Insmeth Lecture 8 - Gas Chromatography

7/28/2019 Insmeth Lecture 8 - Gas Chromatography

1/33

Gas Chromatography


2/33

General Design of a GC


3/33

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


4/33

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


5/33


6/33

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


7/33

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.


8/33

Capillary

ColumnStationary

Phases


9/33


10/33

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!


11/33


12/33


13/33

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.


14/33

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


15/33

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


16/33


17/33

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


18/33


19/33

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)


20/33

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


21/33


22/33

Injection types


23/33

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.


24/33

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.


25/33

Dont Forget SPME (Solid Phase Microextraction)


26/33

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.


27/33


28/33


29/33

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


30/33

FID D t ti l l t


31/33

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


32/33


33/33

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

Insmeth Lecture 8 - Gas Chromatography

Documents

Transcript of Insmeth Lecture 8 - Gas Chromatography