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

2. 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

3. 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

4. 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)

5. 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

6. 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

7. 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)

8. 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

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

10. 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, (2002).

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

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

13. 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 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

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

15. Close-up of Droplet Formation at the Needle Tip

16. Electrospray Process Viewed Through a High Resolution Microscope With Increasing Voltages at the Needle Tip

17. 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

18. Example Applications

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

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

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

22. 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

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

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

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

26. Thank-you
Questions/Comments?