1594
Anal. C 9 m . 1981, 53, 1594-1598 H
O,!N -,H -,,
181 "TCH2
O."-O
H2F$$; kJ "*coo-
H
The better linearity observed for histidine indicates that fewer complex species predominate at the fixed pH evaluated. The curvature observed for the other amino acids must indicate the formation Of mixtures Of complex species at the pH values studied. Despite the curvature, however, we have shown that the metal electrodes give reproducible response by making use of rapid flow analysis where the electrode surface is continuously washed with electrolyte, allowing quantitative determinations of amino acids in the pH range evaluated. The effect Of acids at copper metal in to the copper membrane electrodes may therefore be useful in simplifying HPLC detection of amino acids. Loscombe et al. (IO)have already demonstrated the usefulness of the Cumembrane electrode for this purpose but with the requirement of postcolumn addition of Cu2' to the eluate. In future studies, we propose to we the electrodes as HPLC d&Xtors for amino acid quantitation.
(3) Saad-S. M. Hassan; Zaki, M. T. M. Mikrochim. Acta 1070, I , 137- 144. (4) Van der Linden, W. E.; Oostervlnk, R. Anal. Chlm. Acta 1078, 101, 419-422. (5) Ross, J. W.; Frant, M. S. Anal. Chem. 1069, 41, 1900-1902. (6) Olson, V. K.; Carr, J. D.; Hargens, R. D.; Ken Force, R. Anal. Chem. 1078, 48, 1228-1231. (7) Van der Meer, J. M.; den Boef, G.; Van der Linden, W. E. Anal. Cbim. Acta 1076, 85, 309-316. (8) Westall, J. C.; Morel, F. M. M.; Hume, D. N. Anal. Chem. 1970, 51, 1792-1798. (9) Sekerka, I.; Lechner, J. F. Anal. Lett. 1978, All, 415-427. (IO) Loscombe, C. R.; Cox, G. B., Dalzlel, J. A. W. J. Chromatogr. 1078, 166, 403-410. (11) Toribara, T. Y.; Koval, L. Talanta, 1070, 17, 1003-1006. (12) Ferrel, E.; RMgion, J. M.; Riley, H. L. J . Chem. Soc. 1034, 1440- 1447. (13) Alexander, P. W.; Seegopaul, P. Anal. Chem. 1080, 52, 2403-2406. (1.4) vogei, A. I., "A Textbook of QuantitativeInorganic Analysis": 3rd ed.; Longmans: London, 1961; p 35. (15) Lange, N. A., Ed. "Handbook of Chemistry", 10th ed.; McGraw-HIII: New York, 1961; p 952. (16) Mldgley, D.; Torrance, K. "Potentiometric Water Analysis"; Wlley: New York, 1978; pp 17, 126-127. (17) Laitinen, H. A.; Harris, W. E. "Chemical Analysis", 2nd ed.; McGrawHill: New York, 1975; pp 227-233. (18) Keenan, A. 0.; Webb, C. A.; Kramer, D. A.; Compton, K. G. J. Ekctrochem. SOC.1976. 123, 179-182. (19) Heijne, G. J. M.; van der Linden, W. E. Anal. Chim. Acta 1078, 06, 13-22. (20) Doran, M. A.; Chaberek, 8.; Marteii, A. E. J. Am. Chem. Soc. 1064, 86, 2129-2135. (21) Williams, D. R. J. Chem. Soc., Dalton Trans. 1072, 790-797.
LITERATURE CITED (1) ECTaraz, M. F.; Pungor, E.; Nagy, G. Anal. Chim. Acta 1076, 82, 285-292. (2) Nikolelis, D. S.; Papastathopoulos, D. S.; Hadjiloannou, T. P. Anal. Chlm. Acta 1070, 78, 227-232.
RECEIVEDfor review May 21, 1980. Accepted May 29, 1981. The authors are grateful to the Government for the award of a Colombo Plan Fellowship to C.M.
Radiochemical Multielement Neutron Activation Analysis of High-Purity Niobium with Short-Lived Indicator Radionuclides Werner
G. Falx,
Rostlslav Caletka, and Viliam Krlvan"
Sektion Analyfik und Hochstreinigung, Universitat Ulm, Oberer Eselsberg N26, 0-7900 Ulm, Federal Republic of Germany
A rapid radiochemical neutron activation technique based on = the utilization of short-lived indicator radionuclides ( 2.2-18.6 mln) is described whlch enables the analysis of hlgh-purity niobium for ?hetrace element impurities Mg, AI, TI, V, Co, Cu, Se, Rb, and Mo. the essential feature of the technlque is the use of an anion exchange procedure whlch allows one to separate directly from the HF/HN03 decomposition solution the radionuclides of niobium, tantalum, and, to a great extent, tungsten, representing the dominating radioactivity In the irradiated sample. By use of sample amounts of about 100 mg, the whole procedure from the end of the irradiation to the beginning of the counting takes an average of 7 mln. For the above sample amount, an irradiatlon time of 10 min, and a thermal neutron flux of 8 X IOi3 n cm-'s-', the limits of detection are between 0.1 ng/g for V and 60 ng/g for Mo.
In recent years, niobium has become a material of great scientific and technological significance. Many of the properties of niobium, whether of scientific interest or relevant for its applications, can be influenced by trace impurities (1-3). Thus, the development of powerful techniques for analytical characterization of high-purity niobium is very important. Niobium represents a difficult matrix for trace analysis via techniques requiring chemical processing of the sample prior 0003-2700/8 1/0353-1594$01.25/0
to actual measurement. Therefore, we are developing highly efficient techniques for the analysis of this matrix based on instrumental and radiochemical activation analysis with neutrons and charged particles, The unique features of the activation techniques are freedom from ordinary blank and the possibility to remove even preirradiation surface contamination involved prior to analysis. This is the principal advantage of activation analysis in the determination of elements at extremely low levels (kg/g and below), since experience shows that the blank is the primary factor not only dictating the limit of detection but also limiting the accuracy of all analytical techniques (4). In the most cases, instrumental neutron activation analysis (NAA) of niobium permits only the determination of the main impurities, Ta and W, because the corresponding target nuclides are strongly activated by thermal neutrons and the formed indicator radionuclides give rise to a very complicated spectra: lEhTa emits 9 and lS2eTa emits 24 y-rays of different energies. In a previous communication, the simultaneous determination of Ti, V, Cr, Fe, Zr, Mo, Hf, Ta, and W in niobium by instrumental proton activation analysis was reported (5). In this case, limits of detection between 4.5 pg/g (for Ta) and 0.04 Mg/g (for Ti) were obtained. Considerable progress has been achieved in the analysis of niobium by using postirradiation radiochemical separations. For instance, as little as 10 pg/g of Cr, 1.5 ng/g of Fe, and 4 pg/g of Co can be detected by radiochemical NAA involving a long irradiation in a medium-flux reactor and a 0 1981 Amerlcan Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 11, SEPTEMBER 1981
1595
Table I. Data on Production and Properties of the Indicator Radionuclides isotope abundance, element
principal1 reaction
%
11.01
barn 0.033
Uth,
Ti/z, min
9.46
Mg
Z6Mg(n,Y)z7Mg
A1
27Al(n,i/)28Al
100.0
0.230
2.246
Ti
5Ti(n,r)5LTi
5.3
0.179
5.8
V
"V( n,Y ) 52V
9!3.75
4.88
3.75
co
59C~(n,'~)60mCo 100.0
cu
V u (n,'Y )66 Cu
30.9
2.17
Se
7aSe(n,?')79Se
23.5
We( n,?')81Se 85Rb(n,Y)86mRb
Rb Mo
87Rb(n, Y ) "Rb l"Mo(n,Y)'olMo
major 7-rays, MeV 0.843 76 1.014 40 1.778 80 0.320 00 0.928 50 1.434 20
intens, % interference reactions 72.0 28.0 100.0
95.0 5.0 100.0
5.1
0.058 36 1.332 52 1.039 00
99.75 0.24 9.0
0.33
3.9
0.095 7
10.0
50.0
0.53
18.6
0.275 94
72.17
0.05
1.2
27.03 9.6
20.0
0.12 0.199
10.5
17.8 14.6
0.555 0.898 1.836 0.192 0.590
80 04
13 00
80
1.012 40
specific separation of the indicator radionuclides (6). Similar improvement of the proton activation analysis could be achieved by its radiochemical performance (7). In this work, a rapid radiochemical N U technique for the determination of Mg, ,41, Ti, V, Go, Cu, Se, Rb, and Mo in niobium, making use of short-lived indiicator radionuclides (T1l2= 2-20 min), is presented.
EXPERIMENTAL SECTION Chemicals and Apparatus. Reagents used in the development of the separation procedure as well as those used in the activation analysis prior to irradiation were (of "suprapure" grade. The concentrations of hydrofluoric acid and nitric acid used were 40% and 65%, respectively. The separation procedure was developed by using the following radioisotopes as radioactive tracers: W,V ,WO,&,Cu,'%e, &Rb,94Nb,%o, l8Va, and lmW. They either were produced by irradiations of the appropriate elements or their suitable compounds of high-purity grade in the FR-2 reactor at the Nuclear ]Research Center of Karlsruhe or were supplied by NEN Chemicals GmbH. The radioactive purity was checked by y-ray spectrometry. The separation experiments were carried out on the Dower: 1X8,200-400 mesh (Fluka), a strongly basic anion exchange re,sin in the F- form. In the tracer experiments, a single-chamnel analyzer with a well-type 3 X 3 in. NaI (TI) detector was used for counting. The y-ray spectrometry measurements were performed by using a Ge(Li) detector, having an energy resolution of 1.9 keV fwhm for the 1.332-MeV y-ray ofoC@ ' and efficiency of 20% relative to a 3 X 3 in. NaI (Tl) detector, and a peak-to-Compton ratio of 401. For interference-freecounting of %o, an iintrinsic Ge detector, 16 mm in diameter by 10 mm active depth, Connected to an Ortec 572 amplifier was used. 'The total system resolution was 218 eV fmhm at 5.9 keV and 51.4 eV at 122 keV. The detectors were coupled alternatively to an Canberra 8180 multichannel analyzer. Samples and Standards. The following niobium samples were analyzed: (a) Nb-ES, Heraeus, Hanau, F.R.G.; (b) Nb-WCT, Teledyn Wah Chang, Allbany, OR; (c) Nb-IR-1, Max-Planck-Institut fur Metallforschung, Stuttgart, F.R.G.; (c) Nb-R-2, MaxPlanck-Institut fur Metrillforschung, Stuttgart, F.R.G. Samples of each niobium material (between 50 and 100 mg) were cut with a diamond saw, etched for 20 s in HF/HNOB(91) to remove possible surface contamination, and then packed in polyethylene capsules which were cleaned with high-purity nitric acid. Standards were prepared from solutions containing the appropriate elements. To prevent contamination, the capsules with
0.51 98.2 14.5 21.4 25.0 20.0 25.0
27A1(n , ~ ) ' ~ M g 30Si(n,a)27Mg %i( n,p)28A1 'lP( n,a )28A1 "V( n,p)"Ti 54Cr(n,a )51Ti 52Cr(n,p)52V 55Mn(n,a)52V 60Ni(n,p)60mCo 63C~(n,a)60mC~ 66Zn(n,p)Wu 69Ga(n,a)66C~ 79 Br( n , ~Se) ~ ~ n,a)79Se 81Br(n,p)81Se &Kr(n,a)81Se ffiSr(n,p)ffimRb ,,Y(n,a)"mRb 88Sr(n,p)88Rb 'WRu(n,a)LolMo
standards were placed in another, larger capsule. Irradiation. Irradiations were performed in the FR-2 reactor at the Nuclear Research Center of Karlsruhe using a pneumatic transport system (time for one way about 1min). Samples and standards were irradiated with a thermal neutron flux of 8 X 1013 neutrons cmm2 s-l for 1-10 min. Radiochemical Separation Procedure (Figure 1). On the basis of our systematic studies about the anion exchange characteristics of the elements in HF/HNOs medium (8))the following radiochemical separation procedure was developed: After irradiation, the samples were etched for 10 s in HF/HNOa/H20 (91:2.5) whereby the dissolved portion of the sample was
