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Anal, Chern. 1982, 5 4 , 1229-1231
Nitrogen Dioxide Source. Poly(tetrafluoroethy1ene) tubing of various lengths were filled with liquid nitrogen dioxide and used as permeation tubes to provide low concentrations of nitrogen dioxide. By the use of five different permeation lengths, alone or in combination, and by varying the flow rate of the diluting air stream, desired concentrations of nitrogen dioxide were obtained. Exposure of Reagent Papers. Reagent papers were exposed to (1)various concentrations of ozone at a flow rate of 2100 mL m i d for 300 (Figure 2) and 1000 s (Figure 3), (2) 0.88 ppm of ozone at a flow rate of 2100 mL m i d for varying periods of time (Figure 4), (3) various concentrations of nitrogen dioxide for 500 s at a flow rate of 1460 mL min-' (Figure 5), and (4) a constant 2.85 ppm concentration of nitrogen dioxide at a flow rate of 2100 mL m i d for varying periods of time (Figure 6). Reflectance measurements were made at 540 nm on all samples.
RESULTS AND DISCUSSION
L
Oo
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460
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I200
All the calibration curves were smooth curves. For narrow concentration ranges, segments of the curves could be considered linear; however, direct comparisons of reflectance measurements to the calibration curves would provide a wider response range with minimal error. The reagent papers are slightly more sensitive to ozone than to nitrogen dioxide. The nonlinear response is probably due, at least in part, to an in-depth formation of colored diphenylcarbazone complex, wherein the colored surface layer partially obscures the deeper colored layers. Comparison of samples to a series of permanent color standards for semiquantitative determinations would be the most likely mode of use. The principal application of the reagent papers would be an economical, preliminary way t o monitor potentially harmful concentrations of atmospheric oxidants prior to precise determination by other methods.
Flgure 6. Color response at 540 nm of reagent papers to 2.85 ppm of nitrogen dioxide with various exposure times. Average and range of two determlnations at each exposure Interval are shown. Flow rate of air stream was 2100 r n min-'. ~
energy was read as absorbance in the regular manner. Exposure Vessel. A Kimble low form, cap-style 50 mm inside diameter weighing bottle was modified by sealing 5 mm inside diameter tubing through the bottom as shown in Figure 1. When used in the inverted position, the 55/12 '$ joint cap provides ready access for inserting or removing the reagent paper circle. Insofar as possible, all tubing that carried reactive gas mixtures was borosilicate glass or poly(tetrafluoroethy1ene) tubing. Short lengths of poly(viny1chloride) tubing may be used. Glass-toglass connections were made with Fisher and Porter Teflon seals with O-rings. Ozone Generator. A McMillan Model 1000 ozone generator (ColumbiaScientific Industries Corp., Austin, TX) was calibrated iodimetridy and used to provide known concentrations of ozone.
LITERATURE CITED (1) Lambert, J. L.; Beyad, M. H.; Paukstelis, J. V.; Chiang, Y. C. Anal. Lett. 1981, 14, 863. (2) Wendlandt, W. W.; Hecht, H. G. "Reflectance Spectroscopy"; Intersclence: New York, 1966 p 124.
RECEIVED for review January 4,1982. Accepted March 1,1982. This research was supported in part by National Science Foundation Grant No. CHE-7915217 and by an associated NSF Grant No. SPI-8013291 to Yuan C. Chiang.
Primary Electron Beam Current Regulator for Pulse-Counting Auger Electron Spectroscopy W. S. Woodwtrrd, Dieter P. Griffls, and Richard W. Linton" Department of Chemlstry, University of North Carolina, Chapel Hill, North Carolina 27514
A number of commercial surface analysis spectrometers are in use which accommodate both X-ray photoelectron spectroscopy (XPS) and electron-excited Auger electron spectroscopy (AES). Data acquisition hardware often includes, therefore, both pulse-counting electronics for XPS experiments and a bck-in amplifier for detection of modulated signals in AES (1). Direct hardware differentiation of Auger spectra (d.N(E)/(L?C)is utilized to discriminate against the large background of iacattered primary electrons resulting from relatively large primary beam currents. Alternatively, the pulse-counting electronics may be used to obtain low dose electron-excited Auger spectra directly in the N ( E ) mode. General advantages of pulse counting include the extension of AES to important classes of materials which undergo significant electricdl charging or chemical alteration (e.g., electron stimulated desorption or adsorption, oxidation, reduction, 0003-27Q0/82/0354-1229$0 1.25/0
polymerization, field-enhanced diffusion, etc.) at higher primary electron beam fluxes (2). Further, N ( E ) spectra enable quantitative Auger yield measurements via single integration of line shapes. Although Auger line shapes may be quite sensitive to chemical state (3),total Auger electron yields may be relatively unaffected ( 4 ) . Although subject to problems in background removal, integration of N ( E )spectra to obtain intensity inforrnation may provide a more accurate measure of elemental concentrations than the customary utilization of peak-to-peak heights in dN(E)/dE spectra ( 4 ) . Critical dose8 for detectable electron beam damage in many samples (oxides, insulators, organic films) are on the order of to C/cm2 (2). This corresponds to less than a 10-s irradiation with a 0.3 mm diameter, 1pA electron beam (2). The optimal utilization of pulse-counting AES for beam sensitive materials requires, therefore, stable primary electron @ 1982 Amerlcan Chemical Society
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BIAS RENRN
ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982
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Flgure 1. Schematic diagram of beam current regulator for electron beam source used in AES.
beam currents in the low nanoampere range with corresponding low current densities to minimize beam damage effects and charging problems often encountered in higher current density, derivative mode AES experiments. Instruments equipped primarily for derivative mode AES may lack adequate beam regulation a t the low currents necessary for pulse-counting, low dose AES. This paper describes the modification of the control circuit in a widely used commercial AES electron gun source (1)to provide enhanced capability for pulse-counting AES via improved regulation of primary electron beam current.
EXPERIMENTAL SECTION The primary elements of the Perkin-Elmer Physical Electronics Industries electron source (Model 11-010) are the thermionic filament and emission control cup. Electrons emitted by the f i i e n t are extracted by the emission control voltage maintained between cup and filament. About 97% of the emitted electrons impinge upon the cup; the remainder pass through the source aperture and comprise the primary electron beam. Actual circuitry devised to implement beam regulation based upon emission cup current is shown in Figure 1. Three FET-input operational amplifiers and an ensemble of other integrated and discrete components combine to measure, monitor, and compare cup current (and therefore total filament emission) to an operator-selected value in the range of 10 mA to 1 nA full scale. The result of the setpoint comparison is used to generate a correction signal which is applied to existing filament heating circuits to achieve the desired beam stabilization. A detailed analysis of circuit operation follows. Conversion of the cup current to 0-10 V internal signal is the duty of amplifier A1 and the associated components. Conversion gain is selectable over 7 decades. Available full scale factors range in decade steps from 10 mA to 10 nA. Some measure of protection against the high energy arcs that generally occur in high voltage circuitry is provided by antiparallel IN4004 diodes. Amplifier A2 performs a continuous comparison of the output of A1 to the setting of the beam current potentiometer. This setting is available as a voltage in the range 0-10 V due to the connection of the potentiometer to an AD581 precision reference. The output of A2 slews negative whenever the output of A1 is greater than the setpoint denoting excessive emission. Should the output of A1 be less than the setpoint, insufficient emission is indicated and A2 slews positive. The output of A2 is applied to control transistor 40410 so that negative excursions there result in an increase in the current sourced into a summing point internal to the filament heating circuit. The effect is to reduce the heating current supplied with a resulting cooling of the filament and a reduction in emission. Conversely, a positive excursion by A2 decreases the summing point current and results in a warmer, more emissive filament. Thus, emission stabilization is achieved. This control arrangement has the property that the filament heating current can never rise beyond the filament setting of the filament heating circuit. This serves as a safety measure as it
prevents overheating of the filament in the event of regulator failure, absence of extraction voltage, or any of a number of other causes for misreading emission levels. Amplifier A3 serves as a voltage-to-current converter to drive the vendor-supplied emission panel meter. This provides a convenient monitor of gun operation familiar to instrument users trained prior to the installation of regulator modifications. The design of the 15-V regulated power supply is straightforward and merits little discussion beyond mention of the requirement for high voltage interwinding insulation placed upon the power transformer used. The regulator circuit is electrically "local" to the gun source and therefore sits atop the 5-kV accelerating potential. This dictates such precautions as insulating sections in the control shafts of the amps full scale switch and beam current potentiometer and the aforementioned transformer insulation rating. Auger spectra were acquired with a Perkin-Elmer Physical Electronics Industries Model 548 XPS/AES spectrometer ( I ) operated in the pulse-count mode (Princeton Applied Research Model 1105 photon counter). An electron beam energy of 5 keV was used for all Auger spectra, but the electron beam intensity was varied as described in the text. The electron beam was defocused to a circular area of 0.02 cm2and the current measured by placing a +200 V bias on the sample probe. Compounds including NazS03(B and A, Certified ACS 98.5%) were used to test the performance of the electron beam current regulator. Previous studies (5) suggested that Na2S03was very susceptible to electron beam damage.
RESULTS AND DISCUSSION In the vendor-supplied electron gun control circuit, beam current regulation was attempted through the very indirect method of regulation of the filament heating current. It was hoped that by maintenance of a constant heating current a constant filament power dissipation would result. This was to ensure a constant filament temperature which would, in turn, stabilize filament emission. Actual operating experience, however, indicated that none of the assumptions apparently embodied in the original regulator design was particularly valid. Before installation of the beam regulator modifications described herein, it was routine to observe upward drift of filament emission for long periods after turning on the gun. This drift, while serious for all operating conditions, was particularly severe when experiments were attempted at relatively low emission levels. The chief source of this drift apparently was a gradual rise in temperature of the filament resulting from the slow warming of the gun structures during operation. Our speculation was, thus, that heat transferred from the filament to adjacent structures caused a general rise in the temperature of these structures with a resulting reduction in the rate of heat loss of the filament. With heat loss reduced, the constant heating current maintained in the filament would have caused a rise of filament temperature with an inevitable increase in thermionic emission. This reasoning was supported by the observed aggravation of drift when running at low emission (and consequently low filament temperature) levels. At low filament temperatures, the chief mechanism for the transport of heat from the filament would be conduction, as contrasted with radiation which would tend to dominate a t higher temperatures. The rate of heat loss due to conduction is essentially proportional to the first power of temperature, while that due to radiation is dependent upon the fourth power of temperature. Thus, the temperature rise and associated increase in electron emission needed to offset a given reduction in rate of heat loss resulting from a warming of filament environs would be larger in a cool, conductiondominated operating regime than in a hot, radiation-dominated one. The consequence of this reasoning was that an acceptable level of beam stability could be expected only if the emission control circuitry were modified to provide more accurate
1231
Anal. Ghem. 1082,54. 1231-1233
Flgure 2. Pulse count Auger spectra for Na,SO, as a function of 5 keV primary electron beam current: (A) 10 nA, (B) 200 nA.
regulation of source current. It was quickly decided that regulation should be achieved via manipulation of filament heating current. The alternative of control by variation of extraction voltage was rejected because of concern about possible effects of changes in intragun potentials upon beam focus. A feedback parameter for monitoring of filament temperature, total emission, or actual beam current remained to be selected. Direct measurement and control of filament temperature was strongly considered due to prior experience of one of the authors with this technique in field-desorption sources for mass spectrometry (6). Rejection of this method was based primarily upon the extent of modification to existing filament heating circuitry that i t would have required. Monitoring of actual beam current was likewise rejected as an unfeasible technique. Measurement of the beam current would have required electrically floating the high voltage acceleration power supply in the gun control chassis. Achieving isolation of this complex, power-consuming circuitry adequate to permit the accurate measurement of beam currents as low as I1 nA seemed unacceptably difficult. Total emission, as approximated by the current drawn by the emission control cup, seemed the most readily available estimator of beam current and was, therefore, the feedback parameter chosen for regulator control. Isolation of the cup
for the purpose of current measurement was already available to that instrument modification in the area of source optics was unnecessary. The fact that cup current was about 30 times greater than beam current promised to simplify needed measurement circuitry (currents on the order of tens to thousands of nanoamperes rather than picoamperes were involved) and to reduce sensitivity to a host of such error sources as cable leakage. The utility of the beam current regulator for pulse-counting AES is demonstrated by the results for Na2S03 The low beam currents available with the current regulator permitted acquisition of the Na2S03spectrum (Figure 2A). Higher beam currents precluded pulse-counting experiments as the result of severe electrical charging effects (Figure 2B) which were also encountered using beam currents (1MA)sufficient for direct derivative mode data acquisition. The lowest electron beam doses required for pulse-counting AES (2 X C/cm2) did not produce any detectable beam damage to the Na2S03 surface. However, higher electron beam doses (>4 X C/cm2) resulted in the desorption of volatile surface hydrocarbons and increasing surface sulfate formation as observed in correlative XPS measurements (7). In summary, the primary electron beam current regulator permits pulse count Auger spectra to be acquired using beam currents down to the low nanoampere range, thus minimizing the electron beam dose and current density required for electron-excited AES. Improved beam stability also should enhance the precision of integral methods for Auger quantitation at low beam currents or conventional derivative methods when relatively high beam currents can be used.
LITERATURE CITED Paimberg, P. W. J. Vac. Sci. Technol. 1075, 12, 379-384. Pantono, C. G.; Madey, T. E. Appl. Surf. Sci. 1081, 7 , 115-141. Madden, H. ti. J. Vac. Sci. Technol. 1081, 18, 677-689. Sickafus, E. N. Surf. Sci. 1080, 100, 529-540. Turner, N. H.; Murday, J. S.; Ramaker, D. E. Anal. Chern. 1080, 5 2 , 84-92. (6) Fraley, D. F.; Woodward, W. S.; Bursey, M. M. Anal. Chern. 1080, 52, 2290-2293. (7) Griffis, D. P.; Llnton, R. W., unpublished results, University of North Carolina-Chapel tiiii, 1961.
RECEIVED for review November 18,1981. Accepted February 18,1982. The authors wish to thank the Hercules Chemical Co. (Wilmington, DE) for a grant-in-aid in support of this research. The XPS/AES facility was funded in part by the National Science Foundation (Chemical Instrumentation) and the North Carolina Board of Science and Technology.
Amperometric Titrations with Hydrodynamic Modulation for End Point Detection Joseph Wang” and Bassam A. Frelha Department of Chwnistry, New Mexico State University, Las Cruces, New Mexico 88003
One of the most extensive applications of electrochemistry has been for end point detection in titrations. Amperometry is particularly useful as a method for detection in titrations of species at the milli- and submillimolar concentration levels. In its most common form amperometric titration consists of adjusting the potential of a microelectrode, usually the dropping mercury electrode, to be in the limiting current region for the species being monitored (analyte, product, or titrant). Sophisticated and sensitive electroanalytical techniques, such as ac polarography (I), differential pulse polarography (21, or anodic stripping voltammetry (3), have been incorporated with mercury electrodes to lower the detectability 0003-2700/82/0354-123 1$01.25/0
of amperometric titrations to the submicromolar concentration level. However, as applied to analyses of many important oxidizable substances, which cannot be determined a t the mercury electrode (due to its limited anodic potential range), the titrations are performed with solid electrodes (usually platinum) operated in the dc mode, resulting with detectability of only around the 1.O4 M concentration level (4). The purpose of the following work is to examine the feasibility of combining amperometric titrations with solid electrode end point detection based on hydrodynamic modulation voltammetry (HMV). HMV is proving to be a versatile technique for trace analysis at solid electrodes (5, 6). The 0 1982 American Chemical Society
