un og old ▁lab elling ) ▁may ▁be ▁used ▁prior ▁to ▁T EM ▁observation ▁to ▁select ively ▁depos it ▁electron ▁dense ▁atoms ▁in ▁or ▁on ▁the ▁sample ▁in ▁desired ▁cell ular ▁or ▁protein ▁region . ▁This ▁process ▁requires ▁an ▁understanding ▁of ▁how ▁heavy ▁met als ▁bind ▁to ▁specific ▁bi ological ▁t issues ▁and ▁cell ular ▁structures . ▁ ▁Mechan ical ▁mill ing ▁Mechan ical ▁pol ishing ▁is ▁also ▁used ▁to ▁prepare ▁samples ▁for ▁imag ing ▁on ▁the ▁T EM . ▁Pol ishing ▁needs ▁to ▁be ▁done ▁to ▁a ▁high ▁quality , ▁to ▁ensure ▁constant ▁sample ▁thick ness ▁across ▁the ▁region ▁of ▁interest . ▁A ▁diam ond , ▁or ▁cub ic ▁bor on ▁nit ride ▁pol ishing ▁comp ound ▁may ▁be ▁used ▁in ▁the ▁final ▁stages ▁of ▁pol ishing ▁to ▁remove ▁any ▁scratch es ▁that ▁may ▁cause ▁contrast ▁fl uct u ations ▁due ▁to ▁varying ▁sample ▁thick ness . ▁Even ▁after ▁careful ▁mechanical ▁mill ing , ▁additional ▁fine ▁methods ▁such ▁as ▁ion ▁et ch ing ▁may ▁be ▁required ▁to ▁perform ▁final ▁stage ▁thin ning . ▁ ▁Chem ical ▁et ch ing ▁ ▁C ertain ▁samples ▁may ▁be ▁prepared ▁by ▁chemical ▁et ch ing , ▁particularly ▁met all ic ▁spec im ens . ▁These ▁samples ▁are ▁th inned ▁using ▁a ▁chemical ▁et chant , ▁such ▁as ▁an ▁acid , ▁to ▁prepare ▁the ▁sample ▁for ▁T EM ▁observation . ▁Dev ices ▁to ▁control ▁the ▁thin ning ▁process ▁may ▁allow ▁the ▁operator ▁to ▁control ▁either ▁the ▁voltage ▁or ▁current ▁passing ▁through ▁the ▁spec imen , ▁and ▁may