2732
Anal. Chem. 1984, 56,2732-2735
High Sensitivity Measurement of Atomic Emission Spectra with an Applied Voltage Modulation Technique Kazuaki Wagatsuma a n d Kichinosuke Hirokawa*
The Research Institute for Iron, Steel, and Other Metals, Tohoku University, Sendai 980, Japan
A vonage modulation technique was applled to the hlgh sensltivlty measurement In atomic emission spectrometry using a glow discharge source. The emisslon from the glow discharge plasma can be modulated by a cyclic variation in input power to effect discrimination against the predomlnant nolse sources on the discharge. Compared to a dc detection method, the slgnal-to-noise ratio was improved by a factor of 20-50. I t was found that the relative Intensities of the emission lines from sputtered elements were enhanced wRh the voltage modulation method. This lndlcates that the Sputtering process Is greatly affected by the modulatlon.
Table I. Performance of Instruments and Operating Conditions spectrometer
Hitachi Model 808; spectral band-pass, 0.1 nm photomultiplier R-446 (Hamamatsu Photonics, Ltd.) dc power supply Model PAD lk-0.2L (Kikusui Electronics Corp., Japan) 0-1500 V, 0-200 mA ac power amplifier Model POW 70-2 (Kikusui Electronics Corp., Japan) 0 to f70 V, 0 to k200 mA lock-in amplifier Model LI-574A (NF Electronics Instruments Corp., Japan) sensitivity, 1.0, 2.0, and 5.0 mV/fs
band-pass filter When improvements in detection limits are attempted in atomic spectrometric elemental analysis, it is important to detect the signals as effectivelyas possible. If signal intensities are very weak, the essential problem is whether the signals can be separated from the background fluctuation or not. Modulation spectroscopy, with which faint signals can be selectively detected in the presence of the high background levels and/or excessive noise, has been successfully applied to several spectroscopic methods, for example, Auger electron spectroscopy, infrared spectroscopy (I, 21, and atomic fluorescence spectroscopy ( 3 , 4 ) . Rotary choppers or sector mirrors have been employed to modulate light, for example, in optical spectrometry. The hollow-anode-type glow discharge lamp developed by Grimm (5, 6) is widely used as a light source for atomic emission spectrometry. The emission lines are sharp (7) and the self-absorption is small compared to other light sources (8)principally because the discharge is a t reduced pressure. Furthermore, the sample introduction into the plasma is based upon cathode sputtering (9). With this particular sampling mechanism, the Grimm-type glow discharge lamp has been employed for surface analysis by many investigators (10-14). We have studied the surface of some binary alloy systems with glow discharge spectrometry (GDS) and reported the elemental analysis and sputtering properties deduced from the emission intensities when the glow lamp was operated at low power levels (15, 16). In the low-power GDS, mild sputtering conditions are realized and, therefore, the resolving power in depth is improved compared to that obtained with conventional GDS implementations. However, a decrease in emission intensities, which results from a drop in the excitation temperature or the amount of sputtered atoms, occurs in the low-power GDS. An applied-voltage modulation technique was applied to lowpower glow discharge spectrometry to gain enhancement of the intensities of emission lines. By periodically varying the voltage supplied to a glow discharge tube, the radiation emitted from the glow discharge plasma can be modulated according to the period of input power. The modulated radiation can be selectively detected with a phase-sensitive detection method. EXPERIMENTAL SECTION Principle for Measurement. Figure 1 shows a schematic diagram of supplied voltage patterns. While a glow discharge lamp
Model FV-651 (NF Electronics Instruments Corp., Japan) function generator Model E-1202 (NF Electronics Instruments Corp., Japan) 5 Hz to 500 kHz
has been operated in a direct voltage mode (Figure la), an alternating voltage (ac) is superimposed on a dc bias voltage (V,) for the purposes of modulation (Figure Ib). The amplitude of the alternating wave is relatively small compared to the bias voltage. If these voltage patterns ( Vof A v ) are supplied to the lamp, the excitation temperature of the plasma is expected to vary periodically. Therefore, the radiation emitted from the plasma can be modulated according to the frequency of the ac voltage. The wave form, the amplitude, and the frequency of the ac voltage are significant experimental parameters. Apparatus. A schematic diagram of the apparatus is shown in Figure 2. The wave patterns of input power were generated with a dc power supply and a series-connected bipolar power amplifier. The radiation from the glow discharge lamp was dispersed with a grating spectrometer. The modulated signals were obtained from the overall signals with a lock-in amplifier tuned to the modulation frequency. The performance of the instruments and their operating conditions are summarized in detail in Table I. Our glow discharge lamp was made according to the original model suggested by Grimm (6). The inner diameter of the hollow anode was 8.0 mm and the distance between the anode and cathode was adjusted to be 0.4-0.6 mm. The lamp was evacuated to 4.0-1.3 Pa and then argon gas (99.9995% purity) was introduced. Argon pressure of 9.3 X lo2 Pa was selected and fixed. Samples and Surface Treatments. Pure nickel (99.9% purity) and copper (99.99% purity) plates were prepared. The surfaces were polished with waterproof emery papers (no. 600 to no. 1500) and finished to mirror faces with emery cloth. Predischarges for 3-5 min at about 550 V (20 W) were carried out to remove contaminations and initial oxides.
THEORETICAL SECTION If an equilibrium temperature ( T ) can be assumed to represent plasma conditions, it is expected that when the voltage patterns as described above are supplied to create a glow discharge, a periodic variation (AT) appears in the resulting plasma temperature (Tof AT). The modulated components in the total emission intensity, which are observable with a phase-sensitive detection technique, are related to this periodic variation (AT) in the plasma temperature. On the other hand, the intensity of emission line depends on several factors, for example, the excitation temperature, transition probability, atomic density in the plasma, etc. However, because the factors which are not influenced by the
0003-2700/84/0356-2732$01.50/00 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56,NO. 14,DECEMBER 1984
2733
1
S
8 I
TIME
10
Figure 1. Wave patterns of input voltage for (1)a dc amplification method and (2)a voltage modulation method. Chwr
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OUtpVf
Figure 2. Schematic diagram of the modulation spectrometer: GDL, glow discharge lamp; PM, photomultiplier; PA, preamplifier.
ac component in input voltage (e.g., transition probability, etc.) can be ignored, the detectable component ( A I ) in the overall intensity is given by
AI = f(Wan? where AN means a periodic variation in the atomic density caused by the alternating voltage. Though background emission is also modulated, little change in the background intensity level occurs when the amplitude of the ac component is relatively small compared to the dc bias voltage. Neutral Ar atom density in the plasma may be independent of the ac component under the condition of fixed argon pressure, because the majority of particles in the plasma are neutral Ar atoms
AI = f ( A T ) , for emission lines of Ar atoms
(2)
The ionization probability of argon is sensitive to the change in plasma temperature; therefore, the number of created Ar ions may be changed by AT. The emission intensity of Ar+ ions can be modulated by a periodic change in both the plasma temperature and the density of Ar+ ions. The atomic density of sputtered elements in the plasma strongly depends on the number and kinetic ion energy of projectiles (primary ions, Ar+ in our experiments). Accordingly, the emission intensity of these elements can be modulated by a periodic variation in the plasma temperature, the number, and the kinetic ion energy of Ar+
AI = f ( A T , ANSP"),N = f(h"4,+,M A r + ) , for emission lines of sputtered elements (3) where Mspu and m,+ denote the variations in the density of sputtered particles and argon ions, respectively, and ah+ is the periodic change in the kinetic ion energy of Arf.
13
,
, , ,
,,,,I
I
lo3
MODULATION FREQUENCY
: ,
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Figure 3. Modulation frequency dependence of the peak intensities of (a) Cu I 324.7and (b) 327.4 nm: dc bias voltage, 350 V; dc lamp current, 5.8 mA; amplitude of the ac component, f10 V.
RESULTS AND DISCUSSION Selection of Operating Conditions. Figure 3 shows semilog plots of the observed intensities against the modulation frequency when Cu 1324.7 and 327.4 nm are monitored as the analytical emission lines. The detection efficiency of the phase detector in our lock-in amplifier decreases in the very low frequency range (below 50 Hz). Therefore, it is considered that the decrease in the detectable signals (see the low frequency side of Figure 3) results from the reduced efficiency of the phase detector. On the other hand, the sensitivity drops which appear in the relatively high frequency side (more than 1kHz) are related to atomic excitation and/or sputtering processes created with ac voltages in the glow discharge plasma. If the number and energy of Ar+ ions perfectly follow the cyclic change in input power, the volatge modulation for cathode sputtering is the most effective; therefore, the maximum signals can be detected on such conditions. Accordingly, as the modulation frequency increases, the Ar+ ions will be less able to follow the rapid variations in input voltage. Therefore, the intensity decreases, as shown in Figure 3. In our experiments, the modulation frequency was determined to be 120,130, or 140 Hz from the frequency dependence of both the signal intensity and external noises. The amplitude of the ac voltage (AV in Figure 1) is also an important experimental factor. When the frequency is fixed, the peak intensity measured with the applied voltage modulation technique (AVM) increases monotonically with the amplitude of the ac component. However, an increase in the ac component also provides an overall increase in average power to the glow discharge lamp. An average peak intensity, which represents the nonmodulated components, can be simultaneously measured with a conventional direct current amplification method (dc). If the AVM signals vary with the amplitude but only a small change in the dc signals occurs, variation of AVM amplitude can contribute to the net modulation effects. Figure 4 shows the relation between output voltage ratio (AVM signals/dc signals) and the amplitude of ac voltage. The output voltage ratio rises up to the amplitude of ca. 40 VM, indicating that the maximum modulation effect is obtained at more than 40 Vpsak. Improvement of Signal-to-Noise Ratio. It is difficult to detect emission signals with a usual amplification method as the supplied power decreases. With the AVM technique, the modulated components can be discriminated from the total emissions. As shown in Figure 5(2), an AVM spectrum was recorded over the wavelength range from 340 to 290 nm when a copper cathode was sputtered under very low power
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 14,DECEMBER 1984
I I ) DC.
0
20
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30 40 AMPLITUDE
50
60 Pooh VolWe
Flgure 4. Relation between the amplitude of the ac component and
the output slgnal ratio (AVM/dc) at Cu I 327.4nm: the dc lamp current gradually increases with the amplltude, that is, from 7.6 to 10.4mA; dc bias voltage, 365 V; modulation frequency, 130 Hz.
I
I
WAVELENGTH Flgure 6. Dc spectrum (1) and AVM spectrum (2)of copper emission lines and OH emission bands measured in the range of wavelength from 330 to 300 nm: dc bias voltage, 310 V; dc lamp current, 5.6 mA; modulation frequency, 130 Hz; amplitude of the ac component, f30 V.
I
I
WAVELENGTH
Flgure 5. AVM spectrum (2)and dc spectrum (1) of Cu I327.4and 324.7nm emission lines: dc bias voltage, 280 V; dc lamp current, 1.4 mA; modulation frequency, 120 Hz; amplitude of the ac component, f30 V.
discharge conditions (280 V dc, 400 mW). A dc spectrum was also monitored under the same conditions (Figure 5(1)). While two copper emission lines (Cu 1324.7 and 327.4 nm) clearly appear on the AVM spectrum, these intensities are comparable to the level of the background fluctuations on the dc spectrum (see X5 expansion chart in Figure 5). A series of emission lines which exists over the range of wavelengths from 320 to 300 nm is assigned to a band spectrum of OH radicals and another series in the range of 339-332 nm to N2 or NH radicals (17). Although these radicals are created from minor impurities in the lamp, the band spectra are easily emitted even when input power is very weak. By use of the AVM method, the signal-to-noise ratios for these Cu lines were improved by a factor of 20-50. Relative Intensity of Emission Lines from Sputtered Elements. As already described in the Theoretical Section, it is expected that the parameters for creating the modulation (e.g., the amount of modulated species) may differ for the kind of emitting species, for example, gaseous radicals, Ar atoms, Ar ions, or sputtered particles. The intensity ratio of the Cu emission lines was calculated to the most intense band head (308.9 nm) of OH radicals. When the glow lamp is operated at 310 V dc (1.7 W), sufficient intensity for the Cu lines can be obtained even by the dc
~~
WAVELENGTH
Flgure 7. AVM spectrum (1)and CM spectrum (2)of Cu I and Ar I 1 emisslon lines In the wavelength range 300-280 nm. The intensity difference spectrum (3) Is computed from (AVM - CM) with a data processor: dc bias voltage, 380 V; dc lamp current, 10.0mA; voltage modulation frequency, 140 Hz; amplitude of the ac component, f30 V; chopper frequency, 44 Hz.
method, as shown in Figure 6(1). Figure 6 shows that the intensity ratio (Cu I 327.4/0H 308.9 nm) is about 5.0 for the AVM detection and 1.4 for the dc detection, respectively. By use of the dc intensity ratio as a relative intensity scale between emission lines, it is found that the Cu I 327.4 nm emission line is modulated more strongly than OH 308.9 nm with AVM detection; that is, while the emission lines of sputtered particles are sensitive to the periodic variations in both the atomic excitation and sputtering conditions, those of OH radicals are insensitive to the sputtering conditions, as suggested by eq 1-3. Another type of light modulation can be realized with a rotatory chopper as illustrated in Figure 2. With the chopper modulation (CM) used, the signal-to-noise ratios increases by a factor of more than 10 compared to the dc method; however, in contrast with the AVM method, the relative intensities
Anal. Chem. 1984,56,2735-2740
2735
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, 3 1 8 , 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
