Simple, Fast, and Highly Precise δ¹³C and δ²H Analysis of Organics via Dual Picarro CRDS Integration

Stable isotope analysis of organic materials is essential in environmental, geochemical, and food authenticity research, offering insights into carbon sources and product origins. Traditional Picarro Combustion Module–Cavity Ring-Down Spectroscopy (CM-CRDS) systems provide reliable, cost-effective δ¹³C analysis; furthermore, enabling simultaneous δ²H measurement greatly expands their utility for many applications.

We present a straightforward extension of the CM-CRDS system, integrating two dedicated analyzers: the Picarro G2201-i for δ¹³C and the Picarro L2130-i for δ²H. Key modifications include removing the water trap, heating the transfer tubing, and adding a heated buffer volume, enabling direct isotopic analysis of water vapor alongside carbon dioxide. This setup maintains the original sample delivery for carbon isotope analysis, while a simple software adjustment allows precise peak integration for hydrogen isotopes.

Performance was validated using a range of international standards representing diverse organic materials: USGS88 (marine collagen), USGS89 (porcine collagen), USGS90 (millet flour), USGS91 (rice flour), and IAEA CH7 (polyethylene foil). The system demonstrated excellent δ²H linearity (with slopes of 1.040, 1.074 and 1.017 on three separate days and R² values exceeding 0.99) while maintaining the high accuracy of δ¹³C measurements. Precision was assessed with hexamethylenetetramine (HMT), yielding a δ²H standard deviation of 0.26‰ and δ¹³C of 0.04‰ over 50 replicates. We chose HMT to determine the precision because it does not exchange hydrogen isotopes during storage and analysis and is used to determine the carbon-bound non-exchangeable hydrogen in fructose and glucose in honey [1]. Calibration procedures and best practices for hydrogen isotope analysis are discussed.

Our findings highlight the potential of combining the G2201-i and L2130-i analyzers with a CM in a coordinated analytical workflow for dual isotope analysis. This methodology opens new opportunities for isotope studies for environmental as well as food authenticity and food origin studies, and is a low-cost, easy to use alternative to IRMS analysis.

Reference

[1] Li et al., 2024, A new approach to detecting sugar syrup addition to honey: Stable isotope analysis of hexamethylenetetramine synthesised from honey monosaccharides (fructose and glucose). Food Chemistry 434.