Situation 5, the constant Knen is the only important constant in any of the fluorescence systems discussed whose value might not be known or considered in the original selection of the excitation wavelength to give maximum sensitivity in the normal situation. Sometimes when the values for RD and E D are known, greater selectivity for ML+”-’ with perhaps some loss in sensitivity may be realized by choosing different wavelengths for excitation and measurement than those originally selected. B y judicious selection of the depth of solution to be measured, of conditions which affect the chemical equilibria of the system, and of wavelengths for excitation and measurement, tremen-
dous sensitivity and selectivity can be attained in fluorometry using a vertical-axis transmission-type filter-fluorometer. LITERATURE CITED
(1) Bowen, E. J., Analyst 72, 379-82 (1947). -. z . \--
(2) Bowen, E. J., Quart. Rev. Chem. SOC. 1, 1-15 (1947). (3) Bowen, E. J., Wokes, Frank, “Fluorescence of Solutions,” Longmans, Green, New York. 1953. (4) Danckwort, P. W., “Lumineszenz
Analyse im Filtrierten Ultravioletten Licht,” Akademische Verlagegesellschaft, Leipzig, 1940. (5) Ellinger, P., Holden, M., SOC.Chem. I n d . Trans. 63. 115-21 (1944). (6) Fletcher, M: H., ANAL. CHEW 35, 288 (1963).
(7) Lothian, G. F., J . Sci. Instr. 18, 200-2 f 1941). (8) LotGan, G. F., J . SOC.Chem. Ind. 61, 58-60 (1942). (9) Milkey, R. G., Fletcher, &I. H., J . Am. Chem. SOC.79, 5425-35 (1957). (10) Nichols. E. L.. Merritt. E., Phvs. (11) Parker, C.‘A., Barnes, W. J., Analyst 82, 606-18 (1957). (12) Parker, C. A., Rees, W. T., Ibid., 85, 587-600 (1960). (13) Pringsheim, P$er, “Fluorescence and
PhosDhorescence. Interscience. New York: 1949. (14) Weber,-G., Teale, F. W. J., Tran8. Faraday SOC.53, 646-55 (1957). (15) Ibid., 54, 640-8 (1958).
RECEIVEDfor review June 11, 1962. Accepted December 5, 1962.
A Vertical-Axis Transmission-Type Filter-Fluorometer for Solutions MARY H. FLETCHER U. S. Geological Survey, Washington 25, D. C.
F A vertical-axis transmission-type filter-fluorometer for solutions is described and scale drawings are presented. Data from studies of several chemical-fluorescencesystems show that filters effectively isolate fluorescence from exciting energy. Other data illustrate the relationships between fluorescence intensity and solution depth.
I I I - II
Primary Filters-
A
model of a simple vertical-axis transmission-type filter-fluorometer has been useful in this laboratory. The instrument was built several years ago in connection with a study of the thorium-morin system ( S ) , and has been used here to obtain data from several chemical-fluorescence systems and to resolve the mixture of complexes in the beryllium-morin system. The theoretical and practical reasons for the superiority of the transmission over the perpendicular arrangement, the advantages of a vertical axis, and the importance of the design to theoretical studies where the data must be used in calculations have been discussed (1). Fundamentally, highest sensitivity and selectivity for a fluorometric method depend upon the chemical-fluorescence systems, and the characteristic properties of the substance being determined are best exploited when the wavelength bands used for excitation and measurement are smallest. So long as the excitation wavelength band is sufficiently inN EXPERIMENTAL
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ANALYTICAL CHEMISTRY
diaphragm -4
-------
0 -
Optical Cell
-L
Glass standard +-#-da’phragm Secondary Filters
Shuiter --c
1
Figure 1. Arrangement of parts in transmission fluorometer
tense for the instrument’s detector and measuring systems t o measure accurately the fluorescence which can be attributed to the reagents, increasing either the intensity of excitation or signal amplification by a large amount serves no useful purpose. The lamp used as the primary excitation source in this instrument emits a focused light beam having such a high intensity that even when narrow wave-
length bands are selected for excitation of fluorescence, the blank fluorescence from all systems studied so far has been great enough to be measured accurately a t low to medium levels of signal amplification. The transmission arrangement on a vertical axis and the relatively large distances between the optical cell and the excitation source and phototube are the important basic features of the fluorometer design. By virtue of these features, experimental fluorescence intensities conform t o the general equation for fluorescence over a wide range of light absorption. Parallel light rays are required for excitation because concentration, solution depth, and light absorption are related according to the Beer-Bouguer law in the general equation for fluorescence. d small beam of sufficiently parallel light is obtained without lenses in the prototype fluorometer, by using a spot lamp 17 inches above the optical cell. The phototube is also about 17 inches from the optical cell. This distance was made large to minimize changes in V , the geometric factor in the general equation for the cone of fluorescence reaching the phototube ( 1 ) ; and also to minimize any changes in fluorescence intensity, which could result in conformity with the inverse square law, when the position of the fluorescent volume is shifted with respect t o the phototube. This provision is necessary because the distance between the fluo-
o diameter
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IF------
m
3;"
+I
I
optical axis
I
40
19
IO 10 12
I*
2
I
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Figure 2. Front elevation fluorometer body
*
of brass
rescent volume and the phototube changes .when different volumes of solution are used or with the same solution volume when there is a large difference in the amount of light absorbed. With the optical cell used in this laboratory, the surface of the solution moved slightly more than 3 cm. Rhen the volume was increased from 1 to 25 ml. The description of the fluorometer which follows is intended to serve as a guide for the design of more sophisticated instruments. Exact measurements for the fluorometer body are unimportant to the basic over-all design, if the body is completely light-tight and all moving parts can be positioned accurately. Similarly, the exact nature of some of the parts is unimportant; equivalent or superior light sources, detectors, and measuring systems could well be substituted for those described. DESCRIPTION OF FLUOROMETER
The basic design showing the vertical arrangement of excitation source: optical cell, and detector and their positions relative to other fluorometer parts is given in Figure 1. Side and front elevations of the fluorometer body are presented in Figures 2 and 3. VOL 35, NO. 3, MARCH 1963
289
The fluorometer housing is attached to a small tripod stand with adjustable legs which level the instrument. The stand rests on the base of a Model V transmission fluorometer (2). The silvered tube, 18, passes through a hole in this base, which is supported by four steel rods about 16 inches above a lower base where the phototube rests. The lamp support and housing from the Model V transmission fluorometer are used; the bottom of the lamp housing is about 5 inches above the primary filters. Complete shop dranings for the lamp support and housing have been published ( 2 ) .
Positive positioning of optical cell or uranium glass standard is accomplished simply. The optical cell is in the light path when its slide carrier is pulled t o its extreme forward position; the uranium glass is in the light path when the slide carrier is pushed in. Similarly, the shutter is closed when pushed in and open when pulled out. A stop, not shown, limits the forward movement of the shutter. A GE 100-watt high pressure CH-4 mercury spot lamp is the excitation source. Vnrious lines from the mercury spectrum can be isolated, or as the lamp also produces a fairly high in-
tensity of white light, narrow wavelength bands from this continuous radiation can be isolated in regions where there are no mercury lines. The lamp is operated from a Sols S o . 301,883 constant wattage transformer and cooled with a fan. The aperture in the diaphragm above the primary filters is smaller than the cross section of the optical cell; consequently, all fluorescence is confined t o the cylindrical volume within the solution that coincides n-ith the pnth of the exciting light. As the latter does not reach the side walls of the optical cell,
O p t i c a l Axis
i
-____-
I
13
I 1 I
I I
I I
Figure 3. 1. 2. 3. 4. 40.
Side elevation of fluorometer body
Carrier for primary filters, 3, and diaphragm, 2 inches in diameter Diaphragm over primary filters, aperture 1 Primary filters, 2 X 2 inches Front top section, stationary Rear top section. Carries- primary filters and slides toward rear to give access to Interior. Optical cell inserted into instrument through this opening 5. Baffle 6. Shield for optical cell, which fits snugly within it. Cell has capacity of 25 ml., diameter of 3.1 34 cm., and Is 4.204 cm. high 7. Aluminum slide carrier with brass base for optical cell and glass standard, 9. Shown with glass standard in light path 8. Handle to slide carrier. When unscrewed and removed, front o f fluorometer, 14, slides upward to glve access to secondary filters, 1 1 , and glass standard, 9 9. Glass standard, 2 X 2 inches, uranium glass 10. Aluminum slide carrier for secondary filters. Can b e moved only after section 14 i s removed loa. Diaphragm over secondary filters with aperture 1 I/, inches in diameter 1 1 . Secondary filters, 2 X 2 inches 12 Baffle above shutter with 1 '/a-inch diameter aperture 13. Shutter, in closed position. Aperture in shutter 1 '/r-inch diameter 14. Sliding front, can be removed only after 8 is removed 140. Rubber gasket 15. Screw fitting attached to silvered tube 16. Base 17. Rear wail 18. Brass tube with silvered interior, connecting fluorometer housing through 15 to phototube housing. 13 inches long with inside diameter of 1 ' / 4 inches
290
ANALYTICAL CHEMISTRY
no spurious fluorescence in the side walls of the cell is produced. The aperture in a second diaphragm above the secondary filters as well as the shutter opening confines the path of the fluorescence after i t leaves the optical cell. The wells for the urimarv and sccondary filters are deep knough t o hold four filters. -4 Photovolt GAB interference filter in combination with several Corning glass filters isolates narrow wavelength bands of liqht for excitation and measurement,. Yarrow wavelength bands of light are required if absorption is to follow the Beer-Bouguer law; i u addition, they greatly increase the sensitivity and selectivity of analytical methods and reduce light contributing t o the blank. An RC.2 1P21 multiplier phototube is the detect,or. -kn -Atomic Instrument Co. Model 306 PR high voltage supply furnishes reguiat'ed power for the phototube. The photocurrent is measured with an RCA SO.TTT.V.-84 ultrasensit,ive c1.c. microammeter. This meter has a scale of 50 divisions and the full scale can i ~ emade equiv,!ent to 0.01, 0.10, 1.00, 10.0,or 100 pa. INSTRUMENTAL PERFORMANCE
1
i
-0 5
10 5 lC?(d-p-
If the detector and measuring device of a fluorometer are aufficiently sensitive and stable to measure the fluorescence accurately, the attainable sensitivity of a fluorometric determination depends principally upon the nature of the chemical-fluorescence system, the excitation wavelength, and that part of the total blank which does not result from the complex-forming ligand. For want of a better term this portion of the blank is called the instrument blank and includes stray light', instrument noise, filter-leakage, a i d epurious fluorescence from filters, optical cell, and reagents other t,lhari the coinp1e:r-forming ligand. If F,is the i n s t r u r ~ ~ eblank, nt F E is the reagent blank, F B is the t o t d blank (F, FR)>an(! F.1, is the fluorescence reading fur a mixture of a, metal complex and the reagent, the potential sensitivity for the determination is greatest at the excitation wavelength where the ratio (F'M - F B ) / F R is greatest ( I ) . HomTever, the attainable experin-ient,aI sensitivity varies with the size of the rat,io (FM-FB)/FB. Therefore, the potential sensitivity of the cheniltal system can be realized on!y if the tn-o rat.ios are equal. 'This occurs when P, is zero or when F d F , is large enough for the instrument blank to be insipnifii:n,nt. F n / F ; also determines whether or not F R can be measured accuratdj-. The size of the instrument blank is therefore of priil1ary i.rrqortance and an attempt has always been made to keep i t as low as possible. Historically this usually involved placing the phototube on a n axis perpendicular t o tlie path of the e x i t i n g light, because this limited the amount of exciting energy reaching
+
--*I
Figure 4. Fluorescence as function of solution depth 443.5 X 10 -6 mole BC in all solutions 1. 0.02366 X mole morin 2. 0.05915 X 10-6 mole morin 3. 0.1 183 X 1 mole morin 4. 0.2366 X 1 0 H mole morin 5. 0.4732 X mole marin 6. 1.183 X mole morin 7. 2.366 X mole morin
the phototube. I n the earliest fluorometers the perpendicular arrangement was almost mandatory because of the inadequacies of the filters and measuring devices available. With inndern equipment, filters can isolate the fluorescence from the exciting light very effectively, and sufficiently low instrument blanks can now be obtained with the transmission arrangement. Instrument blanks for several primary and tiecondary filter systems in a transmission fluorometer are given in Table I, where other d a t a illustrate the interrelationships among excitation wavelength, instrument blank, and relative sensitivity for three chemicalfluorescence systems. The data for each system were obtained with a single set of secondary filters and two sets of primary filters. The excitation wavelength, 365 mp, was used for all systciils. The second excitation wavelength for each system is the one where a solution containing excess metal ion and reagent gave the greatest positive absorbance difference from the reagent blank. It does not necessarily give the greatest absorbance difference when the solutions contain excess reagent, and definitely did not for the b e q Ilium-quinizarin
system. Wavelengths of 365 and 405 mM, which correspond to emission lines in the mercury spectrum, were isolated with conventional filters. In all other instances, narrow wavelength bands from the continuous radiation from the lamp were isolated with a combination of interference and conventional filters. As indjcat'ed in the table, the secondary filters for the different chemical systems isolated wavelength regions of varying purity for measurement; a narrow wavelength region was measured only in the case of the beryllium-morin system. Although wide wavelength bands were measured for two of the Rystems, Table I shows t,hat the instrument blank, Pi, and even F B , the total blank for the thorium-morin system, are all low, with the single exception of F , for 365-mp excitation of the berylliumquinizarin system, where all wavelengths greater than about 610 mp were measured. The data for the beryllium-quinizarin system illustrate how seriously the instrument blank can limit the sensitivity of a fluorometric method. With the secondary filters used with this system, t.he ratio ( F M- F B ) j F ~ i n d i c a t e s t h a t the potential sensitivity should be about 18% greater with 365-mp excitat,ion than with 525-mp excitation. However, the ratio ( F M - FB)/FBshows the actual sensitivity to be about 6.6 times more sensitive with 525-mp excitation than with 365-mp excitation. Moreover, the ratio F R / ' F ~ is large enough with 525-mp exc:tation to measure F E accurately, whereas with :365-mp excitation F R I F , is so small t h s t this would be almost impossible. 11-1contrast, in the beryllium-morin tern where a IiarroK wavelength band of fluorescence was measured, F R I Fis~ so large for both excitation wavelengths that the ratios, ( F M - F B ) / F R and - F B ) / F s , are essentially the same. All values clearly show greater sensitivity for 44:j-mp excitation. Here the inst>rumentblank would not be a limiting factor in an anaiyt,ical met.hod. Solution depth plays the major role in determining the value of the sensiof the fluorescence t i ~ i t yleve! (S.L.) ( 1 ) . However, changes in depth are accompanied by changes in the position of the fluorescent volume in relation to the phototube, and these changes could effect the relationship between fluorescence intensity and so1ut)iondepth. The extent of these effects is shown in Figure 4,where log fluorescence is plotted u s . log (cm. depth), depths calculated from volumes used, for each solution in a series that contained a large excess of beryllium and 0.02366 X l o + to mole per liter of morin. 2.366 X Each of the seven curves is a straight line with a slope of 1.0 when the depth is 1.296 cm. or less and S.L.is 2.28
*
VOL 35, NO. 3, MARCH 1963
291
1%. Why the top point of each curve corresponding to 3.241-em. depth (25ml. volume) is too large to fall on the curves is not known. Nevertheless, the constant slope of 1.0 and the straight-line relationship which occurs when depths vary from 0.1296 to 1.296 cm. and concentrations range from to 2.366 X mole 0.02366 X per liter, show that the theoretical
relationship between fluorescence and depth is followed for tenfold change in depth over a very wide range of light absorption, even though total absorbance never exceeded 0.01. The instrument design has therefore largely eliminated those effects which could result from varying the depth of solution and affect the relationship between fluorescence intensity and solution depth.
LITERATURE CITED
(1) Fletcher, M. H., AYAL. CHEM.35, 278 (1963). (2) Fletcher, M. H.9 May, Irving, Anderson, looq,J.Pt.W., 12, U. 93-5S. (1954). Geol. Survey Bull.
(3) Mllkey, R, G,, Fletcher, M. H., J . Am. Chem. SOC.79, 5425-35 (1957).
R~~~~~~~ for review J~~~ 11, 1962. Accepted December 5, 1962.
Solvent Extraction Method for Zirconium-97 Use for Evaluating Critical Nuclear Incidents WILLIAM J. MAECK, S. FREDRIC MARSH, and JAMES E. REIN ldaho
Atomic Energy Division, Phillips Pefroleum Co., ldaho Falls,
b Radiochemical analysis is an important means of determining the magnitude of critical nuclear incidents. O n the basis of experience with three incidents in which large levels of fission products were present prior to the incidents, three fission product CeI43, and ZrgT-are nuclides-Mo99, recommended as monitors. A method is described for Zrg7 based on TTA extraction of zirconium activity followed b y milking and counting of the Nbg7 daughter activity. Relative standard deviation is 1.1 %.
A
means of determining the magnitude of a critical nuclear incident is radiochemical analysis of the postcritical material for selected fission product nuclides. Desirable nuclear properties of nuclides for this purpose are (1) a high fission yield, (2) a well-known, preferably simple, decay scheme, (3) the absence of gaseous precursors in the decay chain, and (4) a half life of 1 to 100 hours. The nuclides should be nonvolatile, even at elevated temperature. Reliable methods of separation and dctermination are needed. From the standpoint of the radiochemist, critical nuclear incidents can be classified in two types, characterized by either the presence or absence of fission product. prior to the incident. For example, in a reactor that is temporarily shut down or in a processing plant in which U*35 or Puz39 is recovered from spent fuel, copious quantities of fission products are present. This dictates that the half life of the selected criticality-measuring nuclide be sufficiently short that significant quantities N IMPORTANT
292
ANALYTICAL CHEMISTRY
are not present just prior t o the incident. However, the half life should be a minimum of about 1 hour, because a critical incident invariably contaminates the environs and samples are not obtainable until hours or even days afterwards. The upper half-life value is governed by the age of the fission products in the precritical material. For instance, spent fuel usually is cooled a t least 6 months before processing, and a nuclide with a half life of 100 hours is essentially decayed. Lyon, Reynolds, and Eldridge (3) have suggested nine nuchdes for the radiochemical evaluation of critical incidents. These are, listed according to decreasing half life, 65-day Zr95, 51-day Srsg, 33-day Ce141, 12.8-day Ba", 67-hour M099, 33-hour CelJ3, 17-hour Zrg7, 9.7-hour Sr9l, and 84minute Ba139. On the basis of experience in this laboratory with the evaluation of three incidents, the SL-1 reactor (8) and two in processing vessels in the Idaho Chemical Processing Plant (1, 9 ) , all characterized by the precriticality presence of large levels of fission products. six of these nine nuclides were not suitable. The four longest lived nuclides-Zrg5, SrSg.Ce141, and Bal4--especially the first three, were present in large amounts prior to the incidents. Two nuclides, Ba139 and Sr91, which have long-lived gaseous precursors were spewed from the environs as a result of the high temperature, rapid expansion, and, in the case of the SL-1 reactor, rupture of the fuel cladding. These were found in air samples several miles from the incident areas. I n the decay chain, over 80yoof the Ba'39 is formed through 41-second Xe139 ( 2 ) , and about 60%
of the Srgl is formed through 10-second KrQ1. Also, 60% of Ba140 is formed through 16-second Xe14 (2). Of the three nuclides suitable for the evaluation of the type of incident being discussed, applicable radiochemical methods for molybdenum-99 and cerium-143 have been reported ( 5 , 7 , I I ) . The main purpose of this paper is to describe the development of a method for ZrQ7. In addition to its 17-hour half life, this nuclide has desirable nuclear properties of a 5.9% U*36 thermal fission yield and no significant gaseous precursors. EXPERIMENTAL
Apparatus a n d Reagents. Extractions were made in 25-ml. screw-top test tubes (Kimax K45066A), with Teflon stopcocks sealed to the bottom of the tube, on a 33-r.p.m. extraction wheel (spinnerette model, S e w Brunswick Scientific Co., Ken. Brunswick, N. J.). The strip solutions were collected in 1-inch-diameter x 4-inch cylindrical plastic tubes (Lermer Plastics, Inc., Garwood, S . J.) and counted with a 3 X 3 inch N a I (Ti) crystal coupled to a Nuclear Data 512 channel analyzer. Reagent grade chemicals and the 2-thenoyltrifluoroacetone (TTA) (Peninsular Chemical Research, Inc., Gainesville, Fla.) were used without purification. T o prepare the zirconium carrier, dissolve 6.0 grams of ZrOCl? 8H20 in 6 X HKOa. Boil until the solution clears and nitrate decomposition ceases. Cool and dilute to 500 ml. with conand water to make centrated "03 the final concentration 6-11 in nitric acid. Prepare the niobium strip solution by dissolving 131 mg. of KsNbsOlg 16H20 in 500 ml. of water, adding 17 ml. of 30% HzO~,and diluting to 1 liter with 6M HClOI.
