Adduct formation in LC–ESI–MS of nonylphenol ethoxylates: mass spectrometrical, theoretical and quantitative analytical aspects Original Research Article

The analysis of nonylphenol ethoxylate (A9PEOn) surfactants with LC–ESI–MS was investigated in a detailed study of the formation of different types of adducts. Part of the observations was explained by calculating their relative stabilities using molecular dynamics techniques. Strong differences in adduct formation behaviour were found for different oligomers. Beside the common sodium adducts, surfactant dimer adducts [2 × A9PEO1,2 + Na]+, adducts including a solvent molecule [A9PEO1,2 + MeOH + Na]+ and doubly charged adducts [A9PEO>11 + 2 × Na]2+ were found. Molecular dynamics calculations showed that the A9PEOn molecule wraps itself around the complexing sodium ion in a way that negative electronic charges on oxygen have optimum electrostatic interaction with this ion. van der Waals interactions between alkyl chains are of less importance for the stability of these adducts. Both [2 × A9PEO2,5 + Na]+ dimer and [A9PEO2,5 + Na]+ monomer adducts turned out to be stable from an energetic point of view with adducts of A9PEO5 being more stable than adducts of A9PEO2. Only for the monomer adduct the latter is in accordance with experimental observations. Consequences of the formation of several adducts per A9PEOn oligomer for the quantitative analysis of environmental samples were evaluated. In clean samples, it was found that the presence of short-chain A9PEO1,2 can cause an overestimation of long-chain A9PEO>2. In real environmental extracts, other processes like matrix effects have a stronger influence on the quantitative result, and therefore no significant influence of adduct formation processes could be observed. However, inclusion of [A9PEO1,2 + MeOH + Na]+ adduct signals does improve the detection limits of the two short-chain oligomers. Correct quantitative results are obtained when A9PEO1 and A9PEO2 are quantified separately, and longer oligomers with a molar calibration followed by correction of the average molar weight of the A9PEO>2 in the sample.