Determination of Wear Metals m Engine Oils by Atomic Absorption Spectrometry with a Graphite Rod Atomizer R. D. Reeves,' C. J. Molnar, M. T. Glenn, J. R. Ahlstrom, and J. D. Winefordner* Department of Chemistry, Uniuersity of Florida, Gainesuille, Fla. 32601
The concentrations of Ag, Cr, Cu, Fe, Ni, Pb, and Sn in used jet-engine and reciprocating-engine oils have been determined by atomic absorption, using atomization from a cavity in a heated graphite rod. The samples analyzed were taken from those provided by the United States Air Force Spectrochemical Oil Analysis Program (S.O.A.P.) during the period May 1969-December 1971. This enabled a comparison to be made between the present results and those obtained by flame atomic absorption by the laboratories participating in S.O.A.P. Excellent agreement between the two sets of results was found for Ag, Cu, Fe, Ni, and Pb. The use of the graphite-rod atomizer led to results for Cr that are considerably higher than those found with an air-acetylene flame, but are comparable with results obtained with a nitrous oxide-acetylene flame. It appears that most of the Sn concentrations are near, or below, the levels detectable with flame atomization, and in some cases Sn was also undetectable with the graphite-rod atomizer.
APPLICATION OF TRACE ELEMENT ANALYSIS to the determination of wear-metal particles suspended in the lubricating oils of railroad and aircraft engines has been firmly established for many years. Early methods of analysis included colorimetry (after ashing and dissolution of the sample) ( I ) , and a number of direct emission spectrographic techniques, discussed by Fry (2). Direct-reading emission spectrographs continue to be widely used. The past decade has seen the development of flame atomic absorption spectrometry (3-10) and preliminary investigations of flame atomic fluorescence spectrometry (11, 12). The range of applications of engineoil analysis for wear metals has been reviewed by Middledorf (13). Attention has recently been given to the possible advantages of using flameless atomization systems for oil analysis On leave, Department of Chemistry and Biochemistry, Massey University, Palmerston North, New Zealand. 1 Author to whom reprint requests should be sent. (1) A.S.T.M. Standards on Petroleum Products and Lubricants, Method D810 (1960), American Society for Testing and Ma-
terials, Philadelphia, Pa. (2) D. L. Fry, Appl. Spectrosc., 10, 65 (1956). (3) S. Sprague and W. Slavin, At. Absorption Newslett., No. 12 (April 1963). (4) J. A. Burrows, J. C. Heerdt, and J. B. Willis, ANAL.CHEM.,37, 579 (1965). (5) E. A. Means and D. Ratcliff, At. Absorption Newslett., 4, 174 (1 965). (6) S.Sprague and W. Slavin, ibid., p 367. (7) Ibid., 5, 106 (1966). (8) J. E. Schallis and H. L. Kahn, ibid., 7, 84 (1968). (9) D. R. Jackson, C. Salama, and R. Dunn, Can. Spectrosc., 15, 3 (1970). (10) W. B. Barnett, H. L. Kahn, and G. E. Peterson, At. Absorption Newslett., 10,106 (1971). (11) R. Smith, C. M. Stafford, and J. D. Winefordner. Can. Spectrosc., 14, 38 (1968). (12) R. L. Miller, L. M. Fraser, and J. D. Winefordner, Appl. Spectrosc., 25, 477 (1971). (13) A. J. Middledorf, Spex Speaker, 13, 1 (1968).
by atomic absorption. Brodie and Matougek (14) have investigated the use of a graphite mini-furnace for the determination of Ag, AI, Cu, Cr, Mg, Ni, and Pb in lubricating oils, and have obtained good sensitivity using undiluted samples no larger than 1 pl. Alder and West (15) have studied graphite-rod atomization for copper and silver in dieselengine lubricating oils, and Omang (16) has reported the use of a graphite-tube furnace for determining nickel and vanadium in crude petroleum. The present work is an investigation of the behavior of oilbased samples o n a graphite-rod atomization system which has been described in detail elsewhere (17). Seven elements of major practical importance (Ag, Cr, Cu, Fe, Ni, Pb, Sn) have been studied, and analyses have been carried out on a larger number of real samples than have been reported previously. The oil samples chosen for analysis were a selection of used jet-engine and reciprocating-engine oils provided by the United States Air Force Spectrochemical Oil Analysis Program (S.O.A.P.) between May 1969 and December 1971. Independent analyses of these samples by standard flame atomic absorption methods have been performed by up to 50 US. military laboratories participating in the program, and by several other military and commercial laboratories in North America and Europe. The results of these determinations allow detailed comparisons to be made between flame and non-flame atomization for the elements studied. EXPERIMENTAL
Instrumentation. The graphite-rod atomizer described by Molnar et al. (17) was used without further modification. The atomizer was mounted in place of the burner in a PerkinElmer 303 Atomic Absorption Spectrophotometer equipped with a recorder readout unit and a Sargent T R recorder. The photomultiplier output was also connected to a digital integrator (Autolab 6230, Vidar Corp., Mountain View, Calif.) with print-out facility. Both the recorder and the integrator were operated in modes giving the most rapid response. During the course of this work, the use of an integrated absorption signal generally provided no increase of precision over that obtained by using peak absorbances taken from the recorder chart, and the use of the integrator was eventually discontinued. Single-element and multi-element hollow-cathode lamps (Perkin-Elmer, Varian Techtron, Aztec) were operated at the manufacturer's recommended maximum currents. Graphite rods were machined from Poco FX9I graphite (Poco Graphite Inc., Decatur, Texas 76234), and a cylindrical cavity 1.4 mm in diameter and 1.0 mm deep was drilled in the center of the top of each rod. The cavity volume (about 1.5 pl) was suitable for containing and ashing the 0.3- to 0.8-pI samples used in this work. (14) K. G. Brodie and J. P. Matougek, ANAL.CHEM.,43, 1557. (1971). (15) J. F. Alder and T. S. West, Anal. Clzim. Acta, 58, 331 (1972). (16) S. Ornang, Anal. Chim.Acta, 56, 470 (1971). (17) C. J. Molnar, R. D. Reeves, J. D. Winefordner, M. T. Glenn, J. R. Ahlstrorn, and J. Savory, Appl. Spectrosc., in press.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 13, NOVEMBER 1972
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40
t 10
20
2.0
71-98
- 0 71-98
I
(a)
Cu
I
(b)
71-8A
I,
71-84
Sn
Figure 1. Recorder tracings from ashing and atomization of standards and oil samples. Atomic absorption peaks preceded in each case by molecular absorption and scattering from hydrocarbon combustion products (a) Cu in 20 pg ml-1 standard and in sample 71-9B (b) Sn in 1 pg ml-1 standard and in sample 71-SA
Samples were dispensed from a Hamilton 1.0-p1 syringe (7101 N-CH, Hamilton Co., Whittier, Calif. 90608) with a Chaney adaptor and a tungsten plunger extending to the tip of the needle. The use of this type of syringe minimized syringe cleaning problems and eliminated air- or vacuumbubble problems that were occasionally encountered in withdrawing oil samples with syringes having a plunger extending only to the end of the barrel. Power was supplied to the graphite rod from a 250-A, 10-V dc supply (SCR Power Supply, Electronic Measurements, Inc., Oceanport, N.J.), with the current-time sequence controlled from a pre-set program in a n adjustable timing circuit. The atomizer was used in conjunction with a flow of argon (7.2 1. min-I) and hydrogen (1.2 1. min-I). The argon/hydrogen/entrained-airflame that ignites when the rod reaches about 600 "C is valuable in preserving the atom populations for a considerable distance above the rod (17-19). Reagents. The standards used in this work were solutions of organometallic compounds in a neutral base oil (Phillips Petroleum Company Condor 105) supplied by the U S . Air Force Spectrochemical Oil Analysis Program. The following certified N.B.S. organometallic compounds were used : silver 2-ethylhexanoate, tris(l-phenyl-1,3-butanedionato)chromium (111), bis(l-phenyl-1,3-butanedionato)copper(II), tris(1phenyl-1,3-butanedionato)iron(III), nickel cyclohexanebutyrate, lead cyclohexanebutyrate, dibutytin bis(2-ethylhexanoate). Batch 10 solutions (March 1970) were used, containing each metal in the following concentrations: 0, 1, 2, 3, 5, 10, 15, 20, 50, 100 pg mP1. The 50 pg m1-l solution contained only 20 pg ml-' of silver, while the 100 pg ml-' solution contained no silver. Magnesium and aluminum were also present in all solutions. Solutions with 35 and 75 pg ml-I of all elements (except silver) were also prepared from those provided. Procedure. A suitable volume (0.3-0.8 ~ 1 of ) a standard solution was placed in the cavity o n the graphite rod, and ashed for 18 seconds a t about 440 "C. The current was then rapidly increased to a higher value for 3 seconds to give a temperature high enough to atomize the element being determined. The absorption-time record was checked to ensure that the absorption returned t o zero after completion of the (18) M. D. Amos, Amer. Lab., August 1970, p 33. (19) B. M. Patel, R. D. Reeves, C. J. Molnar, and J. D. Winefordner, University of Florida, unpublished work, 1972. 2206
Table I. Experimental Conditions Atomizing ConcenWave- temper- Sample tration Ele- length/ ature, range, Sensitivity volume, ment reductionb nm pl pg ml-l Oca 0-5 (iv) Ag 328.1 1400 0.5 Cr 357.9 1920 0.5 0-10 ... Cu 324.7 1640 0.5 0-30 (i), (iv) Fe 372.0 1920 0.5 0-50 (i), (ii), (iv) Ni 232.0 2050 0.5, 0 . 8 0-5 ... Pb 283.3 1440 0.5, 0 . 3 0-100 (iii), (iv) Sn 286.3 1600 0.8 @5 ... All standards and type A jet-engine oils were ashed for 18 sec at 440 "C. All type B jet-engine oils were ashed for 23 sec at 440 "C. A drying time of 10 sec was used. b (i) = Solutions diluted with isooctane. (ii) = less sensitive spectral line used. (iii) = smaller sample volume used. (iv) = graphite rod lowered. Q
ashing and before the atomizing step was carried out. The absence of a memory effect in a blank run immediately after the first standard confirmed that a sufficiently high atomizing temperature had been used. All samples were run in duplicate, and, in general, if the duplicates did not agree to within 4z, a third determination was made. Typical recorder tracings of the ashing and atomization signals are shown in Figure 1. Some analytical curves obtained with the present system for organometallic solutions in oils diluted with isooctane have been published elsewhere (17). Figure 2 shows some analytical curves obtained with both diluted and undiluted oils in the present work. The used jet-engine oils were treated in an identical manner to the standards. The heavier reciprocating-engine oils required a n additional 5 seconds of ashing at about 500 "C to remove the less volatile organic constituents. Table I gives details of instrumental conditions, sample sizes, and optimum concentration ranges for each element. It should be noted that it was not always necessary or desirable to operate under conditions giving the maximum sensitivity. Several different methods of reducing the sensitivity were used when necessary. The methods available included dilution of the standards and samples with an organic solvent such as isooctane or methyl isobutyl ketone; use of a lesssensitive spectral line; use of a smaller sample volume; and increasing the vertical distance between the graphite rod and the light beam from the hollow-cathode lamp. Each method has certain disadvantages or limitations. Dilution is timeconsuming, and vitiates one of the main reasons for using flameless atomization in the first place ; suitable less-sensitive lines are not available for all elements; the use of sample volumes smaller than about 0.3 p1 increases the relative error of volume measurement and dispensation and increases the danger of non-representative sampling ; lowering the graphite rod until it is 3-7 mm below the bottom of the beam from the hollow-cathode lamp is satisfactory, but beyond this region the reproducibility may be severely impaired. By using a Hamilton 1.O-p1 syringe with Chaney adapter, syringe cleaning was simple and no air or vacuum bubble problems arose. Also, the reproducibility of measurements on 0.5-pi oil samples was as good as that for the standards--i.e., the relative standard deviation was 4 % or less. The methods actually chosen in each case are discussed in the next section under the appropriate elements. The atomizing temperatures quoted in Table I are those reached in the cavity a t the end of the 3-second atomization step, and are therefore higher than the temperatures a t which atomization actually occurs.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 13, NOVEMBER 1972
Table 11. Iron, Copper, and Silver in S.O.A.P. Correlation Samples Cu concentration, pg ml-1 Ag concentration, pg ml-l Fe concentration, pg ml-l Air Force Air Force Air Force Sample This work mean, std dev This work mean, std dev This work mean, std dev 15.6 14.7 f 0 . 6 0.55 0.64 f 0.26 18.7 18.4 f 1 . 4 69-5 6.8 6.8 0.6 4.80 4.88 i. 0.35 69-6A' 22.5 25.0 f 3.0 4.82 5.23 f 0.37 17.3 18.5 f 2.4 6.5 6.2 f 0.5 69-7A 1.96 1.90 f 0.23 32.7 28.9 f 6 . 4 18.9 18.9 f 1 . 8 69-8A 0.39 0.40 f 0.15 46.3 45.4 f 7.0 3.9 4.4 f 0 . 3 69-9A 2.5 2.4 f 0 . 3 0.20 0.23 f 0.13 69-10A 23.2 23.3 i 2.7 4.8 4.4 i 0.3 0.91 0.96 i. 0.18 36.3 38.3 i 3.2 69-12A 2.6 2.2 0.6 0.51 0.44 f 0.17 19.3 20.3 i 2.3 70-2A 1.17 1.22 i. 0.21 35.8 36.8 f 4.4 30.0 29.2 f 2.8 70-7A n.d. 32.5 f 3 . 6 1.20 1.20 i. 0.13 35.7 37.7 i 3.3 71-1A 2.0 1.8 f 0.3 0.88 0.73 i 0.12 n.d.b 67.8 i 15.8 71-3A 3.9 4.4 f 0.4 0.40 0.40 f 0.14 16.8 17.0 it 2.4 71-7A 4.5 4.2 f 0.4 0.38 0.32 i 0.14 14.0 16.6 + 1 . 9 71-8A 0.35 0.44 If 0.23 19.2 22.6 i. 2 . 8 3.3 3.2 f 0.3 71-9A 8.8 8.1 i 0.7 0.52 0.48 i 0.15 71-IOA 20. I 20.7 i. 4 . 0 1.30 1.21 i. 0.23 21.2 25.1 i. 3 . 7 13.4 14.2 f 0 . 9 71-1 1 A 18.1 18.9 i 5.0 3.6 3.5 f 0.6 1 .oo 0.82 i 0.19 71-12A 4.4 4.2 f 0.5 1.71 2.16 i. 0.25 71-8B" 17.0 14.9 f 2.9 1.16 1.59 i 0.47 50.5 47.2 i 8.1 22.5 21.7 i 1 . 7 71-98 1.57 1.76 i 0.27 21.4 22.1 f 2.9 9.1 8.5 f 0.9 71-10B 37.1 35.1 f 4 . 7 17.7 16.5 f 1 . 3 1.10 1.27 f 0.31 71-llB 1.84 2.06 i. 0.38 21 .o 19.7 f 3 . 8 8.5 8.0 i 1.3 71-12B a A: Jet-engine 011; B: Reciprocating-engine oil. * n.d. = not determined.
* *
Figure 2. Analytical curves Ag, Ni, Sn: undiluted oils Cu, Pb: undiluted oils, graphite rod lowered Fe: diluted oils (1 : 2 oil : isooctane); graphite rod lowered
RESULTS AND DISCUSSION Iron. Because the iron concentrations in the used engine oils were in the 10-50 pg ml-l range, it was apparent that the 248.3 nm line would be too sensitive. The 372.0 nm line was chosen as a suitable alternative (about one-fifth as sensitive), and the standards and samples were diluted with spectrophotometric grade isooctane (Mallinckrodt SpectrAR) in the ratio 1 : 2 oi1:isooctane. In addition, instead of passing the light beam across the graphite rod at a grazing incidence, the rod was lowered 3.2 mm below the bottom of the beam to give a further reduction of sensitivity. Under these conditions, the analytical curve that resulted was al-
most linear, with absorbances of approximately 0-0.100 for the concentration range 0-16.7 pg ml-1 in the diluted solutions. Reproducibility was high, the average deviation from the means over all duplicate and triplicate samples being 1SZ. The analytical results are shown in Table 11, together with the mean values' and standard deviations for the (air-acetylene) flame atomic absorption determinations by the 25-50 laboratories participating in S.O.A.P. For iron, agreement among these laboratories was good, the relative standard deviations being 8-26 %, and averaging about 15 % over the period May 1969-December 1971. The table shows that,
ANALYTICAL CHEMISTRY, VOL. 44, NO. 13, NOVEMBER 1972
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0 0
2
1 Ag c o n c e n t r a t i o a / ( r g
3 ml'l)
4
5
I i r force C o r r e l a t i o n Average
/
10
0
30
20
~e concentration//rg
mr')
40
50
60
Figure 5. Correlation of Ag values
Uir Force C o r r e l a t i o n ~ r c r r g a
-
__
____
y = 1 . 0 5 ~- 1.54 (least-squares line) y = x
+
y = 0 . 9 3 ~ 0.02
--.-y = X
Figure 3. Correlation of Fe values
(least-squares line)
siderably. Good results were obtained by diluting standards and samples with isooctane in the ratio 1 :10 oil: isooctane. Even a t this dilution, high reproducibility was achieved, the average deviation from the means for all duplicate and triplicate samples being 1.6%. There was, therefore, no evidence of inhomogeneity of the copper metal dispersions in the used engine-oils when 0.9-p1 samples were taken at levels down to
