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?

Liquid- Solid - TLC, column, HPLC

Liquid-Gas - Gas Chromatography

stationary

mobile

Based on partition between two phases:

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

• 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

• Chromasorb T - Teflon beads

• Other - charcoal, alumina, silica, etc.

Least Inert

Most Inert

I. B. Examples of Liquid Phase Materials

Liquid Phase Type PropertyMax Temp. Limit (°C)

Carbowax 20M Hydrocarbon wax Polar 250

OV-17Methylphenyl

siliconeIntermediate

polarity300

QF-1 FluorosiliconeIntermediate

polarity250

SE-30Silicone gum

rubberNonpolar 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?

• Thermal Conductivity (TC)

• Ionization– Flame Ionization ( FID )– Electron Capture ( EC )– Mass Spectrometer ( MSD )– Other [ flame photometry, coulomeric, etc. ]

Detectors:

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.

• Longer Rt means greater solubility of component in liquid phase..

• Chromatogram should be labeled with column temperature, flow rate, and liquid phase….

• Measured in minutes(given on chromatogram!)

Distance = Rt

Chart Speed

Retention Time (Rt)

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 = ( t2 - 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 thewidth 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)

General Procedure will include…1. The GC will be set up by the instructor.2. Record all pertinent set-up factors.3. Follow the directions for the software in the Procedure

handout.4. Draw up 0.5-1 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.