Procedure for Bromination-EDXA

Prior to bromination, small sticks of wood (1.5 mm radial 1.5 mm tangential 5 mm longitudinal) are extracted overnight with benzene-ethanol (2 1, v/v) in a Soxhlet extractor. The extracted sample is dehydrated sequentially with ethanol and water mixtures with increasing ethanol concentration up to 100%, and finally with chloroform. The sample is brominated at room temperature in a nonaqueous system with a solution of bromine in chloroform (0.3 ml in 20 ml). The bromine solution is slowly dropped into a 125-ml Erlenmeyer flask that contains 70 ml of chloroform and 1 g of wood sticks with stirring. After stirring for 30 min, the brominated wood sticks are washed several times with an excess of chloroform at room temperature to remove unreacted bromine and then with ethanol. 

The brominated sticks are embedded in Spurr epoxy resin (Spurr 1969) and sectioned with a diamond knife or glass knife mounted on an ultramicrotome to give cross sections of 0.15//m thickness for TEM-EDXA or 0.5//m thickness for SEM-EDXA. The sections are then placed on a carbon-coated collodion film on the specimen support grid. The specimens are carbon-coated again to avoid charging. 

The brominated specimens may be studied by a SEM-EDXA or a TEM-EDXA system. Since the intensity of the X-ray emission is directly proportional to the illuminating current, it is essential, for quantitative assay, to ascertain the current during the analysis. A Faraday cup (Howitt et al. 1976) in conjunction with an electrometer (e.g., Keithley 616 Digital Electrometer) may be used. The illuminating current should preferably be in the range of 10-9-10-11) amperes. 

Other variables, such as the distance between X-ray detector and specimen, tilting angle of specimen holder, specimen height, and take-off angle, should be 

Thin sections may readily be examined with TEM-EDXA or SEM-EDXA in the transmission mode (STEM-EDXA). However, to use conventional SEM-EDXA, some modifications of the equipment are required. As shown in Fig. 4.4.3, the electrons transmitted through a thin specimen impinge upon the carbon plate underneath. This causes the emission of a strong continuous spectrum of white X-rays. To minimize this problem, a carbon grid holder with a Faraday cup should be used. If the depth of the cylinder in the Faraday cup is more than 20 times the size of aperture, the electrons can be effectively collected (Grubb 1971, Howitt et al. 1976), thus reducing a large portion of the white X-rays emitted. 

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