Immunoaffinity column clean-up and thin layer chromatography for determination of ochratoxin A in green coffee

Abstract

An immunoaffinity clean-up-based method for determining ochratoxin A (OTA) in green coffee aiming at one-dimensional thin layer chromatography (TLC) analysis was established. OTA was extracted with a mixture of methanol and aqueous sodium hydrogen carbonate solution, purified through an immunoaffinity column, separated on normal or reversed-phase (RP) TLC plates and detected and quantified by visual and densitometric analysis. The linear equation of the standard calibration curve by densitometric analysis gave R2 > 0.999 (0.04–84 ng). The mean recovery (R) of OTA from spiked samples (1.8–109 µg kg−1) by densitometric and visual analyses were 98.4 and 103.8%, respectively. The relative standard deviations (RSD) for densitometric and visual analysis varied from 1.1 to 24.9% and from 0.0 to 18.8%, respectively. The RSD for naturally contaminated samples by densitometry (three levels of contamination, n = 3) varied from 11.1 to 18.1%. The correlation (R2) between high-performance liquid chromatography (HPLC) and densitometry, and between visual and densitometric analysis for spiked samples were > 0.99. The limit of detection (LOD) of the method was 0.5 µg kg−1 for normal TLC. Toluene-ethyl acetate-88% formic acid (6:3:1 v/v/v) and acetonitrile-methanol-water-glacial acetic acid (35:35:29:10 v/v/v/v) were regarded as the suitable TLC solvents for eluting both standards and samples on normal and RP TLC plates, respectively. Toluene-acetic acid (99:1 v/v) was chosen as the spotting solvent among several others for giving the best sensitivity and resolution of OTA on TLC plates as well as the best recovery of OTA from standard and sample extract residues. Preliminary studies were carried out to investigate the reuse of the immunoaffinity column and the interference of caffeine in the OTA recovery.