Scanning Microscopy


Magnesium, potassium, calcium and phosphorus are stored in seed tissues in spherical particles called globoid crystals. The main component of globoid crystals is phytin, a salt of myo-inositol hexaphosphoric acid. Based on chemical similarity, commercially available phytates may be suitable standards for quantitative energy dispersive X-ray (EDX) microanalysis of globoid crystals. The stability of globoid crystals and commercial phytates was different when analyzed under identical conditions. Phytates in globoid crystals from Cucurbita maxima cotyledons, were relatively stable in the electron beam during EDX microanalysis at room temperature, but there was a loss of potassium. No loss of potassium occurred during analysis at low temperature, even with repeated analyses on the same spot. Commercial phytates showed considerable beam damage in the form of raised mounds at the sites of analyses. The extent of the damage was much reduced with analysis at low temperature. Although there was some variation in the peak-to-background ratios of potassium and sodium with various analytical conditions, there was no differential loss of potassium or sodium as occurred with EDX analysis of globoid crystals. Reasons for differences between in situ and isolated phytates are unclear. Provided analyses are carried out at low temperature, commercial phytates have potential as standards but further research is required to determine how to control moisture content during sample preparation and analysis.

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