i'm analyzing fatty acids (FA),c2-c18, in ethanol and aqueous/ethanol solutions, at various FA concentrations. i'm getting an interesting phenomenon when the concentrations of the FA are about 10-20ppm or less (at any dilution, the concentrations of all FA are about the same). the peaks of the first run of a 4 or 5 sequence (same sample) run are up to an order of magnitude greater than the peaks of the second run; the peaks of the subsequent runs of the same sample are either about the same or less than the peaks of the second run. this phenomenon is more pronounced at the higher molecular weights. any thoughts about this?