ANALYTICAL CHEMISTRY, VOL. 50, NO. 6, M A Y 1 9 7 8
cases, and misinterpretation due to phase misadjustment (Figure 7 ) is unlikely. We anticipate a variety of N M R applications for the DISPA plot covering the full range of line-broadening mechanisms treated in the preceding paper in the near future.
DISPERSION
. ..
* *
.
HDO
LITERATURE CITED
PHASE MISADJUSTED
-_
"' 2
767
1 "
ABSORPTION
Figure 7. DISPA plot based on the 'H NMR spectrum of the residual HDO in 99.7% D,O. Absorption and dispersion spectra were produced by Fourier transformation of the same data set as in Figure 1, but this time with deliberate phase misadjustment of 10'
a 10"phase misadjustment: the noticeable displacement from a reference semicircle (diameter = maximum apparent "absorption" peak height) is obvious. However, the asymmetry of the associated absorption spectrum was also obvious, and phase misadjustment was easily reduced to values for which t h e DISPA plot differed insignificantly from the semicircle of Figure 1. In conclusion, it is experimentally clear t h a t a n apparently (visually) symmetrical absorption line shape contains insufficient dispersion components to disturb a DISPA plot. and that DISPA plot distortions due to phase misadjustment are experimentally negligible.
CONCLUSION T h e eight experimental data sets plotted in this paper conclusively demonstrate the diagnostic value of the DISPA plot in detecting various mechanisms for NMR spectral line-broadening. Reliability of the plot is good (Figure l), agreement with theory is excellent (Figure 1-3, 5 , 6) for test
A. G. Marshall, L. D. Hall, M. Hatton, and J. Sallos, J . Magn. Reson., 13. 392 (1974). I.D. Campbell,C . M. Dobson, R. J. P. Williams, and A. V. Xavier, J. Magn. Reson., 11, 173 (1973). A. De Marco and K. Wuthrich, J , Magn. Reson., 24, 201 (1976). H. A. Resing, Adv. Mol. Relaxation Processes, 1, 109 (1967-68). B. Boddenberg, R. Haul, and G. Oppermann. Adv. Mol. Relaxation Processes, 3, 61 (1972). A. Zipp, T. L. James, I.D. Kuntz, and S. B. Shohet, Biochim. Biophys. Acta, 428, 291 (1976). E. Hsi and R. G. Bryant, J . Phys. Chem., 81, 462 (1977). R. Cooke and I. D. Kuntz, Ann. Rev. Biophys. Bioeng.. 3, 95 (1974). I . D. Duff and W. Derbyshire, J , Magn. Reson., 17, 89 (1975). D. E. Woessner and B. S. Snowden, Jr., J . Colloid Interface S c i . , 34, 290 (1970). D. E. Woessner, B. S. Snowden. Jr., and Y.-C Chiu, J . ColloidInterface Sci., 34, 283 (1970). A. Allerhand and H. S. Gutowsky, J . Chem. Phys., 41, 2115 (1964). A. Carrington and A. D. McLachbn. "Introduction to Magnetic Resonance", Harper & Row, New York, N.Y., 1967, pp 205-208.
RECEIVED for review October 7 , 1977. Accepted January 16, 1978. This work was supported by grants to A.G.M. from the National Research Council of Canada (A-6178), the University of British Columbia (21-9222), and t h e Alfred P. Sloan Foundation. D.C.R. gratefully acknowledges a National Research Council of Canada Postdoctoral Fellowship (1974-75). A.G.M. is an Alfred P. Sloan Foundation Research Fellow (1976-78). Portions of this work have been presented a t the Sixth International Symposium on Magnetic Resonance (Banff, Alberta, M a y 1977), and a t the 32nd Northwest Regional Meeting, American Chemical Society, Portland, Ore., J u n e 1977.
Direct Determination of Phosphorus in Gasoline by Flameless Atomic Absorption Spectrometry Daniel J. Driscoll," Dwight A. Clay, Crystal H. Rogers, Robert H. Jungers, and Frank E. Butler United States Environmental Protection Agency, Source Fuels and Molecular Chemistry Section, Analytical Chemistry Branch, Environmental Monitoring and Support Laboratory, Research Triangle Park, North Carolina 2771 1
A new method is presented for the determination of phosphorus in gasoline using flameless atomic absorption. Lanthanum nitrate solution is inserted in a graphite furnace prior to direct addition of gasoline. The organic matrix is charred prior to atomization of the phosphorus. The sensitivity of this method is 20 ng of phosphorus per 90-pL aliquot of gasoline. Repeated analysis of NBS triphenyl phosphate standard at the 80-ng level resulted in 2 % relative standard deviation. Each determination requires less than 2 min.
Organophosphorus compounds were originally added to gasoline as detergents to prevent fouling of spark plugs. The Clean Air Act of 19'70 ( I ) limited phosphorus compounds in gasoline to 0.005 g/gal because of adverse effects to catalytic converters. In support of this legislation, the Environmental Protection Agency (EPA) Fuels Laboratory analyzes gasoline for This paper not subject to U.S. Copyright.
phosphorus. The procedure used is the molybdate-hydrazine method ( 2 ) . It is a time-consuming colorimetric wet chemistry test. T o minimize analysis time, the graphite furnace was adapted for a screening test for the direct determination of phosphorus in gasoline. Previous studies dealing with flameless atomic absorption (AA) determination of elements in petroleum described conversion of the organic matrix to the aqueous phase prior to analysis (3). Limited tests were made on the effect of organic matrices on flameless AA analysis ( 4 ) . However, no direct analytical method for phosphorus in gasoline using the graphite furnace has been reported.
EXPERIMENTAL Instrumentation. The following Perkin-Elmer equipment was used in this investigation: A model 403 double beam atomic absorption spectrophotometer equipped with a deuterium background corrector and a phosphorus electrodeless discharge lamp (EDL), an HGA 2100 Graphite Furnace, a model 56 recorder, Published 1978 by the American Chemical Society
I
I
DRY CYCLE
60SEC@ llO°C
CHAR CYCLE
50 SEC @ 160OOC
ATOMIZING CYCLE WAVELENGTH BANDWIDTH
4.0 nm
PURGE GAS
ARGON, INTERRUPT MODE
REAGENT
15% LANTHANUM NITRATE I N 25% AQUEOUS ETOH
REAGENT VOLUME SAMPLE I
T 0
400
800
1200
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ZOO0
1Opl ADDED FIRST 90 p l
-
P H O S P H O R U SF R E E G A S O L I N E
0
5 SEC 0 27OOOC
214.2 nrn. DEUTERIUM CORRECTION USED
1
2400
2800
L A N T H I N U M gI
Figure 1. Effect of lanthanum on phosphorus signal response
and an M-2 calculating integrator. Direct Injection Experiments. NBS grade triphenyl phosphate (TPP) is dissolved in phosphorus-free gasoline. The standard solutions range in concentration from 0.001g/gal to 0.010 g/gal phosphorus. The effect of lanthanum on phosphorus signal response is illustrated in Figure 1. Ten pL aliquots of various concentrations of lanthanum nitrate dissolved in 25% EtOH were injected into the furnace. The volume of both the phosphorus-free gasoline and the 0.9-pg phosphorus standard was held constant at 90 pL. As can be seen, concentrations in excess of 2 mg lanthanum do not appreciably interfere with absorption in the EDL beam. Maximum phosphorus signal response occurs when 960 pg of lanthanum is present in the graphite tube. Ten p L of a 15% lanthanum nitrate solution in 25% EtOH (960 pg lanthanum) is injected into the graphite furnace ahead of 90 pL of sample or standard. This amount of lanthanum enables us to achieve a lower detection limit of 0.001 g/gal (20 ng). Lower detection limits can be achieved by reducing the volume of the gasoline sample prior to injection. A stream of clean air is bubbled through a sample of gasoline. Within 15-30 min, the volume of the sample can be reduced by 50%. A 90-pL sample of 0.004 g / g a l phosphorus (80 ng) standard will produce a peak height of 3.0 before concentration. After reduction in volume. the above standard will produce a peak height of 6.0. Three purge gases were evaluated for their response to concentrations of phosphorus ranging from 20-200 ng. Helium: the lowest molecular weight purge gas used resulted in no response over the entire range of phosphorus standards. At a concentration of 200 ng phosphorus, nitrogen yielded a relative area count of 300 while argon produced a relative area count of 1000.Therefore argon was chosen as the optimum purge gas. The method of Manning (5) was used to pyrolytically coat the interior of graphite tubes. Using this technique, we deposited an average of 112 mg of pyrol>tic carbon onto each tube. However peak heights of 50% less were obtained using pyrolytically coated tubes rather than uncoated tubes. Final Procedure. Table I outlines the operating conditions for the direct injection method. Due to the refractory nature of phosphorus, the charring cycle can be operated at 1600 "C. The interrupt gas flow mode is used for purge gas operation. In this manner of operation, the purge gas flow ceases during the atomization cycle; therefore the phosphorus is retained longer in the EDL beam, thereby increasing sensitivity. When the phosphorus content of a sample is above 0.01 g/gal the atomization time should be increased to 15 s. This will atomize the phosphorus quantitatively off the graphite tube and result in better signal precision. The 4.0 nm bandwidth lowers the background noise to 0.5 of a scale division, thereby allowing a signal of 1.0% scale deflection to become the minimum detection limit. The operator procedure for direct injection follows. A graphite tube which has been thermally conditioned for 15 s is injected with 10 pL of 15% lanthanum nitrate. This injection is followed by an injection of 90 pL of sample or standard. The temperature
program is initiated and the phosphorus signal is analyzed in terms of ahsorhance at the moment of atomization. The absorbance signal is displayed on a recorder as peak height. Each tube may be used for two standards and 10 analyses. The lowering of the signal response to the standard necessitates the replacement of the graphite tube.
RESULTS AND DISCUSSION The early developmental work performed in this laboratory on the phosphorus method utilized aqueous standards of potassium phosphate. Initially the phosphorus solutions analyzed in the graphite furnace yielded low signal responses. T h e signals were enhanced by the injection of 20 pL of 5 % lanthanum nitrate. In the subsequent direct injection approach. the volume of lanthanum nitrate was reduced to 10 pL and the concentration of lanthanum nitrate was increased to 15%. T h e reduction in the volume of lanthanum nitrate allowed us to increase the sample size from 80 to 90 pL thereby increasing sensitivity. Runnels et al. (fi) pretreated graphite tubes with lanthanum chloride and determined that 600 p g of lanthanum yielded the maximum signal in the determination of beryllium in petroleum. Our experimental data show measurably increased phosphorus signal response is obtained when 960 fig of lanthanum (10 pL of 1 5 7 ~lanthanum nitrate) are added to the graphite tube before addition of the standard or sample. T h e mechanism by which lanthanum enhances the phosphorus signal is unknown. However, the enhancement effect must be cancelled by the atomization process since the lanthanum must be replenished before each analysis. T h e reduced sensitivity obtained by using pyrolytically coated tubes was most surprising since other investigators ( 7 ) using coated tubes reported a fivefold increase in sensitivity in the determination of refractory elements, other than phosphorus. We determined that the only conditioning of the graphite tube necessary is a 15-s high temperature purge. Because of the low surface tension of organic liquids, a loss of gasoline from the micropipet tip can occur when transfering the sample to the furnace. This can result in poor reproducibility. Other investigators (3)have improved precision by chemically pretreating the disposable pipet tips before usage. Instead of chemical pretreatment, we wet the micropipet tip with gasoline sample prior to injection. This technique enables us to achieve a precision of 2.0%. T h e relative standard deviation is calculated from 10 injections each of 90 pL of 80 ng of TPP phosphorus standard in gasoline. Earlier flameless ,4A determination of elements in gasoline was accomplished by elimination of the organic matrix. Kaegler dry ashed 1-g samples of gasoline prior to determination (3). Beaty wet ashed gasoline to extract phosphorus into the aqueous phase prior to determination in the graphite furnace (8). Similar experiments in this laboratory involved acid extraction of phosphorus from gasoline and ashing of gasoline on metal oxides. Results were variable and the procedure was time-consuming. Therefore experiments in-
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previously. T h e lower detection limit of the molybdate-~ hydrazine method is 0.004 g/gal (80 ng) using 3 mL of gasoline. During the development of the method for determination of gasoline phosphorus by direct injection, 500 samples were comparatively tested by both the graphite furnace and the molybdate-hydrazine procedures. T h e latter method is a lengthy procedure involving burning the sample off, burning carbon off in the muffle furnace, acid digestion, and further heating and cooling. T h e average time required to process an individual sample in this mariner is 20 min. By contrast, the graphite furnace method can process a sample in less than 2 min. Equipment costs between the two methods are comparable if the laboratory already possess an AA equipped with a graphite furnace. The cost per graphite tube is $6.00 making the cost per replicate $0.60. Increased analytical capability and decreased personnel time per analysis are added benefits. Further work using the direct injection of gasoline into the graphite furnace will be done. The techniques described here should be applicable to other trace elements in gasoline.
LITERATURE CITED
PHOSPHORUS
g 911
Figure 2. Response curve
volving direct injection of gasoline into the furnace were undertaken. Figure 2 shows t h e phosphorus response curve for 90-pL aliquots of gasoline injected directly into the graphite furnace. Each injection is preceded by t h e injection of 10 pL of 15% lanthanum nitrate in 25% E t O H (960 pg lanthanum). T h e response is linear in the region of 0.001 g/gal (20 ng) to 0.006 g/gal (120 ng). Phosphorus can be detected in the graphite furnace to a lower detection limit of 0.001 g/gal using 90 pL of sample. Still lower concentrations of phosphorus can be detected by concentrating t h e sample prior to analysis as described
(1) Clear Air Act of 1970, Sec. 211. (2) Fed. Regist., 39, No. 131 (1974). (3) S. H. Kaegler, "Atomic Absorption Spectroscopy Detection Limits of Fhmeless AAS & AAS with Flame and Application", ErdOslKohk, Erdgas, Petrochem. Brennst-Chem., 28 (5), 2327 (1975). (4) A Varner, "Effect of Miscible Organic Solvents on Determination of Trace Metal Concentrations by HGA", Sohio Research Lab., Cleveland, Ohio, 1976, unpublished. ( 5 ) D. C. Manning, Perkin-Elmer Atomic Absorption Report $602, December 1975. (6) J. Runnels, R. Merryfield, and H. Fisher, "Analysis of Petroleum for Trace Metals-A method for Improving Detection Limits of Some Elements with the Graphite Furnace Atomizer", Anal. Chem.. 47, 1258 (1975). (7) R. Ediger, Perkin-Elmer Atomic Absorption Report $51 1, August 1975. (8) Oral communicationswith Richard Beaty. Pekin-Elmer Corp., Gaithersburg, Md.
RECEIVED for review December 15, 1977. Accepted February 17, 1978. This paper was presented in part a t the 1977 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy. Mention of proprietary names does not constitute an endorsement by EPA.
Determination of Rhodium in Platinum-Rhodium Loaded Automotive Catalyst Material by Graphite Furnace Atomic Absorption Spectrometry N. M. Potter Analytical Chemistry Department, General Motors Research Laboratories, Warren, Michigan 48090
An analytical method to determine rhodium in automotive catalyst materials used to reduce nitrogen oxide emissions is described. Because of the low level of rhodium (-0.002%), a sensitive method is required. Atomic absorption spectrometry (AAS) using a flameless (heated graphite) furnace was used to determine low levels of rhodium with a minimum amount of sample treatment. Complete sample dissolution in sulfuric and hydrochloric acids followed by flameless AAS measurement resulted in a relatively simple method for the determination of rhodium with a relative standard deviation of 4 % and an accuracy of %YO. 0003-2700/78/0350-0769$01,00/0
With the continuing development of catalytic converters to control automotive emissions, analytical methods are required to measure the rhodium content of platinum-rhodium loaded automotive catalyst materials. Because of the low levels of rhodium (-0.002%), sensitive analytical techniques are required, while research and future economic considerations necessitate accurate methods. Methods for the accurate determination of platinum in automotive catalyst materials are discussed elsewhere ( I , 2). Several studies have been undertaken to develop methods to determine rhodium, but most have dealt with geological C 1978 American Chemical Society
