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CHEM 211 > Unit 5 > Flashcards

Unit 5 Flashcards

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1
Q

Elution chromatography

A

Column is filled with stationary phase .
Sample is added at the inlet and moved over the stationary phase by a mobile phase (ie solvent).
Analyte partitions between the mobile and stationary phase, moving down the column because of the time spent in the mobile phase.

KA = [A]stat/[A]mob

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2
Q

Describe solid-phase micro-extraction

A

Support coated with a hydrophobic extraction phase (ex.polymer)
Amount of analyte extracted to its concentration in a sample.
Simple, suitable for on-site sampling.
Can pre-concentrate analyte

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3
Q

Larger partition coefficient means

A

More partitions into the stationary phase

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4
Q

Define components of the partition coefficient

A

KA = [A]stationary/[A]mobile

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5
Q

Give examples and properties of solid adsorption media

A

Silica (SiO2): Polar, slightly acidic

Alumina (Al2O3): Polar, neutral or slightly basic

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6
Q

Steps of thin-layer chromatography

A

Spot a mixture onto plate of silica/alumina (stationary phase)

The bottom big the plate is immersed in solvent, which then travels up the plate by capillary action (mobile phase)

The mixture separated into spots or bands
Visualizing colourless compounds (uv-shadowing, charring, chemical staining

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7
Q

Elution Adsorption chromatography (preparatory technique for chemical synthesis): steps

A

Column packed with silica gel or alumina
Load sample mixture as narrow band at top of column.
Elute with organic solvent as mobile phase bands for each component separate out
Collect volume fractions

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8
Q

What can separation science do?

A

Separate and isolate components of the mixture. The amount of each analyte can then be determined.

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9
Q

What can chromatographic techniques mitigate?

A

Poor intrinsic selectivity of other analytical techniques.

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10
Q

Benefits/downsides of tlc

A

Quick, simple, low cost, but only qualitative

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11
Q

Block diagram: HPLC instrumentation

A

HPLC pump (solvents;mobile phase in), injector (sample in), HPLC Column, Detector (waste out), electronics/computer

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12
Q

HPLC pump function

A

Forces mobile phase through column at high pressure

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13
Q

Injector

A

Introduces a reproducible volume of sample onto the column

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14
Q

Column

A

Effects the separation of analytes

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15
Q

Detector function (HPLC)

A

Produces a measurable signal when analytes elute from the column

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16
Q

Function: inlet and inline solvent filter

A

Removes particulate matter and air bubbles (ie. Degas) in mobile phase

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17
Q

Pre-column filter

A

Remove particulate matter introduced with sample

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18
Q

Guard column

A

Protects analytical column. Removes particulate matter and chemical components that would irreversibly bind to the analytical column

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19
Q

What would happen without HPLC filters?

A

Top segment of stationary phase in the analytical column would be rapidly deteriorated

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20
Q

Back pressure regulator

A

Prevents bubbles from forming in the detector cell as the mobile phase exits the column

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21
Q

Draw out an HPLC diagram

A

See slide 5 under high performance liquid chromatography

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22
Q

Benefits and trade-offs of decreased particle size

A

Column more densely packed, separation efficiency increase

Tradeoff: very high back pressures. Gravity insufficient to move mobile phase through column. Pumps required

Slide 6 under high performance liquid chromatography

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23
Q

Analytical column

A

Used for trace analysis and detection

Diameters between 1-10mm and lengths on order of 10-10^2 um

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24
Q

Preparative and semi-preparative columns

A

Used for purification of synthesized compounds (laboratory scale)

Column diameters and particle sizes are typically about an order of magnitude larger than analytical columns

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25
Q

Bonded stationary phase: column is packed with:

A

Silica particles. Functional particles are bonded to silica particles to modify their polarity. Slide 10 high performance chromatography

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26
Q

What type of stationary and mobile phase is in normal phase HPLC? Also provide examples.

A

Polar stationary phase: silica, amino, cyano

Non-polar mobile phase: hexanes, diethyl ether, Ethyl acetate, dichloromethane, etc.

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27
Q

What type of stationary and mobile phase are in reverse HPLC? Also provide examples

A

Non-polar stationary phase: C18, C8, phenyl, etc.

Polar mobile phase: water, methanol, acetonitrile, propanol, etc.

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28
Q

Predict the order of elution of the following compounds on a reverse phase column for LC.
Benzene, 1-hydroxybenzene, 1,3-dihydroxybenzene

A. Benzene (first), 1-hydroxybenzene, 1,3-dihydroxybenzene (last)
B. Benzene, 1,3-dihydroxybenzene,1hydroxybenzene
C. 1,3-dihydroxybenzene, 1-hydroxybenzene, benzene
D. 1,3-hydrobenzene, benzene, 1-hydroxybenzene
E. The 3 compounds co-elute

A

C. 1,3-dihydroxybenzene, 1-hydroxybenzene, benzene

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29
Q

Which of the following methods will extract an analyte from a 50 mL aqueous solution with the highest efficiency?
A. One 50 mL portion of organic solvent
B. One 150 mL portion of organic solvent
C. Two 75 mL portions of organic solvent
D. Three 50 mL portions of organic solvent

A

D. Three 50 mL portions of organic solvent

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30
Q

Examples of Commercial Systems

A
  1. Solvent reservoirs
  2. Pumps and valves
  3. Autosampler and injector
  4. Column in thermostated oven
  5. UV-vis detector
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31
Q

Sparging

A

Helium is bubbled through the solvent reservoirs to sweep out dissolved air. Helium is insoluble in most solvents. Slide 16 under high performance liquid chromatography.

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32
Q

Solvent proportioning valve

A

Mixes different solvents together in defined and controllable proportions.

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33
Q

Draw and Label: Single wavelength detector (HPLC UV-Vis Detector)

A

Slide 17 under High Performance Liquid Chromatography

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34
Q

Draw and label: Diode Array (HPLC UV-Vis Detector)

A

Slide 18 under High Performance Liquid Chromatography

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35
Q

Design (shape) of the flow cell and what its good for

A

Z-shaped. offers long path length (better sensitivity) without diluting the sample or significantly affecting flow.

36
Q

Advantages of Diode Array

A
  1. Speed
  2. The diode array permits acquisition of full spectra as analytes elute from the column (or acquisition of several wavelengths in parallel)
37
Q

Commonly monitored wavelengths and the groups they are associated with:

  1. 190-210 nm
  2. 210-220 nm
  3. 240-260 nm
A
  1. 190-210 nm: Isolated double bonds
  2. 210-220 nm: Carbonyl groups, dienes
  3. 240-260 nm: Aromatic groups
38
Q

UV-visible detectors are sensitive to

A

Concentration and non-destructive

39
Q

Fluroescence detector

A

Very sensitive, but often requires derivatization of non-fluorescent analytes

40
Q

Refractive index detector

A

Universal detector, but not very sensitive

41
Q

Evaporative light scattering detector

A

Mass sensitive. Useful with almost all non-volatile analytes.

42
Q

Charged aerosol detector

A

Mass sensitive. Useful with almost all analytes

43
Q

Electrochemical detector

A

Voltammetric measurements on eluent. Sensitive to species that can be oxidized or reduced.

44
Q

Mass spectrometer

A

HPLC-MS is a hyphenated technique that combines HPLC with mass spectrometry. The method is both sensitive and powerful, providing information about molecular structure and identity while also permitting quantitative analysis.

45
Q

HPLC Chromatogram

A

Analytes can be differentiated and identified on the basis of their retention time.
The size of the peaks indicates something about the quantity of the corresponding analyte.

46
Q

Draw and label: HPLC Chromatogram

A

Slide 25 under High Performance Liquid Chromatography

47
Q

Dead Time (tm)

A

Time required for the mobile phase to travel the length of the column.

48
Q

Retention time (tr)

A

Time required for the analyte to elute from the column

49
Q

Adjusted retention time, with formula

A

Retention time corrected for dead time.

t’R = tR - tM

50
Q

Retention factor (k):

A

A measure of the relative amount of time an analyte spends in the stationary phase.
k = (tR - tM)/tM
k = KVs/VM
k = time in stationary phase/time in mobile phase

51
Q

Resolution

A

The degree to which two peaks are separated in a chromatogram. See slide 27.

52
Q

Theoretical plate

A

Conceptual representation of a separation step. More theoretical plates, better separation.
See slide 28

53
Q

Tailing

A

Ubiquitous (found everywhere) in liquid chromatography. Often results when there is more than 1 retention mechanism for the analyte. For ex, acidic silanol groups (w/o C18 modification) can support ion exchange and hydrogen bonding interactions (in addition to partitioning with C18).
Other common causes include a sample dilution solvent that is a stronger eluent than the mobile phase, contamination of the stationary phase, and sample overload. Slide 29

54
Q

Fronting

A

Opposite of failing; relatively rare in liquid chromatography. Usually caused by bad column packing (fix by replacing the column)

55
Q

Rate theory

A

Band broadening and its impact on separation efficiency

56
Q

The longer an analyte remains on column, the…

A

Broader the peak shape becomes. Total area remains constant

57
Q

Higher number of plates (N); higher plate height means

A

There are narrower distribution of carbon numbers from each trap (or plate). Higher number of plates means the narrower “peak” obtained from that trap

58
Q

Plate height formula

A

N = L/H

59
Q

Abbreviated Form of the Van Deemter Equation, define each term

A 
H = A + B/v + Cv where A,B,C are constants
A = "Eddy" Diffusion (Anastomosis)
B = Molecular Diffusion
v = linear flow rate
C = Rate of Mass-Transfer (Non-Equilibrium Effect)
60
Q

Draw a Van Deemter Grraph

A

Slide 36 under high performance liquid chromatography

61
Q

As particle size decrease and monodispersity increases, what happens to various paths through the column?

A

The various paths through the column become more uniform in distance

62
Q

As particle size decreases, what happens to diffusion lengths?

A

Decrease.

Diffusion length: the distance an analyte must travel to reach the stationary phase.

63
Q

Isocratic elution + examples

A

The same mobile phase composition is used throughout the separation. Example: Reverse phase HPLC, Solvent A = water, Solvent B = acetonitrile. Slide 40

64
Q

Gradient Elution

A

The mobile phase composition is gradually changed during the separation, used to maintain good resolution all around

65
Q

In reverse phase HPLC, the mobile phase is gradually made…

A

Less polar

66
Q

Draw linear gradient, segmented gradient, and step gradient

A

Slide 40 under higher performance liquid chromatography

67
Q

How to identify: Low organic content in RP-HPLC

A

Good early peak resolution, Poor late peak resolution (lag)

68
Q

How to identify: High organic content in RP-HPLC

A

Poor early peak resolution (mass transfer is inadequate), good late peak resolution

69
Q

Draw and label: Gas Chromatography column

A

Slide 3 under GC

70
Q

GC: does the gaseous mobile phase act as a solvent

A

No

71
Q

GC: what drives analytes into the gas phase

A

Temperature

72
Q

Analytes of GC can remain in…. condense in….. dissolve in…

A

Remain in vapour phase, condense on stationary phase, dissolve in stationary phase

73
Q

Predict the order of elution of the following compounds on a GC column with a moderately polar stationary phase:
A. Hexane (bp 68C), 1-butanol (bp 117C), octane (bp 125C)
B. Hexane, octane, 1-butanol
C. Octane, hexane, 1-butanol
D. 1-butanol, octane, hexane
E. The butanol and octane will co-elute

A

C. Octane, hexane, 1-butanol

Compounds with lower b.p. move through column more quickly; have shorter retention times.
More polar = more likely to remain on stationary phase

74
Q

Describe Gas Chromatographs

A

Gaseous Mobile phase, liquid/solid stationary phase, partitioning generally independent of mobile phase gas, temperature is critical

75
Q

Draw and label: Gas chromatograph block diagram

A

See slide 5 GC

76
Q

Draw and label: GC Capillary Column

A

Slide 7 GC

77
Q

FSCOT/WCOT

A

Fused silica/Wall-coated open tubular column:

High efficiency, low capacity, most common type

78
Q

For narrow capillary columns, what is required for sample injection?

A

Splitter system

79
Q

Split/Splitless injection

A

Inject liquid sample into a heated port with a syringe.
Rapid vaporization of sample.
Some of sample bled off to waste, some enters column, the amount of which is controlled by valve

80
Q

What does flame ionization detector do?

A
  • Air/hydrogen flame.
  • Hydrocarbons burned to produce ions (most prominent =formylium) and electrons
  • ions and electrons collected by electrode assembly help at high potential
  • current measured is proportional to amount of material
81
Q

Flame ionization detector is insensitive to

A

Most inorganic compounds, eg. O2, N2, SO2, NH3, CO2

82
Q

Flame ionization detector has no response to

A

Fully oxidised carbons, ex. carbonyl, carboxyl

83
Q

Detection limit and dynamic range of flame ionization detector

A

Detection limit as low as 10^-12 g C/s;

Dynamic range of ~10^7

84
Q

Draw the effect of flow rate on Van Deemter Plot

A

Slide 10 GC

85
Q

Temperature programming is the GC equivalent to

A

Gradient elution in LC

86
Q

Difference between isothermal and temperature ramp plot

A

Slide 11 GC

87
Q

Comparison: Liquid Chromatography and Gas Chromatography

A

Liquid Chromatography:

  • Applicable to any soluble compound (eg ions, small molecules, polymers, biomolecules)
  • More versatile optimization than GC (can vary mobile phase and stationary phase)
  • purified compounds can be collected afterwards
  • Often less preparation than GC

Gas Chromatography:

  • Requires volatile and thermally stable compounds
  • Faster than HPLC (minutes vs. tens of minutes)
  • More sensitive, better resolution than HPLC
  • More “universal” detectors, less expensive

CHEM 211 (5 decks)

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