Using aromatic polyamines with high proton affinity as “proton sponge” dopants for electrospray ionisation mass spectrometry

Proton sponges are polyamines with high proton affinity that enable gentle deprotonation of even mildly acidic compounds. In this study, the concept of proton sponges as signal enhancing dopants for electrospray ionisation is presented for the first time. 1,8-Bis(dimethylamino)naphthalene (DMAN) and 1,8-bis(tetramethylguanidino)naphthalene (TMGN) were chosen as dopants, using methanol and acetonitrile/methanol as solvents. Individual standard compounds, compound mixtures and a diesel fuel as a complex sample matrix were investigated. Both proton sponges enhanced signal intensities in electrospray ionisation negative mode, but TMGN decomposed rapidly in methanolic solution. Significantly higher signals were only achieved using the acetonitrile/methanol mixture. On average a more than 10-fold higher signal intensity was measured with 10⁻³mol l⁻¹ DMAN concentration. A stronger signal increase of alcohol functionalities was observed compared to acid functionalities. All compound classes which were detected in the diesel fuel (CH– and CHO_x–class) received roughly 100-fold higher signal intensities when using DMAN as a dopant. Furthermore, the number of detected compounds as well as the double bond equivalent of the detected compounds increased. The compound class distribution shifted when adding DMAN and the formerly dominant CHO₂–, CHO₃–, and CHO₄– classes received similar relative intensities as formerly less accessible classes. The findings depict DMAN as a promising additive for electrospray ionisation negative analysis of at least mildly acidic compounds, even within complex sample material.