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Biological/biomedical accelerator mass spectrometry targets. 1. optimizing the CO2 reduction step using zinc dust.

Kim SH, Kelly PB, Clifford AJ - Anal. Chem. (2008)

Bottom Line: 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 degrees 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.The GCIP reliably produced strong (12)C (-) currents and accurate and precise F m values.

View Article: PubMed Central - PubMed

Affiliation: Department of Nutrition, University of California Davis, One Shields Avenue, Davis, California 95616, USA.

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 (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 degrees 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 delta (13)C was -17.9 +/- 0.3 per thousand. 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.

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Overview of the reduction of CO2 to graphite by our previous graphitization method,(15) which used ∼100 mg of Zn dust (a reductant), ∼10 mg of −400MSIP (a catalyst), and 525 °C (average temperature, 2 cm from bottom) for 6 h. The reduction temperature varied from 480 ± 7 (top Cu heating plate) to 560 ± 7 °C (bottom Cu heating plate). During the reduction, the Zn dust was converted to Zn cake (right bottom SEM), two Zn mirrors (right two middle SEMs) plus two Zn bands (right two top SEMs), and the graphite-coated Fe (GCI) as a fuzz (right the second SEM from the bottom).
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fig1: Overview of the reduction of CO2 to graphite by our previous graphitization method,(15) which used ∼100 mg of Zn dust (a reductant), ∼10 mg of −400MSIP (a catalyst), and 525 °C (average temperature, 2 cm from bottom) for 6 h. The reduction temperature varied from 480 ± 7 (top Cu heating plate) to 560 ± 7 °C (bottom Cu heating plate). During the reduction, the Zn dust was converted to Zn cake (right bottom SEM), two Zn mirrors (right two middle SEMs) plus two Zn bands (right two top SEMs), and the graphite-coated Fe (GCI) as a fuzz (right the second SEM from the bottom).

Mentions: In optimizing the reduction step of our previous method,(15) Ox-2 was used as the AMS standard rather than the ANU whose supply was limited. Oxidation of the 1-mg aliquots of solid C (which has traces of H2O vapor) to CO2 was conducted as described by our previous method.(15) Transfer of the CO2 to septa-sealed vials that had an inner vial containing a catalyst that sat atop glass beads that sat atop a reductant (Zn dust) was also the same as by our previous method(15) except that several amounts of reductant, catalyst, types of catalysts, heating temperatures, and heating durations were evaluated as summarized in Table 1. Criteria for evaluating the AMS data included a high, reliable, and stable n13C+ current along with an accurate Fm value. The reductant levels ranged from 1 to 300 mg of Zn dust in increments of 25 mg. The five catalysts included three of iron and two of cobalt, each at only one level, 10 mg. Then, six levels of the best performing (defined by ion current) of the five catalysts that ranged from 1 to 10 mg were evaluated. Three reduction temperatures that ranged from 500 to 550 °C in increments of 25 °C and 12 reduction durations that ranged from 0.5 to 6 h in increments of 0.5 h were tested. Graphitization yield and isotopic fractionation were measured using a PDZ Europa ANCA-GSL elemental analyzer interfaced to a PDZ Europa 20−20 isotope ratio mass spectrometer (Sercon Ltd., Cheshire, UK).(18) The resulting graphite was tamped into AMS target holders and analyzed at the LLNL as by our previous method.(15) The 13C+ current of the AMS target of interest was normalized by the 13C+ current of the Ox-2 SRM and referred to as normalized 13C+ (n13C+) current. The Fm of a sample of interest was the ratio of (14C/13C)sample/(14C/13C)Ox-2, each normalized for δ13C. The δ13C was calculated as [(13C/12C)sample − (13C/12C)PDB]/(13C/12C)PDB × 1000, where PDB referred to the Cretaceous belemnite formation at Peedee, SC.(19) The δ13C value used in this study was −25 ‰ for biological/biomedical application and −17.8 ‰ for Ox-2 SRM. Four C sources were compared using our previous method(15) and our optimized method.


Biological/biomedical accelerator mass spectrometry targets. 1. optimizing the CO2 reduction step using zinc dust.

Kim SH, Kelly PB, Clifford AJ - Anal. Chem. (2008)

Overview of the reduction of CO2 to graphite by our previous graphitization method,(15) which used ∼100 mg of Zn dust (a reductant), ∼10 mg of −400MSIP (a catalyst), and 525 °C (average temperature, 2 cm from bottom) for 6 h. The reduction temperature varied from 480 ± 7 (top Cu heating plate) to 560 ± 7 °C (bottom Cu heating plate). During the reduction, the Zn dust was converted to Zn cake (right bottom SEM), two Zn mirrors (right two middle SEMs) plus two Zn bands (right two top SEMs), and the graphite-coated Fe (GCI) as a fuzz (right the second SEM from the bottom).
© Copyright Policy - open-access - ccc-price
Related In: Results  -  Collection

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fig1: Overview of the reduction of CO2 to graphite by our previous graphitization method,(15) which used ∼100 mg of Zn dust (a reductant), ∼10 mg of −400MSIP (a catalyst), and 525 °C (average temperature, 2 cm from bottom) for 6 h. The reduction temperature varied from 480 ± 7 (top Cu heating plate) to 560 ± 7 °C (bottom Cu heating plate). During the reduction, the Zn dust was converted to Zn cake (right bottom SEM), two Zn mirrors (right two middle SEMs) plus two Zn bands (right two top SEMs), and the graphite-coated Fe (GCI) as a fuzz (right the second SEM from the bottom).
Mentions: In optimizing the reduction step of our previous method,(15) Ox-2 was used as the AMS standard rather than the ANU whose supply was limited. Oxidation of the 1-mg aliquots of solid C (which has traces of H2O vapor) to CO2 was conducted as described by our previous method.(15) Transfer of the CO2 to septa-sealed vials that had an inner vial containing a catalyst that sat atop glass beads that sat atop a reductant (Zn dust) was also the same as by our previous method(15) except that several amounts of reductant, catalyst, types of catalysts, heating temperatures, and heating durations were evaluated as summarized in Table 1. Criteria for evaluating the AMS data included a high, reliable, and stable n13C+ current along with an accurate Fm value. The reductant levels ranged from 1 to 300 mg of Zn dust in increments of 25 mg. The five catalysts included three of iron and two of cobalt, each at only one level, 10 mg. Then, six levels of the best performing (defined by ion current) of the five catalysts that ranged from 1 to 10 mg were evaluated. Three reduction temperatures that ranged from 500 to 550 °C in increments of 25 °C and 12 reduction durations that ranged from 0.5 to 6 h in increments of 0.5 h were tested. Graphitization yield and isotopic fractionation were measured using a PDZ Europa ANCA-GSL elemental analyzer interfaced to a PDZ Europa 20−20 isotope ratio mass spectrometer (Sercon Ltd., Cheshire, UK).(18) The resulting graphite was tamped into AMS target holders and analyzed at the LLNL as by our previous method.(15) The 13C+ current of the AMS target of interest was normalized by the 13C+ current of the Ox-2 SRM and referred to as normalized 13C+ (n13C+) current. The Fm of a sample of interest was the ratio of (14C/13C)sample/(14C/13C)Ox-2, each normalized for δ13C. The δ13C was calculated as [(13C/12C)sample − (13C/12C)PDB]/(13C/12C)PDB × 1000, where PDB referred to the Cretaceous belemnite formation at Peedee, SC.(19) The δ13C value used in this study was −25 ‰ for biological/biomedical application and −17.8 ‰ for Ox-2 SRM. Four C sources were compared using our previous method(15) and our optimized method.

Bottom Line: 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 degrees 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.The GCIP reliably produced strong (12)C (-) currents and accurate and precise F m values.

View Article: PubMed Central - PubMed

Affiliation: Department of Nutrition, University of California Davis, One Shields Avenue, Davis, California 95616, USA.

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 (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 degrees 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 delta (13)C was -17.9 +/- 0.3 per thousand. 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.

Show MeSH
Related in: MedlinePlus