Nanobubbles in Electrospray Ionization Mass Spectrometry

Abstract

Nanobubbles (NBs) are very small gas-filled cavities in solvents, and when their sizes reach diameters around 200 nm, they remain in solution for extended periods of time, featuring special chemical and physical properties. Here, we investigate the application of NBs in electrospray ionization. We show that the addition of CO₂ or N₂ NBs into spray solvents significantly improves signal responses of small molecules in both positive and negative modes during ESI-MS. The magnitude of the increase depends on analyte identity, solvent system, NB gas composition, and the method for preparing the NBs. When NBs are used to analyze proteins, both signal intensities and charge states increase. This is tentatively attributed to NB's increasing the total area of the hydrophobic gas-liquid interface, on which proteins can unfold, and improved transport of analytes to the droplet surface. This increase in the interface is likely also a contributing factor in the further enhancement of the rate at which reagents are converted into products when NBs are present compared to those measured from accelerated reactions from microdroplets during reactive-ESI experiments. Moreover, NBs can overcome solubility barriers when one of the reagents is gaseous and, thus, can be incorporated into an NB. This was demonstrated in the reaction between N,N-dibutyl-1,3-propane diamine and carbon dioxide, where the reaction conversion rate could be significantly improved when N₂ NBs were present in solution, but even more so when the bubbles were composed of CO₂.

Figure 1

Representative spectra of (a) caffeine, (b) hydrocortisone, and (c) ibuprofen in 50% MeOH with CO₂ NBs (red traces) and control without CO₂ NBs (black traces). The intensities provided indicate the absolute signal intensity.

Figure 2

Average signal intensities of (a) caffeine with (orange traces) and without (blue traces) CO₂ NBs and (b) Rhodamine signal with increasing caffeine concentration with CO₂ NBs (orange trace) and without (blue trace) in a 50% MeOH solvent system.

Figure 3

Representative ESI-MS spectra of 10 μM cytochrome c in (a) 100% water and (b) 100% water containing CO₂ NBs. The normalization level (NL) indicates the absolute signal intensity.

Figure 4

Conversion ratio of 10 μM N,N-dibutyl-1,3-propane diamine (DBPA) in different solvent systems when using (a) N₂ as the nebulizing gas and (b) CO₂ as the nebulizing gas. Conversion ratio is calculated as the ratio of intensity of the product ion over that of the reagent ion.