- iii - LBL - 647 TRACE ELEMENT ANALYSIS WITH SEMICONDUCTOR DETECTOR X - RAY SPECTROMETERS *

A method of obtaining high sensitivity and precision in x-ray fluorescence analysis using semiconductor detector spectrometers is discussed. Monoenergetic exciting radiation is employed to generate characteristic x-rays from trace elements in thin, uniform specimens. Corrections for absorption effects are determined; enhancement effects are omitted as they are negligible for many thin specimens. A single element thin-film standard is used to calibrate for the x-ray geometry, and theoretical cross sections and fluorescent yield data are employed to relate the x-ray yields for a wide range of elements to the thin-film standard. Various corrections which affect the accuracy of the method are discussed including the method for determining x-ray spectral background. Results obtained in the analyses of biological and geological specimens, and of air particulate filters are reported. Using a single excitation energy, the concentrations of more than fifteen trace elements may be simultaneously determined during a fifteen minute interval for concentrations of 1 ppm or less. This corresponds to less than 10 ng/cm 2 on air particulate filters. INTRODUCTION The analytical technique of x-ray emission spectroscopy depends upon the ability to excite and accurately measure characteristic K and L x-rays emanating from the specimen. Prior to 1966, energy separation was usually achieved by using wavelength dispersive spectrometers. Since the first utilization of semiconductor detectors for x-ray spectrometry (1), major advancements have been made. Progress in electronic design over the past several years has significantly improved the energy resolution and count rate performance of semiconductor detector x-ray spectrometers (2,3,4). More recently, the advent of guard-ring detectors (5) has drastically reduced x-ray spectrum background resulting from the degradation of signals due to incomplete charge collection in the detectors. As a result of these improvements, semiconductor detectors are now applicable to many analytical problems, including trace element analyses. Some of the principal attributes of semiconductor detector spectrometers for energy dispersive analysis are: (1) simultaneous detection of a wide range of energies allowing the intensities of many characteristic x-rays and their respective spectrum backgrounds to be determined together; (2) interelement interference due to overlapping x-ray lines are clearly shown; (3) compact excitation radiation-specimen-detector geometries are permitted, minimizing the required intensity of excitation radiation; (4) high detection efficiencies over a wide energy range; and (5) no requirements for moving parts or mechanical alignments. The energy resolution capabilities of semiconductor detector spectrometers are more than sufficient for most analytical applications, although x-ray spectrometers using crystals for energy dispersion provide somewhat higher