Lecithin---sphingomyelin ratio in amniotic fluid, as assessed by a modified thin-layer chromatographic method in which a commercial pre-coated plate is used.
Commercially available pre-coated thin-layer chromatographic plates consisting of silica gel supported on aluminum sheets were used to determine the lecithin-sphingomyelin ration in samples of amniotic fluid. These lipids were separated in chloroform-methanol-water (65:25:4 by vol) solvent system (lecithin Rf = 0.33; sphingomyelin, Rf = 0.25), and the spots were made visible in 5-15 min by heating to 180 degrees C after spraying with a solution of copper acetate (30 g/liter) in aqueous phosphoric acid (80 g/liter). The lipids were measured densitometrically after "peeling off" the adsorbent containing the spots by using a transparent cellulose ("Scotch brand") adhesive tape. Virtues of the original Gluck procedure are retained without need to heat the plates to 280 degrees C after spraying with corrosive aqueous sulfuric acid (698 g/liter) to make the lipids visible. Results for synthetic mixtures in concentrations within the physiological range and with a ratio of 0.5 to 3.0 showed an accuracy of 99% pluss or minus 7% (SD.).