Chapter 22. GC & LC. 22.1 Gas Chromatography. Schematic diagram. 22.1 Gas Chromatography. Columns : open tubular columns. 22.1 Gas Chromatography. m.p.(gas) - s.p. s.p.: solid （ using adsorption ） ex: SiO 2 column ages: Si-O-H cause tailing peak.
GC & LC
column ages: Si-O-H cause tailing peak.
2) s.p.: liquid（GLC, using partition）
a range of polarities (Table 22-1), “like dissolves like”
Decrease thickness of stationary phase leads to
Like dissolves like (a) S.P: nonpolar, b.p. dependent (b) S.P: polar
B) The effects of column polarity on separation
Figure 22-4 Resolution of trans fatty acids in hydrogenated food oil improves when the stationary phase is changed from DB-23 to HP-88 (aryl group)
C) Common solid s.p. :
a) Porous carbon :larger molecules bind more tightly than small ones, flexible molecules bind more than rigid ones
b) Molecular sieves : inorganic materials with nanometer-size cavities that retain & separate small molecules : H2, O2, N2, CO2, CH4. (Fig. 22-5)
c) Guard column
Collect nonvolatile components that would otherwise be injected into a column and never be eluted.
packed column vs. open tubular column
lower sample capacity
temp of column v.p. solute,
isothermal :constant temp.
temp. programming (gradient) :
raise the column temp. during the separation.
Figure 22-6 (a) Isothermal and (b) programmed temperature chromatography of linear alkanes through a packed column with a nonpolar stationary phase.
4. Carrier Gas
Figure 22-7 Injection port operation for (a) split, (b) splitless, and (c) on-column injection into an open tubular column.
split injection (350℃) (only 0.1-10% sample)
could cause thermal decomposition
splitless injection (220℃) (80%)
For quantitative analysis and for analysis of trace components of mixture
solvent trapping (Tsolvent < 40℃) for dilute sample
cold trapping (Tsolute < 150℃) for high-boiling solutes
on-column injection(50℃) (100%)
best for thermally unstable solutes.
Qualitative analysis :mass spectrometer, IR
Quantitative analysis :area of a chromatographic peak.
a) Thermal conductivity detector:
-most general way
-responds to everything
-not sensitive enough for high resolution.
b) Flame ionization detector :
-mainly responds hydrocarbons (C-H)
c) Electron capture detector :
-for compounds containing atoms with high electron affinities.
-sensitive for halogen, C=O, NOx, & orgaometallic compounds.
d) Mass Spectrometric Detection and Selected Reaction Monitoring :
- A mass spectrometer is the single most versatile detector.
- Total Ion Chromatogram (TIC)
- selected ion monitoring (SIM) at on value of m/z
- selected reaction monitoring (SRM) = tandem mass =
- Multiple reaction monitoring (MRM)
Precursor ion (parent ion) vs. Product ions (daughter ion)
Solid phase extraction (SPE)
1. open, gravity-feed column
2. closed column (under high pressure) packed with micron-size particles. (HPLC)
3. stationary phase :
a. adsorption : silica (SiO2xH2O), alumina (Al2O3xH2O),
b. molecular exclusion,
c. ion-exchange, affinity
compete with▲for binding on s.p.
the more strongly bind to s.p.eluent strength
4. Eluent strength : Table 22.2
The more polar solvent
5. Gradient elution : increased the eluent strength during the separation in liquid chromatography.
1. Through a closed column, and needs high pressure.
2. s.p. particles size
microporous particles of silica
with diameters of 1.5-10 um
s.p. m.p. faster,
i.e. C in van Deemter eqn.
3. Stationary phase
a) Normal-phase chromatography :polar s.p. and less polar solvent. Eluent strength is increased by adding a more polar solvent.
b) Reversed-phase chromatography :low-polarity s.p. and polar solvent. Eluent strength is increased by adding a less polar solvent.
c) Bonded stationary phase.
polar vs. nonpolar
d) Optical isomers
D- & L-amino acids
for drug industry
see p.494 for R = polar or nonpolar
d) Optical isomers separation
ex: for ant-inflammatory drug Naproxen
a) Isocratic elution :
elution with single solvent or a constant solvent mixture
b) Gradient elution :
solvent is changed continuously from a weak eluent strength to a strong eluent strength by mixing more and more of a strong solvent to a weak solvent during the chromatography.
A : KH2PO4(aq)
The gradient can be used to resolve all peaks by reducing the time from 2 h to 38 min.
- Ultraviolet detector
- Electrochemical detector
- Fluorescence detector
- ESI (Electrospray ionization)
- APCI (atmospheric pressure chemical ionization)
Figure 22-23 Atmospheric pressure chemical ionization interface between liquid chromatography column and mass spectrometer.