ESI-MS: Electrospray Ionization Mass Spectrometry Jason Russell USD - Chem. 730 Dec. 4th, 2006

Presentation Overview What is ESI-MS? Features of ESI-MS Brief history of the origins of ESI How ESI-MS works Two theories of gas phase ion formation from liquid droplets Example Applications

What is Electrospray Ionization Mass Spectrometry (ESI-MS)? The technique of electrospray transfers ions present in solution into the gas phase Gas phase ions can then be analyzed in a mass spec The mechanism(s) of ionization will be addressed later

Features of ESI-MS Can produce multiply charged molecules, decreasing m/z allowing the mass determination of up to 10^7 g/mol Considered a “soft” ionization method Can preserve solution phase noncovalent interactions in the gas phase Can study 3-D molecular conformations Can obtain the molecular mass Conditions can be adjusted to promote fragmentation usable for molecule identification (usually by using a triple-quad MS)

Feature of ESI-MS (cont.) Samples are introduced in solution and at atmospheric pressure Can be interfaced with solution-based separation methods HPLC/UPLC CE Two basic modes of operation Positive mode – usually for basic molecules Negative mode – usually for acidic molecules

Origins of ESI Electrospray – well known phenomenon since the 1800’s John Zeleney (Univ. Minnesota, Yale) Early 1900’s: Able to see that that charged droplets broke apart after the solvent had evaporated Confirmed the prediction made by Rayleigh in the 1800’s, known as the Rayleigh limit Coulombic repulsion at a droplet surface would reach a critical value overcoming the surface tension Result was smaller daughter droplets

Origins of ESI (cont.) Malcolm Dole (Northwestern, Baylor) Pioneered the work that laid the foundation for what is now known as ESI-MS Theorized that continued Rayleigh Limit occurrences in charged droplets containing multiple molecules would eventually yield a droplet containing a single charged molecule Result was a gas phase ionized molecule 1968, paper describing the ESI technique using polystyrene and a magnetic sector MS Developed the charge residue model (CRM)

Origins of ESI (cont.) John Fenn et. al., Built upon the work of Dole Countercurrent desolvating gas Experimented with smaller molecules and used a quadrupole MS Successfully mated ESI-MS with LC

How ESI-MS Works There have been several modifications from the original design of Fenn et. al., but the general concept remains the same

J. Fenn. J. Biomolecular Techniques. 13, 101-118(2002). Diagram of a Second-generation ESI-MS Apparatus from Fenn’s Lab (Late 1980’s) J. Fenn. J. Biomolecular Techniques. 13, 101-118(2002).

N. Cech & B. Enke. Mass Spec. Rev. 20,6, 362-387(2001).

P. Kebarle, J. Mass Spec. 35, 804-817(2000).

Droplet fission may occur with the ejection of up to 20 or more daughter droplets Radius = 1/10 of parent droplet Can be observed with when r > 1um Daughter droplets can be produced with r = 10 nm in 100-500 us Daughter droplets carry away 2% of the parent mass, but 15% of the charge R. Cole. “Electrospray Ionization Mass Spectrometry: Fundamentals, Instrumentation, and Applications”. John Wiley and Sons, Inc. 1997

P. Kebarle, J. Mass Spec. 35, 804-817(2000).

Close-up of Droplet Formation at the Needle Tip http://www.newobjective.com/electrospray/index.html

Electrospray Process Viewed Through a High Resolution Microscope With Increasing Voltages at the Needle Tip http://www.newobjective.com/electrospray/spray_anim.html

Theories of Ion Transfer to the Gas Phase Charge Residue Model (M. Dole) Ion Evaporation Model (Iribarne and Thomson) With very small radii (< 1nm), the two processes are currently indistinguishable The IEM model is viewed as the most likely mechanism, although there is experimental evidence to support both

Example Applications

Very Large Mw Molecules J. Fenn & T. Nohmi. J. Am. Chem. Soc. 114, 3241-3246(1992)

N. Cech & B. Enke. Mass Spec. Rev. 20,6, 362-387(2001). Proteins (Myglobin) N. Cech & B. Enke. Mass Spec. Rev. 20,6, 362-387(2001).

Multiple Charging and Deconvolution – Bovine Serum Albumin R. BakhTiar & F. Tse. Mutagenesis. 15(5), 415-430(2000).

Estimation of protein surface areas in Solution Determination of dissociation constants for protein-ligand complexes Relating polarity character of peptides and small proteins by their ESI response Amino acid analysis

CE-ESI-MS Amino Acid Determination T. Soga & D. Heiger. Anal. Chem. 72, 1236-1241(2000)

Monitoring Electrochemical Reactions G. Van Berkel & K. Asano. Anal. Chem. 76, 1493-1499(2004)

G. Van Berkel & K. Asano. Anal. Chem. 76, 1493-1499(2004)

Thank-you Questions/Comments?