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Effect of Microstructure of Graphite on the Nonreductive Thermal Ion Emission in Thermal Ionization Mass Spectrometry H. Z. Wei,†,‡ S. Y. Jiang,*,‡ and Y. K. Xiao‡ State Key Laboratory for Mineral Deposits Research, Department of Earth Sciences, Nanjing UniVersity, Nanjing 210093, P.R. China, and Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining 810003, P.R. China ReceiVed: October 7, 2009; ReVised Manuscript ReceiVed: December 27, 2009
The emission behavior of polyatomic ions in the ionization source of thermal ionization mass spectrometry (TIMS) was investigated. The results suggest that the presence of a graphite promoter plays a key role for the formation and stable emission of polyatomic ions, such as M2X+, M2BO2+, Cs2NO2+, and Cs2CNO+. Our data further implied that the intensity of M2X+ and M2BO2+ increases and the emission temperature decreases with increasing cationic and anionic radius. During the boron isotopic measurement using the Cs2BO2+-graphite-PTIMS method, the isobaric interference ion Cs2CNO+ cannot be transformed from nitrate or organic compounds containing an amide group but can be induced by the existence of trace amounts of boron because of its special electron-deficiency property (B3+). Characterization on the planar crystalline structure of various graphite samples with SEM, TEM, and Raman spectroscopy confirmed the relationship of the emission capacity of polyatomic ions and the crystal microstructure of graphite and provides direct evidence that graphite with a perfect parallel and equidistant layer orientation shows a beneficial effect on the emission of polyatomic ions in TIMS. The mechanism study on the formation of polyatomic ions opens the possibility to establish high precision methods for isotopic composition analysis of more nonmetal elements with the TIMS technique. 1. Introduction The well-known allotropes of carbon, including graphite, diamond, and fullerenes, exist in a variety of materials with differing properties. The most common are based on the graphite structure, consisting of ideally infinite sheets of graphene stacked parallel. Three types of stacking are known to exist in graphite: the ABAB, ABCABC, and AAAA types,1,2 where the most common form of graphite with a two-dimensional grahene sheet has the ABAB stacking and the carbon atoms in graphite are all sp2 hybridized with an intraplanar C-C bond length of 1.42 Å and interplanar space of 3.354 Å (i.e., D002). As shown in Scheme 1, the atomically ordered hexagonal plane containing the a axis is commonly known as the “basal plane”, while the irregular surface parallel to the c axis is known as “edge plane”. The edge and basal planes differ greatly in chemical and physical properties, and the dimensions of the crystallites are denoted by parameters of La, the in-plane crystallite size, and Lc, perpendicular to the grapheme planes (Scheme 1). In particular, the layers in graphite are roughly parallel and equidistant but are not completely oriented, and there are some irregularities and defects within the graphene layers. The difference in the microstructure patterns of turbostratic graphite and ideal graphite are shown in Scheme 2. Carbon materials with large specific surface area (sometimes called active, amorphous, or microcrystalline carbon) are assuming increasing importance in various fields, such as in the control of pollution, in purifying and controlling the general chemical environment, and in certain biomedical applications.4 * Author to whom correspondence should be addressed. Phone: +86 (25) 83596832. Fax: +86 (25) 83592393. E-mail:
[email protected]. † Nanjing University. ‡ Qinghai Institute of Salt Lakes, Chinese Academy of Sciences.
SCHEME 1: Schematic Representation Stacked Graphene Sheets Including the Atomically Ordered Hexagonal Plane, i.e., Basal Plane, and the Irregular Surface Parallel to the c Axis, i.e., Edge Plane
All applications of carbon involve adsorption and catalysis, physical-chemical processes which may depend upon the crystalline structure, microscopic physical structure, electronic properties, surface chemistry, and the presence of impurities within the carbon. Graphite has been extensively used in thermal ionization mass spectroscopy (TIMS) as a reducing agent for isotopic measurement of uranium,5,6 vanadium, rare earth elements, and cerium7,8 at high temperature. It is interesting to note that graphite is being applied on isotopic measurement of boron,9 chlorine,10 and nitrogen and oxygen11 with its peculiar nonreductive characteristics, which first was introduced by Xiao et al. who found the intensity of Cs2BO2+ emitted from Cs2B4O7 can be increased to 2-orders of magnitude when loading graphite on the filament in TIMS at low temperature ( no. 1 > no. 3 > no. 2 > no. 6. The numerical analysis on X-ray diffraction data revealed a relationship of the special characteristics of graphite with the maximal distortions of the crystal lattice. The emission of polyatomic ions with the promoter of graphite mainly depends on the crystal microstructure of graphite, and samples with a perfect planar crystalline structure has a superior feature.30 To further investigate the fine crystal structure of graphite, characterization of the planar crystalline structure of various graphite samples is performed by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) as well as Raman spectroscopy. 3.2.1. Morphology Characterization of Various Graphite and Carbon Samples with SEM. Magnified SEM images of various graphite and carbon samples are shown in Figure 8. Clearly, all graphite samples including the carbon sample have a two-dimensional characteristic (i.e., planar surface), but the planar layers in all samples are in curls to a certain extent except for graphite no. 5, which means for an artificial graphite it is very difficult to get an absolute planar crystalline structure as a perfect natural graphite crystal. Relatively, the planar crystalline structure in sample no. 5, followed by graphite no. 1, is more perfect than others, and obvious cylindraceous shape is present in samples no. 3 and no. 4. For sample no. 2, it is very hard to capture any single plane structure because of very dense overlap of the curled layers as in the carbon sample (no. 6). The results from SEM images of all selected graphite samples match the data of the maximal distortion of crystal lattice obtained from XRD characteristics very well, i.e., the graphite with the lowest value in the maximal distortion shows the better planar crystalline structure. Analytical comparison between the
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Figure 8. SEM images of various graphite and carbon samples. Samples no. 1 to 5 are graphite powder from different origins, and no. 6 is the carbon sample.
microstructure images further confirmed the microstructureproperty relationship among various graphite samples. 3.2.2. Crystallite Characterization of Various Graphite and Carbon Samples with Raman Spectroscopy. Raman spectroscopy is sensitive to slight changes in graphite microstructure; therefore, it is employed for the characterization of crystalline, nanocrystalline, and amorphous carbon phases. All carbon materials show common features in Raman spectra in the 800-2000 cm-1 region, the so-called G and D peaks, which lie at around 1560 and 1360 cm-1, respectively, for visible excitation while both bands are assigned to sp2 orbital hybridization in π-conjugated ring structures. The G peak is due to the bond stretching of all pairs of sp2 atoms in both ring and chains;60 the D band is symmetry forbidden in graphite, and its presence is associated with structure disorder.61 As shown in Figure 9, two characteristic bands are observed in the Raman spectrum of graphite and carbon samples: G-band (∼1579 cm-1) and D-band (∼1364 cm-1). It is evident that the Raman spectrum of graphite sample no. 5 only shows a strong G-band, which is a typical Raman spectrum of single crystal of graphite and consistent with completely ordered (crystalline) structure of graphite as characterized with SEM (Figure 8). In contrast, there is a strong D-band and G-band that appear for
the carbon sample. The sample no. 4 has a very weak D-band and a stronger G-band, indicating that it has fine crystalline structure while partial disordered structure exists in the sample. A relatively strong D-band exists in samples no. 1, 2, and 3. The appearance of a D-band and the increase in peak intensity obeyed the TK rules, i.e., I (D)/I(G) ∝ 1/La. Accordingly, the parameters of crystallite size of all samples were estimated from the calibration of Raman intensities versus data of La.62 As listed in Table 2, the in-plane crystallite sizes of La of all graphite samples are higher than 250 Å, and four to ten times larger than that of carbon sample, which is in agreement with the results that the parameter of La for randomly oriented graphite is ∼300 Å and that for carbon black is ∼20 Å as reported by Yoo et al.63 In addition, both the Raman G-position and the D to G intensity ratio I(D)/I(G) of all graphite samples is smaller than that of the carbon sample, indicating the main trend of disordering from 0% sp3 (i.e., graphite) to 85% sp3 (i.e., amorphic carbon) according to the three-stage model of increasing disorder proposed by Ferrari et al.60,64,65 Considering that the full width at half-maximum of the G peak (FWHM G) is also a critical index for the extent of disordering of carbon materials, i.e., it increases continuously as the disorder increases, a plot of FWHM G versus La of all
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Figure 9. Raman spectra of graphite samples obtained with 488 nm laser light at range of 100 to 2000 cm-1.
TABLE 2: Properties of Various Graphite Samples and Carbon Obtained from Raman Spectroscopy sample no. 1 2 3 4 5 6
D002 (Å)30 3.364 3.355 3.368 3.358 N.D. 3.428
Raman G position (cm-1)
I(D)/I(G)
La (Å)
FWHM G (cm-1)
1575 1577 1576 1576 1579 1586
0.163 0.139 0.222 0.088 0.086 0.938
325 270 308 599 613 56
16.7 23.0 22.2 13.3 12.2 55.6
graphite and carbon samples shows a linear relationship (R2 ) 0.633), as shown in Figure 10, which shows a similar trend of FWHM G-La curves as reported by Ferrari et al.64 Therefore, the ordering sequence of the selected graphite and carbon samples is the following: no. 5 > no. 4 > no. 1 > no. 3, no. 2 > no. 6 (carbon). The conclusion from Raman analysis is very consistent with the results of SEM observation and the capacity sequence of graphite from the isotopic determination and points to the fact that the mutually oriented parallel layers in the microstructure of graphite are essential for the formation and stable emission of polyatomic ions in the ionization source of PTIMS.
3.2.3. Characterization of Parallel Plane Orientation of Various Graphite and Carbon Samples with TEM. To complement the morphological result of graphite and carbon samples with SEM, the parallel plane orientation was further characterized by employing of TEM technique, with which either the basal plane or the edge plane of each sample was observed (Figure 10). It is interesting to note that sample No 5 shows a perfect parallel and equidistant layer orientation, indicating an ideal single crystal graphite without any irregularities within the graphene layers, which is exactly the same result from that from the Raman spectra. An obvious turbostratic orientation in the graphene layers was present in the carbon sample (no. 6)
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J. Phys. Chem. A, Vol. 114, No. 7, 2010 2435 plane gives the same information as that from the Raman spectrum. The distortion in graphene layers destroys the ordered orientation in graphite sample no. 2, and it further affects the capacity of the emission of thermal polyatomic ions of graphite in TIMS. The difference in the structural pattern among the selected graphite and carbon samples is evident from the TEM characterization. It is clear that the turbostratic stacked sheets mainly exist in carbon sample no. 6 while the parallel oriented sheets predominate in graphite sample no. 5. The distortion of graphene layers exists in all graphite samples more or less, and the capacity of graphite for the stable emission of polyatomic ions is dramaticlly affacted by the distortion extent in the structural pattern. 4. Conclusions
Figure 10. Variation of the full width at half-maximum of the G peak (fwhm G) with decreasing in-plane crystallite sizes of La. Dashed lines represent the ordering trajectory of the carbon materials.
that is not characterized by well-defined separation of the planes from SEM. The view on the edge planes of graphite samples no. 3 and no. 4 shows that the graphene layers are mutually oriented and some marked irregularities exist within the interlayers. It is very difficult to observe any edge plane in graphite samples no. 1 and no. 2, but the view on the basal
The investigation of emission behavior of polyatomic ions in the ionization source of TIMS indicated that the presence of a graphite promoter plays a key role for the formation and the stable emission of polyatomic ions, such as M2X+, M2BO2+, Cs2NO2+, and Cs2CNO+. It not only induces the formation of the M2X+ series and Cs2NO2+ and Cs2CNO+ ions at a lower ionization temperature but also enhances the stable emission of Cs2BO2+ for a long time. Even though the mechanism for the formation of such polyatomic ions under ionization conditions is unknown, a similar emission trend was observed in
Figure 11. TEM graphs of various graphite and carbon samples. Samples no. 1 to 5 are graphite powder from different origin, and no. 6 is the carbon sample.
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which the intensity of M2X+ and M2BO2+ increases with increasing cationic and anionic radius. A study on the isobaric interference ion Cs2CNO+ (m/e ) 309 and 308) during the determination of boron isotopic composition with Cs2BO2+-PTIMS proposed that an organic compound possessing an amide group would be one possible source. Both nitrate and organic matter cannot be transformed to the isobaric interference ion Cs2CNO+ directly in the presence of graphite, and the existence of trace amounts of boron induced the reaction because its special electron-deficiency property (B3+). On the basis of a comparable evaluation carried out by selecting various graphite and carbon samples from different sources, we have concluded that, while the addition of graphite promotor is a determining factor in the formation of polyatomic thermal ions, obvious discrepancy in the determination of isotopic composition has been observed by using Cs2Cl+-graphite-PTIMS and Cs2BO2+-graphite-PTIMS methods. After understanding the dependence of the capacity of graphite on the crystal microstructure from previous investigations, further characterization on the planar crystalline structure of various graphite samples with SEM, TEM, and Raman spectroscopy confirmed the relationship of the emission capacity of polyatomic ions and the crystal microstructure of graphite. Results from SEM on the morphology characterization and TEM on the parallel plane orientation characterization are mutually complementary and provide direct evidence that the graphite sample that has a beneficial effect on the emission of polyatomic ions in TIMS has a perfect parallel and equidistant layer orientation while the carbon sample that cannot ensure a stable emission for polyatomic ions shows an obvious turbostratic orientation in the graphene layers. The crystallite characterization with Raman spectroscopy gives the ordering sequence in fine crystal structure of all selected graphite and carbon samples, i.e., no. 5 > no. 4 > no. 1 > no. 3, no. 2 > no. 6 (carbon), which is exactly the same as the ordering of the emission capacity of polyatomic ions of graphite in TIMS. The in-plane crystallite size La combined with the full width at half-maximum of the G peak in Raman spectra is a direct index for the microstructure of carbon materials. In future work, it will be of great interest to develop new methods for the high precision isotopic composition determination of light mass elements with polyatomic ions using the TIMS technique, such as the determination of lithium isotopes with the polyatomic ion CsLiI+, and the coinstantaneous determination of nitrogen and oxygen isotopes in geological samples with Cs2NO2+ ion. If the mechanism of the formation of polyatomic ions in the ionzation source is understood, it will allow the development of high precision methods for isotopic composition analysis of other elements with the TIMS technique. Acknowledgment. The authors are grateful to the National Natural Science Foundation of China (no.40776071 and no. 40973002) for support of this research. References and Notes (1) Yoshizawa, K.; Yumura, T.; Yamabe, T.; Bandow, S. J. Am. Chem. Soc. 2000, 122, 11871. (2) Ruuska, H.; Pakkanen, T. A. J. Phys. Chem. B 2001, 105, 9541. (3) Muradov, N.; Smith, F.; Bokerman, G. J. Phys. Chem. C 2009, 113, 9737. (4) Radovic, L. R. Chemistry and Physics of Carbon; Dekker: New York, 1994; Vol 24. (5) Studier, M. H.; Sloth, E. N.; Moore, L. P. J. Phys. Chem. 1962, 66, 133. (6) Fassett, J. D.; Kingston, H. M. Anal. Chem. 1985, 57, 2474.
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