Gas Chromatography
A separation science...
What is Chromatography?
• The separation of a mixture of two or
more compounds or ions by
distribution between a stationary phase
and a mobile phase.
• Chromatography is not an
identification method!!
Types of Chromatography?
Based on partition between two phases:
Liquid- Solid
- TLC, column, HPLC
Liquid-Gas
- Gas Chromatography
mobile
stationary
Components of a mixture
partition between being
dissolved in a stationary
liquid phase or in a mobile
gas phase.
Gas Chromatography (GC)
GC is used to separate mixtures of volatile compounds
whose boiling points may differ by as little as 0.5 °C.
• Analytical tool to HELP identify components in a
mixture.
• Preparatory applications when quantities of pure
compounds are desired.
What does a Gas Chromatograph Look Like ?
Where does the separation occur?
- in the column!
Heat
PARAMETERS AFFECTING
SEPARATION
I.
Nature of Stationary Phase
II. Mobile Phase (inert carrier gas)
III. Size (length) of Column
IV. Rate of Flow (of mobile phase)
V. Temperature (of column)
Parameters Affecting Separation in GC
I.
Stationary Phase
A.
Solid Support
- inert, high surface area, resistant to
crushing under operating pressures
B. Liquid Coating
- high boiling point, low viscosity at
operating temperature, polarity
“LIKE DISSOLVES LIKE”
I. A. Examples of Solid Support Materials
Least
Inert
• Chromasorb P - Pink diatomaceous earth metallic contaminants - pH 6-7 -relatively soft
• Chromasorb W - White diatomaceous earth acid washed to remove metals
• Chromasorb G - Chromasorb W which has
been chemically cross-linked for physical
strength
Most
Inert
• Chromasorb T - Teflon beads
• Other - charcoal, alumina, silica, etc.
I. B. Examples of Liquid Phase Materials
Liquid Phase
Type
Property
Max Temp. Limit
(°C)
Carbowax 20M
Hydrocarbon wax
Polar
250
OV-17
Methylphenyl
silicone
Intermediate
polarity
300
QF-1
Fluorosilicone
Intermediate
polarity
250
SE-30
Silicone gum
rubber
Nonpolar
375
Table 6.1, pg. 197 in textbook
*Above max temp., liquid phase will vaporize and bleed
from column w/the mobile phase
I. B. How much Liquid Phase ?
• Rule of Thumb - The mass of stationary
phase should be about 30 times that of the
sample to be separated.
Columns should be labeled as, for example:
10% OV-1 on Chromasorb W
(This means the stationary phase is 10% by weight of
OV-1 on Chromasorb W.)
I. B. Two important factors that determine
solubility of a gas in a liquid:
1. Vapor pressure at column temp.
i.
As VP increases, solubility decreases.
2. Magnitude of the gas interactions with
the liquid phase.
i.
“Like dissolves like”
Parameters Affecting Separation in GC
II.
Mobile Phase
A.
Carrier Gas
- should be pure, dry, and inert
Examples: He and nitrogen
(sometimes hydrogen)
Parameters Affecting Separation in GC
III.
Size of Column
Preparatory = larger internal diameter and
shorter length - b/c greater loading capacity
Packed = 1/4 to 3/8 inch diameter - 6 to 12
ft long -copper, stainless steel, or glass
Capillary = 0.1-0.2 mm diameter - 30 ft good separations but small loading capacity
Resolution increases with:
• Larger column length (better ΔRt between bands)
• Smaller diameter (narrower bands)
Parameters Affecting Separation in GC
IV.
Rate of Flow
Too Slow = diffusion and poor separation
Too Fast = bunching-up and poor separation
Parameters Affecting Separation in GC
V.
Temperature
Higher Temperature = sample spends less
time in the stationary liquid phase - if too
high, there will be no separation
How do you monitor the
column?
Detectors:
• Thermal Conductivity (TC)
• Ionization
–
–
–
–
Flame Ionization ( FID )
Electron Capture ( EC )
Mass Spectrometer ( MSD )
Other [ flame photometry, coulomeric, etc. ]
Thermal Conductivity Detector
The smaller the
molecules in
the gas phase,
the better they
cool the
filament…
As temperature
changes so
does
resistance...
UNIVERSAL DETECTOR !
Flame Ionization Detector
• The carrier gas and sample are burned in a hydrogen
flame to create ions.
• These ions pass through a capacitor.
• Charged particles conduct electricity from one pole to the
other creating an electric current.
• The millivolt current is measured.
• Since combustion produces low molecular weight gases
such as CO2 and H2O and requires the input of air (nitrogen
and oxygen) these gases can not be measured by this
method.
• This is not a universal detector and requires destruction
of at least part of the sample.
Electron Capture Detector
• A beta emitter gives off electrons.
• As the organic compounds pass through the stream of
electrons, some of them become ions.
• The ions produced conduct current from one plate to the
other of the detector and the current is measured.
• This method works very well for compounds containing
highly electronegative elements such as the halogens.
However, there are some substances which do not eagerly
accept the electrons.
• This is not a universal detector and requires destruction of
at least part of the sample, as well as AEC license.
What are the variables ?
•
•
•
•
•
Retention Time
Resolution
Theoretical Plates
Percent Area
Others - peak type, peak width, chart speed,
area reject, etc.
• Rate of flow
Retention Time
(Rt)
• Measured in minutes
Distance
= Rt
Chart Speed
• Longer Rt means greater solubility of component in
liquid phase..
• Chromatogram should be labeled with column
temperature, flow rate, and liquid phase….
Retention Time (Rt)
Greater retention time = better separations
To increase retention time:
• lower the temperature
• use a longer column
• change to another liquid phase
Rt of a component is independent of the presence/absence of
other components in the mixture.
Resolution
• Resolution = quality
of separation
Resolution =
( t 2 - t1 )
(avg. width at base)
Resolution <1 means overlap
Resolution > 1 = no overlap
Resolution
…is quality of separation.
To increase resolution:
• decrease the temperature
• longer column
• greater % liquid phase
• different liquid phase
Theoretical Plates
Theoretical Plates – as in distillation, is measure of number
of equilibria encountered.
• GC has many more theoretical plates than fractional distillation.
Theo Plates = 16 [ Rt / (peak width at base)]2
All units must cancel !!
Used to monitor column aging ….
Theoretical Plates
Theoretical Plates -- more plates means greater efficiency!
To increase the number of theoretical plates:
• If the column has been used for a while, replace
with a newer column. OR…
• longer column
• greater % liquid phase
• different liquid phase
Percent Area
% Area is approximately the % composition….
If you do not have an electronic integrator, you may determine the
area by measuring the height of the peak and multiplying by the
width of the peak at half height.
% Area would then be the area of one peak divided by the total area
of all peaks.
This assumes that each compound creates the same current for the
same mass of sample. i.e. response/weight factors
Flow Rate
How to calculate Flow Rate…
- Some air will undoubtedly get into the column during
injection.
- However, this air will not be retained by the column, and
will simply be moving along with the carrier gas (at the
same rate). This air peak usually appears within the first
few seconds.
Flow Rate = (Length of column) / (Rt of the air peak)
To Prepare Your NB…
• Table of Properties – still only working with
ethyl and butyl acetate, so just reference the pg. #
in your NB where you have all the info for both
chemicals (in simple distillation experiment).
• No Chemical Reactions
• Print and tape both handouts (on Moodle) into
the Procedure section. One handout gives an
overall summary of what you’ll be doing, and the
other is VERY important b/c it tells you
specifically how to use and operate the GC and
its software.
• No Figures needed.
General Procedure will include…
1. The GC will be set up by the instructor.
2. Record all pertinent set-up factors – carrier gas, flow rate,
oven temperature, detector temperature, which column,
liquid phase, polarity, attenuation, etc. (DATA section)
3. Follow the directions for the software on the
HANDOUTS that you have been provided on Moodle.
(print and tape into NB).
4. Draw up 1 microliter of the sample into the syringe – be
careful not to have bubbles.
5. Insert the syringe needle fully into the appropriate inlet
and quickly push the plunger, remove the needle, and start
the integrator. Be careful not to bend the needle or plunger.
6. Wait long enough to ensure that all the material you have
injected is out of the column, then end the run.
7. Repeat for all samples.
EXAMPLE OF A TYPICAL GC PLOT:
Calculations Section of NB
1. Attach all printouts to the notebook!!
2. Compile a table using the data from the printouts:
5 columns – Sample ID, % Ethyl acetate, % Butyl acetate,
Resolution, Theoretical Plates.
3. Calculate Resolution for all samples where the peaks for
the two components are both significant and put them in
the table. Give at least one clearly labeled sample
calculation.
4. Calculate the number of Theoretical Plates for the GC that
reflects the most pure Ethyl acetate sample and for the
most pure Butyl acetate sample, and put them in the table.
Give at least one clearly labeled sample calculation.