Biological/Biomedical Accelerator Mass Spectrometry Targets. 1. Optimizing the CO(2) Reduction Step Using Zinc Dust

Biological and biomedical applications of accelerator mass spectrometry (AMS) use isotope ratio mass spectrometry to quantify minute amounts of long-lived radioisotopes such as (14)C. AMS target preparation involves first the oxidation of carbon (in sample of interest) to CO(2) and second the reduction of CO(2) to filamentous, fluffy, fuzzy, or firm graphite-like substances that coat a −400-mesh spherical iron powder (−400MSIP) catalyst. Until now, the quality of AMS targets has been variable; consequently, they often failed to produce robust ion currents that are required for reliable, accurate, precise, and high-throughput AMS for biological/biomedical applications. Therefore, we described our optimized method for reduction of CO(2) to high-quality uniform AMS targets whose morphology we visualized using scanning electron microscope pictures. Key features of our optimized method were to reduce CO(2) (from a sample of interest that provided 1 mg of C) using 100 ± 1.3 mg of Zn dust, 5 ± 0.4 mg of −400MSIP, and a reduction temperature of 500 °C for 3 h. The thermodynamics of our optimized method were more favorable for production of graphite-coated iron powders (GCIP) than those of previous methods. All AMS targets from our optimized method were of 100% GCIP, the graphitization yield exceeded 90%, and δ(13)C was −17.9 ± 0.3‰. The GCIP reliably produced strong (12)C(−) currents and accurate and precise F(m) values. The observed F(m) value for oxalic acid II NIST SRM deviated from its accepted F(m) value of 1.3407 by only 0.0003 ± 0.0027 (mean ± SE, n = 32), limit of detection of (14)C was 0.04 amol, and limit of quantification was 0.07 amol, and a skilled analyst can prepare as many as 270 AMS targets per day. More information on the physical (hardness/color), morphological (SEMs), and structural (FT-IR, Raman, XRD spectra) characteristics of our AMS targets that determine accurate, precise, and high-hroughput AMS measurement are in the companion paper.