The use of an energy dispersive spectrometer (EDS) X-ray analyser on a standard scanning electron microscope (SEM) is described and evaluated for the elemental analysis of ultramicrosamples on the order of 10-10 1. volumes of biological fluids. The method incorporates several modifications in the preparation of ultramicro fluid samples as previously described for diffractive microprobe X-ray analysis which extend the sensitivity of EDS quantitation and facilitate the routine preparation of such samples. The method provides quantitation of most elements of biological interest present in concentrations of about 1mM or more, and demonstrates good linearity (r > 0.99) throughout a wide range of commonly encountered biological concentrations. When not limited statistically by count rate, the reproducibility of the analysis is on the order of 2%. In the presence of 150mM NaCl, the minimal determinable concentration (MDC) is between 0.5mM and 1.0mM for S, P, K, and Ca and slightly more than 1.0mM for Mg. The MDC for all elements is approximately 0.3 weight percent of total desiccated sample solute. The validity of the method was verified by comparing results of EDS analysis of diluted and undiluted samples of sweat, saliva, and urine with the results of analyses of the same samples performed by conventional methods. Except for C1 in concentrated urines and Mg in low concentrations, good agreement was found between the methods. © 1978.
