gas chromatography - isen · - l. polite, practical gas chromatography think back: • what...

Gas Chromatography

Modern Techniques in Heterogeneous Catalysis Research

Prof. Neil M. Schweitzer

Updated 2-4-16

• General technique used to separate gas into pure components based on interactions with a stationary phase in a column

Gas Chromatography

• Why use GC?

– Easy to quantitate

• High Sensitivity

– High resolution

– High reproducibility

– Very Fast!- The Essential Chromatography & Spectroscopy Guide, GC and GC/MS, Agilent Technologies

Gas Chromatography

Oven - L. Polite, Practical Gas Chromatography

These are the general parts of aGC, and the focus of ourdiscussion…

GC Injection Ports

• What is the purpose of the Injection port?

– Introduce the sample onto the column - Temperature

– Control the flow of gases through the column - Pressure

• Many exist, but most common is the split/splitless injector

- L. Polite, Practical Gas Chromatography

There is no flow controller on the column… flowrate through column is controlled by the head pressure of the column based on its characteristics (diameter and length)

Keeps air from a leaky septum out of the column

Valve controls percent of analyte vented based on assigned split ratio

If GC can’t maintain column pressure, always check septum first!

Fixed volume of gas is injectedinto the GC inlet when the 6-wayvalve turns back and forth

On-line Gas Injection

• Injection amount determined from Ideal-gas law

– Highly reproducible

• Accomplished using a 6-way valve and a fixed sample loop

Columns• How do columns separate gas components?

• What influences gas retention time?

– Keq

– Temperature

– Flowrate

Assuming fast mass transport, theanalyte in the gas phase and thatadsorbed on/in the stationaryphase quickly reach equilibrium

The analyte in the mobile phasespasses through the small columnvolume to the next volume

The analyte continues to travelthrough the entire length of thecolumn to the detector in this way,propelled by the carrier gas in themobile phase

At 𝑡 = 0, a plug of analyte gas isinjected into the first small volumeof the column

The mobile phase and stationaryphase come to equilibrium again

A carrier gas (the mobile phase)continually passes through thecolumn containing a stationaryphase (black and white layer)

0 2dL

time

a) b) c) d) e) f)

dL 0 2dLdL

t = 0

ColumnsLiquid Columns:

Solid Columns:

Porous Layer Open Tubular

- Agilent: The Essential Chromatography and Spectroscopy Catalogue

Analyte absorbs INTO the stationary phase

Analyte adsorbs ONTO the stationary phase

Detectors• Many to select from, each with various selectivities and sensitivities


Thermal Conductivity Detector(Universal Detector)

Mass-spec Detector

Nitrogen PhosphorousDetector

Flame Photometric Detector(Phosphorous and Sulfur)

Infrared Detector(IR sensitive)

Photoionization Detector(UV-vis sensitive)

Flame Ionization Detector(Hydrocarbons)

Flame Ionization Detector

• Cons:

– Destructive

– Can’t detect O2, CO2, CO, etc..

• Pros:

– High sensitivity to hydrocarbons

– Signal roughly proportional to number of C atoms


Analyte exits column and mixes with hydrogen

Analyte enters hydrogen flame, combusting and producing carbon cations

Carbon cations are detected by an electrometer

Thermal Conductivity Detector300 K 400 K 500 K 600 K

--- Air 26.2 33.3 39.7 45.7

Ar Argon 17.9 22.6 26.8 30.6

H2 Hydrogen 186.9 230.4 - -

H2O Water 18.7 27.1 35.7 47.1

H2S Hydrogen sulfide 14.6 20.5 26.4 32.4

NH3 Ammonia 24.4 37.4 51.6 66.8

He Helium 156.7 190.6 222.3 252.4 8

Kr Krypton 9.5 12.3 14.8 17.1

NO Nitric oxide 25.9 33.1 39.6 46.2

N2 Nitrogen 26.0 32.3 38.3 44.0

N2O Nitrous oxide 17.4 26.0 34.1 41.8

Ne Neon 49.8 60.3 69.9 78.7

O2 Oxygen 26.3 33.7 41.0 48.1

O2S Sulfur dioxide 9.6 14.3 20.0 25.6

CO Carbon monoxide 25.0 32.3 39.2 45.7

CO2 Carbon dioxide 16.8 25.1 33.5 41.6

CHCl3 Trichloromethane 7.5 11.1 15.1 -

CH4 Methane 34.1 49.1 66.5 84.1

CH4O Methanol - 26.2 38.6 53.0

C2H2 Acetylene 21.4 33.3 45.4 56.8

C2H4 Ethylene 20.5 34.6 49.9 68.6

C2H6 Ethane 21.3 35.4 52.2 70.5

C2H6O Ethanol 14.4 25.8 38.4 53.2

C3H6O Acetone 11.5 20.2 30.6 42.7

C3H8 Propane 18.0 30.6 45.5 61.9

C4H10 Butane 16.4 28.4 43.0 59.1

C5H12 Pentane 14.4 24.9 37.8 52.7

C6H14 Hexane - 23.4 35.4 48.7

• Pros:

– Non-destructive

– Universal detector

• N2, O2, CO2, CO, Ar, ect.

• Cons:

– Identifies a property of gases, not the gases themselves…

– Relatively low sensitivity compared to other detectors

• Detecting H2? Use any inert carrier…

• Detecting anything else? Use a He carrier…

TCD detects the change in the thermal conductivity of a gas mixture (the carrier gas plus eluted analyte) relative to a reference gas (the carrier gas alone)

NOTE: The schematic above represents a general TCD configuration, it does not represent an Agilent TCD configuration

We can quantify this seemingly abstract term, and use this understanding to develop optimum methods!

Method Development

• What makes this an ideal Chromatograph?– High retention

– High selectivity

– High efficiency

HIGH RESOLUTION!

• Always start with an established method!

Method Development

𝑅𝑆 =𝛼 − 1

𝛼×

𝑘

1 + 𝑘×

𝑁

4

Master Resolution Equation:

Selectivity CapacityFactor

Efficiency


THINK BACK:


– Keq

– Temperature

– Flowrate 0 2dLdL

Alpha is only a function of the stationary phase used, based on the equilibrium between the stationary phase and the analyte

Alpha measures the retention of two analytes relative to each other in the column and is independent of temperature and flowrate!

Method Development

𝑅𝑆 =𝛼 − 1

𝛼×

𝑘

1 + 𝑘×

𝑁

4



Efficiency


THINK BACK:


– Keq

– Temperature


tR: total time analyte spends in the column

t0: time analyte spends in the mobile phase

t’R: time analyte spends in the stationary phase

At higher temperatures, equilibrium (regardless of analyte or stationary phase) will favor the gas phase, and gases will elute at shorter retention times because the analyte spends less time in the stationary phase

Essentially, the analyte should spend sufficient time in the stationary phase to achieve adequate separation of analytes

Method Development

𝑅𝑆 =𝛼 − 1

𝛼×

𝑘

1 + 𝑘×

𝑁

4



Efficiency

• Efficiency Increases with:– Longer column

– Smaller Diameter

– Thinner film

– Optimized Flowrate


THINK BACK:


– Keq

– Temperature


Van Deemter Plot

U (cm/s)Take home message… higher efficiency leads to sharper peaks!

• Goals: develop a method that optimizes resolution and time

– Select a column with high Selectivity and high Efficiency

– Optimize temperature program to optimize resolution and minimize time

• Start with a broad temperature ramp

– Optimize flowrate to optimize resolution and minimize time

• Start with an ideal flowrate (~25 cm/s)

Method Development

Acknowledgments:Dr. Lee Polite, Axion Labs: www.axionlabs.com** We highly recommend Practical Gas Chromatography, an

Agilent sponsored instrumentation course**

http://www.axionlabs.com/

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