Strategies in using analytical restricted access media columns for the removal of humic acid interferences in the trace analysis of acidic herbicides in water samples by coupled column liquid chromatography with UV detection

In this comprehensive study, analytical restricted access media (RAM) columns were investigated to determine their performance in decreasing the humic acid interference encountered in the trace analysis of acidic herbicides in environmental water samples when employing reversed-phase liquid chromatography with UV detection (RPLC-UV). The selected analytical column RAM materials are 5-mu m Semi-Permeable-Surface (SPS, Regis), 5-mu m Internal Surface Reversed Phase (ISRP, Pinkerton, Regis), and 5-mu m Hisep (Supelco); a precolumn containing 25-mu m alkyl-diol-silica (ADS) RAM material (Merck) was also included in this study. Different LC configurations were investigated, including the single RAM column mode and column switching (LC/LC), employing one RAM column in combination with an analytical C18 column, or two RAM columns. Concerning methodology, both the single-residue method (SRM) approach and the multiresidue method (MRM) approaches were tested by analyzing SPE C18 extracts of reference water samples spiked with the analyte(s) at the 0.5-1.0 mu g/L level and containing dissolved organic carbon (DOC) between 3 and 12 mg/L Mecoprop (MCPP) was used as a model compound in SRM, and the more polar compounds metsulfuron methyl, bentazone, bromoxinyl, and MCPA were included to investigate the MRM approach. With regard to SRM using the LC/LC mode, the use of at least one analytical RAM column improved the resolution between the analyte and the humic interferences sufficiently to trace the analyte in extracts from water samples containing a high level of DOC (12 mg/L) without a preceding cleanup. In the MRM approach, ISRP/C18 was most favorable for the simultaneous analysis of the heterogeneous group of pesticides in water with a high DOC level. For samples with a medium DOC content (6 mg/L), C18/SPS using short columns (50 x 4.6 mm i.d.) and isocratic elution is an efficient alternative, The performance of the useful MRM procedures was tested by analyzing water samples containing 6 and 12 mg/L DOC and spiked with the analytes at the 0.5-1.0 mu g/L level. Mean recoveries (n = 5, each DOC level) ranged between 86 and 102%, with RSDs between 3 and 7.0%, The use of RAM columns in the different LC/LC configurations tested allows us to develop rapid, selective, and sensitive methods for the analysis of acidic herbicides, which are very adequate for screening purposes in environmental water samples.