(4) Bernstein, R. B., Iiatz, J. L., Ibid.,
ACKNOWLEDGMENT
11,46 (1953). 15) Cohen, B. L.,. Phvs. Rev. 81, 184 (1951). ‘ (6) Coleman, R. F., Analyst 86, 39 (1961). (7) Coleman, It. F., Perkin, J. L., Ibid., 84, 233 (1959). (8) Everling, F., Koenig, L. ;?., Mattauch, H., 1960 Kuclear J. H. E., Wapztra, A4. Data Tables, Part 1, Xational Academy of Sciences, National Research Council, February 1961. ( 9 ) Fuller, E. S., Kaiser, W., Thurmond, C. D., Phys. Chem. Solids 17, 301 (1961). (10) Gatos, H. C., Am. Inst. Chem. Engrs. Conf., New York, N. Y., December 1961. (11) Heath, R. L., AEC Research and Development Rept. IDO-16408, July 1, 1957.
The authors thank J. R. Woolston for the mass spectrometric analysis and L. R. Weisberg for supplying the ingot. LITERATURE CITED
(1) Anders, 0. V., ANAL.CHEM.33, 1706
(1961).
( 2 ) Bate, 1,. C., Leddicotte, G. W.j Pittsburgh Conference on Analytical
Chemistry and Applied Spectroscopy, March 1958. (3) Beard, D. B., Johnson, R. G., Bradshaw, W. G., Aucleonics 17, 90 (1959).
(12) Koch, R. C., “Activation AnalJsis
Handbook,” ASTIA DOC f.214941, December 15, 1958. (13) bhrkowitz, 5. S., Mahony, J. D., ANAL.CHEM.34,329’(1962). (14) Morrison, G. H., Cosgrove, J. F., Ibid., 27,810 (1955). (15) Osmond, R. G., Smales, A. Anal. Chim. Acta 10, 117 (1964). (16) Smales, A. A Pate, B. P., ANAL CHEW24,717 (1952). (17) Steele, E. L., Rfeinke, W. W., Ibid., 34, 185 (1962). 118) Thommon. B. A., Ibid., 33, 583 . (1961). ’ (19) Veal, D. J., Cook, C. F., Ibid., 34, 178 (1962). RECEIVEDfor review July 24, 1962. Accepted March 22, 1963.
Quantitative Interpretation of Color Quenching in Liquid Scintillator Systems HARLEY H. ROSS’ and ROGER E. YERICK2 O a k Ridge Institute of Nuclear Studies, Oak Ridge, Tenn.
b A general approach to the quantitative interpretation o f color quenching in liquid scintillator systems i s developed and applied to the specific case of carbon-1 4 in dioxane scintillator. The observed linear relationship between per cent quenching and concentration of color-quenching species permits the evaluation o f quenching coefficients. In the perfectly general system, a complete series solution requires the use of a computer program. For systems in which the absorption spectrum of the colored material i s simple, good agreement between predicted and observed quenching can b e obtained b y determining only a few coefficients. In the system investigated, agreement i s achieved with the evaluation of only three coefficients. The linearity of color quenching makes it possible to estimate easily the amount o f quenching in systems in which only one quenching agent of known identity i s present. In any system, a comparison o f the predicted color quenching with the observed total quenching permits calculation of the amount o f chemical quenching within the system.
T
methods of sample preparation for liquid scintillation counting often rcsult in systems that are considerahly less than ideal from the standpoint of counting efficiency. Several techniques have been investigated for correcting for total quenching by R ~ internal standard method (6, 11, 12) o r by an approximation tcchiiique (‘7). \1/ hile work has been done on cheiiiical
794
quenching (2, 8),little has been published on the general phenomenon of color quenching. Some specific instances have been noted. Leffingwell, Riess, and Melville (IO) observed the effect of color on iron-59 systems containing the colored tris-l,lO-phenanthroline iron(I1) chelate but noted an enhancement of photon yield, relative to their colorless standards, that was chelate-concentration dependent. Baille (1) found a difference in over-all magnitude between chemical quenching and color quenching at higher concentrations, and Herberg (9) investigated the effect of color in obtaining suitable background solutions for certain types of biological materials. Helf and White (6) studied the quenching effects of organic nitrocompounds as a function of their ultraviolet absorption spectra but did not suggest a way to correct for this other than to change the chemistry of the counting system. Ilalvorsen (4) used an extrapolation technique to correct for color quenching in tissue samples. However, this technique can be used only for the same or similar samples. In the present study, a n attempt was made to correlate color quenching with concentration and wavelength absorption maximum of the quenching material for a particular liquid scintillator sy st em .
HE USUAL
ANALYTICAL CHEMISTRY
counts recorded were t h e sum of t h e red and green registers. T h e photomultiplier high voltage mas selected t o give balance point operation in t h e red channel for (2-14in a n unquenched sample. Screwcap 20-ml. glass vials were used as sample bottles. Spectral measurements were made with a Rausch & Lomb Spectronic-20 colorimeter with matched I/*-inch test tubes. The scintillating medium consisted of a dioxane solution containing 200 grams per liter of naphthalene, 7 grams per liter of 2,Bdiphenyloxazole (PPO), and 0.3 gram per liter of 1,4bis - ( 5 - phenyl - 2 - oxazolyl) - benzene (POPOP). Twenty milliliters of scintillating solution were used in each sample. Coloring agents used mere watersoluble FD&C coal-tar dyes. Aqueous solutions of these mere prepared of such dilution that from 10 p1. to 700 pl. were required to give the desired absorbances in the liquid scintillator medium. The weight of dye in these solutions varied from ahout 1 pg. to 100 pg. Activity was introduced as toluene-l-CI4 dissolved in toluene (0.1 /Jc. to 0.01 pc.). Absorbances of the colored solutions were measured against a dioxane blank, because the liquid scintillator blank proved to have too high an absorbance fcr proper spectrophctometer operation. The indirect method of measuring absorbances of liquid scintillator alone us. dioxane blank, and liquid scintillator plus dye against dioxane blank, and
EXPERIMENTAL I
Apparatus and Reagents. C‘ounting w a s done wit,h :? F:irkaril Tri-CIarh liqiiicl sc:iiitill:tticrii spcctriomcltr,
Modo1 314. 1~isciiriiiii:ttor scttiiiys were 10, 50, and 100 yolts, and t h e
1 Present address, Analytlral C‘hmnstrvThvision, Oak Ridge National I,:thoratiirj,
P.O. Box X, Oak Ridgc, Tenn.
2 Prcsent address, Lamar State Collq!c~ of Technology, Beaumont, Texas
scintillator system, i t was expected that the red and yellow solutions Fvould show the greatest quenching effect. That they do is s h o m in Figure 2 . By the bame criteria, it was expected the yellow solutions would show the greatest quenching. However, the particular spectra overlap for the yellow and red dyes most probably accounts for the quenching curves obtained. I n order to evaluate inathematically the total color quenching in a system, it must be recognized that there IS a coiitribution to quenching by every M avelength component of the absorbing species. This is analogous to the total light absorption of a multicomponent system that follows Beer’,. law, for which the expression for total absorbance takes this form, assuming some constant light path :
L 0.61
YELLOW DYE
WAVELENGTH, M Y
Figure 1.
Absorption spectra of dyes
where finally calculating alworbance due to coloring agent s h o w d negligible permutation due to presence of liquid scintillator. Procedure. Comrlete visible spect r a were obtained fcr t h e dyes used. Aliquots of these dyes Fere added t o unquenched counting samples of C14 toluene a n d t o samples of pure dioxane with t h e same volume as t h e counting sample (20 nil.). T h e activities of t h e colored counting samples were determined a n d t h e per cent quenching noted. Absorbance measurements were made on the dyed dio u n e samples. The per cent quenching values and the absorbance of the samples (at selected wavelengths) were used to calculate quenching coefficients from equations derived in the following section. Once the quenchin,; coefficientq had been determined for the scintillator system, i t was possible to calculate the quenching of a n unknown svstem from absorbance mcasuremtants only. Quenching coefficitntq werc determined for various leveli of activity (0.1 pc. to 0.01 p c . ) and a t different instrument settings. KO +@cant variation in quenchin: coefficients was obierved. Absorption spectra of the dyes used m shomn in Figuirl 1. The data c d e c t e d iri thc que ncliing meaiureiiicnts arc 4ioivn in Figirc 2.
exhibits an average path length. When the absorbance of the solution is low, this average path length remains constant. As absorbance increases, a point is reached where some total absorption takes place. When this occurs, the effective average path length changes and deviations from linear response are observed. Practical applications indicate that only the linear portions of the curves need be considered. Because of the wavelength characteristics of the light emitted in this
A t = total measured absorbance B = molar absorptivity of ndicated
c
=
component concentration of indicated component
It is conventional in these systems to measure A t , obtain values of E from independent data, and calculate the concentrations of the various components by solving a series of simultaneous equations. The total color quenching in a sys-
I
RESULTS AND D SCUSSION
T h e quenching curT’es, as illustrated Figure 2, are linegr functions until about 60% quenching is reached. The nonlinear deviations in the highly absorbing solutions of red and yellow dye can be explained by the geometry of the transmission system. JT7hen a small amount of color is precent in the scintil1:itor vial, tltc e i n i t t d photons rriiiqf, travel through i m n y different optical path lengths. The srrtein, in effect, 111
AB SO RBAN CE Figure 2.
Quenching as a function of dye concentration 0 Red dye
4-
Yellow dye
0 Blue dye VOL. 35, NO. 7, JUNE 1963
795
Y = ahsorbancr of yellow solution at
tern is represented by the following integral :
designated wavelength R = absorbance of red solution a t designated wavelength /3 = absorbance of blue eolution a t designated wavelength I , ) bliie solution was due to the small peak a t 410 m p and the absorbance a t 510 in which the subscripts indicate values nip. The value of K s was too small to at specific wavelengths. I n order to be computed with the available preevaluate the various K J s , it is necewwy ckion of the data. to prepare n different solutions, measure Assuming, that for all practical purthe absorbances of each solution at ?L poses, any absorption above 510 rnp did different oavelengths, and determine not contribute to the over-all quenching, the per cent quenching for each solua new set of equations involving data at tion. These data give n equations in 400 inp, 455 nip, and 510 m.3 was den unknown. Obviously, if n is very veloped. The 4 5 5 - m ~ region was large, a computer would be derirable. arbitrarily chosen, being half-way beSince any Wavelength region can be tween the other two: dividrd into any number of segments, the value chosen for n is purely arhiQt, = ( K l ) (Y m ) ( K 2 ) (Y ~ s s ) MI)( YSIO) trary. As n gets smaller, the values of Q? = (KI)(ItmO) ($2)(”455) (K3)(R610) K become more and more approximate Q b = (KINB400) ( k z ) ( & s ) (K,)(B510) and represent averages of all possible values for the segment chosen. n.here the terms have thc same signifiSince solutions of three different cance as before except that colors were used in this study, an atK , = quenching coeficient at 400 nip tempt to determine only three K-values K t = quenching coefficient a t 455 m p was made, employing the wavelength K3 = quenching coefficient at .?I 0 m p absorption maxima for the yellow (400 nip), red (510 nip), and blue (625 nip) Solution of this set of equations gave the following values for the quenching solutions. The equations to be solved took the form: coefficients : X =
wavelength
+
+
+
++ +
++ +
KI
=
lis
= =
Ii2
where Q = observed quencliing by the desig-
nated solution, yellow, red, or blue
Table 1.
80g,Q/unit il 38YcQ/unit A 80xQ/unit A
Frorii these values it was possible to estimate the degree of color quenching for various systems by means of this relationship :
Comparison o f Calculated and Observed Quenching for Various Colored Substances
r Absorbance __ Dye Bromcresol purple
400 mw 455 mp 510 mp 0.48 0.07 0.01
Q, 70 Calcd,-01;;3d. 41.9
Methylene blue
0.00
0.01
0.02
2.0
Methyl red
0.14
0.30
0.17
36.2
F D and C yellow dye FD and C blue dye Methyl orange
0.27
0.15
0.06
32.1
0.08
0.05
0.00
8.3
Azur I1 Eosin
0.01
0.01
0.02
2.8
+
796
ANALYTICAL CHEMISTRY
i
i
Ll-Q]
42.2 43.0 2.8 1.5 50.6 50.7 33.7 30.5 11.8 10.4 1.8 0.7
Calcd.
Obsd.
1.72
1.73
1.02
1.02
1.57
2.02
1.47
1.47
1.09
1.12
1.03
1.01
&e
= (Ki)(Aica) f
(Kz)(Ataa)
+ (KaJ(A6io)
where ralculated color quenching 80YGQ/unitA 3870Q/unit A 80yoQo&/unit A A measured absorbance of solution as designated wavelength The data in Table I compare predicted and observed quenching for several colored systems. Three solutions containing the same amount of each dye were prepared, two in liquid scintillator and one in dioxane. Absorbance measurements were made on the dioxane solutions. Fifty microliters of carbon-14 in toluene were added to each of the scintillator solutions. I n order to correct an observed counting rate for color quenching, the follorving equation is used : Qe
KI K3 K3
= = = = =
[1
Rateoarr= Rateobsd -
(31
Calculated - . - and observed values for the factor are in good agreement except in the case of methyl red. Continuing studies indicate that methyl red is not stable in the scintillator solution at the concentrations used. If the colored compound has a low molar absorptivity, higher concentrations nill be needed to produce significant color quenching, and chemical quenching could be ignored since only microgram quantities of dyes were used. However, this may not be the case in all systems. Once it ha.; been established that color quenching may be satisfactorily evaluated for a system by a procedure such as the one described, a comparison between observed total quenching and predicted color quenching will give an estimation of the chemical quenching in the system. I n the event that color quenching in a given system were to come from a single, known material, evaluation of quenching coefficients would not be necessary. Various amounts of this quenching material could br added t o liquid scintillator systems, the absorbance of each solution measured a t any convenient wavelength (preferably the wavelength corresponding to the absorption maximum), the amount of quenching determined, and a curve similar to one in Figure 2 constructed. It would then be necessary only to measure the absorbance of the unknown solution at the same wavelength and read the per cent quenching directly from the graph. Because the quenching effect is dependent upon the disintegration energy of the particular radioactive tracer being counted ($, different quenching coefficients nill be obtained for different isotopes. Tracer3 with similar disintegration energies should be similarly quenched. Because the mechanisms of energy transfer are complex, different
le=]
quenching cocfficients may be found for the same radioactive iijotope in different liquid scintillator systems. Thus, quenching coefficients are valid only for a specified tracer in E, specified scintillator system. Furthw, the calculated values reflect the ccmbined response efficiencies of the mu1tiplier phototubes and associated electronics of the particular counting equipment used. The approach prescnted here is general for the evaluation of color quenching in any liquid scintillator system. The more complicated the absorption spectrum of the quen:hing species, the larger the number of quenching coefficients that must be evaluated before predicted and observed quenching will agree. The authors do not propose t h a t this technique be used to replace the internal standard rnrthod of quench-
ing correction. However, the technique should be of great value in studying new and old scintillator systems as a means of separating and evaluating the combined effects of color and chemical quenching in a given system. LITERATURE CITED
(1) Baille, L. A., Atomlight pp. 1-7, June 1961. (2) Brown, F. M., Furst, hf., Kallmann, H., “Proceedings of the University of
Kew Mexico Conference on Organic Scintillation Detectors,” U. S. At. Encrgy Comm. Report TID-7612. (3) Guinn, V. P., “Liquid Scintillation Counting,” Proceedings of Northwestern University Conference on Liquid Scintillation Counting, C. G. Bell and F. K. Hayes, eds., p. 173, Pergamon, New York, 1958. (4) Halvorsen, K., “Tritium in the Physical and Biological Sciences,” Vol. 1, p. 313, International Atonric Energy Agency, Vienna, 1962.
( 5 ) Hayes, F. N., Intern. J . A p p l . Radiation Isotoves l . 46 (1956). (6) Helf, S:,Whte, ~C.,ANAL.CHEM.29,
13 (1957). (7) Helf, S. White, C. G., Shelly, R. N., Ibid., 32, 238 (1960). (8) Helmick, M., Atomlight pp. 6-7, February 1960. (9) Herberg, R. J., ANAL. CHEY. 32, 1468 (1960). , \
~
-
-
(10, Leffingwell, T. P., Riess, R. W.,
Melville, G. S., U. S. Dept. Commerce O.T.S., P.B. 148-081, 1960. (11) Okita, G. T., Ypratt, J., LeRoy, G. V., A w l e o n i c s 14,No. 3, 76 ( I 956). (12) Williams, n. L., Hayes, F. N., Bandel, R. J., Rogers, JV. H., Itlid., S o . 1, 62 (1956). RECEIVED for review September 17, 1962. Accepted April 8, 1063. Presented at the 18th Southwestern Regional Meeting, ACS, Dallas, Texas, December 1962. Oak Ridge Xational Laboratory is operated by Union Carbide Nuclear Co. for the U. S. Atomic Energy Commission.
Half Lives of Cesium-137 and Cesium-134 as Measured by Mass Spectrometry L. A. DIETZ, C. F. PACHUCKI, and G. A. LAND General Elecfric Cornpiny, Knolls Atomic Power laboratory, Schenecfady, N. Y.
b Mass spectrometric measurements of radioactive decay rates over a period of l l / z years indicate a half life of 30.35 f 0.313 years for Cs13’ and 2.046 f 0.004 years for CS’~‘. Precisions, quoted as standard deviations, are considered to b e preliminary values, because! the experiment will b e continued.
H
of the important radioactive nuclides CsI37 and C P have been measured many times, b u t the considerable spread in the reported values indicates the presence of unsuspected and/or incompletely corrected systematic errors. I n methods which have been used previoL sly for measuring these half lives i t has been very difficult, if not impossible, to assess experimental errors inherent in m:tss spectrometric measurements of absolute abundance, radiochemical determinations of specific activity, or numerous :orrection factors which must be applied to the data. We have developed :m internal standard technique (4) which eliminates most of these experimenta difficulties and thereby greatly improves the precision of an isotope ratio measured b y thermal ionization mass spectrometry. The only apparent disadvantage of this method for determining a n i itermediate half life, such as that for CkP, is that measurements must be spread over several years to achieve the desired high precision. ALF LIVES
INTERNAL STANDARD TECHNIQUE
Over a very limited mass range we can define a small quantity 6 by which a measured isotope ratio differs from the true ratio as 6 = kAM/M, (1) where k is a bias factor for a given set of experimental conditions and is independent of the mass separation A X of a selected isotopic pair-ie., an isotopic pair of mass separation A M has associated with i t a bias of 6, and a mass separation of 2AM has nearly a bias of 26. The bias factor k is approximately
ISOTOPE
M2
constant during an analysis with one filament, but may have slightly different values for different filaments. The bias factor k depends on optical transmission ( 3 ) ; it may also depend on i5otope effects in surface ionization phrnomena a t the filament and on velocity r)r mass effects in the electron multiplier detection process. Equation 1 is the baqic assumption underlying the internal standard technique. From Figure 1 we see that a true abundance ratio, as derived from ion current measurements, is given by (II- ATl)/12 = (1 - 6)Z1/12---i.e., the observed ratio of two ion currents will differ from the true ratio by a factor (1 - &2), so that n2(true) = (1
- 8L2)r12(observed)
(2)
where rI2(observed) = Il/Z2, An expression similar to Equation 2 applies to the observed ion current intensity of isotope 2 divided by that of isotope 3. Dividing the two expressions defines a ratio of ratios,
( r d n d (true)
n
=
F(rlZ/r*S) (observed) (3) 0.
Figure 1. Schematic representation of mass spectrometer response for voltage scanning of the mass spectrum. Separation between V I and VZ is greatly exaggerated
where F = (1 - kA.W12/M,)/(l kAM2S/M2), and 311 < 3 1 2 < M 3 . I n applying the technique to an actual measurement, i t is not always essential to form such a ratio of ratios to correct for bias. Equation 3 is just a convenient way of showing how the method works. When A M I2 = AMzs, F is VOL. 35, NO. 7, JUNE 1963
797
