Determination of double bond positions in methyl ketones by gas chromatography–mass spectrometry using dimethyl disulfide derivatives

Abstract

Rationale: Methyl ketones are of interest for the application as biofuels. The fatty acid metabolism of different microbes has been rearranged to achieve a sustainable production of methyl ketones. The biofuel properties and possible further chemical modifications of these methyl ketones are influenced by their chain length, as well as their degree of saturation and the corresponding double bond position.Methods: A method based on gas chromatography-electron ionization; mass spectrometry (GC-EI-MS) was used to determine the double bond position of methyl ketones. Derivatization using dimethyl disulfide (DMDS) and an iodine catalyst enabled the activation of the double bonds for selective fragmentation during electron ionization. The cleavage led to key fragments in the Orbitrap high-resolution mass spectrum and allowed the unequivocal localization of the double bond position of the respective monounsaturated methyl ketone. Results:The double bond position of several medium chain length methyl ketones originating from the gram-negative bacterium Pseudomonas taiwanensis (P. taiwanensis) VLB120 was determined. The DMDS derivatives of methyl ketones can yield isobaric fragment ions for different possible double bond positions, which can be distinguished only using high-resolution MS. The double bond position of all methyl ketones deriving from P. taiwanensis VLB120 was the same, counting from the end of the aliphatic chain, and was determined as ω-7. Conclusions:The derivatization of medium chain length monounsaturated methyl ketones with DMDS allowed the determination of the corresponding double bond position via GC-EI-MS. High-resolution MS is needed to differentiate possible double bond positions that yield isobaric fragment ions. | INTRODUCTIONMethyl ketones are used in the fragrance, flavor, pharmacological, and agrochemical industries. The industrial production of these compounds is based on hydrocarbons derived from petroleum. To achieve a sustainable production of methyl ketones, intense research has been carried out to rearrange the fatty acid metabolism of different microbes. [1][2][3][4][5][6][7][8] In this regard, fatty acid-derived methyl ketones are of interest for the application as biofuels. [1][2][3][4] The biofuel properties and possible further chemical modifications of these methyl ketones are influenced by their chain length, as well as their degree of unsaturation and the corresponding double bond position. 9

Methyl ketones: a comprehensive study of a novel biofuel

“…Blend A contained the saturated methyl ketones with a chain length of C 11 to C 15 , while blend B also contained monounsaturated methyl ketones. 88 The EC 50 and LOEC values for blend A were 2.12 mg L −1 and 1.50 mg L −1 , respectively. The EC 50 and LOEC values for blend B were 1.65 mg L −1 and 1.20 mg L −1 , respectively (Table 3).…”