When using cells of mammmalian origin, the amount of biological material available for analytical purposes is often limited (e.g., ~l0⁵ cells/sample, cor responding to few mg tissue). Since most intermediary metabolites are found in a concentratron range of ~50-500 μmol/g in a wide variety of tissues (see, e.g., ref. 1), only nanomolar or subnanomolar quantities can be expected to be present in small samples obtained from isolated cells or cell cultures. Such quantities are at best barely detectable by conventional spectrophotometric or fluorimetric procedures