ance of the solution is caused solely by the 1:2 product, Structure III. That portion of the curve is much like a Beer-Lambert law plot and can be used in practice to calculate the concentration of hydride material directly. Of course, a standard solution of known diethylaluminum hydride content and a minimum of other impurities must be used to construct a working plot. Mixtures of diethylaluminum hydride with triethylaluminum do not complicate the analysis significantly. The diethylaluminum hydride reacts first, quantitatively and rapidly, with the excess isoquinoline to form the 1:l product, Structure II, which in turn coordinates with an unreacted isoquinoline molecule to produce a red color. At the same time, any triethylaluminum present coordinates with another unreacted isoquinoline molecule without color generation. As long as excess isoquinoline exists uncoordinated, no significant competition between the 1:1 diethylaluminum hydride product and triethylaluminum will exist, and no deviation from linearity will be observed. When, however, the free isoquinoline is consumed,
either by reduction or coordination, a competition for the coordinated molecules will result and curvature may be produced. Each of the reactions given by Equations 1, 2, and 3 occur during the positive slope portion of the analytical curve. The negative slope portion of the curve follows similarly in that additional diethylaluminum hydride reacts either with the isoquinoline molecules coordinated with the 1:l product, Compound 111, to disrupt the colored complex directly or reacts with the triethylaluminum complexed isoquinoline molecules with subsequent abstraction of the isoquinoline on the 1:1 product by the freed triethylaluminum to disrupt the colored complex indirectly. Highly stable complexes are necessarily involved. No extraneous or unexplainable products that would indicate nonquantitative reactions have been observed for any of the discussed reactions. Received for review August 2, 1973. Accepted February 22, 1974.
Wear Metal Determination by Plasma Jet Direct Current Arc Spectrometry P. M. McElfresh' and M. L. Parsons2 Department of Chemistry, Arizona State University, Tempe, Ariz. 8528 7
A spectrographic method has been developed for the determination of wear metals in used oils by means of the plasma jet dc arc. The method incorporates force feeding of the oil samples (diluted 1:l with xylene) into an all-He arc and a photographic detection system. Elements determined included AI, Cr, Cu, Fe, and Mg with limits of detection of 0.01, 0.11, 0.08, 0.40, and 0.003 ppm, respectively. The method has been evaluated for accuracy and precision by determination of standard and actual samples, and areas for improvement have been identified.
The determination of trace metals in used lubrication oils has become a useful diagnostic tool for monitoring frictional wear in many systems. The Navy, the Air Force, several railroads, and large trucking firms use trace wear metal determination effectively in the maintenance of their equipment. Atomic absorption spectrometry (AAS) and spark atomic emission (SAE) with a rotating disk electrode are the most widely used techniques for this analysis. Three problems are presented by the oil in these methods; viscosity causes slow sample aspiration (1-3), the combustion of the oil upsets the fuel to oxidant ratio (in AAS), and the oil residue can clog the rotating disk (in SAE). Present address,
T e x a s I n s t r u m e n t s Corp.,
Dallas,
Texas
75222. A u t h o r t o whom a l l correspondence s h o u l d b e addressed (1) S.Slavin and W. Slavin, At. Absorption Newslett., 5, 106 (1966) (2) A. J. Mitteldorf. SpexSpeaker, 13, 1 (1968). (3) D. L. Fry, Appl. Spectrosc., 10, 65 (1956).
The viscosity effects (4-6) are decreased by the addition of MIBK or xylene as solvent. Another major problem is that not all of the metal is necessarily dissolved in the oil; it can be present as particles ranging to several k m in size. The large particles tend to pass through flames unaffected ( 2 ) or settle out in the sample boat of the rotating disk ( 3 ) . A spectroscopic source which has shown ability to eliminate difficult matrices is the dc arc plasma jet (7-16). The plasma jet typically operates in the 7000-10,000 "K temperature range and is designed so liquid samples can be introduced directly into the high energy dc arc. It was felt that this source could be utilized to overcome the problems mentioned above; therefore, it was decided to explore the feasibility of developing a method adaptable to wear metals using this source.
(4) R. Smith, C. M. Stafford. and J. D. Winefordner, Can. Spectrosc.. 14, 2 (1969). (5) R . L. Miller, L. M. Fraser, and J. D.Winefordner, Appi. Spectrosc., 25, 477 (1971). (6) S. Sprague and W. Slavin, At. Absorption Newslett., 4, 367 (1965) (7) M . Margoshes and 8. F. Schribner, Spectrochim. Acta.. 15, 138 (1959). (8) L. E. 0wen.Appl. Spectrosc., 15, 150 (1961). (9) A. J. Mitteldorf and D. 0. Landon, Spex Speaker, 8, 1 (1963) (10) K. Hirokawaand H . Goto, Bull. Chem. SOC.Jap., 42, 693 (1969). (11) G .Collins and C. A. Pearson. Anai. Chem., 36,787 (1964). (12) J. Szivek, C. Jones, E. J. Paulson, and L. S. Valberg, Appl. Spectrosc., 22, 196 (1968). (13) R. J. Heemstra and N. G. Foster, Anal. Chem., 38, 492 (1966) (14) R . J. Heemstra, Appl. Spectrosc., 24, 568 (1970). (15) P. A. Serin and K . H . Ashton, Appl. Spectrosc., 18, 166 (1964) (16) R . Lerner, Spectrochim. Acta., 20, 1619 (1964). A N A L Y T I C A L CHEMISTRY, VOL. 46, N O . 8, JULY 1974
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Table I. E x p e r i m e n t a l A p p a r a t u s and Operating Conditions Spectrograph Plasma jet P o w e r supply Slitwidth Filters Anode Cathode C o n t r o l ring Analytical g a p P h o t o g r a p h i c processing Developing
Microdensitometer P h o t o g r a p h i c plates
Jarrell-Ash 3 . 4 - m plane g r a t i n g (15,000 lines//in nominal) S p e x Industries No. 9030 Hewlett-Packard M o d e l 6477c 10 k W 20 pm T w o - s t e p n e u t r a l density, 100/42 Poco G r a p h i t e N o . P J A L i n d e 2% t h o r i a t e d t u n g s t e n rods, l/&-in. d i a m e t e r P o c o G r a p h i t e N o . PJC 9 mm measured f r o m t o p of u p p e r control ring t o t i p c a t h o d e Jarrell-Ash u n i t 3410 K o d a k D-19, developed 3 m i n at 20 "C, s t o p bath 30 sec, K o d a k R a p i d Fix 3 m i n , 30-min w a t e r wash. Jarrell-Ash M o d e l J A 2100 K o d a k SA-1, 4 X 1 0 in.
EXPERIMENTAL Apparatus. The plasma jet was used with a large-bore capillary nebulizer, and was mounted in a Jarrell-Ash Varisource arc stand. The experimental apparatus and conditions are listed in Table I. To make the power supply compatible with conventional dc arc requirements, an induction ballasting circuit was needed. This circuit (which was designed by Hewlett-Packard and built in this laboratory) consisted of five 1,ohm resistors rated for 30 A connected in series to a 15-mH choke rated for 50 A. Each resistor was connected in parallel to a diverting switch to remove resistance from the circuit as desired. The all He gas flow system was monitored by Brooks Instruments Company rotameters, a No. 6-15-2 for the swirl gas (tangential gas stream) and a No. 5-15-1 rotameter for the nebulization gas flow. Samples were force-fed into the nebulizer with a Sage Instruments Company Model 255-1 syringe pump using 5-cm3B-D glass syringes. Reagents a n d Sample Preparation. ASTM No. 1 oil was used for the blank and diluent oil. Oil-soluble metal caprates of Al, Cr, Cu. Fe, Mg, and P t were prepared according to the procedure of Hearn et al. (17). Standard solutions of the metals were prepared using xylene 1 : l by volume with the blank oil as solvent. All of the standard solutions except P t were analyzed by an atomic absorption method developed in this laboratory. The P t was analyzed by a gravimetric method (18). Standard 100-ppm solutions were prepared for AI, Cr, Cu, Fe, and Mg, and a 1000-ppm solution for P t and K (as KzC03 in 2ethylbutyric acid). Even though acetylacetone and 2-ethylbutyric acid were added for stabilization, the standard solutions were stable for an average of only 20 days. Wear metal samples were obtained from J. D. Winefordner a t the University of Florida. These jet engine oils had been analyzed by AAS by about 30 Air Force laboratories. Another set of wear metal oil samples was obtained from R. C. Trimble of the Aerotechnology Department a t Arizona State University. These samples were analyzed by AAS in Trimble's laboratory. Procedure. Samples were prepared as follows: 2.5 ml of the oil sample was placed into a 5-ml volumetric flask, 0.25 ml of P t solution (internal standard) and 0.5 ml of K solution were then placed in the flask, and the contents were diluted to 5 ml with xylene. The sample was then injected into the plasma jet. The weight of the solvent was determined from its specific gravity which was measured in this laboratory. The plasma jet was ignited by turning on the He swirl gas and striking an arc by touching the tungsten cathode to the graphite (17) W . E. Hearn, R. A. Mostyn, and B. Bedford, Ana/. Chern., 43, 1821 (1971). (18) G . H . Ayres, "Quantitative Chemical Analysis." 2nd ed., Harper & Row, New York, N . Y . , 1968.
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Table 11. Working V a l u e s and Physical Limitations of the E x p e r i m e n t a l Parameters Parameter
Working value
Physical limitations
Current, A Swirl gas, l./min Nebulization gas, l./min Solution flow, m l / m i n
27 29.9 6 .O 0.6
17-30 21-80 0-9 0-0.6
anode ring. The cathode was then raised to about 9 mm above the upper Eontrol ring. One a t a time, 3 of the resistors were taken out of the circuit. A 2-ohm resistance was needed for arc stability. After the current and swirl gas flow were properly adjusted, the nebulization gas was turned on and adjusted. The samples could now be injected by switching on the syringe pump. Triplicate 1-minute exposures were taken of each sample, and three standards were run to serve as a calibration curve for each set of three test samples. The exposed plates were developed as described in Table I. The photographic densities were converted to Seidel intensities using a curve-fitting technique on an HGE425 computer. P a r a m e t e r Study. A study was made of the interactions of the critical parameters which affected the emission signal intensities and background of the metals. The parameters which had the greatest effect were the amperage of the arc, the swirl gas flow rate, the nebulization gas flow rate, and the solution flow rate as determined by the syringe pump. These four parameters are all interdependent on each other, and were optimized simultaneously using a 24 factorial experimental design (19. 20). In the course of optimizing the parameters, it became apparent that the optimum response was beyond many of the physical limitations of experimental apparatus. Table I1 lists these physical limitations as well as the working values for the critical parameters. The lowest value for the current represents the minimum necessary to pyrolyze the oil samples completely; current values below this left unpyrolyzed oil in the plasma jet discharge chamber. The upper current value is the maximum amount of current for which the induction ballasting circuit is rated. The ratio of the swirl gas flow to the nebulization gas flow became an important factor in achieving maximum intensities for the Fe line. When this ratio was about 5 , the gas flow patterns did not interfere with each other, and maximum results were obtained. However, when this ratio approached 10 or 12, the flow patterns interfered to such an extent as to divert a considerable percentage of the oil from the nebulizing gas flow into the swirl gas flow. The lower value for the swirl gas flow is the minimum necessary to make a sufficiently energetic plasma to pyrolyze the oil samples. The 80 l./min value is the maximum output of the rotameter. For the aspiration gas flow the 9 l./min maximum represents the largest amount of nebulization gas which could be used without extinguishing the arc. Associated with this flow rate, the 0.6 ml/min maximum solution flow rate is the largest amount of liquid which can be effectively nebulized in the 6-9 l./ min nebulizing gas flow range. Although these physical limitations did not allow a complete optimization of the parameters, the factorial experiments furnished enough information about the interaction of the four critical parameters to select values which gave the highest emission intensity for the Fe(I1) 2599.40-A line (Table 11). Characterization of the Spectroscopic Source. Temperature measurement of the plasma jet was attempted by the line-pair intensity ratio method. The "temperatures" measured by this method varied greatly. These data indicate that there is no local thermodynamic equilibrium in this system. However, Hwang (21) states that a partial thermodynamic equilibrium exists for elements whose transitions occur from the principal quantum number of 5 or greater in a plasma of this type. The temperature as determined by the Pt(1) 2659.45-Pt(I) 3064.71-A line pair, which conforms to Hwang's criteria, was about 7000 OK. This value is (19) W . J . Youden, "Statistical Methods for Chemists." John Wiley and Sons, New York, N . Y . , 1951 (20) W . G . Cochran and G . M . Cox, "Experimental Designs," 2nd ed., John Wileyand Sons, N e w York, N . Y . , 1964. (21) C. C. Hwang, J. Q u a m Spectrosc. Radiat. Transfer, 12, 783 (1972).
Table IV. Comparison of Results with Air Force Data
Table 111. Comparison of the Limits of Detection Limits of detection, ppm Plasma jet Element A,
.I
This work
3092.84 3082.15 3961 .52 2835.63 4254.33 3247.54 2599.40 3719.94 2852.13 2795.53 2790.79
-
Aqueous
... ...
Aqueousu
0.1
0.01
... 2c
Results (ppmj
Rotating disk
...
...
Air Force
Oilb
...
Sample No.
This work
...
... 0.2 10 0.003d ... ... ... ... 1 0.08 0 .05e 0.2 3 0.40 0.2e 0.005d ... ... ... ... 0:009 ... 0,005 . . . 0.003 0 . 1 c ... 0.2 ... ...
See reference (26). See reference ( 3 ) . See reference (24).'See reference ( 2 5 ) .e See reference (9).
similar to temperatures of similar types of plasma jets which are known to be in thermodynamic equilibrium (22).
RESULTS AND DISCUSSION The strongest emission lines (Table 111) were used to determine the limit of detection for each element, using the criteria of Boumans and Maessen (23). Because some of the lines used in this work were superimposed on a relatively even background, it became useful to modify the limit of detection expression (23) to state that the limit of detection intensity was equal to the background intensity. At the limit of detection, the only signal that will be seen is equal to 3 d g B Inet,where UB is the standard deviation of the background intensity, and Inet is the total intensity a t the line minus the background intensity. The analytical curves for each element were linear from the limits of detection to 50 ppm (the highest concentration studied). Table I11 shows the values of the detection limits obtained and also shows a comparison with other plasma jet work done in aqueous media plus rotating disk work done in both water and oil. A comparison of this work to the rotating disk work in oil ( 3 ) shows about a 100-fold improvement in the limit of detection for each element studied. Interelement Effects. Because of the high effective temperature of this spectroscopic source, no interelement effects were expected to be present. However, for Fe and Cr there was a 13% and 30% depression, respectively, in emission intensity in the presence of 100 ppm of each of the other four remaining elements. Xo similar effect was found for Al, Cu, and Mg. Because ion lines were used for analysis, the interelement effects on Fe and Cr appeared to be due to the suppression of ionization. It is not clear why Mg did not show this effect. In flames, ionization effects are eliminated by adding a large concentration of an easily ionizable species. By repeating the previous experiment with 100 ppm K added to the solutions, the interelement effects were eliminated with no significant change in the limits of detection. Therefore, K was added to all standard and test solutions to give a concentration of 100 PPm. Evaluation of the Analytical Method. The precision of technique was determined by repeated analysis of two
+
S Greenfield,J. D. Jones, and C . T. Berry, Analyst, (London).89, 713 (1964). P. W. J. M Boumans and F . J M . J . Maessen, Fresenius' 2. Anal. Chern , 2 2 0 , 241 (1966). H . Goto and I . Atsuya, Fresenius' 2 .Anal. Chern.,225, 121 (1967) S. E Valente and W. G. Schrenk, Appl. Spectrosc., 24, 197 (1970). W . K . Baer and E. S . Hodge. Appl. Specfosc., 14, 141 (1960)
'5;
Aluminum
2
0.11
Re1 std dev,
Range
Mean
71-09A 71-10A 71-12A 72-01A 72-02A 72-03A
0.24 10.3 1.9 1.5 0.43 0.24
1.7 9.2 7.7 2.1 2.2 1.1
0-3.8 0.1-14.0 1 .O-13. O 0-4. O 0-3.5 0-3 . O
53 27 23 62 36 73
0-2.6 2 .O-5.7 1.6-5.7 0.7-4.5 0.5-5.6 0-5.3
40 30 32 33 38 56
2.4-4.O 5 .O-10.4 3 .0-11.5 5.6-9.5 2.3-9.3 1.5-5.8
9 11 16 11 35 48
15.1-27.4 16 .O-28. O 12.0-30.6 6.0-34.1 7 .O-27.6 6.0-31. O
12 13 19 25 24 33
2.6-4.6 0.9-5.o 1 .O-4.5 1.4-5.1 3.6-7.3 2.3-11.8
14 24 18 17 29 44
Chromium 71-09A 71-10A 71-12A 72-01A 72-02A 72-03A
1.5 3 .O 3.4 2.4 2.9 2.5
1.5 1.6 5.2 2.2 3.6 3.3
Copper 71-09A 71-10A 71-12A 72-01A 72-02A 72-03A
3.2 8.5 8 .O 7.2 3.4 3.1
1.3 4. O 3.4 7.7 3.2 1.3
Iron 71-09A 71-10A 71-12A 72-01A 72-02A 72-03A
31.4 29.9 31.5 30.4 22.4 22.8
22.6 22.1 19.7 18.6 14.1 16.6 Magnesium
71-09A 71-10A 71-12A 72-01A 72-02A 72-03A
3.O 2.5 8.2 3.9 7.1 5.8
3.6 2.9 2.8 3.5 5.2 5.4
Table V. Comparison of Results with Trimble's Data Results, ppm Sample N o .
This work
Trimble
Differences
Aluminum 591 733
0.4 0.6
591 733
1.4 0.8
nd
...
nd
...
Chromium 1 .o nd
0.4 ...
2 .o 2. o
0.o 3.2
Copper 591 733
2 .o 5.2 Iron
591 733
12 15
16 18
4 3
Magnesium 591 733
0.9 1.6
1 .o 1 .o
0.1 0.6
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 8, J U L Y 1974
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Cannon Instruments wear metal standard solutions, and a third sample made from the standard metal caprates to approximate actual wear metal oil concentrations. Data were based on triplicate measurements and were collected on five separate days. The 2659-A line of Pt was found to be the best internal standard line and was used throughout. The relative standard deviations are as follows: Al, 21%; Cu, 9.4%; Fe, 4.2%; Cr, 11%; and Mg, 5.1%. Table IV compares the data from the Air Force jet engines, and Table V compares the data from the internal combustion engine oil from Trimble’s laboratory. There was sufficient jet engine oil for one triplicate analysis; therefore, these data should be considered as preliminary. The two samples from Trimble’s laboratory were run according to a standard procedure and can be considered to be a better indication of the method’s capabilities. These results are generally comparable within experimental error. Several points should be made with respect to Table IV. First, the data for the jet oil have been collected from 30 laboratories and indicate lab to lab variation rather than precision of a particular method. Second, agreement of results from lab to lab is not necessary for a meaningful wear metal analysis program. The important requirement is self-consistent results within a particular laboratory. Third, regardless of the “true” values for the elements in the jet engine samples, the data obtained by the plasma jet technique presented here give answers in the same ball park as those of the other labs. These data generally show acceptable results compared to other work (27). In the case of Al, the results are partly (27) Applications Report, “The Quantometric Analysis of Oil.” Applied Research Laboratories, Sunland, Calif. 1972.
explained by the rather flat analytical curve. The consistently high results for Fe, Cr, and, generally, Mg may in part be due to the metal particles which escape analysis in other wear metal techniques. The overall variations in the results may be the result of the lack of precision in the method rather than its inaccuracy. It must be realized that the specific procedures used in this study are subject to a rather large amount of inherent uncertainty. The more important sources of error were: the standard solution instability; the volumetric sampling procedure; the variation in syringe diameter (1.12 to 1.16 cm); and the variations involved with the photographic process. The variation in syringe diameter results in about 2% relative standard deviation from sample injection alone. Therefore, relative standard deviations of less than 10% can be considered quite good. It is felt that more sophisticated experimental design could lower this variation by a significant amount. Further research with a direct reading instrument using stable standard solutions, gravimetric sample preparation, and a more reproducible sample injection system should substantially improve the results of this technique.
ACKNOWLEDGMENT The authors would like to thank J. D. Winefordner and R. C. Trimble for samples used in this project. Thanks are also due to G. K. Wittenberg for helpful comments and suggestions with the manuscript. Received for review August 16, 1973. Accepted February 28, 1974. The authors would like to acknowledge partial support by an A.S.U. Faculty Grant-in-Aid as well as computer time provided by the University Computing Center.
Fluorescence Detection of Sulfur Dioxide in Air at the Parts per Billion Level F r e d e r i c k P. S c h w a r z ’ and Hideo O k a b e Physicai Chemistry Division. Nationai Bureau of Standards. Washington. D . C. 20234
Julian K. Whittaker Nuclear Sciences Division, Nationai Bureau of Standards. Washington. D . C. 20234
A previously reported detector capable of rapid and continuous measurement of SO2 in air has been modified to extend the detection limit to the low ppb range. The principle of detection is based on photon counting of the SO2 fluorescence excited by the Zn 21 38-A line. Fluctuation of the lamp intensity was accounted for by measuring the ratio of the fluorescence photon counts to that of the excitation source. At 8.6 ppb, the standard deviation is 29% for a counting time of about 1 minute. The detector response is linear from at least 8.6 ppb to 1.8 ppm. The major source of measurement error at low ppb concen1024
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8, JULY 1974
trations is the statistical fluctuation of the low scattered light and signal counts, whereas at high SO2 levels, it is due to fluctuations in the sample preparation. The cell design was modified to reduce the scattered light. The inside of the cell was coated with a non-water-absorbing black Teflon to reduce possible H2O-SO1-wall interactions. With the Zn lamp as an excitation source, the quenching effect of water vapor on the SO2 fluorescence signal previously observed with Cd 2288-A excitation was found negligible. The result can be reasonably explained by the shorter life time of the SO2 fluorescence.
