( electron microscopy. Accurate size ana...)Sub-Micron Gold Particles Are Superior to Larger Particles for Efficient Biolistic Transformation of Organelles and Some Cells

electron microscopy. Accurate size analysis of gold microparticles is particularly difficult because of their high density and their potential to agglomerate. Most suppliers of gold powder determine particle size by sedimentation techniques. This procedure has the advantage of analyzing a very large number of particles as well as being a rapid and relatively simple procedure. However, sedimentation analysis does not distinguish between individual and agglomerated particles, and particle diameter is estimated assuming perfectly spherical particles. While electron microscopy is more labor intensive, it permits visual determination of the particle shape and allows direct measurement of the diameter of individual particles. Therefore, non-spherical batches of gold particles can be excluded from further assay and agglomerated particles can be eliminated from the size calculation for maximum consistency between samples. Figure 1 shows scanning electron micrographs of gold and tungsten particles representative of the those sold by Bio-Rad. Also shown are size distributions for the four sets of particles used in these experiments. The contrast between the two types of particles in terms of the size and shape is particularly evident. Scanning electron microscopy has been used as a part of the quality control procedure to determine the average size, the variation in size, and the sphericity of Bio-Rad gold particles since 1993. Information on the average particle diameter, the standard deviation of the diameter, and the aspect ratio (the ratio of the long axis of the particle to the short axis of the particle a measure of sphericity) is provided with every lot of gold microparticles. Additionally, quality control includes
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