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Lecture 14 LC-MS Ionization GC-MS GC MS Computer LC-MS LC MS Computer LC? MS Analysis MS Gas phase Ions Condensed Phase Ionization Electrospray Ionization (ESI) Atmospheric Pressure Chemical Ionization (APCI) Atmospheric Pressure Photoionization (APPI)

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Lecture 14

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Lecture 14 l.jpg

Lecture 14

LC-MS

Ionization


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GC-MS

GC

MS

Computer


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LC-MS

LC

MS

Computer


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LC?

MS Analysis

MS

Gas phase

Ions


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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


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PCBs

Production started

1920s

Env Detection

1968

Difference?

PFOS

Production started

1930s

Env Detection

2001


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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


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Problem solved…

Electrospray Ionization

(ESI)

John Fenn


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LC

Ionization

Chamber

Capillary

Gas

Heater

MS

Nebulized

Spray

Sample

Cone

Electrospray Ionization(ESI)

ESI Key Features:

- Capillary (LC) 90o to Cone (MS)

- Atmospheric pressure ion source

- Nebulized flow through capillary

- Heat applied to evaporate solvent

- Voltage difference applied between capillary and cone

+

- Detect (+) ions  Cap (+), Cone (-)

- Detect (-) ions  Cap (-), Cone (+)

-

10-6 torr

What happens in the droplet?

760 torr


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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.


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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


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LC

Ionization

Chamber

Capillary

Gas

Heater

Nebulized

Spray

Sample

Cone

Atmospheric Pressure Chemical Ionization(APCI)

APCI Key Features:

- Capillary (LC) 90o to Cone (MS)

- Atmospheric pressure ion source

- Nebulized flow through capillary

- Heat applied to evaporate solvent

- No voltage applied to capillary

- droplets are not charged

- Voltage still applied to cone

- Detect (-) ions  Cone (+)

- Detect (+) ions Cone (-)

MS

-

- Heat evaporates solvent, analytes partition into the gas-phase uncharged

10-6 torr

Corona discharge needle

760 torr

- Need to make ions!


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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


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LC

Ionization

Chamber

Capillary

Gas

Heater

Nebulized

Spray

Sample

Cone

Atmospheric Pressure Photoionization(APPI)

APPI Key Features (same as APCI):

- Capillary (LC) 90o to Cone (MS)

- Atmospheric pressure ion source

- Nebulized flow through capillary

- Heat applied to evaporate solvent

- No voltage applied to capillary

- droplets are not charged

- Voltage still applied to cone

- Detect (-) ions  Cone (+)

- Detect (+) ions Cone (-)

MS

UV lamp

-

- Heat evaporates solvent, analytes partition into the gas-phase uncharged

10-6 torr

760 torr

- Need to make ions!


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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


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Summary LC-MS Ionization

m/z

(MW)

www.agilent.com


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+

-

+

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.


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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


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Desorption Electrospray Ionization(DESI)

Z. Takáts et al.Science 2004, 306, 471-473.


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Direct Analysis in Real Time(DART)


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