Anal. Chem. 1983, 55, 57-64
57
Spatial Discrimination in Spark Emission Spectrochemical Analysis John P. Walters"' and Wllllam S. Eaton* Department of Chemistry, University of Wisconsin, Madison, Wiscons;in 53706
An adjustable wave forim spark source and argon flow jet are used to produce a positionally stable spark traln. The Image of the spark is relayed to an lntermedlate focal plane containing masks of varying dlameters. Background, plasma, and ionized electrode emlsrrlons are mlnlmlzed by positioning the mask In front of the central core of tho discharge, leavlng simpler spectra wlth less nolse to enter the spectrometer. Three- to ten-fold Improvements In slgnal/nolse are reported for common lmpuritles in commercial alumlnum alloys. I t Is shown that lost signal due to masklng can be recovered by rotatlng the dlsk samplle and increasingthe repetltlon rate of the source.
The signal and background levels in a positionally stabilized spark discharge can be decoupled. The discharge displays radial structure, with imany of the common analytical lines emitting in its radial wings (1). Much of the background and its noise is, however, confined more to the central core, along and coaxial with the inkrelectrode axis. If the light observed from the discharge is restricted to that originating in the outer radial regions, the spectra are simpler, the lines narrower, and the signal to noise ratiios higher than observed if all of the radiation is detected. There is additional information in the time domain ( 2 , 3 ) . The analytical signals tend to maximize when the time derivative of the discharge current is negative, while background and plasma ion noise signals tend to maximize when it is positive. Example spectroscopic data showing these characteristics of a positionally stable spark have been published, along with evidence and ideas on their physical causes ( 4 ) . When either spatial or temporal discrimination is used in observing the radiation emitted from a positionally stable spark train, some usable signal is lost. This occurs mainly because the separation in time or space between signal and noise is not abrupt, but rather continuously melds from region to region (5). This need not be a fatal flaw to the discrimination technique if the losses can be compensated by a real increase in the amount of useful signal generated by the spark. We have shown this to occur when the repetition rate of the spark source is increased (6) and the early times in the burn monitored (7). Thus, by combination of higher repetition rates with techniques that use more of the electrode surface to give the equivalent of observation early during a burn, spatial or temporal discriminatijon will show real improvements as mentioned, without significant analytical system losses. To illustrate the discrimination technique, experiments were conducted with Conservative equipment and straightforward procedures. These methods obviously would not be used in a production situation. For example, for exploration of all effects of parameter changes knowledgeably, data were recorded photographically, with darkroom procedures suffiPresent address: Department of Chemistry, St. Olaf College, Northfield, MN 55057. Present address: Rockwell International, P.O. Box 27930, Denver, CO 80227. 0003-2700/83/0355-0057$01.50/0
ciently standardized to allow day-to-day reproducibility on the order of 5 % or less. To produce the discharge train and cover a wide range of repetition rates, we modified an airinterrupter spark source (8) to produce a pulsating, unidirectional current wave form. To stabilize the discharge positionally, we directed argon coaxially with the interelectrode axis, from anode to cathode, using a simple nozzle (9-11). T o capitalize on the increased sampling efficiency that occurs during the first portions of a burn with a positionally stable spark discharge (7), without actually time-resolving the photographically-detected spectra, we rotated the aluminum cathode at -4 rpm during the exposure. This provided a continually refreshed electrode surface, giving the same performance spectroscopically as is observed during the first few seconds of a burn to a stationary electrode attacked by a positionally stable spark. This also allowed a primitive averaging of the radial inhomogenieties in the electrode (12-14). Then a modest adjustment in the repetition rate was done to accent the electrode signal over the background, again due to enhancements in the sampling efficiency (6, 7). Spectral lines for analysis were selected from temporally and radially resolved spectra, obtained from instrumentation that allowed high-fidelity acquisition of such multidimensional spectra (15). All analytical data were acquired in a time-integrated mode. To verify that the line intensities detected in this mode were responsive to the spark source parameters (16),we measured the time integral of the current wave form as a function of changes in the source inductance. Then, the line intensities for selected test lines from the cathodic electrode were also measured as a function of increasing source inductance. Correlation between these data was used to signal proper correspondence between the spark source and electrode sampling. In all, the spark source, while conservative in comparison to present computer-controlled electronic units (17),was well behaved. In this paper, the above introductory remarks will be amplified with experimental parameters. Then data taken with aluminum electrodes sparked in argon will be presented to illustrate the points made. We will also show spectra and microscope data that address the issue of increased current efficiency relative to electrode sampling. Following this, we will show the improvement in spectral simplification and in signal to noise ratios that result from optically masking the central core of the discharge.
EXPERIMENTAL SECTION The apparatus consisted of a modified (8) air-interrupter spark source, a conventional arc-spark stand containing an electrode rotator assembly, a quartz transfer lens, a concave grating eagle spectrograph, conventional darkroom equipment, a recording densitometer, and an assortment of conventional electronics and electrode preparation devices. A complete set of experimental parameters for the above is given in Table I. Electrodes were prepared by machining disks of aluminum to the proper circular shape and then drilling and tapping a small hole in their center. One surface was then turned to a smooth finish. The disk was then mounted on a rotating arbor in the spark stand as shown in Figure 1. The disk was cathodic with respect to the tungsten pin counterelectrode. The counterelectrodewas 0 1982 American Chemical Society
58
ANALYTICAL CHEMISTRY,
VOL. 55.
NO. 1. JANUARY 1983
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Table 1. Experimental Parameters source: National Spectrographic Laboratories. KE-1234, modified as per ref 8 capacitance 0.012uF 10 spark power resistance 0 added 56 p H inductance, L1 inductance, L2 variable, 5-40 p H oscilloscope Tektronix 54711A1 Pearson, Type 110, current current monitor transformer into 1 M n spark stand: Spex Industries, Inc., No, 9010 arclspark stand electrode gap 3 m m typ cathode electrode aluminum flat (see Figure 1 ) anode electrode 3% thoriated tungsten (see Figure) gas argon flow rate 0.6 Llmin cathode rotation 4 revlmin spectrograph: Baird-Atomic RDRS Mod. 6X-1, 3.0 m Eagle Mount slit width 0.050 m m spectralplate 28, with 2.5 m m plate aperture grating angle 3" grating mask 1.8 em height wavelength range 2400-5100 A dispersion 5.5 Almm, first order emulsion: Kodak, type SA-1, 4 x 10 glass plates development Kodak, D-19, 4 min stop Kodak stop, 30 s fix Kodak fix, 10 min wash deionized water, 30 min Drocessor Jarrell-Ash Model 34-301 calibration Baird-Atomic seven-step sector, 3-A DC iron arc densitometer Baird-Atomic, recording, Model RC-2, operated as per slit height 1 m m slit width 0.007 m m scan rate 0.0055 mmls
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Figure 2. Current wave fwms produced fromthe source described h ref 8 urder the following parameters. L1 = 56 pH a L2 as per: (A) residual. (6) 0.1 pH, (C) 2.2 MH.(D) 8.0 pH, (E) 10 pH. (F) 88 pH.
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Flgura 3. Coulombs per spark fw current wave forms varying from ruily osdlatary (len of ikw, A-A) to rub midirmomi(*I 01 he A-A). For the osclllalcfy dlscharges the coulombs apply only to half cycles where the aluminum electrode in Figure 1 was cathodic.
centered in the jet with three screws at 12O0 separations, such that it could be replaced periodically. With this configuration and the air-interrupter spark source, the discharge was sufficiently stable to produce a burn between 2 and 3 m m in diameter at electrode separations between 2 and 4 mm. The gas flow was adjusted about a nominal value of 0.5 L m i d to produce optimum stability. The spark source parametera were fixed, except for the wave-shaping inductance L2 (18). This was adjusted to produce the wave forms shown in Figure 2. The current was measured with a Pearson Electronics Model 110 current transformer into a Tektronix type 547/1A1 oscilloscope/plug-in combination. These wave forms were selected as working standards to be used for all measurements. They allow the range of charge shown in Figure 3 to be delivered to the aluminum electrode. The aluminum and selected impurity lines that were chosen to monitor signal to background changes with respect to spatial discrimination are indicated in Figure 4. It is evident that they are all responsive to the changes in current wave form in reasonable proportion to the changes in the number of coulombs delivered to the electrode (IS21).
Flpm 1. oectmde cunReUa&m and nunhal experknemal parametfor producing stable spark dlscharges.
mounted to a jet assembly. through which argon was delivered coaxially with the tungsten pin. The pin was held in a collet and
Optical discrimination is accomplished hy placing one or another of a series of copper wires at Sirks focus in front of and parallel to the spectrograph slit and then focusing an image of the positionally stable spark channel on the wire using a 20-cm focal length biconvex quartz lens. Corrections for chromatic aberration were not made (22). The experiment as executed is shown schematically in inset A of Figure 5. A wire having a preselected diameter between 0.5 and 2.5 m m is placed between the spectrograph entrance slit and the spark gap, in line with the spark and to one side of the optical axis. The electrode is started rotating and the spark started. After five to
ANALWICAL CHEMISTRY. VOL. 55, NO. 1. JANUARY 1983
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the same changes in SOUICB inductance L2 as reputed in Figure 3. Matrix and sample impurity lines in Insets A. 8. and D respond to the mulanbs ddivwed to llm akmhnm &e@&% h & g d .AI 111. and Ar 111 lines do not (Insets B and C). &,
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F l ~ w e5. Iikrsnation 01 llm maskhg experlnmt (inset A) and me changes in detected that resun from dmerent mask posnions. Spectral simplification for two mask posnlons is shown in inset B. Backqound reducmn for Wee mask wstims is show h hset C. The current wave form is D in Figure 2
six full revolutions, the emission from the spark has stabilized (see Figure 6 ) . The shutter to the spectrograph is then opened and an exposure taken. The spectrograph camera is then racked to a new position, and the identical run repeated with a new electrode mounted in the stand, After each pair of exposures, the mask is shifted in position by at least one diameter closer to the optical axis, and the same complete exposure routine is repeated. A set of exposures usually is taken, corresponding to moving the mask completely through coincidence with the optical axis. Spectra similar to those in Figure 5 result. where each two positions on the plate correspond to duplicate rum at one mask position. A few lines are identified in Figure 5 to illustrate the more obvious spectral simplification that results when the mask has been moved to a position directly coincident with the spark channel. Proper darkroom and densitometer procedures are essential, since each masking experiment may require as many as a separate set of plates per parameter change. A Jarrell-Ash 34-300 photoprocessor was used to develop the plates according to the pa-
* 59
.. Figure 6. Nonmasked spectra observed as the aluminum camode Yown in F W e 1 rotated under Vw pohitlonaiiy stable spark. Masking experiments are begun after the hird or fourlh rotation. See ref 7.
rameters in Table I. After being washed. each plate was carefully wiped with photographic sponges dipped in Kcdak PhotoFlo 200
wetting solution, and dried under gentle heat to a spot-free condition. For quantitation, a conventional seven-step sector method of emulsion calibration (23)was used to convert T to relative intensity. Each development was done on two plates at the same time, and each pair of plates was calibrated before quantitation.
CALIBRATION Two pmta of the experiment were calibrated before spatial discrimination work was begun. The first part was to verify for the matrix lines of aluminum that there was a correspondence between the total amount of charge delivered to the electrode and the integrated line intensities, both in terms of the number of coulomhs per spark (Le., the area of the current wave form) and in terms of the number of sparks per second per exposure (i.e., the repetition rate). This is necessary to allow normalization of spectra that have different degrees of spatial discrimination, so that when a particular signal to noise ratio is observed, it also is known what fraction of the total intensity changes that have occurred can be mmpensated via spark source adjustment. The spark source was calibrated by photographing the current wave forms of interest, graphically measuring their area, and then plotting that area against the relative value of the inductance L2 that was used to produce the different wave forms. When the discharge was oscillatory, only the area of the half cycles when the aluminum electrode was cathodic was measured. Then the same changes in inductor L2 were used to prepare aeries of time integrated spectra. Linea were identified in these spectra that were expected to be useful in the discrimination studies as indexes of the amount of 'signal" present, Le., the analytical signal of choice due to the electrode material 88 opposed to the plasma lines and continuous background. The intensities of these lines were measured and plotted as a function of the m e relative value of the inductor L2. The source data are shown in Figure 3. The abscissa is not linear, and should be interpreted only in terms of the data shown in Figure 4, where relative line intensities over background are plotted against the m e set of source inductances. Data to the left of the vertical dotted lines in both figures correspond to an oscillatory discharge, while data to the right correspond to a pulsating unidirectional current wave form. Correlation plots were not made of the line intensities directly against number of coulomhs because the discontinuity that occurs when the wave form changes from oscillatory to unidirectional changes the physical nature of the discharge as well as the current wave form (18). The data in Figure 4 for the unidirectional current wave forms indicate that most A I I line intensities respond to in-
80
ANALYTICAL CHEMISTRY. VOL. 55. NO. 1. JANUARY 1983
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!=lpm7. Maosmplc comparison 01 hmedalunhun camode sufam erosbn by the spark cwrenl wave forms shown when the electrode is rotated during an exposure and held stlll. See
also ref 7
creases in the area of the current wave form (e.g., inset A), as do selected AI 11, Ti 11, and Cu I lines (insets B and D). Background does not, a t least to the same degree as the above lines, nor do the selected set of AI and Ar I11 lines shown in inset C. We conclude from these data that any signal lost in a discrimination experiment could at least he partially recovered by increasing the number of coulombs delivered to the sample electrode via adjustments in the source waveshaping inductor L2. Because there were cases when it was not desirable to change the current wave form, a second calibration experiment was done on the spark source to determine how time integrated line intensities could be increased by adjustment in spark source repetition rate. Because the literature precedent is against this as an effective parameter for such adjustments (24,25), we report here our fmt investigations on microscopic electrode erosion pattern produced by different current wave forms. The data in Figure 7 show differences in electrode erosion that result on an aluminum cathode held stationary under a positionally stable spark compared to one that is rotated a t approximately 4 rpm for three different wave forms. The "puddled" appearance of the stationary electrode is clear, as is its traceability to a lack of electrode rotation, rather than a unique current wave form (26). We have shown that when an electrode has this "puddled" appearance (7),the emitted light is both noisier and of lower intensity than when the electrode surface erosion has a more granular appearance. Further, we have shown (6) that under the conditions that produce the granular erosion, the spark source repetition rate is effective as a parameter that affects time integrated line intensities. These independent observations are verified here by comparing the effect of repetition rate on the intensities of selected lines for stationary and rotating electrodes. The data in Figure 8 show the effect of source repetition rate on line intensities over hackground for a stationary electrode. It is evident that the main effect of increasing the repetition rate in this case is to increase the intensity of the spectral noise, i.e., the Ar 111, Ar 11, and background radiation. The matrix and analytical impurity lines respond only sluggishly if at all to the increased repetition rate. However, as shown in Figure 9, the situation is much different when the electrode is rotated. In that caw, the analytical line intensities over background increase with increases in source repetition rate at least as much as the plasma and background ratiation, and in some mea to a greater degree. This represents a real inin available signal. From these data,and our previoua work, we conclude that there has been a real increase in the sampling efficiency.
Fbue 8. Effect01 spark source rspetnbn rate on ~ l e c t e dplasma and aluminum camode eIech& Ines lor a nonrotatmg elechode.
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