188.8.131.52. Radiocarbon in Biobased Products (Fuel)
(derived from LS Measuring Procedures [Moebius and Moebius 2012] and HIDEX Application Note [HIDEX 2016])
The determination of 14C finds application in the age determination of archaeological objects, in the determination of biobased content in biopolymers and bioplastics as well as in the analysis of environmental samples.
Biogenic materials as blends with fossil fuels are presently used in fuels, both diesel and gasoline. Because the 14C activity differs in biogenic and fossil fuels, it is used for its characterization. As biofuels have gained increased popularity, there is a growing interest by industry, researchers, and stakeholders in the promise of renewable resources, see [DIN 2014] or [ASTM 2012]. The prove of the vegetable origin of tax-reduced biomass fuels is therefore of importance.
The 14C cycle in nature is summarized as follows:
Radioactive 14C is formed in the upper atmosphere due to cosmic radiation. Nitrogen (14N) captures a high-energy neutron to form 14C by emitting a proton:
14C is further oxidized to 14CO2, which is then absorbed by plants during photosynthesis. The intake of 14C immediately terminates, when the organism dies. From that moment a constant decay back to 14N begins:
Consequently, the ratio between 14C and stable 12C/13C in a deceased organism decreases at a constant rate over time.
For the determination of Radiocarbon in biological samples automatic combustion equipments are commercially available (e.g. Zinsser Analytic). They allow the simultaneous quench free sample preparation of 3H and 14C in dual labeled substances, e.g. by combustion of 14C into 14CO2, which is then trapped into NaOH, as carbamate or into Carbo-Sorb/methanol. However, the procedure is tedious and time consuming. As alternate, a benzene synthesizer system for the conversion of organic and inorganic carbon–based materials into high purity benzene through modern chemical synthesis techniques has been introduced. The resulting 14C containing benzene is ready for quench free liquid scintillation counting (14Culp Consulting LLC), see also [Noakes 2007] by simply adding fluors in solid form.
With the availability of low level and/or luminescence free triple PMT LS counters with figure of merits FOM > 300,000, direct measurement of bioethanol and bio-oil samples are accessible. They avoid the sensitive but complex and cost-intensive method of accelerator mass spectrometry.
A wide range of gasoline and diesel fuels can be mixed with a suitable organic cocktail (e.g. Ultima Gold F, or a mixture of OptiScint HiSafe and Ultima Gold F in ratios 5:15 to 15:5 [L’Annunciata 2012]) for direct measurement. The triple coincidence measurement of the 14C samples in organic solutions avoids luminescence in the low energy part of the spectrum for colored samples without dark storage.
Otherwise, high color quenching demands for effective correction methods or the use of a suitable internal standard (e.g. 14C-toluene or [4-14C]-cholesterol in 100 µL synthetic alcohol).
Current-day bioethanol serves as a reference sample and eliminates the need to correct for ‘bomb carbon’ [Norton and Woodruff 2012].
Care has to be taken that all materials including background samples are free of 14C!
Materials and Equipment
- MaxiLight+, Ultima Gold F or OptiScint HiSafe
- Fossil fuel
- LL TDCR LS counter
(see [DIN 2014] and [Noakes et al. 2007])
- 10 mL organic cocktail and 10 mL diesel fuel or gasoline (measure weight!) are mixed well in a 20 mL LS vial.
- The mixture is measured in a LL LS counter with TDCR option for several hours.
- Background is determined using fossil fuel following the same procedure.
- Efficiency is calculated from the TDCR value; in absence of TDCR the spectrum should be cut below channel 100 in order to reduce luminescence.
Best figure of merit for ratio: diesel fuel / cocktail 14 : 6; pure ethanol / cocktail 12.5 : 7.5
The amount of biobased carbon can be expressed as a fraction of sample mass, total carbon content or total organic carbon content. Products containing only carbon from fossil resources no longer have any 14C activity and are reported as having a biobased content of 0 %. Combining fossil carbon with modern carbon gives intermediate values for the biobased content, which are directly proportional to the concentration of modern carbon in the sample. For the calculation, the current pMC (percentage of modern or present-day carbon) level of bioethanol (to be taken as 100%) should be verified (14.1 dpm/g C in 2010 with a yearly decrease of 0.047 dpm/g C [DIN 2014]). This takes into consideration the current ratio between atmospheric 14C/12C, which has been affected by atom bomb testing in the 1950s and fossil fuel consumption during the last century.
The sample activity is reported in disintegration per minute per gram carbon (dpm/g C) and converted to percent natural (% Nat) through dividing by today’s activity level. Detailed calculations for the values of FAME (Fatty Acid Methyl Ester) and HVO (Hydrogenated Vegetable Oil) in diesel or ethanol and HVO in gasoline can be found in [DIN 2014].
The determination of the biogenic fraction determination in fuels by LSC has recently been reported by different authors [Nikolov et al. 2017], [Kristof et al. 2013], [Krajcar Bronic et al. 2017].
Lower Limit of Detection LLD: 25 mBq per sample (5 g C) for tm = 1 h
L’Annunziata M.F. 2012: “Handbook of Radioactivity Analysis”, Chapter 15, 3rd Edition 2012, Elsevier
ASTM 2012: Standard test methods for determining the biobased content of solid, liquid and gaseous samples using Radiocarbon analysis, ASTM D6866-12, ASTM International
DIN 2014: Liquid petroleum products – determination of the bio-based hydrocarbon content in diesel fuels and middle distillates using liquid scintillation method; DIN 51637 2014-02, Deutsches Institut für Normung
HIDEX 2016: Application Note 413-012, Determination of the 14C content in bio-based products
Krajcar Bronić I. and Barešić J. 2017: Determination of biogenic component in liquid fuels by the 14C method and direct LSC measurement; Abstract “LSC2017 Advances in Liquid Scintillation Spectrometry”, Copenhagen
Kristof R. and Kozar Logar J. 2013: Direct LSC method for measurements of biofuels in fuel; Talanta 111 (2013) 183-188
Moebius S. and Moebius T. L. 2012: Handbook of Liquid Scintillation Spectrometry, DGFS e.V. and Karlsruhe Institute of Technology, Karlsruhe 2012
Nikolov J., Stojkovic I., Tomic M., Mićić R. and Todorovic N. 2017: Development of direct method for biogenic fraction determination in fuels; Abstract “LSC2017 Advances in Liquid Scintillation Spectrometry”, Copenhagen
Noakes J., Norton G., Culp R., Nigat M. and Dvoracek D. 2007: A comparison of analytical methods for the certification of biobased products; in: S. Chalupnik et al. “LSC2005 Advances in Liquid Scintillation Spectrometry”, pp. 259-272, Radiocarbon 2006, Tucson
Norton G.A. and Woodruff M.X. 2012: Simplified radiocarbon analysis procedure for measuring the renewable diesel concentration in diesel fuel blends; J. Am. Oil Chem. Soc. 89 (2012) 797-803