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Chem 241 > Exam 2 > Flashcards

Exam 2 Flashcards

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1
Q

What are the goals of separation?

A

100% recovery and 100% purity

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2
Q

What are the goals of separation in phase transfers?

A

To transfer the compound of interest (analyte) out of the sample matrix and transfer other compounds out of the sample, leaving the analyte.

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3
Q

Solvent-solvent extraction equation

A

Kd= [S]org/[S]aq
Kd: distribution constant
Kd= (1-q(n/Vorg))/(q(n/Vaq))

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4
Q

What is q?

A

Fraction of solute left in the aqueous phase.

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5
Q

What is 1-q?

A

Fraction of organic extracted.

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6
Q

Equation for q

A

q= Vaq/(Vaq+ KdVorg)
q= 1/(1+ Kd(Vorg/Vaq))

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7
Q

True or False: It is better to do many extractions with smaller volumes of organic solvent.

A

True

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8
Q

Fraction left in water phase equation

A

q^n= (Vaq/(Vaq+ KVorg))^n
log(q)= nlog(Vaq/(Vaq+KVorg))

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9
Q

In a solvent-solvent extraction, charged acid-base species, as compared to neutral species are…

A

More soluble in the aqueous phase than the organic phase.

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10
Q

pH effects in solvent-solvent separation

A

D=[org]/[aq]
D= [HA]org/([HA]aq+[A-]aq)
D=Kd[H+]/([H+]+Ka)=Kd alpha HA

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11
Q

What is the impact of the extraction of a weak acid into an organic solvent as the pH of the aqueous phase increases?

A

As the pH increases, the value of D decreases and the percentage of the weak acid left in the aqueous phase increases.

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12
Q

When trying to calculate the fraction of weak base in the form B, extracted from a buffered solution into an organic solvent, which formula for calculating the distribution coefficient would you use?

A

D= (Kd) alpha A-
alpha A- = alpha B

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13
Q

You are trying to develop an extraction scheme to separate a neutral, non-acid-base organic compound (N) from a weak acid (pKa=4.5). Both species are present in an aqueous solution. Do you add NaOH or HCl first before adding organic solvent?

A

NaOH

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14
Q

What would happen to a peak if the flow was stopped and the column was allowed to sit?

A

The peak would broaden and diminish in height.

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15
Q

What is absorption chromatography?

A

The solute molecules equilibrate between the mobile phase and stationary phase surface.

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16
Q

What is size exclusion chromatography?

A

The solute molecules penetrate voids in the stationary phase. The largest molecules elute from the column first.

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17
Q

What is ion-exchange chromatography?

A

The mobile phase ions are attracted to the stationary phase ions.

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18
Q

What is partition chromatography?

A

The equilibration of the solute between the mobile phase and the liquid film stationary phase.

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19
Q

What is affinity chromatography?

A

Specific groups are attached to the stationary phase to attract the solute.

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20
Q

What is a packed column?

A

A hollow tube filled with particles that are coated with stationary phase materials.

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21
Q

What is a linear flow rate?

A

The velocity of the solvent as it travels through the column.

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22
Q

What is retention time?

A

The elapsed time between sample injection and sample detection.

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23
Q

What is an eluent?

A

The fluid that enters the column.

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24
Q

What is a stationary phase?

A

A separation medium that is bonded to the surface of the particles packed in a column.

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25
Q

What is a mobile phase?

A

A gas or liquid solvent that carries that sample through the separation column.

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26
Q

What is an open tubular column?

A

A hollow capillary tube with inner walls that are coated with stationary phase material.

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27
Q

What is an eluate?

A

The fluid that exits the column.

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28
Q

What is volumetric flow rate?

A

The volume of solvent traveling through the column per unit of time.

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29
Q

What is adjusted retention time?

A

The time required by a retained solute to travel though the column beyond the time required by the unretained solvent.

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30
Q

What is a retention factor?

A

The amount of time that a sample spends in the stationary phase relative to the mobile phase.

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31
Q

What is retention volume?

A

The volume of mobile phase required to elute a solute from the column.

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32
Q

What is relative retention?

A

The ratio of adjusted retention times or retention factors of two solutes. Also called separation factor.

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33
Q

What is a partition coefficient?

A

The ratio of solute concentration in the mobile and stationary phases.

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34
Q

Relative retention formula

A

alpha= t’r2/t’r1= Kd2/Kd1= k2/k1

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35
Q

Retention factor formula

A

k= (tr-tm)/tm= ns/nm= CsVs/CmVm= Kd(Vs/Vm)
tr-tm: time solute spends in stationary
tm: time solute spends in mobile
ns: moles of stationary
nm: moles of mobile
Cs: concentration of stationary
Cm: concentration of mobile

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36
Q

Fraction of solute in mobile phase equation

A

qmp= nm/(ns+nm)= 1/(1+k(Vs/Vm))= 1/(1+k)

37
Q

Fraction of solute in stationary phase equation

A

(1-qmp)= k/(1+k)

38
Q

Rate of solute travel equation

A

rate= uxqmp= (ux)(1/(1+Kd(Vs/Vm)))

39
Q

If k is very large (>30), what is true?

A

The solute may not elute from the column because it is strongly retained and the solute spends the majority of its time in the stationary phase.

40
Q

We want plate height to be (blank) so that we have (blank) in our chromatogram and thus achieve (blank) separations.

A

small, narrower peaks, more efficient.

41
Q

In chromatography, you can increase column efficiency by…

A

Increasing N and L and decreasing H

42
Q

Plate height equation

A

H= (sigma)^2/x

43
Q

Number of plates equation

A

N=L/H
N=16(tr/w)^2
N=5.55(tr/w1/2)^2

44
Q

Resolution equation

A

R= delta tr/w avg
R= delta Vr/ w avg
R= .589 delta tr/ w 1/2 avg

45
Q

True or false: Resolution is proportional to the number of plates on the column

A

False
R proportional to N^1/2 proportional to L61/2

46
Q

What are the three primary modes of band broadening that we need to consider in chromatography?

A

Multiple solute paths in column, longitudinal diffusion, and mass transport issues.

47
Q

Van Deemter equation

A

H= A+ B/ux+ Cux
H: plate height
A: multiple paths
B: longitudinal diffusion
ux: linear flow rate
C: mass transport

48
Q

What is the correct van Deemter equation if a GC experiment is run with an open tubular column

A

H= B/ux+ Cux

49
Q

What area on the van Deemter plot is governed by longitudinal diffusion?

A

The sharp negative slope

50
Q

What area on the van Deemter plot is governed by mass transport?

A

The small positive slope

51
Q

What is fronting?

A

It is caused by GC overloading. The soluted moves slower at higher concentrations. Gradual increase at the front end of the peak and normal Gaussian shape at the back end of the peak. Liquid stationary phase film is saturated with solute so SP looks like solute.

52
Q

What is tailing?

A

Overloading in LC/HPLC. Secondary undesirable interactions of solute with SP particles (hot spots) or contaminants in silica particles. Kd decreases, limited retention capacity, and at high solute concentrations, the solute saturates a significant portion of SP sites leaving fewer sites available for retention. Retention of the solute is reduced.

53
Q

What are the basics of gas chromatography?

A

It has a carrier gas mobile phase and a liquid or solid stationary phase. Volatile liquids or gas samples are analyzed.

54
Q

What is absorption chromatography?

A

Gas-solid chromatography; semi-permanent retention of active/polar analytes. Non-linear absorption leads to peak tailing.

55
Q

What is partition chromatography?

A

Gas-liquid chromatography; Wall coated open tubular column or support coated open tubular column.

56
Q

As the temperature of a GC column increases, the (blank) of the analyte increases, the retention time (blank), and the peak shapes (blank).

A

vapor pressure, decreases, sharpen

57
Q

When using a wall coated open tubular capillary, increasing the thickness of stationary phase film does what?

A

Increases the retention factor and H

58
Q

If you had a 0.32 mm inner diameter and a 0.25 mm inner diameter open tubular column, which would have better resolution?

A

The 0.25 mm column

59
Q

Butanol was found to elute much later than octane on a GC column with a polar SP even though octane has a higher bp than butanol. Why?

A

Butanol is polar and can interact more strongly with the stationary phase.

60
Q

When analyzing a mixture of samples by GC, all with a range of boiling points, the following is often used to obtain the most efficient separation?

A

temperature programming

61
Q

What is a thermal conductivity detector?

A

Not destructive, concentration sensitive, and measures voltage

62
Q

What is an electron capture detector?

A

Mostly nondestructive and measures current.

63
Q

What is a flame ionization detector?

A

Destructive, measures current, and best for the analysis of organic compounds.

64
Q

What is a mass spectrometry detector?

A

Destructive, measure intensity vs m/z, and gives structural information of eluted compounds.

65
Q

Flame ionization detectors are…

A

Mass detectors

66
Q

This GC detector is particularly sensitive to halogen containing molecules…

A

Electron capture

67
Q

Where does the “high performance” come from in HPLC?

A

The use of smaller and more uniform stationary phase particles.

68
Q

Liquid chromatography/HPLC basics

A

Uses a liquid mobile phase and a variety of stationary phases. The types of samples are non-volatile

69
Q

What is the stationary phase used in partition chromatography?

A

Liquid

70
Q

What is the stationary phase used in absorption chromatography?

A

Solid

71
Q

What is the stationary phase used in size exclusion chromatography?

A

Gel

72
Q

What is the stationary phase used in ion exchange chromatography?

A

Ion exchange resin

73
Q

What is the stationary phase used in affinity chromatography?

A

Selective stationary phase

74
Q

In liquid chromatography, what can help increase the resolution between two closely spaced peaks?

A

Change the composition of mobile phase, change the composition of stationary phase, and make adjustments to stationary phase particle size to decrease plate height.

75
Q

As the particle size decreases, which terms in van Deemter decrease?

A

A and C

76
Q

What are the parameters for HPLC

A

6000 psi, 3-5 um particle diameter, 1-2 mL flow rate, 10-30 min analysis time, stainless steel column, 4.6 mm diameters, 250 mm column length, and 12 um plate height.

77
Q

What are the parameters for UHPLC

A

20,000 psi, 1.3-2 um particle diameter, 250-750 uL/min flow rate, <2-10 min analysis time, increased throughput, packed capillary columns, less solvent used, 2.1 mm or less diameters, short column length, and 4-6 um plate height.

78
Q

In HPLC, reversed-phase chromatography uses a (blank) stationary phase and a (blank) mobile phase. The more (blank) the mobile phase becomes, the stronger the eluent strength.

A

nonpolar, polar, nonpolar

79
Q

What are the normal phase chromatography polarities?

A

Less polar mobile phase and polar stationary

80
Q

What are the reversed-phase chromatography polarities?

A

More polar mobile phase and nonpolar/weakly polar stationary phase.

81
Q

In HPLC a normal phase chromatography experiment utilizing gradient elution, how should you set up your mobile phase gradient?

A

Start with a nonpolar MP and increase its polarity over time.

82
Q

In size exclusion chromatography, molecules that are smaller than the fraction range have…

A

Vr=Vm and K=1

83
Q

What is Vo?

A

Void volume; determined by running a molecule larger than the stationary phase fractionation range through the column.

84
Q

What is Vi?

A

Volume of mobile phase inside the stationary phase pores; can be determined by running a compound that is smaller than the stationary phase fractionation range though the column
Vi= Vm-Vo

85
Q

What is Vm?

A

Total volume of mobile phase in the column
Vm= Vi+Vo

86
Q

What is Vr?

A

Retention volume

87
Q

Elution equation

A

Kav= (Vr-Vo)/(Vm-Vo)= (Vr-Vo)/Vi
Vr= Vo+ KavVi

88
Q

As Kav increases…

A

The size of the particle is smaller and fits in the stationary phase pore better.

Chem 241 (1 decks)

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