X-ray analysis of biological fluids: contribution of microdroplet technique to biology

Electron probe analysis by x-ray spectrometry is used in biology for simultaneous determination of the concentrations of any elements with a higher atomic number than that of carbon in single samples with volumes of 0.01 to 0.5 nl. In this technique, deposits prepared from identical volumes of biological fluids and standard solutions are totally covered by the electron beam, and the measured x-ray intensities for each element directly compared. The possibility of intensity quantification depends on the thinness of the dried deposits obtainable by various preparatory techniques. Factors affecting the accuracy of the results include droplet stability under the electron beam, the identity of the degree of oxidation of the elements in biological fluids and standards, sample mass thickness, beam voltage, and matrix effects. Minimum detectable concentrations in the 0.05 mmol.l-1 range are now achievable. This technique is the only one applicable in cases where available volumes are too small to determine the concentrations of several elements on the same sample (for instance of Na, Mg, S, P, Cl, K, Ca, Fe and Co), or even to determine the concentration of a single element (e.g. Mg). Although the droplet technique has so far mainly been used in renal physiology, it has also been applied in reproductive and digestive physiology. Isolated cells are analyzed according to the same principle of totally covering the cell by the electron beam. During the last decade, the vast increase in the relevant literature has testified to the contribution of the microdroplet technique to various fields of biology.