Isotopic analysis of rare earth elements by total vaporization of samples in thermal ionization mass spectrometry

International Journal of Mass Spectrometry and Ion Processes, 120 (1992) 163-177

163

Elsevier Science Publishers B.V., Amsterdam

Isotopic analysis of rare earth elements by total vaporization of samples in thermal ionization mass spectrometry J.C. Dubois,

G. Retali and J. Cesario

DPpartementdes ProcPdksd’Enrichissement.Commissariatci I’Energie Atom&e, Centre &Etudes Nuckaires de Saclay. I31191 Gif sur Yvette (France) (First received 30 October 1990; in final form 27 May 1992)

ABSTRACT A major cause of inaccuracies in isotopic analysis with thermal ionization mass spectrometry is the mass-dependent differential vaporization of the isotopes. This problem can be overcome if all the vaporized atoms from a given sample are ionized with a constant yield, and integrated. This requires the use of a multi-collection mass spectrometer and precautionary measures concerning, in particular, the optical parameters adjustment phase. We have applied this method, originally developed for analyses of uranium and plutonium, to analyses of neodymium, samarium, gadolinium and lutetium. A specific software program has been written for real-time data acquisition and processing. Fractionation curves and isotope ratios are presented. Very good precision (relative u values below 10m4)is obtained with sample amounts ranging from 20 to 50 times less than with the conventional measurement method of stabilized currents. A new value is proposed for the normalizing ratio ‘46Nd/‘uNd used by geochronologists i.e. IssNd/“‘Nd = 0.72333 (isotopic ratio). Keyworrls: isotopic analysis; thermal ionization; rare earth elements.

1. ISOTOPIC

FRACTIONATION

A major cause of inaccuracies in thermal ionization mass spectrometry (TIMS) is isotopic fractionation. This is because, for a given element, the isotope ratio readings are affected by an error proportional to the relative ,differences in the atomic masses: the lighter isotope vaporizes more rapidly during heating of the specimen in a vacuum, in accordance with an M-Ii2 dependence (Rayleigh’s law). At the start of the analysis, the ratio of the vaporized fluxes is always biased in favour of the lighter isotopes. The quantity of the specimen remaining, which is of finite dimensions, thus becomes increasingly depleted in these isotopes, and the light to heavy ratio Correspondence to: J.-C. Dubois, Departement des Pro&d& d’Enrichissement, Commissariat zl 1’Energie Atomique, Centre d’Etudes NuclCaires de Saclay, F91191 Gif sur Yvette, France.

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value tends first towards the true average value then falls below it, subsequently approaching an infinitely small value. In practice, a certain stability of the reading is observed in some cases after significant variation in the first hour. The specimen is generally obtained by drying one or more micro droplets deposited in the centre of a metal strip which can be heated using the Joule effect. The area of the deposit (l-3mm’) gives a thickness equivalent to around 1000 single layers for a 10e6 g deposit and a single layer for a deposit of lop9 g. Under these conditions numerous parameters, which are difficult to isolate, e.g. the vapour pressure and the stability of the chemical compound on the filament at the temperature involved, the liquid or solid state of the deposit, isotopic re-homogenization owing to thermal excitation or diffusion etc., affect the vaporization rate of each isotope. Simplified descriptions have, nevertheless, been proposed. Let us consider two isotopes, A and B, of a given element, E, with atomic masses MA and MB, where NA and NB etc. are the numbers of atoms of A and B present on the filament before the start of the evaporation. If AM = MA - MB (assumed to be positive) is small compared with MB, the ratio of isotopic abundances, R,, measured at the beginning of the analysis is given, to a first approximation, by: R, = R,(l -kAM)

where k = 1/2M and R, = NA/NB . More thorough theoretical analyses of the phenomenon have been proposed [l-3]. However, the uncertainties concerning the various parameters involved (the amount of material deposited, the chemical form of the salts deposited, the vaporization temperature of the deposit, the contingent dissociation of the vaporized molecules, the amount of time available for the analysis, etc.) make it only rarely possible to use these methods to obtain quantitative corrections. The degree to which the isotope readings vary depends on the analytical procedure used. In practice, two methods are available to analysts for making allowance for isotopic fractionation, comparison with a reference sample and internal standardization. 1.1 Comparison with a reference sample

For a given element, a calibration specimen or a reference material with a known isotopic composition is required. Its analysis makes it possible to determine a correction coefficient for each isotope pair measured. The analytical parameters are carefully recorded to establish the operating procedure to be used subsequently for analysis of unknown samples of the same element. The accepted procedure is then to apply the same correction coefficients with a margin of uncertainty depending on the method by which they were derived.

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It is difficult to assess the degree of similarity between the operating method finalized during analysis of the calibration sample and that adopted for the test sample. In particular, the latter is frequently less pure and the amount deposited is less accurately known. Experience is necessary to quantify the resulting increase in the uncertainty. 1.2 In ternal standardization

To avoid the uncertainty resulting from the fact that it is impossible to guarantee exactly the same analytical conditions for the reference material and the test material, we need only analyze both simultaneously using a reference material which consists of a pair of specific isotopes which lie in the spectrum of the isotopes of the elements studied. A number of different cases are possible: (i) The two isotopes are already present in the sample in its natural state, and their abundance ratio is measurable and invariable. Comparison between the reading for this ratio and a value conventionally assigned to it (a value which may or may not be the true one) makes it possible to calculate a standardization coefficient kAm. To correct the measured isotopic ratio, Ri, of two other isotopes separated by a mass difference, Ami, a simple linear relationship (in fact, the correction coefficients are equal for identical values of Am/m”*, and not for constant Am differences. As the resulting differences are, generally, not significant in the mass range of a given element, this correction may be neglected. However, some authors use more complex laws that depart from linearity.), based on the above considerations is applied:

This method of standardization is mainly used by isotope geologists. For example, using the Rb/Sr method, the ratio of the relative numbers of *%r and **Sr atoms is conventionally taken to be 0.1194, and the standardized *’Sr/86Sr ratio is used as a measure of the time-integrated decay of *‘Rb. (ii) The two isotopes are not present in the unknown sample. In such cases, they must be added after having prepared a mixture with a known ratio. If the value of this ratio is determined by a method independent of mass spectrometry providing the required degree of accuracy, then a true calibration of mass fractionation is possible. For example, for the isotopic analysis of uranium in nuclear fuels, it is possible to use the 233U/236U ratio to standardize the 235U/238Uratio and, with iterations, the 233U/238Uratio [4]. The methods described above have made thermal ionization an accurate measure of isotopic composition. They are, however, extremely limiting and in many cases they cannot be used as there is no suitable reference material.

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166

1.0675 -. 1,067o -. 1.0665 -. 1x=0,:::;;;; 35

43

::::; 51

59

67

75

i:; 63

91

i:;:;; 99

107

;:I 115

123

131

Tii (mini

Fig. 1. Isotopic fractionation

The total consumption alternative.

of samarium during analysis.

method

described

below constitutes

a valuable

2. TOTAL CONSUMPTION

As theory indicates, a plot of fractionation against time (Fig. 1) shows that, for a given pair of isotopes, the apparent degree of enrichment of a lighter isotope is greater at the start of the analysis and then begins to fall. It may be postulated that total vaporization of the sample and integration of all the ions formed by the isotopes being measured will give accurate values for the isotopic ratios. It is necessary that the ionization yield is kept constant throughout the vaporization and that only one species of ion is formed during the analysis. For example, the result is necessarily biased by the production of M+ and MO+ ions in variable proportions with different fractionations. This hypothesis has been verified experimentally by Callis [S] and by the present authors in the analysis of samples of NBS-certified uranium and plutonium reference materials. The total consumption method gives satisfactory results with these two elements for samples of nuclear fuel [5-7] and it was, therefore, of interest to check its validity for analyzing rare earth elements. 3. EXPERIMENTAL

METHOD

The spectrometer used was a VG 354 fitted with five identical Faraday cups in the same plane.

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TABLE 1 Weight of each deposit (ng)

Fractionation curve Total vaporization Normalized analysis

Neodymium

Samarium

Gadolinium

Lutetium

300 20 300

300 20 300

500 50 500

300 20 -

3.1 Calibration of the collection channels The relative calibration of the five electrometry channels can be carried out using a highly stable electronic current generator whose output is successively applied to each amplifier input. Successive measurement in each channel of the same intense stable ion current makes it subsequently possible to assess the relative values of the efficiencies of the collectors. In addition, a relative calibration method combining static multi-collection and mass switching (peak jumping) during the measurement of a multi-isotope element was applied [ 81. The relative calibration values obtained with these three methods are not significantly different in terms of the uncertainty in la = 1 x 10m4. Although the methods are not independent, it is possible to deduce from the readings that the uncertainty in the relative efficiencies of the five channels is + 1 x 10-4. In routine work, calibration is limited to the use of a current generator before each sample is analyzed. This makes it possible to measure, and possibly compensate, for any drift in the electrometer amplifiers, the most sensitive components of the channels. 3.2 Analytical procedure The triple tilament analysis technique was chosen as being the only one capable of ensuring an ionization yield value independent of deposit vaporization temperature variations. Rare earth elements having a natural isotopic composition were supplied by Johnson Matthey in the form of oxides. After dissolving weighed aliquots in 3N nitric acid, a few ~1 of adequately-diluted solution are deposited at the centre of a 99.99% pure rhenium filament obtained from Rhenium Alloys. The quantities deposited are given in Table 1. With the gadolinium deposits the filament is brought to red heat for a few seconds. This increases the ion current (intensity) by 30%. During heating of the deposit under vacuum in the spectrometer source, the nitrates thus deposited are gradually transformed into oxide, which is probably

J.-C. Dubois et al./Int. J. Mass Spectrom. Ion Processes 120 (1992) 163-177

168 Amperes

l,OOE-10 l,OOE-11 l,OOE-12

C

A

l,OOE-13 l,OOE-14 l,OOE-15 l,OOE-16 10

0

30

20

40

50

timecminl

Fig. 2. The three phases of analysis using a VG mass spectrometer: (A) adjustment (B) acquisition phase; (C) shutdown phase. For full details see text.

phase;

the vaporized chemical form. The oxide molecules coming into contact with the ionizing filament, which is raised to an extremely high temperature (2300K), are to a large extent dissociated. The M+ ions are preponderant. Under our measurement conditions the following situation prevailed: I MO+&+

< 1O-4

The VG mass spectrometer does not have software have, therefore, developed a specific program components of the VG software (e.g. control of the the magnetic field). The analysis consists of three

for total vaporization. We which uses some of the filament power supply and phases (Fig. 2).

3.2.1 Adjustment phase The ionization filament temperature, which determines the ionization yield of the atoms of the sample, is adjusted indirectly by measuring the positive le7Rh ion current. It is then kept at the same value for the rest of the analysis. An initial adjustment of the source is made. As a cyclic sweep of the magnetic field makes it possible to examine the mass range of the element analyzed, the vaporization filament temperature is increased until a total ion current in the region of lo- l5A is obtained. The source parameters are then once more adjusted and the magnetic field to mass ratio is reset for the most abundant isotope of the element measured. All these adjustments are carried out under mono-collection using the Daly electrode which enables low currents to be measured accurately. As we shall see, the amount of sample consumed during the first phase is negligible compared to the amount of sample used to obtain the reading. Depending on the masses of the isotopes to be measured, the collectors are automatically positioned at the required locations in the focal plane.

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169

3.2.2 Acquisition phase Simultaneous continuous measurement of the ion currents is initiated. The vaporization filament temperature is increased by fixed or variable steps until I max, the total ion current set by the operator is reached, generally some 10-i' A. Continuous monitoring of the ion current decay rate governs the temperature steps, up to a limiting value of the pre-determined heating current. Integration of the currents is interrupted at regular intervals for measurement of the base line of the electrometry channels and the transfer of data to the computer. Allowance is made for these periods. As readings can be taken simultaneously by the multi-collection system, ionic emission stability is no longer a determining factor in the precision of the isotopic analysis results. 3.2.3 Shutdown phase The analysis is terminated when the ion current drops below a level set by the operator. For the amounts of deposits shown in Table 1, the total length of the analysis is around 40 min, of which 25 min are devoted to adjustment and 15 min to acquisition and integration. The observed ratio of the ions integrated in the acquisition phase to the ions lost during the adjustment and shutdown phases is greater than 10’. The computer calculates the isotopic ratios on the basis of the integrals of the current measured throughout the acquisition for each isotope. The ion current integrals can easily be converted into the number of ions received by the collectors. It is thus possible to evaluate a purely statistical uncertainty value for the total quantity of each isotope measured, in the form &n. 4. RESULTS

For each of the four rare earth elements studied, the following are reported. 4.1 A curve showing the fractionation as a function of time The isotope ratio value readings are measured at regular intervals during the analysis. Each value is then conventionally related to a value with the same ratio obtained in analyses carried out under total consumption conditions. The values of the coefficient, k, calculated for Am = 1 u are plotted on the curves (Figs. 3-6). The same procedure was carried out for the following isotope ratios: ‘46Nd/‘44Nd; ‘49Sm/‘47Sm; 160Gd/“* Gd; ‘76Lu/‘75Lu. Each point on the curve is the mean of the values obtained in the analysis of three samples. The curve obtained for neodymium shows a deviation that is probably due to a chemical transformation of the deposit at the temperature involved.

J.-C. Dubois et al./Int. J. Mass Spectrom. Ion Processes 120

170

48

56

64

k'lOa+4 -5 i

72

80

88

96

1CS-f~~120

128

136

144

(1992)

152

163-I

77

150

.A .I'\_.-~'

-10

./

-15 / -20 1 TiIlW~min~

Fig. 3. Fractionation

coeffkient, k (Am = 1 u), for neodymium.

For the four elements, the analysis is terminated before the deposit is exhausted. The amplitude of the variations of the discrimination factor observed under these conditions is about 2 x 10V3to 3 x 10m3for 1 u. 4.2 The results of a series of total consumption analyses 4.2.1 Interferences

The analyses were carried out using pure elements (see Section 3.2) to eliminate the risk of isobaric interferences. The results are given in Tables 2-5. We have, however, checked the absence of interfering ions using the arrangement of the collectors shown in Scheme 1.

k'lOw4

Fig. 4. Fractionation

coefficient, k (Am = 1 u), for samarium.

J.-C. Dubois et aLlInt. J. Mass Spectrom. Ion Processes 120 (1992) 163-177

171

lo T

: k’e.4

: 48

;

: 56

:

: 64

:

: 72

;

:/ev=+-

:

_.,BQgd

Tii

Fig. 5. Fractionation

:

lminl

coefficient, k (Am = 1 u), for gadolinium.

Elements Analysed

Collectors

L2

b

AX

HI

H2

143

144 147

145 149

146 152

‘47Sm

155 ‘73Yb

157

158

160

“‘Dy

176

‘77Hf

Neodymium Samarium

‘46Nd

Gadolinium Lutetium

175

Scheme 1.

No signal that was significantly different from the background was detected in any of the channels that could record isotopes of other elements. An unidentified interference was observed at m 155 in the analysis of gadolinium. Eugster et al. [9] mention an interference due to BaF, which we 10 8 -. 6 -. 4 -. 2 -.

Timeimini Fig. 6. Fractionation

coefficient, k (Am = 1 u), for lutetium.

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J.-C. Dubois et al./Int. J. Mass Spectrom. Ion Processes 120

(1992)

163-177

TABLE 2 Neodymium isotopic ratios, by total vaporization Sample

Ratio 1431144

1451144

1461144

1 2 3 4 5 6 7

0.51099 0.51099 0.51098 0.51100 0.51098 0.51097 0.51104

0.34871 0.34871 0.34871 0.34870 0.34871 0.34868 0.34870

0.72337 0.72334 0.72317 0.72338 0.72333 0.72333 0.72340

Mean Standard deviation

0.51099 0.00002

0.34870 0.00001

0.72333 0.00008

142/144

148/144

HO/144

did not detect, at m 157 (for the oxides see Section 3.2). The measured value for the ‘46Nd/‘44Ndratios = 0.72333 f 0.00007 (The standard deviation in the successive analysis of seven samples of the same material is 0.000076. The confidence in the result (95%) should be 2.45 x O.O00076/J7 = 0.00007, not including any systematic error involved in the calibration of the collectors.) differs from the value normally adopted by geochronologists [lO,ll]: ‘46Nd/ lUNd = 0.7219. Wasserburg et al. [12] give 0.72413 normalized with respect to 146Nd/142Nd= 0.63615. The analyses were carried out with NdO which implies a correction for TABLE 3 Samarium isotopic ratios, by total vaporization Sample

Ratio 147/149

152/149

1 2 3 4 5 6

1.08664 1.08699 1.08688 1.08658 1.08682 1.08691

1.93038 1.92934 1.92988 1.93073 1.93042 1.92982

Mean Standard deviation

1.08680 0.00016

1.93009 0.00051

J.-C. Dubois et aLlInt. J. Mass Spectrom. Ion Processes 120 (1992) 163-177

173

TABLE 4 Gadolinium

isotopic ratios, by total vaporization

Sample

Mean Standard deviation

Ratio 160/158

157/158

0.87880 0.87868 0.87855 0.87872 0.87888 0.87811 0.87870 0.87856 0.87870

0.63086 0.63083 0.63077 0.63076 0.63083 0.63073 0.63080 0.63116 0.63082

0.87863 0.00022

0.63084 0.00013

oxygen [12]. Moore [13] in the analysis of NdCl, had found the ‘46Nd/142Nd ratio = 0.6369. The ensemble of the values for the 146Nd/‘44Ndratio are inside the range of representative values published by the Commission on Atomic Weights and Isotopic Abundances [14].

TABLE 5 Lutetium atoms isotopic ratios, by total vaporization Sample

Ratio 1761175

1 2 3 4 5 6

0.026538 0.026549 0.026544 0.026543 0.026548 0.026550

Mean Standard deviation

0.026545 0.000005

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J.-C. Dubois et al./Int. J. Mass Spectrom. Ion Processes 120 (1992) 163-177

TABLE 6 Neodymium isotopic ratios, normalized Sample

Ratio 142/144

143/144

145/144

146/144

148/144

HO/144

1 2 3 4 5 6

1.13961 1.13985 1.13979 1.13966 1.13948 1.13957

0.51099 0.51097 0.51104 0.51091 0.51095 0.51094

0.34874 0.34876 0.34875 0.34873 0.34872 0.34872

0.72333

0.24302 0.24288 0.24295 0.24316 0.24307 0.24304

0.23824 0.23810 0.23819 0.23830 0.23824 0.23818

Mean Standard deviation

1.13966 0.00014

0.51097 0.00005

0.34874 0.00002

0.72333 0.00000

0.24302 0.00010

0.23821 0.00007

4.3 The results of a series of analyses carried out with single collection These results are standardized relative to an isotopic ratio found under total consumption conditions, except for lutetium (see Tables 6-8). 4.3.1 Isotope fractionation laws Three types of equations were tried to correct for isotope fractionation: linear, exponential and power. The variations obtained in the isotopic ratios in using these different equations are not significant. For the low fractionation correction values “The linear law is essentially equivalent to the power law” [12]. We have, therefore, used a linear fractionation equation. TABLE 7 Samarium isotopic ratios, normalized Sample

Ratio w/149

147/149

148/149

150/149

152/149

154/149

1 2 3 4 5 6

0.22392 0.22369 0.22387 0.22398 0.22392 0.22362

1.0680

0.81424 0.81416 0.8 1423 0.81422 0.81422 0.81409

0.53373 0.53361 0.53368 0.53385 0.53367 0.53341

1.93047 1.93009 1.93001 1.93057 1.93087 1.93040

1.64006 1.63968 1.63968 1.64040 1.64016 1.64051

Mean Standard deviation

0.22383 0.00014

1.08680 0.00000

0.81419 0.00006

0.53366 0.00015

1.93040 0.00032

1.64008 0.00035

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175

TABLE 8 Gadolinium

isotopic ratios, normalized

Sample

Ratio 152/158

154/158

155/158

156/158

157/158

160/158

1 2 3 4 5

0.00830 0.00828 0.00829 0.00826 0.00827

0.08828 0.08824 0.08820 0.08819 0.08819

0.59800 0.59797 0.59774 0.59778 0.59775

0.82614 0.82599 0.82583 0.82588 0.82590

0.63086 0.63096 0.63083 0.63087 0.63088

0.87863

Mean Standard deviation

0.00828 0.00002

0.08822 0.00004

0.59785 0.00013

0.82595 0.00012

0.63088 0.00005

0.87863 0.00000

4.3.2 Comments on the results It must be emphasized that the agreement is excellent between the results obtained with multi-collection combined with total consumption and conventional single collection (see Scheme 2). Isotopic ratio

Total consumption

Standard TIMS

‘43Nd/l”Nd ‘52Sm/‘49Sm ‘57Gd/“‘Gd

0.51099 1.93009 0.63084

0.51097 1.93040 0.63088

Scheme 2.

It should be noted that the results are not completely independent since the normalization ratio resulting from the total collection is inserted in the standard TIMS single collection measurements. 4.3.3 Reproducibility This is expressed by the standard deviation:

c=JC(Xi-.Z)2/n-

1

calculated for each isotopic ratio. These external reproducibilities are very good but show some anomalies. There is not always a correlation between the isotope ratio value and the standard deviation. Several explanations can be proposed: (i) Low values of the populations. (ii) No test for the rejection of anomalous data was used. For example, in total consumption if the ratio IhNd/ l”Nd = 0.723 17 is dropped, the standard

176

J.-C. Dubois et al./Int. J. Mass Spectrom. Ion Processes 120 (1992) 163-177

TABLE 9 Comparison of the isotopic ratios found in the present work, with published values Neodymium

Ratio 142/M

143/144

145/144

146/144

148/144

150/144

1.1399 1.13830 1.14189 1.13966

0.5118 0.51185 0.51097

0.3487 0.34896 0.34839 0.34874

0.7223 0.72413 0.72190 0.72333

0.2420 0.24307 0.24156 0.24302

0.2370 0.23862 0.23645 0.23821

144/149

147/149

1481149

150/149

152/149

1541149

Ref. 18 (IUPAC) Ref. 12 This work

0.22 0.22249 0.22383

1.09 1.08507 1.08680

0.82 0.81348 0.81419

0.54 0.53399 0.53366

1.93 1.93477 1.93040

1.64 1.64609 1.64008

Gadolinium

Ratio 152/148

154/158

155/158

156/158

157/158

160/158

Ref. 19 (IUPAC) Ref. 9 This work

0.0080 0.00817 0.00828

0.0878 0.08782 0.08822

0.5837 0.59593 0.59785

0.8241 0.82410 0.82595

0.6300 0.63024 0.63088

0.8800 0.88036 0.87863

Lutetium

Ratio

Ref. Ref. Ref. This

17 (IUPAC) 12 11 work

Samarium

Ratio

176/175 Ref. 15 Ref. 16 This work

0.0266 0.02677 0.026545

deviation is reduced to 0.00003 and there is good correlation between the ratio and the reproducibility. (iii) Interferences (see the value for 155Gd/‘58Gd). (iv) The linear law for isotope fractionation is not exact. We recall that it is applied only to single collection analyses. The measurements of the isotopic ratios for neodymium, samarium, gadolinium and lutetium given by different authors are listed in Table 9. 5. CONCLUSIONS

The total consumption method is of interest as an alternative to the traditional TIMS method for the analysis of rare earth elements. It is particularly

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177

appropriate for nuclear applications in combination with isotope dilution. Thus for the calculation of burnup, precise measurements of ratios such as Nd/U and Gd/U can be obtained by using the total consumption method for the two elements in each pair. The excellent measurements obtained with lutetium made it possible for us to determine the volume of fuel reprocessing tanks. There is a restriction as to the number of isotopes that can be measured simultaneously. It depends on the spectrometer used (up to nine collectors with the FISON-VG and FINNIGAN-MAT instruments). The risks of isobaric interference necessitate good separation chemistry. The total consumption method is rapid and sensitive, totally eliminating the effect of isotopic fractionation bias on the results. It involves using very small quantities of material, requiring that great care be taken to avoid contamination. REFERENCES

5

6 7

8

9 10 11 12 13 14 15 16 17 18 19

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