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Gas Chromatography
Introduction
1.) Gas Chromatography Mobile phase (carrier gas) is a gas
- Usually N2, He, Ar and maybe H2
- Mobile phase in liquid chromatography is a liquid
Requires analyte to be either naturally volatile or can be converted to a volatile
derivative- GC useful in the separation of small organic and inorganic compounds
Stationary phase:- Gas-liquid partition chromatography – nonvolatile liquid bonded to solid support
- Gas-solid chromatography – underivatized solid particles
- Bonded phase gas chromatography – chemical layer chemically bonded to solid
support
Magnified Pores in activated carbon Zeolite molecular sieveBonded phase
Gas Chromatography
Introduction
2.) Instrumentation Process:
- Volatile liquid or gas injected through septum into heated port
- Sample rapidly evaporates and is pulled through the column with carrier gas
- Column is heated to provide sufficient vapor pressure to elute analytes
- Separated analytes flow through a heated detector for observation
AA
A
AA
A
A
A
B
B
B
B
B
BB B
Liquid Stationary Phase
B A
A A
A
A
A
A
AA
B BB
B
B
B
B
B
Capillary Tube
He Carrier gas
Time0
Immediately after injection
He Carrier gas
After several minutes Resulting chromatogram
Compounds A and B interact with the stationary phase through intermolecular forces:
(van der Waals or dipole-dipole forces, including hydrogen bonding).
A interacts more strongly with the stationary liquid phase and is retained relative to B,
which interacts weakly with the stationary phase. Thus B spends more time in the gas
phase and advances more rapidly through the column and has a shorter retention
time than A.
Typically, components with similar polarity elute in order of volatility. Thus alkanes
elute in order of increasing boiling points; lower boiling alkanes will have shorter
retention times than higher boiling alkanes.
GAS CHROMATOGRAPHY
Sample Injection
System
• For quantitative work, more
reproducible sample sizes for
both liquids and gases are
obtained by means of a rotary
sample valve.
• Errors due to sample size can be
reduced to 0.5% to 2% relative.
• The sampling loop is filled by
injection of an excess of sample.
• Rotation of the valve by 45 deg
then introduces the reproducible
volume ACB into the mobile
phase.
Column Configurations
• Two general types of columns are encountered in gas chromatography, packed and
open tubular, or capillary.
• Chromatographic columns vary in length from less than 2 m to 50 m or more. They are
constructed of stainless steel, glass, fused silica, or Teflon. In order to fit into an oven
for thermostating, they are usually formed as coils having diameters of 10 to 30 cm.
• Column temperature is an important variable that must be controlled to a few tenths of
a degree for precise work. Thus, the column is ordinarily housed in a thermostated
oven. The optimum column temperature depends upon the boiling point of the sample
and the degree of separation required.
Gas Chromatography
Instrumentation
1.) Open Tubular Columns Commonly used in GC
Higher resolution, shorter analysis time, and greater sensitivity
Low sample capacity
Increasing Resolution- Narrow columns Increase resolution
- Resolution depends directly from column length
Easy to generate long (10s of meters)
lengths of narrow columns to maximize
resolution
Gas Chromatography
Instrumentation
1.) Open Tubular Columns Increasing Resolution
Decrease tube diameter
Increase resolution
Increase Column Length
Increase resolution
Gas Chromatography
Increase Stationary Phase Thickness
Increase resolution of early eluting compounds
Also, increase in
capacity factor and
reduce peak tailing
But also decreases
stability of stationary
phase
Instrumentation
1.) Open Tubular Columns Increasing Resolution
Gas Chromatography
Instrumentation
2.) Choice of liquid stationary
phase: Based on “like dissolves
like”
Nonpolar columns for
nonpolar solutes
Strongly polar columns for
strongly polar compounds
Experimental Organic Chemistry D. R. Palleros, Wiley, NY, 2000
GC – Stationary Phase
Gas Chromatography
Instrumentation
3.) Packed Columns Greater sample capacity
Broader peaks, longer retention times and less resolution- Improve resolution by using small, uniform particle sizes
Packed columnOpen tubular column
Gas Chromatography
Retention Index
1.) Retention Time Order of elution is mainly determined by volatility
- Least volatile = most retained
- Polar compounds (ex: alcohols) are the least volatile and will be the most
retained on the GC system
Second factor is similarity in polarity between compound and stationary
phase
Gas Chromatography
Retention Index
2.) Describing Column Performance Can manipulate or adjust retention time by changing polarity of stationary
phase
Can use these retention time differences to classify or rate column
performance- Compare relative retention times between compounds and how they
change between columns
Can be used to identify unknowns
Gas Chromatography
Temperature and Pressure Programming
1.) Improving Column Efficiency Temperature programming:
- Temperature is raised
during the separation
(gradient)
- increases solute vapor
pressure and decrease
retention time
Temperature gradient improves
resolution while also decreasing
retention time
Gas Chromatography
Temperature and Pressure Programming
1.) Improving Column Efficiency Pressure Programming:
- Increase pressure increases flow of mobile phase (carrier gas)
- Increase flow decrease retention time
Pressure is rapidly reduced at the end of the run- Time is not wasted waiting for the column to cool
- Useful for analytes that decompose at high temperatures
Van Deemter curves indicate
that column efficiency is
related to flow rate
Flow rate increases N2 < He < H2
Detection Systems
Characteristics of the Ideal Detector:
The ideal detector for gas chromatography has the following characteristics:
1. Adequate sensitivity
2. Good stability and reproducibility.
3. A linear response to solutes that extends over several orders of magnitude.
4. A temperature range from room temperature to at least 400oC.
5. A short response time that is independent of flow rate.
6. High reliability and ease of use.
7. Similarity in response toward all solutes or a highly selective response toward one or
more classes of solutes.
8. Nondestructive of sample.
Flame Ionization Detectors (FID)
The flame ionization detector is the most widely used and generally applicable
detector for gas chromatography.
• The effluent from the column is mixed with hydrogen and air and then ignited
electrically.
• Most organic compounds, when pyrolyzed at the temperature of a hydrogen/air
flame, produce ions and electrons that can conduct electricity through the flame.
• A potential of a few hundred volts is applied.
• The resulting current (~10-12 A) is then measured.
• The flame ionization detector exhibits a high sensitivity (~10-13 g/s), large linear
response range (~107), and low noise.
• A disadvantage of the flame ionization detector is that it is destructive of sample.
GC-MS
GC-MS
Gas Chromatography
1.) Qualitative and Quantitative Analysis
Compare retention times between reference sample and unknown- Use multiple columns with different stationary phases
- Co-elute the known and unknown and measure changes in peak area
The area of a peak is proportional to the quantity of that compound
210641 wtpeak heigh. peak Gaussian of Area
Peak A
rea
Concentration of Standard
Peak area increases proportional
to concentration of standard if
unknown/standard have the
identical retention time same
compound
GC – Peak Areas and Resolution
Method Development
Basic parameteres
• Sample injection (split- splitless)
• Column selection (packed or capillary column)
• Selection of gradient or isothermal program
• Detector selection
Inlet temperature, detector temperature, column temperature and
temperature program, carrier gas and carrier gas flow rates, the
column's stationary phase, diameter and length, inlet type and flow
rates, sample size and injection technique
GC - Derivatization
Why is chemical derivatization needed?
GC is best for separation of volatile compounds which are thermally stable.
Not always applicable for compounds of high molecular weight or containing polar
functional groups. These groups are difficult to analyze by GC either because they
are not sufficiently volatile, tail badly, are too strongly attracted to the stationary
phase, thermally unstable or even decomposed.
Chemical derivatization prior to analysis is generally done to:
• increase the volatility and decrease the polarity of compounds;
• reduce thermal degradation of samples by increasing their thermal stability;
• increase detector response
• improve separation and reduce tailing
Derivatizing Reagents Common derivatization methods can be classified into 4
groups depending on the type of reaction applied: • Silylation
• Acylation
• Alkylation
• Esterification
The Retention Index / Kovats index
The retention index I was first proposed by Kovats for identifying solutes from
chromatograms. The retention index for any given solute can be derived from a
chromatogram of a mixture of that solute with at least two normal alkanes having
retention times that bracket that of the solute. That is, normal alkanes are the standards
upon which the retention index scale is based. The retention index for a normal alkane
is equal to 100 times the number of carbons in the compound regardless of the column
packing, the temperature, or other chromatographic conditions. Within a homologous
series, a plot of the logarithm of adjusted retention time t`R (t`R = tR - t`M) versus the
number of carbon atoms is linear.
I = Kovats retention index,
n = the number of carbon atoms in the smaller
alkane,
N = the number of carbon atoms in the larger
alkane,
tr' = the adjusted retention time.
The Retention
Index / Kovats
index
CHROMATOGRAPHY
Preparative vs Resolution vs Speed vs Expense