. ▁ ▁Material s ▁that ▁have ▁dimensions ▁small ▁enough ▁to ▁be ▁electron ▁transparent , ▁such ▁as ▁pow der ed ▁subst ances , ▁small ▁organ isms , ▁vir uses , ▁or ▁nan ot ub es , ▁can ▁be ▁quickly ▁prepared ▁by ▁the ▁de position ▁of ▁a ▁dil ute ▁sample ▁containing ▁the ▁spec imen ▁onto ▁films ▁on ▁support ▁gr ids . ▁Bi ological ▁spec im ens ▁may ▁be ▁embedded ▁in ▁res in ▁to ▁with stand ▁the ▁high ▁vac u um ▁in ▁the ▁sample ▁chamber ▁and ▁to ▁enable ▁cutting ▁t issue ▁into ▁electron ▁transparent ▁thin ▁sections . ▁The ▁bi ological ▁sample ▁can ▁be ▁st ained ▁using ▁either ▁a ▁negative ▁st aining ▁material ▁such ▁as ▁u ran yl ▁ac et ate ▁for ▁b acter ia ▁and ▁vir uses , ▁or , ▁in ▁the ▁case ▁of ▁embedded ▁sections , ▁the ▁spec imen ▁may ▁be ▁st ained ▁with ▁heavy ▁met als , ▁including ▁os m ium ▁t etro x ide . ▁Altern ately ▁samples ▁may ▁be ▁held ▁at ▁liquid ▁nit ro gen ▁temper atures ▁after ▁embedding ▁in ▁vit re ous ▁ice . ▁In ▁material ▁science ▁and ▁met all ur gy ▁the ▁spec im ens ▁can ▁usually ▁with stand ▁the ▁high ▁vac u um , ▁but ▁still ▁must ▁be ▁prepared ▁as ▁a ▁thin ▁fo il , ▁or ▁et ched ▁so ▁some ▁portion ▁of ▁the ▁spec imen ▁is ▁thin ▁enough ▁for ▁the ▁beam ▁to ▁pen et rate . ▁Con stra ints ▁on ▁the ▁thick ness ▁of ▁the ▁material ▁may ▁be ▁limited ▁by ▁the ▁scatter ing ▁cross - section ▁of ▁the ▁atoms