Anal. Chem. 1984, 56, 1517-1519
ticeable leaching of the modifier occurred for the CoPC CMEs, and it appears that the CoPC electrodes should have approximately the same solvent compatibility in LCEC as conventional carbon paste electrodes. Without question, the detection and quantitation of hydrazine in a flow injection or LCEC context are greatly facilitated by the use of CoPC-containing CMEs. At this point, the nature of the catalytic processes involved has not yet been elucidated. Also, because high pH is required for the catalysis to proceed optimally, provision must of course be made for the postcolumn addition of strong base to the effluent stream before this approach can be used to form the basis of a practical LCEC procedure. However, in view of the wide range of catalytic activity exhibited by phthalocyanines toward a variety of analytically important species, we anticipate the application of various phthalocyanine-containing CMEs for several systems in addition to the hydrazines. In a wider sense, this work clearly illustrates some of the advantages which can be expected by use of judiciously selected electrocatalytic CME systems in LCEC for the quantitation of high overvoltage analytes. Registry No. CoPc, 3317-67-7; NzH4, 302-01-2; carbon, 7440-44-0.
LITERATURE CITED (1) Kissinger, P. T. Anal. Chem. 1977, 4 9 , 447A-456A.
(2) Synder, L. R.; Kirkland, J. J. “Introduction to Modern Liquid Chromatography”, 2nd ed.; Wiley-Interscience: New York, 1979; pp 153-158.
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(3) Sneii, K. D.; Keenan, A. G. J. Chem. Soc., Chem. Soc. Rev. 1979, 8, 259-282. (4) Murray, R. W. Acc. Chem. Res. 1980, 73, 135-141. (5) Sittampalam, G.; Wilson, G. S. Anal. Chem. 1983, 55, 1608-1610. ( 6 ) Ravichandran, K.; Baldwin, R. P. Anal. Chem. 1983, 55, 1782-1786. (7) Zagal, J.; Fierro, C.; Rozas, R. J. Nectroanal. Chem. 1981, 719, 403-408. (8) Zagal, J.; Monoz, E.; Ureta-Zanartu, S. Electrochim. Acta 1982, 2 7 , 1373-1 377. (9) Green, J. M.; Faulkner, L. R. J. Am. Chem. Soc. 1983, 705, 2950-2955. (10) Ravichandran, K.; Baldwin, R. P. J. Electroanal. Chem. 1981, 726, 293-300. (11) Ravichandran, K.; Baldwin, R. P. Anal. Chem. 1983, 5 5 , 1586-1591. (12) Rollmann, L. D.; Iwamoto, R. T. J. Am. Chem. Soc. 1968, 9 0 , 1455-1463. (13) Shepard, V. R., Jr.; Armstrong, N. R. J. Phys. Chem. 1979, 8 3 , 1268- 1276. (14) Behret, H.; Binder, H.; Sandstede, G.; Scherer, G. G. J. Elecfroanal. Chem. 1981, 777, 29-42.
Kelvin M. Korfhage K. Ravichandran Richard P. Baldwin* Department of Chemistry University of Louisville Louisville, Kentucky 40292 RECEIVED for review October 17, 1983. Resubmitted and accepted March 5, 1984. This work was supported by the University of Louisville Graduate School. I t was presented in part at the 186th National Meeting of the American Chemical Society, Washington, DC, Aug 1983.
Consequences of Light Beam Misalignment in Background Corrected Atomic Absorption Spectrometers Sir: The recently commercialized Smith-Hieftje (S/H) atomic absorption background correction system outputs the difference in absorbance signals measured when a single hollow cathode lamp is first operated to produce a narrow spectral line and then is momentarily run in a mode which both broadens and “self-reverses” the line (I). It utilizes a “double pulse” measurement cycle; i.e., a low current lamp pulse is followed by a high current pulse. These two pulses produce the sample beam (SB) and reference beam (RB), respectively. The author’s alternate “one pulse” approach differs by utilizing the light emitted during the first 10-20 M S of the single, high current, lamp pulse used per cycle as the SB (2). An advantage usually cited by proponents of S/H-based spectrometers is that the use of a single source lamp for both light beams makes optical alignment simple. Yet, the writer’s experience with both types of S / H instrumentation indicates that while optical alignment is indeed “simple”, it is not necessarily “perfect”. The purpose of this correspondence is to identify some of the causes for light beam misalignment in atomic absorption spectrometers and to point out possible consequences. EXPERIMENTAL SECTION The optical components used were those of a standard Varian Techtron Model AA6 atomic absorption spectrometer. Two similar spherical biconvex quartz lenses are used in a symmetrical, linear, configuration. The first is positioned to form a magnified image (1:4/1) of the front of the lamp cathode in the plane of the atomizer; the second lens focuses that image on the entrance slit of the monochromator while reducing its size by the same factor. A piece of stainless steel shim stock with a round 1.2-mm diameter hole drilled through it was used as a light baffle. This baffle was
affixed to the top of a burner head mounted in the standard adjustable burner mount at the focal point situated between the hollow cathode lamp and the monochromator. Moving this baffle vertically through the optical path while monitoring the light throughout permits measurement of the spatial emission profile of the hollow cathode lamp. The electronic circuitry of the single pulse instrument is described in detail in ref 2. The double pulse instrument substantially duplicates the signal measurement protocol of the spectrometer originally described by Smith and Hieftje: the SB is measured during an initial 10 ms long, 10 mA lamp current pulse; the RB is measured during the last 100 ws of a subsequent 220 p s , 220 mA pulse ( I ) . The frequency of data collection cycles is 30 Hz. Detailed circuit diagrams of either instrument are available from the author upon request. Both instruments function by first gating the photomultiplier tube’s (PMT) signal response into separate sample and hold amplifiers at appropriate times during an individual instrument cycle. The stored light signal voltages in each channel (SB and RB) are next summed with appropriate offset-correcting voltages and then input to a balanced differential logarithmic amplifier. The criterion used to null out offset/gain errors is the apparent “absorbance” response of the instrument to a reduction of PMT voltage (about 25%) sufficient to cause a 10-fold reduction in the magnitude of the raw “light” intensity signals in each channel. After careful calibration, the response of both instruments was less than 0.002 absorbance unit to these PMT voltage changes. Signal voltages at appropriate points in the circuitry were measured with the Hewlett-Packard data acquisition system used previously ( 2 ) . Standard hollow cathode lamps from three different manufacturers were used.
RESULTS AND DISCUSSION The relevant criterion for correct alignment in this context is that the imposition of any nonatomic scatterer or absorber 0 1964 Amerlcan Chemlcal Society
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 8,JULY 1984
Table I. Calculated Response of Background Corrected Spectrometers to Vertical Nonatomic Absorbance Gradientsa shape and size of gradient
BAFFLE POSITIOK
Flgure 1. Beam intensities measured vertically across the focal plane of the atomizer: (A) dotted line, lead lamp, low current SB light beam; (6) dashed line, lead lamp, high current RB llght beam; (C) solid line, Varian Techtron D, continuum lamp, normal current. r0.1,-
4 0-.
0
5 10 POSITION mm
Flgure 2. Relative sample and reference light beam Intensities observed as a function of the position of a baffle in the focal plane. The log,, (SWRB) values are normalized to zero at position 5: (A) dotted line, Westinghouse lead lamp, 283.3 nm; (B) dashed line, Instrumentation Laboratories, Inc., lead lamp (new model) 283.3nm; (C) solid line, Varian Techtron copper lamp, 324.7 nm.
(e.g., “smoke”, a molecular gas, a knife edge, etc.) into the light path causes no change in instrument response other than an increased level of AC noise. Both of the S / H instruments at this writer’s disposal fail the knife-edge test. The reason for this is demonstrated in Figures 1 and 2. These figures show the effect of vertical movement of the small light baffle upon light signals measured with the “dual pulse” instrument. Figure 1 gives the raw intensity data observed with both a conventional lead hollow cathode lamp (both signal channels) and a hollow cathode-type Dz continuum lamp (only the low current signal channel). Figure 2 shows plots of the apparent absorbance (log,, (SB/RB)) values observed a t the different baffle positions with three different hollow cathode lamps. Each point on the curves in these figures represents the mean of 200 individual data points collected by the programmable DVM. The data clearly indicate that the spatial distribution of light from conventional hollow cathode lamps changes substantially under the different excitation conditions used to produce the two light beams. Because a single, fixed set of optical components is used with S / H instruments, these emission zone position changes within the lamp necessarily cause imperfect RB/SB light beam coincidence within the atomizer. Qualitatively similar results are observed with the one-pulse instrument which indicates that the position of maximum light emission within the lamp changes significantly during individual high lamp current pulses. A bias will result if the two light beams sample inequivalent zones within the atomizer. The existence of extremely steep free atom concentration gradients for some hard-to-atomize elements in flames has been well documented (3). Holcombe
shape
bottom
L L L L L L E E E
0 1.0 0 1.0 0
1.0 1.0 1.0 1.0
top 1.0 0
1.0 0
1.0 0
0.368 0.135 0.368
furnace diam-
eter,c mm 5 5 3 3 1 1
5 5
3
response, absorbance unit
S/H
KIP
+0.0317 -0.0169 +0.0515 0.0283 0.0066 -0.0052 -0.0133 -0.0165 -0.0210
+0.0595 -0.1157 0.0159 -0.0715 -0.0174 +0.0038 -0.0510 -0.0518 -0.0274
a The intensity vs. position data of Figure 1 was used for Key: L, linear; E, exponenboth types of instrument. tial; numerical value is in absorbance units. The furnace itself was assumed to be the limiting field s t o D .
et al. have measured substantial vertical free atom concentration gradients across a Varian Techtron Model 90 graphite furnace atomizer for a number of elements (4-6). While there has been no systematic study published proving this point, it is reasonable to assume that the radial atomic concentration gradients within a furnace used with a L’vov platform are somewhat steeper than they are in the same atomizer when no platform is used. The reason for this is the formation of a vertical temperature gradient across the light path caused by the presence of the relatively cool platform at the bottom of the graphite tube (7). The abrupt temperature gradient found above open “filament” or “boat” nonflame atomizers is responsible for the steep atom population gradients which have been measured above them (8). The slopes of these gradients depend upon the chemical reactivity of the species in the gas phase which is, in turn, influenced by the nature of the sample matrix. The existence of radial concentration gradients in the polyatomic species responsible for the background absorbance signals seen in furnace AAS work has not been verified-or even studied-to this writer’s knowledge. However, it is reasonable to assume that such gradients do exist and that the shapes of their concentration profiles across the light path will generally not exactly match those of the free analyte atoms. The radial lamp intensity data shown in Figure 1 were used to calculate spectrometer response errors due to the presence of radially nonuniform absorbance gradients. This is done by first numerically integrating the light transmission values across each light beam and then calculating the resultant log,, (RB/SB) value for each of the assumed absorbance gradients. The difference between these values and that obtained with a spatially uniform absorber(s) distribution is the “error”. A program compatible with the Hewlett-Packard 9825 computer was written to facilitate these calculations. Table I lists the calculated responses of two types of “background corrected” atomic absorption instruments when vertical (linear or exponential) nonatomic absorption gradients are formed across furnaces of different diameters. In these examples no analyte (lead in this case) atoms were assumed so the “ideal” response in every case is zero. In this table “ K / P refers to the spectrometer utilizing the continuum lamp type of RB originally suggested by Koirtyohann and Pickett in 1965 (9). The calculations indicate that the magnitude of this type of error signal seen with well-aligned K / P instruments is generally greater than that expected with S / H spectrometers. The relative abilities of the two types of instruments to ignore
Anal. Chem. 1984, 56,1519-1521
A
LAMP
Figure 3. Two ways to spatially homogenize the light beams from the lamp: (A) diffuse-reflectance integrating sphere; (B) “scrambled” op-
tical fiber. spectrally structured background signals differ by a far larger factor. The analytical consequences of these biases are proportional to the ratio of the time integrated error signal to that of the integrated analyte atom signal. Spatially isothermal, tubular, atomizers having lengths which are large relative to their diameters will be least affected because any radial concentration gradients initially formed will disappear in a time which is short relative to the total duration of the absorption signal. On the other hand, open “filament type” atomizers represent a “worst case” scenario. Detection of these biases in actual practice will be quite difficult. Unless the bias is both in a negative direction and large enough relative to the analyte atom signal to produce a negative net “absorbance” response, the bias will generally go undetected during routine analyses of *unknowns”. It should be realized, too, that errors of this sort are not compensated for by using the popular “standard additions” technique. The writer must stress that the values listed in Table I are calculated errors based on hypothetical concentration gradients. Truly realistic estimates of actual analytical biases must wait until quantitative time-resolved data on the radial concentration gradients of all of the light absorbing species formed in real atomizers atomizing real samples becomes available. The optical apparatus developed by Holcombe et al. ( 4 ) would be ideally suited for obtaining the necessary data. This writer’s sole experience with an analytical bias unambiguously attributable to imperfect light beam coincidence was a base line shift that occurred when the light baffle plate of a Varian Techtron Model 90 atomizer workhead had not been perfectly aligned with the atomizer tube. The base line shift occurred during the time that the analyte volatilized and consequently caused difficulties in obtaining accurate signal integrals. The shift was caused by lateral movement of the atomizer tube, the side of which was inadvertently serving as a field stop (because one electrode rod post of this atomizer is fixed while the other “gives” to prevent graphite breakage due to thermal expansion, there is substantial lateral move-
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ment of the tube when the system is heated). In this instance the moving graphite behaved as a radially asymmetric, nonatomic absorber (or in other words, like a knife edge). An analogous situation occurs with Massmann-type furnaces when sampling “probes” are used to create nearly isothermal atomization conditions (IO). Noncoincidence of SB and RB light beams may be caused by optical components other than the source lamp(s). Imperfections in either the quality or alignment of mirrors, lenses, beam combiners, polarizers, etc., may cause significant RB/SB noncoincidence, especially if the components move during an individual instrument cycle. For this reason even Zeeman background corrected instruments may not be absolutely immune to this problem. A spectrometer’s relative sensitivity to errors of this sort may be gaged by slowly passing a knife edge vertically through the light path at the point normally occupied by an atomizer. The writer can suggest several ways to reduce the magnitude of errors traced to noncoincident light beams. The simplest of these is to deliberately not sharply focus the light beams on the center of the atomizer-the usual practice. Doing this has the effect of broadening the beams which reduces the slopes of the emission intensity gradients across the atomizer. Unfortunately, in small-tube graphite furnace applications, doing so also causes a loss in total light throughput, increasing noise. Another solution is to restrict light measurements to only the central portions of the light beams. This can be done by placing a field stop at the atomizer or by simply limiting the height of the entrance slit of the monochromator. However, doing so will also reduce the total light intensity that the spectrometer has to work with. Two other possible fixes require the use of additional optical components at an additional focal point situated between the hollow cathode lamp and the atomizer. They both serve the purpose of rendering the light beams spatially homogeneous. The first approach (Figure 3A) utilizes a diffuse reflectance integrating sphere; the other (Figure 3B) uses a length of “image-scrambling” fiber optic light pipe.
LITERATURE CITED (1) Smith, S. B.; Hieftje, G. M. Appl. Spectrosc. W83, 37, 419. (2) Siemer, D. D. Appl. Spectrosc. 1983, 37, 552. (3) Willard, H. H.; Merritt, L. L.; Dean, J. A. “Instrumental Methods of Analysis”, 5th ed.; D. Van Nostrand: Princeton, NJ, 1974; p 372. (4) Holcombe, J. A.; Rayson, G. D.; Akerlind Spectrochim. Acta, Part 6 1982, 378, 319. (5) Rayson, G. D.; Holcombe, J. A. Spec*rochim. Acta, Part 6 1983, 386,987. (6) Holcombe, J. A.; Rayson, G. D. Prog. Anal. At. Spectrosc. 1983, 6, 225. (7) Siemer, D. D.; Lundberg, E.; Frech, W. Appl. Spectrosc., in press. (8) Anderson, R. G.; Johnson, H. N.; West, T. S. Anal. Chim. Acta 1971, 57, 281. (9) Koirtyohann, S. R.; Plckett, E. E. Anal. Chem. 1965, 37, 601. (10) Manning, D. D.; Slavln, W.; Myers, S. Anal. Chem. 1979, 57, 2375.
Darryl D. Siemer Westinghouse Idaho Nuclear Co., Inc. P.O. Box 4000 Idaho Falls, Idaho 83403
RECEIVED for review January 6,1984. Accepted March 9,1984.
Secondary Ion Mass Spectrometric Analysis of Polymer and Coal Surfaces for Trace Inorganic Elements Sir: Applications of secondary ion mass spectrometry (SIMS) to the study of organic polymeric surfaces have increased rapidly in the past few years. Particular emphasis 0003-2700/84/0356-1519$01.50/0
has understandably been placed on the production and detection of secondary ion molecular fragments, which are characteristic of the polymeric structure. Experience gained 0 1984 American Chemical Society
