X-ray mapping is now an important qualitative and quantitative aspect of X-ray microanalysis of biological specimens, and has been the subject of a number of recent research papers and review articles (Chang, Shuman & Somlyo, 1986; Fiori, 1986; Fiori et al., 1988; Ingram et al., 1987; Lamvik et al., 1989; Sauberman & Heyman, 1987). The related technique of electron energy loss mapping has also been the subject of much recent interest (Jeanguillaume et al., 1983; Leapman & Ornberg, 1988) and is discussed separately by de Bruijn et al. in this volume.

X-ray mapping is the technique whereby an image is formed using the X-ray signal from a specified energy range, in order to show elemental distribution within the sample. X-ray mapping in its simplest form has been available since the early days of X-ray microanalysis. The technique has evolved in sophistication over the past 20 years, the main milestones being the development of digital mapping in the late 1970s and quantitative mapping in the late 1980s. The methods currently available are:

1. (a) analogue dot mapping,

2. (b) qualitative digital mapping,

3. (c) quantitative digital mapping.

Analogue dot mapping

This procedure was developed in the mid 1960s. Dot maps are formed when X-rays generated within a specified energy range appear as bright dots on the electron microscope display as the electron beam scans the sample. The advantage of the technique is that maps are very quick to set up and acquire.