A Plane=GratingTime-Resolved Spectrometer for Basic and Analytical Emission Spectrometry J. P. Walters Department of Chemistry, University of Wisconsin, Madison, Wis. 53706 A photographic-photoelectric spectrometer specifically designed for time-resolved detection of emission from transient repetitive discharges is described. By using a synchronously rotated camera mirror in an off-plane Ebert configuration, time-resolved photography of transients as short as 0.2 psecond, and as long as 90 pseconds, over a wavelength range up to 1000 A per exposure, is possible using both conventional spectrometric and Polaroid emulsions. Photoelectric detection is achieved from the same grating through an additional pair of mirrors and an auxiliary system of slits arranged in an in-plane Ebert configuration. Interchangeable optical benches, coupled with a precision sine-bar grating-drive, allow rapid alternation between photographic and photoelectric modes for both exploratory and final analytical use.
MANYEXPERIMENTAL APPROACHES have been used in the past for obtaining time-resolved spectra from transient discharges, particularly those of analytical concern, such as high-voltage sparks ( I ) . The two most popular methods have used gated amplifiers or photomultipliers for photoelectric detection, while a synchronized rotating mirror and prism spectrograph allowed photographic detection. The exploratory value of a time-resolved photographic display is clear, because in few other spectrometric techniques is it more important to have a wide range of lines from a single exposure stored for detailed analysis. However, the usual problem of determining intensities with an emulsion, involving laborious graphical calculations, is greatly magnified for time-resolved work, where literally hundreds of measurements need be made per line. Thus, in any instrument designed specifically for time-resolved work, both photographic and scanning photoelectric display and detection should be incorporated in a manner allowing rapid and reproducible accessibility to either mode. Such instrumentation has not appeared previously. When apparatus is constructed for time-resolved spectrometry, it should be suitable for analytical use also, because the equipment involved proves too costly to duplicate for applied extensions of basic studies. For example, the optical configuration of the spectrograph should allow a wide yavelength coverage with an operating resolution of at least 1 A. Optical benches, excitation stands, focusing optics, and other external devices should be simple and adaptable to a wide variety of sample types, electrode holders, controlled atmosphere devices, and the like. A wide variety of emulsions should be accepted, in addition to high-speed film valuable for exploratory studies. Any electronics necessary to fire the light source, plus any special optics associated with synchronous triggering, shouid be simple and require a minimum of adjustment and calibration. When photoelectric detection is used, integratiou of radiation at specific times after the start of the discharge should be possibie anc convenient, with a time resoiutior; adjustable for the analytical application. A convenient and (I) K. Laqua ane W . Li. Hagenat. “10th Co1:oquiun: Spectrciscopicum Internotionale.” Spart-r Hooks, Washington. D. @, 1963, p. 91.
precise method for scanning the spectrum should be included, with direct, calibrated wavelength readout. Considering only the time-resolved aspects of such instrumentation, a wide time range is desired, covering at least 100 pseconds. Time resolution over this range should be on the order of 0.1 psecond to allow basic studies of short transients and accurate determination of spectral appearance times, as well as complete resolution of high-frequency oscillatory sparks popular in routine analysis. Rapid photographic readout should be implicit, because many more photographs are required for a thorough time-resolved study than are encountered in conventional spectrography. Of prime importance would be the ability to isolate the time-dependent radiation from various regions over the light source with good positional resolution and reproducibility. Finally, the instrument should have a reasonably high photographic speed. Long exposure times are an inconvenience analytically and pose a fundamental limitation for basic studies of those light sources that involve perishable electrodes. When electrode surfaces and gas compositions change during the course of an exposure, the interpretation of time-resolved spectra is not impossible, but is made unnecessarily difficult. Of previous time-resolved spectrometers reported (2-5), none have successfully combined all of the above criteria. The instrument described here was designed to satisfy all of the above criteria for photographic or photoelectric time-integrated or time-resolved spectrometry, with a minimum of awkward or sensitive adjustments. Of the compromises made in its design, none seriously limit its convenient analytical use. PHOTOGRAPHIC DETECTION
The optical configuration of the instrument is shown schematically in Figure 1 and pictorially in Figure 2. Critical operating specifications are given in Table I. The basic principle of the optical configuration has been reported previously by King (6). When operating in the time-resolved mode with photographic detection, the over-and-under or off-plane Ebert configuration is used, with the lower or camera mirror rotating. Choice of this spectrographic configuration was based on its known high-quality focusing properties (7), the lack of stray light resulting from directgrating reflection (8), and the unrestricted optical path to the camera. One important application of the instrument is the study of spark discharges using spark sources that are fired at a multiple or submultiple of the mains frequency. For synchronization of the rotating camera mirror to this (2) R. A. Hill and E. H. Beckner, Appl. O p f . ,3,929 (1964). (31 A H. Gabriel and W. A. Waller, J. Sci. Instr., 40,10 (1963). (4) F. L. Curzon and J. R. Grieg, Ibid,,38, 239 (1961). (5) I-. R. Schwartz and B. J . Pernick, Rev. Sci. lnstr., 33, 765 (1962). ( 6 ) C. W. King, J . Scr. Insrr., 35,11(1958). (7) R. F. Jarrell, J . Opt. SOC.Am., 45,259 (1955). (8) L Wolf. “Progresa n Optics, Voi. 4,” Wiley, New York, 1965, p. 241.
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firing frequency, it is necessary that it be driven by a synchronous motor. The over-and-under optical configuration allows synchronous rotation speeds of the camera mirror with a time dispersion twice that resulting with an in-plane Ebert or side-by-side configuration. Figure 3 shows that this is due to striking the camera mirror at a fixed angle of incidence from above its normal, giving a reflected beam sweeping relative to the incident beam at an angular rate twice the rotational speed of the mirror. Thus a 1-meter optical path may be used for compactness and wide wavelength coverage with sufficient time dispersion to allow short duration studies. Basic Operation. Photography of time-resolved spectra with this instrument requires a triggered light source. The source is triggered when the camera mirror is in a position to focus diffracted light from the grating somewhere near
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Table I. Specifications for the Time-Resolved Speclrometer Spectrograph section Variable, 5 to 1oM microns Vertical slit width Fixed, 100 or 70 microns Horizontal slit width Plane, 102 mm square Grating geometry 600 lines/mm Grating ruling A, first order Grating blaze Parabolic, 1/20 wave Hg green Mirror geometry Mirror diameter 114 mm 1.0 meter Mirror focal length loo0 A, first order, 102-mm span Wavelength coverage 16,4 Almm. first order Wavelength dispersion Wavelength handpass 1 A, first order, 50-micron slita Camera mirror speed Synchronous at 60 rev/second Time range 135 pseconds, 102-mm span Time dispersion 1.33 @econds/mm Time resolution 0.2 #second, 100-micronslitb Monochromator section Entrance-exit slit widths Variable, 5 to loo0 microns, unganged Mirror geometry Parabolic, 1/20 wave Hg green 114 mm Mirror diameter 0.75 meter Mirror focal length 0 to 2:,oo0 A, first order Wavelength range 22.0eA/mm, first order Wavelength dispersion 0.5 A, first order, 15-micron slits Wavelength handpass 1600, 800, 400, 200, 100, 50 &minute Scanning speeds See Table I1 for typical results Scanning accuracy Scanning repeatability 0.1 A Measured with camera mirror rotating, full wavelength span, full time span. b Worst case time resolution, full wavelength span, full time span. ~~
Figure 3. Definition of the optical sweep rate, d y/&, arising in the off-plane Ebert configuration the bottom of the photographic emulsion. Further rotation of the camera mirror streaks the discharge radiation up the emulsion and defines the time axis. With the camera mirror stationary, the height of each spectral line is equal to the height of the horizontal slit placed behind the vertical entrance slit. With a slit height of 0.1 mm, and the camera mirror sweeping out 1.33 pseconds per mm at the emulsion, it will be possible to just resolve emissions separated by 0.13 &second,as the total length of each line is a composite of continuously overlapping images of the horizontal slit. Use of parabolic mirrors minimizes on-axis aberrations and astigmatism (9-11), a situation experimentally v@ied here for first-order wavelength spans up to loo0 A and spectral heights to 0.1 mm. MultipleSweep Photography. If the radiation from every light source investigated were of sufficient intensity at all times during its lifetime to overcome the exposure inertia of the emulsion chosen, only one sweep of the camera mirror would be required to produce a readable photograph. Few ignited light sources meet this requirement. Thus it is necessary to rotate the mirror several times, ignite the light source at precisely the same angular position of the mirror, (9) W. T.We1ford.J. Opt. Soe. Am., 53,766(1963). (10) C. S . Baker, Ihid., 54,271 (!964). (11) A. S . Filler, Ihid.,54,424(1964). VOL 39, NO. 7, JUNE 1967
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assume the radiation from each ignition temporally identical, and superimpose the time-dispersed patterns on the emulsion until some background is apparent a t all times of interest. Use of this multiple-sweep technique allows an exposure to be selected for a particular radiation intensity, enabling a wide range of emulsion contrasts and latitudes to be chosen. However, mechanical restrictions are placed on the instrument that would not exist in a single-sweep application. One significant restriction is the high degree of precision with which thencamera mirror must be spun if wavelength resolution of 1 A is to be accomplished. This is particularly true for this optical system, where a 15,000-lines-per-inch grating is chosen for 1000 A wavelength coverage, and a 1meter-focal-length camera mirror chosen for 100-Hsecond time coverage at synchronous mirror rotation speeds. If only one sweep were required, the mirror would need only rotate in a uniform trajectory to streak our straight spectral lines, not necessarily repeating the same trajectory on the next rotation. The additional mechanical requirements for both repeatable and uniform mirror trajectories are indicated in Figure 4. If a photographic sweep distance of 4 inches is chosen, limiting the time range to a maximum of 133 pseconds, it is necessary that the camera mirror rotate only Z " 5 5 ' to cover the entire time range. However, if the nonrepeatable wobble of the crossed-entrance slit image (th? image dot in Figure 4) is not to exceed the equivalent of 1-A wavelength dispersion (0.001 inch) during the sweep, it is necessary that the total angular wobble at the bearing centers in the mirror drive motor be restricted to less than 0.00009 inch. Further, the nonangular axial play in the mirror shaft must be restricted to less than 0.0005 inch. Neither of the above tolerances car? change or drift during the course of an exposure due to temperature changes in the mirror shaft support or bearing housings. The size of the camera mirror used creates a windage problem, requiring a large motor. A 0.1-hp unit was chosen, To prevent displacement of sensitive grating and collimator mirror positions, the assembly was designed to run with minimal vibration and then finely balanced. The necessity for proper balance is compounded by the fact that the rnirro: is spun about an axis tangent to its reflecting face, and not through its center of mass. The camera mirror was ground to less than diffraction-limited tolerances on quartz blanks, requiring that the housing for the spinning mirror prevent deflections of the blank greater than a few millionths of an inch, while the mirror is rotating. All of the above requirements were met here with success through the adaptation of a precision grinding spindle (disigned and manufactured by the Whitnon Manufacturing Co., Farmington, Conn.) to an integral mirror holder rnrcchined for balance and mirror-deflectior, restreir! Til:. 772 *
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mirror assembly, including an auxiliary triggering mirror to be described below, is shown in Figure 5 . Under slow rotation, the total nonrepeatable camming action at the end of the mirror holder was measured at less than 30 millionths of an inch. Running periods up to 4 hours have failed to reveal significant changes of the system with temperature. The synchronous motor reaches full speed (3600 rpm) from a dead start in less than 1 minute, firmly locking in on the mains frequency. Electromechanical phase adjustments are conveniently made by briefly breaking the driving power and restarting the motor. The entire assembly is now commercially available through the Whitnon Co. To test the image trajectory at the focal plane and detect any deflection of the mirror under full rotation speed, a 3-A iron arc was focused on the spectrograph entrance slit assembly, and the continuous radiation photographed on Polaroid Type 5 5 P/N film with the mirroz rotating. A 50micror. vertical slit was used, giving a 0.8 A calculated bandpass on nonbroadened lines in the first order. The distance between the rotating mirror and the camera was adjusted to provide optimum time focus at two regions separated by 2 inches along the 4-Inch focal surface, a cylinder in the time direction. Densitometer tracings were then taken at the top, bottom, and middle of the 4-inch-long iron lines resulting from a 3-minute exposure. While first order lines separated by siightly Iess than 1 A were visibly resolved, lines separated by 1.5 A were sufficiently resolved for analytica! densitometry aver the fui! time arid wavelength range. \12> J . P. Waltrrs and H. V . Malmstad:, App!. Spcciry , 20, SS (1966).
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Figure 7. First-order timsresolved spectrum for a zinc-graphite, pin-to-pin spark Inset shows magniRed view of the start of the discharge mlh 1-rrseeond time markem indiested
Discharge Triggering. When using multiple-sweep photography, time resolving power depends both on the width of the horizontal entrance slit and the time jitter in the discharge firing point with respect to the angular position of the camera mirror. Precise synchronization of the igniting trigger signal with the mirror position is required. This is accomplished here through the use of a small trigger mirror, shown in Figure 5, attached to the end of the rotating mirror drive shaft. This mirror images the filament of a tungsten lamp on a photodiode as shown in Figure 1, generating an electrical pulse a t a particular camera mirror position. Currently, spectra of spark discharges are being taken, the trigger circuit being used to fire a high-voltage thyratron in a Bardocz-type spark source (12). The electronics used to shape the 35-V trigger pulse from the photodiode to an amplitude sufficient for thyratron triggering are shown in Figure 6. The photodiode is used to trigger a monostable multivibrator through a cathode follower, the 2M)-V multivibrator output then being delivered to the light source in use through a cathode-follower output stage. A siliconcontrolled switch is used as an auxiliary line-phased trigger in the event time-integrated photography with a stationary camera mirror is desired. For firing light sources of lower power, the output of the photodiode may be taken directly from the first cathode follower. By setting the angular relation between the triggering and camera mirrors, the position of the time-resolved spectrum on the emulsion may be adjusted. The fine position is then adjusted by raising or lowering the photodiode or light source, each mounted on a calibrated racking mechanism. In this instrument the need for any delay electronics between the photodiode and discharge generator has been eliminated, greatly reducing the problem of trigger jitter, totally eliminating the problem of slow trigger drift, and greatly improving the time resolution routinely seen. Also, because no delay electronics are required, it is not necessary to use precision electronic components or regulated power supplies, reducing the complexity of the electronics to a trivial level. Once set, the positions of the triggering elements need not be changed, eliminating any calibrated electronic adjustments
and providing a fixed spectral position relative to any external reference mark on the emulsion. Time Calibration. Calibration of the sweep rate at the emulsion is straightforward with the above triggering system. Because there is no drift or change in the starting point of the discharge, any radiation corresponding to the first discharge ignition may be used to establish a zero-time reference mark on the emulsion. This, then, may be used to locate the start of the emission from any other light source by comparison to an external reference. The rotational speed of the camera mirror is determined with a delayed-sweep oscilloscope by monitoring the output from the tungsten trigger lamp with the photodiode for at least 2 different vertical positions of either the lamp or diode. The discharge of interest is then started and 2 time-resolved exposures are taken on the same emulsion, separated in time by a convenient interval, such as 10 microseconds. This separation is easily and accurately accomplished through a measured change in position of either the photodiode or tungsten lamp. The resulting photograph has 2 time-resolved spectra superimposed on it whose starting points are separated in time by 10 pseconds, and the densitometer or projection system used to read the photograph is easily calibrated. Because there is no change in magnification with wavelength in this off-plane optical configuration, the calibration may be made at a wavelength where the radiation corresponding to the start of this discharge is sharply defined, and then used at any other wavelength in any order. In a spark discharge in air, the starting point is signaled by the short-duration nitrogen second positive band radiation (13). Wavelength Resolution. One distinct disadvantage of the off-plane or over-and-under optical configuration is the spectral line tilt from the vertical slit position as the angle of diffraction increases. On short focal length instruments such as this, the angular spread of the diffraction pattern cannot exceed more than a few degrees from the normal of the camera mirror before the tilt becomes apparent. In con(13) J. P. Walters and H. V. Malmstadt, ANAL.CH~M., 37, 1477 (1965). VOL 39, NO. 7. JUNE 1967
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Figure 8. Densitometer tracings of time resolved spectra for three typical index lines in an aluminumgraphite pin-to-pin spark discharge Spectra taken just off the surface of the A1 electrode when C = 0.063 pF, L = 30 pH, R = 5s2, A1 cathodic on the firstcurrent half-cycle, and a 4-mm electrode separation
ventional time-integrated spectrography, the tilt presents little problem, because the stigmatic nature of the mount allows partial superposition of comparison spectra at all wavelengths (14). However, when the camera mirror is spun, tilt of the spectral lines appears as an effective widening of the vertical entrance slit, causing the wavelength resolution to suffer. Forfhis reason, the 15,000-linegrating was chosen, giving a 1000 A coverage in 4 inches of emulsion width and small angular spreads around the camera mirror normal. As the spectral region is changed, and the angle of diffraction falling directly at the camera mirror normal increases, i t is necessary to tilt the spectrograph entrance-slit assembly. This tilt seldom exceeds 10 degrees, and the horizontal time slit may be simultaneously tilted with essentially no change in time resolving power. A precision Spiroid gear and turns counter is built into the entrance slit assembly, proper slit tilt for a particular wavelength range being determined photographically. The experimental procedure is identical t o that used to test the spectral quality of the lines with a spinning camera mirror and iron-arc light source. The major compromises resulting from a spinning camera mirror used in the off-plane Ebert configuration have been presented above. When spectra of high complexity are encountered, or in analytical applications where discharge mechanisms are known past the exploratory stage, higher diffraction orders and selected time intervals are used for line separation and to minimize the background. Longer exposure times for off-blaze wavelengths and decreased wavelength coverage obviously result. Camera Systems. Several interchangeable cameras have been designed for use with this instrument. For basic studies, where radiation from various regions in the discharge is examined one exposure at a time, a single photograph often amounts to a small part of the overall experiment. It is important, then, that exploratory photographic data be obtained rapidly. This is emphasized further when hysteresis studies of a discharge system are done, spectra of as few as (14) S . S . Berman, P. Tymchuk, and D. S . Russell, Appl. Specfry., 15, 124 (1961;.
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Figure 9. Densitometer tracings of time-resolved spectra for two index lines in different gap regions in an aluminum-graphite pin-to-pin spark discharge Discharge parameters identical to Figure 8. Dotted lines indicate arrival of material from a cathodic parent electrode in the observation region 5 discharges being sequentially photographed during the course of a typical 3000-discharge analytical exposure. For these reasons, Polaroid emulsions (Type 57 for exploratory work and Type 52 for qualitative analysis) are extensively used. The instrument uses a Graflex 4 X 5 camera back into which are inserted holders for Polaroid 4 X 5 emulsions, 4 X 5 cut film, and 4 X 5 spectroscopic plates. Kodak 4 X 5 plates (Types 103-0, 103-a-0, 1-N, 111-F,SA-1, and SWR) are used for densitometry. Polaroid Type 55 P/N film is used for preparing lecture slides and illustrations such as Figure 7. A 35-mm-roll film adapter also fits the Graflex back. There is no shift in the focal plane in converting between Graflex back combinations. The entire Graflex assembly can be removed and replaced with another camera when conventional 2 x 10 and 4 x 10 spectroscopic plates are used for analysis purposes. The positip of the entrance slit is then changed to bring at least 1000 A into clear focus on the plate. The camera used with 10-inch plates has provision for warping the plates both along the time and wavelength axis, as well as a uniform tilt of the entire plate along the wavelength axis. Spectral Results. Time-resolved spectra similar to those shown in Figure 7 are routinely and simply obtained with the instrument. Figure 8 demonstrates with spark discharges the importance of linear time dispersion coupled to a long time range. Here, lines in 3 spectra of aluminum have been recorded on the densitometer, the tracings then directly superimposed using the nitrogen second positive band system as a common zero time reference. The spectral behavior with time, particularly with regard to those times when the current is changing direction, is quite clear, even on a compressed time scale. Neutral atom pumping from the singly-ionized species, they in turn being fed by the relaxation of doubly-ionized species (13), is very evident, particularly toward the end of the discharge. The time required to prepare these data was approximately 0.5 hour, the bulk of it spent in obtaining densitometer tracings suitable for reproduction. The exposure was complete on Polaroid Type 55 P/N film after the passage of 200 spark discharges. Figure 9 further emphasizes the importance of short exposures. Here the arrival of material from a cathodic parent electrode at the surface of an anodic counter electrode in an oscillatory spark is shown, both in the first- and secondcurrent half cycles (13). These tracings were made from 150
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discharge photographs. If the exposure is increased to 500 discharges, and the entrance slit of the spectrograph narrowed to keep the background intensity roughly the same, the abrupt increase in emission intensity as electrode material arrives in the observation zone is not evident, being replaced instead by a gradual trend. This is due to physical changes in the parent electrode that occur with sparking, altering both the direction and velocity distribution of vapor cathodically ejected from its surface PHOT0E:LECTRIC DETECTION
Strictly speaking, the optical requirements for photoelectrically detecting and displaying time-resolved spectra are secondary to the limitations imposed by the electronics used. Considering he1,e only repetitive discharges detected monochromatically, t h t basic requirements for the monochromator are linear scanning, reasonable wavelength resolution, and moderate optical speed. Linear scanning is desirable for obtaining, full wavelength displays at various times after the start of a discharge, good resolution and accurate wavelength positioning allowing a time scan to be made at a fixed and Icnown wavelength. In this work, a gated amplifier system is used following the photomultiplier, and a moderate aperture system is required to ensure that large load resistors in the multiplier anode circuit are not required to obtain a strong signal. The specifications for the monochromator section of the instrument are given in Table I, related to the optical schematic shown in Figure 1. Both photoelectric time display and time resolution are accomplished through E. Tektronix Type 547 oscilloscope and
Type 1.31 sampling plug-in. The plug-in acts as a gated amplifier for the voltage developed across the multiplier anode load resistor, sampling as a function of time or at a fixed time after the start of the discharge. The signal is transmitted from the photomuliplier to the plug-in with good fidelity using a Tektronix Type P-6032 cathode-follower probe. One sample is taken per discharge, time resolution being determined by the width of the sampling gate. The gate is normally one tenthousandth of the total time display presented to the oscilloscope, typically being 100 picoseconds. In most applications, the response characteristics of the detector and load resistor limit the time resolution. The 1S1 plug-in has a storing memory, allowing the time-sampled output of the photomultiplier to be stored in an analog fashion and recorded with a conventional millivolt strip chart recorder. Here, a Heathkit Type EUW-301 recording electrometer is used, wired to operate as an operational amplifier (15) for purposes of integration. Time Selection. One of the distinct advantages of using the above complement of commercial apparatus for time resolution is that the position in time of the sampling gate, relative to an external trigger pulse or the start of the discharge, is determined by the value of an analog reference voltage that may be supplied externally to the plug-in. This gives a way to automatically control the time at which a sample is taken. Figure 10 shows the analog generator
H. V. Malmstadt, R. M,. Barnes, and P. A Rodriguez. J. Chem. Educ., 41, 263 (19641.
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built here for this purpose. The system is buiIt around a Heathkit Type EUW-19A operational amplifier module. The control waveforms begin with a variable dc voltage, called the initial time setting and used to set a fixed sampling time. Summed with this is a low frequency ramp, called the main time sweep and used to scan in time from the fixed reference. At scan rates as slow as 1 V per minute, a complete time scan, requiring 10 V for horizontal oscilloscope deflection, may easily be followed with a 1-second pen-response recorder, When the simple ramp is used, the time gate advances a fixed amount for every discharge, followed on the oscilloscope screen, at a rate determined by the slope of the ramp. A triangular wave is generated also, called the gate sweep, and summed into the output for randomly driving the sampling gate back and forth a controlled amount in time about either a fixed time, set by the initial time setting, or gradually driven to increasing times by the main time sweep. The time span randomly swept by the triangular wave is determined by its amplitude, and the frequency of the wave is adjusted in relation to the time interval swept to ensure a lack of synchronization with the repetition rate of the light source. The purpose of the triangular wave is to set electrically the time resolution desired for signal intergration. Signal Integration. Integration of a time-resolved signal at a particular time is desirable for reducing the normal noise associated with photomultiplier use. When spark discharges or other electrically ignited systems are studied, radio frequency interference on the sensitive electronics is invariably present. Further, with spark discharges, both the normal light fluctuations and the positional instability of the plasma must be considered. For example, in the spark source used here, neither the capacitor voltage. nor the peak current in each discharge show more than a few per cent variation, Still the light output for ion- and neutral-atom lines varies randomly as much as 2 0 0 x . Mainly. this results from the 776
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ANALYTICAL CHEMISTRY
Figure 12. Photoelectric time-resolved response obtained in an aluminum-graphite pinto-pin spark discharge Spectra taken just off the surface of the graphite electrode when C = 0.025 pF, L = 30 pH, R = 5n, AI cathodic on the first-current half-cycle, and a 3-mm electrode separation. Upper trace = first 14,000 discharges Lower trace = next 14,000 discharges
different amounts of material sampled on an individual spark basis in addition to the different paths followed by the material as it traverses the spark gap. These fluctuations may be reduced to a degree by damping the output of the sampling system at the strip chart recorder, but this brute force technique does not allow control of the averaging process and at best is a poor substitute for a gated integration of the signal. This is particularly true for faint light signals of short duration, where the single electron response of the photomultiplier is observed, and recorder damping merely diminishes the signal amplitude. To maintain time perspective in an integrated display, it is necessary to set the time interval over which the integration is carried out to be substantially less than the total length of the transient studied. The procedure used here is shown in Figure 11. Here a faint 0.3-psecond light pulse was derived from an incandescent light source. The noise superimposed on top of the signal is typical for faint light sources. After such a scan has been prepared with the sampling system, the amplitude of the triangular wave shown in Figure 10 is set to give an integration gate 50 nanoseconds wide for the pulse shown ir, Figure 11. The output of the photomultiplier is then integrated for a sufficient number of discharges or pulses to give reliable statistics, in this case 1800 sufficing, The integration interval is then delayed in time 50 nano-
seconds and the process repeated. The final result is a histogram composed 0: nine, 50-nanosecond randomlysampled integrated segments. With spatially instable spark discharges, the integration interval becomes sufficiently long so that the electrodes drastically change. In this case partial spatial integration of the discharge light over discrete radial zones along the length of the discharge axis is used. This is accomplished here with 2 parabolic image transfer mirrors, separated in height by 2 inches and ground for a 5-inch focus. They are diagrammed in Figure 1. Because they are illuminated at a substantial angle, they transfer a point image from the center of the discharge column astigmatically to the spectrometer slit (16), separating the 2 focal planes by 1.5 inch. In use then, the vertical focus, corresponding to zone thickness along the axis of the discharge, is adjusted to fall in the plane of the spectrograph horizontal slit. The horizontal focus then falls inside the instrument, and the radial variations in light intensity around the discharge axis are averaged, while changes up and down the axis are transferred stigmatically to the detector. With this system, heavy positional instability can be tolerated in the discharge without extending the time for gated integration, or direct observation, past that necessary to compensate for natural intensity va::iations. This is illustrated in Figure 12, where 2 repeated scans down the Al(II1) 3612 line were made, requiring the passage of more than 25,000 discharges. If simple direct image transfer is done, using a lens, these traces are sufficiently noisy so as to be uninterpretable. Wavelength Location. When desired, the position of the time sampling gate can be fixed and a wavelength scan done, with or without gated integration depending on the application. Such an applica1:ion is shown in Figure 13, where a scan was made for an oscillatory spark between an aluminum parent electrode and a silver counter electrode. Here the sampling gate was positioned 8.2 pseconds after the start of the discharge, and the mdiation observed in the region of the counter electrode just as the current changed direction. The intense neutral line:; of aluminum are evident, in addition to the first ion spectrum of silver, both spectra resulting from relaxation of parent species in higher ionization states (1.3). To carry out a fixed-time wavelength scan, or locate a particular line for a time scan, the instrument is designed with an accurate, precise sine-correcting grating drive, shown in exploded form in Figure 14. This particular sine correcting drive, described by Badger et al. (17) is an ideal harmonic linkage (18). For this mechanism to provide a conversion from linear rotation of the lead screw to sinusoidal rotation of the grating, a line drawn from the center of rotation of the grating to the center of the ball making contact with the flat on the lead-screw follower nut must be set perpendicular to a line defining the direction of travel of the nut down the lead screw when the grating is in the zero-order position. Also, the contact flat on the follower nut must move down the lead screw in a fashion such that its surface always remains parallel to its zero-order position. If the above conditions are accurately met, the drive is sinecorrecting within the lead-screw pitch-error. To assure that lead-screw alignment could be used as a reference, not re~~~~
(16) F. A. Jenkins and H. E. White, “Fundamentals of Optics,” McGraw-Hill, New York, 1957, p. 95. (17) R. M. Badger, L. R. Zumwalt, and P. A. Giguere, Rev. Sci. Instr., 19, 861 (1948). (18) A. Svobcda, “Computing Mechanisms and Linkages,” Dover, New York, 1965, p. 58.
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Pr! m m
r u Ir
Figure 13. Wavelength scan made in an aluminum-silver pinto-pin spark discharge at 8.2 pseconds after the start of the discharge Spectra taken just off the surface of the silver electrode with sparking parameters identical to those in Figure 8
quiring adjustment or angular measurement, the lead screw and guide bar were preassembled in two bearing support blocks that fit snugly into an alignment slot in the plates supporting the grating yoke and main bearings. The alignment slot was accurately machined to be perpendicular to the back edge of the support plates, which were then firmly fastened to the front wall of the spectrometer. The grating itself is cemented to a support plate and the dimensions of the assembly are accurately determined with an optical comparator. The grating is then inserted into the grating yoke, and pressed up against three locating buttons in the rear plate of the yoke. The locating buttons are set mechanically such that the face of the grating is on the axis of rotation of the grating yoke. It can be seen in Figure 14 that this axis passes directly through the center of the grating bearings and preload rings; they in turn accurately fit into two precisely aligned holes in the parallel support plates. Thus, once the grating is installed in the yoke, its face is a known distance from the back edge of the support plates. It is then a simple matter to advance the follower nut on the lead screw VOL. 39, NO.
7; JUNE 1967
777
END ASSEMBLY FOR CALIBRATING DIRECT WAVELENGTH READOUT
REAR PLATE a LOCATING BUTTONS
GRATING ON SUPPORT PLATE POSITIONING THE TlNG AT ZERO ORDER
Figure 14. Isometric diagram of the grating drive system used for wavelength location and linear scanning
Parts called out are double scale the central diagram until the face of the contact flat is at the same distance, minus the radius of the contact ball on the end of the sine drive arm. A line from the center of the ball to the axis of grating rotation is now perpendicular to the direction of travel of the follower nut, accurately defining the mechanical zero-order position, To bring the optical zero order into coincidence with the mechanical zero-order position, it is necessary to rotate the grating without changing the position of the sine drive arm. Here, the adjustment is made by advancing a differential screw, which in turn rotates the grating yoke through the differential arm. The sine drive arm is then coupled through the differential drive arm to the grating yoke. The grating lines are placed parallel to the axis of rotation by decoupling the two drive arms and manually rotating the grating from positive to negative orders, adjusting the tilt of the grating blank with three iocating buttons in the yoke until the image ride on the exit slit is zero for all orders. All the alignment adjustments are made without disturbing the mechanical alignment of the sine drive arm, greatly simplifying setting the internal optics of the instrument with the proper wavelength drive condition. To have a direct wavelength readout of the grating position, or the wavelength at the exit slit of the monochromator, it is necessary only to evaluate the proportionality between revolutions of the lead screw and the ked-position exit-beam wavelength. The length of the sine drive arm is then adjusted to meet this proportionality at at least three wavelengths, preferably over three orders of diffraction. A digital counter is belted to the lead screw, coupling ratios being chosen to match the number of angstroms per revolution of the screw. In this instrument, a 28-threads-per-inch screw is used for a sine drive-arm length around 5 inches, giving 200 A per screw revolution for a 15,000-line grating in the first order. The screw is cut with a pitch error of 0.0001 inch ner 5 inches travel, and a soft Srcnze follower nut is used with ri; !east 20 threads in contact with :be screw at al! times. 2.. FjS8
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ANASYTICAL CHEMISTRY
hardened flat, ground to a high flatness tolerance is brazed to the nut, contacting the carbide ball on the sine drive arm. To test the tracking accuracy of the entire drive assembly, a mercury spectrum was recorded using a Heath Model EUW-301M recording electrometer sensing the photomultiplier current (15). The electrometer is equipped with a stepping-motor chart drive. The error in wavelength determined represents the total error beginning at the drive motor on the lead screw and ending at the stepping motor on the recorder chart. The data shown in Table I1 indicate that there is a fixed error around 1%,in identifying all wavelengths, resulting from failure to properly synchronize the start of the recorder with the start of the wavelength scan. A random error for each wavelength, resulting in an average deviation of 0.3 A, represents the true total error of the entire system, including measurement of line position on the recorder tracing. In practice, it has always proved possible to locate a line with no difficulty, either on the counter on the instrument or on a strip chart recording, to within an angstrom either side of its maximum. OPERATIONAL CONSIDERATIONS
Excitation Apparatus. From an analytical viewpoint, rapidly changing any special excitation apparatus, such as an arc-spark stand or controlled-atmosphere device, is as significant as obtaining high-quality spectra. Such considerations are equally important for exploratory research, as well as long-term basic studies. Here, where photographic and photoelectric detection systems involve different optical paths, it becomes mandatory that entire optical benches be interchangeable. In an academic laboratory, such versatility allows individual experiments and projects to be breadboarded OR the instrument and simply attached for readout purposes. On this instrument, not only can photographic and Thotoelectric systems be set up with entirelj different
TRIGGER
OPTICS
Figure 15. External view of the instrument set up for photoelectric time-resolved studiff of spark discharges For SperOgrSphic~lse,the o p t i d bench on the left is transferred to the front of the instrument experiments, but the whole instrument can also be simultaneously shared merely by alternate use of a particular wavelength region or setting. Figure 15 shows an external view of the instrument where one optical bench is set up with image transfer mirrors and the arc-spark stand in current use. The spectrograph slit and a 35-mm-film casette are visible on the front end of the instrument. When it is necessary to change the optical bench, or replace it with another experiment, the entire assembly is readily removed from the side of the instrument by unscrewing two lug bolts. It is then attached to the front of the instrument with the same bolts, kinematically locating itself on two ball sockets and two flats. A support post is placed at the back end of the optical bench for those applications where very heavy objects are used, although in most cases the post has not proved necessary. In all instances, optical alignment is retained. The arc-spark stand in use now is constructed for precisely locating a region in the spark gap. The electrode jaws are water-cooled, bilateral stainless-steel assemblies, racking on precision screws along the discharge axis to a reproducibility less than 0.001 inch. The position of each jaw relative to the optical axis may be set by a demountable projection system, or through the use of a height gauge referenced to the surface of the optical bench. Displacements of the electrodes from the optical axis are followed with two 5-inch-travel-dial indicators, set to read the position of the electrode jaw relative to any desired reference. One initial calibration, made by backlighting the optical path, places all measurements on an absolute basis. Each electrode jaw assembly may be easily removed from the stand, being replaced with inert atmosphere
Table II. Typical Wavelength Readout Accuracy’ Experimental wavelength read
Handbook value
2537.2 2653.2 2654.7 2656.0 2700.2 2731.3 2754.0 2805.5 2806.4 2895.7 2926.5 2968.8 3022.9 3024.8 3126.7 3132.8 3342.7 3651.5 3656.0 3664.1
2536.5 2652.0 2653.1 2655.1 2699.5 2730.3 2752.8 2803.5 2804.5 2895.3 2925.4 2967.6 3021.5 3023.5 3125.7 3131.8 3341.5 3650.2 3654.8 3663.3
Difference
Deviations from average difference
+0.7
-0.5 0.0 -0.2 -0.3 -0.5 0.0 +0.8
+1.2 +1.0
f0.9
+0.7 fl.O +1.2 f2.0
+0.8
+2.0
-0.8 -0.1 0.0 f0.2 +0.2
f0.4
fl.1 f1.2 f1.4 +1.3
+o.
+1.0 +1.0 +1.2
1
-0.2 -0.2 0.0
+o.
f1.3 fl.2 f1.8
1 0.0 f0.6
Average = +1.2
Average =0.3
.Light source: General Electric, 4-Wmercury germacidal lamp. 35 microns, entrance and exit. Slit widths: *an rate: I M&minute.
VOL. 39, NO. 7, JUNE 1967
779
devices, a rotating disc electrode assembly, or a Petrey table. Future accessories strictly for analytical use are now being fabricated. An instrument function that was used extensively in initial photographic focusing and alignment and is of analytical importance involves slowly rotating the spectrograph camera mirror using the worm gear wheel attached to the trigger mirror shown in Figure 5 . The synchronous motor for highspeed mirror drive is turned off, and a small clock or stepper motor is engaged to the worm wheel through a sliding platform and gear reduction train. The slow drive motor is programmed with relays and timers to rotate one sixth of a revolution at adjustable time intervals, which when coupled through a 720:l reduction ratio allows rapid and precise spectral displacements on the order of 2 mm at the camera. The slow drive motor may also be run continuously, sweeping out a continuous tracing. This function allows programmed split-burn and moving-camera arc-discharge studies, as well as pre-spark and hysteresis studies on time-integrated spark discharges. SUMMARY
It has been shown previously with an oscillatory spark discharge (13, 19) that it is possible to extend analytical sensitivity, substantially simplify qualitative analysis, reduce or even eliminate matrix effects, and successfully use rather atypical discharge regimes for analysis. The above ends were (19) R. M. Barnes, Ph.D. Thesis, University of Ilhois, Urbana,
1966.
achieved by combining a time-resolved approach to spectrometric analysis with some leading knowledge about the discharge mechanism. In many cases, simplicity and convenience were sacrificed, the analysis or study requiring long exposure times and sensitive manipulations of the apparatus. The instrument discussed here was designed to retain highquality time-resolved readouts, while at the same time eliminating sensitive adjustments, decreasing photographic exposure times, extending time ranges into the afterglow for many electrical discharges, and emphasizing the optical characteristics of modern analytically-oriented spectrographs and monochromators. ACKNOWLEDGMENT
The author thanks Ivel Reithmeyer, Russell Riley, and Robert Schmelzer for their many helpful contributions to the construction and design of the apparatus. James B. Peters, and Robert Darlington of the Whitnon Co., were instrumental in the design and construction of the sine-bar and rotatingmirror mechanisms, respectively. The donation of two highvoltage spark sources by the General Motors Research Laboratories is appreciated. RECEIVED for review January 16, 1967. Accepted March 17, 1967. Mid-American Symposium on Spectroscopy, Chicago, Ill., May 1967. The research was supported in part by the Wisconsin Alumini Research Foundation, and by the National Science Foundation under Grant GP-5073. Special financial assistance was given by the Research Corp. and the U. S. Rubber Co.
Use of X-Ray Spectrometry in Activation Analysis: Determination of Bromine Cesia Shenberg, Jacob Gilat, and Harmon L. Finston' Nuclear Chemistry Department, Soreq Nuclear Research Centre, Yavne, Israel X-ray counting of *OBr and EomBris applied to the determination of submicrogram amounts of bromine in neutron-irradiated samples containing a large excess of common interfering elements. The precision of the method is about 2%, and an accuracy of 2 4 % can be attained for Na/Br or K/Br ratios up to 500:l. The method also provides a means of fast analysis of milligram amounts of bromine in organic matter. The advantages of x-ray counting and spectrometry over conventional scintillation gamma spectrometry in nondestructive neutron activation analysis are discussed.
THEPOSSIBILITY OF nondestructive analysis of selected elements in a complex matrix is one of the most attractive features of radioactivation analysis. The prerequisites for such analyses are that some of the characteristic radiations emitted by the activated species should be discernible in the presence of large amounts of other radiations. Countless examples of the uses of characteristic gamma rays, delayed neutron emission, Israel AEC Fellow; present address, Brooklyn College, City University of New York, New York, N. Y. 780
a
ANALYTICAL CHEMISTRY
positron decay, etc. can be found in the literature (1-5). However, little attention has been paid to the analytical applications of x-rays emitted in radioactive decay. The two main processes responsible for the production of x-rays in radioactive decay are electron capture and internal conversion of gamma rays. In both cases, vacancies are created in the low-lying electron shells (mainly the K shell) of the decaying atoms. The filling of these vacancies results in the emission of x-rays characteristic of the decaying element. Thermal neutron activation leads mainly to isotopes on the neutron excess side of the stability line. Therefore, relatively few isotopes which decay by electron capture (e.g., 37Ar, 61Cr, 55Fe, 7lGe, 7sSe,*OBr, g3M0) can be produced in the predominantly thermal neutron flux of a reactor. Large internal conversion coefficients are expected only for high multipolarity gamma transitions (isomeric transitions). Thus the probability of x-ray emission from nuclides produced in a (1) W. W. Meinke, ANAL.CHEM., 32 (3,104 R (1960). (2) G.W .Leddicotte, Ibid.,34 (9,143 R (1962). (3) G. W .Leddicotte, Ibid.,36 (5), 419 R (1964). (4) W. S.Lyon, E. Ricci, and H. i-I. Ross, Ibid.,38 (3,251 R(1966). ( 5 ) F. Adam and J. Hoste, AI. Energy Reu. 4 (2), 113 (1966).
