to tablet. 20 tablets of each sample were ground to a powder, and a representative sample of this powder was analyzed. The ASA content was determined by comparison to the fluorescence emission of a single standard of USP reference standard aspirin rather than by use of a calibration curve. The absolute difference between the USP results and filter fluorometric results ranged from 3 to 8 mg of ASA per tablet as compared to a difference between USP results and spectro-
fluorometric results ranging from 1 to 5 mg of ASA per tablet (4). The filter fluorometric results were lower than the USP results in three out of four cases; the spectrofluorometric results were always slightly higher than the USP results (4).
RECEIVED for review February 2, 1972. Accepted April 11, 1972.
Accuracy of Quantitative Electron Probe Microanalysis with Energy Dispersive Spectrometers D. R. Beaman and L. F. Solosky Dow Chemical U.S.A., Midland, Mich. 48640 The accuracy in the measurement of chemical composition using an energy dispersive spectrometer was determined. After considering a large number of analyses performed by a number of investigators and comparing them with a series of analyses performed in our laboratory, it was concluded that if the concentration is greater than about 20 weight per cent, a relative error of &6% of the amount present could be expected. At lower concentration, large positive errors were often encountered. The EDS results were compared with those obtained with a crystal spectrometer. Possible sources of error were examined.
ENERGYDISPERSIVE X-RAY SPECTROMETERS are now being widely used as analytical tools in electron probe analyzers and scanning electron microscopes. The energy dispersive spectrometer (EDS) has found its greatest utility in the scanning electron microscope (SEM) where the high sensitivity (counts per second per nanoampere of beam current) of the EDS was needed because most existingoinstruments were designed for optimum resolution (
