Anal. Chem. 1980,
while the oxide is formed when the nitrate salt is used. The depressing effect of metals used in the nitrate form on Cr(V1) absorption may occur because the metals are transformed into the refractory oxides mixed with the Cr(V1). On the other hand, the metals used in the chloride form are transformed from the chlorides into atoms, some of which are subsequently converted into oxides by combining with oxygen in the flame; therefore, conversion of the Cr(V1) atom into the oxide is depressed. The enhancement of Cr(V1) absorption may occur a t the expense of the production of coexisting metal atoms. Calcium, Mg, and Al, not only in the chloride form but also in the nitrate form, enhanced Cr(V1) absorption. The result seems to be connected with the fact t h a t these metals are remarkably stabilized by formation of the oxides, as compared with other metals. Changes in flame condition gave changes in the degree of interference, but no change in the type of interference, depression, or enhancement. Serious interferences were observed under fuel-rich flame conditions which gave a high absorption signal for Cr(V1). Under the hotter, fuel-lean flame condition a t an acetylene flow rate of 1.25 dm3/min, the interferences were almost eliminated. When the same concomitants were present with either SDS or DTAC, each interference on Cr(V1) absorption was almost eliminated. T h e interference-suppressing effects of DTAC under the optimum flame condition are summarized in the second column of Table 11, where results are similar to those obtained by use of SDS in our previous work (7). I t is also apparent t h a t the interference-suppressing effects of the surfactants are related to their enhancing effects which are not disturbed by the presence of the concomitants. As seen in Table 11, the surfactant shows a larger enhancement in Cr(V1) absorption than each concomitant. Thus, when the concomitant coexists with the surfactant, the enhancing effect of the concomitant may be masked by that of the surfactant, and consequently, only the enhancement caused by the surfactant will be observed. On the other hand, the suppressing effects of the surfactants on depressions in Cr(V1) absorption
52,2361-2365
2361
caused by the concomitants in the nitrate form may be attributed to the increase in the rapidity of vaporization of the refractory compounds, the size of which is reduced by the surfactants. The interference-suppressing effects of the surfactants were complete for all flame conditions.
ACKNOWLEDGMENT We express thanks to Professor Kenji Yamaji of Kwansei Gakuin University for his guidance in taking photomicrographs and also for the apparatus to catch droplets. We thank Professor Masaji Miura of Hiroshima University for his measurement of surface tension. We also thank Ms. Diane Lawrence of the Perkin-Elmer Corp. who edited this manuscript for this journal.
LITERATURE CITED (1) (2) (3) (4) (5) (6)
Dean, J. A.; Carnes, W. J . Anal. Chem. 1982, 34, 192. Allan, J . E. Spectfochim. Acta 1981, 17, 467. Willis, J. B Spectfochim. Acta, PartA 1987, 23A, 811. Stupar, J.; Dawson, J . B. Appl. Opt. 1988, 7 , 1351. Lockyer, R.; Scott, J. E.: Slade, S. Nature (London) 1981, 189,830. Robinson, J . W. Anal. Chim. Acta 1960, 23, 479. (7) Kodama, M.; Shimizu, S.; %to, M.; Tominaga, T . Anal. Lett. 1977, 70, 591. (8) Nukiyama, S.;Tanasawa, Y. Nippon Kikai Gakkai Ronbunshu 1939, 5 ,
68.
(9) Miura, M.: Kodarna, M. Bull. Chem. SOC.Jpn. 1972, 45,428. (IO) Ekwall, P; Mandell, L.; Fontell, K. J . CollaidInterface Sci. 1989, 29, 639. (11) Horri, K. "Studies of Fogs"; Kozima, K., Ono, T., Yamaji, K., Eds.; (12) (13) (14)
(15) (16) (17) (18) (19)
Institute of Low Temperature Science, Hokkaldo University: Satsuporo. 1953; p 303. Shinoda, K.; Nakagawa, T.; Tamamushi. B.; Isemura, T. "Colloidal Surfactants"; Academic Press: New York, 1963; p 58. Mugele, R. A.; Evans, H. D. Ind. Eng. Chem. 1951 43, 1317. Shinoda, K.; Nakagawa, T.; Tamamushi, B.; Isemura, T. "Colloidal Surfactants"; Academic Press: New York. 1963; p 181. Kodama, M.; Miura, M. Bull. Chem. SOC.Jpn. 1972, 45, 2265. Takeuchi, T.; Swuki, M. "Atomic Absorption Spectrochemical Analysis"; Nankoudo: Tokyo, 1969; p 70. Ottaway, J. M.; Pradhan, N. K. Talanta 1973, 20, 927. Fujiwara, K.; Haraguchl, H.; Fuwa, K. Anal. Chem. 1975, 47, 743. Hurlbut, J. A,; Chriswell, C. D. Anal. Chem. 1971, 43, 465.
RECEIVED for review May 17,1979. Accepted August 20,1980.
Determination of Ultratrace Ammonium, Nitrite, and Nitrate Nitrogens by Atmospheric Pressure Helium Microwave-Induced Plasma Emission Spectrometry with Gas Generation Technique Kiyoshi Tanabe, Kazuko Matsumoto, Hiroki Haraguchi, Department of Chemistry, Faculty of Science, University
of
and Keiichlro Fuwa
Tokyo, Bunkyo-ku, Tokyo
Ammonium, nitrite, and nitrate nitrogens were generated as nitrogen gas from the solutions and introduced into the plasma. This method made It possible to determine nanogram per milliliter level of these nitrogen compounds.
Ammonium, nitrite, and nitrate nitrogens are found in soils, waters, biological substances, and other materials. The determination of nitrogen in these chemical forms, especially in water, is of importance because these nitrogen compounds have great influences on biological activities and environment. The conventional methods such as titration, azotometry, and colorimetry have usually been used for the determination of the nitrogen compounds. 0003-2700/80/0352-2361$01 .OO/O
113, Japan
Recently, spectrometric methods for the determination of ammonium nitrogen using molecular emission or absorption have been reported (1-7). Cresser has utilized the ultraviolet molecular absorption of gaseous ammonia after its liberation from strongly alkaline sample solutions (1-3). Belcher et al. used a similar generation technique in molecular emission cavity analysis and observed the emission from NH2 species at 500 nm (4).Butcher and Kirkbright observed the emission from N H species in hydrogen-nitrogen diffusion flame by use of a similar generation technique ( 5 ) . Alder, Gunn, and Kirkbright introduced nitrogen gas which was evolved by the hypobromite oxidation reaction to an inductively coupled plasma and subsequently observed the emission from nitrogen-containing species (6). In general, these spectrometric methods are rather convenient, compared with other con1980 American Chemical Society
2362
ANALYTICAL
CHEMISTRY, VOL. 52, NO. 14, DECEMBER 1980 Table I. Instrumental Components components
7,
microwave generator microwave cavity discharge tube monochromator
Figure 1. A schematic diagram of nitrogen gas generation-plasma emission measurement system: (F,, F2)flow meter, (S)reaction agent solution, (V,, V), three-way valves, (R) reaction vessel, (I) silicone rubber septum, (D) dry t u b e [Mg(ClO,),], (Q) silica tube, (C) cavity, (P) microwave generator, (L) lens, (M) monochromator.
ventional methods. However, almost all the detection limits obtained by the spectrometric methods are a t the level of 0.2 pg of N / m L , which is comparable with those of the conventional methods. Recently, the present authors reported the molecular absorption method, in which a heating device in the ammonia gas generation step and a liquid-nitrogen-cooled gas trap for the condensation of gaseous ammonia were employed for effective gas generation and preconcentration (7). A detection limit of 7 ng of N / m L was achieved by the system. This gas trap system, however, appears t o be rather complicated in experimental procedure. In recent years, atmospheric pressure helium microwaveinduced plasma (MIP), which was developed by Beenakker ( 8 ) , has been investigated as a new excitation source for spectrochemical analysis of nonmetallic or metallic elements ( S 1 3 ) . Actually, efficient excitation of nonmetallic elements by the atmospheric pressure helium MIP has been demonstrated by some workers (9-11). Recently, the present authors showed the potential use of the helium MIP for analyses of ultratrace ammonium and nitrite nitrogens in solution (14). Hence, this paper reports the convenient, sensitive and selective methods for the direct determination of ammonium and nitrite nitrogens in solution by the helium MIP emission spectrometry with nitrogen gas generation technique. Ammonium ion is oxidized t o nitrogen gas by sodium hypobromite in a n alkaline medium (15), and nitrite ion is reduced t o nitrogen gas by sulfamic acid in an acidic medium (16). Nitrate ion is reduced to nitrite ion with a Cd-Cu column and determined by the same method as that for nitrite nitrogen. In addition, the methods developed have been successfully applied t o the analyses of real samples.
EXPERIMENTAL SECTION Chemicals. All chemicals used were of analytical reagent grade. The stock solutions of ammonium ion (1mg/mL), nitrite ion (1 mg/mL), and nitrate ion (1mg/mL) were prepared by dissolving ammonium sulfate, purified sodium nitrite, and potassium nitrate in distilled water, respectively. All the analytical standard solutions were obtained by diluting the stock solutions with distilled water to the series of proper concentrations. The standard solutions of nitrite ion at concentrations lower than 10 pg/mL were analyzed within 5 h. A 10% sodium hypobromite solution (extra pure reagent from Kanto Chemical Co., Ltd.) was used for the nitrogen gas generation from ammonium ion solutions. A 0.2 M sulfamic acid solution was used for the nitrogen gas generation from nitrite ion solutions. In order to remove dissolved nitrogen, the reaction agent solutions were degassed with helium gas before use. Commercial pure helium gas (99.995%) was used for the plasma gas without further purification. Instrumentation. A schematic diagram of the present experimental system is shown in Figure 1, and the instrumental components are summarized in Table 1. The instrumental setup consists primarily of the plasma system, the optical measurement system, and the gas generation system. Plasma System. The microwave cavity with TMolomode was constructed from copper metal after Beenakker’s description (8) with some modification ( 1 4 ) . The inner diameter and thickness
lens
model or size
manufacturer
MR-3S
Ito Chotanpa Co., Ltd.
TM,,, type 6 mm o.d., 3 mm i.d. JE-5OE
laboratory constructed laboratory constructed
photomultiplier
f:60 mm, 2 5 mm 0 R 787
power supply picoammeter chart recorder
a part of IC AP-5 0 0 B-181H
Nippon Jarrell-Ash Co.. Ltd. Hamamatsu T V Co., Ltd. Nippon Jarrell-Ash Co., Ltd. Rigakudenki Co., Ltd.
of the designed cavity were 92.6 and 10 mm, respectively, and the BNC connector was mounted on the cavity. A transforming connector (N-P-a/U-BNC) was used for connecting the cavity to a 5 0 4 coaxial power cable. Although the reflected power could be tuned to a minimum with the tuning screws on the cavity, it remained rather high (10-20’70). No additional tuning equipment was used in the present system. The microwave generator, which provided 20-200 W of microwave power a t 2.45 GHz, was generally run at 7 5 W forward power in the present experiment. The carrier helium gas for the gas generation was normally used as the plasma gas, and the flow rate of helium gas was in the range of 0.25-1.5 L/min, when a silica tube of 6 mm 0.d. and 3 mm i.d. was used as the plasma discharge tube. Optical Measurement System. An Ebert-type monochromator (0.5 m focal length) with a reciprocal linear dispersion of 1.6 nm/mm a t 300 nm was used for spectral measurement. In the measurement of nitrogen atomic line, the slit width and slit height of the entrance slit were 50 pm and 1.5 mm, respectively, and those of the exit slit were 50 ym and 10 mm, respectively. In the molecular band emission measurements, the slit width and slit height of the entrance slit were 10 ym and 1.5 mm, respectively, and those of the exit slit were 10 pm and 10 mm, respectively. A 1:l image of the plasma (axially viewed) was focused on the entrance slit by using a quartz lens with a 6-cm focal length and 2.5-cm diameter. A photomultiplier tube (R 787 from Hamamatsu T V Co., Ltd.), a high voltage power supply, and a picoammeter were used for the signal detection. The output of the picoammeter was recorded on a strip-chart recorder. The time constant of the readout system was 0.3 s. Gas Generation System. As shown in Figure 1, there are two gas flow systems. Main flow system (I) for nitrogen gas generation was connected to the plasma discharge tube. The other gas flow system (11) was used for degassing the dissolved air from the reaction agent solutions. A reaction vessel (25 mL volume) was constructed from a gas washing trap, where a side wall port (10 mm id.) was made. A silicone rubber septum was fitted on the port for the injection of the samples, pH control, and reaction solutions with the syringes. A drying tube (12 mm i.d. and 20 cm long) packed with grains of magnesium perchlorate was used to remove water vapor, which caused instability of the plasma. The path change of the helium gas flow between the reaction vessel and bypass was done by switching two synchronous electromagnetic three-way valves. Procedure. Experimental procedures are summarized in Table 11. A t first, the carrier gas was flowed through the bypass. Then, the washed reaction vessel was connected to the gas generation system, and the carrier gas was directed to the reaction vessel by switching the three-way valves. First, this caused the plasma to be blown off by the excess air in the vessel making it necessary to switch the gas flow from the reaction vessel to the bypass. A few seconds after the switching, self-initiation of the plasma occurred. After the self-initiation of the plasma, the carrier gas was directed again to the reaction vessel. This procedure had to be repeated several times until the air was almost completely removed and the plasma could be stably sustained with the carrier gas flowing through the vessel. After these procedures, the sample solution and the solution for pH control were injected with syringes
ANALYTICAL CHEMISTRY, VOL. 52, NO. 14, DECEMBER 1980
2363
Table 11. Experimental Procedures for Nitrogen Gas Generation purging the air in the reaction vessel
1 1
injection of sample ( 2 mL) injection of pH adjustment solution ( 0 . 5 mL) 0.25 N NaOH solution" 1 N HC1 solution
1 1 I
purging the air in the solution injection of reaction agent solution (0.1 mL) 10% NaOBr solutiona 0.2 M sulfamic acid solution measurement a In the case of ammonium nitrogen. nitrite nitrogen.
In the case of
0.5pg.N/rn L
02
1
Y
30s Figure 2. Analytical signals obtained at 391.4 nm (the N2+ "main" system bandhead) at various concentrations of ammonium ion.
into the vessel through the side wall rubber septum. About 1 min later, these solutions were degassed. After the completion of the degassing was confirmed by monitoring the emission signal from the nitrogen-containing species, the reaction agent solution (0.1 mL) was quickly injected with a syringe. The emission signal was immediately observed, and the signal was measured as the peak height on the recorder chart. Typical emission signals obtained a t 391.4 nm (the N2+ "main" system bandhead) at different concentrations of ammonium ion are illustrated in Figure 2. The silicone rubber septum was usable for about 50 measurements.
RESULTS A N D D I S C U S S I O N Optimization of E x p e r i m e n t a l Conditions. According t o the previous works employing mercury cold vapor generation techniques (17,18),the influences of the carrier gas flow rate, the solution volume in the reaction vessel, and the volume of the reaction vessel on the analytical sensitivity must be generally taken into account when the gas generation technique is employed for sample introduction. Furthermore, it has been reported that the sensitivity of helium MIP emission spectrometry is significantly dependent on the plasma gas flow rate (9-11). Therefore, the experimental conditions for nitrogen gas generation and plasma parameters were examined t o achieve better analytical sensitivity for nitrogen analysis. p H Optimization. The dependence of analytical sensitivity on the p H of the reaction solution was studied for the determination of ammonium and nitrite nitrogens. The analytical sensitivity for ammonium nitrogen was almost constant in neutral and alkaline regions, while the shift of the base line level toward the negative was observed in the acidic region. Considering these results, 0.05 N NaOH alkaline condition was chosen for the nitrogen gas generation from ammonium nitrogen. The p H adjustment was carried out by adding 0.5 mL of 0.25 N NaOH solution to the sample solution. The negative signal for the acidic solution may be due to the evolution of Br2 gas which caused the plasma overload and the decrease of the background emission from nitrogen-containing species due to the impurity in the helium gas.
L05
1.0
IS
Flow Rate (Lmin-1)
Figure 3. Dependence of analytical sensitivities on the flow rate of helium gas (observed at 391.4 nm): (0)ammonium nitrogen, 2 mL of 0.9 pg of N/mL sample solution; ( X ) nitrite nitrogen, 2 mL of 0.6 kg of N/mL sample solution.
The analytical sensitivity of nitrite nitrogen was almost constant in the acidic region lower than p H 1.2. The sensitivity decreased significantly a t higher pH, and no signal was observed for alkaline solution. Considering the pH dependence of the sensitivity mentioned above, 0.2 N HCl acidic condition was chosen in the case of gas generation from nitrite nitrogen, where 0.5 mL of 1 N HC1 solution was added to the sample solution. V o l u m e of Reaction Vessel. According to the previous works (17, 18),the use of the smaller volume reaction vessel provided better sensitivity in the gas generation experiment. this may be due to less dilution of generated gas in the dead space of the reaction vessel. Therefore, in this experiment, a 25-mL reaction vessel was used. Solution Volume. The dependence of analytical sensitivity on the volume of the solution in the reaction vessel was investigated by injecting 1-8 mL of the sample solutions containing the same amount of analyte ion, where 0.25-2 mL of the pH adjustment solutions and 0.1 mL of the reaction agent solution were used. The sensitivities for both ammonium and nitrite nitrogens became poorer when the quantity of the solution in the vessel was increased, and the tendency was slightly more notable in the case of nitrite nitrogen. There was, however, little change in sensitivities when the volume of the solution in the vessel was less than 3 mL. Consequently, the volume of the sample solution and the solution for p H adjustment were decided to be 2 and 0.5 mL, respectively, in the following experiments. The amount of the reaction agent solution used was 0.1 mL. This amount was enough to react with 2 mLl of 500 pg/mL analyte ion solutions. The use of small volumes of the reaction agent solutions allows rapid and reproducible injection and also minimizes the blank signal from nitrogen gas dissolved in the reaction agent solutions. Flow Rate of Helium Gas. T h e dependence of analytical sensitivity on the flow rate of helium gas was studied in the range of 0.25-1.5 L/min. The results are shown in Figure 3. The sensitivity for ammonium nitrogen became significantly better with the increase of the flow rate and was found t o become constant a t flow rates greater than 1.0 L/min. However, a t the higher flow rates, the reproducibility of the signals was not good and the lifetime of the drying tube became shorter. On the other hand, the analytical sensitivity for nitrite nitrogen was best a t the flow rate of 0.5 L/min and decreased with the change of the helium gas flow rate. Therefore, the flow rate of helium was maintained a t 0.5 L/min for the analysis of both ammonium arid nitrite nitrogens. Detection Limits a n d D y n a m i c Ranges. Four wavelengths listed in Table I11 were used for the determination of nitrogen. These peaks of nitrogen-containing species provided different detection limits and dynamic ranges. The detection limits and dynamic ranges obtained for ammonium and nitrite nitrogens are also summarized in Table 111. The
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 14, DECEMBER 1980
Table 111. Detection Limits and Dynamic Ranges for Ammonium and Nitrite Nitrogens at Different Emission Lines and Bands
waveiength, nm
ammonium N detectn lim, pg dynamic of N/mL range
0.008 0.004
0.02
0.009
-5000
0.01
0.006
336.0 (NH) 746.8 ( N )
0.004
re1 sensitivity ammonium nitrite
nitrite N detectn lim, p g dynamic of N/mL range
-3000 -200 -50
337.1 (N,) 391.4 ( N 2 + )
Table IV. Interferences of Inorganic Ions and Organic Compounds with Determination of Ammonium and Nitrite Nitrogens (Observed at 391.4 n m )
0.006
concomitant
-1000
- 200 - 100
-3000
~
detection limit was defined as the signal level corresponding to twice the standard deviation of the blank signals. As can be seen in Table 111, the N2+“main” system bandhead at 391.4 nm provided the lowest detection limit, while the N2 “second positive” system bandhead at 337.1 nm and the nitrogen atomic line at 746.8 nm were observed to have wide dynamic ranges (more than 3 orders of magnitude). The emission signal of t h e N H band maximum (“A” system) a t 336.0 nm was t h e largest of those of the four analytical wavelengths, but the detection limits of this band, especially for ammonium nitrogen, were not so good because of the significantly large blank signals at 336.0 nm. T h e relative standard deviation of eight replicate measurements was about 7% at the 10-fold concentration of the detection limit, and was about 4% at the 100-fold concentration for all the analytical wavelengths. Nitrate nitrogen was reduced to nitrite nitrogen with a Cd-Cu column and analyzed by the same method as that for nitrite nitrogen. Thus the detection limit and dynamic range were almost similar to those for nitrite nitrogen. As mentioned previously, the analytical sensitivity generally became poorer when the amount of the solution in the reaction vessel was increased. I n order to get the maximum concentrational sensitivity, however, we increased the sample volume. In the case of ammonium nitrogen, when 8 mL of the sample solution (this volume was the maximum allowable for the 25-mL reaction vessel) was used for the analysis a t 391.4 nm, t h e detection limit of 1 ng of N / m L was obtained for ammonium nitrogen. In the case of nitrite nitrogen, when 1.5 M sulfamic acid solution was used as the reaction agent solution, and when 4 m L of the sample solution was used for the analysis at 391.4 nm, the detection limit of 2 ng of N / m L was obtained for nitrite nitrogen. Interference Studies. The effects of various concomitants possibly coexisting in water samples and soil extracts were investigated. T h e samples containing 0.4 pg of N / m L ammonium nitrogen or 0.3 pg of N / m L nitrite nitrogen were analyzed in the presence of 10C-20 OOO pg/mL of the inorganic ions. Of the organic compounds, amino acids and urea were examined, where the analysis was done in the presence of 0.5-100 times concentration (in molar concentration) of the concomitants. T h e results are summarized in Table IV. Table IV shows that only a few inorganic ions and organic compounds interfere with nitrogen analysis by helium M I P emission spectrometry. Determination of Ammonium Nitrogen in Soil Extracts. The present method was applied to the determination of ammonium nitrogen in soil extracts. Three different soil samples were taken from the surface (Gl cm) and 1-5 cm and 5-10 cm in depth in a rice field. A 10-g sample of wet soil was shaken with 100 mL of 10% potassium chloride solution for YO min. After filtration, the filtrate was used for the analysis. The analysis was done by three different methods, i.e., t h e conventional p H titration method utilizing t h e steam-distillation technique, ion-selective electrode method, a n d the present plasma emission method. The results are
concn
none Na’ K’ Mg 3 + Ca ,+ A1 3 + Fe 3 +
13000 yg/mL 20000 pg/mL 500 pg/mL 1000 pg/mL 100 pg/mL 1000 pg/mL 100 pg/mL 100 pg/mL 100 pg/mL 100 pg/mL 1 0 0 yg/mL 20000 pg/mL 1500 yg/mL 4000 yg/mL 500 hg/mL 7 mM 1 mM 7 mM
Fe *+
Mn”
co ,+ c u *+ Zn *+ C1~ NO;
so,*~
0
~
3
-
alanine serine glutamic acid urea
Na
Nb
l0OC
l0OC 115 105
100
98 98 97 103 96
104 102 102 101
84
71 99 106
103 99
107
114
100 105 107 102
102 100 107
99 70 100
97 95
1 mM
90
7 mM 1 mM 7 mM
94
99 99
0.015 mM
114
a The concentration of the test solution was 0.4 y g of N/mL. The concentration of the test solution was 0.3 Defined as 100. y g of N/mL.
Table V . Analytical Results in the Determination of Ammonium Nitrogen in Soil Extracts ammonium nitrogen content, p g of N/mL sample ammonium no. a plasma emission electrode pH titration 1
0.32
i-
2 3
2.88
i
6.6
t
0.02
0.25
?-
0.01
0.09 0.3
2.81
i
0.03
6.2
*
0.1
0.48 2.39 5.1
?
0.03 0.01 0.1
i i
Samples 1, 2, and 3 refer to the soils taken from the surface (0-1 cm) and 1-5 cm and 5-10 cm in depth in the rice field, respectively. Measured a t 391.4 nm. a
Table VI. Analytical Results in the Determination of Nitrate Nitrogen in Pond Water sample no.a
nitrate nitrogen content, p g of N/mL plasma emission colorimetry
1
0.010
I
2
0.019
i-
0.002 0.002
0.013 i 0.001 0.021 i 0.001
a Samples 1 and 2 refer to the surface water samples taken from the Sanshiro-ike pond at the University of Tokyo. Measured at 391.4 nm.
summarized in Table V. The analytical data obtained by the p H titration method were slightly different from others, but the data obtained by the electrode and the plasma emission method were in agreement with each other. Determination of Nitrite and Nitrate Nitrogens in Pond Water. The method developed was also applied to the determination of nitrite and nitrate nitrogens in pond water. Two samples were taken from two surface points of the Sanshiro-ike pond in the University of Tokyo. The analysis was carried out by two different methods, i.e., the conventional colorimetric method and the present plasma emission method. Nitrite nitrogen in pond water was not detected by either
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 8 1 9 7 8 , 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
