Purification of CH 3Cl from CH 3I using cold trap with sealed 2,2,4-trimethylpentane for δ 37Cl measurement

Abstract

A cryogenic separation method of chloromethane (CH 3Cl) from methyl iodide (CH 3I) for δ 37Cl 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. CH 3Cl can be separated from CH 3I at the TMP melting point (−107 °C) based on the difference in the vapor pressure between CH 3Cl (322.6 Pa) and CH 3I (lower than 1.3 Pa) at −107 °C. After two-step separation processes, the yields of CH 3Cl purified from CH 3Cl–CH 3I mixture are 96–101%, and the difference between the δ 37Cl original and δ 37Cl After separation is from −0.06 to +0.06‰ (0.01 ± 0.04‰). These results suggest that CH 3Cl is completely separated from CH 3I with no change of δ 37Cl 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.