Biological/Biomedical Accelerator Mass Spectrometry Targets. 1. Optimizing the CO₂ Reduction Step Using Zinc Dust

Abstract

Biological and biomedical applications of accelerator mass spectrometry (AMS) use isotope ratio mass spectrometry to quantify minute amounts of long-lived radioisotopes such as 14C. AMS target preparation involves first the oxidation of carbon (in sample of interest) to CO2 and second the reduction of CO2 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 CO2 to high-quality uniform AMS targets whose morphology we visualized using scanning electron microscope pictures. Key features of our optimized method were to reduce CO2 (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 δ13C was −17.9 ± 0.3‰. The GCIP reliably produced strong 12C− currents and accurate and precise Fm values. The observed Fm value for oxalic acid II NIST SRM deviated from its accepted Fm value of 1.3407 by only 0.0003 ± 0.0027 (mean ± SE, n = 32), limit of detection of 14C 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.

Quality of Graphite Target for Biological/Biomedical/Environmental Applications of 14C-Accelerator Mass Spectrometry

“…Although differences of F m between the AMS targets of GCIP (T s carbon) and GCIP (nongraphitic carbon) were not significant ( P < 0.9804), the AMS target of GCIP (T s carbon) had less accurate and precise F m value (relative error of 0.57%) than the AMS target of GCIP (nongraphitic carbon, 8,11 relative error of −0.02%) (Figure 6c). In addition, the AMS target of GCIP (T s carbon) produced an ≈40% lower 12 C − , 13 C + , and normalized 13 C + (n 13 C + ) currents ( P < 0.0001) than the AMS target of GCIP (nongraphitic carbon 8,11 ) (Figure 6d−f).…”

“…The AMS target of ICM(11) was produced using C/Fe ratios that ranged from 1:1 to 1:15 and 400 °C. The AMS target of ICM did not show the graphite reflection peak (G-002) at 2Θ of ≈26°.…”

Application of zinc sealed tube graphitization on sub-milligram samples using EnvironMICADAS

“…Recently we have adopted an alternative sealed tube method for 1 mg carbon sample size, where the zinc reagent is used as sole reduction agent (Zn-method) [9][10][11] and optimized it for the MICADAS AMS instrument (under publication). This alternative method has already been successfully applied and used in other labs for micrograms sample size [12][13][14].…”

“…The temperature gradient, which is formed along the length of the reaction tubes is highly required in case of the Zn-method [9,10]. For some reason, the Zn-method does not work for us in muffle furnace, which is fully capable for the TiH 2 -Zn method; therefore a new heating block and control device has been developed for the Zn-method by Isotoptech Zrt.…”

Microdosing studies using accelerated mass spectrometry as exploratory investigational new drug trials

“…For 14 C quantification assays, samples should be subjected to a highly purified graphitization process in which the human bodily fluid is combusted. Various pretreatment protocols are required before AMS analysis, depending on the necessity of sample drying or reduction (Kim et al, 2008(Kim et al, , 2010Arjomand, 2010;Salehpour et al, 2010). These processes involve unique manipulations that have been developed according to the pretreatment technology available at a given institution.…”