[en] Tagitinin C, an antiplasmodial compound, identified as one major compound of the subtropical medicinal plant, Tithonia diversifolia, was, determined by FT-IR spectroscopy method. The crude ether extracts from aerial parts of the plant were evaporated to dryness and re-dissolved in tetrachloroethylene (C2Cl4) before analysis. The magnitude of the absorbance of the very specific C=O stretching vibration (v(c=o)) at 1664.8 cm(-1) was exploited in order to quantify tagitinin C. The determination coefficient (r(2)) of the calibration scale was 0.9994, the detection limit was lower than 3 mug ml(-1) and the quantification limit was lower than 10 mug ml(-1). Recovery values from 100.5 to 101.7% were found for spiked concentration levels from 19.91 to 89.95 mug ml(-1). The main characteristics of the curves obtained from the calibration standards and from the standard addition technique were not statistically different (Student t-test) suggesting that matrix effects were negligible. The results obtained for the determination of tagitinin C in the crude ether extract from aerial parts of T diversifolia by LC and FT-IR spectroscopic method agreed well: 0.76 +/- 0.02 and 0.773 +/- 0.009, of tagifinin C in dried plant respectively. (C) 2003 Elsevier B.V. All rights reserved.