Coceth sulfate characterization by electrospray ionization tandem mass spectrometry

Rationale: The anionic surfactants, among which are alkyl ether sulfates (AESs), are the most used class of surfactants in cleansing applications. The negatively charged head group of AESs is a sulfate moiety linked with a variable number of ethylene oxide units, i.e. a polyethylene glycol chain. The hydrophobic part of an AES is constituted by a linear alkyl chain of carbon atoms, generally obtained from natural fatty acids. Coconut oil fatty acids, including the sodium salts of coceth sulfate (CES) with chemical formula CxHy (OCH2CH2)nOSO3Na, are widely used as feedstock for AESs synthesis. CES is added to many cleaning products and detergents defined as non-aggressive. Currently, no detailed structural information concerning the alkyl chain length x and, more importantly, the degree of ethoxylation n has been reported. Methods: A commercial standard solution of CES was characterized by tandem mass spectrometry, employing direct injection into the electrospray ionization (ESI) source of a a linear quadrupole ion trap mass spectrometer. Results: Two series of oligomeric species, characterized by a C12 and C14 alkyl chains, i.e. [C12H25(OCH2CH2)nOSO3]− and [C14H29(OCH2CH2)nOSO3]− with n ranging from 0 to 7, were successfully identified. The interpretation of these data was very useful for CES identification in three commercial dishwasher cleaning products. Conclusions: Direct injection MS/MS analysis of CES revealed a well-defined molecular weight distribution and allowed the alkyl chain composition and the number of ethylene oxide units to be to identified.