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Chem 453 > Chapter 4 - Fundamentals of LC in LC-MS - part 1 > Flashcards

Chapter 4 - Fundamentals of LC in LC-MS - part 1 Flashcards

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1
Q

GC separation involves a

A

mobile phase and a stationary phase.

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2
Q

In GC, the stationary phase is usually

A

an immobilized liquid of varying hydrophobicity coated onto the inner walls of fused silica capillary tubing.

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3
Q

GC has ________ options for improving selectivity when compared to LC.

A

less

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4
Q

In LC, the mobile phase is usually

A

a liquid mixture of water and an organic solvent plus other additives.

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5
Q

In LC, the stationary phase is usually

A

materials of varying hydrophobicity chemically bonded to particles of silica as a solid support.

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6
Q

With reverse phase LC, the amount of water in the mobile phase will

A

determine how strongly a hydrophobic analyte is repelled into the stationary phase.

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7
Q

An LC system consists of the following components:

A
  1. mobile phase reservoir
  2. vacuum degasser
  3. pump
  4. injector
  5. column
  6. detector
  7. data system
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8
Q

LC grade solvents are required in order to

A

prevent column degradation and minimize background signals.

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9
Q

A filter in LC excludes _____ micro meter matter and in HPLC it excludes ______ micro meter matter.

A

> 0.5 (LC)

>0.2 (HPLC)

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10
Q

Outlet degassing occurs when

A

the mobile phase moves from the small volume of the capillary tubing to the large volume of the flow cell.

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11
Q

Most effective degassing technique

A

Offline refluxing (arduous and has risks, so rarely used)

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12
Q

Least favored degassing technique

A

Offline ultrasonic degassing (least efficient, and risks loss of volatile mobile phase components).

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13
Q

Degassing technique that removes 80% of dissolved gases.

A

Helium sparging

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14
Q

Most popular degassing technique

A

Vacuum degassing (low pressure of vacuum chamber draws dissolved gases out through a gas permeable tubing)

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15
Q

Isocratic elution

A

uses a constant mobile phase composition.

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16
Q

RPLC uses a _________ stationary phase and a ________ mobile phase.

A

nonpolar, polar

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17
Q

As the organic solvent content increases,

A

mobile phase polarity decreases, resulting in increase of affinity of the analytes for the mobile phase and a decreased retention time.

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18
Q

General problems of isocratic elution

A

Early eluting analytes may be poorly resolved.
Late eluting analytes may have broad peak shape and low sensitivity.
Not always suitable to separate complex mixtures with a wide range of chemistry and polarity.

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19
Q

3 fundamentals of an organic solvent that affect chromatographic selectivity.

A

Polarity (dipole moment)
Acidity (proton donor)
Basicity (proton acceptor)

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20
Q

3 organic solvents commonly used in RPLC whose properties differ significantly.

A

Methanol
Acetonitrile
Tetrahydrofuran (THF)

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21
Q

Nomograms

A

can be used to alter the separation such that the overall retention time remains similar, but the selectivity is altered.

22
Q

THF is the ________ while methanol is the __________ of the three organic solvents commonly used in LC.

A

strongest, weakest

23
Q

Gradient elution

A

gradually increases the content of organic solvent to increase eluting strength.

24
Q

Advantages of gradient elution:

A 
Improved resolution
Increased detection
Ability to separate complex samples
Shorter analysis times
Decrease in column deterioration
25
Q

Disadvantages of gradient elution:

A

More expensive instrumentation

Re-equilibration of column after each analysis adds time to analysis time

26
Q

Most common length of LC column

A

15 cm

27
Q

Inner diameter of LC column is between

A

1-5 mm

28
Q

Most common LC column inner diameter

A

2.1 mm

29
Q

Most common support of LC stationary phase is

A

highly pure, spherical, microporous particles of silica that have a surface area of several hundred square meters per gram.

30
Q

Irregular particles provide poorer column efficiency than spherical particles due to

A

much poorer packing homogeneity.

31
Q

The smaller the packing material particle in LC column,

A

the lower the plate height, the better column efficiency, the higher the optimum linear velocity of the mobile phase.

32
Q

The presence of metal ions within silica can cause

A

peak tailing.

33
Q

The composition of silica used for LC is

A

SiO2 xH2O

34
Q

Most silica cannot be used above pH

A

8 because it dissolves in base.

35
Q

Silanol groups are protonated at pH

A

~ 2-3

36
Q

All silanol groups interact with

A

polar and ionic analytes to different extents.

37
Q

Acidic silanol groups give the strongest reaction and the

A

worst peak tailing.

38
Q

The siloxane bond (Si-O-SiR) hydrolyzes below pH

A

2 (generally limited to use between pH 2-8)

39
Q

Monomeric stationary phases

A

are highly efficient, but not as chemically robust as those produced from di- or trichlorosilanes (which are known as polymeric bonded phases).

40
Q

Manufacturers may choose to end-cap the column surface, which involves

A

reacting the silanol groups with a sterically small but highly reactive reagent that ‘caps’ the polar surface silanol with a non-polar trimethylsilyl group.

41
Q

Longer chain silica provide more steric hinderance and achieve

A

a lower surface coverage.

42
Q

RPLC is characterized by having a stationary phase that is

A

less polar than the mobile phase.

43
Q

In RPLC, ____ is the most popular stationary phase.

A

C18 (highly robust hydrophobic phase which produces good retention with non-polar analytes).

44
Q

Shortening the alkyl chain of RPLC stationary phase will

A

shorten the retention time.

45
Q

The use of more polar RPLC stationary phases show

A

altered selectivity compared to alkyl phases, and can interact with polar analyte functional groups via dipole-dipole interactions.

46
Q

Traditional reverse phase columns are not suitable for use with aqueous mobile phases above _____% water due to phase collapse (or self-association)

A

95

47
Q

Stability in highly aqueous mobile phases is achieved by

A

the adsorption of a layer of water at the silica surface.

48
Q

The method of introducing stability in 100% aqueous mobile phases is to use

A

polar embedded ligands.

49
Q

Polar embedded ligands contain a modification to the alkyl chain which is usually

A

an amide, carbamate, or other suitable polar functional group.

50
Q

Water wettable phase advantages

A

polar group that gives an alternative selectivity
ability to separate polar, ionizable and highly basic compounds
enhanced robustness over shorter chain and polar bonded phases

Chem 453 (11 decks)

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