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Atmospheric Pressure Chemical Ionization (APCI). APCI is an ionization technique using gas-phase ion-molecule reaction at atmospheric pressure.

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slide1

Atmospheric Pressure Chemical Ionization(APCI)

APCI is an ionization technique using gas-phase ion-molecule

reaction at atmospheric pressure.

slide2
The nebulizer consists of three concentric tubes, the eluent is pumped through the inner most tube and nebulizer gas and make-up gas through the outer tubes.
  • The combination of the heat and gas flow desolvates the nebulized droplets, producing dry vapor of solvent and analyte molecules.
  • The solvent molecules are then ionized by a corona discharge
  • The results of these reactions produce water cluster ions, H3O+(H2O)n or protonated solvent, such as CH3OH2+ (H2O)n(CH3OH)m with n + m < = 4.
slide3
These ions enter in gas-phase ion-molecule reactions with an analyte molecules, leading to (solvated) protonated analyte molecules.
  • Subsequent declustering (removal of solvent molecules from the protonated molecule) takes place when the ions are transferred from the atmospheric-pressure ion source towards the high vacuum of the mass analyzer.
  • Proton transfer reactions are major process, while other reactions such as adduct formation and charge exchange in positive ion mode or anion attachment and electron capture reactions in negative ion mode are also possible.
slide5

Atmospheric Pressure Ionization (and APcI)

Ion Evaporation

Chemical ionization

slide6
APCI
  • Analogous to CI
  • For compounds with MW about 1,500 Da
  • Produce monocharged ions
slide7

Electrospray ionization (ESI)

Method used to produce gaseous ionized molecules from a liquid solution by creating a fine spray of droplets in the presence of a strong electric field.

Electrospray ionization/mass spectrometry (ESI/MS) which was first described in 1984 (commercial available in 1988), has now become one of the most important techniques for analyzing biomolecules, such as polypeptides, proteins having MW of 100,000 Da or more

in the eyes of fenn
In the eyes of Fenn…

“Although ESI is now in daily use all over the world, its component processes and mechanisms, especially the dispersion of the sample liquid into charged droplets, and the formation of gas phase ions from those droplets are poorly understood”

Professor John B. Fenn

slide9

Several kilovolts

Few µl/min

320-350 K, 800 torr

100 ml/s

slide10
Iribarne-Thomson Model:
  • Charge density increases
  • Raylaeigh limit (Coulomb repulsion = surface tension)
  • Coulomb explosion (forms daughter droplets)
  • Evaporation of daughter droplets
slide11
Special features of ESI process:
  • Little fragmentation of large and thermally unstable molecules
  • Multiple charge
  • Linear relationship between average charge and molecular weight
  • Easily coupled to HPLC
slide13
Applications:

Determination of MW and charges for each peak (Smith et al. Anal. Chem., 1990, 62, 882-899):

Assumptions

  • The adjacent peaks of a series differ by only one charge
  • For proteins, the charging is due to proton attachment to the molecular ion.
  • This has been an excellent (but not crucial) assumption of nearly all proteins studied to data where alkali attachment contributions are small.
  • Ionization of only the intact molecule.
slide14

M/Z

P1

P2

Z2

Z1

Given these assumptions, eq 1 describes the relationship between a multiply charged ion at m/z P1 with charge z1 and molecular weight M.

P1Z1 = M + MaZ1 = M + 1.0079Z1 [1]

Assume that the charge carrying species (Ma) is a proton. The molecular weight of a second multiply protonated ion at m/z P2 (where P2 > P1) that is j peaks away from P1 (e.g. j = 1 for two adjacent peaks) is given by

P2(Z1-j) = M + 1.0079(Z1-j) [2]

Equations 1 and 2 can be solved for the charge of P1.

Z1 = j(P2-1.0079)/(P2-P1) [3]

The molecular weight is obtained by taking Z1 as the nearest integer valve.

slide16

These images are frame captures of a PicoTip spraying 5% Acetic acid in 30% MeOH at 200 nl/min by direct infusion from a syringe pump. Each frame differs by an applied voltage of approximately 100 volts. The tip-to-inlet distance was ca. 5 mm. 900 V - no spray1000 V - Taylor-cone/droplet oscillation, more "drops" than spray1100 V - cone/droplet oscillation. approx 50% spray1200 V - cone/droplet oscillation, on the verge of a stable Taylor cone1300 V - stable cone-jet1400 V - cone-jet on the verge of "jumping", slight instability1550 V - multiple cone-jets

What happens when voltage is applied?

http://www.newobjective.com/electrospray/spray_anim.html

ionization mechanisms
Ionization Mechanisms

Coulomb Fission:

Assumes that the increasedcharge density, due to solventevaporation, causes largedroplets to divide into smallerdroplets eventually leading tosingle ions.

Ion Evaporation:

Assumes the increased chargedensity that results from solventevaporation causes Coulombicrepulsion to overcome the liquid’s

surface tension, resulting in arelease of ions from dropletsurfaces

slide24

HOW MANY AMINO-ACIDS?

~ 1 charge per 1000 Da!

slide25

4 easy steps to ESI:

  • Production of charged droplets from electrolyte dissolved in solvent.
  • Shrinkage of charged droplets by solvent evaporation and droplet disintegration.
  • Mechanism of gas-phase ion production.
  • Secondary processes by which gas-phase ions are modified in the atmospheric and ion sampling regions.
    • Kebarle and Tang, Anal. Chem. 1993, 65, 972A-986A.
slide26

End Plate

-2 to -3kV

Desolvation

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Glass

Capillary

-2 to -5 kV

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Emitter

(Ground)

Coulombic

Explosion

Desolvated

Ions

Electrospray Ionization Process

3-6KV

0.3-2 cm

106V/m

Liquid flow

production of charged droplets
Production of Charged Droplets
  • Voltage difference between the emitter and counter-electrode establishes an electric field (E  106 V/m). For positive ion mode:
    • Emitter grounded, counter-electrode biased –ve (2-6 kV)
    • Emitter biased +ve (2-6 kV), counter-electrode usually +ve a few V.
  • Liquid flowing through capillary is conductive.
electric field at tip e
Electric Field at Tip (E)

4d

2V

E =

ln( )

r

r

r

V

Counter-

electrode

Capillary

d

HV

taylor cone
Taylor Cone
  • Accumulated charge at surface leads to destabilization of surface because ions at surface are drawn toward counter-electrode yet can’t escape surface.
  • Leads to formation of the Taylor cone.

Q = 49.3

Capillary

Taylor cone

surface tension and droplet production
Surface Tension and Droplet Production
  • The cone instability is profoundly influenced by the surface tension (g) of the fluid.
  • The onset voltage (Von) required to initiate charged-droplet emission is related to surface tension by:

4d

Von = 2x105(g r)0.5

ln( )

r

slide31
Thus…
  • Onset voltage is higher for liquids of higher surface tension. 4kV for water, 2.2 kV for methanol
  • Relative ranking:

iPrOH < MeOH < AcCN < DMSO < H2O

  • The higher the voltage, the increased probability of electrical discharge (esp. in negative ion mode)! Corona discharge also increases with decreasing pressure, so this is why ESI is done at atmospheric pressure.
parameters influencing droplet size
Parameters Influencing Droplet Size
  • The radius (R) of an electrosprayed droplet depends upon fluid density (r), flow rate (Vf), and surface tension (g).
  • Thus, higher Vf result in larger initial droplet sizes. Larger droplet sizes lead to lower ionization efficiency because the droplets are not so close in size to the Rayleigh limit

R  (rVf2g)1/3

droplet shrinkage
Droplet “Shrinkage”
  • Now that the charged droplets have been released from the capillary, they are accelerated toward the counter-electrode.
  • Shrinkage of the droplets results as a combination of two factors:
    • Solvent evaporation
    • Droplet disintegration by Coulombic explosions
rayleigh limit
Rayleigh Limit
  • When charge Q becomes sufficient to overcome the surface tension which holds the droplet together, Coulombic explosions begin:

Q2 = 64p2eogR3

where eo is the permitivity of vacuum.

esi advantages
ESI Advantages
  • Soft-ionization technique
  • Controllable fragmentation
  • Readily coupled to liquid separations
  • Produces intact non-covalent complexes
  • Multiple-charging of analyte
  • Capable of ionizing large molecules (to MDa)
slide38

ESI: Protein analysis

Peptide sequencing by nano-electrospray mass spectrometry

slide39

M+17

Electrospray spectrum of horse myoglobin (mw 16,951.5)

Multiply-charged ion distribution from +12 to +24 shown at low resolution.

The +17 charge state at a resolution of about 15,000 showing the resolved isotope peaks.

M+17

slide41

Limitation of Ion Current

  • Electrochemical reactions occur in last few μM.
  • Ions extracted per unit of time to the MS is limited by the current produced by the oxidation or reduction process at the probe tip.
esi is a constant current electrochemical cell
ESI is a constant-current electrochemical cell
  • Too many ions from salts will decrease the abundance of sample ion
  • If too diluted or at very low flow, ion flow from capillary will be insufficient. Oxidation or reduction of solvent or sample will occur, producing radical ions.