Theoretical analysis and experimental evaluation of headspace in-drop derivatisation single-drop microextraction using aldehydes as model analytes

In-drop derivatisation single-drop microextraction approach can constitute, to a certain degree, a low-cost reasonable alternative to the well-known on-fibre solid-phase microextraction. The headspace mode integrates extraction, preconcentration and derivatisation into a single step from the headspace of a sample. In this study, two low-molecular-weight aldehydes are derivatised in a hanging drop containing 2,4,6-trichlorophenylhydrazine, in a headspace single-drop microextraction configuration system. The single organic drop, dispersed in gas phase, is well covered in this study as a locale of the main reaction. The measurement of diffusion and kinetic parameters and their relationship were designed to reveal, for the first time, inherent mechanistic aspects in such an analytical system. The two-film theory of mass transfer is used to discuss the mechanism along with the calculation of characteristic times and specific rates of absorption. All these, together with certain experimental data may ascertain whether the overall process is reaction rate dependent or limited by mass transfer in the gas phase, at the air–water and air–organic interface or in the organic phase. The descriptors of mass transfer and chemical reaction in a single drop are critically reviewed and reconsidered and the practical aspects for the analysis of volatile organic compounds are highlighted. Relative standard deviations for both aldehydes were 3.4% (n = 5) and 4.9% (n = 5) for 1 μM of hexanal and 0.3 μM of formaldehyde, respectively. Detection limits for aqueous samples were 0.1 and 0.03 μM for formaldehyde and hexanal, respectively.