in my lab the highest speed of centrifuge is 20000 rpm but the method of separation mitochondria of rat brain contain centrifuge has 21000 rpm ,so can i use 20000 rpm instead of 21000 rpm in separating rat mitochondria
You definitely can. You probably will lose some lighter mitochondria, but most mitochondria would get pelleted down at 20,000 rpm. For mouse mitochondria, I usually use 10,000g to spin them down.