cm-1 is an unsettled issue. In practice, because a perfect single graphite crystal is hardly ever obtained with high temperature, all graphite materials have defects of different types. Heat treatment of graphite materials can reduce the intensity of the D band at 1350 cm-1, however, the D band is still present even at 3000 °C.35,36 Therefore, a perfect graphite crystal did not show D, D′, or D + G bands because they are defect-induced Raman features.35 In disordered, microcrystalline, and nanocrystalline graphite, the D′ band at 1620 cm-1merged with the G band, and the intensity of the merged bands increased with edge carbon atoms or as the relative content of disordered carbon increased. Also, the D′′ band was inaccurately assigned as a defect-induced feature (artifact).37 Finally, disorder in the graphite structure modiﬁed the C-C bond vibrations, and therefore broadened (increased fwfm) the D and G bands.36-38 The crystal size in graphitic materials was inversely related to the fwhm of D and G bands because the fwhm of D and G bands included defects in graphitic materials. The disordered graphitic materials showed a broader D band (>150 cm-1) than ordered graphitic materials (∼60 cm-1). In addition, the G′ band corresponded to overtone of the D band, and the ratio of ID/IG (not ID′/IG) was inversely related to La. The La can be determined to be ∼2 nm using the TK formula, and it underestimated La due to the domain effect of small crystal compared to XRD measurement.34-41 As a result, ICM was mostly iron carbide (Fe3C, cementite) with only traces of nanocrystalline graphite or disordered graphite because iron carbide-like Fe3C formation precedes graphite structure formation.42 The D, G, D′, and D + G bands of GCI and GCIP were consistent with prior reports.30,35,36 Therefore, the GCI and GCIP were less-ordered graphitic materials rather than single graphite crystal mixed with Fe3C crystal, nanocrystalline graphite, or sp3 carbon structure (D band) that were placed mostly in the edge of GCI and GCIP. Although the formation of graphite and its crystal size increased at higher CO2 reduction temperatures, the difference in La between GCI and ICM formed at 559 ± 7 °C for 6 h compared to GCIP formed at 500 ± 0.3 °C for 3 h was not signiﬁcant statistically. However, the higher CO2 reduction temperature (>585 °C) can produce more ordered graphite crystal without disordered structure. Because our FT-IR and Raman results suggested that ICM, GCI, and GCIP consisted of a mix of nanocrystalline graphite (or g-C), and Fe3C crystal, we further examined the AMS targets using XRD to distinguish between crystal and amorphous structures. XRD. Graphite crystal is found in two forms: hexagonal graphite that has the stacking sequence ABAB and rhomboheral (34) Ferrari, A. C.; Robertson, J. Physc. Rev. B 2000, 61, 14095–14107. (35) Pimenta, M. A.; Dresselhaus, G.; Dresselhaus, M. S.; Canc¸ado, L. G.; Jorio, A.; Saito, R. Phys. Chem. Chem. Phys. 2007, 9, 1276–1291. (36) Hirlimann, C.; Jouanne, M.; Forrie`res, C. J. Raman Spectrosc. 1992, 23, 315–317. (37) Reigh, S.; Thomsen, C. Phil. Trans. R. Soc., London A 2004, 362, 2271– 2288. (38) Escribano, R.; Sloan, J. J.; Siddique, N.; Sze, N.; Dudev, T. Vib. Spectrosc. 2001, 26, 179–186. (39) Knight, D. S.; White, W. B. J. Mater. Sci. 1989, 4, 385–393. (40) Nakamura, K.; Fujitsuka, M.; Kitajima, M. Phys. Rev. B 1990, 41, 12261– 12263. (41) Nikiel, L.; Jagodzinski, P. W. Carbon 1993, 31, 1313–1317. (42) Santos, G. M.; Southon, J. R.; Grifﬁn, S.; Beaupre, S. R.; Druffel, E. R. M. Nucl. Instrum. Methods Phys. Res., Sect. B 2007, 259, 293–302. 7668 Analytical Chemistry, Vol. 80, No. 20, October 15, 2008 graphite that has the layer sequence ABCABC (Figure 1). The natural abundance of graphite type is 70% hexagonal and 30% rhombohedral.2 The unit cell of graphite has dimensions a, b, and c along x, y, and z axes and also an interlayer distance, d. For graphite, a ) b ∼0.246 nm, c ∼0.670, and d ∼0.334 nm.17 The degree of graphite crystal is diminished as the d values increase.12 The graphite crystal is generally formed at temperatures g2000 °C, whereas at 400-1000 °C, the carbon formed a disordered (or less-ordered) structure-like i-C or g-C.1,19,43 Most graphitization methods for AMS targets reduced CO2 to “graphite”,3-5 a fullerene “graphite”,6 or “solid fullerene”11 and sometimes to a MeC, such as cobalt or iron carbide.5,12 Both H2 and Zn (dust) are each commonly used as a reductant. The Zn reduction method produced graphite crystal with a consistent line broadening (0.55°-0.65°) for the G-002 reﬂection peak with some less crystallized material.12 In the beginning, we expected that GCI and GCIP to be graphite crystal (or t-C) and ICM to be mostly Fe3C crystal. However, we did not ﬁnd pure graphite crystal in either GCI or GCIP. If GCI and GCIP contained pure graphite crystal, it would show an intense and sharp G-002 reﬂection peak at ∼26.5°. Furthermore, if GCI and GCIP were t-C, they should also show a little broad G-002 reﬂection peak at ∼26.5° depending on the degree of the defect of graphite crystal. However, the GCI and GCIP may be nanocrystalline graphite that lacked a stacking arrangement, because detecting graphite crystal <20 nm in size using XRD is not practical.35 Based on the absence of CdC stretching in FT-IR spectra, weak D and G bands in Raman spectra, and a clear presence of Fe3C crystal in XRD spectra, we concluded that the ICM was mostly Fe3C crystal with a minimal amount of sp2 (g-C including nanocrystalline graphite) and sp3 (edge-carbon in graphite). The GCI and GCIP were also a mix of nanocrystalline graphite (or g-C) and Fe3C crystal based on FT- IR (CdC), Raman spectra (D, G, G′, and D + G bands), and XRD (Fe3C crystal). The g-C and Fe3C crystal was were preferentially formed over single graphite crystal by our previous7 and our optimized13 methods, formation of perfect graphite crystal might be possible with a combination of higher CO2 reduction temper- ature and smaller mass ratio of C/Fe. Longer reduction time combined with higher CO2 reduction temperature and smaller mass ratio of C/Fe may be useful to also minimize defects of in the graphite crystal. As soon as AMS targets were produced, an overnight cooldown to room temperature (similar to annealing process) did not produce well-ordered larger graphite crystal compared to a 3-min cooldown. The graphitization yield and isotopic fragmentation were not inﬂuenced by the duration of a slow cooldown. Ion currents were affected more by the amount of graphite rather than its crystal size. Consequently a 3-min cooldown facilitated a high throughput for biological/biomedical applications of AMS. Previ- ously a skilled analyst could prepare 60-150 AMS targets per day,5-7 while our optimized method produced as many as 270 AMS targets per d/skilled analyst. CONCLUSIONS AMS targets of GCI and GCIP each produced a mix of nanocrystalline graphite (or g-C), and Fe3C crystal, intense 12C- (43) Onodera, A.; Higashi, K.; Irie, Y. J. Mater. Sci. 1998, 23, 422–428.