Development of the Online Standard Addition Method for Stable Isotope Analysis and Its Validation for Sulfur Isotope...

The standard addition method (SAM) determines sample isotope compositions from mixtures of samples and standards with known isotope ratios, which has been widely applicated for low-mass or low-concentration samples. However, conventional off-line SAM is labor-intensive and can suffer from substantial uncertainty (≥2.5%) in the sample proportion (f value) within the mixture, which compromises isotopic accuracy. Here we present a novel online SAM specifically designed to address these challenges. The reference gas routinely used in conventional analyses is innovatively repurposed as a standard and introduced online directly into the sample stream via the ConFlo IV interface. The isotope ratios of samples are subsequently calculated using an isotope mass balance model. The method is evaluated and validated by measuring the sulfur (S) isotope compositions of standard materials and natural samples using elemental analyzer isotope ratio mass spectrometry (EA-IRMS). The uncertainty in the f value is less than 0.148%, ensuring the accuracy of calculated results. The approach eliminates the need for manual standard addition, significantly enhancing measurement efficiency. The method achieves accuracy and reproducibility comparable to conventional techniques while reducing the sample mass by 75%. Based on the developed approach, coarse to ultrafine atmospheric particulate matter was successfully measured in a haze event, revealing size-dependent S isotope variations. Importantly, this methodology is highly practical and easy to be widely adopted for current standard equipped EA-IRMS laboratories, as it requires no additional labor and financial investments for hardware upgrading proposed in previous publications.