Purification of CH3Cl from CH3I using cold trap with sealed 2,2,4-trimethylpentane for δ37Cl measurement

A cryogenic separation method of chloromethane (CH3Cl) from methyl iodide (CH3I) for δ37Cl measurement with isotope ratio mass spectrometer is described. A cold trap with sealed 2,2,4-trimethylpentane (TMP) as the cryogen is used in this method. CH3Cl can be separated from CH3I at the TMP melting point (−107 °C) based on the difference in the vapor pressure between CH3Cl (322.6 Pa) and CH3I (lower than 1.3 Pa) at −107 °C. After two-step separation processes, the yields of CH3Cl purified from CH3Cl–CH3I mixture are 96–101%, and the difference between the δ37Cloriginal and δ37ClAfter separation is from −0.06 to +0.06‰ (0.01 ± 0.04‰). These results suggest that CH3Cl is completely separated from CH3I with no change of δ37Cl value. This method using the cold trap with sealed TMP is very safe and not harmful, because the liquid organic compound used as the cryogen is contained in the trap and does not evaporate during the separation procedure. This method is also simple and inexpensive relative to the method using a gas chromatograph.