Figure 3. FT-IR transmission spectra of GST, ICM, GCI, and GCIP. All spectra showed a water band at ∼ 1630 cm-1. All spectra, except that of ICM, showed a band at ∼1580 cm-1. This band corresponded to the sp2 bond (CdC bond stretching) in graphite, and it demonstrated that GCI and GCIP each consisted of a carbon hexagonal structure. -400MSIP, often called R-Fe or body-centered cubic lattice iron (bcc-Fe), had the R-Fe (110) reﬂection peak at ∼45° and the R-Fe (200) reﬂection peak at ∼65°. Forty milligrams of R-Fe had a Lc of 29.11 nm and an interlayer distance of 0.202 nm using R-Fe- 110 as reference. Figure 5, panel B summarized XRD spectra of the AMS targets ICM, GCI, and GCIP. Consistent with prior studies,19-21 there was no evidence of a G-002 peak at ∼26.5° in ICM, GCI, or GCIP, detecting only Fe3C crystal with possible traces of FexCy and R-Fe crystals within the range of 30°-70° (2Θ). Even if GCI spectrum lacked a G-002 peak, it still suggested the presence of g-C because FT-IR and Raman spectra of GCI showed a mix with some sp2 and sp3 carbon bond structures. As already seen from Figure 1, the i-C and g-C have an “amorphous (disordered)” and an “less ordered” carbon structure, respectively. The XRD spectrum of GCIP was very similar to those of ICM and GCI, and when the FT-IR and Raman spectra were considered together, the GCIP consisted of g-C and Fe3C crystal. The G-101 refection peak overlapped that of Fe3C and of R-Fe (110), so it was difﬁcult to individually differentiate graphite from R-Fe and Fe3C at ∼45°. Figure 5, panel C showed relative sensitivity toward 1:1, 1:10, and 10:1 ratios of GST/GCIP (including Fe3C and R-Fe crystals) to Cu KR, the X-ray source. A good quality XRD spectrum of graphite crystal (especially, G-002) required an AMS target that contained e2 mg of graphite crystal. In all ratios of GST/GCIP, the G-002 reﬂection peak was much more sensitive than the reﬂection peaks of Fe3C or R-Fe crystals. Finally, the graphite crystal mass of ICM, GCI, and GCIP was e2 mg. The absence of a G-002 reﬂection peak indicated that ICM, GCI, and GCIP each consisted of a mix of Fe3C crystal and g-C rather than of a graphite (19) Jaer, C.; Mutschke, H.; Huisken, F.; Alexandrescu, R.; Morjan, I.; Dumi- trache, F.; Barjega, R.; Soare, I.; David, B.; Schneeweiss, O. Appl. Phys., A: Mater. Sci. Process. 2006, 85, 53–62. (20) Nikitenko, S. I.; Koltypin, Y.; Palchik, O.; Felner, I.; Xu, X. N.; Gedanken, A. Angew. Chem., Int. Ed. 2001, 40, 4447–4449. (21) Chung, U. C.; Kim, Y. H.; Lee, D. B.; Jeong, Y. U.; Chung, W. S.; Cho, Y. R.; Park, I. M. Bull. Korean Chem. Soc. 2005, 26, 103–106. crystal. The ICM had more iron carbide (Fe3C) crystal than g-C, and ICM produced low and variable ion current whereas the opposite was true for GCI and especially so for GCIP. DISCUSSION A variety of graphite-like substances are formed during the reduction of CO2 to AMS targets depending on the conditions used. The types of graphite-like substances include ﬁlamentous,4 ﬂuffy, and ﬁrm or solid graphite;22 these substances differ from one another in a variety of traits such as color, hardness, texture, SEM, FT-IR, Raman, and XRD spectra. To the extent that the traits affect the magnitude of the ion current produced, they are important so, we determined these traits in AMS targets produced using our previous7 and optimized methods.13 Our goal was to identify those traits in AMS targets that produced intense and reliable io1n currents for accurate, precise, reliable, and high- throughput measurements of 14C using AMS. Our previous method formed GCI (black, soft pellet, less uniform, fuzz) 80% of the time and ICM (gray, hard pellet, featureless carbon deposition) 20% of the time.7 On the other hand our optimized CO2 reduction method formed only GCIP (black, soft powder, uniform, fuzz) 100% of the time.13 In general, the presence of a nongraphitic carbon or Fe3C crystal resulted in the formation of a harder carbon structure than a compact graphitized carbon.23,24 Graphite produced ion currents of 13.4 µ A of 12C4+/mg of C while a-C produced ion currents that ranged from 5.4 to 7.4 µ A of 12C4+/mg of C.3 Furthermore, the presence of cobalt carbide in AMS targets produced still lower ion currents whose associated error in the 14C/13C ratio was as high as 10%.5 Because the ion current produced by ICM was 13% lower than that produced by (22) Santos, G. M.; Mazon, M.; Southon, J. R.; Rifai, S.; Moore, R. Nucl. Instrum. Methods Phys. Res., Sect. B 2007, 259, 308–315. (23) Franklin, R. E. Proc. R. Soc. London., Ser. A: Math. Phys. Sci. 1951, 209, 196–218. (24) Wagner, D. B. Iron and Steel in Ancient China; E.J. Brill: London, 1993; pp 335-344.. Analytical Chemistry, Vol. 80, No. 20, October 15, 2008 7665