Journal o f Radioanalytical Chemistry, Vol. 64, No. 1-2 (1981) 65- 72
A CONTRIBUTION TO THE CHEMICAL ASPECTS O F P R E C I S E MASS S P E C T R O M E T R I C A N A L Y S I S OF URANIUM O. J. HEINONEN*, R. KORHONEN
Department o f Radiochemistry, University o f Helsinki, Unioninkatu 35, SF-O0170 Helsinki 17 (Finland)
(Received February 23, 1981) The chemical aspects of isotopic fraetionation in a multiple f'dament thermal ionization source were investigated. Samples of uranium were loaded as nitrates, chlorides and sulphates. The dependence of measured isotopic ratios on the filament material and the chemical form of the loading solution was studied. The dependence of the formation of uranium and its oxide ions on various chemical and instrumental conditions were investigated using tungsten and rhenium Filaments. Introduction
An accurate knowledge of the isotopic composition of actinides is important in nuclear fuel quality control, in plant control and in nuclear safeguards. In spite of recent developments in non-destructive analysis of nuclear materials ~ radiochemical methods are sill most important for such determinations. The radiochemical methods will also undergo intensive development in the future. Not only development of new separation methods but investigation of all chemical phenomena associated with present routine methods are essential before automatization of separation schemas and sample preparation systems, since automatization cuts the possibility for human interference and observation during the analysis. The need for a close understanding of all associated phenomena is necessary in high precision thermal ionization mass spectrometry, 2 which is one of the basic techniques used for the determination of the isotopic composition of uranium and plutonium. An accurate determination of the isotopic composition is achieved not only by computer controlled mass spectrometry but using a standardized analytical procedure starting from sample chemistry and ending in evaluation of the results. This, and the fact there have been few systematic studies of isotopic fractionation in a multiple filament thermal ionization source 3 lead us to investigate the chemical aspects of fractionation. Because chemical conditions and fractionation *Present address: Technical Research Centre of Finland, Reactor Laboratory, Otakaari 3A, SF-02150 Espoo 15, Finland Z Radioanal. Chem. 64 (1981) 5
65
O. J. HEINONEN, R. KORHONEN: A CONTRIBUTION (may) cause systematic errors 2 it may be difficult to observe them during routine analysis. This calls for identification and, if possible, quantification of all parameters causing bias.
Experimental Apparatus The mass spectrometer used in this investigation was a 12 inch radius, 90 ~ sector magnet instrument type MI-1309 (V[O Techsnabexport, USSR). It was equipped with a triple fdament thermal ionization source, which can operate at accelerating voltages up to 3 kV. The instrument was equipped in these investigations with an ion counter type SI-03 (3/[0 Techsnabexport, USSR). Spectra were recorded and the instrument controlled by an on-line desk-top computer adapted to the instrument. 4 Filaments The filament materials used in these experiments were rhenium (Rembar Co, USA) and tungsten (V/O Techsnabexport, USSR). Before use the filaments were washed in ethanol p.a. and preheated in vacuum (10 -s Pa) for 4 hours. In spite of this treatment the tungsten fdament emitted considerable quantities of potassium ions, which may interfere the uranium isotopic measurements, s After 4 hours heating the agK+ signal was still 3 X 10 -12 A in typical uranium analysis conditions. Conversion o f samples The samples used in this investigation were U.S. Department of Commerce National Bureau of Standards SRM-U-500. The powder samples were dissolved in nitric, sulphuric or hydrochloric acid (Merck p.a.). The prepates were made in the following way. Samples in 0.5M acid were dried gently in 10 microliter aliquots containing 10 micrograms of uranium on the filaments by using electric current. The samples were converted into uranium oxides by heating the filament 10 seconds or 10 minutes in open air. The 10 second conversion was obtained using a current of 2.1 A for tungsten and 0.9 A for rhenium filaments. This current heated the filament material in both cases to glowing. The longer conversion time was adjusted by using a current just below the glowing point for 10 minutes. Analyzing conditions The formation of uranium and its oxide ions were recorded as a function of ionization and evaporation temperatures for both fdament materials. The effects 66
J. Radioanal, Chem. 64 (1981)
O. J. HEINONEN, R. KORHONEN: A CONTRIBUTION
of f'dament material and chemical composition to the isotopic ratio of the sample were measured from 23 s U § and 2 a 8 U § ion intensities. The spectra were scanned 10 times using the computer system based on the peak switching technique. The isotopic ratios were calculated according to the principles described elsewhere. 4 The effect of ion source pressure on the formation of different ion species was studied in the following way. A sample loaded on a rhenium filament from 0.5M nitric acid was converted 10 minutes. After heating and stabilization of U§ § current ratio at a pressure of 1 X 10 -s Pa the diffusion pump of the ion source was switched off to increase the pressure. The pressure increased during 15 rain to 2 X 10 -.4 Pa and the ion currents of U § and UO + ions were recorded. Operating pressures in the ion source region were maintained below 2- 10 -s Pa, and pressure in the analyzer region below. 1 - 10-SPa. The heating procedure of the filaments in the isotope ratio measurements of uranium was the following. The current in the ionization f'dament was increased automatically during 40 min to 4.8 and 3.0 amperes for tungsten and rhenium, respectively. The evaporation current was increased during 40 min to 2.0 A (tungsten) or 0.8 A (rhenium). The final adjustment was made manually. After focussing the measurement started 60 min from the beginning of heating. The temperature of filament was measured with an optical pyrometer. The temperatures were controlled during the measurements using 187Re+ and 183W+ signals, too. A working resolution of 400 (M/AM = 400,10% valley definition) for uranium 2 3 s U + w a s used.
Results and discussion The intensity distribution of uranium in atom and oxide ions under different conversion conditions are presented as a function of ionization temperature for rhenium fdaments in Fig. 1. The results obtained using tungsten filaments in analogous experimental conditions are shown in Fig. 2. The results for rhenium f'daments loaded with uranyl nitrate differ from the remits of HEUMANN. s The uranium ion current increases rapidly at ionization temperatures above 1900 ~ but the formation of UO § ions seems to be considerably higher. This can be explained with a higher evaporation temperature (1400 ~ neded because of lower ion transmission of MI-1309 ion source. Another significant difference is the decrease in UO § ion formation at ionization temperatures above 2400 ~ From Fig. 1, can be seen the increase of U § ion yield with increasing conversion temperature and time. The samples loaded as uranyl sulphate showed a higher tendency for UO § ion production compared to other solutions still after 10 J. Radioanal. Chem. 64 (1981)
5*
67
O. J. HEINONEN, R. KORHONEN: A CONTRIBUTION A Conversion
Os
102 - - H N O 3
Conversion
9
102
_
lOs
HNO3
oe
to o 9
o o
o 10
} to
o
-
o
o +0
++ I
o
o
E t _
++
-
9
+
+
+
4-
2
.4+
2
3
Current~A
Conversion
= 102 8
3 Current,A
10rain
HNO 3
~n O
~5 10 - -
~
.f
o
o o
o
g U* ions
9
o UO ~ i o n s
2
Cur rent,
10rain
Conversion
102 -
Conversion
ooOeoe o9
HCI
o
102
--
lOmin
H2SO ~
o~ o
o
9
~-
~5 10
~+§247
o 0
z
9
, 3
Current,A
g§
4-
o
2
9
o
+~++.r
9
--
ooo co~ 9 9
o
4" I
o
o
to
+'t~oOOo O
=c
UO~ ions
+
3 A
I z
4-
9
,
[ a Current, A
,
--
lib
Fig. 1. Intensity distribution o f U § UO* and UO~ ions as a function of ionization current (temperature for rhenium filaments with evaporation temperature 1400 ~ The temperature of the ionization filament was 1900 ~ with a current 2.1 A and 2400 ~ with 3.15 A 68
J. Radioanal. Chem. 64 (1981)
O. J. HEINONEN, R. KORHONEN: A CONTRIBUTION
100
HNO 3
eP
100
HCI
9
~
9~ a ,-
oCi~o 9 -o 10--
~ ~ a '~
o
o.
9
100 - HzSO4
e9
o
.
~ ~ ~2
10
9-
e9
~
oO~ o 9o 1C-
o 9
R
o
9
0§ ~.
§
Conversion 10 rain , I t I * I Ib 4.0 5.0 6.0 Current, A
'
o ConYersion 10 r n i n ~ / , / , i ~,. 40 50 6.0 Current, A
§ Conversion 10 m i l l ~ I , / J / 40 5.0 6.0 Current, A
j~
Fig. 2. Intensity distribution of U*, UO* and UO; ions as a function o f ionization current (temperature) for tungsten filaments with evaporation temperature 1400 ~ The temperature of ionization filament was 1 9 0 0 ~ with a current 4.0 A and 2 4 0 0 ~ with 5.15 A. See
the explanation of symbols in Fig. i min conversion. On the contrary the samples loaded as chlorides generated mostly U § ions at ionization temperatures above 2100 ~ This was obtained even with samples converted for 10 s. In all three cases the samples nonconverted (conversion 0 s) proved to generate higher oxide ion intensities and the total ion currents (U + + UO++ UO~) were lower than the currents of converted samples. The alpha spectrometric measurements, however, showed the uranium yield on the filament to be independent on the chemical form of the loading solution. From a practical point of view the preparation of uranyl sulphate proved to be difficult since the samples had a tendency to deposit at the ends of the Filament. Similar observations can be made in Fig. 2. with tungsten filaments with the exception of samples loaded from chloride solution which seems to generate less oxide ions. The decrease in formation of UO + ions at ionization temperature above 2400 ~ can be seen here, too. In Fig. 3. is presented the formation of U + and UO + ions as a function of evaporation temperature with a constant ionization temperature. The U/UO + ratio seems not to be significantly dependent on the evaporator temperature in typical uranium analysis conditions. Similar observations were made with tungsten and rhenium with all three loading solutions converted 10 rain. The formation of U + ions is strongly dependent on conversion time as shown in Fig. 4. The preparate converted 10 min from 0.SM nitric acid on a tungsten filament reaches maximum U + yield after 40 min. Corresponding loading without conversion does not reach a constant U + emission during a reasonable analysis time. The emission in the latter case is in the beginning mainly UO~" ions, which indicates uraJ. Radioanal. Chem. 64 (1981)
69
O. J. HEINONEN, R. KORHONEN: A CONTRIBUTION
9
100
100
ie
- -
I0C o
~
m
oo
o o
9
o
~
10--
9
o
~
10
9
o o
g 9
E
o
1
o Re,HC[ [ , [ .p. 1.o z.o Current, A
9
Re,HNO 3 I , J. -b ~.o 20 Current, A
W, HNO3 I , l ~.o 2.o C ur rent, A
,
_=.
Fig. 3. Intensity distribution of U*. and UO§ ions as a function of evaporation temperature at ionization temperature of 2200 ~ using rhenium and tungsten filaments. The temperature of rhenium evaporator was 1400 ~ with a current 0.85 A. For tungsten filaments a temperature of 1400 ~ was measured with a current 1.75 A. See the explanation of symbols in Fig. 1 nium is evaporating as uranyl nitrate as in the work o f HEUMANN. s Our measurements show, however, considerably higher emission o f UO + ions i n contradiction to the measurements o f HEUMANN. s This m a y be due to the different experimental conditions. The problem with insufficient conversion is not only the p o o r yield o f U § ions relative to the total uranium ion current, b u t the evaporation in the beginning reduces the U § signal producing statistically weaker ion current. If the operator of an instrument tries to compensate it by increasing the signal with a higher temperature, he will obtain a systematic error as shown by DE B I E V R E ) The chance in the formation o f U § and UO § ions as a function o f pressure was not found to be significant. During a pressure increase from 1.1 9 10 -s Pa to 2 . 0 . 1 0 -4 Pa a difference o f 1% (relative) in the ratio U§ § was observed. This is much less as the differences caused b y improper preparation o f sample as shown above. The variation o f observed isotopic ratio with the loading form o f the salt on tungsten and rhenium filaments is presented in Table 1. The values are based on eight separate loadings o f NBS SRM-U-500 standard. The error presented in Table 1 is external standard deviation (2 X Sext).* With the precision of the whole anal*The term external standard deviation is defined in the following way. When n samples of the material to be measured are admitted to the mass spectrometer (as separate loads) and measured. Each of them yields a value R- for a single measured isotopic ratio. Then a mean P, and an external standard deviation Sext can be calculated as =
and s2 n
70
ext
n(n-
1)
J. Radioanal. Chem. 64 [1981)
O. J. HEINONEN, R. KORHONEN: A CONTRIBUTION Table 1 Measured 23 ~U/2~s U ratios Preparation HN03 HCI H2SO4
Tungsten 1.0036 + 0.0015 1.0078 + 0.0016 1.0076 • 0.0020
Rhenium 1.0077 • 0.0019 1.0085 • 0.0010 1.0093 • 0.0019
lytical system the observed isotopic ratio of samples loaded as nitrates seem to differ from the other salt forms on a tungsten filament. The lower reproducibility of the samples loaded as uranyl sulphate is due to the use of lower ion currents caused by difficulties in preparation, similar observations with isotopic ratios can be seen for rhenium f'daments. Systematic difference of measured isotopic ratios between tungsten and rhenium fdaments may be due to the slightly different sizes of the fdaments causing a systematic error in temperature measurements. As a conclusion of facts presented above The formation of uranium atom and molecule ions is dependent on the chemical form of the loading solution, and on conversion conditions. The pressure change during analysis has a smaller affect on the formation of different ion species compared to conversion. The chemical form of the loading solution has effect on the measured isotopic ratio, even when evaporation and ionization temperatures are kept constant. Impurities, e.g. sulphates, may cause trouble in sample preparation. U*/UO § intensity ratio can be used as an indicator of the quality of the conversion of the sample.
The authors are g~ateful to the Department of Energy, Ministry of Trade and Industry, Finland, for f'mancial aid and to Professor J. K. MIETTINEN, Head of the Department, for his support. Mrs PIPSA VUOLEVI is thanked for skilful operation of the mass spectrometer.
Z Radioanal. Chem. 64 [1981)
71
O. J. HEINONEN, R. KORHONEN: A CONTRIBUTION
References 1. L. STANCHI (Ed.), Proc. 2rid Annual Symposium on Safeguards and Nuclear Material Management, European Safeguaxds Research and Development Association, Varese, 1980. 2. P. DE BIEVRE, in N.R. DALY fEd.), Advances in Mass Spectrometry, Vol. 7 A, Heyden & Son Ltd, London, 1978, p. 393. 3. L. J. MOORE, E. F. HEALD, J. J. FILLIBEN, in N. R. DALY fed.), Advances in Mass Spectrometxy, Vol 7 A, Heyden & Son Ltd, London, 1978, p. 448. 4. O.J. HEINONEN, Kemia-Kemi, (1980) No. 10. 5. K. G. HEUMANN,Int. J. Mass Speettom. Ion. Phys., 9 (1972) 315.
72
J. Radioanal. Chem. 64 (1981J
