Urea measurement by X-ray microanalysis in 50 picoliter specimens

摘要

Studies of the renal concentrating mechanism require a method for defining the concentration of both urea and electrolytes in very small fluid specimens obtained from collecting ducts. Urea in such samples has been measured in the range from 20 to 100 pmoles using a fluorescence technique [1], and from 100 to 500 pmoles by absorption spectrophotometry [2]. These methods require several nanoliters of fluid and many handling steps. The electron probe microanalyzer can measure electrolyte composition in samples smaller than 10-10 liters [3–5], but because it measures only elemental composition, it has not been useful for defining organic chemical composition. We here describe a new method based on the precipitation of urea by thioxanthen-9-ol from microdroplets in the 50-pl range.