Analysis of Petroleum for Trace Metals1-The Determination of Trace Quantities of Manganese in Petroleum and Petroleum Products by Heated Vaporization Atomic Absorption Winston K. Robbins Analytical & Information Division, Exxon Research & Engineering Company, Linden, N.J.
A method is described for the determination of manganese by heated vaporization atomic absorption which is capable of measuring manganese directly in petroleum matrices in concentrations from 10-300 ng of manganese per gram of sample. The technique of micro standard additions is used to overcome matrix effects. A discussion of heated vaporization atomic absorption as related to the characteristics of a Varian Techtron Model 63 Carbon Rod Atomizer is included. The precision of the method is calculated from data obtained using this method at different laboratories.
Only limited data are available about the abundance, distribution, or nature of metallic species, other than nickel and vanadium, in crude oils and petroleum products. Surveys which include concentrations of trace elements in petroleum matrices are characterized mainly by the wide variations reported (1-4). These differences arise both from variations in natural levels in the crudes themselves and from dissimilar production and handling of the oil prior to sampling (2, 3). Even in carefully sampled oils from a limited geological region, variations in trace metal concentrations of two or three orders of magnitude are encountered ( 5 ) . The trace elements of greatest interest are those which represent a potential hazard to the environment. Even part-per-billion levels of metals are significant, because current world-wide production of crude oil exceeds 3 X lo9 metric tons per year (6). The growing awareness of the significance of trace metals in petroleum is reflected in a number of recent symposia a t which several of the most useful analytical techniques were reviewed (7, 8). The data obtained in a recent study conducted by the U.S.National Bureau of Standards for the analysis of metals in fossil fuels varied widely, indicating the need for reliable analytical methods (9). Manganese has been reported to occur over a wide range of concentrations as a natural trace element in crude oils (1-5, 10, 111. Manganese additives have been patented for C o n t r i b u t i o n of T r a c e M e t a l s P r o j e c t P a r t i c i p a t i n g L a b o r a t o C h e v r o n Research ries: A t l a n t i c R i c h f i e l d C o m p a n y , H a r v e y , Ill.; C o m p a n y , R i c h m o n d , Calif.; E x x o n Research a n d E n g i n e e r i n g C o m p a n y , L i n d e n , N.J.; M o b i l Research a n d D e v e l o p m e n t C o r p o r a t i o n , Paulshoro, N.J.; a n d P h i l l i p s P e t r o l e u m C o m p a n y , B a r t l e s ville, O k l a . (1) J. W. McCoy, "Inorganic Analysis of Petroleum." Chemical Publishing Co., New York, N.Y., 1962. (2) 0. I. Milner, "Analysis of Petroleum for Trace Elements," Pergammon Press, New York, N.Y., 1963. (3) B. B. Agrawal and I. 8. Gulati, Petrol. Hydrocarbons, 6, 193 (1972). (4) K. R. Shah, R. H. Filby. and C. W. A. Haller, J. Radioanal. Chem., 6, 185 (1970). (5) B. Hitchon, P. H. Filby. and K. R. Shah, Amer. Chem. SOC.,Div. Petrol. Chem.. Prepr., 18, (4), 623 (1973). (6) L. Auldridge, OllGas J., June 18, 1973, p 25. (7) H. A. Braier and J. Eppolito, Amer. Chem. SOC.,Div. Petrol. Chem., Prepr. 18 (4), 593 (1973). (8) H. A. Briar, Anal. Chem., 45, 196R (1973). (9) Dimension-NBS; "Environment a Question of Quality," October, 1973, p 240. (10) G. Palmai, L. Vajta. I. Szabenyi, and G. Toth, Period. Polytech., Chem. Eng., 13 (12), 99-104 (1969): Chem. Abstr., 72, 456949 (1970). ( 11) E. M. Lobanov and G. G. Mingaliev, Aktiv. Anal. Elem. Sostava Geol. Ob'ektov, 89-92 (1967); Chem. Abstr., 71, 932351 (1969).
use in improving fuel oil combustion (12), and in conjunction with alkyl lead compounds in providing a synergistic octane improvement (13, 14). Combustion of various hydrocarbon fuels has been cited as the source of manganese in some urban atmospheres (15). Generally, the manganese reported in petroleum has been measured as part of trace-element survey analysis by neutron activation (4, 5 ) or emission spectroscopy (1-3, 10, 11).In other matrices, trace levels of manganese have been determined by colorimetric ( 1 6 ) ,kinetic (17 ) , and electrochemical (18) procedures. The application of these techniques for the determination of trace quantities of manganese requires destruction of the hydrocarbon matrix, which introduces the risk of contamination or loss of manganese. One widely used technique which combines sensitivity with selectivity is atomic absorption spectroscopy. A paper has recently appeared which discusses the determination of manganese using conventional flame atomic absorpt.ion and includes references to previous work (19). Heated vaporization atomic absorption (HVAA) is a technique in which micro quantities of sample are dried, ashed, and atomized on a carbon rod, furnace, or filament, and the absorption due to the atomic species is then measured. This technique is more sensitive than flame techniques for most elements and it can readily be adapted to the direct analysis of metals in many matrices (20-25). The application of HVAA to the determination of trace metals in petroleum has recently been reviewed (26). Several applications of HVAA to the determination of trace quantities of manganese in fresh water have been reported (2729). Manganese has also been determined by this technique in human serum (30-32), sea water (33), milk ( 3 4 ) , titanium dioxide (351, and amoeba (36). Some authors (12) A. T. Rolfe. U.S. Patent 3,443,916; Chem. Abstr., 71, 24558e (1969). (13) G. B. Kyriakopoulos, J. lnst. Petrol., 54 (504). 369 (1968): Chem. Abstr., 70, 98450q (1969). (14) D.R. Bailey, F. J. Cordera, and R. G. Tuell, Belg. Patent 613,117; Chem. Abstr., 59, 9731h (1963). (15) J. R. Fancher, "Trace Element Emissions from the Combustion of Fossil Fuels," Cycling and Control of Metals, M. G. Curry and G. M. Gigliotti. Ed., National Environmental Research Center, Cincinnati, Ohio, 1973. (16) W. Ballschmiter and M. Perscheid. Fresenius Z. Anal Chem., 258, 430 (1972); Chem. Abstr., 76, 941816 (1972). (17) F. Tsutomu and T. Yamane. BunsekiKagaku, 168 (1973). (18) V. Fano, Microchem J.. 15, 422 (1970). (19) D. G. Van Ormer and W. C. Purdy, Anal. Chim. Acta. 64, 93 (1973). (20) G. F. Kirkbright. Analyst (London), 96, 609 (1971). (21) T. S. West, Pure Appl. Chem., 26, 47 (1971). (22) H. L. Kahn and S. Slavin, At. Absorption Newslett., IO, 125 (1971). (23) M. D.Amos, Amer. Lab., (5), 57 (Aug. 1972). (24) R. B. Baird, S. Pourian, and S. M. Gabrillian, Anal. Chem., 44, 1887 (1972). (25) W. K. Robbins, Anal. Chim. Acta. 65, 285 (1973). (26) G. Hall, M. P. Bratzel, and C. L. Chakrabarti, Talanta, 20, 755 (1973). (27) T. Takeuchi. M. Yanagisaiva, and M. Suzuki. Talanta. 19, 465 (1972). (28) S. H. Omang, Kjemi, 31, 13 (1971); Chem. Abstr.. 75, 6778x (1972). (29) B. Welz and E. Wiedeking, Fresenius 2. Anal. Chem., 264, 110 (1973). (30) F. Bek, J. Janouskova, and 6.Moldan, ChemListy, 66, 867 (1972). (31) P. A. Ullucci, C. J. Mokeler, and J. Y. Hwang, Amer. Lab, 63, August 1972. (32) B. Welz and E. Wiedeking, Fresenius Z. Anal. Chem., 252, 11 1 (1970). (33) D. S. Segar, Anal. Chim. Acta, 58, (1972). (34) J. Lagathu and J. Descraut, Rev. Tr. Corps. Gras., 19, 169 (1972). (35) C. W. Fuller, At. Absorption Newsleft., 12. 40 (1973). (36) B. Wels, CZ-Chem. Tech., 1, 455 (1972); Chem Abstr., 76, 37450h (1972).
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 14. DECEMBER 1974
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Table I. Atomic Absorption Spectrophotometer Conditions
Table 11. Operating Parameters for Carbon Rod Analyzer
Analytical line
279.4 nm
Background line
280.1 n m
Atomization tube Support e l e c trodes Inert gas Cooling water Injection frequency Sample a h quot
Slit width S p e c t r a l band width P hot0 multipl ie r Recorder T i m e constant Scale expansion Read out
( J a r r e l l - A s h Mn L a m p 8 milliamps) (Jarrell-Ash P b L a m p 8 milliamps)
100 y m 0.2 nm R-106 Speedomax W
0 . 5 sec 10 x
(Hamamatsu) (Leeds & Northr u p Multirange) (0-1 mV on Recorder)
9 m m Pyrolytic FX-9 G r a p h i t e
(Ultracarbon from Varian) (Poco Graphite)
N2 at 4 l./min 4 1./min at 16-20 “C 90 s e c o n d s
1 p l (*Is)
P e a k height
Reproducing Syringe (Pfizer 6443 Micropipette r, P f i z e r Diagn o s t i c s , New York, N.Y.)
Power setting on
have also reported matrix and interelement effects (37, 38). The present paper describes the development of a HVAA procedure for the determination of manganese at the ng/g
level in petroleum matrices.
CRA-63 Program
supply unit’
Dry 2 Ash 6. 5 Atomize 8 a Arbitrary units on CRA-63 power supply.
Seconds
20
30 3
EXPERIMENTAL Standards and Reagents. All the reagents and solvents used were of analytical reagent grade purity. Where used, distilled water was demineralized before use by passage through a mixed bed (blue) ion exchange resin (Barnstead, Boston, Mass.). Manganese sulfonate (5000 ppm W/W, Conostan, Ponca City, Okla.) and cyclohexanebutyrate (13.9% Mn certified, National Bureau of Standards, Washington, D.C.) were each diluted with a 1:l mixture of xylene and 4-methyl-2-pentanone (MIBK) to prepare stock standard solution of 1000 pg Mn/ml. Further dilutions were made in tetrahydrofuran (THF) immediately before use. Preparation of Standards for Interlaboratory Cross-Check Program. Oils typical of those encountered in the petroleum industry were spiked by adding 2 ml of a lO-pg/ml manganese standard (manganese sulfonate in M1BK:xylene) to 622 g of oil in a quart bottle and then agitating for 5 minutes on a paint shaker. Spiked and unspiked oils were then poured into 4-oz Teflon bottles. These samples were used in the interlaboratory cross-check program which is described later. Apparatus and Operating Parameters. For the development of this procedure, a Varian-Techtron Model 63 Carbon Rod Atomizer (Varian-Techtron Corp., Palo Alto, Calif.) was mounted in place of the burner of a Jarrell-Ash 82-532 Atomic Absorption Spectrophotometer (Jarrell-Ash Div., Fisher Scientific Co., Waltham, Mass.). The components and function of the CRA-63 have been previously described (23, 39). The Jarrell-Ash 82-532 was modified by raising the monochromator and the second lens turret I%”,thus bringing all optics into a plane and eliminating the need for a periscope. The monochromator was protected from the atomizer tube emissions by mounting a stainless steel shield with a 2.25-mm hole on the exit side of the CRA-63 workhead. Absorbances were measured from the peak heights recorded under the conditions listed in Table I. A graphite atomization tube and two support electrodes were positioned in the workhead for optimum optical alignment. At this optimum, a maximum amount of light passed through the tube and only a minimal tube blank was obtained during atomization a t full voltage. The operating parameters which gave peak height of -10 X A for 1 pl of a 5-ng Mn/ml standard are listed in Table 11. Procedure. A 2.5-g sample of oil is weighed into a 5-ml volumetric flask and diluted to volume with THF. One microliter of the sample is injected onto the furnace and the program initiated. To allow for the response changes due to standard additions, the manganese peak height for the initial sample solution should be A. If it is not, a more dilute solution is preless than 50 X
s.
(37) L. Ebdon, G. F. Kirkbright, and T. West, Anal. Chim. Acta, 58, 39 (1972). (38) L. Ebdon. G.F. Kirkbright, and T. S. West, Talanta 19, 1301 (1972). (39) C. R. Parker, J. Rowe, and D. P. Sandoz, Amer. Lab., 53, August 1973.
2178
pared either by weighing a smaller sample or by a secondary dilution in T H F before the analysis is continued. If the signals from three independent injections agree within 10% of their average, that average is used; otherwise, the average signal for five injections is used. Once this value is obtained, a 5-pl addition of a 5-pg Mn/ml working standard is added to the solution in the 5-ml volumetric flask, i.e., a 25-ng manganese standard addition, and an average signal is again determined. After the average signal for the sample solution and for the sample plus each of three such standard additions is obtained, the background is measured a t the lead 280.1-nm non-absorbing line. The concentration of manganese in the sample is then calculated by the following equation:
where A0 is the average peak height for the sample solution, B is the signal at the non-absorbing line, i is the number of additions, and A, is the average recorder signal after the ith addition of manganese. The final value is calculated as the average of the values calculated in this manner. The calculation format is not mathematically rigorous. The use ) the of [(i) X 25 ng/Ao - A,] in place of (25 ng/A, - A L - ~weights initial value and levels small variations in the microstandard additions. The average value calculated in this manner is within 10%of that obtained from a least squares plot of the data. Since the quantity of sample removed for analyses is very small with respect to the total volume of the sample, the volume changes are neglected. For example, ten 5-pl additions would be required to cause a 1% volume change in the 5-ml solutions, while the agreement between injections has been reported to be within 2-10% for HVAA measurements as shown by our own work and others (23,26,39,40).
RESULTS AND DISCUSSION Characteristics of the Carbon Rod Atomizer. The power characteristics of the CRA-63 used i n the development of the procedure were determined at t h e workhead. The voltage drop across the workhead terminals was meas u r e d with a 0-10 rectifier t y p e ac m e t e r (Simpson Electric Company, Chicago, Ill.). The c u r r e n t passing t h r o u g h the power cables was measured on the appropriate amperage scale of a clip-on A m m e t e r (Amprobe RS-3A, Amprobe, Lynnbrook, N.Y.). Power curves were obtained b y varying the setting of each control on the power supply and recording the voltage a n d amperage after 10,30, and 3 seconds for the Dry, Ash, and Atomize cycles, respectively (Figure 1). (40) G. Torsi and G. Tessari, Anal. Chem., 45, 1812 (1973).
ANALYTICAL CHEMISTRY, VOL. 46, NO. 14, DECEMBER 1974
r-
IkU
Figure 2. Effect of input power on atomization signal for 1 pl of 25
nanogram Mn/ml standard in THF N2
-~
L
-
4 I /min, ash 0 19 k W for 20 sec
I
T '
I
r
Figure 1. Effect of power supply settings on output power for dry,
ash, and atomize cycles
*-@-*-
I
1 1
I
I
\
I
I
I
T h e use of a measured power, expressed as VXA/(1000) (k W), allowed more reproducible setting between instruments. (Each instrument has its own characteristic power curves.) The power measurement also allowed continuous monitoring during individual runs. Power losses were indicated as the tubes decayed. Typical useful lives before power decrease were in the order of 300 atomizations. Effect of Atomization Power. The signal from 1 p l of a 25-ng Mn/ml standard solution increased regularly, but not linearly, with increasing atomization power (Figure 2). For the procedure, the optimum linear range for manganese was attained a t a power setting corresponding to 1 kW. Effect of Ashing Power. The power used during the ashing cycle also affected the manganese atomization signal (Figure 3). As can be seen, when the power was above 0.23 kW, the atomization signal was lowered. This is consistent with the loss of manganese by volatilization prior to atomization. This type of loss has been observed for copper and nickel under severe HVAA ashing conditions ( 4 1 ) . The mechanism of such losses is unclear. T o avoid the loss of manganese, the power during the ash cycle was kept well below the 0.23-kW level. Ashing crude oils a t power of
