LIQUID-CHROMATOGRAPHY PARTICLE-BEAM MASS-SPECTROMETRIC DETERMINATION OF BETA-CAROTENE BY NEGATIVE-ION CHEMICAL-IONIZATION

The first method for liquid chromatography-particle-beam mass spectrom etry (LC-PBMS) of beta-carotene is described. It uses a reversed-phase (RP) HPLC column and methanol-dichloromethane, 90:10, as mobile phase . For comparison purposes, UV-visible detection was performed at 436 n m to determine linearity, sensitivity and precision; the absolute limi t of detection of the LC-UV method was estimated to be 0.2 ng. The pos sible modes of ionization of beta-carotene were explored for PBMS dete ction; the best selectivity and sensitivity were obtained by use of el ectron-capture negative-ion chemical ionization (NCI). When operating in NCI mode with selected ion monitoring (SIM), the detection limit wa s 0.3 ng for beta-carotene, comparable with that of the UV detector. T he relative standard deviation (RSD) of the LC-PBMS system was 2.6 % a t the 10 ng level (n = 10). For the determination of beta-carotene in real samples absolute calibration and calibration by the method of sta ndard addition were used for both UV and PBMS detection.