Clumped isotope measurements on nanomoles of CO₂ using a hollow core fiber-based spectrometer

The abundance of carbon dioxide (CO₂) clumped isotopologues is an important tracer of environmental conditions in both atmospheric and paleoclimatic contexts. However, the conventional method for accurate CO₂ clumped isotope measurements, based on magnetic sector isotope ratio mass spectrometers (IRMSs), is time-consuming (hours per replicate) and requires large sample sizes (up to 100 µmoles CO₂). The recent development of isotope ratio laser spectrometers (IRLSs) determines CO₂ clumped isotope composition based on the infrared absorption spectra of corresponding CO₂ isotopologues and has significantly decreased the duration of clumped isotope measurements (minutes per replicate). However, existing IRLS instruments still require similar sample sizes to current IRMSs. Here, we present, to the best of our knowledge, a new IRLS prototype that demonstrates the feasibility of performing clumped isotope measurements using only nanomoles of CO₂. The instrument leverages hollow core fiber (HCF) technology and achieves a reduction in sample size of about four orders of magnitude compared to existing IRLSs. We successfully detect and quantify the clumped isotopologues ¹⁶O¹³C¹⁸O and ¹⁸O¹²C¹⁸O absorption using ∼17 nanomoles of CO₂ and measure their abundances to precisions of 0.7‰ and 1.0‰, respectively in 1-1.5 minutes. Combining them with three other measured isotopologue lines we are able to calculate the clumped isotope ratios Δ₆₃₈ and Δ₈₂₈, achieving precisions of 0.7‰ and 1.8‰ in 15-30 seconds. Our results demonstrate the potential of HCF technology for clumped isotope analyses of nanomoles of CO₂ with future technical improvements particularly in mid-infrared fiber technology.