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Analytical Chemistry > General Chromatography > Flashcards

General Chromatography Flashcards

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1
Q

What is goal of separation

A

1.Resolution of peaks (Ability to separate 2peaks)
2.retention time
3. Zone width- efficiency

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2
Q

What is a good resolution value

A

Resolution have to be 1.5 means that we don’t have missing and our peaks are well separated

If Rs is low than 1,5 means that there is mixing of 2 peaks

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3
Q

What affect resolution

A

Different in peak size can cause inseparable problems of peak/ not be able to distinguish them

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4
Q

How to improve resolution

A

-increase retention time of analytes
- improve selectivity by choosing other phases
-improve efficiency

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5
Q

What does retention and selectivity depends on

A

Intermolecular interactions the analytes experience with M and S phase molecules

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6
Q

Why does analytes interact with M phase in LC only

A

Because in GC M phase is gas (N) and they have idle behaviour means they will not interact with analytes.
Gas purposes in GC is to transport analytes away from column where is not dissolved in S phase

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7
Q

What are different chromatographic mode

A

Reverse phase chromatography: when S phase is non polar and M phase is polar.
Normal phase chromatography: when S phase is polar and M phase is less polar

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8
Q

What is retention factor

A

Is how long time analytes stays in stationary phase compared to mobile phase

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9
Q

What affect retention factor and retention time

A

It’s can be affected by flow rate, if we reduce flow rate retention time will increase
Ex: 1 ml/min flow rate give 1 retention factor but if we reduce to 0,5 ml/min flow rate retention factor will be twice

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10
Q

Separation factor

A

Is time two analytes stays in stationary phase

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11
Q

What is the characteristic of separation factor

A

It has always to be great than 1, if less or equal to 1 means that 2 cpds has same retention(co-elute) time and no separation occurs

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12
Q

What does retention factor depends on

A

It depends on distribution constant

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13
Q

Separation factor depends on what

A

Depends on distributions constant ratio of 2 compounds

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14
Q

Can pressure drop affect retention factor

A

In LC no effect- If you have liquid, the compressibility of liquid is insignificant that will not give any effect on the retentativity of a compound Cz the thermodynamic properties will not change by pressure.

In GC pressure have effect- But when mobile phase is compressible, the it can affect retentativity due to change in solubility

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15
Q

Can temperature affect retention factor and. Separation factor

A

In GC, k is affected by temperature and separation factor

  • so the higher temperature the higher vapour pressure and faster analyte will come out.
  • the more volatile the compound is the less time it will spend in the system.
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16
Q

What else does temperature affect

A

It affect kinetic properties of systems and linked to the zone width (broadening of a peaks) .

-increase of temperature lead to increase of diffusivity leading to wider zone

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17
Q

How is temperature affect in LC

A
  • retention factor is reduced at low %(no po)
  • affects kinetic properties increase mass transfer(moving of analytes from M.phase to S.phase ) lead to fast equilbria led to narrow peaks and less mixing of the zones
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18
Q

What does pressure drop

A

In LC increase in temperature reduce viscosity lead to increase of flow rate

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19
Q

What determines efficiency

A

Is determined by the width of zone, better control of zone width better efficiency

20
Q

Effect on N and H

A

The bigger plate number the narrow plate height is and a small plate height the small width of peaks

21
Q

What does plate height depend on

A

Zone broadening causes large plate heights
- the higher the resistance the wider peak becomes

22
Q

Zone broadening

A

The faster flow rate the more zone broadening cause equilibrium resistance increases with the flow rate

23
Q

Mass transfer resistance

A

The analytes close to the boundary will have short time to diffuse into M.phase from S.phase

24
Q

What happened at high pressure in GC

A

At high pressure gas are more compressible lead to the effect on linear velocity and diffusion rate.
So because is compressible it become more viscous which affect it’s flow ability

25
Q

Explain bidding’s equation

A

Ko= measure of the porosity, so the higher the porosity the bigger the contribution of the pores we have to the zone broadening
O = show geometry of pores. so straight pores O=1, pores are tortuous O>1 so the zone broadening become more

26
Q

Small particles size

A

You can increase flow rate without reducing efficiency
- by reducing particle size the minimum of plate height is reduced

27
Q

Big particles size

A

If you increase flow rate you will reduce efficiency

28
Q

Fact that affect zone broadening in extra- column

A
  1. Injected volume : the bigger volume injected the wider zone width resulting in zone broadening
  2. Detection: use cell that are less in % than peak volume
  3. Tubing design: if you use wider tubing peak will be dispose in higher volume lead to zone broadening
  4. Void volume If you have narrow zone of molecules and enter (dissolved ) in bigger volume will lead to wide peak at low flow rate same as high flow rate
29
Q

Cause of mixing chamber

A

It happens when tubil didn’t connect well with cavity so they will creation of mixing chamber that cause wide zone lead to wide peak and zone broadening

30
Q

How can solvent cause zone broadening

A

If we dissolved the analytes in the stronger solvent that eluent this can cause zone broadening

31
Q

Explain concentration effect

A

When 100% of water of solvent is used distribution ratio will be increased and analytes will stick to the stationary phase, as water doesn’t have the same composition as the mobile phase.

But when we use 100% methanol will give opposite effect, distribution ratio will be reduced. So the analytes will follow sample solvent until is mixed with mobile phase as they have the same composition.

32
Q

Zone broadening due to the isothermal effects

A

When we increase the concentration and column is overloaded there will be analyte-analyte interactions and they will contribute to the distribution constant by increasing it lead to the increase in retention time resulting in peak broadening

33
Q

Zone broadening due to the isothermal effect (adsorption chromatography )

A

Analytes interact with s.phase by diffusion but when we overload analyte concentration, analyte will not have more space to stick to s.phase surface leading to increases of analytes concentration in M.phase which reduce distribution ratio And reduce retention time resulting in peak broadening

34
Q

What is the difference between fronting and tailing

A

Fronting: occurs in liquid s.phase, when we overload columns with analyte concentration this will cause analyte analyte interaction which contributes to the increase of distribution ratio lead to increase of retention time resulting in peak broadening

Tailing: occurs in solid s.phase, so when analyte interact with s.phase by diffusion and we overload it there will be no more space for analyte to stick on s.phase surface so this will increase the analytes concentration in M.phase causing the reduction of distribution ratio resulting in peak broadening and as the overload increases we increase the tailing

35
Q

How to solve the fronting and tailing

A

We can solve the problem by diluting our sample

36
Q

Relationships between resolution and efficiency(N), selectivity(&), retentivity(K)

A

Retention time: by increasing retention time to 5 this will increase the resolution but after 5 no need of increasing k cause Rs will be constant (no effect), so the ideally 1<k<5, if k is less than one means that compound spend less time in s.phase than in M.phase leading to bad separation.

Separation factor: small change in & can cause a big change in Rs, Rs increase with &. And & >1 because If is equal to 1 means that there is co-elution means both cpds have same retention time. & is the ratio of retention factor.

Plate number: Rs can be increased by reducing plate height

37
Q

Which factor cause flow rate going from normal to micro column

A

Cross sectional area

38
Q

How can flow rate be obtained in micro columns, by which equipment

A

It can be obtained by splitting flow with micro pumps

39
Q

What happened when we have same velocity in M.phase

A

H,N and Rs will be the same

40
Q

What happens in micro column whe flow rate is 1

A

H will increase leading to decrease of Rs and decrease of N

41
Q

What happened to the pressure drop when flow rate is keeper to 1

A

Pressure drop will be higher

42
Q

What happens to k when columns diameter reduces

A

Nothing, k Depends on nature of 2 phase M and S phase and their relative amount

43
Q

What does k depend on

A

M and S phase

44
Q

What happens to retention time in micro column

A

If linear velocity is small no effect. but, when we increase linear velocity with flow rate equals to 1 retention time reduce

45
Q

What happens to the peak height in micro column

A

It goes there with higher concentration lead to height peak

Analytical Chemistry (4 decks)

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