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Anal. Chem. 1981, 53, 1099-1103
LI’I’ERATURE CITED (1) Private communication, Paul D. Hobson, Chairman, Oxygenated Fuels Task Force, Information Document and Proposed Specification for Gasohol, July 11, 1980, American Society for Testing and Materials, Technical Division A of Committee D-2; Mercury Marine, Fond Du Lac, W1
(2) &ke, G. ErdoelKohle 1981, 14(10), 816-20. Chem. Abstr. 1963, 59, 9701b. (3) Aleksandrov, A. N.; Tysovskii, G. I. Neftepereab. Neftekhlm. (Moscow) 1966, 37-39. Chem. Abstr. 1968, 65, 16065q.
(4) Bessler, E. Clem. Cult. (Sa0 Paulo) 1977, 29 (8), 928-930. Chem. Abstr. 1978, 88, 9357w. (5) Chem. Mark. Rep. 1979, 216(25), 5. (6) Wiiks, P. A., Jr. Am. Lab. (FalrfieM, Conn.) 1980, 99-102. (7) Harrick, N. J. “Internal Reflection Spectroscopy”; Wliey: New York, 1967; Chapter 7.
for review November 19,1980. Accepted March 9, 1981.
Efficiencies of Sample Introduction Systems for the Transport of Metallic Particles in Plasma Emission and Atomic Absorption Spectrometry Costandy S. Saba innd Wendell E. Rhlne University of Dayton, Research Institute, Dayton, Ohio 45469
Kent J. Eisentraut” Materials Laboratory, Ab Force Wrbht Aeronautical Laboratories, United States Air Force, Wright-PattersonAir Force Base, Ohio 45433
A study Is made of tlhe factors influencing the transport of metallic particles In lubricatlng oils to the emission source of a plasma spectrometor or the flame of an atomic absorptlon spectrometer. A filtration technique was successfully used to collect the parllclos reachlng this region. The ceramic nebulizer/cylindricaI spray chamber sample introductlon system quantitatlvely transported 7-ym Fe particles and 62% of the 7-14-ym partlcies. A comprehensive comparison Is made of the capablllties of irarlous nebuiizer/spray chamber combinations to transport metaiilc particles to the atomirlng device. The merits of atomlc absorption, dlrect current and Inductively coupled plasma spectrometers are compared In the analysis of metalilc particles In aircraft engine lubricating olis.
Metal particulates are generated in oil as a result of wear of engine parts. Depending upon the type and severity of wear, wear metals can be present in oil as dissolved species or as particulates ranging in size from submicron to 100 pm or larger. The effective spectrometric @nalysisof wear metal particles in used lubricating oils depends largely upon the efficiency of the samlple introduction system employed to transport the analyte to the atomizer, the analyte residence time in the atomizer, the atomizer temperature, and the viewing region. Most sipectrometers designed for the analysis of homogeneous liquid samples are supplied with sample introduction systems consisting of a nebulizer/spray chamber combination. Although wear metal particles are not uniformly distributed in oil, they are generally determined by using techniques traditionally applicable to the determination of metals in homogeneous solutions. Atomic absorption spectrometry (AAS), rotating disk electrode atomic emission (RDEAE), inductively coupled plasma (ICP), and direct current plasma (DCP) techniques have been used for the determination of wear metals in used lubricating oils (1-15). Even though plasmas1 produce higher temperatures than flames and arcs, our previous work showed that analyses of
oils containing particles larger than, ca., 3-10 fim, are, in general, not quantitative (3,5,7). Since the presence of larger metallic particulates in oils is inevitable when engine parts undergo severe wear, it is imperative that the spectrometer possess the capability to quantitatively analyze for large as well as small particles. In fact, in cases of severe wear it may be more important to analyze large particles. In the literature, attention has been given mainly to the study of the production of aerosols, their droplet size distribution, and their effect on emission and absorption signals (16-18). Several investigators have reported efficiencies of sample introduction systems used in atomic spectrometry. Olson, Haas, and Fassel have found the efficiency of the ultrasonic nebulizer to be an order of magnitude higher than the pneumatic nebulizer in a desolvated ICP system (14). Peterson (19) and Taylor et al. (15)have determined the efficiency of ICP and flame AAS systems, respectively, for the atomization of iron powder suspended in oil/2-methyl-4pentanone solution. Factors which influence the atomization efficiency in flame AAS were studied by Willis (20) in the direct aspiration of mineral suspensions. Particle size distribution is one main factor which can influence the efficiency of the analyte transported during aspiration. It was observed that as the particle size increases, the efficiency of nebulization decreases (3,5, 15). In this work, experiments were conducted to determine the efficiency of particle transport by several different types of sample introduction systems employed by DCP, ICP, and AAS spectrometers. Iron powder suspensions with specific particle size distributions were used to measure the efficiencies of several sample introduction systems. The particle size distribution of the metal in the aerosol was measured and compared to the particle size distribution of the metal in the sample before aspiration. The effects of nebulizer type, chamber configuration and size, and liquid uptake rate on the efficiency of nebulization were also determined.
EXPERIMENTAL SECTION Apparatus. The plasma spectrometers used were a Spectraspan 111 direct current argon plasma spectrometer with
0003-2700/81/0353-1099$01.25/00 1981 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 7, JUNE 1981
Table I. Nebulizer and Spray Chamber Sources model no. nebulizer all glass concentric Babington type ceramic cross-flow Perkin-Elmer Sonimist Varian variable Varian fixed ultrasonic spray chamber
a
glass cylindrical glass hornlike Perkin-Elmer cylindrical plastic diamond References 21 and 22.
T-230-A3 53061 TN-1 303-0358 6005 01-100064-00 01-100065-00 UNS-1 model no. sc-2 040-0144 53264
Spectrajet I11 source (SMI-111) (SpectrametricsInc., Andover, MA) and a fluid analysis spectrometer equipped with an inductively coupled plasma source (FAS-2PL) (Baird-Atomic,Inc., Bedford, MA). The atomic absorption spectrometer was a Perkin-Elmer Model 305B (Perkin-Elmer Corp., Norwalk, CT). Table I lists the nebulizer and spray chambers tested in this work. Millipore filters (25 mm diameter) with 1.2 pm pore size (Millipore,Bedford, MA, Catalog No. RAWP 02500) fitted in a microsyringe filter holder (Luer inlet, Millipore Catalog No. XX3002500) were used to filter the aerosol in order to collect the metal particles. Stock Suspensions. Several stock suspensions were prepared from iron powder obtained from Atlantic Equipment Engineers (AEX) (Bergenfield,NJ). Three different suspensions of 400,440, and 1336 ppm iron were prepared by allowing the well-shaken nonhomogeneous mixture of 2-3 g of AEE iron powder in di(2ethylhexy1)azelate ester base oil (an aircraft engine lubricating oil used by Air Force) to settle for 1-1.5 h. After decantation, the oil contained smaller particles than the original AEE powder with approximately 60 pm being the largest linear dimension. Stock suspensions containing Fe particles of
