An automated, laser-based measurement system for nitrous oxide isotope and isotopomer ratios at nanomolar levels.

RATIONALE

Nitrous oxide (N O) is an atmospheric trace gas regulating Earth's climate, and is a key intermediate of many nitrogen cycling processes in aquatic ecosystems. Laser-based technology for N O concentration and isotopic/isotopomeric analyses has potential advantages, which include high analytical specificity, low sample size requirement and reduced cost.

METHODS

An autosampler with a purge-and-trap module is coupled to a cavity ring-down spectrometer to achieve automated and high-throughput measurements of N O concentrations, N O isotope ratios (δ N and δ O values) and position-specific isotopomer ratios (δ N and δ N values). The system provides accuracy and precision similar to those for measurements made by traditional isotope ratio mass spectrometry (IRMS) techniques.

RESULTS

The sample sizes required were 0.01-1.1 nmol-N O. Measurements of four N O isotopic/isotopomeric references were cross-calibrated with those obtained by IRMS. With a sample size of 0.50 nmol-N O, the measurement precision (1σ) for δ N , δ N , δ N and δ O values was 0.61, 0.33, 0.41 and 0.43‰, respectively. Correction schemes were developed for sample size-dependent isotopic/isotopomeric deviations. The instrumental system demonstrated consistent measurements of dissolved N O concentrations, isotope/isotopomer ratios and production rates in seawater.

CONCLUSIONS

The coupling of an autosampler with a purge-and-trap module to a cavity ring-down spectrometer not only significantly reduces sample size requirements, but also offers comprehensive investigation of N O production pathways by the measurement of natural abundance and tracer level isotopes and isotopomers. Furthermore, the system can perform isotopic analyses of dissolved and solid nitrogen-containing samples using N O as the analytical proxy.