Oxidation of RIS occurs rapidly under ambient conditions This is especially true for iron monosulfides, which have a higher surface area than iron disulfides. 
Monosulfidic material will change from pitch black to light brown within minutes of being exposed to air (see images below). Consequently, acid sulfate soil samples should be immediately cooled to < 4°C to slow oxidation; and samples containing monosulfidic material should be immediately frozen (e.g., using a portable freezer or liquid nitrogen). The use of dry ice is recommended as the gaseous carbon dioxide should further hinder oxidation. Samples should be tested for ABA within 24 h of sampling. Otherwise, additional preservation techniques (e.g., freezing or drying at > 85°C) should be used.