Answers to Chapter 27/28 Problems
27-6
Temperature programming in GC – increasing the temperature of a gas
chromatography column with time
Low temperature gives reasonable retention for early eluters and then gradually increase
the temperature to make higher boilers elute off the column in a reasonable time.
27-14 Ideal Properties of a Liquid to be a Stationary Phase in GC
1) low volatility - typically use high molecular weight polymer which is crosslinked and
possibly bonded to the capillary wall
2) thermal stability
3) chemical inertness
4) solvent characteristics (interaction parameter χ) which yield suitable k’ and α for
the analytes
27-15 Effect of film thickness of stationary phase in GC
Two effects: increased retention
poorer efficiency
k′ = K
VSTAT
VMOB
Increasing the film thickness increases the volume of stationary phase,
which increases the retention.
d 2film
B
DM
d P2
H = A + + Cu = const ⋅ d P + const ⋅
+ f (k' )
u + f (k' )
u
u
u
DM
D film
Increasing the film thickness (dfilm) will increase the resistance to mass transfer in the
stationary phase (CS, right-most term above), which will increase the band broadening.
27-25 a) increasing the weight of stationary phase relative to the packing weight
This will increase the film thickness, which as discussed above will increase the
resistance to mass transfer in the stationary phase (CS, right-most term above), which
will increase the plate height (decrease efficiency).
b) decrease the rate of sample injection
Reducing the rate of sample injection will lead to band broadening because not all of
the sample molecules start migrating down the column at the same instant. Thus a
decrease in the rate of sample injection will increase the plate height (reduce
efficiency).
This type of band-broadening is known as “extra-column” band broadening”.
c) increasing the injection port temperature
If the injection port temperature is too low, the sample will take some time to
evaporate. This will in effect decrease the rate of sample injection, and so will
increase broadening (plate height).
Ideally, the injection port temperature should be ≥50°C above boiling point of least
volatile component.
d) Increasing the flow rate
Increasing the flow rate may cause a decrease or increase in H depending upon where
one lies along the van Deemter plot.
e) reducing the particle size of the packing
d 2film
B
DM
d P2
H = A + + Cu = const ⋅ d P + const ⋅
+ f (k' )
u + f (k' )
u
u
u
DM
D film
Decreasing the particle size of the packing results in a decrease in the resistance to
mass transfer in the mobile phase (CM, second last term above), and so a decrease in
the plate height.
f) Decreasing the column temperature
Changes in the column temperature affect the plate height in a number of ways. First,
the k’ will change. Both resistance to mass transfer terms have a nonlinear
dependence on k’. Second, the diffusion coefficients will be inversely dependent upon
temperature. Decreasing the column temperature will decrease the diffusion
coefficients, which will decrease the longitudinal diffusion band broadening, but
increase the resistance to mass transfer (C) terms.
Overall, it would be difficult to predict the precise effect, however in general
decreasing the column temperature results in an increase in plate height.
28-1
Substances to which each of the following kinds of chromatography are most
applicable:
a) gas-liquid – substances that are somewhat volatile and are thermally stable
- typically low molecular weight (<300 g/mol) and non- to intermediate
polarity)
b) liquid-partition – analytes that are nonvolatile or thermally unstable
c) reversed-phase partition – most low to moderate molecular weight organic
compounds that are nonvolatile or thermally unstable
d) ion-exchange – substances that are ionic
e) gel-permeation – high molecular weight hydrophobic compounds (e.g., polymers)
f) gel-filtration - high molecular weight hydrophilic compounds (e.g., proteins)
g) gas-solid – low molecular weight permanent gases (e.g., CO2, CO, NOX, Ar, N2)
h) liquid-adsorption – nonpolar low to moderate molecular weight organic compounds
- particularly useful for isomeric organic species
i) ion-pair – small organic and inorganic ions
- particularly useful for mixtures of ionic and nonionic analytes
28-2
Three general methods for improving resolution in partition chromatography:
i) adjusting the solvent strength (e.g., %acetonitrile) to optimize k’ and α
ii) altering the organic modifier to alter α
iii) change the stationary phase to obtain a change in α
These approaches are listed in order of their desirability or ease. Option iii is an
expensive choice. An additional alternative would be to increase the column efficiency
(use smaller particles or longer column), but these too would require another column, and
we would only get a √N improvement.
28-3
Manipulate retention factor in partition chromatography:
log ki′ = log k w − Sφ B
Polarity of the mobile phase can be varied by adjusting the relative proportions of a weak
solvent (A) and a strong solvent (B). For example, in reversed phase chromatography,
water is a weak solvent and acetonitrile is a strong solvent.
28-4
Manipulate selectivity factor:
a) gas chromatography – by altering the polarity of the stationary phase
b) liquid chromatography – most commonly by altering the specific polar interactions
of the mobile phase by going from say an acetonitrile-water mobile phase to a
methanol-water mobile phase of equal solvent strength
- could also alter column, but as in question 28-2 this is an expensive option
28-6
a) sparging – process of removing dissolved gases from a solution by sweeping it with a
stream of bubbles of an inert gas of low solubility (e.g., He)
b) isocratic elution – solvent composition (and thus solvent strength) is held constant
throughout the separation.
c) gradient elution – solvent composition is altered so as to increase the solvent strength
as the elution proceeds
- can be done gradually (most common) or in discrete steps (if performed manually)
f) extra-column band broadening – band broadening that arises in parts of the
chromatograph other than the column
e.g., injector (question27-25b), detector or connecting tubing (keep short and narrow
internal diameter)
g) reverse-phase packing – nonpolar packing (e.g., C18) used in conjunction with a
more polar mobile phase (e.g., acetonitrile/water)
h) normal phase packing – polar packing (e.g., CN, diol, NH2) used in conjunction
with a less polar mobile phase (e.g., CH2Cl2/hexane)
28-10 Desirable characteristics of HPLC detectors:
• adequate sensitivity
• good stability and reproducibility
• linear response over several orders of magnitude
• short response time (~1s)
• high reliability and ease of use
• low volume (minimize extra-column band broadening)
28-12 Properties and Roles of Mobile Phases
GC – inert gas (e.g., He, N2) that plays no role in the distribution of the analyte between
the mobile and stationary phase.
- role is strictly to carry the sample down the column (hence called “carrier gas”
- can affect detector sensitivity
LC – polarity of the mobile phase has a major impact on k’ and α
- generally choose mobile phases that are compatible with detection (i.e., prefer
methanol, acetonitrile and water because we can monitor in the low (>200 nm) UV
28-15 Type of Liquid chromatography for the following:
a) -typically would try reversed phase. However for structural isomers, adsorption
(normal phase) chromatography often works better. If I weren’t limited to LC, I
would probably try GC on an intermediate polarity open tubular column.
b) Ethanol and propanol – again I would rather use GC. Skoog suggests partition
chromatography, which would work but I don’t know how you would detect them.
Refractive index I quess, but its not very sensitive.
c) Ba2+ or Sr2+ - ion exchange with conductivity detection.
d) Pentanoic and hexanoic acid - Skoog says normal phase partition, but I would use
ion exchange or ion-pair chromatography. Conductivity detection would be best.
e) High molecular weight glucosides – size exclusion (gel-filtration) chromatography