Introduction

Perspective: directly deposited microdroplets

Electron probe X-ray microanalysis (EPXMA) is capable of yielding quantitative data from excited specimen volumes < 1 μm3. Thus, the analysis of fluid microvolumes in the range 10−11 to 10−10 litres is comfortably within the scope of this technique. For this reason, EPXMA was advocated at an early stage in its technological evolution for the analysis of nanolitre fluid volumes (Ingram & Hogben, 1967). Subsequently a number of laboratories have independently pursued physiological questions by the analyses of diverse biological fluids. This family of related microdroplet methods is described fully elsewhere in this volume by Roinel and Rouffignac (Chapter 11). However, in order that the sprayed microdroplet technique be seen in a realistic perspective a brief resumé of the more conventional ‘direct deposition’ microdroplet methods is provided here.

In general, two major variants of the microdroplet method are practised (Fig. 10.1).

1. 1. Microdroplets of known volume. Microdroplets of known volume are dispensed from calibrated micropipettes, mounted on polished solid (bulk) support materials, then dehydrated by freeze-drying to produce the all-important microcrystallites that minimise X-ray absorption. They are then individually analysed by wavelength-dispersive spectrometry (WDS).

2. 2. Microdroplets of unknown volume. Microdroplets of samples and standards of indeterminate volume are dispensed from constant-volume micropipettes, mounted on thin-film supports, then dehydrated by flash evaporation and subsequently analysed by energy-dispersive spectrometry (EDS). A selected list of the combinations of available options adopted by some of the major laboratories in this specialised field is presented in Table 10.1.