A Method for Trace Analysis with an Electron Microprobe
A “chopped beam” system of analysis has been devised for a scanning electron microprobe equipped with a 400-channel analyzer. The system has been programed with circuitry which places a signal onto the X or Y scanning coils of the probe, so that the beam jumps back and forth between two analytical areas. The same signal is used to activate alternate halves or quadrants on the 400-channel analyzer in a synchronous manner. The analyzer accumulates X-ray intensity data in the appropriate halves and quadrants as the electron beam oscillates between sample and standard or, in the case of trace analysis, between the unknown and the pure major constituent for background correction. The probe may be left unattended while it is gathering information in this manner. The dwell time of the probe on a given analytical area is 6 sec “live” time.
Errors due to instrumental drift and sample contamination are nullified or minimized by this technique. Consequently, theoretical precision is closely approached for extended counting times. 100 ppm levels of aluminum, silicon, nickel, and iron in uranium have been determined to precisions as good as ± 10 ppm at the 95% confidence level.
KeywordsTrace Analysis Live Time Carbon Contamination Instrumental Drift Digital Printer
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