1. Lecture 14
LC-MS
Ionization

2. GC-MS GC MS
Computer

3. LC-MS LC MS
Computer

4. MS Analysis MS
Gas phase
Ions
LC?

5. Condensed Phase Ionization
Electrospray Ionization (ESI)
Atmospheric Pressure Chemical Ionization (APCI)
Atmospheric Pressure Photoionization (APPI)
Matrix Assisted Laser Desorption/Ionization (MALDI)
Desorption Electrospray Ionization (DESI)
Direct Analysis in Real Time (DART)
Liquid phase
(LC)
Solid phase

6. Difference? PCBs PFOS Production started 1920s Env Detection 1968
2001

7. MS Analysis GC-MS and LC-MS ionization are very different
GC – Producing ions from gas phase analytes
Producing gas phase compounds from charged analytes
LC –
Necessary for certain important analytes:
Biological molecules:
Proteins
Amino acids/nucleic acids
sugars
Strong acids without the ability to absorption UV-vis

8. Electrospray Ionization
Problem solved…
Electrospray Ionization
(ESI)
John Fenn

9. Electrospray Ionization (ESI)LC
ESI Key Features:
- Capillary (LC) 90o to Cone (MS)
Capillary
Ionization
Chamber
- Atmospheric pressure ion source
- Nebulized flow through capillary
- Heat applied to evaporate solvent
Heater
Gas
- Voltage difference applied between capillary and cone
Nebulized
Spray + MS
Sample
Cone
- Detect (+) ions  Cap (+), Cone (-)
- Detect (-) ions  Cap (-), Cone (+) -
10-6 torr
What happens in the droplet?
760 torr

10. Electrospray Ionization (ESI)+ + +
Interested in Positive Ions
- Apply (+) Voltage to capillary
- (+) ions repelled from capillary
- (-) ions attracted to capillary
- Results in droplets enriched in (+) ions + + +
P. Kebarle, J. Mass Spectrom. 2000, 35, 804–817.

11. Electrospray Ionization (ESI)
Summary
Ions are formed in condensed phase
In mobile phase droplets
Ions move into the gas phase by two mechanisms:
Partition into gas phase due to columbic repulsion in droplet
Solvent in droplet evaporates leaving ion behind
Works well for compounds that can be charged in the condensed phase
Strong or weak
Acids and Bases

12. Atmospheric Pressure Chemical Ionization (APCI)
APCI Key Features: LC
- Capillary (LC) 90o to Cone (MS)
- Atmospheric pressure ion source
Capillary
Ionization
Chamber
- Nebulized flow through capillary
- Heat applied to evaporate solvent
- No voltage applied to capillary
- droplets are not charged
Heater
Gas
Nebulized
Spray
- Voltage still applied to cone
- Detect (-) ions  Cone (+)
- Detect (+) ions Cone (-)
Sample
Cone MS -
- Heat evaporates solvent, analytes partition into the gas-phase uncharged
10-6 torr
Corona discharge needle
760 torr
- Need to make ions!

13. Atmospheric Pressure Chemical Ionization (APCI)
Summary
First step is solvent evaporation
Compounds in gas-phase as neutrals
Ions produced in the gas-phase
Corona discharge ionizes carrier gas and mobile phase
Acid/base reactions in the gas phase with mobile phase
Works well for compounds that are protic but neutral at a pH between 2 and 9
Weak
Acids / Bases

14. Atmospheric Pressure Photoionization (APPI)
APPI Key Features (same as APCI): LC
- Capillary (LC) 90o to Cone (MS)
- Atmospheric pressure ion source
Capillary
Ionization
Chamber
- Nebulized flow through capillary
- Heat applied to evaporate solvent
- No voltage applied to capillary
- droplets are not charged
Heater
Gas
Nebulized
Spray
- Voltage still applied to cone
- Detect (-) ions  Cone (+)
- Detect (+) ions Cone (-)
Sample
Cone MS -
UV lamp
- Heat evaporates solvent, analytes partition into the gas-phase uncharged
10-6 torr
760 torr
- Need to make ions!

16. Atmospheric Pressure Photoionization (APPI)
Summary
First step is solvent evaporation
Compounds in gas-phase as neutrals
Ions produced in the gas-phase
UV lamp ionizes compound by photon absorption or
Charge exchange with dopant or
Dopant ionizes mobile phase, followed by;
Acid/base reactions in the gas phase with mobile phase
Works well for compounds that are absorb UV radiation or are protic but neutral at a pH between 2 and 9
Weak Acids / Bases
Nonpolar compounds that are UV active

17. Summary LC-MS Ionization
m/z
(MW)

18. Matrix-Assisted Laser Desorption/Ionization (MALDI)MS
MALDI Key Features:
Vacuum
- Analyte deposited on surface with excess matrix compound + -
- Shine laser on the surface
- hn aborbed by matrix molecules
- Matrix molecules transfer energy to analyte molecules
- Results in desorption/ionization of both matrix and analyte
Laser beam
Analyte molecule
Matrix molecule
Adapted from Dass, C. Fundamentals of Contemporary Mass Spectrometry, Wiley-Interscience, 2007.

19. Matrix-Assisted Laser Desorption/Ionization (MALDI)
Common dopants:
Analytes of interest:
Proteins
Polymers
High MW, ionization not possible using ESI, APCI, APPI
High MW separates these compounds from the background signal of the matrix

20. Desorption Electrospray Ionization (DESI)
Z. Takáts et al. Science 2004, 306,

21. Direct Analysis in Real Time (DART)