtained with Design I furnace are shown in Figure 3. The water droplet scatter was measured by aspirating only water through the nebulizer and the scatter due t o unvaporized solute particles was measured by aspirating 1000 pg/ml of aluminum chloride. The scatter signals were the highest when measured near the top of the furnace but decreased steadily with height above the furnace top. At the optimum height of 1.5 cm, the scatter signal due to 1000 pg/ml of aluminum chloride was negligible over the useful linear portion of the analytical curves of all analyte species studied (see Table 11). Scatter due to unvaporized water droplets, however, was significant (but small) at the measurement height of 1.5 cm and had t o be corrected for by subtracting the background scatter signal from the total "fluorescence signal" at each concentration of analyte. The scatter signal 1772
obtained with compounds like aluminum chloride being introduced into the platinum furnaces is certainly small compared to the introduction of similar materials into low temperature flames (16). Scatter signals were similarly measured with furnace Design I1 at 1100 "C. The relative scatter signal due t o unvaporized solute particles (also measured with aluminum chloride) was of the same order of magnitude as that obtained with Design I furnace. However, the background o r scatter signal due t o water droplets was approximately 5-fold higher than with Design I. Analytical Curves. Analytical curves for Cd, Zn, Hg, Cu, and Fe were obtained for Design I platinum tube furnace temperatures at 1350 "C and 1600 "C and for Design I1 tube furnace at 1100 "C. A summary of the pertinent characteristics of these analytical curves for Cd, Zn, Hg, Cu, and Fe atomized by both furnace designs are given in Table 11. Effect of Furnace Temperature-Design I Furnace. The fluorescence signals were greater when the furnace was operated at the higher temperature, for example, 1600 "Crather than 1350 "C. The effect of temperature of the furnace o n the fluorescence signal and its variation with height of measurement was studied for 0.1 pg/ml of C d and 100 pg/ml of Fe. Fluorescence measurements were made a t three different furnace temperatures, 1150 "C, 1350 "C, and 1600 "C (filament currents of 30-40A a t several volts) at various heights above the top of the furnace. An increase in the S/N ratio was observed as the temperature increased. At the optimum measurement height of 1.5 cm, there was a 4-fold increase in the C d SIN ratio and a 10-fold increase in the Fe SIN ratio for a furnace temperature of 1600 O C as compared t o 1350 "C. Because of the lower volatility of iron and its compounds, an increase in the temperature shculd have a greater effect upon increasing the atomization efficiency than for the more volatile cadmium system. At a furnace temperature of 1150 "C, the Fe fluorescence signal was not detectable. It is also significant t o note that as the furnace temperature is increased, the optimum height for measurement also increased. As the temperature of the (16) M . Bratzel, R . Dagnall, and J. Winefordner. ANAL.CHEM.. 41, 7 1 3 (1969).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1 9 7 1
Table 111. Comparison of Atomic Fluorescence Detection Limits Obtained for Cadmium, Zinc, Copper, Mercury, and Iron with Design I and I1 Furnaces with Conventional Atomic Emission (AE), Atomic Absorption (AA), and Atomic Fluorescence (AF) Flame Spectrometry Detection limits (pgiml) Wavelength, Design I furnacea Design I furnacea DesigyII furnace Element nm 1350 "C (AF) 1600 "C (AF) 1100 "C (AF)" Flame (AF)* Flame (AA)b Flame (AE)b 1 x 10-2 2 x 100 1 x 10-6 3 x 10-4 4 x 10-3 2 x 10-4 228.8 Cadmium 2 x 10-3 5 x 10' 4 x 10-5 5 x 10-4 4 x 10-4 8 x Zinc 213.9 4 x 10' 1 x 10-1 5 x 10-1 8 x 4 x 10-1 2 . x 10-1 253.7 Mercury 5 x 10-3 1 x 10-2 5 x 10-3 8 X 4 x 10-2 324.8 2 x 10-2 Copper 5 x 10-2 2 x 10-1 5 x 10-3 4 x 100 1 . 4 X 10' 1 x 10' Iron 248 3 a This study. These values taken from tabulated values in Reference ( I ) . Table IV. Comparison of Interference Effects (Expressed in Percentage Recovery) for 1 pg/ml Cadmium (228.8 nm) at Design I Furnace Temperatures of 1350 "C and 1600 "C with Those of Furnace Design I1 at 1100 "C and 1400 "C Design I1 __Design I ____1350 "C 1600 "C 1100 "C 1400 "C Interferent, pg/ml 64 45 79 71 50 80 33 8 5 40 16 41 10 2 0 10 5 14
10
100
1000
furnace increased, the extent of solute vaporization increased, resulting in a greater fluorescence signal higher above the furnace top. Also, dilution with entrained air and compound formation with oxygen and quenching should increase with height. Thus, the optimum height is a compromise between these competing factors. Effect of Furnace Temperature-Design I1 Furnace. I n contrast t o the results for Design I furnace, a peak S/N ratio for all elements resulted a t a n optimal furnace temperature. For Cd, the peak S/N ratio occurred at 1100 " C and for Fe at 1400 "C (filaments currents of 80-90 A at less than 1 V). However, a n increase or decrease in furnace temperature by 100 "C had only a small effect (about 13z decrease in S/N ratio for C d and 3 for Fe). The decreasing S/N ratios with increasing furnace temperature a t high furnace temperatures may be due t o a deflection of heated aerosol downward and out of the concentric tube furnace as a result of increased gas viscosity within the concentric tube--i.e., the viscosity of the heated gas flowing through the concentric tube furnace increases at high temperatures and has partially closed off the furnace annulus to the flow of aerosol. By increasing the sheathing gas-flow, the S/N ratio for all elements decreased. Also, by adding additional argon gas to the sample aerosol, the S/N ratio for all elements decreased. Therefore, the above explanation based upon gas viscosity increase with temperature is probably only partially correct in rationalizing the temperature dependence of the S/N ratios for Design I1 furnace. Certainly the residence time of particles and the efficiency of heat transfer t o the particles is more efficient in Design I1 furnace than in Design I-[.e., in Design 11, the aerosol is heated as it passes
z
98 103 100 102 97 100 88 87 80 89 81 90 41 37 35 43 26 51
101 99 98 98 100 98 91 80 78 81 7s 80 46 47 40 35 46 44
100 100 101 98 103 97 101 95 94 95 85 100
67 65 59 59 65 61
through the stainless steel jet prior t o the concentric tube furnace and when within the concentric tube furnace, the particles experience longer residence time and more heated surface per unit gas volume than in Design I. The Design I1 furnace resulted in greater S/N ratios than Design 1 furnace even when the latter was 5 0 0 'C hotter than the former. Limits of Detection. Detection limits are defined as that concentration resulting in a S/N ratio of 2. F o r each determination, five 30-second measurements (with a time constant of 1 second) were made of the water signal and the lowest detectable solution concentration. Detection limits measured for each element with each furnace design a t several temperatures are shown in Table 111. The limits of detection determined with both furnaces are compared with those previously reported for flame atomizers. Although the melting point of platinum is 1779 "C, 1600 O C was the highest temperature t o which the platinum could be heated without hot spots forming in the metal. It can be seen from the data in Table I1 that an increase in the Design I furnace temperature decreased the limits of detection for all the elements. This is most likely a direct result of more effective atomization a t the higher temperature. The detection limits for Design I furnace a t 1600 "C and Design I1 furnace at 1100 'C are similar and comparable t o those reported for conventional atomic absorption and atomic emission flame spectrometry. Interference Effects. The influence of lo-, loo-, and 1000fold excesses of chloride, sulfate, phosphate, carbonate, silicate, and barium salts on the fluorescence signal of 1 pg/ml cadmium was studied a t two temperatures. The results, expressed as percentage recovery with respect t o the 1 pg/ml
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same magnitude). Furnace Design IT, which had a larger heated surface area and allowed a longer residence time for the sample particles, was a more efficient atomizer than Design I. Because its operational temperature was lower than that of Design I and yet similar results were obtained, better heat transfer must have been occurring. However, Design I1 could not be operated above 1100 "C without producing a decrease in the fluorescence S/N ratio. Modifications are needed that will incorporate a n effective heat transfer without the adverse effects of the previously proposed viscous flows and radial sample injections. These changes are now being studied. Even so, the present platinum furnaces are effective atomizers for atomic fluorescence siudies of volatile elements. Advantages offered by this technique of atomization over previous nonflame methods are the simplicity of sampling, the precision of measurement, and the relative freedom from chemical interferences. With slight modifications of the furnace and the use of an ultrasonic nebulizer, this nonflame method should be applicable in the trace analysis metals in real samples such as biological fluids and jet lubricating oils.
cadmium without excess interferents, are given in Table IV for two different furnace conditions for both furnace designs. All the results indicate the interferents depressed the fluorescence signal, When the Design I furnace temperature is increased from 1350 OC to 1600 OC, the interference effects are decreased considerably; the fluorescence depression due t o the 100-fold excess interferents is almost completely removed. Also detection limits of all analytes studied with Design I furnace a t 1600 "C are about 20-fold lower than for the same furnace a t 1350 "C (see Table 111). The recoveries of 1 pg/ml C d with Design I1 furnace a t 1100 "C are approximately identical with those of Design I furnace a t 1600 O C (see Table IV). When the Design I1 furnace temperature was increased t o 1400 "C, the recoveries were increased further with a concomitant decrease in the S/N ratio for the 1 pg/ml C d solution. CONCLUSIONS Atomization of the sample is a function of heat transfer efficiency. This was illustrated when furnace Design 11, operated at 1100 O C , produced the same, and in some cases lower detection limits than obtained with furnace Design I operated a t 1600 O C (the interference effects were also of the
RECEIVED for review May 7, 1971. Accepted July 19, 1971. This work was supported by AF-AFOSR-70-1880B.
X-Ray Photoelectron Spectroscopy of Molybdenum Corn pounds Use of ESCA in Quantitative Analysis William E. Swartz, Jr., and David M. Hercules Department
of Chemistry, Unicersiry of Georgia, Athens, Ga. 30601
The Mo (3d,2-3d512)electron binding energies have been measured as a function of oxidation state for a series of molybdenum compounds. A linear relationship is found to exist between the binding energies and oxidation state. The binding energy shift between the Mo(3d) electrons in Mooz and Moo3 was large enough (1.7 eV) to allow measurement of one oxide in the presence of the other. This permitted development of a quantitative analytical procedure for bulk analysis of MoOrMoOs mixtures. The resulting analysis shows a relative standard deviation of 2%.
X-RAY PHOTOELECTRON SPECTROSCOPY (ESCA) is a technique for determining the binding energies of core-electrons. ESCA measures the kinetic energies of electrons ejected from a molecule by a mono-energetic beam of X-radiation ( I ) . The binding energies of the core-electrons are dependent upon the oxidation state of the atom. Few ESCA data have been reported for the transition metals. Fadley et al. have studied multiplet splittings of the core-electron binding energies for several of the transition metals (2, 3). Fadley and Shirley have also studied the densities of states and coreelectron energy levels of Fe, Co, Ni, Cu, and Pt ( 4 ) . N o extensive investigation has been reported on a given transition (1) K. Seigbahn et d.. "ESCA 'Atomic, Molecular and Solid State Structure Studied by Means of Electron Spectroscopy,' " Almquist and Wiksells, Uppsala, 1967. (2) C. S . Fadley, D. A. Shirley, A. J. Freeman, F. S. Bagus, and J. V. Mallow, Phys. Reo. Lett., 23, (24), 1397 (1969). (3) C. S. Fadley and D. A. Shirley, Phys. Ret. A, 2, 1109 (1970). (4) C. S . Fadley and D. A. Shirley, Phys. Rer. Lett., 21, (14), 980 (1968). 1774
metal although Cook et al. (5) have recently reported a detailed study of platinum compounds. We wish t o report an investigation of molybdenum (3d3/2-3dS,2) core-electron binding energies as a function of molybdenum oxidation state for ca. 20 compounds. ESCA has been shown to offer possibilities for quantitative chemical analysis. Siegbahn et al. ( I ) have reported some quantitative studies in which they were able to determine the C:Cl:S ratios in a number of amino acids and insulin. Siegbahn et a/. ( I ) have also analyzed brass samples containing zinc (10-50%), copper (50-90x), tin (ca. 0.7%), and lead (ca. 0 . 8 z ) . Kramer and Klein (6) have attempted quantitation of frozen solutions containing K3Fe(CNh, K4Fe(CN)a, and NaC1. For concentrated solutions (ca. l M ) , a reasonable calibration was demonstrated. The analysis of mixtures of MOOSand Moos has always been accomplished using time consuming wet chemical analyses. An instrumental technique has not yet been reported for such oxide mixtures, since no established technique has been able to distinguish one oxide from the other. We wish t o report development of an analytical technique which employs ESCA t o analyze mixtures of Moo3 and MoOZ. EXPERIMENTAL Apparatus, The electron spectra were obtained with a 30-cm, double focusing iron-free electron spectrometer of the __-
Y.Wan, U. Gelius, K. Hamrin, G. Johansson, E. Olsson, H. Siegbahn. C. Nordling, and K. Siegbahn, .I. Ameu.
( 5 ) C. D. Cook, K.
Chem. SOC.,93, 1604 (1971). (6) L. N. Kramei and M. P. Klein, J. Cliem. Phys., 51, 3620 (1969).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1 9 7 1
