I need to do RNA extractions of a large number of samples from developing wheat grain, and plan to do them in the batches of 12. Is is sensible to keep the homogenised samples (with pestle and mortar using liquid nitrogen) on dry ice whilst the entire batch is homogenised before proceeding to the extraction protocol? Keeping in liquid nitrogen would probably be best but is dry ice also safer? The first/foremost sample will have to be on dry ice for roughly 1-hour by the time all 12 are grinded.