Calculating Radiation Exposures during Use of ¹⁴C-Labeled Nutrients, Food Components, and Biopharmaceuticals To Quantify Metabolic Behavior in Humans

¹⁴C has long been used as a tracer for quantifying the in vivo human metabolism of food components, biopharmaceuticals, and nutrients. Minute amounts (≤1 × 10 ⁻¹⁸ mol) of ¹⁴C can be measured with high-throughput ¹⁴C-accelerator mass spectrometry (HT ¹⁴C-AMS) in isolated chemical extracts of biological, biomedical, and environmental samples. Availability of in vivo human data sets using a ¹⁴C tracer would enable current concepts of the metabolic behavior of food components, biopharmaceuticals, or nutrients to be organized into models suitable for quantitative hypothesis testing and determination of metabolic parameters. In vivo models are important for specification of intake levels for food components, biopharmaceuticals, and nutrients. Accurate estimation of the radiation exposure from ingested ¹⁴C is an essential component of the experimental design. Therefore, this paper illustrates the calculation involved in determining the radiation exposure from a minute dose of orally administered ¹⁴C-β-carotene, ¹⁴C-α-tocopherol, ¹⁴C-lutein, and ¹⁴C-folic acid from four prior experiments. The administered doses ranged from 36 to 100 nCi, and radiation exposure ranged from 0.12 to 5.2 μSv to whole body and from 0.2 to 3.4 μSv to liver with consideration of tissue weighting factor and fractional nutrient. In comparison, radiation exposure experienced during a 4 h airline flight across the United States at 37000 ft was 20 μSv

Biological/Biomedical Accelerator Mass Spectrometry Targets. 1. Optimizing the CO₂ 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 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 <i>F</i>_m values. The observed <i>F</i>_m value for oxalic acid II NIST SRM deviated from its accepted <i>F</i>_m value of 1.3407 by only 0.0003 ± 0.0027 (mean ± SE, <i>n</i> = 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

Accelerator Mass Spectrometry Targets of Submilligram Carbonaceous Samples Using the High-Throughput Zn Reduction Method

The high-throughput Zn reduction method was developed and optimized for various biological/biomedical accelerator mass spectrometry (AMS) applications of mg of C size samples. However, high levels of background carbon from the high-throughput Zn reduction method were not suitable for sub-mg of C size samples in environmental, geochronology, and biological/biomedical AMS applications. This study investigated the effect of background carbon mass (<i>m</i>_c) and background ¹⁴C level (<i>F</i>_c) from the high-throughput Zn reduction method. Background <i>m</i>_c was 0.011 mg of C and background <i>F</i>_c was 1.5445. Background subtraction, two-component mixing, and expanded formulas were used for background correction. All three formulas accurately corrected for backgrounds to 0.025 mg of C in the aerosol standard (NIST SRM 1648a). Only the background subtraction and the two-component mixing formulas accurately corrected for backgrounds to 0.1 mg of C in the IAEA-C6 and -C7 standards. After the background corrections, our high-throughput Zn reduction method was suitable for biological (diet)/biomedical (drug) and environmental (fine particulate matter) applications of sub-mg of C samples (≥ 0.1 mg of C) in keeping with a balance between throughput (270 samples/day/analyst) and sensitivity/accuracy/precision of AMS measurement. The development of a high-throughput method for examination of ≥ 0.1 mg of C size samples opens up a range of applications for ¹⁴C AMS studies. While other methods do exist for ≥ 0.1 mg of C size samples, the low throughput has made them cost prohibitive for many applications

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

Catalytic graphitization for ¹⁴C-accelerator mass spectrometry (¹⁴C-AMS) produced various forms of elemental carbon. Our high-throughput Zn reduction method (C/Fe = 1:5, 500 °C, 3 h) produced the AMS target of graphite-coated iron powder (GCIP), a mix of nongraphitic carbon and Fe₃C. Crystallinity of the AMS targets of GCIP (nongraphitic carbon) was increased to turbostratic carbon by raising the C/Fe ratio from 1:5 to 1:1 and the graphitization temperature from 500 to 585 °C. The AMS target of GCIP containing turbostratic carbon had a large isotopic fractionation and a low AMS ion current. The AMS target of GCIP containing turbostratic carbon also yielded less accurate/precise ¹⁴C-AMS measurements because of the lower graphitization yield and lower thermal conductivity that were caused by the higher C/Fe ratio of 1:1. On the other hand, the AMS target of GCIP containing nongraphitic carbon had higher graphitization yield and better thermal conductivity over the AMS target of GCIP containing turbostratic carbon due to optimal surface area provided by the iron powder. Finally, graphitization yield and thermal conductivity were stronger determinants (over graphite crystallinity) for accurate/precise/high-throughput biological, biomedical, and environmental¹⁴C-AMS applications such as absorption, distribution, metabolism, elimination (ADME), and physiologically based pharmacokinetics (PBPK) of nutrients, drugs, phytochemicals, and environmental chemicals