Manipulation of electrospray generated droplets using various vapors

Volatile reagents can be entrained in desolvation gas to manipulate electrospray generated droplets. These reagent vapors are admitted into the atmospheric interface region of a QqTOF tandem mass spectrometer. The electrospray droplets interact with the reagent vapors between the curtain and orifice plate for approximately a millisecond, which is the time required for analyte desolvation. A variety of vapors, particularly polar aprotic vapors, can be used to reduce alkali metal adduction on proteins. This effect is due to the alkali metals interacting with the vapor to form alkali metal adducted reagent vapor cluster ions that are ionized via the ion evaporation model, which reduces the amount of alkali metal in the electrospray droplet available to adduct to proteins that are ionized via the charge residue model. Polar vapors can be used to stabilize noncovalent protein complexes via evaporative cooling of the analyte of interest and lengthening the electrospray ionization desolvation process. These vapors interact with peptide containing ESI droplets to form peptide ions that are adducted to the respective vapor that was doped into the curtain gas. Additionally, these vapors interact with ESI droplets to form unique doubly charged cluster ions of the form [nX+H+Y] 2+ where X is a reagent vapor molecule, n = 5-6 and Y is a sodium or potassium ion. Acidic and basic vapors can also be introduced into the desolvation gas to alter the ESI response of analytes derived from a mixture in the positive and negative polarities. In general, in the positive polarity, the ESI response of acidic bioanalytes in a mixture is enhanced by introducing acidic vapor into the curtain gas. In the negative polarity, the ESI response of basic analytes in a mixture is enhanced by doping basic vapor into the curtain gas. The ESI response is most universal when acidic vapor is entrained in the curtain gas in the positive polarity and when basic vapor is entrained in the curtain gas in the negative polarity. Acidic and basic vapors shift the ESI selectivity via changing the droplet pH during the ESI process. The main variables in the ability of acidic or basic vapors to enhance ESI response for analytes depends on the ionization polarity, the analyte isoelectric point, and the reagent vapor pKa and vapor pressure. For protein mixtures, the propensity of proteins to ionize via the chain ejection model versus the charge residue model may also influence the ESI response observed upon exposing the electrospray droplets to acidic or basic vapors