Molecular distribution of some commercial nonylphenol ethoxylates using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Nonylphenol ethoxylates (NPEs) belong to a group of nonionic surfactants that are collectively referred to as alkylphenol ethoxylates (APEs). APEs find widespread use in heavy‐duty commercial and household cleaning formulations, shampoos, and industrial processing, i.e. textile manufacture. Their environmental impact depends on the molecular distribution and the extent of their biodegradation in municipal sewage systems, waterways and rivers. We have established two sample preparation methods that have enabled the determination of the molecular distributions of six commercial nonylphenol ethoxylates using matrix‐assisted laser desorption/ionization (MALDI) time‐of‐flight (TOF) mass spectrometry (MS). In both methods, α‐cyano‐4‐hydroxycinnamic acid, dissolved in acetonitrile/tetrahydrofuran, was used as the matrix. In one set of experiments, the NPEs were dissolved in an acetonitrile/tetrahydrofuran solvent system prior to mixing with the matrix solution, and the resulting MALDI‐TOF mass spectra produced mostly sodiated molecules [M + Na]+. The NPEs, all having the formula 4‐(C9H19)‐C6H4‐(OCH2CH2)n‐OH, are Surfonic ®N‐95, N‐100, N‐102, N‐120, N‐150 and N‐300. Surfonic N‐95 and N‐100 gave n values of 5–20; Surfonic N‐102, N‐120, N‐150, and N‐300 gave n values of 5–21, 5–22, 8–25 and 15–40, respectively. In order to develop a sample preparation method that could be used with less polar NPEs, we dissolved the NPEs (except N‐300) in pentane prior to mixing with the matrix solution, and found that the MALDI spectral quality was unaffected by the solvent systems. Copyright © 1999 John Wiley & Sons, Ltd.