Anal. Chem. 1984,56,2735-2740
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the AVM technique, indicating that the sputtering process is greatly influenced by the applied voltage modulations. This enhancement effect can be utilized generally for the selective measurement of sputtered elements among the other emitting species. The intensity difference spectrum (AVM - CM), which is computed with a data processor, is also shown in Figure 7(3). The copper emission lines appear upward from the base line level. The broad band near 283 nm, which is assigned to OH radicals (17), can be distinguished from the Cu I intense line at 282.2 nm, because this band structure appears downward. The nickel difference spectrum which was observed in the wavelength range from 370 to 300 nm is shown in Figure 8. Most of the Ni I emission lines ( B )for , example, 341.5,352.5, or 361.9 nm, etc., are easily separated from Ar I or Ar I1 lines (lower parts in Figure 8) on the spectrum chart.
CM 4 4 t h AVM I2OHz 3OV
LITERATURE CITED
WAVELENGTH
Figure 8. Intensity difference spectrum (AVM - CM) for Ni cathode sputtering: dc bias voltage, 365 V; dc lamp current, 9.5 mA; voltage modulation frequency, 120 Hz; amplitude of the ac component, f30 V; chopper frequency, 44 Hz.
among various emission lines are very similar to those measured by the dc method. Both the AVM and CM signals can simultaneously observed with a second lock-in amplifier tuned to the chopper frequency (44 Hz in our experiments). When a copper target was sputtered a t 380 V dc (3.8 W), the AVM and CM spectra were recorded in the wavelength range from 300 to 380 nm as shown in Figure 7(1) and Figure 7(2). The emission lines at 282.2 and 295.8 nm can be assigned to the electronic transitions of copper atoms and the lines at 297.6 and 294.0 nm to argon ions. It is found that when the relative intensities of Cu I to Ar I1 emission lines are calculated, these intensities measured by the AVM method are rather different from the corresponding values by the CM method. For ekample, the intensity ratio (Cu I 282.2/Ar I1 294.0 nm) is estimated to be 1.6 for the AVM detection, and 0.90 for the CM detection, respectively. Accordingly, the relative intensities of Cu I emission lines are enhanced with
(1) Bradshaw, A. M.; Hoffman, F. Surf. Scl. 1975, 5 2 , 449 (2) Golden, W. G.; Dunn, D. S.; Overend, J. J. Phys. Chem. 1978, 82, 643. (3) Winefordner, J. D. J. Chem. Educ. 1978, 5 5 , 72. (4) Johnson, D. J.; Plankey, F. W.; Winefordner, J. D. Anal. Chem. 1978, 5 0 , 360. ( 5 ) Grimm, W. Naturwlssenschaften 1967, 54, 566. (6) Grimm, W. Spectrochim. Acta, Part B 1988, 2 3 8 , 443. (7) Hlrokawa, K. Bunko Kenkyu 1972, 22, 317. (8) West, C. D.; Human, H. G. Spectrochim. Acta, Part6 1978, 318, 61. (9) Boumans, P. W. J. M. Anal. Chem. 1972, 4 4 , 1219. (IO) Berneron, R.; Charbonnier, J. C, S I A , Surf. Interface Anal. 1981, 3 , 134. (11) Ohashl, Y.; Yamamoto, Y.; Tsumoyama, K.; Kishidaka, H S I A , Surf. Interface Anal. 1979, 1 , 53. (12) Waitlevertch, M. E.; Hurwitz, J. D. Appl. Spectrosc. 1976, 3 0 , 510. (13) Belle, C. J.; Johnson, J. D. Appl. Spectrosc. 1973, 2 7 , 116 (14) Wagatsuma, K.; Hirokawa, K. S I A , Surf. Interface Anal. 1984, 6 , 167. (15) Wagatsuma, K.; Hirokawa, K. Anal. Chem. 1984, 5 6 , 412. (16) Wagatsuma, K.; Hlrokawa, K. Anal. Chem. 1984, 5 6 , 906. (17) Pearse, R. W. B.; Gaydon, A. G. "The Identification of Molecular Spectra"; Chapman and Hall: London, 1965. (18) Zaidel, A. N.;Prokof'ev, V. K.; Raiskii, S. M. "Spektraltabellen"; VEB Verlag Technik: Berlin, 1961.
RECEIVED for review July 9, 1984. Accepted August 27, 1984. This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Japan.
Evaluation of the Graphite Spark Technique Using Time Resolution James C. Williams,* Jeffery E. Kuehn, Jerry T. Coleman,' and Teresa A. Mausert Department of Chemistry, Memphis State University, Memphis, Tennessee 38152
The graphite spark method is briefly reviewed. Pulse height distributions produced by individually recording the signal from each spark show that most of the useful signal Is produced in less than 100 sparks when the sample is deposited on a 3/le In. electrode. Electronic integration for a selected period after gap breakdown improves the detection limits for Cu by a factor of 20 compared to reported photographic work. An absolute detection limit for Cu of 24 pg Is reported and an ultimate limit of 1.2 pg is imposed by photon and detector noise. The method may be useful for very small samples but will require the use of an Internal standard.
'Present address: E. I. du Pont de Nemours, Plant, Aiken, SC 29808.
Savanah
River
The graphite or copper spark technique (1,2) was one of the earliest methods developed for trace element determination in microsamples. In the graphite spark method, graphite electrodes are rendered nonporous by depositing a layer of grease on the electrode surface. The analyte solution is placed on the electrode, evaporated to dryness, and then sparked with approximately 25 000 high voltage, high inductance sparks. With this procedure, samples deposited on copper and graphite electrodes differ only in background spectra. In both cases, evaporation of aqueous solutions results in an uneven deposition of solids on the electrode surface, most commonly collecting around the periphery of the electrode (3). Sample evaporation rate from the electrode surface depends
0003-2700/84/0356-2735$01.50/00 1984 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
Table I. Experimental Facilities and Operating Conditions spark power supply and electrode stand NSL Model 6448, 0.005 pF, re8idual inductance, 4 mm auxiliary gap, peak voltage 14000 V, one spark per half cycle spectrometer 0.5 m Czerny-Turner, McKee-Pederson Model MP-1018, f / 8 , 35 A/mm, 100 pm slits electrodes Spex 4072 flats or 4040 rounded end multichannel analyzer (MCA) Nuclear Data Model 2200 with ADC, 4K memory, and magnetic tape storage photomultiplier tubes (PM) RCA 1P28, biased at -970 V with 300 k resistors in the divider chain, and 0.1-pF capacitors across the last three dynodes for pulse operation. PM HV supplys Ortec Model 446 discrimator Ortec Model 455, constant fraction timing single channel analyzer, modified for adjustable delay of 0-100 p s amplifiers Ortec Models 452 and 485 preamplifiers Ortec Model 113 delay generators Ortec Models 416A (0-110 p s ) and 416 (modified to give 0-60 ps delay) scalers Ortec Models 431 and 484 solid-state switches AD7512 and AD7511, Analog Devices markedly on spark voltage and electrode diameter, with high voltage and small diameter being most effective. Nakajima and Kawaguchi (3) showed that, with commonly used excitation conditions, 40% of the sample remained on the 6-mm electrode after 20000 sparks; 10% of the sample remained on a 3- or 4-mm electrode after 20 s (9600 sparks) using primary voltages of 140 to 200 V and an insulating sheath of fluorocarbon to prevent sparks flying to the side of the small electrodes. Wetting agents give a more even distribution of solids on the electrode and electrode rotation ensures uniform sparking of the electrode surface ( 4 ) but there is little improvement in precision over previous methods and detection limits are poorer than those reported by Morris and Pink (2). Detection limits in the nanogram range can be realized for a large number of elements, although good precision requires the use of internal standards. The validity of the method for multielement analysis has been demonstrated using photographic (2) and photoelectric detection (4). Other methods, such as flame emission, atomic absorption, atomic fluorescence, and the inductively coupled plasma source, offer significantly better sensitivity along with good precision such that the graphite spark has received little attention as a trace method in recent years. Recent work here shows that a large part (25-50%) of the useful signal from the graphite spark method is produced by a relatively few (
