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Analytical Chemistry > Unit 3: Separations > Flashcards

Unit 3: Separations Flashcards

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1
Q

Distribution Constant K

A

[S]org/[S]aq = [(1-q)m/Vorg]/[qm/Vaq]

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2
Q

What is qn

A

The fraction in aqueous phase after n separations

qn = (Vaq/(Vorg+K*Vorg))^n

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3
Q

what is tm

A

Dead/void time.

Time for only mobile phase to pass through

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4
Q

What is tr

A

tr is the amount of time a specific substance is on a column

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5
Q

what is ts

A

Time on stationary phase.

Given by tr -tm = ts = tr’ (adjusted retention time)

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6
Q

What is retention factor k

A

= ts/tm = (tr-tm)/tm

Used to correct for variations in flow rate, column conditions, etc.

Can also be
= K (Vs/Vm)

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7
Q

Selectivity factor alpha

A

= tr’2/tr’1

Alpha must be > 1 so tr’2 much be larger than the other

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8
Q

How to improve resolution

A
  1. Make peaks narrower

2. Space peaks further apart

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9
Q

Define resultion mathematically

A

delta tr/Wav = delta Vr/Wav
= 0.589 delta tr/W(1/2)avg

Wav represents width @ base
W1/2 avg represent width @ 1/2 peak
delta tr = seperation between peaks

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10
Q

What is baseline resolution

A

Res > 1.5

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11
Q

What does plate height H describe

A

= varience/length column packing

Describes column efficiency
Smaller = better

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12
Q

What is the van deenter eq

A

A + B/u + u(Cs + Cm)

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13
Q

Define the variables in the van deenter eq

A

A: diffusion (multple path)
B: longitude diffusion
Cs: rate of diffusion in stat. phase, lowered by thin uniformly coated SP
Cm: rate of movement through MP, lowered by uniform packing, small particles

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14
Q

Importance of uniform, small particle packing

A

Lowers the Cm contribution in van deempter eq.

Also decreases static pools

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15
Q

How to improve resolution? Based on van deempter eq

A
  1. Reduce packed particle size (reduces Cm and A)
  2. Skinnier column (reduces A and B)
  3. Thinner stationary phase (reduces Cs)
  4. Optimize flow rate (helps u term)
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16
Q

Ways to vary retention factor k

A 
Increase k2 (at expense of time)
change in temp (GC) 
Change in MP comp. (LC)
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17
Q

Variations in alpha

A

Change SP comp
Change column T
Change MP comp
Chemical rxns (Ag+ captures)

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18
Q

Understanding electron capture detector

A

Sample eluate passes over radioactive beta particle emmitter
Ionizes carrier gas (N2 - necessary)
Selective for organics w halogens

19
Q

Why choose LC?

A

For non-volatile or thermally unstable species (inorganics/biologicals)
Allow for more MP interaction, changing composition allows better seperation

20
Q

Why is porosity important in LC?

A

Maximizes the area of stationary phase. Superficially porous is best

21
Q

Liquid bonded phase partition chromatograph

A

Immobilizes viscous liquid (SP) by covalent attachment on solid support (particles w pores)
Covalent attachment is
Si - O - Si - R where R group is SP often alkane

22
Q

Define isocratic and gradient separation

A

Isocratic: single MP comp. (different temps)
Gradient: changing MP composition
Can use gradient profile

23
Q

Parts of HPLC

A
  1. Solvent reservoir
  2. Solvent degasser
  3. Gradient valve and solv mix
  4. Sample injection loop
  5. Guard column
  6. Analytical Column
  7. High pressure pump
  8. Detector
  9. Waste
  10. Data acquisition
24
Q

Importance of Solvent reservoir

A

Ports for gas line on bottles (sparging, removes dissolved gasses)

25
Q

Solvent degasses

A

Bubbles create dead volumes and change flow rate.

26
Q

Gradient valve/mixing:

A

Typically increase relative amount of more strongly elution solvent (normal elution)

  1. Binary gradient pump
  2. Single pump w/ proportioning valve
27
Q

High pressure pump:

A

Steady flow, no pulsing

Often pulse dampener

28
Q

Switch Valve:

A

6 way connection that changes connections in order to go from a fully filled sample loop to loading it on the column

29
Q

Guard column

A

is a short column w/ the same stationary phase between sample and analytical column to protect from contaminants

30
Q

Detection methods in HPLC

A 
UV
Fluoresence (derivatization) 
Refractive index 
Electrochem 
Mass-spec
31
Q

Extending path length in HPLC (band broadening)

A

Z-cell helps with that

32
Q

HILIC

A

Hydrophilic molecules too polar for revers HPLC (for small bio molecules)
SP: high polar
MP: mixed w aq buffer

33
Q

Ion exchange column

A

Anion exchange = anions attract
Cation exchange = attact cations
Conductivity detector

34
Q

Size exclusion column

A

Large molecules elute faster (small molecules must go through pores)
For proteins/desalting

35
Q

Affinity Column

A

Specific interactions bind molecules of interest

36
Q

Hydrophobic interaction column

A

Agarose gel w polar groups in pores

Elute w/ decreasing salt concentration ( increases solubility of pore)

37
Q

Developing new techniques

A
  1. 0.5 <= k <= 20
  2. res >= 2 (2 closest peaks
  3. pressure <= 15 mPa
  4. 0.9 <= asymp factor <= 1.5

Optimize plates, retention factor, and column/solvents

38
Q

Define asymmetric factor and improvement stategies

A

Factor = B/A (areas split down peak)

  • new mobile phase/column
  • clogged frit
  • guard column
  • sample+MP discrepency
  • Overloading
39
Q

Isocratic elution

A

Can switch elution order with MP

Use the isocratic profile to hit the extremes of %solvent comp and temperature

40
Q

Normograph

A

Different volume % of different solvents have same eluent strength. Shown graphically

41
Q

Gradient Elution strategy

A

Start w scout run. Gradually increase %solvent composition over 40 min. Gradients not useful are eliminated

42
Q

When to use Gradient or isocratic?

A

Calculate delta t/tg
where delta t = retention time between first and last peak
tg = gradient time

Use gradient if delta t/tg > 0.40
Isocratic if delta t/tg < 0.25
Use either if between

43
Q

Segment gradient:

A

Hold different %solvent comp over time and elute

Analytical Chemistry (1 decks)

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