Determination of Ultra-Trace Impurities in Electronic Grade Nitrogen by Mass Spectrometry with Evaluation of Measurement Uncertainty

A determination method for the ultra-trace impurities in electronic grade nitrogen was developed by using atmospheric pressure ionization mass spectrometry (APIMS) with standard addition method. 5 impurities (CO, CH₄, H₂, O₂, CO₂) in high purity nitrogen were detected after purifying via a high-efficiency purifier. The standard gas used for calibrating APIMS was gravimetrically prepared in the lab according to the rules in ISO 6142 & ISO Guide 34, and it was added into the ionization source with different concentrations by dynamic dilution, then the fitting curve was built based on the data from different dilution points. Furtherly, the measurement uncertainty was evaluated through the method described in Guide to the expression of uncertainty in measurement (GUM) and Monte Carlo Method (MCM). The results show that all the 5 impurities have good linearity (R²>0.998) in the dynamic range of 0.1 nmol/mol, and the high sensitivity is achieved, the detection limits (LOD) reached to the low level of several to tens of pmol/mol with the expanded uncertainty of 37% (k=2). By compared with the previously reported data in the articles, the LOD values of magnitude for CH₄ and CO are improved 1 and 2 orders, and the LOD values for other three impurities H₂, O₂, CO₂ are the same levels as the reported data. It indicates that the method can be used for the relevant electronic gases for ultra-trace analysis with the traceability supporting.