overdetermination; the long wavelength regions of infrared absorption spectrometry are known to contain information about bond types and strengths, but interpretation to date has been quite difficult; X-ray fluorescence studies are again overdetermined and contain more bonding information than is presently utilized; the further resolution and interpretation of complex X-ray and gamma-ray spectra would enhance nuclear techniques; other instrumental data sources as NMR, ESR, etc., could use the most sophisticated data evaluation methods available. Hence, for scientific data
evaluation, as well as in other areas of information handling, the learning machine approach should prove to be an important and powerful technique. ACKNOWLEDGMENT Acknowledgment is gratefully given to W. S. Chilton for his helpful suggestions regarding this work. RECEIVEDfor review June 6, 1968. Accepted October 3, 1968.
Determination by Neutron Activation Analysis of the Burn-Up Indicator Neodymium-148 in Irradiated Uranium Dioxide-PI utonium Dioxide M. R. Monsecour and A. C . Demildt Studiecentrum coor Kernenergie, Centre #Etude d e I‘Energie NuclPaire, Mol, Belgium
A radiochemical method has been developed for the isolation of Nd from irradiated UOz-PuOzfuel elements and measurement of ImNd by neutron activation analysis. The method is based on separation by reversed phase chromatography, with di(2 ethy1)hexylorthophosphoric acid (DZEHPA) as thestationary phase, and anion exchange on Dowex 1-X4 columns mixed with PbOz. The chemical yield, after the subsequent steps, is determined by adding 14’Nd tracer at the start of the separation procedure. The 14SNd (n,r) 149Nd neutron activation reaction carried out in the BR 2 reactor is used for the radiochemical determination.
To MEASURE the burn-up of a reactor fuel element, it is sufficient to determine one or more fission product nuclides whose concentrations at every point in the fuel are directly proportional to the number of fissions. In fuel elements working at relatively low temperatures are (500 “C and lower) the concentrations of 13’Cs and proportional to the fission density and these nuclides may be used as bum-up indicators. However, in high temperature fast reactors the longitudinal thermal diffusion of these nuclides may become important when long residence times in the reactors are envisaged. The ideal fission indicators are the stable isotopes of the more refractory elements provided they do not have very high burn-out cross sections and are not formed simultaneously by neutron capture from the lower chains. This kind of element provides the additional advantage of avoiding all absolute y - or @-raymeasurements which on a routine basis, are difficult to perform with sufficient accuracy. For these reasons certain isotopes of neodymium and molybdenum have been proposed (1-3). The nuclide 148Nd is particularly useful as a fission indicator because of its almost identical fission yield for 235U and 239Pu (1.7%). Although (1) J. E. Rein and B. F. Rider, U. S. At. Energy Comm. Rept., TID-17,385 (1963). (2) W. J. Maeck and J. E. Rein, ibid., IDO-14,656 (1965). (3) B. F. Rider, ibid., GEAP-4053-2, 2nd quarttrly report (1962). (4) W. J. Mc Gonnagle, “Nuclear Materials Management,” Proceedings of a Symposium, Intern. At. Energy Agency, Vienna,
STI/PUB/110,851(1966).
McGonnagle ( 4 ) proposed and Rider (5) mentioned the possibility of neutron activation analysis, no experimental data were available; therefore, such a method was developed. The composition of irradiated nuclear fuels is a complex mixture of elements. Apart from U, Pu, and certain transplutonium nuclides, all radioactive and stable fission products are present as well as the cladding constituents such as stainless steel and zircaloy. The nuclear data of interest in the 148Nddetermination by neutron activation are listed in Table I. The amounts of 14*Nd and 149Nd resulting from (n,f) reactions in the fuel should be several orders of magnitude larger than the quantity originating from neodymium present in the fuel as impurity. The expected 149Nd activities in fuel materials are listed in Table 11. Because one megawatt-day per ton (MWd/t) equals 2.695 X l O I 5 fissions gram-’or 1.126 X 10-2pg 148Ndgram-‘ fuel, it is easily seen that a depleted reactor fuel which dissipated 10.000 MWd/t energy contains 112.6 pg 148Nd gram-’. This amount forms, during a neutron activation of 5 minutes, 5.1 x 1010 atoms 149Ndgram-1 if a neutron flux (4) of 1014 n ern+ sec-l is available. Comparing this figure with the 149Ndproduced by 235Uand 239Pufission during the activation, it is easily seen that a decontamination of the order of 10’ is required if the fission contribution during the activation analysis has to be kept below 0.1 %. A chemical procedure had to be developed to separate sufficiently pure neodymium from the depleted fuel material. The required decontamination factors for uranium and plutonium are so severe that a quantitative recovery of neodymium can not be achieved. A 147Nd tracer is used to determine the over-all chemical yield. Use of this tracer imposes two limitations: the fuel must be cooled a long enough time to get rid of all the 147Ndresulting from fission and the tracer used must be sufficiently free from I4*Nd so that this addition may be neglected in comparison with the 148Ndformed during the burn-up. For a natural UOz-l % Pu02 mixture for example, (5) B. F. Rider, J. P. Peterson Jr., C. P. Ruiz, and F. R. Smith,
U. S. At. Energy Comm. Rept., GEAP-4621, 10th quarterly report (1964). VOL. 41, NO. 1, JANUARY 1949
27
Target nuclide 46Nd l4sNd
Table 1. Nuclear Data of the Neodymium Isotopes and Their Daughters Total fission yield of Activation product thermal Most prominent nuclide, absorption gamma radiation Daughter cross-section (235U/ nuclide energies, keV 289PU) Half-life (barn) Product (10-12) Half-life (6) nuclides (6, 8, 9) (6) (6) 1.8 14’Nd 2.6 12.2 91.06, 120.5, 275.4, 147Pm 2.65 years 11.1 days 319.4, 531.0 149Pm 53 hours 1.72 0.02 114, 154, 187, 210, 2.54 =!= 149Nd 1 . 3 11.4 240, 268 hour (5) 0.18
z
(7)
l50Nd
1. 5
l5lNd
0.4510.8
142Nd 143Nd
18 240
...
4Nd 145Nd
5
la3Nd la4Nd 145Nd
60
140Nd
... ... ...
12 minutes
85.4, 117.2, 139.0
15lPrn
28.4 hours
...
... ... ...
... ... ... ...
stable stable stable stable
Table 11. Number of 149NdAtoms Produced by (n,f) Reaction of Different Fuel Compositions Irradiated for 5 Minutes at a Flux of 1014n cm-* sec-’ a Number of lagNdatoms formed Reactor fuel irradiated 4.41 X 10l2 1 gram Unat 1 gram 235U (90% enrichment) 5.3 x 1014 4.54 x 1013 1 gram Una$- 4% Pu 2.09 x 1014 1 gram Unat- 20% Pu a Assumed effective fission cross-sections for a thermal reactor: 235U-591 b, Z39Pu-974 b. having a 5000 MWd/t burn-up, a cooling time of one year is estimated to reduce its original I47Nd activity to about 0.3 nC gram-’. 147Nd with high specific activity is obtained by irradiating 96.19z enriched 146Ndin a high flux BR 2 reactor at 1014ncm-2 sec-’(TableIII). The resulting 147Nd must have a very high specific activity t o keep the added amount of 148Ndvery low compared to the concentration in the fuel. Because of the over-all activity of the sample and, specifically, its own 147Nd content, an adequate activity of 147Nd isotope must be added to overcome that from fission. EXPERIMENTAL
Chemical Procedure. Because activation analysis has been adopted as a n analytical technique, a complex separation scheme had to be developed t o isolate neodymium from all active and stable fission products and the different fertile and fissionable nuclides. A three-column separation procedure has been developed. Figure 1 shows the detailed analytical scheme. (6) Nuklidkarte, Chart of the Nuclides, Bundesminister fur wissenschaftliche Forschung, Bad Godesberg, Germany Zweite Auflage (2, Nachdruk 1965). (7) B. F. Rider, C. P. Ruiz, J. P. Peterson Jr., and F. R. Smith, U. S . At. Energy Comm. Rept., GEAP-4716, 11th quarterly report (1964). (8) E. K. Hyde, “The Nuclear Properties of the Heavy Elements,” 111, Prentice Hall, Englewood Cliffs, N. J., 1964. (9) E. H. Fleming Jr., U. S. AI. Energy Comm. Rept., UCRL50243, Vol. I (1967). (10) A. Backlin and S. C. Malmskog, Aktiebolaget Atomenergi, Report AE-265, Stockholm, Sweden (1967). (11) C. M. Lederer, J. M. Hollander, and I. Perlman, “Table of Isotopes,” 6th ed., Wiley, New York, N. Y., 1967. (12) C. E. Crouthamel, “Applied Gamma-Ray Spectroscopy,” Pergamon Press, New York, N. Y., 1960. 28
ANALYTICAL CHEMISTRY
...
Most prominent gamma radiation energies, keV (1, 11) pure 6 emitter 285.7 143.4, 163.6, 168.0, 177.1, 340, 445, 638, 671, 718
...
... ... ...
COLUMN 1. Preliminary separation of the trivalent rare earths was by reversed-phase partition chromatography columns with Kieselguhr as the inert carrier and di(2 ethyl)hexyl-orthophosphoric acid (D2EHPA) as the stationary phase. The detailed method of preparation of the column has been described by Sochaka and Sikierski (13). By activation analysis of uranium and a. spectrometry of plutonium, the residual amounts present in the rare earth fraction were determined and decontamination factors of 5 x l o 4 and 5 X lo2, respectively, were found for both nuclides after passing through the column. COLUMN 2. The separation of Ce from the rare earths can be achieved by percolating the mixture through an anion exchange column mixed with PbOz (14). During this step, Ce is oxidized t o the four-valent state and retained by the anion exchanger together with traces of Pu(1V). COLUMN 3. Neodymium can be isolated from its neighboring rare earths Pr and Pm but remains partly contaminated with 241Am. However, this nuclide does not disturb the measurement of 14’Nd during the yield determination. After irradiation of the Nd fraction, the presence of 24Na and j6Mn impurities, resulting mainly from the Kieselguhr and DZEHPA, interfere with the 149Ndy spectrum. A final extraction from 0.1M H N 0 3 with D2EHPA-nheptane gives a pure N d spectrum; the yield of the extraction is over 99%, either with or without addition of Nd carrier. Neutron Activation Analysis of ‘MNd. As can be derived from Table I, a mixture of neodymium and promethium radioisotopes is obtained either from a natural source or as the result of fission. Table 111 provides the fission yields and the isotopic composition of the different stable neodymium nuclides according to their origin. The laNd concentration can be determined by measuring the activity of either 149Nd(tllz = 1.72 hours) or la9Pm( f 1 1 2 = 53 hours). Several factors influence the choice between a short irradiation which implies the measurement of 149Ndand a long irradiation and measurement of 53 hours for 149Pm. Because the 149Pmdecay contributes less than 2% y emission of 285 keV, which interferes with the 275 and 277 keV y rays of lslPm and la7Nd, respectively, it was found necessary to separate neodymium from promethium before counting 149Pm with a NaI detector providing a suitable decay time in order to eliminate lj1Pm (12). From this survey it follows that the measurement of the 211 keV gamma ray of 149Nd provides the highest sensitivity without any further separation of the lanthanides. (13) R. J. Sochaka and S. Sikierski,J. Ctiromatogr. 16, 376 (1964). (14) P. F. Roberts, U. S. At. Energy Comm. Rept., HW-SA-3082
(1963).
Table 111. Isotopic Composition of the Stable Neodymium Nuclides and Their 235U/2agP~ Fission Yields Neodymium species lazNd 143Nd l14Nd 145Nd 46Nd 148Nd Natural isotopic composition (3) 27.11 12.17 23.85 8.30 17.22 5.73 0.5 0.44 Isotopic composition of enriched ld6Nda 1.46 96.19 0.97 0.43 0 5.88 Neodymium 235Ufission yields (15) 5.31 3.87 2.94 1.68 Isotopic composition of the neodymium z35U fission mixtureb 0 28.95 26.14 19.05 14.47 8.27 3.93 Neodymium 238Pufission yields (8, 16) 0 4.57 3.13 2.60 1.73 0 4.453 3.787 3.000 2.483 1.671 Isotopic composition of the neodymium 23sPufission mixtureh (8, 16) 0 26.93 23.16 18.44 15.32 10.19 23.260 0 22.271 18.331 15.151 10.208 a Enriched isotope brought at Oak Ridge National Laboratory. * Without burn-out correction.
SAMPLE HOLDER AND CONTAINER. Activation analysis with medium lived isotopes in the neutron flux of IOl4 n cm-2 sec-I surrounding the hydraulic conveyer of BR 2 raises the problem of the choice of packing material and irradiation container. The materials used should have a low capture cross-section, permit rapid and quantitative recovery of the samples, and be highly radiation resistant, In such high neutron flux the residence time of polyethylene containers in the reactor is limited to 15 minutes while Ertalon (trade name for superpolyamides, ERT'A Plastics, Tielt, Belgium) proved to be resistant for one hour or 3.6 X IO1' n cm-2 (integrated flux: +t where t stands for time in seconds).
S a m
pie A/-[
' I
I
t-l
1 I
Stable + radioactive F. I? Cladding elements: Fe-AI-Zr +I4'Nd tracer
'
,
I60Nd 5.62 0.1 0.634 3.12 1.01 0.945 5.95 5.786
At longer neutron exposures, sealed aluminum capsules have t o be used but dismantling should be carried out in a hot cell. This operation is time consuming. Several half-lives of the 149Ndisotope may pass before the post-irradiation recovery procedure can be started, which implies again a considerable loss of activity and no real gain in sensitivity can be expected. Graphite, quartz, aluminum, and filter paper, Whatman No. 1, were investigated as packing materials for the samples. None of them showed a sufficiently low activity to allow direct counting of the samples. Filter paper is the easiest material t o handle, for destruction or leaching with diluted nitric acid permits the quantitative recovery of Nd while a loss of sample was observed in A1 and quartz capsules. The filter paper used became brittle after an irradiation time of 10 minutes and quantitative transfer was questionable; therefore only 5-minute irradiations were performed. STANDARDS. Ndz03 is not a primary standard and Rider mentions the existence of basic carbonates through the
I
Column 1
- 30% 2Kieselguhr-D2'EHPA 0x1cm
(15) W. J. Maeck and J. E. Rein, U. S. at Energy Comm. Repr., IDO-14,660 (1965). (16) B. F. Rider, C . P. Ruiz, J. P. Peterson Jr., and F. R. Smith., ibid. GEAP-5403, 20th quarterly report, September-November (1966).
4 Figure 1. Separation scheme Sample Evaporate to dryness and take up in 20 ml0.1M HCI
V
Column 2 2ox0,4cm Dowex 1 P b 0 2 ( 50mg Pb02 g-l Dowex)
Column 3 20x0,4 cm Kieselguhr- D2EHPA -10%
% Ex tract ion
Column 1 1. wash column with 100 mllSMHC1, and 25 ml O.1MHCI 2. Load the sample on the column and fixation of U, Pu, Zr, Nb, Y, Ru (part) Al, Fe 3. Wash with 200 ml0.1M HCI, and elution of Cs, Sr, Ba, AI, Fe 4. Elute with 100 ml 1M HCI, elution of R.E., Am, Cm, Ru (part), evaporate to dryness, and take up in 2 ml8M "03 Column 2 1. Wash column with 8M " 0 3 2. Load the 8 M HNOI fraction on the column 3. Elute with 10 ml 8M "03, fixation of Ce (IV), Pu, Ru, elution of 111-valentR.E., Am, Cm, and Ru (part), evaporate to dryness, and take up in 2 ml0.1M HCI Column 3 1. Wash column with 1.5M HCI, and 0.1M HCI 2. Load the 0.1M HCI fraction on the column 3. Wash with 0.1M HCI and elution of Ru 4. Elute with 0.2M HCI and chromatographic separation of R.E.: 5.
+
La, Pr Am, Nd, Pm, Sm Neutron activation
Extraction Extraction with D2EHPA-n-heptane and removal of 24Naand 56Mn VOL. 41, NO. 1, JANUARY 1969
29
Table
IV. Comparison between Added and Determined '"Nd
Number 1 2 3a 3b 4a 4b
148Nd added, fig 0
7.564 15.128
15.128 18.910 18.910
Yield I47Nd, % 19.85 15.85 7.52 13.40 3.17 8.78
I4*Ndfound %
ILg
Man error
0
7.648 14.89 14.73 18.47 18.09
101.1 98.4 97.3 97.6 95.4
1.1
-1.6 -2.7
-2.4 -4.6
Table V. Results of Burn-Up Determination by Means of laNd Activation Analysis 14sNd Num14?Nd in 5-ml l48/gram ber yield, sample, p g UO2-PuO2, p g MWd/t 59 * 99 5327 la 8.51 12.73 5403 12.91 60.84 lb 8.046 5425 12.96 61.08 2a 12.063 5190 12.40 58.44 2b 5.399 59.38 5274 3a 16.118 12.61 5134 12.26 58.78 3b 2.506 Mean value: 12.64 59.58 5292 Relative standard deviation: 2.2%.
absorbance of COS and H2O from the air. Spectroscopically pure Nd203 (Johnson & Matthey) heated at 1000 "C showed a loss in weight of 6.5% (7) and could then be used as standard. From a solution of a preliminary known weight of Nd203 the Nd standards were pipetted, with calibrated 25-pl micropipets, upon filter papers and sealed in plastic bags together with the unknown fission product Nd samples. The standard deviation in the pipetting amounted to 0.6% in ten attempts. Although there is a considerable difference in isotopic composition between natural neodymium and fission product neodymium (Table 111), especially for certain isotopes with considerable cross-sections (Table I), no flux depression in the samples occurred under these experimental conditions. FLUXVARIATIONAND FLUXDEPRESSION. Preliminary test on the homogeneity of the irradiation flux in BR 2 showed differences in induced activity up to 26% when four Ni samples sealed in quartz vials were packed in an aluminum can. To avoid flux differences between samples and standards and to eliminate the addition of flux monitors, filter papers were packed in a folded polyethylene bag and piled in bricklike arrangement so that all samples were subjected to the same flux. This simple technique allows the irradiation and easy recovery of a great number of samples. Three series of six filter papers, each containing 4 pg laNd (as natural Nd) were irradiated for 5 minutes at 1014n sec-1 and examined. After irradiation, the Nd was leached from the filter paper with 0.1M H N 0 3 and extracted with 50% D2EHPA-n-heptane. The solution was examined by integral counting and y ray spectrometry of the 211 keV y peak. The over-all standard deviation amounts to 1.7% including the pipetting error, the recovery from the filter paper, the activity measurement, and the decay corrections. Because the high cross-section of and 145Nd(18, 240, and 60 barn, respectively) flux depression and self-shielding corrections were included (17) but shown not to be necessary in the described experimental conditions. Six filter papers containing 1, 2, 4, 8, 16, and 32 pg 14Nd (natural Nd) were irradiated and analyzed in the way described. From the specific activities measured no flux depression was detected. 142,14ai
(17) S. A. Reynolds and W. T. Mullins, Znt. J. Appl. Radiat. Isoropes, 14, 421 (1963). 30
rn
ANALYTICAL CHEMISTRY
Apparatus. All gamma ray spectra were registered with a 3" X 3" NaI(T1) crystal-photomultiplier detector (7.3x resolution) connected to a 400 channel RIDL, Model 34-12 B, multichannel analyzer. The chromatographic fraction collector was of a commercial type (Pleuger Universal Fraction Collector) but adapted so that it would be controlled by an electronic unit constructed by the authors. Reagents and Chromatographic Columns. DZEHPA. The double 2 ethyl-hexyl ester of orthophosphoric acid was purified according to Peppard's method (18). KIFSELGUHR (MERCK8119). This is treated with analytical grade H N 0 3 to elute the major fraction of metallic ions. After washing with double-distilled water and drying, the silica is made hydrophobic with dichlorodimethyl silane. The 147Ndtracer with high specific activity is prepared by neutron irradiation of highly enriched 146Nd. The 1eNd standard is prepared by dissolution of natural spec pure Nd203(Johnson & Matthey) after heating to 1000 "C. The heating decomposes all nonstoichiometric bounded H 2 0 and COS. All other products are analytical grade reagents. SAMPLING.Although 0.1 pg of laNd is sufficient to carry out a neutron activation analysis, samples between 1 and 10 pg are more suitable to avoid interferences from impurity. Because of the necessary separation after irradiation, filter paper gives little disturbing neutron-induced activities and is easily recoverable after 5-10 minutes of neutron irradiation. The container in which the paper is irradiated consists of separately sealed small polyethylene bags which are introduced in an Ertalon can. Procedure. A known amount of 147Ndtracer (=lo5 cps) is added to an aliquot of sample which contains about 10 pg 1BNd. After mixing, evaporate to dryness, dissolve in ca. 20 ml 0.1M HCl, and load on column 1. Wash the column with 200 ml 0.1M HCl. Elute lanthanides and 111-valent actinides with 100 ml 1 M HC1. Evaporate the 1 M HC13 fraction to dryness, take up in 2 ml 8M "OB, load on column 2, and wash with 10 ml 8 M H N 0 3 . Evaporate the 8M H N 0 3fraction to dryness, take up in 2 ml 0.1M HCl, load on column 3 and wash with 100 ml 0.1M HCI. Elute with 0.2M HC1 and collect fractions of 20 drops with a fraction collector connected to a drop counter. The separation of americium (241Am daughter of 241Pu) and neodymium may not be quantitative. Only those fractions freed from 241Amare used for activation. Analyze the fractions with a multichannel analyzer, combine the pure la7Ndfractions and evaporate them to dryness. Take up 100 p1 0.5M HNO, and transfer the solution on two or more filter papers with single use 25-pl pipets. Determine the chemical yield against 25 p1 147Ndpipetted on a filter paper. After packing the Nd samples in a polyethylene bag, irradiate 5 minutes at l O I 4 n ern+ sec-'. Remove Nd activity from the filter paper by repeated washings with 0.1M H N 0 3 and extract quantitatively with 50% D2EHPAn-heptane. Measure the 211 keV 149Ndpeak with a gamma ray spectrometer and calculate the Nd content of the sample. RESULTS AND DISCUSSION
Synthetic Mixtures. A series of samples was prepared containing 237 mg natural U and 10 mg Pu, increasing amounts of natural neodymium, and l47Nd tracer. The procedure described was applied and the results are listed in Table IV. Decontamination factors of U and Pu were excellent as no 148Ndcould be detected in the blank sample. Table V shows that in the absence of other fission products prior to neutron activation, the complete separation scheme (18) D. F. Peppard, G. W. Mason, J. L. Maier, and J. W. Driscoll, J. Inorg. Nucl. Chem., 4, 334 (1957).
and radiochemical analysis enables one to determine the 148Nd with an accuracy of 2 %. Irradiated U02-Pu02. One of the fuel elements of the BR3 reactor, irradiated for eight months at 10’3 n cm+ sec-l, was sectioned into 15 pieces and the burn-up distribution determined with l37Cs, 9%r, and 144Ce. Section V41/7 was analyzed for 148Nd as a check for the burn-up results obtained with the radioactive fission monitors (5676 MWd/t). This section, containing 42.438 grams U02-Pu02(natural U enriched with 0. 9 6 z Pu), was dissolved and diluted to 1000 ml; three 5-ml samples were subjected to the Nd procedure and the results listed in Table V. Sensitivity. For a burn-up of 1000 MWd/t about 25 mg samples are required to carry out the analysis as described because the sensitivity is limited to l pg. Half saturation of 149Nd activity will be achieved after a 1.72-hour irradiation period. However, the irradiation time is limited to 5 minutes for radiation damage to the capsule material which interferes with the quantitative recovery of the Nd in the samples. If a more radiation resistant substrate could be found, such as pure Be metal, for example, which still assures the quantitative recovery, irradiations could be performed up to 75 minutes in Ertalon containers providing an increase in sensitivity by a factor of 15. As a result of higher sensitivity, smaller columns could be used subsequently
shortening time required for separations. Another advantage of increasing the sensitivity is that the work in a shielded a-P-7 cell can be avoided and replaced by easier glove-box work. Applications. It should be remembered that the chemical procedure for isolation of the radioactive fission monitors (137Cs and 144Ce) is easier than for lrsNd, but the latter should be used when other fission monitors are excluded, namely for long irradiations at high temperatures. Furthermore, the absolute accuracy of the burn-up determination is more reliable with 148Nd than with radioactive fission monitors because decay, absolute detector geometry, and efficiency corrections are avoided. The main limitation of the 148Nddetermination is that no 148Ndmay be present in new or reprocessed fuel material to be analyzed. ACKNOWLEDGMENT
The authors express their appreciation to L. H. Baetsl6 for many helpful discussions and critical review of the manuscript, R. Boden who performed the preliminary separations of the active samples in the a-P-T-cell, and Miss Vreys who carried out much of the radiochemical work. RECEIVED for review June 17, 1968. Accepted September 9, 1968.
Metastable Ion Characteristics. Measurements with a Modified Time-of-Flight Mass Spectrometer W. F. Haddon’ and F . W . McLafferty2 Department of Chemistry, Purdue Unioersity, Lafayette, Ind. 47907
The sensitivity for the measurement of ions produced by metastable decompositions in the flight tube of a time-of-flight mass spectrometer has been increased by a factor of up to 200 by defocusing the interfering normal ions. The method provides unique identification of the precursor ion and the metastable decompositions which it undergoes. A computer technique is described which yields accurate values of daughter ion masses. “Flat-topped” metastables can be observed, and the method appears advantageous for detecting small values of kinetic energy released in the formation of such peaks. The method is particularly valuable for the study of collision-induced metastabletransitions; thereisa high probabilityfor reaction in the long drift region, and the neutral decomposition products which can be observed serve as a measure of the drift region pressure.
METASTABLE IONS are an important feature of the mass spectra of organic molecules because they provide unique confirmation of the occurrence of a particular ion decomposition reaction ( I ) , and can aid in the quantitative analysis of complex mixtures (2). In addition, particular characteristics of meta1
Present address, Celanese Research Co., Box 1O00, Summit,
N. J.
2 To whom correspondence should be addressed at the Department of Chemistry, Cornel1 University, Ithaca, N. Y.14850.
(1) J. H. Beynon, “Mass Spectrometry and Its Applications to
Organic Chemistry,” Elsevier, Amsterdam, 1960. (2) F. W. McLafferty and T. A. Bryce, Chem. Commun., 1967,
1215.
stable ions can be used to establish the structures of their precursor ions and to distinguish between isomers (3-6). Beynon has recently reviewed such uses of metastable ions (7). The demonstrated importance of metastable ion data has resulted in a substantial effort to develop convenient and general methods for obtaining such information. Probably the most sensitive method available is the Barber-Elliott (8, 9) technique in which the normal ions are defocused by the electrostatic sector of a double-focusing mass spectrometer. The method detects all metastable transitions which lead to a particular daughter ion, and can determine the mass of the daughter ion with high accuracy. However, determination of the precursor mass can be ambiguous, and rather complex procedures are necessary to determine the kinetic energy released in forming the ions (10, 11). (3) T. W. Shannon and F. W. McLafferty, J. Amer. Chem. SOC., 88, 5021 (1966). (4) F. W. McLafferty and W. T. Pike, ibid., 89, 5951 (1967). ( 5 ) W. F. Haddon and F. W. McLafferty, ibid., 90, 4745 (1968). (6) F. W. McLafferty and R. B. Fairweather, ibid., p 5915. ( 7 ) J. H. Beynon, “Advances in Mass Spectrometry,” E. Kendnck, Ed., Vol. 4, Institute of Petroleum, London, 1968. (8) M. Barber and R. M. Elliott, ASTM E-I4 Conference on Mass
Spectrometry, Montreal, June 1964. (9) T. W. Shannon, T. E. Mead, C. G. Warner, and F. W. Mc39, 1748 (1967). Lafferty,ANAL.CHEM., (10) T. W. Shannon, F. W. McLafferty, and C. R. McKinney, Chem. Commun., 1966, 478. (11) M. Barber, K. R. Jennings, and R. Rhodes, 2.Narurforsch., 22a, 15 (1967). VOL. 41, NO. 1, JANUARY 1969
31
