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ANA CHEM 2 > Chromatography > Flashcards

Chromatography Flashcards

(68 cards)

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1
Q

is a separation technique based on the different interactions of compounds with two phases, a mobile phase and a stationary phase, as the compounds travel through a supporting medium

A

Chromatography

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2
Q

a solvent that flows through the supporting medium

A

Mobile phase

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3
Q

a layer or coating on the supporting medium that interacts with the
analytes

A

Stationary phase

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4
Q

a solid surface on which the stationary phase is bound or coated

A

Supporting medium

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5
Q

The analytes interacting most strongly with the stationary phase will take longer to pass through the system than those with weaker interactions. What does this mean?

A

High affinity

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6
Q

GC; type of stationary phase
1. Gas-solid chrom.
2. Gas-liquid chrom.
3. Bonded-phase gas chrom.

A
  1. solid, underivatized support
  2. liquid- coated support
  3. chemically-derivatized support
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7
Q

LC; type of stationary phase
1. Adsorption chrom.
2. Partition chrom.
3. Ion-exchange chrom.
4. Size exclusion chrom.
5. Affinity chrom.

A
  1. solid, underivatized support
  2. liquid coated/underivatized support
  3. support containing fixed charges
  4. porous support
  5. support w/ immobilized ligand
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8
Q

tr
tm
Wb
Wh

A

retention time
void time or dead time
baseline width of peak
half height width of peak

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9
Q

The separation of solutes in chromatography depends on two factors:

A

(a) a difference in the retention of solutes (i.e., a difference in their time or volume of
elution

(b) a sufficiently narrow width of the solute peaks (i.e, good efficiency for the separation
system)

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10
Q

The volume of the mobile phase that it takes to elute a peak off of the column.

A

Retention volume (Vr)

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11
Q

Is directly related to the
strength of the solute’s interaction with the mobile and stationary phases

A

Retention volume (Vr)

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12
Q

The amount of mobile phase that it takes to
elute a non-retained component.

A

Void volume (Vm)

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13
Q

Theory of chromatography:

A
  1. Typical response obtained by chromatography (i.e., a chromatogram)
  2. Solute retention
  3. Efficiency
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14
Q

Retention on a given column pertain to the particulars of that system:

A
  • size of the column
  • flow rate of the mobile phase
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15
Q

Column length formula/ average linear velocity

A

v = L/tr

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16
Q

Capacity factor (k’): more universal measure of retention, determined from tr or Vr

A

k’ = (tr -tm)/tm

k’ = (Vr - Vm)/Vm

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17
Q

fundamental definition of k’

A

k’ = moles A (stationary phase)/ moles A (mobile phase)

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18
Q

k’ is directly related to the strength of the interaction between a solute with the stationary
and mobile phases.

A
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19
Q

represents the amount of solute present in each
phase at equilibrium

A

Moles A (stationary phase) and moles A (mobile phase)

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20
Q

When k’ is less than or equal to 1.0,

When k’ is > 30,

When k’ is = 2-10,

A

separation is poor

separation is slow

separation is optimum

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21
Q

equilibrium constant for the distribution of A between the mobile
phase and stationary phase

A

Kd

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22
Q

Assuming local equilibrium at the center of the chromatographic peak:

A

k’ = [A]stationary phase Volume stationary phase/ [A] mobile phase Volume mobile phase

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23
Q

k’ = Kd Volume stationary phase/ Volume mobile phase

A
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24
Q

As KD
increases, interaction of the solute with the stationary phase becomes more
favorable and the solute’s retention (k’) increases

A
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25
peak separation also represents different changes in free energy
26
is related experimentally to a solute’s peak width - an efficient system will produce narrow peaks - narrow peaks = smaller difference in interactions in order to separate two solutes
Efficiency
27
is related experimentally to a solute’s peak width - an efficient system will produce narrow peaks - narrow peaks = smaller difference in interactions in order to separate two solutes
Efficiency
28
related theoretically to the various kinetic processes that are involved in solute retention and transport in the column - determine the width or standard deviation (sigma) of peaks
Efficiency
29
Wb
4 sigma
30
Wh
2.354 sigma
31
compare efficiencies of a system for solutes that have different retention times
Number of theoretical plates (N)
32
The larger the value of N is for a column, the better the column will be able to separate two compounds.
- the better the ability to resolve solutes that have small differences in retention - N is independent of solute retention - N is dependent on the length of the column
33
compare efficiencies of columns with different lengths:
Plate height or height equivalent of a theoretical plate (H or HETP)
34
Plate height or height equivalent of a theoretical plate (H or HETP)
H = L/N
35
H simply gives the length of the column that corresponds to one theoretical plate
36
Why Do Bands Spread?
a. Eddy diffusion b. Mobile phase mass transfer c. Stagnant mobile phase mass transfer d. Stationary phase mass transfer e. Longitudinal diffusion
37
What will you do if there are overlapping peaks?
change the design or method Peaks that are moderately overlapped can often be resolved by increasing column efficiency — by increasing the column plate number to sharpen the peaks (reduce peak volumes)
38
caused by column overload/overpacking
Fronting peak
39
caused by underpacking or sample is too viscous
Tailing peak
40
a process that leads to peak (band) broadening due to the presence of multiple flow paths through a packed column.
Eddy diffusion
41
As solute molecules travel through the column, some arrive at the end sooner then others simply due to the different path traveled around the support particles in the column that result in different travel distances
Eddy diffusion
42
a process of peak broadening caused by the presence of different flow profile within channels or between particles of the support in the column.
Mobile phase mass transfer
43
A solute in the center of the channel moves more quickly than solute at the edges, it will tend to reach the end of the channel first leading to band broadening
Mobile phase mass transfer
44
The degree of band-broadening due to eddy diffusion and mobile phase mass transfer depends mainly on:
1) the size of the packing material 2) the diffusion rate of the solute
45
band-broadening due to differences in the rate of diffusion of the solute molecules between the mobile phase outside the pores of the support (flowing mobile phase) to the mobile phase within the pores of the support (stagnant mobile phase).
Stagnant mobile phase mass transfer
46
Since a solute does not travel down the column when it is in the stagnant mobile phase, it spends a longer time in the column than solute that remains in the flowing mobile phase.
Stagnant mobile phase mass transfer
47
The degree of band-broadening due to stagnant mobile phase mass transfer depends on:
1) the size, shape and pore structure of the packing material 2) the diffusion and retention of the solute 3) the flow-rate of the solute through the column
48
band-broadening due to the movement of solute between the stagnant phase and the stationary phase.
Stationary phase mass transfer
49
Since different solute molecules spend different lengths of time in the stationary phase, they also spend different amounts of time on the column, giving rise to band broadening.
Stationary phase mass transfer
50
The degree of band-broadening due to stationary phase mass transfer depends on:
1) the retention and diffusion of the solute 2) the flow-rate of the solute through the column 3) the kinetics of interaction between the solute and the stationary phase
51
band-broadening due to the diffusion of the solute along the length of the column in the flowing mobile phase
Longitudinal diffusion
52
The degree of band-broadening due to longitudinal diffusion depends on:
1) the diffusion of the solute 2) the flow-rate of the solute through the column
53
H =A + B/u + Cu
Van Deemter equation
54
Plot of van Deemter equation shows how H changes with the linear velocity (flow-rate) of the mobile phase
55
where H has a minimum value and the point of maximum column efficiency; is easy to achieve for gas chromatography, but is usually too small for liquid chromatography requiring flow-rates higher than optimal to separate compounds
Optimum linear velocity
56
Measures of Solute Separation: parameter used to describe how well two solutes are separated by a chromatographic system:
separation factor
57
Does not consider the effect of column efficiency or peak widths, only retention.
separation factor
58
a second measure of how well two peaks are separated:
Resolution (Rs)
59
is preferred over separation factor since both retention (tr) and column efficiency (Wb) are considered in defining peak separation.
Resolution (Rs)
60
represents baseline resolution, or complete separation of two neighboring solutes = ideal case.
Rs is greater than or equal to 1.5
61
considered adequate for most separations.
Rs is greater than or equal to 1.0
62
affinity = strength of adhesion
63
the property of how well a component of the mixture sticks to the stationary phase
Adsorption
64
the property of how well a component of the mixture dissolves in the mobile phase
Solubility
65
Higher the adsorption to the stationary phase, the slower the molecule will move through the column.
66
Higher the solubility in the mobile phase, the faster the molecule will move through the column.
67
The nonpolar solvent acts as the mobile phase. Nonpolar solvents interact more with the mobile solvent, travelling quickly along the polar stationary phase, while polar solutes are attracted to the stationary phase and travel more slowly. This property allows for separation based on polarity.
68
normal phase chromatography, where the stationary phase is polar, polar molecules will spend more time adsorbed on the stationary phase, while less polar ones will be carried more quickly by the non-polar mobile phase.

ANA CHEM 2 (3 decks)

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