Anal. Chem. 1980, 52, 2365-2370
method. Nitrate nitrogen was analyzed after reduction t o nitrite nitrogen with the Cd-Cu column. The results are presented in Table VI. The analytical values obtained by these two methods are consistent with each other.
2365
Takahashi, M.; Tanabe, K.; Saito, A.; Matsumoto, K.; Haraguchi, H., Fuwa, K. Can. J. Spectrosc. 1980, 25, 25-28. Beenakker, C. I. M. Spectrochim. Acta, Part 81978, 378, 483-4SU. Beenakker, C. I. M. Spectrochim. Acta, Part 8 1977, 328, 173-107. Quimby, 6. D.; Uden, P. C.; Barnes, R. M. Anal. Chem. 1978, 50, 2112-2118. Quimby, B. D.; Delaney, M. F.; Uden, P. C.; Barnes, R. M. Anal. Cbem. 1979, 57, 875-880. Zander, A. T.; Hieftje, G. M. Anal. Chem. 1978, 50. 1257-1260. Mulligan. K. J.; Hahn, M. H.; Caruso. J. A,; Fricke, F. L. Anal. Chem. 1979, 57, 1935-1938. Tanabe, K.; Matsurnoto, K.; Haraguchi, H.; Fuwa, K., Abstract, The 28th Annual Meeting of the Japan Society for Analytical Chemistry, 1979; p 616. Stehle, R . L. J . Bioi. Chem. 1920, 45, 223-228. Hassan, S. S. M. Anal. Chim. Acta 1972, 58, 480-483. Tanabe, K.; Takahashi, J.; Haraguchi, H.; Fuwa, K. Anal. Chem. 1980, 52, 453-457. Hawley, J. E.; Ingle, J. D., Jr. Anal. Chern. 1975, 47, 719-723.
ACKNOWLEDGMENT T h e authors are indebted t o K. Uehara and Y. Inubushi, Department of Agricultural Chemistry, University of Tokyo, for providing the soil samples and analytical data of ammonium-nitrogen by the p H titration method. They also thank H. Tao and K. Chiba, Department of Chemistry, University of Tokyo, for providing the analytical data of ammonium nitrogen by the ammonium electrode method. LITERATURE CITED Cresser, M. S. Anal. Chim. Acta 1976, 85, 253-259. Cresser. M. S. Analvst(London) 1977. 702.99-103. Cresser, M. S. Lab..Pract. $977, 26,'19-21. Belcher, R.; Bogdanski, S. L.; Calokerions, A. C.; Townshend, A. Anaiyst (London) 1977, 102, 220-221. Butcher, J. M. S.; Kirkbright, G. F. Analyst (London) 1978, 103, 1104-1 115. Alder, J. F.; Gum, A. M.; Kirkbright, G. F. Anal. Chim. Acta 1979, 92, 43-48.
RECEIVED for review June 19, 1980. Accepted September 8, 1980. This research has been supported by the Grant-in-Aid for Environmental Science under Grant No. 403023 from the Japan$ and partly Ministry Of Education$Science and supported by the Kurata Science Foundation.
Particle Size Independent Spectrometric Determination of Wear Metals in Aircraft Lubricating Oils John R. Brown, Costandy S. Saba, and Wendell E. Rhine University of Dayton Research Institute, Dayton, Ohio 45469
Kent J. Eisentraut" Materials Laboratory, Air Force Wright Aeronautical Laboratories, Air Force Systems Command, United States Air Force, Wright-Patterson Air Force Base, Ohio 45433
A method for the particle size independent determination of Ni, Fe, Mg, Cu, AI, Sn, Mo, and Ti in synthetic ester lubricating oils by spectrometric analysis is presented. Partial dissolution of Cr, Si, Pb, and Ag also occurs. Used ester lubricating oils as well as samples prepared with -325 mesh (44 pm) or -200 mesh (74 pm) metal powders were reacted with a small amount of hydrofluoric acid and aqua regia at 65 OC for 45 min with ultrasonic agitation. The reacted mixture was then dlluted with a methyl isobutyl ketone and isopropyl alcohol mixture and analyzed spectrometrlcally. The recoveries for metal powder suspensions of Ni, Fe, Mg, Cu, AI, Sn, Mo, and Ti ranged from 97 to 103% and the relatlve standard devlatlon ranged from 4 to 10%. I n addition to the metal powder suspensions, over 200 aircraft oil samples were analyzed. The method is much faster and more convenient than previously reported particle size independent methods using ashing techniques.
Spectrometric oil analysis (SOA) is a preventative maintenance technique used by the military services and some commercial and industrial enterprises. It utilizes spectrometric analysis techniques to monitor the generation of wear metals in oil-wetted lubrication systems. On the basis of the wear metal concentrations found in the lubricating oil, equipment failure can be predicted and appropriate equipment maintenance performed. T h e ability to predict equipment failure 0003-2700/80/0352-2365$01 .OO/O
reduces equipment maintenance costs, improves equipment reliability, and enhances operational safety. However, a distinct limitation also exists in SOA. Small wear metal particles (less than 1 pm) produced by normal wear can be readily analyzed by spectrometric methods. However, several cases have been documented where large metallic wear particles produced by more severe wear are not quantitatively analyzed (1-5). In some of these cases SOA failed to predict aircraft oil-wetted component failure ( 2 , 5 ) . I t has also been shown t h a t the spectrometers typically used for SOA, spark/arc rotating disk electrode atomic emission and flame atomic absorption, are limited in particle size detection capabilities ( 4 , 6-15). The breakdown of SOA to predict impending aircraft engine failure (2, 5 ) may be related t o the particle size detection limitations of SOA spectrometers. Saba and Eisentraut have previously reported rapid acid dissolution procedures for the particle size independent analysis of T i (24) and Mo (15) using atomic absorption spectrophotometry (AAS). However, analytical methods which would permit the particle size independent determination of several wear metals simultaneously in lubricating oils were required and, therefore, were investigated. Several methods were identified for the particle size independent determination of wear metals in lubricating oils. Many authors have reported the use of wet ashing and dry ashing techniques which can be quite effective (16-18). However, wet ashing and dry ashing methods are very time consuming and laborious. Bartels and Kriss (11) reported a 0 1980 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 14, DECEMBER 1980
Table I. AAS Instrumental Conditions instrument burner control box grating nitrous oxide tank regulator burner regulator flow rate acetylene tank regulator burner control flow rate
uv
4 2 psig 30 psig 12.5 L/min
1 2 psig 8 psig 6.3 L/min
lamp wavelength, element current, mA nm Ni
Cr Fe Mg cu A1 Sn Mo Ti
25 30 30 30 30 30 30 30 40
Table 11. DCP Instrumental Conditions
Perkin-Elmer 305B Perkin-Elmer Model 303-0678
341.5 357.9 372.0 285.2 324.7 309.3 286.3 313.3 365.3
instrument argon tank sleeves nebulizers electrodes anodes cathode entrance slit cassette slits
slit, nm 0.2 0.7 0.2 0.7 0.7 0.7 0.7 0.7 0.2
particle size independent procedure which involved diluting samples with an acidified diluent with subsequent determination by atomic absorption spectrophotometry. This method was much simpler than the wet ashing and dry ashing techniques. However, it was only effective for iron particles less than 3 pm and required 16 h for complete iron particle dissolution. I n view of the inconvenience of the above procedures, a rapid multielement particle size independent method (PSIM) was needed. T h e approach used in developing this method was to acid digest the metals, in situ, followed by multielement atomic emission spectrometric or sequential single element atomic absorption spectrophotometric analyses. The method developed is reliable for the analysis of Ni, Fe, Mg, Cu, Al, S n , Mo, and T i and has been used to analyze these elements in u p to 60 samples/day using the dc plasma. The reaction conditions affecting this method were studied and the results of these experiments are presented. T h e precision and accuracy of this method were determined by using metal powder suspensions a n d authentic oil samples from operational aircraft. EXPERIMENTAL SECTION Apparatus. The atomic absorption spectrophotometer used was a Perkin-Elmer Model 305B equipped with a corrosion-resistant nebulizer and a nitrous oxideacetylene 5-cm slot burner. The output signal was fed into a Leeds and Northrup Speedomax W strip-chart recorder. Integration time was 10 s and absorbance was obtained from (number of chart divisions)/(lO X scale expansion). Concentration was read directly from the working curve of absorbance vs. standard concentration. The fuel/oxidant flow rates (L/min) were obtained from the Perkin-Elmer Model 303-0678 burner control box flow readings, using the manufacturer's reported plots of gas flow vs. control box flow meter reading (29). The instrumental conditions used for each element are given in Table I. A Spectrametrics, Inc., Spectraspan I11 (SMI), equipped with the Spectrajet I11 direct current argon plasma (DCP) source and multielement capability, was used for plasma atomic emission determinations. Table I1 reports the SMI instrumental conditions used to analyze all samples. The plasma and grating positions were adjusted t o maximize the iron emission signal for the 259.9-nm line. The instrument was calibrated by using a 50-ppm standard and a blank. The working curve for each element was checked and found to be linear between 0 and 100 ppm. The emission signal of each sample was integrated three times for 10 s each, and the results were printed on a Texas Instruments, Inc., Silent 700 data terminal. A Varian fixed concentric nebulizer was used for the analysis of oils. The acid resistant ceramic nebulizer
Spectraspan 111, Spectrametrics, Inc. 52 psig 49 psig Fixed Varian or Ceramic graphite 5% Th doped W 300 pm x 50 pm (vertical x horizontal dimensions) 25 pm
element
wavelength, nm
element
wavelength, nm
Ni Cr Fe Mg cu A1
231.6 283.6 259.9 280.3 324.8 309.3
Sn Mo Ti Si Pb Ag
284.0 313.3 337.3 251.6 283.3 328.1
supplied by Spectrametrics, Inc., was used to analyze the aciddigested samples. Reagents. The mixture of HF and aqua regia was prepared by adding 95 mL of aqua regia to 5 mL of reagent grade concentrated (48%) HF. The aqua regia was prepared as a mixture of 82 mL of reagent grade concentrated (37%) HC1 and 18 mL of reagent grade concentrated (70%) "OB. For convenience the aqua regia was not used until the evolution of gas had ceased (about 2 days). A di-2-ethylhexylazelate ester base oil, a common Air Force lubricating oil, was used for blank and standard preparations. The diluent used was a mixture of 10 mL of reagent grade methyl isobutyl ketone (MIl3K) and 90 mL of reagent grade isopropyl alcohol, to ensure the solubility of most lubricants encountered. Standards for t h e Particle Size Independent Method. High-purity metal powders (Research Organic/Inorganic Chemical (ROC/RIC) Corp., Sun Valley, CA), -200 or -325 mesh, were used to prepare the calibration standards. The metal powders of Ni, Fe, Mg, Cu, Al, Sn, Pb, and Mo were dissolved with aqua regia, Cr was dissolved with concentrated HC1, Si and Ti were dissolved with a mixture of 80 mL of concentrated "OB and 20 mL of concentrated HF, and Ag was dissolved with concentrated "OB. The appropriate amount of the above concentrates was mutually diluted with aqua regia to obtain a nine-element (Ni, Fe, Mg, Cu, Al, Sn, Pb, Cr, and Mo) standard of 750 ppm for each element. The Ti, Si, and Ag 750 ppm standards were prepared separately by diluting the concentrate with the appropriate acid. The single-element concentrates are stable indefinitely and the 750 ppm stuck solutions have been kept for 6 months with no adverse effects noted. Oil calibration standards were prepared by adding an appropriate amount of the 750 ppm stock solution and aqua regia to 15 g of ester oil. Twenty-eight grams of MIBK/isopropyl alcohol was then added. In this manner, standards of 100, 50, 20, and 10 ppm were prepared which had a final dilution ratio of 15:2:28 by weight (oil:750 ppm stock + aqua regia:diluent). The blank was prepared by using aqua regia. The 100, 50, 20, and 10 ppm standards were always freshly prepared from the 750-ppm stock solutions since the diluted standards are not stable for more than 1 day. S t a n d a r d s for t h e Analysis of Used Lubricating Oils. Conostan organometallic single-element concentrates (Conostan Division, Continental Oil Co., Ponca City, OK) were blended together to prepare multielement standards for the direct spectrometric analysis of undigested oil samples. The Conostan concentrates were diluted with ester oil to obtain single-element 5000-ppm standards. The single-element 5000-ppm standards were then mixed to prepare 13- and 7-element standards. The two groups of standards, C13 (Ag, AI,Cr, Cu, Fe, Mg, Na, Ni, Pb, Sn, Si, Ti, and Mo) and C7 (B, Ba, Be, Mn, V, Zn, and Cd), were added together to obtain 20-element (C20) standards. The two major contaminants present in each concentrate, Fe and Na, were compensated for in preparing the C13 and C20 standards. The multielement standards were diluted further with ester oil to
ANALYTICAL CHEMISTRY, VOL. 52, NO. 14, DECEMBER 1980
2367
Table 111. Effect of Acid Mixture o n Metal Recoveries (%) aqua regia Ni Si
cr Fe Mg
cu A1 Pb
Sn Mo Ti Ag
.__-
98+3 9 t 1 71-4
99i 97i 96t 103 t
1ooi 93
3 2
3 3 4
5 5 25r 5 9 0 f 11 i
loo?
HF/ aqua regia HFIHNO, 96c 4 140t 1 5 6 0 + 30 100 t 3
94 i- 3 96t 4 106i 4 1 1 6 . 23 97 5 98c3 loo* 4 8 0 i 10
72i 3 1 2 4 c 10 61-3 85c 3 9oi 5 992 4 100. 5 97 1- 5 95-t 5 1OOt 3
1ooi
5
95t7
HF/HC1 69i 5 3 0 + 10 100. 8 61 i 6 95t 3
98i 4 1035 5 100i 5 9oi 2 23. 2 loot 4
0 V
C Y W
50 kz W 0
rc W
20
I
20
40 REACTION
60
,
BC
65
TEMPERATURE
('C)
Figure 2. Effect of reaction temperature on percent recovery: (W) AI; ( 0 )Cu; ( 4 ) Sn; (A)Mo; (A)Mg; (0)Ni; (0) Cr; ( 0 ) Fe.
4c 4 5
ac RECCTOl
33 - M
4:
?T
Ylh
Figure 3. Effect of reaction time on percent recovery: Cu; ( 6 ) Sn; (A)Mo; (A)Mg; (0) Ni; ( 0 )Cr; ( 0 )Fe
(H)AI; ( 0 )
reactions usually occurred after dilution with the MIBK/ isopropyl alcohol solvent. Aqua regia or a mixture of aqua regia and concentrated H F (19:l by volume) proved to be the most effective acids for dissolving the metal powders suspended in oil (Table 111). Figures 1-3 show the results of experiments designed t o optimize acid amount, reaction temperature, and reaction time, respectively. The optimum reaction conditions selected from these experiments were 0.4 g of acid/3 g of oil (0.133 g of acid/g of oil) with a reaction temperature of 65 OC and 45 min of ultrasonic agitation. T h e optimum diluent mixture was found by varying the percent, by volume, of MIBK in isopropyl alcohol and observing the homogeneity of aqua regia with different manufacturers' ester oils in the diluent. T h e solution ratio, by weight, was 3.00.45.6 (oikaqua regia:hIIBK/isopropyl alcohol).
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 14, DECEMBER 1980
Table IV. Percent Metal Recoveries for M12-100 Suspensions Using HF/Aqua Regia
I30
t
P
120
% recovery
I
2 7 1 "
~
element Ni
Fe Mg
cu AI
Sn Mo
Ti
DCPa 99i 103 i 99t 97 i971 98i 99i 97 t
AAS 98 i 1 0 7 i89i 98i92t 99i98t
10 7 4 5 4
10
4 4 3
3 6 2 62i 2
6 5 4
Average of 20 DCP determinations. AAS determinations.
Average of 6
a
~~
~~
~
Aqua regia was added to separate solutions of four different ester oils diluted with a 1:9 M1BK:isopropyl alcohol mixture. A clear solution could not be obtained for one of the supplier's ester oils with the above or any MIBK/isopropyl alcohol mixture. Although a clear and homogeneous oil/acid/diluent solution was not possible with this ester oil, analysis of this oil sample was still possible by using the optimum 1:9 M1BK:isopropyl alcohol diluent mixture and with vigorous sample agitation immediately before analysis. Aqua regia did not give high recoveries for Ti. For inclusion of T i in the method a small amount (5%) of H F was added t o the aqua regia which substantially improved the recovery of T i (Table 111). However, if T i is not an important wear metal, the use of aqua regia is recommended. Results for Metal Powder Suspensions. Each element's mean percent recovery (M12-100 suspension) and its relative standard deviation are reported in Table IV. The percent recoveries reported for DCP analyses are based on 20 determinations. The results for atomic absorption analyses are also included in Table IV and are the average of six determinations. T o further illustrate the need for and effectiveness of the particle size independent method, iron powder was suspended in lubricating oil and filtered through 1-,3-, 5-, 8-, lo-, and 12-pm Nuclepore polycarbonate membrane filters. The filtrates, plus the original unfiltered sample, were analyzed by the PSIM. In addition each sample was analyzed by rotating disk emission (RDE) (Baird Atomic's FAS-2), atomic absorption (AA),and dc plasma instruments. The samples were diluted 1:4 with MIBK for AA and DCP analysis and each instrument was calibrated with a single-element (Fe) Conostan standard. The results of these analyses are shown in Figure 4 where the Fe concentration found in each filtrate is plotted vs. the pore size of the filter used. The results show the limitations of direct spectrometric analysis of metallic particulates. If the spectrometers could quantitatively analyze the particles present, the analyses obtained by DCP, RDE, and AAS would superimpose that obtained by the PSIM. For the analysis of samples containing metal particles, the particle size independent method is clearly superior. The metal particle size limitations of several currently available spectrometric methods will be reported separately. T h e effectiveness of the procedure for the determination of Si, Cr, Pb, and Ag was also investigated. The recoveries of Si, Cr, and P b using HF/aqua regia were erratic as indicated by their very high standard deviations (Table 111) and recoveries of Si and P b were usually over 100%. The average percent recovery of Cr and Ag was 60 and 80%, respectively. Additional work is currently under way to develop a method which gives reliable results for Si, Cr, P b , and Ag. T h e large difference between the AAS and the DCP recovery of Ti is probably due to the incomplete reaction of Ti with the HF/aqua regia. Preliminary work (8)has shown that the DCP can analyze Ti metal particles more effectively than
'
4 '6 ' 8 IO' I ' r ' l i ' FILTER PORE SIZE i microns1
Figure 4. The effect of particle size on the analysis of iron powder by (0)PSIM, (0)DCP, (+) RDE, and (0)AA.
the AAS. Therefore, about 40% of the Ti, after acid treatment, remained in a form not analyzable by AAS. While the DCP is capable of quantitatively analyzing Ti by this procedure, the alternative T i method developed by Saba and Eisentraut (14) is the recommended method for T i AAS analysis. Under the reported reaction conditions, the percent recoveries for elements other than Mg were not affected by the presence of HF. T h e results for the M12-10 suspension were comparable t o those reported for the M12-100 suspension. In all samples analyzed by the particle size independent method (metal powder suspensions and authentic oil samples), orange crystals appeared about 6 h after the samples were heated and ultrasonically agitated. T h e orange crystals did not appear in the calibration standards which were not subjected to heat or ultrasonic agitation. Spectrometric analysis of the orange crystals confirmed t h a t the crystals did not contain any metal. Any changes in the sample due to the formation of these crystals does not affect the metal analyses by the reported procedure. R e s u l t s for A u t h e n t i c Oil Samples. The particle size independent method was tested with authentic used oil samples. These used ester oil samples were spectrometrically analyzed by the PSIM. Table V reports the results for each element before and after acid dissolution. All three samples contained wear metal particles which were not quantitatively analyzed prior to acid dissolution. Precisions and percent recoveries for authentic oil sample determinations were comparable to the precision and percent recoveries found for the metal powder suspensions. The Ti method developed by Saba and Eisentraut (14) was also used to analyze sample 3, and the results agreed with the average DCP T i value. In addition to the samples reported in Table V, over 200 used aircraft engine oil samples were analyzed by using this procedure. T h e average PSIM concentrations were plotted against sample concentrations determined before acid addition by using A A S and DCP analyses. The results for Fe analysis are shown in Figure 5 (PSIM vs. direct SMI (DCP) determinations), T h e solid line in this figure was calculated from the data by using the method of least squares. T h e broken line in this figure is termed the line of perfect correlation and would be achieved if all the wear metal particles in these samples were detected by direct spectrometric analyses. The line calculated by the method of least squares in Figure 5 diverges significantly (slope = 0.850) below the line of perfect
ANALYTICAL CHEMISTRY, VOL. 52, NO. 14, DECEMBER 1980
Table V. Comparative Results for Aircraft Oil Samples by DCP and AAS" sample 1 PSIM no acid
Ni
cr Fe
Mg
cu A1
Sn Mo
Ti
1 1
1 2
64 19 19 6 2
63 17 12
1
