but that attack by nitrosonium ion produces only the synnitrolic acid. Although nitrous acid is a much weaker electrophile than nitrosonium ion, it may be the attacking electrophile if the ratio of nitrous acid t o nitrosonium ion is very large. Conditions in which this would be the case include: Use of solvents of low dielectric constant, in which ionic species are unfavorable; reacting the aci-nitroparaffin with a nitrous acid solution at a p H somewhat higher than the pK, of nitrous acid to decrease the nitrosonium ion concentration; use of a cold nitrous acid solution, thereby decreasing the ionization of nitrous acid. It is found that these conditions d o indeed increase the ratio of anti- to sgn-nitrolic acid. Figures 2 and 3 give the suggested mechanisms for both
types of attack. Nitrosonium ion is expected t o produce an intermediate containing a ring structure by intramolecular hydrogen bonding, thereby favoring formation of the synnitrolic acid. Nitrous acid attack, however, produces an intermediate species which may rearrange to produce either a syn- o r an anti-nitrolic acid. Work is currently underway to isolate and charxterize both the species having absorbance maxima at 330 mp and the species having absorbance mdxima at 370 mp. RECEIVED for review May 31, 1968, Accepted August 8, 1968. Work supported by a grant from the National Science Foundation.
Spectrophotometric Determination of Trace Amounts of Silver(1) Mohamed T. El-Ghamry and Roland W. Frei Department of Chemistry, Dalhousie Uniuersity, Halifax, N . S., Canada Two simple, rapid, sensitive, and selective methods are proposed for the spectrophotometric determination of trace amounts of silver(1) in aqueous and nonaqueous media. These methods are based on the formation of a ternary complex between the silver(1) ions, 1’10-phenanthroline, and 2,4,5,7-tetra-bromofluorescein. The optimum pH ranges for these color reactions are pH 4-8 in aqueous solution and pH 8 in organic solution. Beer’s law is obeyed over the range of 0.10-2.00 ppm of silver(l), with molar absorptivities of 35,000 in aqueous and 55,000 in nonaqueous media at 550 mp. The color reaction i s instantaneous and the complex remains stable for over 15 days in aqueous solutions and for twelve hours in nonaqueous solutions. Of the 33 cations and 15 anions examined for interference, only iridium(lV) and cyanide showed serious effects.
MOSTof the published methods for the spectrophotometric determination of silver(1) (see Table I) involve binary complex formation between the silver(1) ions and the chromogenic reagent (1-8). The standard method based o n p-dimethylaminobenzylidenerhodanine has serious drawbacks because of its extreme sensitivity to changes in acidity while the dithizone method has been deemed unsatisfactory because of its extreme sensitivity t o variations in laboratory conditions and because the silver complex extract undergoes photodecomposition (9). Pyrogallol Red was proposed for the spectrophotometric determination of silver by Dagnall and West (IO). Although this method is more reliable than the dithizone and the p-dimethylaminobenzylidenerhodanine (1) S. S. Cooper and M. L. Sullivan, ANAL.CHEM.,23, 613 (1951)’ (2) G. C. B. Cave and D. N. Hume, ibid., 24, 1503 (1952). (3) J. H. Yoe and L. G. Overholser, ibid., 14, 148 (1948). (4) J. Michal and J. Zyka, Anal. Absfr., 4, 2122 (1957). ( 5 ) J. D. Dux and W. R. Fairheller, ANAL.CHEM.,33,445 (1961). (6) I. M. Kolthoff and P. J. Elving, Eds., “Treatise on Analytical Chemistry,” Part 11, Vol. 4, Interscience, New York, N. Y.,
1966. (7) G. Charlot, “Les Methodes de la Chimie Analytique,” 5th ed., Masson, Companie, Paris, 1966. (8) E. B. Sandell, “Colorimetric Determination of Traces of Metals,” 3rd ed., Interscience, New York, N. Y.,1959. (9) T. S. West, Analyst (London), 87,630 (1962). (10) R. M. Dagnall and T. S. West, Tuluntu, 8, 711, (1961). 1986
ANALYTICAL CHEMISTRY
methods, it lacks sensitivity because its molar absorptivity is only about 10,000. Recently, 2-amino-6-methylthio-4-pyrimidine carboxylic acid was described as a spectrophotometric reagent for silver(I), with a molar absorptivity of about 2090 (11). This method is not sufficiently sensitive and is subject to interferences from at least 24 cations and two anions. Another method based on the binary complex formation between silver(1) ions and 4, 4-bis-(dimethylamine)thiobenzophenone (thio-Michler’s ketone) has been described by Cheng (12). A molar absorptivity of about 9.38 X l o 4 is claimed. This method is the most sensitive present. However, it is extremely light and p H sensitive (to 1 0 . 2 0 pH unit), and subject to interference from most of the halogens. The authors did not examine the effect of the other noble metals. The formation of ternary complexes eliminates many of the discussed drawbacks of binary complexes. This has been discussed in greater detail by West (13, 14). The first method for the spectrophotometric determination of trace amounts of silver(I), involving ternary complex formation, was described by Dagnall and West (15, 16). This method is based on the formation of the silver/l,lOphenanthroline/bromopyrogallol red ternary complex which has molar absorptivities of about 51,000 and 32,000 in aqueous and organic media, respectively. This method is applicable over a wide p H range and only cyanide ions interfere seriously. Advantage was taken of this fact for the selective and sensitive indirect determination of cyanides ( I 7, 18).
(11) (12) (13) (14)
0. K. Chung and C. E. Meloan, ANAL.CHEM., 39, 383 (1967). K. L. Cheng, Mikrochim. Acta, 5,820 (1967). T. S. West, Analyst (London), 91,69 (1966). T. S. West in “Trace Characterization, Chemical and Physical,” W. W. Meinke and B. F. Scribner, Eds., National Bureau of Standards Monograph, 100, U. S. Government Printing Office, Washington, D. C., pp 215-98, 1967. ( 1 5 ) R. M. Dannall and T. S. West. Tuluntu.. 11.1533 (1964). , .~ i16j Ibid., p 1627. (17) R. M. Dagnall, M. T. El-Ghamry, and T. S. West, ibid., 13,
1667 (1966). (18) R. M. Dagnall, M. T. El-Ghamry, and T. S. West, ibid., 15, 109 (1968).
Table I. Sensitivities of the Spectrophotometric Methods of Silver(1) Molar absorptivity Solvent
Reagents Pyridine, 2,6-dicarboxylic acid Tetraphenylporphine 2-Amino-6-methylthio-4-pyrimidine
carboxylic acid Tetraphenylchlorine Diethyldithiocarbamate Pyrogallol red Tetraphenylporphine p-Dimet hylaminobenzylidenerhoodanine Diphenylthiocarbazone (Dithizone) Tetraphenylporphine Dit hizone ],lo-Phenanthroline/bromo pyrogallol red (PHEN/BPR) 1,lO-Phenanthroline/tetrabromofluorescein (PHEN/TBF) PHEN/BPR PHENiTBF Thio Michler’s ketone
However, the color of the Ag/l,lO-phenanthroline/bromopyrogallol red ternary complex is only stable for about thirty minutes. The authors did not investigate the interferences from the other noble metals [except gold(III)], which may be serious because of the vicinal hydroxyl groups in the bromopyrogallol red (15). Because the ternary complex formation provides not only a selective but also more sensitive spectrophotometric method-e.g., the palladium/pyridine/tetrachloro-(P)-tetraiodo-(R)-fluorescein ternary complex has a molar absorptivity of 125,000 (19)-the authors investigated the possibility of developing a new ternary complex system for the spectrophotometric determination of trace amounts of silver(I), which would eliminate the drawbacks of the BPR method. A series of amines and a wide range of spectrophotometric reagents were examined. The most satisfactory system found consisted of 1 ,IO-phenanthroline (PHEN), and 2,4,5, 7-tetrabromo-fluorescein, TBF, (Eosin). In the absence of PHEN, no significant color change was observed, while in its presence a deep pink color was formed between the silver(1) ion and TBF. EXPERIMENTAL
Apparatus. Absorbance measurements a t varying wavelengths were recorded with a Spectronic 505 recording spectrophotometer. Absorbance measurements a t a fixed wavelength were made with a Unicam S.P. 500 spectrophotometer set at high constant sensitivity with a slit width of about 0.04 mm. Matched glass cells of 1.OO cm path length were used. A radiometer p H meter Model 28A with a saturated calomel-glass electrode system was used for all p H measurements. (19) R. M. Dagnall, M. T. El-Ghamry, and T. S. West, Tala!zfa* in press. (20) H. Hartkamp, Z . Atzul. Chern., 184,98 (1961). (21) “Spectrophotometric Data,” Commission on Spectrochemical and Other Optical Procedures for Analyses, International Union of Pure and Applied Chemistry, Butterworths, London, 1963.
(XlOOO)
Aqueous Aqueous Pyridine Pyridine Benzene
0.809 0.841 0,900 1.40 1.75
Aqueous Benzene Carbon Tetrachloride Aqueous Pyridine
2.09 3.20 5.40
Wavelength (mw) 920 512 600 655 550
375 610 340
10.00 20.00
460
Aqueous Carbon Tetrachloride Benzene Chloroform
23.20
595
27.20 =t1 . 8 28.00 29.00 =t1
462 425 465
Nitrobenzene
32.00
590
Aqueous Aqueous Nitrobenzene Aqueous
35.00 51 .OO 55.00 93.80
550 635
390
550
530
All analytical weighings were made on a n Oertling analytical balance. Reagents. Na2/EDTA12H20from G. F. Smith Chemical Co. was used to prepare a 10-IM solution. An 18% Na2HP04.12Hz0 (British Drug House) solution was prepared as the buffer solution. Nitrobenzene and all other organic solvents were Fisher Certified reagents. A 10-3M 1,lo-phenanthroline (PHEN) solution was made up from Fisher reagent. Silver nitrate (analytical reagent grade) was obtained from Mallinckrodt Chemical Works (Montreal). The 10-2M standard silver nitrate solution was standardized against a 10-2M sodium chloride solution with dichloro fluorescein as an adsorption indicator. Sodium acetate tri-hydrate was obtained from Nichols Chemical Co. The acetate buffer was prepared by dissolving 34.02 grams of the sodium acetate in 250 ml of water. 2,4,5,7-tetra bromo fluorescein (TBF) from Aldrich Chemical Co. was used t o prepare a 10d4Msolution (647.9 mg) in 1 liter of water. Double distilled water was used throughout and all other reagents were reagent grade. Recommended Procedures. (a) AQUEOUS MEDIUM. A series of solutions was prepared containing 1 ml of 10-1M EDTA, 1 ml of 10d3M PHEN, 0-20 ml of 5 X 10-jM AgNOo, 1 ml of the acetate buffer and 15 ml of 10d4MTBF. The solutions were then diluted t o mark with water in 50-ml volumetric flasks. After thoroughly mixing the solutions, the absorbance measurements were made a t 550 m p with matched 1-cm glass cells against a reagent blank similarly prepared but containing no silver. N o standing time is required. By plotting the absorbance readings cs. micrograms of silver(I), a straight line passing through the origin was obtained. 1.00 ml of 5.00 X 10-5M A g N 0 3 = 5.39 pg Ag(1) When examining the effect of interfering ions, portions of the solutions containing the ions were pipetted first, then followed by the silver nitrate, EDTA, PHEN, acetate buffer, and TBF. The above procedure was followed. VOL. 40, NO. 13, NOVEMBER 1968
1987
580
560
540
520
!NO
400
460
440
WAVELENGTH (mp)
Figure 1. Absorption spectra of AgjPHENiTBF ternary complex in aqueous solution 1. TBF (a), Ag/TBF (b), PHENiTBF (c); 2. Ag/TBF (b) us. TBF (a); 3. Ag/PHEN/TBF (d); 4. Ag/PHEN/TBF (d) us. PHENiTBF (c)
(b) SOLVENT EXTRACTION.A series of solutions was prepared by pipetting 1 ml of 10-lM EDTA, 1 ml of 10-aM PHEN, 0-20 ml of 2.5 X 1OU5MAgN03, 1 ml of the phosphate buffer, and 15 ml of 10-4M TBF into 100-ml separatory funnels. These solutions were brought to volume (25 ml) with water. After adding 25 ml of nitrobenzene t o each separatory funnel, the contents were mixed thoroughly by continuous inversion shaking for about one minute. The separation of the two phases took place after standing for about fifteen minutes. A small portion of each extract was placed into clean, dry 50-ml beakers. Absorbances were measured immediately at 550 m p against a reagent blank. The plotting of the absorbance readings obtained us. micrograms of silver gave a straight line which passed through the origin. 1 ml of 2.5 X 10-5M AgN03
=
2.97 pg AgO)
RESULTS AND DISCUSSION
Absorption Spectra. The absorption spectrum of a solution containing 10 ml of 10F4MTBF(a) solution and 5 ml of 10-1M EDTA solution per 50 ml (reagent blank) shows a peak at 505 mp, Figure 1, curve I. The addition of 9 ml of lO-4M silver nitrate(b) to this solution results in the identical spectrum and the light orange color of the reagent blank remains unchanged. However, the addition of excess silver nitrate to TBF gives a slight color change from light orange to pinkish orange. A plot of (b) cs. (a) results in curve 2 of Figure 1. The addition of 1 ml of lOU3MPHEN(c) t o solution (a) gives no color change and its absorption spectrum coincides with curve I of Figure 1 . 1988
0
ANALYTICAL CHEMISTRY
The addition of 9 ml of 10-4M silver nitrate solution (d) to solution (c) gives a deep pink color. Its absorption spectrum indicates a decrease in the peak at 505 mp while another peak is established at 550 mp, Figure 1, curve 3. The order of addition of the reagents was not important. A plot of (d) cs. (c) shows a sharp peak at 550 mp, Figure 1, curve 4. Optimum pH. p H from 1 to 11 was examined. It was observed that the pink color was formed from p H 3 to 10, while a t p H 1,2, and 1 1 n o complex formation was apparent. The optimum p H was from p H 4 to 8. A p H of about 7 was chosen for the aqueous method because of the simplicity of the required buffer system. A 1 M sodium acetate buffer solution was used. Optimum Time. An investigation of the optimum time for color development was carried out by preparing a solution containing 1 ml of IO-IM EDTA, 1 ml of 10-3M PHEN, 1 ml of sodium acetate buffer, 10 ml of 5 X IO-jMsilver nitrate, and 10 ml of 10-4M TBF per 50 ml. The absorbance measurements against a reagent blank were made at 550 mp over a period of fifteen days. The intervals between measurements were 5, 10, 15, 30, and 60 minutes; 1, 2, 3, 4, 6 , 8, 12, and 24 hours. After that, the absorbance readings were taken daily for fifteen days. It was found that the color was formed instantaneously and the absorbance readings remained almost unchanged ( A = 0.35 + 0.01) for fifteen days after which the absorbance measurements were discontinued. The stability of the developed color was tested, over the above period, under normal laboratory conditions-ie., room temperature, artificial light, daylight, and darkness, etc. Effect of Masking Agents and Extraneous Ions. The effect of EDTA, as a mass-masking agent, o n the sensitivity of the proposed method was examined. It was found that the addition of EDTA up to 10,000-fold molar excesses over silver(1) had no effect on the sensitivity. This is attributed to the high stability of the bis-phenanthrolonium-silver(1) coordinated complex, stability constant = 15.04; over Ag/EDTA chelate, stability constant = 7.32 (22). Consequently, EDTA was chosen as a masking agent in the study of the extraneous ions. Thirty-three cations Al(III), As(III), Au(III), Ba(II), Bi(III), Ca(II), Cd(II), Co(II), Cu(II), Fe(III), Fe(II), Hg(II), Ir(V), K, La(III), Li, Mg(II), Mn(II), Mo(VI), Na, Ni(II), Os(IV), Pb(II), Pt(IV), Rh(III), Sr(II), Th(IV), U(VI), W(VI), Zn(II), Zr(IV), and 1 5 anions, Br-, C1-, C ~ H ~ O Z CN-, -, C032-, czo42--, F-, HP04’-, I-, NOn-, N03-, S2-, SCN-, S03’-, and SO4*-,were investigated for their interferences in presence of 3000-fold molar excesses of EDTA over silver(1). It was found that the only cation and anion which do interfere seriously are iridium(1V) and cyanide, which, consequently, must be absent if the method is t o be used successfully. On the other hand, the method can be applied successfully in the presence of the other coinage metals--e.g., copper(I1) and gold(II1)-and the other noble metals, such as palladium(I1) and platinum(IV), which renders the method highly selective toward the silver(1) ions. The high selectivity of this method over the 1,IO-phenanthroline,’bromopyrogallo1 red method is due to the absence of vicinal hydroxyl groups in TBF, while their presence in the bromopyrogallol red renders the latter reagent less selective. (22) “Stability Constants of Metal-Ion Complexes,” Special Publication No. 17, International Union of Pure and Applied Chemistry, The Chemical Society, London, 1964.
Nature of the Complex. The fact that the deep pink colored complex can be obtained only in the presence of 1,lO-phenanthroline indicates that we are dealing with a ternary complex system. In order to investigate the structure of this ternary complex both Job’s Continuous Variation Plots and the Mole Ratio methods were employed. The results obtained are as follows: Ag:PHEN ratio, in presence of excess TBF: Job’s Plots show a complex ratio of 6.65:13.35 or 1 : 2 ratio between silver(1) and PHEN, respectively. The Mole Ratio plots show that the silver(1):PHEN ratio is 3:5.6 or approximately 1: 2. Ag:TBF ratio, in presence of excess P H E N : From Job’s Plots, the Ag:TBF ratio is found about 13.3:6.7 or approximately 2: 1. The Mole Ratio Plots show a ratio of 8:4 or 2 : l . Hence, the 2 : l Ag:TBF ratio can be deduced from both Job’s Plots and the Mole Ratio method. PHEN/TBF ratio, in presence of excess silver(1): Job’s Plots indicate a ratio of 15.85:4.15--i.e., 3.82:l or approximately 4: 1. Therefore, it can be concluded that the overall ratio of the A g : P H E N : T B F complex is 2:4:1. These results indicate that the compound under investigation is a ternary complex rather than a binary one and its formula could be suggested as follows: [PHEN
Ag
- PHEN]Z+, TBF2-
Little is known of the mechanism of this reaction. The order of addition of the reagents is not important. The deep pink color is obtained whether PHEN is added before or after TBF. Two hypotheses could, therefore, be advanced for the formation of this silver/PHEN/TBF ternary complex. Silver(1) is known to form a co-ordinated complex with 1,IO-phenanthroline which is of the bis-phenanthroloniumsilver(1) type with one positive charge. The addition of TBF to the AgjPHEN solution could result in the adsorption of TBF on the surface of the [Ag (PHEN)$ species. Silver(1) may form a chelate with tetra bromo fluorescein (TBF2-). This can be confirmed by the slight color change resulting from the addition of large excesses of silver(1) to the solution of TBF. Then, the 1,lO-phenanthroline reacts with the chelate by taking up co-ordination sites on the chelated silver ions, leaving the TBF2- ions free for adsorption on the surface of the [Ag (PHEN)# micelles t o give the AgjPHENiTBF ternary complex. However, the analytical application of ternary complexes in spectrophotometric analysis is relatively new and further investigation is needed t o explain the behavior of such complexes. In view of the findings of other investigators (15) and the results obtained for the complex ratio, it can be concluded that the bis-phenanthrolinium-silver co-ordinately-bound positively charged complex is formed when PHEN is added before or after TBF. Then this reacts with the counter ion TBF2- to form the ion association system, [Ag (PHEN)z]+2, TBF2-. Solvent Extraction. Several organic solvents have been examined for their ability to extract the AgjPHENjTBF ternary complex. The solvents investigated were amyl acetate, benzene, bromobenzene, carbon disulfide, carbon tetrachloride, chloroform, cyclohexane, ethyl acetate, heptane, iso-butyl methyl ketone (hexone), and nitrobenzene. Of all the solvents tested, only nitrobenzene was capable of extracting the AgjPHENjTBF ternary complex, while in the other cases a pinkish precipitate was observed a t the inter-
0.70}
0.60-
= I < 040
K
O
0301
I1
I
0.0 WAVELENGTH (mp)
Figure 2. Absorption spectra of AgjPHENiTBF ternary complex in nitrobenzene 1. PHENiTBF; 2. Ag/PHEN/TBF; 3. Ag/PHEN/TBF (2) us. PHENiTBF (1)
face. Hence, nitrobenzene was chosen as the organic solvent to be used in extraction. The silver complex was extractable into nitrobenzene in slightly acidic, neutral, and alkaline media. With the use of the acetate buffer system (which is used in aqueous medium), the complex was extracted in nitrobenzene, but a very high absorbance was observed for the reagent blank. The acetate buffer is, therefore, not suitable because TBF is extracted into nitrobenzene. Other buffer systems were investigated and the disodium hydrogen phosphate duodecahydrate buffer (pH 8) was found more suitable, The addition of EDTA up to 10,000 fold molar excesses over silver had no effect on the extraction process. Therefore, EDTA can be used as a mass masking agent which provides, in combination with solvent extraction, an extremely selective method for the determination of trace amounts of silver(1). Another important advantage of the extraction method is the higher molar absorptivity obtained ( E = 55,000) compared with the aqueous method ( E = 35,000). The extraction occurred instantaneously and the complex remained stable for twelve hours under the laboratory conditions. The absorption spectrum obtained from a solution containing 1 ml of 10-’M EDTA, 1 ml of 10-3M PHEN, 1 ml of phosphate buffer, and 10 ml of 10-4M TBF, extracted into 25 ml of nitrobenzene (reagent blank) shows a small peak at 550 mp, with a shoulder at 510 mp, Figure 2, curve 1. The color of this extract is orange. The addition of 5 ml of 5 X 10-jM silver nitrate t o the above solution, followed by extraction into 25 ml of nitrobenzene, results in a peak at 550 mp with a shoulder at 510 mp, Figure 2, curve 2. In this case the color of the extract is pinkishviolet. A plot of the second c’s the first reveals a sharp peak at 550 mp, Figure 1, curve 3. Hence, maximum VOL. 40, NO. 13, NOVEMBER 1968
1989
Table 11. Accuracy of the Ag/PHEN/TBF Method in Aqueous Solution Absorbance against reagent Sample blank at no. %Om@
Silver(1)content Found Certified Difference 1.23% 1.220% +O.Ol% 1 0.282” 7.19% 1.220z -0.03% 2 0.28W a Each absorbance reading in columns 2 and 3 represents the mean of two absorbance readings obtained from two separate solutions. = 550 mp, and all subsequent absorbance measurements of the extracts were made at this wavelength. Calibration Curves and Beer’s Law. By following the recommended procedures, the validity of Beer’s law was examined in both aqueous and organic media. In aqueous medium, it was found that Beer’s law was obeyed over the range 5.39-107.87 micrograms of silver(1) per 50 ml, or approximately 0.1-2.0 ppm of silver(1) in the final concentration with molar absorptivity of 35,000 at 550 mp. I n the case of solvent extraction, it was found that Beer’s law was valid over the range 2.69-53.93 micrograms of silver(1) per 25 ml or approximately 0.1-2.0 ppm of silver(1)
in the final concentration with molar absorptivity of 55,000 at 550 mM. Application of Method. Spectrophotometric analyses were performed o n four solutions obtained from a silver alloy (Analyzed Silver Alloy Let. No. 10, Thorne Smith Co., Michigan). This alloy was analyzed as recommended (23). Two stock solutions were prepared, each containing 50.00 mg of the alloy. The solutions were diluted with doubly distilled water to 250 ml. From each solution a 3-ml aliquot was pipetted into a 50-ml volumetric flask and neutralized with 1M ammonium hydroxide. The sample was then treated according to the recommended procedure. The absorbance measurements were made at 550 mp against a reagent blank. Duplicate solutions were prepared from each stock solution of the alloy. The results obtained are summarized in Table 11. RECEIVED for review March 29, 1968. Accepted July 18, 1968. Work supported by a grant of the National Research Council of Canada. M.T.E. is grateful for financial support as a Postdoctoral-Fellow from the same grant and a special research grant from Daihousie University. Presented at the 51st conference of the Canadian Institute of Chemistry, Vancouver, June 1968. (23) A. I. Vogel, “Quantitative Inorganic Analysis,” 3rd ed.,
Longmans, London, 1961.
Neutron Capture Gamma-RayActivation Analysis Using Lithium Drifted Germanium Semiconductor Detectors S. M. Lombard and T. L. Isenhour Department of Chemistry, Unicersity of Washington, Seattle, Wash. 98105 The performances of a planar and a coaxial Ge(Li) semiconductor gamma-ray detector are compared with that of a Nal(TI) scintillation detector for neutron capture gamma-ray activation analysis. The superior resolution of these large volume Ge(Li) diodes more than compensates for their lower efficiency because post-irradiation chemical or half-life resolution is not possible in the capture gamma-ray method. Data are given for the efficiency, resolution, and observed background of the detectors; several sample containers are compared; and detection limits for 18 elements in aqueous solution are presented. CONVENTIONAL activation analysis is a method of elemental analysis in which the sample is irradiated by neutrons, charged particles, or gamma rays produced in nuclear reactors or accelerators, and the resultant delayed radiation is analyzed, often after chemical isolation of the desired products. Neutron capture gamma-ray activation analysis is based on the instantaneous decay by gamma-ray emission of nuclear excited states produced by the capture of thermal neutrons. I n this method, the irradiation of the sample and the measurement of its activity must be done simultaneously. The radiative capture of thermal neutrons was first observed in 1934 by Lea (1) and by Amaldi and his associates (1) D. E. Lea, Nature, 133, 24 (1934).
1990
ANALYTICAL CHEMISTRY
( 2 ) . It has since been one of the principal sources of information on nuclear energy levels. Initially thermal neutrons were produced by radium-beryllium sources [using the Be(a,n) reaction] imbedded in a moderator such as paraffin. Samples were positioned in the area of highest thermal neutron flux within the moderating material. These sources produced very low thermal neutron fluxes and were of little value for capture gamma-ray studies. The advent, in the early 1940’s, of nuclear reactors made high thermal neutron fluxes available for study of capture gamma reactions. Most capture gamma-ray spectra have been obtained from samples placed in a collimated thermal neutron beam external to the reactor shielding. Thermal neutron fluxes available at these external beam ports are typically of the order of lo6 n/cm* sec. Particle accelerators are also available which produce thermal neutron fluxes of about the same magnitude. The energy of neutron capture gamma radiation ranges from about 75 keV to about 10 MeV and the spectra of most nuclides are fairly complex. The relatively low neutron fluxes available for thermal neutron capture result in rather low sample activity. For these reasons the most important
(2) E. Amaldi, 0. d’Agostino, E. Fermi, B. Pontecorvo, and E. SegrC, Recerca Sc., 2, 461 (1934).
