改写下列句子使其表达更地道：Plasma aliquots of 250 μL were obtained by collecting whole blood from each subject in tubes containing EDTA The blood was then centrifuged at 2500 rpm 4°C for 15 min and stored at −80°C

We collected whole blood from each subject in tubes containing EDTA to obtain plasma aliquots of 250 μL. Afterwards, the blood was centrifuged at 2500 rpm, 4°C for 15 min and stored at −80°C.