Applying Isotope Metrology to the Authentication of Polymers Produced through Biotechnology

L.A. Currie (NIST Scientist Emeritus), D.B. Klinedinst, and R.M. Verkouteren

Objective: To provide U.S. industry with the high metrology needed to help protect investments in the manufacturing and marketing of polymers produced from renewable feedstocks.

Problem: Polypropylene terephthalate (PPT) is a superior alternative to polyethylene terephthalate (PET, a.k.a. "polyester"), and the DuPont Company had recently succeeded in developing a new route to its manufacture involving the biosynthesis of an essential intermediate, 1,3-propanediol (PDO), metabolized through a bioengineered bacteria feeding on glucose in corn syrup. This new route was more economical (and "greener") than using the alternative synthesis of PDO from petroleum-sourced acrolein. However, since the PDO products from either source were chemically indistinguishable, no "composition of matter" patent was possible and future high investments in PET-to-PPT plant conversion were jeopardized. Measurements were needed to distinguish PDO and PPT made by the unique bio-assisted process - measurements with NIST traceability that could be used for legal authentication and protect fair commerce.

Approach: Since carbon from bio-sourced feedstocks is isotopically distinct from that derived from petroleum, measurements of carbon-13 and carbon-14 enabled the differentiation needed to authenticate the new PPT and PDO. The challenge was to design the relevant isotopic measurements having the highest possible traceability. A CRADA was created for this purpose, relevant to the CSTL Biomolecules and Materials Program.

Results and Future Plans: Replication and appropriate isotope reference materials were part of the experimental design. The G-PDO (PDO derived from glucose) had a modern C-14 abundance (with detectable variations due to yearly "vintage") and a depletion in C-13 expected from the enzymatic bio-conversion of glucose. The A-PDO (PDO derived from acrolein) was devoid of C-14. The PPT samples showed similar but lesser differences, owing to the fact that PPT is derived from two monomers, PDO and terephthalic acid (TPA), the latter always derived from petroleum. While the A-PPT was devoid of C-14, the G-PPT had an intermediate dual-isotopic signature predicated by the stoichiometry of the synthetic process. This provided isotopic closure and a means to authenticate the materials. The method was tested through blind analysis of a G-PDO sample provided by DuPont - point "X" in Fig. 1 mapped within the dual isotopic G-PDO space signifying bio-sourced monomer, and also indicated a distinct production based on a new glucose batch. This fact was later verified. The isotopic method may benefit other sectors of the chemical manufacturing industry when similar barriers to commercialization are encountered.

Figure 1. Verification and authentication of PDO and PPT derived from glucose (versus acrolein) by dual isotope (13C + 14C) stoichiometry: G-PPT = 3/11(G-PDO) + 8/11(TPA)

Publications:
Currie, L.A., Klinedinst, D.B., Burch, R., Feltham, N., and Dorsch, R., "Authentication and Dating of Biomass Components of Industrial Materials; Links to Sustainable Technology," Nucl. Instrum. Meth. Phys. Res. B172 (Nov. 2000).