insmeth lecture 8 - gas chromatography
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Gas Chromatography
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General Design of a GC
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Some of the designdetailsGas supplies usually have either in-line or instrument
mounted traps to remove any water, oxygen,hydrocarbons or other contaminants fromcompressed gases
Instruments can have multiple injectors, detectors orcolumns
Injectors and detectors usually have their owntemperature controlled zones (small heaters)
The GC oven has a large fan and a vent door to helpwith rapid cooling of the oven
Data collection (and integration) can be done usinga chart recorder, integrator or a computerized datasystem
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Separation Processes in GC The analyte is in the gas phase in the GC and partitions between
the mobile phase (carrier gas) and the liquid stationary phase thatis coated on the inside of an open-tubular capillary column or onparticles inside a packed column
Some packed-column GC uses non-coated solid stationaryphases, in which case one is performing gas-solid adsorptionchromatography
Capillary, open-tubular (WCOT specifically) column GC is theprimary type of GC used in quantitative analysis: higher resolution = greater ability to discriminate between
components smaller capacity of the column is not important as long as sufficient
analyte is available for detection
pg/mL (ppt) to g/mL (ppm) concentration range for liquid analytes
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The Objective in
Chromatography (all types) Separate your analytes in the shortest amount of
time possible and detect them.
How can we do this in GC? Use different columns for different analyte types
stationary phase
diameter of column, stationary phase thickness
column length
Use different injection types/temperatures to optimizethe process of loading the sample on the column
Use different temperature (or pressure) programs forthe column
Select and use a detector that is suitable for theanalyte(s) of interest
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GC Columns (concentrating on open-tubular
capillary columns)
Column frame constructed of fused silica tubing Polyamide coating on the outside gives it strength
Liquid stationary phases coated or bonded to the inside ofthe tubing
0.1 - 0.53 mm + ID, 5-100 meters in length, stationary
phases usually 0.10 to 1.5 m in thickness Mounted on a wire cage to make them easier to handle
5-150 meters long.
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Capillary
ColumnStationary
Phases
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Choosing a GC Column Is the column compatible with your analytes
polar analytes require polar stationary phases so theywill spend some of their time in the stationary phase
non-polar analytes require non-polar stationary phases
You usually have to compromise on the stationaryphase to get a good column for your analytes (whichare probably a mix of polar and non-polar)
DB-5, HP-5, EC-5, RTX-5 (5% dimethyl, 95% diphenylpolysiloxane) most common general use column.
Temperature range, solvent and carrier gascompatibility
Sample capacity versus resolution usually determines packed vs.. capillary
GCs usually setup for either packed or capillary
Lets say you choose a capillary column, theresmore to think about!
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For capillary GC columns. Increased length = greater N, therefore a
greater R
expense is possible band broadening if analytes are onthe column too long!
Increased length leads to longer separations. Do youhave the time?
Increased stationary phase thickness and
column diameter provides increased samplecapacity and can provide increased resolution tradeoffs are a longer analysis time and more column
bleed with thicker stationary phases
Is the column compatible with the detector? Thick stationary phases bleed more and will
contaminate a mass spectrometer.
For most analytical work, a best compromisecolumn is chosen and other variables (temp,etc.) are altered to optimize the separation.
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Capillary vs. Packed ColumnsCapillary Columns:
Higher resolution (R)Greater HETP and N
Shorter analysis time
Greater sensitivity
Most common in
analytical laboratory GCinstruments
Smaller sample capacity
Higher cost/column
Columns more
susceptible to damage
Packed ColumnsGreater sample capacity
Lower cost (can make yourown)
More rugged
Most common in process labsor separating/determining
major components in asample (prep GC)
Limited lengths reduces R andN
Not compatible with some
GC detectors
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Temperature Programming in GC The simplest way to alter the separation in GC
is to alter the temperature program in the oven.
You can also alter the pressure of the carrier gas,but this is less common (much).
Isothermal = constant temperature
Gradient = varied temperature
By altering the temperature, you vary the rate of
the reaction for any analyte: they spend more or less time in the stationary phase the greater the difference in the times between
analytes, the better the separation!
AnalyteAnalyte phasestationaryphasemobile
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The traps of temperature
If your temperature at a given time is too high,you can cause the peaks to co-elutepoor resolution vs but a faster separation
If your temperature at a given time is too low,
you can get still get a good separationadequate resolution, but a separation that takes
very long
You have to choose a compromisetemperature program
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GC Carrier Gases (the mobile phase) Usually inert gases (dont react with analytes except
sometimes in the detector)
Purpose: sweep sample through the column
protect column from oxygen exposure at temperature
assist with function of the detector
Most common: Helium (available relatively pure without extensive purification
after it leaves a compressed gas cylinder)
Nitrogen (usually requires an oxygen and water trap)
Hydrogen
normally used only with flame ionization detectors (FID) since theFID needs it as fuel for the flame
still rarely used due to safety concerns (and chromatographicones)
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GC Injection. Samples are injected through a septum:
keeps oxygen out of the column
provides a seal to keep the carrier gas pressure up atthe head of the columncarrier gas flow rate is determined by the pressure or the
gas at the opening of the column
Many different (mostly proprietary) materials
red rubber (bleeds at about 250 C)Thermogreen (good up to about 300 C)
High-temperature blue (good a little over 300 C)
The injector is usually lined with a de-activatedglass liner prevents metal injector-sample reactions that would
alter analytes or damage the metal of the injector
Can be cleaned/replaced regularly
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Injection types
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On-Column Injection: used widely in packed-column GC, less in capillary GC
sample is deposited directly on the column
Good for thermally unstable compoundsGood for quantitative analysis at low concentrations
all sample is available to travel to the detector
BUT, you can inject only a relatively small amount ofsample in capillary GC anyhow.
Splitless Injection: Sample is vaporized in the injector itself and ALL of the
sample is swept onto the column by the carrier gas
Again, relatively small samples are injected (10 L orless in capillary GC)
Sample spends a large amount of time in the injector Best for trace (1 -100 ppm range) concentrations of
high boiling point analytes in low boiling point solvents
extra time in the injector helps volatilize the analytes.
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Split Injection:the injection is split, with only a portion of the
sample (usually 1% - 20%) actually making it
to the columnthe most common method of injecting
samples onto small diameter, open-tubularcolumns.Even if you inject 20 L, only a fraction
(adjustable) makes it on to the columnNot good for analytes with a wide range of
boiling pointssome may be swept out the split vent before they
are volatilized
Modern capillary GCs come with aSplit/Splitless injectors standardyou switch between modes by changing
the split vent gas flow and using a different
injection liner.
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Dont Forget SPME (Solid Phase Microextraction)
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GC Detectors A dozen or more varieties (some obscure)
Must be: sensitive to the analytes of interest compatible with the column, carrier gas, solvent, etc.
rugged enough to withstand general unattended used
Ive run our new GC for 36 hours straight without touching it!
Should have a known linear range if the detector response is very linear, you can use a
response factor instead of a calibration curve forquantitation!
Usually require separate gas supplies (other than the
carrier gas), have their own temperature control.Measure nothing more than a voltage or a current.
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ThermalConductivity (TCD)
The carrier gas has a knownthermal conductivity.
As the thermal conductivityof the column eluent (gasflow in) changes, the
resistance of the filamentchanges.
The presence of analytemolecules in the carrier gasalter the thermal conductivityof the gas (usually He)
There is normally a secondfilament to act as areference (the carrier gas issplit)
Increased sensitivity withdecreasing temperature
(detector), flow rate andapplied current.
Filaments will burn out(oxidized) in the presence ofoxygen if hot!
Non-destructive
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FID D t ti l l t
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FID Destructive, sample lost. Analytes containing C
burn in a hydrogen-oxygen flame andproduce ions
CHO+ ions are collectedon a cathode and thecurrent they produceresults in the signal
WILL NOT detect non-C
containing compounds! Requires H2 supply (tank or
generator) and O2 supply(compressed air)
H2 carrier gas can be
used, eliminating theneed for a supply for thedetector
A makeup gas can alsobe required!
-FlameO,H
eCHOOCH22
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ECD Particularly sensitive to halogens
nitriles, carbonyls, nitro compounds
Analytes pass through a cell, in whichelectrons are traveling between a 63Ni
electrode and a collector electrode As analytes with electron capturing
ability pass through the cell, the flowof electrons is interrupted.
The change in current, due toreduced flow of electrons, isrecorded.
EXTREMELY SENSITIVE TO HALOGENS
could ruin detector with 1 ppm
hexachlorocyclohexane by
contaminating it with excess analyte
Widely used for the determination ofpesticides, herbicides and PCBs inenvironmental samples.
Non-destructive
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