Elektronmikroskop

Et transmisjonselektronmikroskop.

Elektronmikroskop er et viktig redskap for å sjekke resultatet av forskning, eller biologiske prøver.

Elektronmikroskop kan hovedsakelig deles opp i to kategorier. Transmisjonselektronmikroskop (TEM) og Skanningelektronmikroskop (SEM).

I motsetning til et vanlig mikroskop hvor en sender synlig lys og lar øyet se det reflekterte lyset direkte, så bruker et elektronmikroskop elektroner i avbildningen. Elektroner blir sendt ut fra et filament (en wolfram-tråd) i den blanke sylinderen midt i bildet, de blir så akselerert med en spenning på opptil 30kV(SEM) eller 3MegaV(TEM), og bombarderer prøven. Et transmisjons mikroskop lar elektronene gå gjennom prøven (tykkelse på prøven ~60nm - noen få mikrometer) og blir avbildet på en fluoriserende skjerm. I Skanning elektronmikroskopet er det en detektor som ser elektronene som blir reflektert fra prøveoverflaten, og gjør det den ser om til et bilde på en TV-skjerm. Siden elektroner har mindre bølgelengde enn synlig lys[trenger referanse], kan vi se mange flere detaljer. I tillegg blir kontrastene bestemt av de elektriske egenskapene til prøven i større grad enn ved synlig lys. Det vil si at f.eks. gull som har en høy elektrontetthet vil vises avbildet som helt svart i TEM (slipper ikke elektroner til fluoriserende skjermen) eller helt hvit i SEM (reflekterer mange elektroner til detektor).

Elektronmikroskopet kan forstørre opp til ti millioner ganger, det vil si at en kan se cellestrukturer og til og med kjemiske bindinger, vel å merke ikke enkelt og problemfritt. Det er mulig å se detaljer ned i området tiendedels nanometer.

Et elektronmikroskop er i prinsippet bygd på samme måte som et lysmikroskop, med strålekilde (lampen), kondensator, preparatholder med justeringsmuligheter, objektiv, tubus, okular og sensor (øyet). Men strålekilden i elektronmikroskopet er en elektron-akselerator/kanon og linsene er bygd opp av elektromagnetiske spoler. Vi kan heller ikke benytte øyet for direkte observasjon. Fordi elektronene bremses svært effektivt opp av luft, må det benyttes vakuum inne i mikroskopet. En annen forskjell fra lysmikroskopet er at materialet som brukes i elektronmikroskop er forbehandlet i større eller mindre grad. Det er fordi elektronstrålen kan destruere eller endre ubehandlet materiale slik at det man ser i mikroskopet ikke blir reelt.

Kilder

  • Goodhew, P.J. & F.J. Humphreys (1988). Electron Microscopy and Analysis (2nd ed.). Taylor & Francis, London
  • Flood, Per R. (1976): Veileder i Elektronmikroskopisk prepareringsteknikk for biologer (4. utgave). Anatomisk institutt, Universitetet i Bergen.

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