Lecture note : Gas chromatography [1] ?????????

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Dong-Sun Lee/ CAT-Lab / SWU
2012-Fall version

Chapter 31
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Gas Chromatography (GC) Introduction Gas
chromatography is a chromatographic technique
that can be used to separate volatile organic
compounds. Two types of GC are encountered
gas-solid chromatography(GSC) and gas-liquid
chromatography(GLC). GLC is finds widespread use
in all fields of science, where its name is
usually shortened to GC. A gas chromatograph
consists of a flowing mobile phase, an injection
port, a separation column containing the
stationary phase, and a detector. GSC is based
on a solid stationary phase on which retention of
analytes is the consequence of physical
adsorption. GLC is based on partitioning behavior
of the analyte between the mobile gas phase and
the liquid stationary phase in the column.
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Characteristics of GLC 1. Sensitivity
mg pg (103 109 g) 2. Versatility
from rare gases to liquids and solids in
solution with
8001000 MW 3. Speed of analysis
typically 5 30 min ,
complex 3 30 mixture
separation 4. Reproducibility
qualitative Accuracy quantitative
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Schematic diagram of a gas chromatograph system.
Basic components of Gas Chromatograph
Carrier gas supply Sample
introduction inlet Column and
controlled-temperature oven Detector
oven Recorder
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Carrier gas system in gas chromatograph The
purpose of the carrier is to transport the sample
through the column to the detector. The selection
of the proper carrier gas is very important
because it affects both column and detector
performance. The detector that is employed
usually dictates the carrier to be used. From a
column performance point of view a gas having a
small diffusion coefficient is desirable (high
molecular weight, e.g., N2, CO2, Ar) for low
carrier velocities while large diffusion
coefficients (low molecular weight, e.g., H2, He)
are best at high carrier velocities. The
viscosity dictates the driving pressure. For
high-speed analysis, the ratio of viscosity to
diffusion coefficient should be as small as
possible. H2 would be the best choice, followed
by helium. The purity of the carrier should be
at least 99.995 for best results. Impurities
such as air or water can cause sample
decomposition and column and detector
deterioration. In temperature programmed runs,
impurities in the carrier gas such as water can
be retained at low temperatures but are then
eluted at higher temperatures impairing the
baseline. Many instrument problems have been
traced to contaminated carrier gases. The carrier
must also be inert to the components of the
sample and the column.
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Properties of common carrier gases Gas
molecular weight Thermal conductivity
Viscosity
? 105 at 100oC
? 106 at 100oC
(g-cal/sec-cm- oC )
(?P) Ar 39.95
5.087
270.2 CO2 44.01
5.06
197.2 He 4.00
39.85
234.1 H2 2.016
49.94
104.6 N2 28.01
7.18
212.0 O2 32.00
7.427
248.5 at 99.6oC at 100.5oC
at 99.74oC
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Using the correct carrier and detector gases are
an important factor in installing a new GC. The
five gases commonly used as carrier gas and
detector fuels in capillary gas chromatography
are helium, hydrogen, nitrogen, argon-methane,
and air. The types of gases necessary are partly
determined by the detection system used. Factors
to consider for each individual gas are discussed
below. Carrier Gas Choice Carrier gases that
exhibit a broad minimum on a van Deemter profile
are essential in obtaining optimum performance.
Volumetric flow through a capillary column is
affected by temperature. When temperature
programming from ambient to 300oC, the flow rate
can decrease by 40 percent. A carrier gas that
retains high efficiency over a wide range of flow
rates and temperatures is essential in obtaining
good resolution throughout a temperature
programmed run. Figure 1 shows the van Deemter
profile for hydrogen, helium, and nitrogen
carrier gases.
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Van Deemter curves for GC of n-C17H36 at 175oC,
using N2, He, or H2 in a 0.25 mm diameter 25 m
long wall coated column with OV-101 stationary
phase
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Hydrogen is the fastest carrier gas (uopt), with
an optimum linear velocity of 40cm/sec, and
exhibits the flattest van Deemter profile. Helium
is the next best choice, with an optimum linear
velocity of uopt 20cm/sec. Nitrogen's
performance is inferior with capillary columns
because of its slow linear velocity, uopt
12cm/sec. Argon-methane has a slower optimum
linear velocity than nitrogen and is not
recommended for use as a carrier gas with
capillary columns. Air is not recommended as a
carrier gas because it can cause stationary phase
oxidation.       With hydrogen and helium as
carrier gases, the minimum H.E.T.P. values can be
maintained over a broader range of linear
velocities than with nitrogen, and high linear
velocities can be used without sacrificing
efficiency. Nitrogen is beneficial only when
analyzing highly volatile gases under narrow
temperature ranges where increasing stationary
phase interaction is desirable. Otherwise, the
use of N2 results in longer analysis times and a
loss of resolution for compounds analyzed on a
wide temperature range. http//www.restekcorp.co
m/gcsetup/gcsetup3.htm
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• Exert Caution when Using Hydrogen as a Carrier
  Gas
• Hydrogen is explosive when concentrations exceed
  4 in air. Proper safety precautions should be
  utilized to prevent an explosion within the
  column oven. Most gas chromatographs are designed
  with spring loaded doors, perforated or
  corrugated metal column ovens, and back
  pressure/flow controlled pneumatics to minimize
  the hazards when using hydrogen carrier gas.
  Additional precautions include
• Frequently checking for leaks using an
  electronic leak detector.
• Using electronic sensors that shut down the
  carrier gas flow in the event of pressure loss.
• Minimizing the amount of carrier gas that could
  be expelled in the column oven if a leak were to
  occur by installing a flow controller (needle
  valve) prior to the carrier inlet bulkhead
  fitting to throttle the flow of gas (for head
  pressure controlled systems only) as shown Fig.
  2.
• Fully open the flow controller (needle valve)
  and obtain the proper column head pressure, split
  vent flow, and septum purge flow rates. Decrease
  the needle valve flow rate until the head
  pressure gauge begins to drop (throttle point).
  Next, increase the flow controller (needle valve)
  setting so that the right amount of flow is
  available to the system. Should a leak occur, the
  flow controller will throttle the flow,
  preventing a large amount of hydrogen from
  entering the oven.

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Make-up and Detector Fuel Gases Gas added to the
stream after the column is called makeup gas.
Choosing the correct make-up and detector gases
will depend on both the detector and application.
Most GC detectors operate best with a total gas
flow of approximately 30ml/min. to ensure high
sensitivity and excellent peak symmetry. Refer to
your GC manual for optimum flow rates on
different instruments. Carrier gas flows for
capillary columns range from 0.5 to 10ml/min.
which are well below the range where most
detectors exhibit optimal performance. To
minimize detector dead volume, make-up gas is
often added at the exit end of the column to
increase the total flow entering the detector.
Make-up gas helps to efficiently sweep detector
dead volume thereby enhancing detector
sensitivity. Make-up gas can be added directly
to the hydrogen flame gas for flame ionization
detectors (FID), nitrogen phosphorous detectors
(NPD), and flame photometric detectors (FPD) or
added to the column effluent by an adaptor
fitting. However, GCs such as Perkin-Elmer and
Fisons do not require make-up gas.      
Combustion type detectors (FID, NPD, FPD) use
three gases make-up, hydrogen (fuel gas), and
air (combustion/oxidizing gas). For
non-combustion detectors, such as the thermal
conductivity detector (TCD), electron capture
(ECD), and photo ionization detector (PID), only
carrier and make-up gases are required. In the
case of the electrolytic conductivity detector
(ELCD), the make-up gas is hydrogen, as a
reaction gas in the halogen and nitrogen mode or
air in the sulfur mode. Table I shows recommended
gases for various detectors.
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Carrier gases and detector fuel gases for use
with various GC detectors
TCD ECD FID NPD FPD ELCD PID
Carrier Gases He O O O O O O O
H2 O - O - O O O
N2 O O O O O - O
Combustion/Reaction Gases     H2 - - O O O O -
Air - - O O O - -
Make-up Gases   N2 O O O O O - O
He O - O O O - O
ArCH2 - O - - - - -
http//www.restekcorp.com/gcsetup/gcsetup3.htm
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Effect of impurities - Impurities such as
hydrocarbon, oxygen, water contribute to
unwanted noise levels, excessive baseline
drift. - Molecular sieve --- Moisture trap,
Oxygen trap, Chemical filter Effect of water on
column efficiency - Carrier gas dryness is very
important !! (use anhydrous sodium sulfate) -
Water can and usually does react with some
portion of the column. This results in loss
of resolution and tends to produce asymmetric
or tailing peaks. Unwanted components or
ghost peaks may also appear. Another effect
is a net loss of sensitivity.
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Gas purifiers The trap will remove any water
vapor or oils that may have been introduced in
the filling process since a number of gases are
water pumped. The contaminants removed by the
trap could otherwise interact with the column
packing material to produce spurious peaks. In
addition the contaminants can cause increased
detector noise and drift. The traps should be
reconditioned (about twice a year ) by heating
to 300 oC for 48 hr with a stream of gas
passing through it or in a vacuum oven.
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1. Hydrocarbon trap 2. Moisture trap 3. Oxygen
trap 4. Indicating oxygen trap
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Gas purifier recommendation for GC
applications Capillary column GC Carrier
Hydrocarbon, Moisture, Oxygen
with any detector Make-up
None - all detector but

ECD moisture oxygen
Air for FID
Hydrocarbon
H2 for FID None
ELCD reaction gas
Hydrocarbon Packed column GC Carrier
Hydrocarbon, Moisture, Oxygen
with FID or TCD Packed column GC
Carrier Hydrocarbon,
Moisture, Oxygen with ECD, FPD, NPD, MSD

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Flow requirements 1. Stable 2.
Reproducible 3. Convenient The more
constant the flow rates, the more precise
and accurate the results. Flow controller (
Pressure controller ) --- to maintain
precise and accurate flow rates
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• Effect of decreased flow rate or lower
  temperature
• - All peaks have shifted to longer retention
  times
• - Apparent loss of peak height
• - The base of each peak is wider,
• however, individual peak area remain
  constant.
• Effect of increased flow rate
• Sample components are squeezed toward
• the injection point
• Cause two components to elute together,
• appearing as single peak

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Regulations of carrier gas Carrier cylinder
bottled at about 2500 psi(150-160 atm) Two stage
pressure regulator - first stage high
inlet pressure - second stage low outlet
pressure ( set at
40100psi) Gas generators
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Gas flow rate control A 1 change in carrier gas
flow rate will cause a 1 change in retention
time. For all these reasons it is important to
keep the flow of the carrier gas constant. 1.
Control of carrier gas inlet pressure 2. Control
of carrier gas flow rate In isothermal operation
the means of regulation is immaterial because
both means provide constant inlet pressure as
well as constant flow rate. In temperature
programmed runs, however, the situation is quite
different. If one maintains the inlet pressure
constant the flow rate will change. Therefore,
with temperature programming of the column, the
flow rate must be controlled. Pressure
controllers 1. The second stage regulator on the
cylinder 2. A pressure regulator mounted in the
GC 3. A needle valve(variable restrictor) mounted
in the GC 4. A fixed restrictor mounted in the GC
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Flow measurement 1) Rotometer The column
flow rate is typically indicated by a rotometer
. ( Calibrate equilibrium position indicating
the flow ) Rotometer is operated by the
volume of gas passing a ball in a tapered
cell. 2) Bubble meter 3) Electronic flow sensor
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Relationships between inside diameter, column
length, mesh size, and carrier gas flow for
packed column Inside diameter Mesh size for
Mesh size for Carrier
flow mm length up to 3m
length over 3m N2, ml/min He or H2,
ml/min 2 100120
80100 815
1530 3 100120
80100 1530 3060
4 80100 6080
3060 60100 John A.
Dean, Analytical Chemistry Handbook, McGraw-Hill,
1995, p. 4.31 .
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Comparison of 1/8-in(0.316 cm) packed, wide bore,
and WCOT columns
1/8 in packed Wide
bore WCOT Inside diameter, mm
2.2 0.53
0.25 Film thickness, ?m
5 15
0.25 Phase volume ratio(?)
1530 130250
250 Column length, m
12 1530
1560 Flow rate, ml/min
20 5
1 Effective plates(Neff) per
meter 2000 1200
3000 Effective plate height (Heff),mm
0.5 0.6
0.3 Typical sample size
15 ?g
50 ng John A. Dean, Analytical
Chemistry Handbook, McGraw-Hill, 1995, p. 4.31 .
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Inlet requirements 1. Temperature
controlled 2. Low volume (total swept by
carrier ) 3. Inert construction Column
Overload If too large an sample were allowed to
enter small bore(capillary) columns column
overload and a loss of resolving power would like
occur.
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Liner All liners help protect the vaporized
sample form contacting the metal wall of the
inlet as sample flows onto the column.
Deactivated glass wool may be used as an aid for
sample vaporization, to minimize discrimination
based on boiling point, and to provide a surface
on which non0volatiles can be trapped. The
simplest liner is a straight tube, which gives
all-around good performance at low cost.
Single-taper liners improve on a straight tube by
minimizing sample vapor contact with metal at the
bottom of the injection port, although they are
somewhat more expensive. Liners are deactivated
borosilicate glass, except quartz where noted.
Liners are guaranteed inert for phenols, organic
acids and bases. Why is Glass Wool Added to an
Injector Liner? - General GC The Glass wool
serves three major purposes. The Glass wool will
prevent the small pieces of septa from reaching
the column. The presence of Glass wool will help
the injected sample stay in the liner a little
longer which will help the sample to vaporize and
mix more thoroughly with the carrier gas. If
positioned properly it will wipe the outer
surface of the syringe needle and improve the
precision of the liquid injection.
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Inlet configuration 1. Direct column inlet
--- 1/8 " OD or larger column sampling
syringe is actually inserted into the end of the
column needle guide / cap / spring /
septum mounting holes / carrier gas in /
inlet body column Swagelock ferrules /
Swagelock nut 2. Splitter inlet --- open
tubular column or less than 1/8" OD column
Because of the limited capacity for sample of
these small bore columns and the
difficulty of injecting extremely small volume
samples, a large portion of the injected
sample is vented to atmosphere by the inlet.
septum / preheated carrier gas / mixing tube
/ restrictor buffer volume / tapered
needle / gold gasket / column fitting
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Injection port for split injection into an open
tubular column. The glass liner is slowly
contaminated by nonvolatile and decomposed
samples and must be replaced periodically. For
splitless injection, the glass liner is a
atraight tube with no mixing chamber. For dirty
samples, split injection is used and a packing
material can be replaced inside the liner to
adsorb undesirable components of the sample.
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Common injection techniques 1) Hot flash
vaporization Direct
Cold-trap Split Splitless 2)
Direct cold on column Split or on-column
Split 1) Simple 2) High column efficiency
3) Column may be protected On-column 1)
Best accuracy 2) Thermolabile compounds 3)
Trace analysis
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Representative injection conditions for split,
splitless, and on-column injection into an open
tubular column.
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Direct injector 1) Good sensitivity
2) Low column efficiency 3) Best for thick
films, widebore column ( 0.53 mm ) Hot on-column
injectors 1) Reduced column efficiency
2) Best with thick films, widebore columns
3) Nonvolatiles may damage column 4)
Cold on-column injector may be used with 0.1 to
0.53 mm i.d. columns Advantage of on-column
1) Best reproducibility Quantitative
results 2) No split, no loss of high
boilers 3) "Cold" on-column injection
available
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Advantage of splitless 1) High
sensitivity ( 95 of sample on column )
2) Solvent effect produces narrow sample bands
3) Same hardware as split injection Disadvant
age of splitless 1) Slow sample
transfer to column 2) Must dilute sample
with volatile solvent 3) Time consuming
must cool column 4) Poor for thermolabile
compounds
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Split and splitless injections of a solution
containing 1 vol methyl isobutyl ketone (bp 118
oC) and 1 vol p-xylene (bp 138 oC) in
dichloromethane (bp 40 oC) on a BP-10 moderately
polar cyanopropyl phenyl ,ethyl silicone open
tubular column(0.22 mm I.d., 10 m long, 0.25 ?m,
column temperature 75 oC).
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Common injection methods Syringe injection
Valve injection Sampling Syringe 0
1 µL --- the sample is totally confined to the
needle 0 5 / 0 10 µL
needle / barrel / plunger Gas sampling valve
Sample in Sample loop --- 1/4 or 10 mL loop
size compatible
with needs Sample vent Carrier gas to
column
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Solvent effect t-1
t-2
time t-1 --- just after injection, solvent and
sample are condensed in a
long plug at the front
of the column.
The column temperature must be cold enough to
condense the
solvent. time t-2 --- after some time,
the column temperature has been raised,
most of
the solvent has evaporated, and the solvent
effect has left the
sample molecules concentrated in a narrow band.
As the column is
further heated, the remaining solvent and
sample molecules are rapidly
vaporized --- resulting
in high column efficiency and narrow peak.
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Syringe for solid phase microextraction.
Sampling by SPME and desorption of analyte from
the coated fiber into a gas chromatograph.
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Purge and trap apparatus for extracting volatile
substances from a liquid or solid by flowing gas.
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GC column Parts of Column 1) Tubing material
Stainless steel--- reactive ( steroids,
amines, free acids ) Glass ------------
can be made inert, difficult handling
Fused silica ---- flexible
most inert
most popular
high resolution 2) Stationary phase Solid
support --- carefully sized granular Liquid
phase --- active portion of the column
Porous polymers Adsorbents
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Important column parameters 1) Inside
diameter 2) Length 3) Film
thickness 4) Stationary phase
composition 5) Flow rate
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Column diameter i.d.
Resolution Speed Capacity Ease
100 micrometer
(narrow bore )
250, 320
(mid bore)
530
(wide
bore) Column length N œ L
R œ L1/2 tR œ L

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Column Glass wool --- both ends of the column
1- ½ (inlet side)
1/4 (detector
side)
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Fused silica surface made by the reaction of
SiCl4 and water vapor in a flame - SiO2
contains 0.1 OH groups - Very inert
- Uniform chemical surface Fused silica
- High tensile strength - Flexible -
Sheath of polyimide - Very inert
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Fused Silica Capillary ColumnsA fused silica
capillary column is comprised of three major
parts (Figure 1). Polyimide is used to coat the
exterior of fused silica tubing. The polyimide
protects the fused silica tubing from breakage
and imparts the amber-brown color of columns. The
stationary phase is a polymer that is evenly
coated onto the inner wall of the tubing. The
predominant stationary phases are silicon based
polymers (polysiloxanes), polyethylene glycols
(PEG, Carbowax) and solid adsorbents. Figure 1.
Capillary columns have to be properly installed
to maximize their performance and lifetime. You
can obtain enhanced column performance and
lifetime by following these recommended
installation guidelines. More detailed
installation, operational and troubleshooting
information can be found in the following
references
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WCOT Wall Coated Open Tubular invented
and patented by Dr Marcel Golay Tubing
- Fused silica - Glass
- Stainless steel
Liquid phase coating WCOT - - - High
resolution Film thickness 0.5 to 5.0
micrometer i.d. 0.10, 0.25, 0.32,
0.53 mm Length 10 to 60 m
Open tubular GC column
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Operational guideline for open tubular GC
columns
WCOT

narrow intermediate wide
bore Column inner diameter, mm
0.25 0.32
0.53 Maximum sample volume, ?l 0.5
1
1 Maximum amount for one component, ng
250 375
5100 Effective plates(Neff) per
meter 30005000 25004000
15002500 Trennzahl(separation) number per 25 m
40
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25 Optimum flow for N2, ml/min 0.51
0.81.5 24 Optimum
flow for He, ml/min 12
12.5 510 Optimum flow for
H2, ml/min 24 37
815 Optimum velocity is 10
to 15 cm/s for each column Optimum velocity
is 25 cm/s for each column Optimum velocity
is 35 cm/s for each column
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Other types of capillary columns SCOT
Support Coated Open Tubular Solid
support Celite Liquid phase
Not available fused silica tubing PLOT
Porous Layer Open Tubular Porous
adsorbent alumina or molecular sieve
Molecular sieve --- efficient for H2, Ne, Ar,
O2, N2, CO, CH4.
Porous carbon stationary phase ( 2 ?m thick) on
inside wall of fused silica open tubular column.
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Capillary column vs Packed column
Capillary Packed
Length 60 m 2 m
Theoretical plates(N/m) 3000-5000 2000
Total plates length(N/m) 180000-300000 4000
Advantages of capillary column 1) Not packed
long lengths high resolution 2) Thin film
efficient, fast 3) Used silica --- inert surface,
better results
Disadvantages of capillary column 1) More
expensive 2) Limited liquid phase Requires
very small samples 3) Dedicated instrumentation
--- capillary inlets, septum purge
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(Left) Gas chromatogram of alcohol mixture at 40
oC using packed column ( 2mm I.D., 76 cm long
containing 20 Carbowax 20 M on a Gas-Chrom R
support and FID. (Right) Chromatogram of vapors
from headspace of beer can, obtained with 0.25 mm
diameter, 30 m long porous carbon column oerated
at 30 oC for 2 min and then ramped up to 160 oC
at 20 oC/min.
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Component Separation with the Column lt The
process of separation gt A series of
partitions Dynamic In-and Out (or Stop-and-Go)
All
differential migration process. The most
volatile components usually pass through the
column first, the least volatile or
highest boiling emerges last.
Mobile phase( Driving force)
?? ?
?
?
?
?
?
Analytes
Stationary phase ( Resistive force)
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Capillary Column Installation Steps 1. Check
traps, carrier gas, septum, liner 2. Place the
nut and ferrule on the column and carefully cut
the column end 3. Install the column into the
injector 4. Turn on the carrier gas 5. Install
the column into the detector 6. Inspect for
leaks 7. Confirm carrier gas flow and proper
column installation 8. Condition the column 9.
Accurately set the carrier gas velocity 10. Bleed
test 11. Run test mix Click here for a complete
listing of tools available from JW, including
magnifiers and cutting tools. http//www.jandw.com
/gccolumn.htmFused Silica Capillary Columns
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Recommended Installation Tools and Supplies 1.
Cutting tool such as a diamond or carbide tipped
pencil, sapphire tipped pencil or ceramic
cleaving wedge 2. Magnifier (10-20X) 3. Ruler 4.
Wrench 5. Ferrules 6. Vial of solvent 7. Clean
syringe 8. Supply of an appropriate non-retained
compound 9. Column test mixture 10. Flow
meter 11. Other supplies septa, clean injector
liners, liner ferrules/O-rings, etc.
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Conditioning of the ColumnOnce the column has
been checked for proper installation and the
absence of leaks, it is ready for conditioning.
Heat the column to its isothermal, upper
temperature limit (temperature limits listed
below) or a temperature 10-20 oC above the
highest operating temperature of your particular
method. Do not exceed the upper limit or column
damage will result. Heat the column rapidly -
slow temperature programming is not necessary.
After the column has reached the conditioning
temperature, plot the baseline. Keep the baseline
on scale so that it can be observed. The baseline
should be elevated at first then start to drop
after 5-10 minutes at the conditioning
temperature. The baseline will continue to drop
for 30-90 minutes then stabilize at a constant
value. If the baseline does not stabilize after
2-3 hours or does not start to significantly
decrease after 15-20 minutes, either a leak is
present or a contamination problem exists. In
either case, immediately cool the oven down below
40 oC and resolve the problem. Continued
conditioning will result in column damage or the
inability to obtain a stable baseline. Excessive
conditioning of the column may result in a
shortened lifetime. In general, polar stationary
phases and thick film columns usually require
longer times to stabilize than less polar and
thinner film columns. GS PLOT columns require a
different conditioning procedure than liquid
stationary phase columns.
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Temperature Limits The temperature limits define
the range over which the column can be safely
used. If the oven is operated below the lower
temperature limit, poor separation and peak shape
problems will be evident, but no column damage
will occur. Upper temperature limits are usually
given as two numbers. The first or lower
temperature of the two is the isothermal limit.
The column can be maintained at this temperature
for indefinite periods of time. The second or
higher temperature is the program limit. The
column can be heated to this temperature for
short periods of time (lt10 minutes). Exceeding
the upper temperature limits will significantly
reduce column lifetime.
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• Liquid Phase(Stationary phase) Classes
• 1. Non-polar phase gives boiling point order
  separation
• 2. Selective phase separates components that have
  close b.p. and
• small structural differences
• 3. Polar phase depends on internal functional
  groups to separate
• compounds that have reactive OH, NH2 or
  other polar radicals
• 4. Each stationary phase retains solutes in its
  own class best
• Raising percentage of stationary phase leads to
• Greater capacity for solute
• Longer retention time
• Increased HETP

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?-Cyclodextrin for chiral column.
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Column oven and temperature control Oven
size column sits in an oven Inner volume
laboratory GC about 22-25 L
process GC about 40 L
Isothermal The oven temperature is kept
constant during the entire analysis
Practical temperature lt 250oC Maximum
temperature lt 400oC Temperature programming
The oven temperature is varied during the
analysis Linear
Non linear High temperature GC (HTGC)
Working limit conventional GC --- 330oC
HTGC --- 450oC
Masses of analysed substrates 600-1000
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Temperature programming With homologues, the
retention time increases exponentially with the
number of carbon. As retention time increases,
width increase and the height decreases, making
detection impossible after a few peaks have
eluted. Since solubility of gas in a liquid
decreases as temperatures goes up, we can
reduce the retention of a compound by increasing
column temperature. General steps to create a
program assuming that the separation is
possible 1) Determine initial temperature and
time based on best possible separation offirst
few peaks 2) Report for the last few peaks to
find the best final temperature and time 3)
Experiment with various ramps to account for the
rest of the components
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Temperature programming Factors to consider
Variations in solubility of solutes
Changes in volatility of solutes Stability
of solutes Flow rate changes
Stability of stationary phase Must stay within
Tmin/Tmax of column Other factors are found
experimentally
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A temperature program Ex. 40 oC(5 min) 10oC/min
250oC(10 min)

• Raising column temperature
• Decrease retention time
• Decrease resolution
• Sharpens peaks

A initial temperature and holding time B ramp
(oC/min) C final temperature and holding
time Some GCs will allow for a more complex
program.
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Comparison of isothermal and programmed
temperature chromatography. Each sample contains
linear alkanes run on a 1.6 mm 6 m column
containing 3 Apiezon L (liquid phase) on a
100/200 mesh VarAport 30 solid support with He
flow rate of 10 ml/min.