was to identify the traits of AMS targets that produced intense and reliable ion currents for accurate, precise, reliable, and high throughput for biological/biomedical applications of AMS. EXPERIMENTAL SECTION Reagents. The reagents were the same as previously described7,13 except for the following: Potassium bromide (KBr, CAS No. 7758-02-3, g99%, FT-IR grade) and graphite standard (GST, CAS No. 7782-42-5, <20 µ m, synthetic powder) were from Sigma-Aldrich (St. Louis, MO). AMS Target Preparation. AMS targets were prepared by our previous method7 as well as by our optimized method13 for which we built a new heating block with 7 rows, 14 holes apiece, to perform at 525 °C ± 0.3 (hole to hole).13 AMS Target Characterization. AMS target color values were measured using a Minolta Color meter (Minolta, Ramsey, NJ). The instrument was calibrated against white reﬂector plate (reference calibrator). The color values represented as L*, a*, and b* color values. The L* deﬁnes the lightness, and a* and b* deﬁne red-greenness and blue-yellowness, respectively.14 Only the lightness values were relevant to the present application. The hardness of the AMS targets was examined by the TA- XT plus Texture Analyzer (Texture Technologies Corp., Menlo Park, CA). The hardness was expressed as a maximum force (lb) to crush the carbon materials.15 The morphological investigation by SEM was the same as previously described.13 The FT-IR transmission measurements were performed on the AMS targets using a Nicolet 6700 FT-IR spectrometer (Thermo Scientiﬁc, Inc., Waltham, MA) at 23 °C over the range of 4000-400 cm-1, spectral conditions were 4-cm-1 resolution and 64 scans. Before FT-IR analyses, ∼3-mg aliquots of AMS targets were ground for ∼2 min with 150 mg of KBr (that had been stored in a desiccator). The KBr and -400MSIP were precalibrated before each measurement. Raman spectra were performed using a Renishaw microspec- trometer (Renishaw Inc. Hoffman Estates, IL) in the continuous scanning mode in the range of 40-4000 cm-1. The 514.5-nm excitation of an Ar ion laser with 5-mW power was introduced with an objective lens (×50 magniﬁcation). The 514.5-nm excitation of the Ar ion laser was more sensitive to ﬁrst- and second-Raman spectra of various graphitic materials than the 632.8 nm of a He-Ne laser or the 780 nm of a NIR diode laser.16 The laser was focused to a 15-25-µ m diameter at the AMS target surface. Peak positions and integral intensities of Raman bands were determined using the SpectraMax microplate spectrophotometer, MaxLine application Note 32 (Molecular Devices Sunnyvale, CA). The XRD analyses were conducted using the Scintag XDS 2000 (Scintag Inc., Cupertino, CA). The X-ray radiation was from Cu KR (λ ) 0.15418 nm) with an accelerating voltage of 45 kV and a current of 40 mA. A computer-controlled diffractometer collected crystal data in the range (2Θ) of 5°-70° at a speed of 5°/min. The interplane distance d was determined using the Wulff-Bragg (14) Gil, M. I.; Holcroft, D. M.; Kader, A. A. J. Agric. Food Chem. 1997, 45, 1662–1667. (15) Mitcham, B.; Cantwell, M.; Kader, A. A. Perishables Handling Newsl. 1996, 85, 1–5. (16) Sadezky, A.; Muckenhuber, H.; Grothe, H.; Niessner, R.; Po¨schl, U. Carbon 2005, 43, 1731–1742. 7662 Analytical Chemistry, Vol. 80, No. 20, October 15, 2008 formula,17 λ ) 2d sinΘ where λ is the wavelength of the X-ray characteristic radiation, d is the basal plane interlayer spacing, and Θ is the diffraction angle. The average crystal size was calculated using the Scherrer equation,18 t ) Kλ/(B cosΘ), where t is the averaged dimension of crystallites (or Lc, stacking height of crystallite to c-direction), K is the Scherrer constant whose value is ∼0.9 (a somewhat arbitrary value in the range 0.87-1.0), λ is the wavelength of X-ray radiation, and B is the half-height width of the diffraction intensity distribution (measured in radians). RESULTS Figure 1 is a carbon classiﬁcation by which AMS targets could be characterized in a relative way. The classiﬁcation enabled us to rank the degree of crystal (high to low) as graphite crystal > t-C > g-C > i-C, the i-C is highly disordered and approximates amorphous carbon (a-C). The isotropic carbon (i-C), graphitizable semiorder carbon (g-C), turbostratic carbon (t-C), and graphite panels were from a previous publication,1 with permission. Figure 2 summarized an overview of the second step of AMS target preparation using our previous7 and optimized13 methods. The left panel shows the setup and appearance of the -400MSIP and Zn dust before the reduction. The center and right panels show what happened to the -400MSIP and Zn dust during the reduction using our previous7 and optimized13 methods, respec- tively. Prior to the CO2 reduction step, the Zn dust and the -400MSIP consisted of loose spherical particles (left panel). After reduction by our previous method7 (center panel), the Zn dust appeared as a ﬁbrous (rubbery) Zn band-1 at 260 °C, a plasticized or plastic-like Zn band-2 at 380 °C, and two Zn mirrors (mirror 1 at 480 °C and mirror 2 at 525 °C), which appeared to be metallic Zn on the inner wall of the septa-sealed vial. Furthermore, the remaining Zn dust appeared as a Zn cake stuck to the bottom of the septa-sealed vial. Finally, the AMS targets appeared at 540 °C as a graphite-coated iron (GCI) fuzz or a featureless ICM on the -400MSIP at the bottom of the borosilicate inner vial. After reduction by our optimized method,13 the Zn dust was deformed and appeared as a thinner (compressed) and more ﬁbrous Zn band at 240 °C, a Zn mirror at 380 °C, and a softer Zn cake with less sinter at 500 °C that stuck to the bottom of the septa-sealed vial. The AMS targets appeared as a graphite-coated Fe powder (GCIP) fuzz on the -400MSIP at the bottom of the borosilicate inner vial at 500 °C. Table 1 summarized the physical and morphological charac- teristics of the GST as purchased from Sigma-Aldrich and -400MSIP as purchased from Sigma-Aldrich. It also summarized the physical and morphological characteristics of AMS targets of iron-carbon materials (ICM) (produced 20% of the time) and GCI (produced 80% of the time) by our previous method7 and of GCIP (produced 100% of the time) by our optimized method.13 The materials were ranked by the lightness of their color as -400MSIP > ICM > GST ) GCI ) GCIP. The ICM was four times harder than the GCI (p < 0.0001); hardness of powders was not measurable. The photos showed that only the GST, -400MSIP, and GCIP were powders. In comparing SEM of GST to that of -400MSIP, GST was graphite sheets, while -400MSIP was (17) Stanjek, H.; Ha¨usler, W. Hyperﬁne Interact. 2004, 154, 107–119. (18) Cullity, B. D. Elements of X-ray Diffraction, 2nd ed.; Addison Wesley: Reading, MA, 1978.