Sample preparation
If only a qualitative analysis is desired, the sample may be mounted with double-stick conductive carbon tape on the sample holder. For precise quantitative analysis, however, careful sample preparation is essential to ensure that the geometrical requirements for wavelength dispersive spectrometry are met. For good results, the surface at the spot being analyzed must be horizontal and the electron beam must be vertical. Hence, the surface must be well polished first with coarse SiC-paper and then with fine diamond paste or corundum (Al2O3) grit slurry. The final grit size should be at least 0.25 micron and preferably 0.06 micron. For samples that dissolve in water, dry polishing with fine polishing paper is recommended. Except for water-soluble samples, the polished sample is then washed with clean water in an ultrasonic cleaner and dried in air and with blow-duster.
The surface of the sample must be electrically conductive to avoid charging by the electron beam. Therefore, insulating samples are carbon coated in vacuum by passing electric current through high purity graphite rods (electrodes). Carbon evaporates at the point of contact of the two electrodes and deposits on the surface of the sample. A polished brass surface is used to monitor the coating thickness. A thickness of ~250 Angstrom that corresponds with a deep blue color on brass (see below) is recommended.
