Anal. Chem. 1995, 67, 1517-1520
Trace Element Bmpurities in C ~ O C,~ Oand , Graphite soot Tibor Braun*lt and Henrik Rauschs Institute of Inorganic and Analytical Chemistry, L. EDNOSUnivesity, H- 1443 Budapest, P.0. Box 123, Hungary, and KFKI Atomic Energy Research Institute of the Hungarian Academy of Sciences, P.0. Box 49, H- 1525 Budapest, Hungary
Trace element concentrations in graphite soot, c60, and c 7 0 have been determined by instrumental neutron activation analysis. It has been shown that all these materials contain trace element impurities at parts-per-billionand even parts-per-millionconcentrationlevels. As structural and physical properties are known to be sensitive to such dopant concentrations of trace element impurities, it is argued that future investigations on electrical, magnetic, and the properties of Uerenes will have to consider the concentrationof trace element impurities in these materials. The purity of fullerenes is in general defined as the relationship of the main component (e.g., )C , to the higher fullerenes (GO, Ca, etc.) and in some cases to other, usually polycyclic, organic compounds.' To the best of our knowledge, data on trace element impurities in commercial fullerenes are not yet published. It is known that trace and ultratrace element impurities significantly affect the electrical, magnetic, optical, and other properties of modem conductors, semiconductors, and superconductors. The chemical identity, concentration level, spatial distribution, and lattice position of each type of impurity are important parameters in the study of these materials. Neutron activation analysis competes favorably with other trace element techniques in terms of sensitivity, accuracy, and precision. Instrumental neutron activation analysis (INAA) is a useful method for the simultaneous determination of about 35-50 major, minor, and trace elements at the parts-per-million,parts-per-billion, and parts-per-trillion levels in a multitude of material^.^-^ As a side line of our investigations on the effect of nuclear radiations on the structural properties of solid fullerenes, elemental impurity levels in C a and C70 samples produced and commercialized by Russian and German companies were determined by INAA. EXPERIMENTAL SECTION
For the INAA measurements, typically 10-50 mg samples were enclosed in polyethylene capsules (for short neutron irradiation periods, maximum 300 s) or in analytical grade SUPRASYLAN type quartz ampules (for long neutron irradiation periods, 1-24 L. Eatviis University. KFKI Atomic Energy Research Institute of the Hungarian Academy of Sciences. (1) Lieber, Ch. M.; Chia-Chun, C. In Preparation of Fullerenes and FullereneBased Materials; Ehrenreich, H., Spaepen, F., Eds.; Solid State Physics Fullerenes; Academic Press: Boston, 1994; p 121. (2) Ehman, W. D.; Robertson, J. D.; Yates, S. W. Anal. Chem. 1990,62,50R70R. (3) Guin, V. P. J. Radioanal. Nucl. Chem. 1 9 9 2 , 160, 9-19. (4) De Bruin, M.J. Radioanal. Nucl. Chem. 1992, 160, 31-40. +
0003-2700/95/0367-1517$9.00/0 0 1995 American Chemical Society
Table 1. Concentrations of the Impurities in Cm (Hoechst) Fullerene Determined by INAA Uslng Short(300 s) and Long-Term Neutron Irradiations and y Spectroscopy
element
Al Ar Au
isotope (half-life) y lines QeV) (2.24 m)
41Ar(1.83 h)
Br
Ig8Au (2.695 d) 139Ba(83.3 m) 82Br(35.4 h)
Ce C1
143Ce(33.7 h) 3C1 (37.21 m)
cu Mg
W u (5.10 m) lZ*I (24.99 m) 42K (12.36 h) 27Mg(9.48 m)
Mn
56Mn(2.582 h)
Mo
99Mo(66.2 h)
Na
9 W c (6.02 h) ' 24Na(14.959 h)
U
239U(2.35 d)
V Zn
52v
Ba
I K
(3.755 m) 69mZn(13.9 h) @Zn (243.8 d)
1778.9 1293.6 411.8 165.9 554.3 619.1 698.4 776.5 827.8 1044.0 1317.5 1474.9 293.3 1642.4 2167.5 1039.8 442.8 1524.7 843.7 1014.0 846.8 1810.7 181.0 739.7 140.5 1368.6 2754.1 106.1 228.2 277.6 1434.0 438.9 1115.5
concn
SD (%)
2.66 pg/g 137.8 p d g 0.18 ng/g 3.1 p d g
6.8
26.Opglg
Y.3
4
8.8 18.6 7.9
134.0 ng/g 10.4 ,ug/g
11.1 7.2
331.0 ng/g 18.2 ng/g 30.7 ng/g 6.96 pg/g
7.5 11.1 16.3 8.2
140.2 ng/g
8.1
681.0 ng/g
8.6
702.4 ng/g 9.6 pg/g
10.5 6.4
13.4 ng/g
10.9
23.9 ng/g 285.0 ng/g 279.4 ng/g
12.5 11.2 8.6
h) to determine element impurities producing short- and longlived isotopes. The samples were neutron irradiated in the recently reconstructed WWR-M type 10 MW nuclear research reactor of the Atomic Energy Research Institute (AERI), Budapest. Short-term neutron irradiations were performed by means of a pneumatic fast rabbit system running between the NAA laboratory and the reactor core. At the irradiation position, a thermal neutron flux of 8.09 x 1013 n-cm-2*s-1and a Qs/Qeflux ratio of 42.6 was available. During the neutron irradiation, the samples were cooled by carbon dioxide gas. Long-term neutron irradiations were carried out in one of the numerous vertical channels of the reactor at a thermal neutron flux of 1.17 x 1014ncm-2.s-1 and at a QJQJ flux ratio of 22.6. Prior to y counting, the neutron-irradiatedquartz ampules were cleaned by chemical etching in 10 vol % hydrofluoric acid solution. Analytical Chemistty, Vol. 67, No. 9,May 7, 7995 1517
Table 2. Concentrations of the Impurities in C70 (Hoechst) Fullerene Determined by INAA Using Short. (300 8 ) and Long-Term Neutron lrradlations and y Spectroscopy
Table 3. Concentrations of the Impurities in Soot Carbon (Hoechst) Determined by INAA Using Short. (300 s) and Long-Term Neutron lrradlations and y Spectroscopy
element
Ar Au Br
isotope (half-life)
41Ar(1.83 h) lg8Au(2.695 d) 82Br (35.4 h)
c1
38C1(37.21 m)
cu Mn
@Cu (5.10 m) 42K (12.36 h) j6Mn (2.582 h)
Na
24Na(14.959 h)
Ti V W
51Ti(5.76 m) j2V (3.755 m) I8?IcT (23.9 h)
Zn
69mZn(13.9 h) 65Zn(243.8 d)
K
y lines (keV)
concn
SD (%)
element
isotope (half-life)
y lines (keV)
concn
SD (%)
1293.6 411.8 554.3 619.1 698.4 776.5 827.8 1044.0 1317.5 1474.9 1642.4 2167.5 1039.8 1524.7 846.8 1810.7 1368.6 2754.1 320.1 1434.0 134.2 479.6 685.7 438.9 1115.5
136.0pg/g 9.1 ng/g 5.85 ,ug/g
9.3 7.3 6.3
AI
28AI (2.24 m) 41Ar (1.83 h) 76As(26.3 h) Ig8Au (2.695d)
1778.9 1293.6 559.1 411.8 165.9 123.7 216.0 373.1 496.3 554.3 619.1 698.4 776.5 827.8 1044.0 1317.5 1474.9 293.3 1642.4 2167.5 320.1 1039.8 121.8 344.3 964.2 629.9 833.9 442.8 1524.7 328.8 487.0 815.8 1596.5 843.7 1014.4 846.8 1810.7 181.0 739.7 140.5 1368.6 2754.1 137.2 155.0 564.0 889.3 1120.5 332.0 320.1 106.1 228.2 277.6 1434.0 134.2 479.6 685.7 438.9 1115.5
39.2 pg/g 65.9 pg/g 203.2 ng/g 3.9 ng/g 24.8 pglg 26.7 pg/g
6.1 9.9 9.5 16.9 11.4 9.5
297.0 ng/g
6.3
616.5 ng/g 25.7 pg/g
11.1 8.5
1.72 ,ug/g 40.2 pglg 160.0 ng/g
12.4 8.3
27.2 ng/g
8.9
52.1 ng/g 18.5,ug/g 289.0 ng/g
18.0 7.3 5.8
65.6 pg/g
8.8
2.18 ,ug/g
5.9
7.4 m / g
9.5
7.9 pglg 10.9 ,ug/g
7.7 6.4
25.6 ng/g 23.5 ng/g 57.0 ng/g 87.4 ng/g
8.5 8.4 7.9 8.0
24.3 pg/g 37.6 ,ug/g 92.8 ng/g
9.6 11.3 10.6
171.5 ng/g 226.0 ng/g
10.1 12.0
27.3 pg/g 24.5 pglg
9.6 8.7
8.2 pg/g 565.0 ng/g 19.4 ng/g 91.5 ng/g 6.9 pg/g
Ar As Au Ba
139Ba(83.3 m) 131Ba (11.5 d)
Br
82Br(35.4 h)
Ce
14Te (33.7 h) 38Cl (37.21 m)
Cr cu
Eu
51Cr(27.71 d) %u (5.10 m) lszEu (12.7 y)
Ga
72Ga(14.1 h)
I K La
42K (12.36 h) I 4 O L a (40.27 h)
Mg
27Mg(9.48 m)
Mn
56Mn(2.582 h)
Mo
99Mo(66.2 h)
Na
9 9 T c(6.02 h) 24Na(14.959 h)
7.5 14.7 11.1 7.6 6.6
960.0 ng/g 19.5 ng/g 152.0 ng/g
10.7 13.0 13.7
227.5 pg/g 214.5 pg/g
4.7 8.7
The y spectroscopy measurements were performed by means of a Canberra spectrometer installed with a HPGe detector (13.6%) and NIM analog electronics as well as with an ACCUSPEC/B type 16K MCA board plugged in to an IBM compatible PC/AT486DX33 machine. The energy resolution of the spectrometer was 1.82 keV for the 6oCo1332.5 keV line. For the evaluation of the y spectra collected in 8K channels, both the Canberra Genie-PC basic spectroscopy system and the HYPERMETS program adapted to an IBM PC/AT machine were used. The quantitative evaluation of the INAA measurements was based on the ko standardization concept6 using gold and zirconium reference comparators cc-irradiated with the samples. Concentrations of the element impurities were computed by the NAACNC program developed in our NAA laboratory (AERI). According to the INAA method outlined above, samples produced by the F & J Co., Moscow, Russia (C, of 99.5% organic punty), and by Hoechst A.G., Frankfurt/M, Germany (C, and C70 of gold grade as well as graphite soot), were analyzed.
c1
Re
Sb sc Sn
Ti U
v W
lz81 (24.99 m)
lS6Re(90.0 h) lSRe (16.8 h) lz2Sb(2.72 d) 46Sc(83.9 d) 125mSn (9.2 m) 5’T1(5.76 m) 239U(2.35 d)
5 2 v (3.755 m) (23.9 h)
RESULTS AND DISCUSSION
The concentrations of element impurities in the investigated (260, C ~ Oand , graphite soot samples of different proveniences are shown in Tables 1-4. The range of concentrations is quite broad, stretching from ultratrace (e.g., Au) to considerably high amounts (e.g., Zn). As is to be expected, most of the element impurities appear in the graphite soot qable 3), some of them (As,Cr, Eu, Ga, La, Re, Sb, Sc, and Sn) being eliminated during the extraction, i.e., puritication, process leading to CSOand C70. It is, however, (5) Phillips, G. W.; Marlow, K. W. Program h”ERMET for automatic analysis of gamma-ray spectra; Naval Research Laboratory (NRL) Memorandum Report No. 3198, 1976. (6) De Carte, F.; Simonits, A; De Wispelaere, A; Elek, A J. Radioanal. Nucl. Chem. 1989, 133, 3-130.
1518 Analytical Chemistry, Vol. 67, No. 9, May 7, 7995
Zn
69mZn(13.9 h) 65Zn(243.8 d)
interesting to note that 10 elements (Ar, Au, Br, C1, Cu, K, Mn, Na, V, and Zn), in the Hoechst A.G. products “survive” all separation steps and remain present in the C ~ and O C ~ final O products. The presence of Ar is easily explainable, since the final separated C, and C ~materials O are stored under an Ar atmosphere according to information received from Hoechst A.G. Seven element impurities (AI,Ba, Ce, I, Mg, Mo, and V), although present in the soot and in the Ca product, do not appear in the C7o samples. On the other hand, Ti and W appear in the
~~
~~
~
~
Table 4. Concentrations of the Impurities in C a (99.5%) Fulierene (Russian Type) Determined by INAA Using Short(300s) and Long-Term Neutron Irradiations and y Spectroscopy
element
As
Al Ar Au Br
isotope (half-life) 11omAg
657.8 763.9 884.7 937.5 1384.3 1778.9 1293.6 411.8 554.3 619.1 698.4 776.5 827.8 1044.0 1317.5 1474.9 1642.4 2167.5 1173.2 1332.5 320.1 1039.8 1099.2
341.6 ng/g
28Al (2.24 m) 41Ar(1.83 h) Ig8Au(2.695 d) 82Br(35.4 h)
38C1(37.21 m)
co
'%o (5.272 y)
Cr
51Cr(27.71 d) %Cu (5.10 m) 59Fe(44.63 d)
Fe
concn
(249.76 d)
C1
cu
y lines (kev)
7.7 pg/g 10.8 pg/g 2.9 ng/g 1.8 pg/g
SD (%)
element
isotope (half-Tie)
8.9
8.8 9.2 9.7 8.4
660.0 p d g
6.4
77.9 ng/g
7.9
3.78 pg/g 1.7 pg/g 19.4 pg/g
6.4 7.6 12.6
I
lZ8I (24.99 m)
In
l1'jmIn(54.2)
K
42K(12.36 h)
La
140La (40.27 h)
Mn
=Mn (2.582 h)
Na
24Na(14.959 h)
Sb Ta
Ti
lnSb (2.72 d) 18Ta (114.43 d) 51'17(5.76 m) l87W (23.9 h)
Zn
&Zn (13.9 h) &Zn (243.8 d)
w
1000
y
concn
SD (%)
1.9 pg/g 43.9 ng/g
6.5 6.6
8.8 pg/g 18.2 ng/g
10.5 16.4
435.0 ng/g
6.8
10.8 pg/g
6.4
28.4 ng/g 14.9 ng/g 37.6 pg/g 106.0 ng/g
11.69 15.7 11.3 12.0
240.6 pg/g 248.0 pg/g
6.0 5.2
lines (kev) 1291.6 442.8 4 16.9 818.7 1097.2 1524.7 328.8 487.0 815.8 1596.5 846.8 1810.7 1368.6 2754.1 564.0 1121.3 320.1 134.2 479.6 685.7 438.9 1115.5
. . .
...
... ........ . . . . ....... ..... . .
. . . . . . . . . .. ... . . .
. .
.
.
....
100
10
1 0.1 0.01
AI Ar Ba Br CI Cu I K MgMnNa Ti V Zn ELEMENT IMPURITIES
Figure 1. Distribution of the main element impurities in various types of fullerene and soot samples. H, Hoechst A.G. gold grade; R, F & J, Moscow, Russia.
soot and in the c70 product but do not appear in the Cm samples. In this latter case, it is supposed that in the c70 samples these elements become introduced during the chromatographic separation process. As can be seen in Tables 1-4 and Figure 1,the graphite soot, the Cm, and the C70 samples of different proveniences contain nonnegligible amounts of trace element impurities. It seems that the presence of these elements at dopant concentrations has not yet been taken seriously into account in studies of physical and physicochemical properties of fullerenes, although it is well known that minute amounts of such impurities might substantially affect those properties. The influence of trace organic impurities on physical properties of the fullerenes has, however, been taken into
account in recent investigations? The exact impurily levels in graphite soot, Cm,and c70 samples will probably vary according to the producer company's technology and €?omproduct batch to batch. The present paper simply demonstrates the potential presence of such impurities in fullerenes and fullerene precursors. The trace element impurities, as shown in Tables 1-4, have probably entered into Cm and C70 materials during the different phases of their production processes. Thus, in cases when structural, electrical, and/or magnetic properties of the fullerenes ~~
(7)McGhie, A R; Fischer, J. E.; Heiney, P. A; Stephens, P. W.; Cappellettri, R L; Neumann. D. A; Mueller, W. H.; Mohn, H. phys. Rev. B 1994.49. 12614-12618.
Analytical Chemistry, Vol. 67, No. 9, May I , 1995
1519
are studied, it is highly recommended to take special care of using purified raw materials (graphite), reagents (solvents), equipment (reactor), sorbents, and vessels which do not contribute to the contamination of the fullerenes end-products. This, as an altemative, seems to be simpler than the postpuriiication of the already prepared Cm and C70. With the data in Tables 1-4 at hand and on the basis of the above-mentioned facts, one could, of course, speculate on the location of the detected element impurities in or on the materials investigated. It is our presumption that some of them are present in the Cw and even C ~materials O in endohedral positions. A more detailed analysis of this presumption is under way in our laboratory.
1520 Analytical Chemistry, Vol. 67,No. 9,May 7, 1995
ACKNOWLEDGMENT
Hoechst A.G., Frankfurt/M, Germany, is kindly acknowledged for the donation of the samples of graphite soot and gold grade Csoand C70. This research has been supported by the Hungarian National Research Fund ( O W ) by Grant T 7642.
Received for review October 11, 1994. Accepted February 3, 1995.@ AC941004N
@
Abstract published in Advance ACS Abstracts, March 15, 1995.
