RESULTS AND DISCUSSION
The oscilloscope traces representing the TRT of filaments and inductively heated ferromagnetic wires under various operating conditions and different measuring systems are shown in Figures 3 and 4. TRT of filaments measured with a photomultiplier for two conditions of excitation are shown in Figure 3. TRT as fast as 15 msec were achieved with the capacitor discharge mode of excitation ( I , 6) (Figure 3b). The TRT of the same filament excited to T,, by applying a constant voltage to its terminals was found to be 10-1 1 sec (Figure 3a). Because the emission of electrons from hot metal surfaces is a function of the temperature, monitoring the emission current may in principle be used for measurement of TRT. Using the system based on emission of electrons described earlier, emission currents of about 5 x 10-l0A were produced by the Pt filaments when heated to about 850 “C. The response time of the electrometer in this arrangement was found to be 300 msec, thus limiting the usefulness of this method to rise times in excess of 300 msec. The TRT of a 0.5-mm diameter ferromagnetic wire measured with a thermocouple was found to be 1.3 sec (Figure 4a) when heated by induction with the Philips pyrolyzer and 120 msec when heated with a 2.5-kW Lepel rf generator. In order to determine if the thermocouple itself is heated by the rf field, the low-mass thermocouple was placed in the rf coil without the ferromagnetic wire. No temperature increase was noted. The TRT of this wire using the photomultiplier tube for measurement and the Lepel high frequency generator for excitation, was again found to be 120 msec indicating that the response of the low mass thermocouple was sufficiently fast. The advertising literature supplied by the manufacturers of the Philips pyrolyzer implies that TRT of 30 msec
can be achieved. Inquiry revealed (11)that the power output of the Philips pyrolyzer-Le., the high frequency generator, is considerably lower (30 W) than the generator originally used by Simon and Giacobbo (3) (1.5 kW) who developed this pyrolyzer. Willmott (12) recently reported similar observations. It appears that the TRT of ferromagnetic wires can be measured with low mass thermocouples; however, in each case it is necessary to ascertain that their response is sufficiently fast and that the heating of the ferromagnetic material is not perturbed by the presence of the thermocouple within the rf field. Because the photomultiplier has a very fast response, it proved to be a most suitable system for measurement of temperature rise times of pulse pyrolysis units. The response of the photomultiplier is sufficiently fast (1.4 nsec) to follow the rise time of any pyrolyzer, and the voltage output is compatible with oscilloscopes. However, a system equipped with a regular photomultiplier tube is applicable only when the equilibrium temperature is higher than the temperature at which light emission begins (about 550 “C). The use of photomultiplier tubes which are sensitive in the IR would extend the application of the method to lower temperatures. ACKNOWLEDGMENT
The authors express their gratitude to W. L. Kester for his valuable advice. RECEIVED for review April 14, 1969. Accepted June 20, 1969. This research was conducted under the McDonnell Douglas Independent Research and Development Program. ( 1 1) W. Simon, private communication, October 1968. (12) F. W. Willmott, “Advances in Chromatography 1969,” A. Zlatkis, Ed., Preston Technical Abstracts Co., Evanston, Ill., 1969, p. 222.
An Image Intensifier Spectrograph Stanley Ness and David M. Hercules Department of Chemistry and Laboratory for Nuclear Science, Massachusetts Institute of Technology, Cambridge, Mass. 02139
IN GENERAL there is much interest in studying the spectra of weak light sources which occur in a large number of physical, chemical and biological phenomena. Fields as diverse as electrical discharges and their afterglows, scintillations, coronae, atomic flames, night sky and upper atmosphere emissions, Raman effect, shock tubes, bioluminescence, chemiluminescence and triboluminescence are examples for which there often exists relatively little spectrometric information. A spectrograph of large aperture is needed but the problem is such that, even with very fast commercial spectrographs (which often are not readily available) it is frequently necessary to provide integrations of the order of many hours to provide a level sufficient for adequate recording. Also to obtain an adequate signal-to-noise ratio, wide slit widths are often used which result in a smearing out of spectral detail, sometimes even losing it altogether. Also in these cases with either film or an integrating photometer as a detector, the long exposure times do not permit recording spectral changes that occur over short time intervals and hence, the diagnostic value of spectrographs in such situations is very limited. It is thus desirable to reduce integrating times as much as possible. Fortunately, detectors such as sensitive image intensifier tubes have become commercially available, and require few
incident photons to produce detectable signals under high contrast conditions. Coupling such an image tube with a fast spectrograph it has been possible to reduce usual photographic exposure times by several orders of magnitude. With this additional stage of detection, a further dynamic range limitation is introduced whose lower limit corresponds to the equivalent noise input flux and the upper to saturation of the tube. THE APPARATUS
A large aperture spectrograph using commercially available optical components was designed and constructed similarly to one reported earlier by Bass and Kessler ( I ) . In conjunction with an R.C.A. C70021 HP2, three stage cascaded image tube, the required system was produced. The arrangement of components is shown in Figure 1. Light from the source illuminates a slit which is located at the focal point of the collimator, an f/4.5, f = 300-mm lens. The collimated beam is dispersed by a plane grating and the resulting spectrum is focused on the photocathode of the (1) A. M. Bass, K. G. Kessler, J. Opt. Sac. Amer., 49, 1223 (1959). VOL. 41, NO. 11, SEPTEMBER 1969
1467
SPECTROGRAPH SENSITIVITY
I
R C A C 70021 H P 2
WATER FOR COOLING FOCUSSING C O I L
DCR 1 5 0 - 1 5 A
,
Wavelength
('I
nrn
COLLIMATOR
Figure 2. Sensitivity as a function of wavelength in quanta per unit wavelength bandwidth. Calibration is at 5 A bandwidth
Figure 1. The apparatus Series 125-fj1.9 - 1:O.g Mag. oscilloscope camera, Tektronix Inc., Portland, Oregon. Minolta SR-1 (Model V) 35 mm. SLR camera with MC RokkorPF f11.4,f = 58 mm lens, Minolta Camera Co. Ltd., Japan. Image tube RCA, Lancaster, Pa. Auxiliary network C33009E, RCA, Lancaster, Pa. Solenoid C33035, RCA, Lancaster, Pa. Solenoid power supply: Raytheon Co., Sorenson Operation, Norwalk, Conn. High voltage power supply: Model LAB 60 PN, Spellman High Voltage Co., Inc., Bronx, New York. Objective lens: Komura, Japan. Grating: Catalog no. 980-44-30-22, Jarrell-Ash Co., Waltham, Mass. The grating has 590 grooves/mm. on a 102 X 102 mm ruled area. Collimator, Accu-rapid. Micrometer slits, Edmund Scientific Co., Barrington, N. J.
image tube. Focusing within the tube is brought about by adjustment of the potentials of the interstage focusing electrodes and by adjusting the axial magnetic field, until the electron transit time from the cathode to the final screen is equal to the time for an electron with a transverse component of velocity to make two loops around the magnetic field vector. Double-node instead of single node focusing was chosen because this gives improved edge resolution. A Minolta SR-I 35-mm single lens reflex camera provided with a Rokkor-PF f/1.4, f = 58-mm lens is focused on the P-I1 phosphor screen. By using extension tubes between the body and the lens it is possible to make the image of the screen fill a whole 35-mm frame. Alternatively, a Tektronix oscilloscope camera has been used, which has the advantage that the results are rapidly available, although in positive
Table I.
Maximum Focal Lengths and Minimum Apertures Required.
Angle of inci300-6430 nm 300-800 nm 300-1000 nm Focal length and minimum aperture dence 10" 135 mm, f/1.6 81 mm, f/O.8 58 mm, f/0.5 20" 135rnm,f/1.8 81mm,f/l.O 58mm,f/0.7 81 mm, f i l . 2 ... 30" 135 mm, f/2.1 a Complete specifications of lenses required t o cover region indicated, angle of incidence specified and with an incident collimated beam diameter of 66.7 mm.
1468
ANALYTICAL CHEMISTRY
form using high speed Polaroid Type 107 film. The resulting spectrum has a height which is roughly the product of the slit height and the ratio of the focal lengths of the focusing and collimating lens and a length depending on the spectrograph dispersion as determined by the grating and optical parameters. The two lenses are held at their larger ends in a cylindrical brass lighttight housing which encloses the grating. The grating is mounted in a three point suspension which is fastened to a turntable having an axis coincident with the front surface of the grating and normal to the plane of the lenses. With an f = 135-mm focusing lens the dispersion is about 10 nmjmm and the spectral range covered on a single frame is 300 to 600 nm. By rotating the grating and using a filter to prevent the interference of second order effects from shorter wavelengths, the near infrared can also be examined. Unfortunately, because glass is used throughout the system the spectrograph functions only in the near ultraviolet, visible and near infrared. A smaller region can be covered in the second order and further rotation of the grating permits the use of higher orders. Although much of the relevant theory and equations to calculate the sizes of the various components has been reported by Bass and Kessler ( I ) , for those interested in different spectral regions Table I has been included. To use good, inexpensive commercial lenses in the regions indicated it is necessary to have an angle of incidence of at least 30" and lens apertures of 70 mm. To image the 300-600 nm region (adequate for many of the experiments in chemiluminescence) a focal length of 135 mm is required. RESOLUTION
The fact that the lenses are achromatized for the visible spectrum means that only a small part of the spectrum is in best focus for any particular setting of the collimator. The focusing was usually adjusted with reference to the 577- and 579-nrn yellow lines of the mercury spectrum and seemed to be very adequate for the regions of interest. Of course, outside the immediate neighborhood of these yellow lines the resolution will decrease slightly but with the system used the ultimate resolution of about 0.5 nm appeared to be determined by the resolution of the image tube. The manufacturer gives 25 line pairs/mm in the center and 18 line pairs/mm at 1.5 cm from the major axis of the tube in single node operation and expects slightly better resolution for double node operation.
(a)
CalibrationLinesfrom mercury gaseous dischargelamp
+
(b) Emission from (’A, %g)* double molecular state in the H.O$& reaction; the peak occurs at 4780 A. Exposure: 35 see. 0.d. of filter at 4780 A: 0.07
4
(e) Emission from the (‘Zg)** state at 3810 A in addition to the 4780 and 5800 A. Exposure: 45 see. 0.d. of filter at 3810 A: 0.07 and at 4780 A and 5800 A: 0.d. > 2.0
(d)
Electrochemiluminescence of lucigenin in acetonitrile showing h t h the fluamscence of n-methyl acridone and the re-emission from lueigenin by a process ofenergytransfer. Exposure: 15 see
(e) Elecirochemildneseenceof a 10-*M solution of anthracene in acetonitrile switching from anion to cation formation. Six flashes are superimposed with a 1second generation ofeach. The emission contains anthracene fluorescence and a significant amount ofa longer wavelength component (3)
(f)
Chemiluminesceme from the isomerization of 9,9’-dehydrodianthracene to bianthrylat200°C. (4)
Figure 3. Sensitivity of apparatus SPECTRAL SENSlTIVITY
avalanche from the cathode is able to ionize atoms of residual
The sensitivity as a function of wavelength was obtained using an NBS 6201 tungsten lamp operating at a color temperature of 2854 “K. The sensitivity curve which takes into account the transmission characteristics of the glass, the efficiency of the grating at various wavelengths and the spectral response of the image tube, was generated from the known spectral distribution of the lamp and the observed distribution from a densitometer scan of the photographic negative. It is shown in Figure 2 and has a peak sensitivity at 500 nm. This curve can be used to give corrected spectra containing no experimental artifacts but only with reservation because to use it properly the exposure range of the film must be the same as that from which the sensitivity curve was estimated.
gas in the tube producing ions, metastable atoms and mole-
IMPROVEMENTS It has not yet been necessary to cool the tube as the background is tolerable at the light levels of interest requiring up to about 4 minutes of exposure at fJ1.9 on 3000 ASA film. In nearly all cases the tube is operated at 29 kV for the best resolution, the maximum recommended operating voltage with the photocathode at ground potential for minimum noise. It must be noted here that the tube is usually operated in the dark for several hours with the voltage applied to all electrodes before taking any photographs. This greatly reduces positive ion feedback to the cathode, The electron
cules which then migrate to the cathode and cause the simultaneous emission of several electrons. This amplified signal causes scintillations of the anode phosphor and generally increases the background. Operation for some hours gradually removes this contribution. Cooling of the tube to reduce thermionic emission, to decrease ion current from alkali metal vapors in the tube in proportion to their reduced vapor pressures and the optimization of some of the electrode potentials is at present under consideration. SENSlTIVITY A good example of the sensitivity of the apparatus is given in Figure 3. Figures 3b and 3c are the emission spectra in part of the visible region from excited oxygen produced by the reaction between chlorine and hydrogen peroxide in alkaline solution. Arnold, Ogryzlo, and Witzke (2) suggested that a simultaneous transition could occur in two excited oxygen molecules with the combined energy being emitted in a single (2) s. J. E. A. OgrYZh Chem. PhYs., 4, 1769 (1964). (3) E. A. Chandross, J. w. Longworth, R. E. Visco, J. mer. Chem. SOC.,81,3259 (1965).
(4) N. M. Weinshenker, Ph.D. Thesis, M.I.T., Cambridge, Mass, VOL. 41, NO. 11, SEPTEMBER 1969
1469
photon. Many bands have been detected (5-12) and assoIZg)* ciated with transitions from these ('Ag),* and ('Ag double molecular states with zero, one or two quanta of vibrational excitation. However, to date emission from the ('Zg),* state has never been detected. In experiments conducted with S. R. Abbott, a series of bands were recorded agreeing very well with those previously reported in the literature and in addition, a very weak broad luminescence band at about 380 nm was also observed. (See Figure 3c) This band which has evaded other experimenters is probably due to emission from the ('Zg),* states. The exposure was 45 seconds on 3000 ASA film at f/1.9 with
+
( 5 ) A. U. Khan, M. Kasha, ibid., 39,2105 (1963). (6) R. J. Browne, E. A. Ogryzlo, Proc. Chem. SOC.,117 (1964). (7) A. U. Khan, M. Kasha, Nature, 204, 241 (1964). (8) L. W. Bader, E. A. Ogryzlo, Discussions Faraday SOC.,37, 46 (1964). (9) J. S. Arnold, R. J. Browne, E. A. Ogryzlo, Phoiochem. Phoiobiol., 4,963-969 (1965). (10) R. E. March, S. G. Furnival, H. I. Schiff, ibid., 4, 971-977 (1965). (1 1) A. U. Khan, M. Kasha, J. Amer. Chem. SOC.,88, 1574 (1966). (12) P. Douzon, J. Capette, J. P. Gout, Compi. Rend. 266C, 993, (1968).
the gain of the image tube at 80,000, corresponding to an exposure of more than one month with a conventional spectrograph of similar aperture. Other examples are also given in Figure 3 and are described in their captions. The exposure times are noted and are all at f/1.9 on 3000 ASA film with the gain of the image tube at 80,000. ACKNOWLEDGMENT
We thank Jack Chang for his assistance in early stages of construction and Seth Abbott for his assistance in the singlet oxygen studies. We thank F. D. Greene and Ned Weinshenker for the sample used in Figure 3f. We also thank T. C. Werner and K. D. Legg for the spectra 3e and 3d, respectively. RECEIVED for review May 16, 1969. Accepted July 11, 1969. Paper presented at the International Chemiluminescence Conference, Desert Hot Springs, Calif., March 1969. This work was supported in part through funds provided by the U.S.Atomic Energy Commission under Contract AT(30-1)905.
Spe'ctrophotometric Determination of Thiocyanate with Rhenium Robert E. Neas and John C. Guyon Department of Chemistry, University of Missouri, Columbia, M o . 65201
NUMEROUS spectrophotometric procedures are available for the determination of the thiocyanate ion but each method suffers from serious interferences or some other undesirable feature. A few selected examples of the more widely applied existing systems and their undesirable aspects include: (a) the iron(II1)-thiocyanate complex systems (1-3) is limited by color instability problems and a variety of interferences (b) the copper-pyridine (4-6) and the methylene blue (7) systems require an extraction step and are susceptible to many interferences (c) the benzidine-pyridine systems (8, 9) exhibit color instability and a very serious objection to all methods using benzidine has arisen because benzidine has been shown to be carcenogenic (3). No previous use of the rhenium-thiocyanate interaction to determine thiocyanate was found. This paper reports the results of a study carried out with the objective of developing a simple, sensitive, spectrophotometric method for determining thiocyanate that would circumvent many of the problems associated with existing methods. (1) B. B. Brodie and M. M. Friedman, J. Biol. Chem., 120, 511 (1937). (2) E. Ginsburg and N. Benotti, ibid., 131,503 (1939). (3) T. G. Whiston and G. W. Cherry, Analyst, 87, 819 (1962). (4) K. C. Bailey and D. F. H. Bailey, Proc. Roy, Irish Acad., 37B, 1 (1924). ( 5 ) J. M. Kruse and M. G. Mellon, ANAL.CHEM., 25,446 (1953). (6) R. S. Danchik and D. F. Boltz, ibid., 40,2215 (1968). (7) T. Koh and I. Iwasaki, Bull. Chem. SOC.Jap., 40(3), 569 (1967). (8) W. N. Aldridge, Analyst 69, 262 (1944). (9) I. Nusbaum and P. Skupeko, Sewage and Ind. Wasies, 23, 875 (1951). 1470
ANALYTICAL CHEMISTRY
EXPERIMENTAL Apparatus. Absorbance measurements were made with a Cary Model 15 recording spectrophotometer with matched 1-cm quartz cells using deionized water as the reference. Blanks were measured and subtracted for all data reported here unless otherwise indicated. Glass hypodermic syringes (without needles) of 1, 2, 5 , 10, 20, and 30-1111 capacities were used for the addition of the various reagents except for potassium thiocyanate solutions which were added with volumetric pipets or burets. Volumetric glassware was also used for potassium perrhenate additions in the mole ratio and continuous variations studies. A water bath with a Sargent Model 3554 thermoregulating unit was used to control reaction temperature at 25 f 1 "C. N o temperature control is required in the recommended procedure so long as room temperature does not deviate more than about five degrees from 25 "C. Reagents. Aqueous potassium perrhenate stock solution was made to be 4.00mM KReOd (greater than 99.9% pure K R e 0 4from the University of Tenn.). Aqueous potassium thiocyanate solutions were prepared using Fisher Certified Reagent KSCN. Stannous chloride reagent was prepared by dissolving 47.6 grams of Mallinckrodt A.R. SnC12.2H20in 40 ml deionized water and 93.5 ml Mallinckrodt A.R. concentrated HC1 yielding about 150 ml solution to which was added 1 gram Fisher Certified Reagent Tin Metal. Deionized water was used for all dilutions and all solutions were stored in glass bottles. At the beginning of the investigation irreproducible results were obtained that seemed to be related to the daily aging of the stannous chloride reagent. Serious deterioration of
