Thermospray (TSP) mass spectra and chromatographic characteristics of the prostanoids PGF,, , Cketo-PGF,, , 2,3-dinor-6-keto-PGF1, and their methyl esters and methoximes are reported. The spectra of these compounds are dominated by ions of the type [ M + H + xNH, -nH,O] + in the positive-ion mode and [M -H + xAcOH -nH,O]in the negative-ion mode (n ranges from 0-4 and x is 0 or 1). The relative abundances of these ions depend on the particular structure, source and interface temperatures and eluent composition. In addition, these compounds show losses of 44 mass units, mainly due to the loss of the C-1O-C-11 moiety, as demonstrated by deuterium labelling of PGF,, . Methoximated derivatives of Cketo-PGF,, also show low-abundance ions due to fragmentation in the position a to the methoxime group. The liquid-phase equilibria of tautomers of 2,3-dinor-6keto-PGF,, or products thereof which direct its particular chromatographic behavior, can be effectively blocked by methoximation of the keto group thus improving its detectability. Detection limits of 20 ng on-column may be obtained for this derivative. Adsorption and hydrolysis in the interface are the principal factors responsible for this relatively high detection limit.