Introduction

Determination with an electron probe of the elemental composition of liquid samples of ‘small’ volumes is not microanalysis sensu stricto. Indeed, these volumes are in the range of 10−10 to 10−11 litre, i.e. still several orders of magnitude larger than the 1 μm3 volume normally excited in conventional X-ray microanalysis. However, dealing with nanolitre volumes is a common situation for a biologist, who then faces the problem of determining a number of elements in so ‘small’ a volume. This explains why the technique of X-ray analysis of droplets has been so extensively used over the past two decades. This chapter will not focus on the technical details of sample preparation. Two recent review papers (Quamme, 1988; Roinel, 1988) have abundantly dealt with this problem. After a brief overview of the technique (sample preparation and characteristics, sensitivity, accuracy and precision), we will show how efficient the use of the technique has been in renal physiology, especially in vivo for the study of the handling of magnesium by the kidney and in vitro for studies involving the microperfusion of isolated tubules.

Although the droplet technique has so far mainly been used in investigations of the renal physiology of mammals, it has also been applied to the study of the excretory function of insects, and to the study of other physiological functions in man (Ferrary et al., 1988), mammals and various animals (see bibliographic reviews in Roinel & Rouffignac, 1982, and more recently Quamme, 1988).