Novel stepwise gradient reversed-phase liquid chromatography separations of humic substances, air particulate humic-like substances and lignins

RP-HPLC using stepwise gradients of dimethylformamide (DMF) in buffered aqueous mobile phase and a wide-pore (30 nm) octadecylsilica column has been applied to the analysis of soil, peat and air particulate humic and humic-like substances (HSs), as well as lignin, in order to demonstrate the usefulness of the approach for their characterization even at trace concentration level. Tandem combination of spectrophotometric (DAD) and fluorimetric detection was used to get more detailed information on chromatographic behaviour of HSs. The results showed that ten-step gradient can induce distinct features of HSs and lignins. Combination of very good DMF solvating and disaggregating properties for HSs and lignins together with wide pore RP sorbent improves surface interactions of the analytes and suppresses influence of size exclusion effects. Thus it provides reproducibility of characterisation profiles and robustness of the method. Very good reproducibility of retention times (from ±0.12 to ±0.36% RSD), of peaks enforced by the step gradient shape supports well defined characterization and/or fractionation of HSs. Evaluated limit of quantitation (S/N=10) of selected soil humic acid working standard using fluorimetric detection (ex.470 nm/em.530 nm) was 9.7 μg/ml. This corresponds to 0.5 μg of the humic acid per injection. Calculated limit of detection (S/N=3) was 3.3 μg/ml, what corresponds to 0.15 μg of the humic acid per injection and enabled determination of 0.1% (m/m) humic-like substances in 20 mg of air particulates under defined rules and conditions.