Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

CHEM312 > Liquid Chromatography > Flashcards

Liquid Chromatography Flashcards

1
Q

Effect of decreasing size of SP particles

A

Decreasing size of SP particles increases the efficiency of packed columns (but require high pressure)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

Describe Modern LC/HPLC

A

High Performance (Pressure) Liquid Chromatography

Uses high pressure to force liquid solvent through a packed column containing fine particles giving high resolution separations

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

LC vs GC characteristics

A

LC = mobile phase is a liquid that interacts with the solutes, the stationary phase is is coated onto small particles in packed column

GC = mobile phase is an inert gas, stationary phase is on column walls

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What kind of analytes will stay on a normal phase column longer?

A

More polar analytes

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

Reverse phase LC
-SP/MP

A

Non-polar SP
Polar MP

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

Normal phase LC
-SP/MP

A

Polar SP
Non-polar MP

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Order of elution for normal phase LC

A

Most non-polar (first)
Most polar (last)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

How to increase retention times of a normal phase LC?

A

Decrease MP polarity

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

Why has normal phase been replaced by reverse phase?

A

Normal phase SP (Si-OH or Al2O3) have poor selectivity
Normal phase has poorer column efficiency (low N, high H) compared to reverse phase

Often observe peak tailing (particularly for more polar compounds due to differences in activity of silanol sites)
Many MP organic solvents are hazardous

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

List some SP’s and MP’s for reverse phase LC

A

SP = octyldecyl silane (C18) or octyl silane (C8)
MP = MeOH, ACN, THF

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What kind of analytes will stay on a reverse phase column longer?

A

More non-polar analytes

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

How can we alter polarity of SP in reverse phase LC?

A

Increase R (alkyl) chain length, SP will become more non-polar

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

Carbon load

A

Refers to the % (by weight) carbon content of the stationary phase bonded to the support material

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What does carbon load increase with?

A

Alkyl group chain length (more non-polar SP)
The amount of R-group bonded to the silanol groups

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

How does alkyl chain length of SP effect retention times (for non-polar analytes)?

A

Retention times increase (for non-polar analytes) with increasing alkyl chain length

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

What does increasing carbon loading do (RPLC)?

A

Increases k’ and tr

15
Q

What does increasing polarity of SP do (RPLC)?

A

Decrease k’ and tr

16
Q

Endcapping

A

If residual Si-OH groups are present on a RPLC column then peak tailing will be observed, often worse for more polar analytes such as alcohols and amines

Polar analytes can interact (eg H bond) with SiOH groups

Endcapping deactivates these groups
(residual SiOH groups)

17
Q

For what analytes is endcapping more likely needed?

A

Analytes with OH or NH2 groups

18
Q

How can we increase tr of analytes in RPLC by varying the MP?

A

To increase retention of the analytes (keep analytes longer on the column), you need to increase the polarity of the mobile phase (remember water is the most polar)
1. increase the % of water
2. decrease % of organic solvent

19
Q

How does increasing the percent of organic solvent change P’mix and k’ in RPLC?

A

As analyte polarity decreases, k’ increases
As MP polarity decreases, k’ decreases

Increasing the amount of organic solvent will decrease P’ and tr

20
Q

Can different solvents cause different elution order in RPLC?

A

Yes
Solvents have different proton acceptor/donor properties as well as dipole-dipole interactions that can impact selectivity

21
Q

What is (generally) the greatest effect on solute retention in HPLC?

A

MP composition

22
Q

Isocratic elution

A

MP composition is constant

23
Q

Gradient elution

A

MP composition is varied

24
Q

HPLC Injectors

A

Manual injector
(low cost, few samples)
Autosampler injection system
(automated injection, more costly, good for large number of samples, unattended sample injection)

25
Q

HPLC Detectors

A

Ultraviolet/Visible (UV/Vis) Detectors
- Fixed wavelength
- Variable wavelength
- Photodiode array
Fluorescence Detector
Refractive Index Detector
Conductivity Detector (Ion-exchange chromatography)
Mass Spectrometry (not discussed in the course)

26
Q

How do photodetectors work?

A

The absorbance of a sample will be proportional to the number of absorbing molecules in the detector cell
Molar absorptivity due to organic chromophores

27
Q

What functional groups are chromophores?

A

Double (pi) bonds, functional groups, and aromatic systems

28
Q

Ultraviolet/Visible (UV/Vis) Detectors

A

Most common, inexpensive, rugged
Wide range of solute detection
Good linear range, good sensitivity

Analytes must absorb in UV or visible region

29
Q

Different types of ultraviolet/visible (UV/Vis) detectors

A

Fixed Wavelength: measures at one wavelength, usually 254 nm (Mercury Lamp)
Variable Wavelength: measures at one wavelength at a time, but can detect over a wide range of wavelengths (you can adjust what wavelength you want)
Photodiode Array: measures a spectrum of wavelengths simultaneously

30
Q

Advantages/disadvantages of photodiode array detector

A

Advantage: greater selectivity than single or variable wavelength detectors
Disadvantage: slightly lower sensitivity

31
Q

Fluorescence Detector

A

Greater sensitivity and selectivity over UV-Vis but the analyte must fluoresce

High absorbance, aromatic, fused rings, electron donating groups all make good fluorophores

32
Q

What causes a molecule to fluoresce?

A

Several combined aromatic groups, or plane, or cyclic molecules with several π bonds
More rigid structures

33
Q

Refractive index detector

A

Universal detector for LC applications
- responds to differences in refractive index of mobile phase and solute(s)
Advantage - response to almost all analytes
- reliable and unaffected by flow rate (pressure changes)

Disadvantage - low sensitivity, cannot use MP gradients (µg levels as compared to ng levels with UV-vis)
- not used with gradient elution

CHEM312 (10 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions