Analytica Chimica Acta 531 (2005) 217–228 Adduct formation in LC–ESI–MS of nonylphenol ethoxylates: mass spectrometrical, theoretical and quantitative analytical aspects Niels Jonkers, Harrie Govers, Pim De Voogt ∗ Department of Environmental and Toxicological Chemistry, Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands Received 26 May 2004; received in revised form 13 October 2004; accepted 13 October 2004 Available online 24 December 2004 Abstract The analysis of nonylphenol ethoxylate (A 9 PEO n ) 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 × A 9 PEO 1,2 + Na] + , adducts including a solvent molecule [A 9 PEO 1,2 + MeOH + Na] + and doubly charged adducts [A 9 PEO >11 +2 × Na] 2+ were found. Molecular dynamics calculations showed that the A 9 PEO n 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 × A 9 PEO 2,5 + Na] + dimer and [A 9 PEO 2,5 + Na] + monomer adducts turned out to be stable from an energetic point of view with adducts of A 9 PEO 5 being more stable than adducts of A 9 PEO 2 . Only for the monomer adduct the latter is in accordance with experimental observations. Consequences of the formation of several adducts per A 9 PEO n oligomer for the quantitative analysis of environmental samples were evaluated. In clean samples, it was found that the presence of short-chain A 9 PEO 1,2 can cause an overestimation of long-chain A 9 PEO >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 [A 9 PEO 1,2 + MeOH + Na] + adduct signals does improve the detection limits of the two short-chain oligomers. Correct quantitative results are obtained when A 9 PEO 1 and A 9 PEO 2 are quantified separately, and longer oligomers with a molar calibration followed by correction of the average molar weight of the A 9 PEO >2 in the sample. © 2004 Elsevier B.V. All rights reserved. Keywords: Surfactants; Nonylphenol ethoxylates; Adduct formation; Liquid chromatography–mass spectrometry; Molecular dynamics; Quantification 1. Introduction Liquid chromatography–mass spectrometry (LC–MS) has in recent years become the preferred analytical instrument for analyzing relatively polar organic molecules. The reasons for this development are the high amount of information and low detection limits which can be obtained with this technique, as well as decreasing prices of the instrumentation. An impor- ∗ Corresponding author. Tel.: +31 20 525 6565; fax: +31 20 525 6522. E-mail address: pdevoogt@science.uva.nl (P. De Voogt). tant additional advantage of LC–MS compared to GC–MS is that there is no need to derivatize the often non-volatile analytes in LC–MS applications. For non-ionic surfactants, the trend is no different, and it is becoming increasingly difficult to find any recent pub- lication in which LC–MS has not been applied. Alkylphe- nol ethoxylates (APEO n ) are one of the principal groups of non-ionic surfactants, and their environmental relevance lies in the aquatic toxicity, in the estrogenic activity caused by some of their metabolites, and in the large amounts in which APEO n are still being produced. APEO n consist of 0003-2670/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2004.10.031