Scanning Electron Microscope


1.      Introduction and Definition

The first scanning electron microscope (SEM) invented in 1938 by Von Ardenne with the first commercial instruments around 1965. Scanning Electron Microscope is instrument that use a focused beam of high energy electrons to generate a variety of signals at the surface of solid specimens to examine objects on a very fine (tiny) scale. We can get data and information from SEM about:

–      external morphology (texture)

–     2-dimensional image (Areas ranging from approximately 1 cm to 5 microns in width can be imaged and with magnification ranging from 20x to approximately 30,000x)

–      chemical composition

–      crystalline structure

–      orientation of materials making up the sample

A Scanning Electron Microscope suitable for producing an image with high resolution by detecting secondary electrons and backscattered electrons generated from a specimen. The main signals which are generated by the interaction of the primary electrons (PE) of the electron beam and the specimen´s bulk are secondary electrons (SE) and backscattered electrons (BSE) and furthermore X rays.

– Secondary electrons generated when the specimen is scanned with primary electrons is based on electrons discharge from the surface of specimen
– Backscattered electrons detector for detecting generated backscattered electrons is disposed on the trajectory of the backscattered electrons

2.      Principles

Electrons in SEM carry significant amounts of kinetic energy, and this energy is dissipated as a variety of signals produced by electron-sample interactions when the electrons are hitting the solid sample. The signal-producing system is to generate a probe of the smallest diameter possible and of maximum brightness when hitting the surface of the specimen. It consists of an electron gun, (cathode – Wehnelt cylinder – anode), lens system (lenses, apertures, beam deflection coils and stigmator coils) and the specimen chamber. And pumps are needed to make pressure to a vacuum.

-Samples must be SOLID and they must fit into the microscope chamber.

-Samples have to be prepared carefully to withstand the VACUUM inside the microscope.

-Sample must be COATED with a very thin layer of gold by a machine called a sputter coater.

–          After the samples is ready, next the sample is placed inside the microscope’s VACUUM column through an air-tight door.

–          After the air is pumped out of the column, an ELECTRON GUN EMITS a beam of high energy electrons. This beam travels downward through a series of magnetic lenses designed to focus the electrons to a very fine spot.

–          Near the bottom, a set of scanning coils moves the focused beam back and forth across the specimen, row by row.

–          As the electron beam hits each spot on the sample, secondary electrons are knocked loose from its surface. A detector counts these electrons and sends the signals to an amplifier.

–          The final image is built up from the number of electrons emitted from each spot on the sample.

–          The Scanning Electron Microscope is revealing new levels of detail and complexity in the amazing world of micro-organisms and miniature structures.

SEM shows very detailed 3-dimensional images at much higher magnifications than is possible with a light microscope. The images created without light waves are rendered black and white.

REVIEW JOURNAL SCANNING ELECTRON MICROSCOPE

Observation of Bacterial Exopolysaccharide in Dairy Products using Cryo-Scanning Electron Microscopy

            SEM pada jurnal ini digunakan untuk melihat mikrostruktur produk susu fermentasi dan keju Karish dan Feta (dairy product) yang menggunakan bakteri asam laktat yang memproduksi Exopolisakarida (EPS) dan yang tidak memproduksi EPS.  Persiapan sampel yang harus dilakukan adalah :

–          Penghilangan air pada produk

Produk tersebut sangat tinggi kandungan air nya sehingga airnya harus di evaporasi karena produk yang dianalisa dengan menggunakan SEM harus padat, kering dan vakum. Penguapan air ini mencegah sampel menghasilkan pembacaan yang berbeda (tidak valid) dan mencegah kontaminasi dan kerusakan pada alat tersebut.

–          Penghilangan Lemak dan Senyawa Volatil

Selain air, yang harus dihilangkan adalah komponen lemak dan senyawa-senyawa volatile yang harus diekstrak karena ditakutkan akan meleleh setelah terkena tembakan electron dari SEM.

Tahap persiapan sampelnya adalah:

–          Sampel difiksasai di larutan glutaraldehid

–          Diberi OsO4 untuk menahan globula lemak

–          Dehidrasi alcohol bertingkat

–          Defatted dengan kloroform

–          Critical point dried

–          Di vacuum kan pada chamber

–          Dilapisi dengan emas

Pengeringan menggunakan pembekuan cepat dengan Nitrogen cair suhu -2700C

Di dalam jurnal ini menggunakan sampel keju dan susu yang telah di fermentasi. Penelitian ingin melihat mikrostruktur 3 dimensi dari 2 jenis keju yaitu Karish dan Feta serta susu fermentasi yang difermentasi menggunakan strain bakteri asam laktat yang memproduksi Exopolisakarida (EPS) dan yang tidak memproduksi EPS.

Exopolisakarida (EPS) sangat berpengaruh terhadap REOLOGI & TEKSTUR dari produk, meskipun mekanismenya masih belum ditemukan.

–          Yoghurt yang dibuat dengan menggunakan EPS-producing cultures akan lebih sulit mengalami sineresis, lebih kental, dan memiliki water holding  capacity yang tinggi.

–          Keju yang dibuat dengan menggunakan EPS-producing cultures akan menghasilkan softer cheese curd dan keju Mozarella yang memiliki daya ikat air yang tinggi.

TO SEE THE PICTURE, CONTACT ME🙂

           Gambar diatas menunjukkan struktur mikro dari keju Karish dengan EPS producing dan non-producing. Pengaruh EPS dalam koagulasi protein pada keju adalah membentuk massa filament dengan pori yang besar.

Hasilnya dapat dilihat dengan SEM konvensional. Sedangkan EPS jika dilihat dalam cyro-SEM menunjukkan massa filamen yang terpisah dari jaringan protein. Padahal biasanya jika menggunakan SEM konvensional filamen-filamen tersebut tercampur merata dalam jaringan protein.

REFERENCES

Argast, Anne and Tennis, Clarence F. 2004. A web resource for the study of alkali feldspars and perthitic textures using light microscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy. Journal of Geoscience Education 52 (3) p. 213-217

Beane, Rachel. 2004. Using the Scanning Electron Microscope for Discovery Based Learning in Undergraduate Courses. Journal of Geoscience Education vol 52 (3), p. 250-253

A.N. Hasan, J.F. Frank, M.Elsoda. 2003. Observation of Bacterial Exoplysaccharide in Dairy Products using Cryo-Scanning Electron Microscopy. International Dairy Journal vol 13, p. 755-762

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