I t should be mentioned that Reticon indicates that the LDA unit is intended for prototype applications. As such, this unit was probably designed and intended for a broad variety of applications and performance may be improved by designing circuitry for specific applications. An example of an application has been the analysis of SO2 in the wavelength range of 280-315 nm. Sensitivity of 100-2000 ppm of SO:! in a cell of 50 cm length was reported (7). We predict widespread usage of the solid state array detectors because of the attractiveness of being able to electronically (vs. mechanically) control all aspects of the spectral acquisition.
LITERATURE CITED (1) R . E. Santini. M. J. Milano, and H. L. Pardue, Anal. Chem., 45, 915A (1973). (2) J. W. Strojek, G. A. Gruver, and T. Kuwana, Anal. Chem., 41, 487 (1969). (3) F. Hawkridge and T. Kuwana, Anal. Chem., 45, 1021 (1973). (4) R. M. Wightman, R. L. Scott, C. N. Reilley. and Royce W. Murray, Anal. Chem., 46, 1492 (1974). (5) Gary Horlich and Edward G. Codding, Anal. Chem., 45, 1490 (1973). (6) Raymond E. Dessey, "Computers in Chemical and Biochemical Research," Vol. 2, Academic Press, New York. 1974. (7) J. W. Strojek and T. Kuwana, Rocz. Chem., 49, 379 (1975).
RECEIVEDfor review February 3, 1975. Accepted December 4, 1975. We ghtefully acknowledge financial support provided by National Science Foundation Grant No. MPs 7304882 and by USPHS Research Grant GM 19181.
Plasma Emission Detection of Chlorinated Pesticides in Inert Matrices D. J. Runser and C. W. Frank* Department of Chemistry, University of Iowa, Iowa City, Iowa 52242
A self-initiating and continuous-flow electrodeless discharge was used to produce spectra of organic compounds. The discharge was driven by a 300-W, 13.56-MHz radiofrequency generator. Chlorinated pesticides were monitored at the 2780-A CCI radical emission band. Absolute detection limits for DDT in alumina and soil were 5.9 and 8.2 pg, respectively. Interferences from inorganic chlorides (NaCI) and urea at concentrations up to 10 000 ppm were negligible.
The first application of electric plasmas to spectrochemical analyses of pesticides was made by Bache and Lisk ( 1 ) in 1965. Using the method suggested by McCormack et al. ( 2 ) ,an argon microwave plasma was used as a detector for gas chromatographic analysis of organophosphorous pesticides. This work was extended to organic bromine, chlorine, iodine, phosphorous, and sulfur pesticide residues using a helium microwave plasma ( 3 ) . Organic mercury compounds have also been studied ( 4 ) ,where sensitivities in the nanogram range have been reported. However, as with other existing analytical procedures for pesticides, preliminary sample extraction, isolation, and pre-concentration were required prior to detection. Studies in our laboratory have been carried out with a radio-frequency electrodeless discharge cell used for studying the emission spectra of organic compounds ( 5 ) . Using this emission source, an investigation was made to study the feasibility of a one-step method of detection of chlorinated pesticides in inert matrices by monitoring the 2780-8, CC1 radical emission band. The work reported here describes the results of this initial study with DDT in soil and alumina samples.
EXPERIMENTAL An argon radio-frequency electrodeless discharge was used in this study (Figure 1). The discharge cell was a self-initiating and continuous flow cell which produced an intense, stable, and reproducible plasma flame and striated column for spectral excitation. Details of cell construction and performance are described elsewhere (6). 514
ANALYTICAL CHEMISTRY, VOL. 48, N O . 3, MARCH 1976
The discharge was sustained using a Tracerlab RFG-300 radiofrequency generator with an operating frequency of 13.56 MHz. A Tracerlab PA-301 Plasma Activator served as the radio-frequency tuner. Emission spectra were studied using a three-meter grating spectrograph with a modified Eagle mounting and a reciprocal linear dispersion of 5.59 A/mm. Kodak Spectrum Analysis No. 1 Plates were used to record the data. Union Carbide, Linde, high purity dry argon, purity 99.996% was used as the carrier gas. Technical grade DDT recrystallized from isopropyl alcohol with a melting point of 108.5-109.5 "C was used. (Literature m p 108.5-109 "C) ( 7 ) . DDT-alumina and DDT-soil samples were prepared using Fisher Scientific Levigated Alumina and 40-mesh brown soil, respectively. Five percent samples were made by weighing both components to five-place accuracy, dissolving in thiophene free benzene, mixing for 5-7 h, and evaporating the solvent under aspirator vacuum using a Rinco rotating evaporator. The amount of DDT present was confirmed by extraction from the inert matrix with benzene, using a micro Soxhlet extractor, and analyzed by quantitative gas chromatography with a Bendix Chroma-lab, Series 2100, Gas Chromatograph using a 6-ft X %-in. stainless steel column packed with 5% OV-1 on 60/80 mesh chromosorb W and a thermoconductivity detector. Samples were weighed out in a triangular tantalum boat which slid over the tantalum grounding strip already in position in a radio-frequency cell (Figure 2). The sample boat was held in place by tabs A and B. The two sections of the cell (A and B, Figure 1) were joined together and evacuated. Once the argon carrier gas flow was established, the plasma flame and striated discharge were initiated. When the striated discharge arced toward the tantalum ground, it heated and vaporized the sample. Vaporization was complete within 1-2 s. Sample sizes varied from 10-0.1 mg.
RESULTS AND DISCUSSION Using this radio-frequency cell and the sample procedure described, intact samples of DDT in alumina and in soil were studied for the presence of the intense 2780-8, CC1 radical emission. The occurrence of CC1 emission from CC14 in gas discharges has been well documented (8, 9) and the presence of CC1 emission from simple aromatic and nonaromatic chlorinated hydrocarbons including DDT, Aldrin, Endrin, and Heptachlor in the radio-frequency discharge has been observed in our laboratories ( 5 ) . By monitoring the 2777.68, 2778.76, 2786.67 and 2788.39-A transitions of the 2780-A band of the CC1 radical emission, the
14.0rnm.
!
TO VACUUM PUMP
-1
Figure 1. The radio-frequency discharge cell (A) Low pressure region, (B) High pressure region, (C) Optical path, (W) Tungsten wire, (Ta) Tantalum strip
Table I. Results of Inorganic Chloride on the Analysis of DDT in Soil CCl
Soil sample
Present Absent
x
1. Unwashed in H,O 2 . Unwashed in H,O, containing 1 0 000
X
ppm NaCl 3. Washed in H,O 4. Washed in H,O, containing 5000 ppm urea 5. Washed in H,O, containing 10 000 ppm NaCl 6. Unwashed soil containing 1 0 000 ppm NaCl and 1 0 000 ppm urea 7. Washed in H,O, containing 6500 ppm NaCl and 5000 ppm urea 8. n'ashed in H,O containing 6500 ppm NaCl and 1000 ppm urea 9. Washed in H,O, containing 3500 ppm NaCl and 5000 ppm urea
x X
x Trace X X
-
presence or absence of DDT in the alumina and soil samples was confirmed. Using this method, absolute detection limits of 5.9 fig and 8.2 fig for DDT-alumina and DDT-soil samples, respectively, were found. With this emission source, no spectral background interference was observed in this region. Sampling of intact samples was simple, and instantaneous volatization of samples was possible by the action of the hot arcing striated column produced by the radio-frequency discharge. Pressure wave effects, produced by the arcing striated discharge, were minimal and samples were not ejected from the sampling boat. Interference effects from inorganic chloride were also investigated. I t was believed that in the radio-frequency discharge, contamination of soluble inorganic chlorides, such as NaCl, may interfere with an analysis of chlorinated pesticides in soil by producing CCl emission from the combination of atomic chlorine from the salt with atomic carbon from the organic components. From our studies, it was found that chlorinated and nonchlorinated hydrocarbons produced atomic carbon emission a t 2478.57 8, in the radiofrequency discharge. I t has also been found that chloride salts of calcium, magnesium, and sodium can accumulate in soil ( I 0). A qualitative investigation of the effect of NaCl on the emission spectra of naphthalene and urea in the radio-frequency discharge was made. All samples were reagent grade. Both naphthalene and urea produced atomic carbon emission a t 2478.57 8,. No CCl emission was observed. However, in combination with NaCl, both urea and naphthalene produced CC1 band emission a t 2780 A. Sample sizes of pure material varied from 3-4 mg each. Soil treated with varying amounts of NaCl and urea were
I.Ornrn.
30.0 m m .
I
Figure 2. Tantalum sampling boat
prepared and studied in the radio-frequency discharge. Using 40-mesh brown soil and following the same preparation as for the DDT-alumina samples, soil samples of 1.0, 0.65, and 0.35% NaCl in combination with 1.0, 0.5, and 0.1%,and 0.5% urea, respectively, were made. Soil with 1.0% NaCl and 0.5% urea, respectively, was also studied. Urea was chosen as the organic additive because of its frequent use in fertilizers (11, 12). Sample sizes varied from 5-10 mg. The results are shown in Table I. The results show at levels of 10000 ppm NaCl with 10 000 ppm urea CC1 emission was barely detectable. Below these inorganic chloride levels, no detectable CCl emission was observed. Soil washed in distilled water was compared with unwashed soil to determine if the unwashed soil sample contained any impurities which would contribute to CC1 emission. No CC1 emission was observed. These NaCl concentrations were chosen as extreme levels of soluble chloride content in soil based on data in the literature ( 1 3 ) . These data describe the effects on crops for different levels of salinity resulting from the total soluble salts in soil such as carbonates, chlorides, and sulfates. The data indicate that soil was greater than 0.65% total soluble salt content cannot be used for growing crops while, below this level, the effects on crop growth and yield vary depending on the salinity level. Very little is reported in the literature on the soluble chloride content in surface soils. However, based on these data ( 1 3 ) and the results obtained in this study, formation of CC1 emission from contaminating soluble chloride salts would be a problem only in strongly saline soils, unsuitable for crops. In conclusion, these results indicate this single step technique, which eliminates extraction, isolation, and pre-concentration, may be a viable method for total carbon-chlorine content. Pesticide identification was made by monitoring the 2780-A CC1 radical emission band. The absolute detection limits for DDT in samples of alumina and soil are 5.9 and 8.2 pg. Soil samples contaminated with inorganic chlorides should not interfere with the analysis.
LITERATURE CITED (1) C. A. Bache and D. J . Lisk, Anal. Chem., 37, 1477 (1965). (2) A. J. McCormack, S. C. Tong, and W. D. Cooke, Anal. Chem., 37, 1470 (1965). ( 3 ) C. A. Bache and D. J. Lisk, J. Assoc. Off. Agric. Chem., 50, 1246 (1967). (4) C. A. Bache and D. J. Lisk, Anal. Chem., 43, 950 (1971). (5) D. J. Runser. "Plasmas in Analytical Chemistry," Ph.D. Thesis, University of iowa, Iowa City, iowa, 1971. (6) D. J . Runser and C. W. Frank, "A New Radio-Frequency Electrodeless Discharge Cell for Studying the Emission Spectra of Organic Molecules," Appl. Spectrosc., 28, 175 (1974). (7) D. E. H. Frear, "Chemistry of the Pesticides," D. Van Nostrand Co., Inc., New York, N.Y., 1955. ( 8 ) T. Horie. Proc. Phys. Math. SOC.,Jpn, 21, 134 (1939). (9) R . D. Gordon and G. W. King, Can. J. Phys., 39,252 (1961). (10) F. E. Bear, "Chemistry of the Soil," Reinhold Publishing Corp., New York, N.Y., 1964. (11) S. I. Tisdal and W. L. Nelson. "Soil Fertility and Fertilizers," The MacmilIan Co., New York, N.Y., 1956. ANALYTICAL CHEMISTRY, VOL. 48, NO. 3, MARCH 1976
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(12) W. V. Bartholomew and F. E. Coark, "Soil Nitrogen", American Society of Agronomy, Inc., Madison, Wis., 1965. (13) "Soil Survey Manual", Soil Survey Staff, Bureau of Plant Industry, Soils, and Agricultural Engineering, US. Department of Agriculture, Washington, D.C.. 1937.
RECEIVEDfor review March 24, 1975. Accepted December 1975. This work was in part by the 53
Petroleum Company. The radio frequency generator was provided on loan from Tracer Lab.
Simultaneous Determination of Wear Metals in Lubricating Oils by Inductively-Coupled Plasma Atomic Emission Spectrometry Velmer A. Fassel,' Charlie A. Peterson, Frank N. Abercrombie, 'and Richard N. Kniseley Ames Laboratory-ERDA and Department of Chemistry, lowa State University, Ames lowa 500 10
The simultaneous determination of 15 different wear metals in lubricating oil by inductively-coupled plasma-atomic emission spectrometry is described. An aerosol formed from a solution of the lubricating oil in 4-methyl-2-pentanone is injected into the axial channel of an inductiveiy-coupied plasma where the atomic spectra are excited. Detection limits range 0.0004 to 0.3 ppm for the elements studied. Low and high viscosity oils (nominally 1.9 X to 2.45 X m2/s, respectively) can be accommodated without biasing the analytical results. Approximately 1.5 min are required for completing the analyticai cycle for one sample. Relevant data on precision and accuracy are included.
The determination of trace metals in used lubricating oil has become an accepted means of monitoring component wear in a variety of oil-wetted systems, especially in diesel and aircraft engines of various types. General discussions on these determinations are found in references (1 t o 4 ) of this paper. A variety of analytical procedures for performing these analyses have been described. In the early years, the published methods generally employed colorimetric, spectrophotometric, polarographic, or arc-spark optical emission analysis of the residue remaining after wet or dry oxidation of the oil sample. Because these procedures involved the possible contamination of the sample by the oxidizing acids andfor loss of the analytes of interest during dry ashing, examination of the oil sample directly or after dilution with an organic solvent has been the favored analytical approach in recent years. For the direct examination approaches, a large number of variations of spark atomic emission, flame or nonflame atomic absorption or fluorescence, and x-ray fluorescence procedures have been proposed. Although the majority of published procedures have now been relegated to the archives, the rotating electrodeatomic emission technique ( 5 ) and several variations of atomic absorption procedures are now extensively employed, and literally millions of determinations are made annually by these two methods. This extensive usage may imply that these two approaches meet all expectations with reference to speed of analysis, simplicity of analytical manipulations, powers of detection, absolute accuracy and precision, but this is not the case. For example, the rotating disk electrode-atomic emission technique is admittedly subject to serious matrix effects arising from changes in the fuel dilution, viscosity, and total composition (base oil plus additives) of the lubricating oil ( 2 ) . For the various flame atomic absorption procedures, dilution of the oil sample Present address, Barringer Research, 304 Carlingview Drive, Rexdale, Ontario, Canada. 516
ANALYTICAL CHEMISTRY, VOL. 48, NO. 3, MARCH 1976
with selected solvents usually reduces the matrix effects to acceptable levels. However, the atomic absorption approach possesses an operational disadvantage. Until the problem of simultaneous atomic absorption analysis is overcome, the determinations must be performed on a sequential basis. The time elapsed to complete the determination of an extended list of trace metals in a large number of samples is, therefore, much greater and unacceptable under some circumstances. The inductively-coupled plasma, atomic emission excitation source possesses certain unique properties and operating characteristics not found in spark or flame atomization systems (6). The relatively high temperatures and residence times experienced by the sample aerosol in the plasma and the inert environment provided by the plasma support and stabilizing gas leads to the expectation that the degree of atomization of the samples, including suspended particulates, should be greater than in flames or spark discharges. Because the atoms are released in a noble gas environment, free-atom depopulation processes, such as monoxide formation, should also be minimized. These favorable environmental factors should, in turn, overcome many of the interelement or matrix interference effects found in flames or arc or spark discharges. These expectations have been confirmed by some observations on several classical interelement interference effects (7). Pforr (8) evidently published the first paper on the direct introduction of oil samples diluted with gasoline into the plasma. In a later preliminary report, Pforr and Aribot (9) reported detection limits of 3.6, 4.2, and 1.5 ppm (by weight), respectively, for Al, Fe, and Ni. In another brief report, Greenfield and Smith (10) indicated that they successfully determined Al, Cr, Cu, Fe, Ti, Ni, Mg, and Mn in samples of engine oil. Neither Pforr and Aribot nor Greenfield and Smith provided any data on the accuracy and precision of their results. These encouraging results induced us to embark on a thorough assessment of the analytical performance on a variety of lubricating oil samples of several instrumenta! ICP-AES (inductively-coupled plasma-atomic emission spectrometry) analytical systems that incorporated recent refinements.
EXPERIMENTAL T h e experimental facilities and operating conditions are summarized in Table I. In both systems, a single channel spectrometer and a multichannel spectrometer observed the plasma along the same optical axis but from opposing directions, Le., a t an angle of 180°. The System I sequential instrument was utilized for the wavelength scans and the viscosity effect studies. T h e System I multichannel spectrometer was employed for the determination of Fe, Cr, Mg, Cu, and Ag in the U.S. Air Force (USAF) Spectrometric Oil Analysis Program (SOAP) samples. An integration time of 30 s was used for these determinations. For the P l a s m a Facilities.
