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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

Lecture 14

LC-MS

Ionization

slide2

GC-MS

GC

MS

Computer

slide3

LC-MS

LC

MS

Computer

slide4

LC?

MS Analysis

MS

Gas phase

Ions

slide5

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

slide6

PCBs

Production started

1920s

Env Detection

1968

Difference?

PFOS

Production started

1930s

Env Detection

2001

slide7

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
slide8

Problem solved…

Electrospray Ionization

(ESI)

John Fenn

slide9

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

slide10

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.

slide11

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

slide12

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!

slide13

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

slide14

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!

slide16

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

slide17

Summary LC-MS Ionization

m/z

(MW)

www.agilent.com

slide18

+

-

+

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.

slide19

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
slide20

Desorption Electrospray Ionization(DESI)

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