Effects of Electrospray Droplet Size on Analyte Aggregation: Evidence for Serine Octamer in Solution

Spraying solutions of serine under a wide variety of conditions results in unusually abundant gaseous octamer clusters that exhibit significant homochiral specificity, but the extent to which these clusters exist in solution or are formed by clustering during droplet evaporation has been debated. Electrospray ionization emitters with tip sizes between 210 nm and 9.2 mu m were used to constrain the number of serine molecules that droplets initially contain. Protonated octamer was observed for all tip sizes with 10 mM serine solution, but the abundance decreases from 10% of the serine population at the largest tip size to similar to 5.6% for the two smallest tip sizes. At 100 mu M, the population abundance of the protonated serine octamer decreases from 1% to 0.6% from the largest to the smallest tip size, respectively. At 100 mu M, fewer than 10% of the initial droplets should contain even a single analyte molecule with 210 nm emitter tips. These results indicate that the majority of protonated octamer observed in mass spectra under previous conditions is formed by clustering inside the electrospray droplet, but <= 5.6% and similar to 0.6% of serine exists as an octamer complex in 10 mM and 100 mu M solutions, respectively. These results show that aggregation occurs in large droplets, but this aggregation can be eliminated using emitters with sufficiently small tips. Use of these emitters with small tips is advantageous for clearly distinguishing between species that exist in solution and species formed by clustering inside droplets as solvent evaporation occurs.

Automated summary

The majority of protonated octamer observed in mass spectra is formed by clustering inside electrospray droplets under specific conditions. Additionally, approximately 5.6% of serine exists as an octamer complex in 10 mM solution, while around 0.6% exists in 100 µM solution.