Impurity analyses for a certified reference material of high-purity sulfur dioxide gas (NMIJ CRM 3402)

A certified reference material (CRM) of sulfur dioxide (SO2) gas, with a certified value of purity, was developed for the preparation of primary reference gas mixtures of SO2 in nitrogen (N2) gas. Impurity analyses by “subtraction method” were performed using a gas chromatograph (GC) with a thermal conductivity detector (TCD), a GC with a flame ionization detector (FID), and a Fourier transform infrared spectrometer (FT-IR), all calibrated with gas mixtures traceable to the Internal System of Units (SI). First, qualitative analysis using the FT-IR was performed on two samples of liquefied SO2 of different quality grades, contained in gas cylinders. Using the FT-IR and the GC-TCD, carbon dioxide (CO2) was detected in the lower- and higher-grade cylinders at approximately 0.5 cmol/mol and 5 µmol/mol, respectively. The FT-IR did not detect any sulfur compound peaks, other than SO2, in the higher-grade sample. Other atmospheric constituents such as N2, oxygen (O2), water (H2O), argon (Ar), and methane (CH4) were also tested for in this sample. The detection limit of H2O in the FT-IR was 49 µmol/mol, while those of N2, O2, Ar, by the GC-TCD were a few µmol/mol, and CH4, using the GC-FID, were better than 1 µmol/mol. The high detection limit of H2O was due to large baseline fluctuations when measuring the FT-IR spectrum of the SO2 gas. Responses corresponding to these analytes were not detected or were detected below the detection limits. For an estimation of purity by the subtraction method, each reported mol fraction was regarded as half the value of each detection limit, except for the CO2. The purity and its expanded uncertainty [k = 2] of the high-purity SO2 gas by the subtraction method were 99.9964% and 0.0028%, respectively.