Chapter 5 - Ionization methods in LC-MS

Question 1

Electrospray ionization (ESI) is a method by which

Answer 1

ions present in solution can be transferred to the gas phase.

Question 2

ESI is highly compatible with analytes possessing

Answer 2

Moderate to high polarity
Up to 100,000 Dalton
Ionize in solution, perhaps with multiple charge

Question 3

3 important processes to transfer ions from LC eluate to gas phase in ESI.

Answer 3

Production of charged droplets at the capillary tip.
Shrinkage of the charged droplets.
Production of gas phase ions.

Question 4

The capillary is used to

Answer 4

introduce the LC eluate into the ESI source.

Question 5

Capillary internal diameter is approximately

Answer 5

0.1 mm

Question 6

Potential difference between capillary and sampling cone

Answer 6

2-6 kV

Question 7

Electric field in the air around the capillary tip.

Answer 7

10^6 V/m

Question 8

In positive ion mode, the capillary is the ______ electrode and the sampling aperture plate is the _______ electrode.

Answer 8

positive, negative

Question 9

In positive ion mode, _______ ions predominately populate the sprayed droplet.

Answer 9

positive

Question 10

Rayleigh Instability Limit

Answer 10

The point at which the surface charge repulsion matches the surface tension of the eluate.

Question 11

Taylor Cone

Answer 11

When Coulombic repulsions overcome the Rayleigh limit.

Question 12

Nebulization

Answer 12

Droplet formation and expulsion into the desolvation region.

Question 13

As the potential difference across the system increases, the size of the droplet

Answer 13

decrease.

Question 14

Axial spray mode

Answer 14

Above a certain applied potential difference, the Taylor cone is formed and small charged droplets are formed from its tip (optimum voltage for the experiment).

Question 15

Rim emission mode

Answer 15

Further increasing applied potential difference causes the liquid cone to vanish and a fine mist of droplets is produced (poor response and irreproducible).

Question 16

Corona discharge

Answer 16

Electrical discharge brought on by the ionization of a fluid surrounding a conductor (produces a noisy baseline).

Question 17

ESI flow rate (when by itself)

Answer 17

10-20 microliters per minute

Question 18

Pneumatically assisted ESI flow rate

Answer 18

Up to 1 mL/min (optimized around 200 microliters)

Question 19

Solvents with low surface tension lead to

Answer 19

smaller droplets, an increase in sensitivity and a reduced risk of rim emission and electrical discharge.

Question 20

The addition of a small amount of _______ or _______ to a highly aqueous LC eluate can often bring about an increase in response.

Answer 20

methanol or acetonitrile

Question 21

LC grade acetonitrile contains a fairly high background level of

Answer 21

sodium ions.

Question 22

The sprayer current is dependent upon the

Answer 22

conductivity of the eluate solution.

Question 23

Low concentration of electrolyte ions in the eluate solution

Answer 23

interferes less with analyte gas phase ion production.

Question 24

Electrolyte ions with low m/z values in the eluate solution

Answer 24

will interfere less with analyte gas phase ion production due to large sphere of hydration.

Question 25

As a droplet shrinks due to solvent evaporation, its radius _________ while its charge _________.

Answer 25

decreases, remains constant

Question 26

Droplet jet fission

Answer 26

When enough solvent has evaporated, electrostatic forces are higher than surface tension, and the droplet undergoes Coulombic Fission.

Question 27

The whole evaporative usually occurs within

Answer 27

a few hundred microseconds and a few milliseconds.

Question 28

Radius of typical first generation droplet radius & charge.

Answer 28

0.10 mm

280 charges

Question 29

Typical 2nd generation droplet radius & charge

Answer 29

0.04 micrometers

10 charges

Question 30

Typical 3rd generation droplet radius & charge

Answer 30

0.003 micrometers

2 charges

Question 31

Doel model

Answer 31

Droplet fission proceeds until a single ion is produced.

Question 32

Iribarne & Thompson model

Answer 32

Below radius 10 nm, the ion is able to ‘evaporate’ from within the droplet (most widely accepted theory).

Question 33

The closer the ion is to the droplet surface,

Answer 33

the more likely it is to evaporate from the droplet.

Question 34

Smaller ions with more charges have _______ spheres of hydration.

Answer 34

larger

Question 35

Larger ions with a single charge will have _______ spheres of hydration.

Answer 35

smaller

Question 36

The rate of ESI production increases with the increase in _________

Answer 36

ion number.

Question 37

The rate of ESI ion production decreases with the increase of

Answer 37

droplet radius.

Question 38

Factors that determine the number of charges in the droplet and the radius.

Answer 38

LC eluate flow rate
LC eluate composition
Nebulization gas flow rate
Capillary voltage
Drying gas temperature

Question 39

Ion suppression

Answer 39

In the presence of a second electrolyte species, the intensity of the analyte can be suppressed.

Question 40

Ion suppression can lead to

Answer 40

artificial and irreproducible reduction in analyte signal.