fere with its determination. Lithium and sodium d o not interfere; however, ammonium, potassium, and rubidium must not be present in excess of the amounts specified, as they will coprecipitate with the cesium. Reducing agents will turn the 12-molybdophosphoric acid blue prior t o precipitation but if not present in large amounts they generally d o not interfere in the precipitation since a large excess of precipitant is used. Interference of some of the transition metals in the ultraviolet method was probably the result of coprecipitation of the mctal resulting in its contribution t o the ultraviolet absorption. Conformity t o Beer’s law for the overall ultraviolet procms was observed for solutions corresponding to 0.05 to 1.413 mg of cesium (based on final volume of 100.0 ml). The optiinum concentration ranges, based on Ringbom plots were 0.1 ‘io0.7 mg of cesium at 226 mp and 0.1 to 0.5 mg cesium at 208 nip. 1)ISCUSSION
An indication of the precision of thismethod wasascertained from the results of 12 samples, run according to the recommended procedure, each containing 0.25 mg of cesium. For the heteropoly blue procedure the mean absorbance value was 0.522 at 805 mp; i.he range was 0.514 t o 0.534. The standard deviation was 0.0082 giving a relative standard deviation of 1.6%. About 50 minutes were required for a series of four determinations. For the molybdate procedure the mean absorbance values
were 0.584 and 1.127 a t 226 and 208 mp, respectively. The range was 0.573 to 0.596 a t 226mp and 1.075 to 1.166 at 208 mp. The standard deviations were 0.0081 and 0.0330 at 226 and 208 mp, respectively, giving corresponding relative standard deviations of 1.4 and 2.9 %. About 30 minutes were required for a series of four determinations. The composition of the precipitate was determined by finding the amount of molybdate resulting from the decomposition of a precipitate and comparing it with the amount of cesium taken initially. A standard calibration curve was prepared using the absorbances from known amounts of molybdate dissolved in the borate buffer. The amount of molybdate in a precipitate was determined by comparing its resulting ultraviolet absorbance with the standard calibration curve. Molar ratios of molybdate t o cesium of 5.8 to 1 and 5.9 to 1 were obtained when the absorbances were measured at 226 mp. These values correspond closely to a formula of C S ~ H P ( M O ~ O for~ ~the ) ~ precipitate (theoretical Mo/Cs = 6.0/1). This formula corresponds to that obtained for the thallium (4) and ammonium ( 9 ) salts of 12-molybdophosphoric acid prepared under similar conditions. RECEIVED for review August 18, 1966. Accepted November 7, 1966. Work supported by a grant from the National Science Foundation for Undergraduate research participation. (9) W. W. Wendlandt, Anal. Chim. Acta, 20, 267 (1959).
Determination of Lanthanide Distribution in Rocks by Neutron Activation and Direct Gamma Counting James C. Cobb Department of’ Chernistrj, Brookhacen National Laboratory, Upton, N . Y .
THELANTHANIDES are a group of elements which are of great geochemical interest because chemically they are so similar, that any relative variations among them, which are thought to be due principally to differences in the trivalent ionic radius, should be indicative of the evolution of various rock types. The separation of the lanthanides is a formidable analytical problem, and until recently, little work has been done. In recent years, the technic ue of neutron activation analysis, coupled with separations by ion exchange chromatography, has been applied to varims rocks by several groups (1-3). Russian geochemists have also been active in this field. Their analytical technique involves a group separation of the rare earths and analysis hy x-ray spectrography (4, 5 ) . It is apparent from the published work that relative variations among the lanthanides are of much more interest than absolute abundances. When plotted against the trivalent ion radius, the relative abundances invariably follow a smooth trend. Therefore, it is not necessary to perform analyses o n (1) L. Haskin and M. A. Grhl, J . Geopliys. Res., 67, 2537 (1962). (2) R. A. Schmitt, R. H. Smith, J. E. Lasch, A. W. Mosen, D. A. Olehy, and J. Vasilevskis, Geocliim. Cosmocliim. Acta, 27, 577 (1963). (3) D. G. Towell, J. W. Winchester, and R. V. Spirn, J . Geophys. Res., 70, 3485 (1965). (4) Y . A. Balashov, A. B. Ronov, A. A. Migdisov, and N. V. Turanskaya, Geochemistry (English trans.), 1964, p. 995. (5) L. K. Gavrilova and N. \’. Turanskaya, Zbid.,1958,p. 163.
11973
all fourteen lanthanides in order to establish the variations in any individual rock. I t is the purpose of this paper to present a method of analysis of six or seven lanthanide elements in various rock types by neutron activation analysis, followed by direct gamma counting of the sample, without any chemical separations, on high resolution lithium-drifted germanium semiconductor detectors. Some of the properties and uses of Ge(Li), detectors with particular reference to nuclear spectroscopy, are described by Ewan and Tavendale (6). Additional descriptions and applications t o neutron activation analysis of impurities in aluminum foil are given by Prussin, Harris, and Hollander (7). The resolution achieved by these detectors is more than an order of magnitude better than the conventionally used Na(T1) scintillation detector. The increased resolution makes it possible to identify gamma photopeaks even though they are close together in energy. The number of elements which can be measured by a combination of neutron activation analysis and direct gamma counting vary with rock type, but gamma photopeaks due to the lanthanide elements are invariably present. The lanthanides which can be measured represent a (6) G. T. Ewan and A. J. Tavendale, Can. J . Pllys., 42, 2286 (1964). (7j S . G. Prussin, J. A. Harris, and J . M. Hollander, ANAL.CHEM., 37, 1127 (1965). VOL. 39, NO. 1 , JANUARY 1967
127
Table I. Description of Samples Analyzed Source Name Westerly, R. I. granite. Circulated by U. S. G- 1 Geological Survey as standard for elemental analysis. Chelmsford, Mass., National Bureau of Standards radium standard No. 4979 granite Graniteville Granite NBS radium standard No. 4981 Diabase from Centerville, Va. Circulated w-1 by U. S. Geological Survey as standard for elemental analysis. NBS radium standard No. 4985 Deccan trap Mid-Atlantic Ridge Location and petrology described by Shand Basalt (8) Burlington Limestone (Mississippean) from Limestone Hickory County, Missouri Hudson River pelite, Dutchess County, New Shale York Garnet separated from metamorphosed Garnet Hudson River pelite, Dutchess County, New York
highly fortunate selection. Published work o n the distribution of lanthanides in rocks ( 9 ) shows that the eight elements gadolinium to lutetium exhibit little relative variations. To define this group, dysprosium, ytterbium, and lutetium can be determined. The six light elements lanthanium to europium generally show smooth relative variations. Of these, lanthanum, cerium, samarium, and europium can be determined. Abundances of manganese, scandium, and thorium will also be given, since these are the chief interfering elements in the measurement of the lanthanides. EXPERlMENTAL
The germanium detector used in this study was made in the Instrumentation Division of Brookhaven National Laboratory. Dimensions are 2.1 cm by 1.3 cm, with a depletion depth of 6 mm. The active volume is thus approximately 1.6 cc. The detector was operated in a vacuum at a temperature of about 80" K with a bias voltage of 800 v. The (8) S . J. Shand, J . Geol., 57, 89 (1949). (9) L. A. Haskin and F. A. Frey, Science, 152, 299 (1966).
Table 11. Decay Characteristics of Nuclides Measured Nuclide Half life Gamma energies, kev l 40La 40.2 h 328, 490, 1600, others lCe 32.5 d 145 lsaSm 47 h 103 lazEu 12.5 y 122, others 165Dy 2.3 h 95, others 169Yb 31 d 178, others 4.2 d 175Yb 396, others 6.8 d '77Lu 208, others 46Sc 890, 1120 84 d 56Mn 2.58 h 850, others 3Pa 27 d 314, others
housing for this and similar detectors made at Brookhaven National Laboratory is described by Chasman and Ristinen (10). The output signal from the detector was fed into a field effect transistor preamplifier and an Ortec Model 410 amplifier. The resultant pulse was then analyzed by a Nuclear Data 1024 channel analyser. Resolution for the T o 1330-kev peak was 4.0 kev. The samples run in this reconnaissance survey include three granites, three basalts, one limestone, one shale, and a garnet separated from a metamorphosed shale. The samples were powders ranging from 50 to 100 mg. Additional descriptions of these samples are given in Table I. These were weighed onto and wrapped in 99.99% aluminum foil for the irradiation. The flux monitors were solutions containing known amounts of the elements to be determined. Fifty p1 were pipetted onto an aluminum foil, evaporated under a heat lamp and wrapped in a manner so as to correspond as closely as possible to the dimensions of the samples. It was found to be possible, in wrapping the samples, to reproduce the dimensions fairly well. This was about 1 cm by 1 cm. In order to minimize the error caused by the slightly different sample sizes and thicknesses, the samples were counted on a shelf position 6 cm away from the face of the detector. Peak intensities were read off from the output tape of the multichannel analyzer, This was corrected for a background estimated by averaging the counts per channel just before and just after the photopeak. (10) C. Chasman and R. A. Ristinen, Nucl. Inst. Methods, 34, 250
(1965). J-sc46
( I 120)
Fe59 (1290)
I
scHfl8' (482)
(890)
1 I
COG0
(1170)
1
co60
(1330)
1
ENERGY ( k e V )
Figure 1. Left. G-1 gamma spectrum; 7.0-day irradiation; count 8 days after EOB. Right. G-1 gamma spectrum; 21 days after EOB 128
ANALYTICAL CHEMISTRY
sc46
l890)-l
+ Z v)
~m'53
I
(I031
sc46
I
3 0 0
J
Fe5' (1290)
xit
ENERGY IkeV)
ENERGY (keV)
Figure 2. Left. W-1 gamma spectrum; 7.0-day irradiation; count 8 days after EOB. Right. W-1 gamma spectrum; 21 days after EOB Two irradiations were performed on each sample. One was 5 minutes long, so as to measure 163Dy, j6Mn, and 1j3Sm, and the other was of seven days duration to pick u p photopeaks due to elements of longer half life. The intensity of each photopeak was measured three to six times at suitable intervals to check the half life. The quoted errors are a combination of the standard deviations of the series of measurements on the sample and the flux monitor. The errors d o not incorporate that due to the differences in cross-sectional area and thickness between the samples and the flux monitor. This geometry error is estimated to be not more than 2%. The count rate and peak to background ratio in the flux monitors. was usually considerably more favorable than in the samples and consequently the statistical errors for the flux monitors of 2 to 3 % were usually significantly less than the counting errors for the samples. Table I1 summarizes the decay characteristics of the nuclides discussed in this paper. Figures 1 and 2 illustrate the gamma spectrum obtained from a seven-day irradiation of the granite G-1 and the diabase W-I, both of which are circulated by the U. S. Geological Survey as standards for trace and major element analysis. Figures la and 2a show the spectrum eight days after the end of the irradiation. Nuclides with half lives between 2 and 7 days are still present. Figures l b and 26 show the spectrum at a later time when these relatively short-lived nuclides have decayed. In Tables 111 and I V the lanthanide abundances found in the different rock types are summarized. Table 111 gives a comparison of lanthanide concentrations in G-1 and W-1 to published data obtained by neutron activation and chemical separation before counting. In the following section the measurement procedure for each lanthanide will be discussed separately. Lanthanum. Three gamma peaks due to 40-hour lroLacan be seen in the gamma spectrum following a long bombardment. The presence of :!4Nacauses considerable background problems so it is necessary to wait until eight days after the end of the irradiation in order to see a measurable 140La signal. The photopeak chosen for measurement is at 1600 kev. The most prominent lJoLapeak at 490 kev is sometimes slightly affected by I81Hf activity at 482 kev. Another 140La peak at 328 kev is at a position where it is difficult to get a good background estimation. The decay can usually be followed for at least two half lives. Cerium. l 4 C e is represented by a photopeak at 145 kev. The decay of the peak flsllows the proper half life from the
100
IA -
. -jI0
.
i
DECCAN TRAP
I
1.14 ILO)
1.07 (Ce)
1.00 0.98
0.92
0.86 0.85
(Sm) (Eu)
(Dy)
(Yb) (Lu)
IONIC RADIUS
Figure 3. Lanthanides in Basalts beginning of counting to as much as three half lives later. 141Ce,however, could not be detected above the background in the case of the basalts and the garnet due to the high background caused by 4%. Samarium. The '53S.m gamma at 103 kev can be detected in both irradiations. It was customarily measured after the short irradiation. 153Sm begins to be seen two to three days after the irradiation after the 24Na activity had decayed sufficiently to reduce the background in the 100-kev region. A much more intense T 3 r n peak is present in the gamma spectrum after the long bombardment. However, particularly in the granites and shale, there is interference from an unknown gamma of a longer half life of 101 kev. This is VOL. 39, NO. 1, JANUARY 1967
129
La Ce Sm Eu
DY
Yb
Lu sc Mn Th
Table 111. Comparison of Lanthanide Abundances in G-1 and W-1, Concentrations in ppm G- 1 w-1 This paper Refi Refh This paper Refa 85 i 5 102 f 7.7 92.4i 3.1 9.6f 0.7 11.7f 1.2 157 i 5 134 f 13 150 f 24 ... 24.3 f 0.2 7.6i 0.3 8.6 f 0.56 8.25f 0.08 3.4f 0.2 3.79 f 0.32 1.3 f 0.1 1.04=t0.11 1.36f 0.05 1.2 f 0.1 1.09 f 0.12 ... 2.52f 0.10 2.1 f 0.4 2.7 f 0.3 ,.. 0.625i 0.06 0.94f 0,075 2.1 i 0.2 2.10f 0.10 0.12i 0.02 0.17f 0.017 0.12f 0.007 0.35 f 0.03 0.325f 0.03 2.7 f 0.2 33 f 1 210 f 15 1370 f 40 46 f 1.5 1 . 8 f 0.4 . I .
Refb 9.3 f 0.37 15.1 f 2.4 3.46 & 0.03 1.29 f 0.05 4.38 f 0.18 2.23 f 0.08 0.35 f 0.2
ReP : Haskins and Gehl (11). Refb: Towell, Volfovsky, and Winchester (12). Table IV. Lanthanide, Sc, Mn, and Th Abundances in Selected Rock Types Concentration, ppm
Graniteville Granite
Chelmsford Granite
La Ce
55 f 2 133 f 8
68 i 4 147 i 10
Sm
Eu DY
Yb Lu sc Mn Th
16 i 0.5 0.27 f 0.03 25 f 1 26 i 1 3.7 f 0.1 0.6 f 0.2 28 f 3 80 f 3
Deccan Trap Rock 14 f 0.6 ...
Shale
Garnet
5.7 f 0.3 4.7 f 0.3
28 f 2 55 i 0.3
8fl
...
...
5.2 i 0.3 1.9 f 0.2
3 . 8 i 0.2 1.5 f 0.2
1.3 f 0.1 0.40f 0.04
3.8 f 0.3 1.0f 0.1
3.2 f 0.2
5.5 f 0.5 '1.8f 0.2
5 . 8f 0 . 8 3.5 i 0.3
6.3 z t 0.6 3.3 & 0.3
1.5 i 0.1 0.80 f 0.2
3.1 f 0.6 2.5 f 0.3
44 f 7 83 f 7
0.23 f 0.01 3.6 f 0.2 220 i 10 45 f 1
0.45i0.05 43 f 2 2070 f 70 2.0 f 0.2
(11) L. Haskin and M. A. Gehl, J . Geophys. Res., 68,2037(1963). (12) D. G. Towell, R. Volfovsky, and J. W. Winchester, Geochim. Cosmochim.Actcz, 29, 569 (9165). ANALYTICAL CHEMISTRY
2.8 i 0.3
Limestone
14.7 i 0 . 8 0.55 f 0.05
not so evident in the spectrum after the short irradiation, because counting begins after about 1 half life of 1j3Sm, while in the long irradiation, counting begins after 4 half lives. Europium. The 15*Eugamma a t 122 kev was used for the measurement. The 1 5 2 Ecounts ~ obtained shortly after the irradiation are usually higher than those obtained later. Therefore, it is advisable to wait about one month before taking data on 15*Eu. Since lj2Eu has a long half life, it is advantageous to wait for some of the shorter lived nuclides to decay, since this improves the peak to background ratio. Other photopeaks due to 15*Eu are sometimes present. Those at 245 and 1410 kev are close to background. The peak at 345 kev is always present, but this is a composite with a ls1Hf gamma of virtually the same energy. Dysprosium. The measurement of 16jDy is difficult. The dysprosium half life is virtually identical to that of j6Mn and the 95-kev gamma is hard to determine in the presence of the bremsstrahlung and Compton distribution due to 56Mn. The dependability of the dysprosium number is a critical function of the Dy/Mn ratio. The agreement with published data in G-1 is satisfactory. Dy in W-1 appears to be erroneously low. Figure 3 illustrates that for W-1 Dy is lower than the values for the neighboring lanthanides. However, in the other samples measured, the dysprosium values are reasonably consistent with its neighbors. Ytterbium. Gammas from I75Yb and I69Yb are generally present in the spectrum after a long irradiation. Both are usually small peaks and require a relatively high background correction. For this reason, the measurement of the I7jYb gamma at 396 kev is preferred to that of 1e9Yb at 178 and 198 kev, since the background in the 400-kev region can be
130
Mid-Atlantic Basalt
0 . 5 6 f 0 . 0 5 0.058i00.0040.16f0.01 38 A 1 0.55 ==! 0.03 17f 1 1460 f 60 220 f 10 450 f 15 ...
