in the case of an organic solvent ( 1 3 ) since the solubility of 0 2 is lower in water (14). The smaller quenching effect of 0 2 in water would favor performing the fluorescence analysis directly in water.
CONCLUSION I t appears that PAH in heavily polluted water near industrial sources is detectable by this technique. T o detect PAH in ground water, the fluorescence technique will have to be made more sensitive or the sample concentrated by a t least two orders of magnitude. There are several problems associated with the direct measurement of PAH in environmental water samples. Suspended matter can increase the light scattering, B , in Equation 2, and reduce the sensitivity of the method. There is also the possibility of heavy metal ion quenching of the fluorescence intensity. This quenching would be similar to that of dissolved 0 2 . These interferences can be removed by gas stripping the PAH and some water vapor out of the water sample. The PAH and water vapor would be collected in a liquid nitrogen trap and analyzed by a fluorescence analysis. This technique is now under investigation. Another problem may be the limited spectral resolution of the excitation and fluorescence measurements in identifying the PAH constituents in a sample mixture. Studies to determine the limitation of this qualitative aspect of the fluorescence method are now in progress. An important feature of the fluorescence technique is that it now appears that the solubilities of other PAH in water can be determined. Since the PAH fluorescence quantum yields are essentially the same in H20 and in ethanol, a comparison of the PAH fluorescence in H20 to its fluorescence in dilute ethanol solutions of known concentrations would yield the solubility in water.
LITERATURE CITED (1) J. Borneff. Der Landarzt, 40, 109 (1964). (2) L. M. Shabad, Vestn. Akad. Med. Nauk. SSR,27, 35 (1972). (3) F.P. Schwarz, H. Okabe, and J. K. Whittaker, Anal. Chem., 46, 1024 (1974). (4) IARC MonoaraDh on the "Evaluation of the Carcinoaenic Risk of Chemicals to Man. ill. Certain Polycyclic Aromatics a& Hetrocyclic Compounds", International Agency for Research on Cancer, World Health Organization, Lyon, France, 1973. W. R. Dodge, J. A. Coleman, S . R. Domen, and J. K. Whittaker, Rev. Sci. Instrum., 37, 1151 (1966). M. Seya and F. Masuda, Sci. Light, 12, 9 (1963). W. H. Meihuish, J. Opt. SOC.Am., 52, 1256 (1962). H. S.Hertz, S. N. Cheder, W. E. May, B. H. Gump, D. P. Enogonio. and S. P. Cram, "Preliminary Proceedings of the Marine Pollution Monitoring (Petroleum) Symposium and Workshop, NBS", Gaithersburg, Md, May 13-17, 1974, p 104. R. B. Cundall and L. C. Pereira, Faraday Trans., 68, 1152 (1972). J. W. Eastman and S.J. Rehfeld, J. Phys. Chem., 74, 1438 (1970). T. Forster and K. Kasper, Z.Electrochem., 50, 976 (1955). E. J. Bowen and A . H. Williams, Trans. Faraday Soc., 35, 765 (1939). T. B. Berlman, "Handbook of Fluorescence Spectra of Aromatic Molecules", Academic Press, New York, 1965, pp 42-189. P. Pringsheim, "Fluorescence and Phosphorescence", interscience Publishers, New York, N.Y., 1949, p 332. J. Perronnet, C. R. Acad. Sci., 243, 65 (1956). H. Zimmermann and N. Joop, 2.Nektrochim., 64, 1215 (1960). M. Moyleand E. Ritchie, Aust. J. Chem., 11, 211 (1958). L. J. Pandya and B. D. Tilak, Chem. hd. (London), 981 (1958). R. N. Jones, C. J. Gogek, and R. W. Sharpe, Can. J. Res., Sect. B, 20, 719 (1946). L. J. Andrews and R. M. Keefer, J. Am. Chem. SOC.,71, 3644 (1949). H.B. Kievens, J. Phys. ColioidChem., IO, 283 (1950). D. C. Locke, J. Chromatogr. Sci., 12, 433 (1974). W. W. Davis, M. E. Krahi. and G. H. A. Clowes. J. Am. Chem. SOC., 64, 108 (1942). C. A. Parker, Anal. Chem., 34, 502 (1962). B. Stevens and M. Thomaz, Chem. Phys. Lett., I , 549 (1968). G. Weber and F. W. J. Teaie, Trans. Faraday SOC.,53, 646 (1957). B. Stevens and M. Thomaz, Chem. Phys. Lett., 1,535 (1968). C. A. Parker, C. G. Hatchard, and T. A. Joyce, J. Mol. Spectrosc., 14, 311 (1964).
RECEIVEDfor review September 10, 1975. Accepted December l, 1975. This work has been supported by the Office of Air and Water Measurement a t the National Bureau of Standards, Washington, D.C.
Evaluation of a Xenon-Mercury Arc Lamp for Background Correction in Atomic Absorption Spectrometry M. S. Epstein" and T. C. Rains lnstitute for Materials Research, Analytical Chemistry Division, National Bureau of Standards, Washington, D.C. 20234
The use of a xenon-mercury arc lamp for background correction in atomic absorption spectrometry is evaluated. The high spectral irradiance of this lamp permits background correction to be extended much further into the visible region of the spectrum than previously possible with hydrogen or deuterium continuum sources. The application of the lamp for background correction using a graphite furnace atomizer is demonstrated.
The need for adequate background correction in atomic absorption spectrometry (AAS) has increased because of the use of graphite furnace or rod atomization techniques. Interferences due to molecular absorption and/or scatter in these techniques appear to be a greater problem than in flame techniques, and simultaneous background correction is essential for precise and accurate analysis ( 1 ) . 528
ANALYTICAL CHEMISTRY, VOL. 48, NO. 3, MARCH 1976
The primary continuum sources used for background correction, the hydrogen hollow cathode and deuterium arc lamps, provide inadequate visible radiant intensity to permit balancing of intensities with many hollow cathode line sources operating a t optimum current levels. The usual procedure in such cases is either to reduce the intensity of the hollow cathode line source below optimum current levels to match the continuum source intensity, or to use an extremely wide slit width. Reduction of hollow cathode lamp intensity may degrade the signal-to-noise ratio (S/N) of a system in which photomultiplier shot noise or electronic noise is predominant. Since amplifier gain must be increased, these noises are magnified. The use of extremely wide slit widths may result in: 1) nonlinearity of calibration curves due to increased nonabsorbable radiation from the line source, 2) increased possibility of spectral interference due to absorption of continuum radiation by matrix ele-
___HOLLOW CATHODE ,
I
LAMP POWER SUPPLY
-.-
VARIABLE NEUTRAL
X E N O N ARC L A M P I P O W E R SUPPLY
OENSITY FILTER
---
dd
MONOCHROMATOR
HOLLOW CATHOOE LAMP
ROTATING MIRROR
=TRANSFORMER ~
~
4.
I
-
-
'
HGA-2100 GRAPHITE iURNACE
3:
2.
REFERENCE SIGNAL
. IOOW POWER AMPLIFIER ~~
__
H P 202c LOW FREOUENCY OSCILLATOR
-~i~
-
-A-
LOCK-IN AMPLIiIER ~~
-
Figure 1. Schematic diagram of instrumentation
ments whose absorption bands are not resolved from the analyte line by the monochromator, and 3) increased photomultiplier shot noise from the furnace blackbody emission, which may overload the photomultiplier tube. The effect of furnace emission is extremely severe for elements like calcium and chromium, which require atomization temperatures of 2500 and 2700 "C, respectively. The resonance lines of these elements lie in the spectral region where the blackbody radiation from the furnace is very intense and where the intensity of the deuterium arc lamp, and especially of the hydrogen hollow cathode lamp, is relatively low. The xenon-mercury arc lamp provides adequate radiant intensity from 190 to 600 nm, except a t 253.7 nm, for matching the radiant intensity of hollow cathode line sources operating a t optimum current levels. Radiation a t 253.7 nm is strongly self-absorbed by mercury vapor in the lamp. The use of the xenon-mercury arc lamp for background correction with a Perkin-Elmer HGA-2100 graphite furnace has been demonstrated, and its advantages and disadvantages have been discussed.
EXPERIMENTAL Instrumentation. A block diagram of the instrumentation is shown in Figure 1 and is described in Table I. The radiation from each source, which must be perfectly coincident, is alternately passed through the graphite furnace by a mirror-chopper (2). The intensity of the xenon-mercury arc lamp is attenuated by a vari-
able neutral density filter. Lock-in amplifier detection synchronized to the mirror-chopper is used.
RESULTS AND DISCUSSION Xenon Arc vs. Xenon-Mercury Arc. The radiant intensities of 150-W xenon arc and xenon-mercury arc lamps were compared over the wavelength region of 190-600 nm. The xenon-mercury arc lamp was found to be superior in intensity and stability, and was therefore used in this investigation. Comparison of Continuum Background Correction Sources. The most important attributes of a background correction source in AAS are good stability and high radiant intensity over a wide wavelength range. If the radiant intensity of the background correction source is not great enough to match the intensity of the elemental line source, the problems delineated in the introduction may occur. If the radiant intensity of the background correction source is adequate to match the line source intensity, the stability of the background correction source is most important. The use of a background correction system in AAS introduces additional contributions from lamp flicker noise, photomultiplier shot noise, and electronic noise, which appear to add to the noise contributions from the line source
I O
190
300 Wovelength
460
.
--__---
500
600
nm
Flgure 2. Comparison of continuum source intensities
Table I. Instrumental Parameters 0.5-m Ebert mount; fi8.6 aperMonochromator ture; grating blazed for 300 nm; adjustable straight slits Detector HTW-R106 photomultiplier; HTV-R166 photomultiplier (solar-blind) Detector power supply 0 to 2100 V , to 30 mA, 0.001% regulation Amplifier P.A.R. phase sensitive look-in amplifier (HR-8) with full-scale sensitivity ranges of 100 nV to 500 mV Recorder 10-mV strip-chart recorder, 0.2s full-scale response time Nonflame atomizer HGA-2100 graphite furnace Background correction Hanovia xenon-mercury arc light source lamp, 150 watts Chopping frequency 7 0 Hz
channel t o give an increased total system noise (3).This results in a poorer S/N.When transmission flicker noise ( 4 ) is significant, as in the case of fluctuation due to flame absorption in flame techniques in the far ultraviolet or nonspecific absorption in graphite atomizer techniques, a background correction system can improve the S/N( 5 ) . A comparison of radiant intensities of a hydrogen hollow cathode lamp operated a t 40 mA, a 25-W deuterium arc lamp operated a t 800 mA, and a 150-W xenon-mercury arc lamp is presented in Figure 2. These are not comparisons of absolute output spectra, but rather are comparisons of a commercial system (deuterium arc lamp) vs. our constructed system (xenon-mercury arc lamp) in the background correction channel. The spectra have been corrected for differences in photomultiplier tube response and grating efficiency by comparison with a hydrogen hollow cathode lamp (in the ultraviolet) and various line sources (in the visible) placed in the analyte channel. Comparisons were made a t equivalent spectral bandpasses of both systems. The radiant intensity of the xenon-mercury arc lamp is completely adequate for background correction in the ultraviolet (except a t 253.7 nm) and is superior to the deuterium arc lamp in the visible region. Practical Application of the Xenon-Mercury Arc Lamp Background Correction System. The application of the xenon-mercury arc lamp for background correction in the analysis of barium (553.6 nm) in the presence of a high concentration of aluminum is shown in Figure 3. The small amount of suppression of the barium absorption by ANALYTICAL CHEMISTRY, VOL. 48, NO. 3, MARCH 1976
529
'AI 1 9 3 7 "rn
.-
: I
0
1
JL
rMjL
A'
B
JL \C
* ID
Figure 3. Application of the xenon-mercury arc background correction system to the analysis of barium at 553.6 nm Recorder tracings of absorption of. ( A ) 2 ng barium. background corrected, (6) 2 ng barium and 500 Fg aluminum, background corrected: (C) 2 ng barium and 500 pg aluminum, no background correction; (D)500 &g aluminum, background corrected
aluminum would have been completely masked without background correction. Stray Light. Because of the intense ultraviolet and visible continuum generated by the xenon-mercury arc, stray light is a significant problem a t wavelengths shorter than 220 nm with our system. At this wavelength, using the ASTM method for estimating stray radiant energy ( 5 ) ,approximately 5% of the radiation reaching the detector was stray radiation. A t 190 nm, stray radiation constituted 60% of the signal. Because the grating angle is small a t low wavelengths, the zeroth order light can strike the focusing mirror and reach the exit slit or reflect off baffling with the same effect (6). This stray radiation will generally not be absorbed or scattered by particles in the graphite furnace to the same extent as the radiation from the elemental line source, resulting in an incomplete background correction. A solarblind photomultiplier tube was used to eliminate stray radiation of visible origin but, below 195 nm, the stray radiation level was still significant, and a t 190 nm was about 10%. The effect of stray radiation is shown in Figure 4. Arsenic (5 ng) was analyzed a t both 193.7 and 197.2 nm in the presence of 50 pg of calcium. The background correction is not valid a t the lower wavelength because of the stray radiation. The uncorrected background signal masks a slight suppression of the arsenic signal by the calcium. At 197.2 nm, where the radiant intensity of the xenon-mercury arc lamp is considerably greater than a t 193.7 nm, stray radiation is negligible and the background correction is valid. Spectral Interferences. Spectral interferences using the xenon-mercury arc lamp for background correction can arise from: 1) overlap of absorption line profiles with the mercury line structure of the lamp, 2) the absorption of continuum radiation by the analyte a t small monochromator spectral bandpasses, or 3) the absorption of continuum radiation by high concentrations of elements whose absorption bands lie within the spectral bandpass of the monochromator. Severe spectral line interference (7) due to direct overlap with the mercury line structure is unlikely. Many of the mercury lines are considerably broadened by the high pressure nature of the source, which increases the possibility of overlap, but decreases the effect of that overlap. A check of wavelength tables (8, 9) revealed no obvious coincidence of the most sensitive AAS elemental lines with the mercury emission lines. Several possible overlaps of minor absorp530
I B 197 2 n m
ANALYTICAL CHEMISTRY, VOL. 48, NO. 3, MARCH 1976
Figure 4. Effect of stray light on the validity of background correction for arsenic at 193.7 n m ( A ) , and 197.2 n m ( B ) Recorder tracings of absorption of: (1) 5 ng arsenic, background corrected: (2) 5 ng arsenic and 50 pg calcium, background corrected: (3) 50 fig calcium, background corrected; (4) 5 ng arsenic and 50 pg calcium, no background correction
Table 11. Absorption of Continuum Radiation by 2 fig of Chromium at 428.97 nm in the Graphite Furnace Absorption, $% 40
31 13 7 3.5
Spectral bandpass, nrn
0.04 0.06 0.14 0.28 0.49
tion lines (Cr 302.156 nm/Hg 302.150 nm; Fe 296.690 nm/Hg 296.728 nm) were investigated by comparing the absorbance of high concentrations of the elements with the xenon-mercury arc lamp and a hydrogen hollow cathode lamp. No difference in absorbance with the two sources was observed, indicating no significant overlap problems in these cases. Interferences due to the absorption of continuum radiation from the background correction source by high concentrations of elements whose absorbing lines lie within the spectral bandpass of the monochromator are a problem with all continuum background correction systems. Table I1 indicates the magnitude of this interference for a hypothetical analysis in which the chromium 428.97-nm line was included in the monochromator spectral bandpass. Even a t relatively large spectral bandpasses, an interference would be observed. The surprisingly high percent absorption (40%) a t a spectral bandpass of 0.04 nm can be explained by a broadening of the absorption line profile in the graphite furnace. Under the same conditions, 100 wg of chromium absorbed 90% of the continuum radiation. Similar results were obtained for high concentrations of other elements. The high radiant intensity of the xenon-mercury arc lamp permits the use of small spectral bandpasses, which can be used to resolve possible interferences. On the other hand, the use of a small spectral bandpass may increase the possibility of error due to absorption of the continuum radiation by the analyte, as well as increase the absorption of any other interfering absorption bands not resolved. In our own work, we have found the xenon-mercury arc lamp to be an excellent system for background correction with our graphite furnace. It has been applied to the analysis of various elements in matrices varying from orchard leaves to brain tissue. If its limitations are recognized, it can provide superior performance as a background correction system.
ACKNOWLEDGMENT
(5) W. B. Barnett and J. D. Kerber, At. Abs. News/., 13, 56 (1974).
LITERATURE CITED
(6) ASTM E387-69T, "Manual on Recommended Practices in Spectrophotometry", 3rd edition, American Society for Testing and Materials, Philadelphia, Pa., 1969. (7) V. A. Fassel, J. 0. Rasmuson, and T. G. Cowley, Spectrochim. Acfa, Part E, 23,579 (1968). (8) NBS Monograph 145, Part II, "Tables of Spectral-Line Intensities". U.S. Government Printing Office, Washington, D.C., 1975. (9) "Flame Emission and Atomic Absorption Spectrometry", Vol. 3, J. A. Dean and T. C. Rains, Ed., Marcel Dekker, New York, N,Y.. 1975.
( 1 ) H. L. Kahn and D. C. Manning, Am. Lab., 4 (8),51 (1972). (2) T. C. Rains, M. S. Epstein, and 0. Menis, Anal. Chern., 46, 207 (1974). (3) M. S. Epstein, T. C. Rains, and 0. Menis, Paper I f , 15th Eastern Analytical Symposium and 12th National Meeting of the Society for Applied Spectroscopy, New York, N.Y., 1973. (4) J. 0. Ingle. Jr., Anal. Chem., 46, 2161 (1974).
RECEIVEDfor review October 23, 1975. Accepted November 19, 1975. In no instance does the identification of commercial products imply recommendation or endorsement by the National Bureau of Standards.
The authors are indebted to K. D. Mielenz of the National Bureau of Standards and A. T. Zander of the University of Maryland for their advice in the preparation of this manuscript.
Determination of Platinum and Palladium in Oxidation Catalysts for Automotive Exhaust by Atomic Absorption Spectrometry Noel M. Potter Analytical Chemistry Department, Research Laboratories, General Motors Corporation, Warren, Mich. 48090
A chemical method of analysis, capable of measuring platinum and palladium in automotive catalyst material with an accuracy of f2%, is described. After dissolution of a representative sample of the material and separation of the noble metals from the substrate material, platinum and palladium are measured by atomic absorption. Radioactive tracers were used to verify completeness of the chemical separation. Synthetic solutions were analyzed to investigate potential sources of interferences in the atomic absorption spectrophotometric measurement. Finally, a precision study using ground catalyst material was performed; for both platinum and palladium, a relative standard deviation of approximately 1 % was found.
With the advent of the catalytic converter t o control automotive hydrocarbon and carbon monoxide emissions, analytical methods are required to measure the platinum and palladium content of the catalyst materials. Because of the low levels of platinum (-0.05%) and palladium (-0.02%), sensitive techniques are required, while economic considerations necessitate methods capable of higher accuracy than is usually required for chemical analyses. Many studies have been undertaken to develop methods for the determination of platinum and palladium, but most investigations have been concerned with either geological materials containing trace amounts of these elements, or synthetic solutions used to evaluate the feasibility of utilizing special reagents or techniques. Instrumental techniques such as x-ray fluorescence ( I ) , x-ray diffraction ( 2 ) , and emission spectrography ( I , 3) have been applied for the determination of platinum and palladium in a variety of materials including alumina-base reforming catalysts. However, for highly accurate work utilizing instrumental techniques, a comprehensive array of specially prepared standards, which would consider differences in the catalyst substrate material and all potential interferences, would be required. Experience gained in wet chemical analysis of reforming catalysts has shown that complete recoveries of the noble metals are obtained when both the metal and support material are dissolved ( 4 ) .Fire assay procedures ( 4 ) have been
used to separate noble metals from various matrix materials, but the equipment required for these procedures is not available in most laboratories. Because of the low levels of platinum and palladium, classical gravimetric procedures ( 5 ) could not be easily applied to this problem. A variety of attractive spectrophotometric procedures (6-9) appear in the literature, but most require that the platinum and palladium be separated from each other as well as the sample solution. In a t least one case (IO),platinum and palladium have been determined in the same solution; however, the conditions for color development appear too stringent for the case of dissolved catalyst material. Various atomic absorption techniques applied to the determination of platinum and palladium appear in the literature (I, I I ) . Each element can be measured in the presence of the other, and conditions for accurate measurement can be easily controlled, once platinum and palladium are separated from the matrix material. For these reasons, it was decided to combine a chemical Separation with atomic absorption. Complete sample dissolution, followed by a chemical separation and atomic absorption measurement, resulted in a method for the accurate determination of platinum and palladium in automotive catalyst materials.
EXPERIMENTAL A p p a r a t u s a n d Operating P a r a m e t e r s . Membrane filters, Metricel Alpha-8, 0.2-w pore size, 47 mm in diameter, regenerated cellulose (Gelman Instrument Co.) were used to collect the precipitated platinum and palladium. Measurements were made using a Perkin-Elmer Model 403 atomic absorption spectrophotometer. Light sources were a Westinghouse Model WL 23474A platinum hollow cathode lamp operated a t 1 2 mA and a Westinghouse Model WL 22969 palladium hollow cathode lamp operated at 14 mA. The platinum resonance line at 266.0 nm and palladium resonance line at 247.6 nm were used. The slit widths were set so that the spectral band pass was approximately 0.2 nm for platinum and 0.7 nm for palladium. A premix burner with a single 10-cm slot burner head was positioned 12 mm below the beam of the hollow cathode lamp. The air flow rate was adjusted to 25.2 l./min and the acetylene flow rate to 7.7 l./min. Solution aspiration rate was between 4 and 5 ml/min. S t a n d a r d s a n d Reagents. All chemicals used were ACS reagent grade. Stock solutions included: lanthanum (100 g/l.), containing 117 g of La203 and 240 ml of HC1 per liter; stannous chloride (250 g/l.), containing 250 g of SnC12.2H20 and 500 ml of HC1 per liter; ANALYTICAL CHEMISTRY, VOL. 48, NO. 3, MARCH 1976
531
