Abstract
Microincinerated, 1-micrometer sections of Tipula iridescent virus (TIV) crystals were analyzed quantitatively with a wave-length-dispersive electron microprobe to determine the chemical composition of high-resolution ash patterns previously obtained from ultrathin sections of the same specimen (Thomas RS: J Histochem Cytochem 29:379, 1981). Parallel analyses were performed on intact sections. In some cases, the same section and probe tracks were analyzed both before and after ashing. The principal elemental constituent of the ash was phosphorus, representing nearly all of the phosphorus found in the unashed sections. This confirmed the likely DNA origin of most of the ash. Other elements--sulfur, calcium, sodium, and perhaps carbon and nitrogen--found in relatively small concentrations in the ash, were partly due to either to incomplete ashing or to a particulate contaminant. Similar plasma ashing and analyses of sectioned model preparations of polymethacrylate containing dissolved triphenyl phosphate confirmed that phosphorus, by itself, could be retained as a stable ash. Analyses of polymethacrylate containing aluminum as well as phosphorus disclosed an unexpected artifact--aluminum inhibited the plasma ashing. These results suggested that the wavelength-dispersive probe, able to analyze for carbon and nitrogen as well as mineral elements, should be a generally useful tool in analyzing plasma microincineration phenomena, where macroscopic results do not apply. Relatively high beam intensity used throughout the probe analyses caused obvious damage to intact sections, particularly when they were mounted on thin-film supports. In contrast, the ash appeared quite stable. This suggested that plasma ashing of biological sections, converting them into mineralogical specimens, may be generally useful in probe analyses of mineral constituents, permitting greater sensitivity via higher beam currents, higher mineral concentrations, and lower backgrounds.