Topic 12 GC and SFC

7/29/2019 Topic 12 GC and SFC

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SKA6014ADVANCED ANALYTICAL CHEMISTRY

TOPIC 15

Gas Chromatography and Supercritical

Fluid Chromatography

Azlan Kamari, PhDDepartment of Chemistry

Faculty of Science and Mathematics

Universiti Pendidikan Sultan Idris


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GC Theory

Mobile-phase flow rates are

much higher in GC (pressure

drop is much less for a gas)

The effect of mobile-phase flow

rate on the plate height (H) is

dramatic

Lower plate heights yield

better chromatography

However, much longer

columns can be used withGC


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GC Instrumentation

Basic layout of a GC:

Injector

Column

Oven

Detector

Carrier Gas


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GC Instrumentation

A typical modern GC the Agilent 6890N:

Diagram from Agilent promotional literature.


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GC Instrumentation

Typical carrier gases (all are chemically inert): helium,

nitrogen and hydrogen. The choice of gas affects the

detector.

Injectors: most desirable to introduce a small plug,

volatilize the sample evenly

Most samples introduced in solution: microflash

injections instantly volatilize the solvent and analytes

and sweep them into the column

Splitters: effectively dilute the sample, by splitting off a

portion of it (up to 1:500)

Ovens: Programmable, temperature ranges from 77K

(LN2) up to 250 C.

Detectors: wide variety, to be discussed shortly


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Headspace GC

A very useful method for analyzingvolatiles present in non-volatile solids

and liquids

Sample is equilibrated in a sealedcontainer at elevated temperature

The headspace in the container is

sampled and introduced into a GC

Needle

Liquid/solid

Headspace


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Columns for GC

Two major types of

columns used in GC Packed

Open

Open columns

work better at

higher mobile

phasevelocities


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Columns for GC Open tubular columns: most common,

also known ascapillarycolumns (inner

diameters of


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Types of Columns for GC

GLC: Gas-liquid chromatography (partition) most common

GSC: Gas-solid chromatography (adsorption)

FSWC: fused-silica wall-coated open tubular columns, very

popular in modern applications (a form of WCOT column)

WCOT (GLC): wall-coated open tubular stationary phasecoated on the wall of the tube/capillary

SCOT (GLC): support-coated open tubular stationary phase

coated on a support (such as diatomaceous earth)

More capacity that WCOT PLOT (GSC): porous-layer open tubular

Packed columns


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Mobile Phases for GC Common mobile phases:

Hydrogen (fast elution)

Helium

Argon

Nitrogen

CO2

The longitudinal diffusion (B) term in

the van Deemter equation isimportant in GC

Gases diffuse much faster than

liquids (104-105 times faster)

A trade-off between velocity and Hisgenerally observed

This is equivalent to a trade-off

between analysis time and

separation efficiency


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Columns and Stationary Phases for GC

Modern column design emphasizes inert, thermally stable support

materials

Capillary columns are made of glass or fused silica

The stationary phase is designed to provide a kand that are useful.

Polarities cover a wide range (next slide).

Stationary phases are usually a uniform liquid coating on the wall

(open tubular) or particles (packed) When the polarity of the stationary phase matches that of the

analytes, the low-boilers come off first

Bonded/cross-linked phases designed for more robust life, less

bleeding often these phases are the result of good polymer

chemistry

Adsorption onto silicates (via free silanol groups) on the silica column

itself: avoided by deactivation reactions, usually leaving an OCH3

group instead.


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Stationary Phases for GC

Target: uniform liquid coating of thermally-stable, chemically

inert, non-volatile material on the inside of the column or on

its particles.

Polysiloxanes

Polydimethylsiloxane

(R = CH3)

phenyl polydimethylsiloxane (R = C6H5, CH3)

trifluoropropyl polydimethylsiloxane

(R = C3H6CF3, CH3)

cyanopropyl polydimethylsiloxane (R = C3H6CN, CH3)

polyethylene glycol

Chiral

amino acids, cyclodextrins

Backbone structure of

polydimethylsiloxane

(PDMS)

HO

O

OH

n

R Si

R

R

O Si

R

R

O Si

R

R

R

n

structure of polyethylene

glycol (PEG)


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Temperature Effects in GC

Temperature programming can be used to speed/slow

elution, help handle compounds with a wide boiling point

range


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Comparison of GC Detectors

Detector Sensitivity

Selective

or

Universal

Common Applications

Flame ionization (FID) 1 pg

carbon/sec

Universal Hydrocarbons

Thermal conductivity (TCD) 500 pg/mL Universal Virtually all compounds

Electron capture (ECD) 5 fg/sec Selective Halogens

Mass spectrometry (MSD) 0.25 to 100 pg Universal Ionizable species

Thermionic (NPD) 0.1 pg/s (P)

1 pg/s (N)

Selective Nitrogen and phosphorus

compounds (e.g. pesticides)

Electrolytic conductivity

(Hall)

0.5 pg/s (Cl)

2 pg/s (S)

4 pg/s (N)

Selective Nitrogen, sulfur and halogen-

containing compounds

Photoionization 2 pg/s Universal Compounds ionized by UV

Fourier transform IR (FTIR) 0.2 to 40 ng Universal Organics


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GC Detectors: FID

The flame ionization detector

(FID), the most common and

useful GC detector Process: The column effluent

is mixed with hydrogen and air

and is ignited. Organic

compounds are pyrolyzed to

make ions and electrons,which conduct electricity

through the flame (current is

detected)

Advantages: sensitive (10-13

g), linear all the way up to 10-4

g), non-selective

Disadvantages: Destructive,

certain compounds (non-

combustible gases) dont give

signals in the FID.


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GC Detectors: Thermal Conductivity

Thermal conductivity

detector (TCD): a non-

selective detector like theFID

Also known as the

katherometer

(catherometer) or hot wire Works by detecting the

changes in thermal

conductivity (also the

specific heat) of a gas

containing an analyte About 1000x < sensitive

than FID

Non-destructive


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GC Detectors: Electron Capture Detector Electron capture: selectively detects halogen-containing compounds

(e.g. pesticides)

Works by ionizing a sample using a radioactive material (63Ni). This material

ionizes the carrier gas but this ionization current is quenched by a

halogenated compound

Detects compounds via electron affinity e.g. I (most sensitive) > Br > Cl > F


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GC Detectors: Other

Atomic emission detector: plasma systems (like ICP, but

often using microwaves) elemental analysis

Sulfur chemiluminescence detector (SCD): reaction

between sulfur and ozone, follows an FID-like process

Thermionic detector: like an FID, optimized and

electrically charged to form a low-temp (600-800 C)

plasma on a special bead. Leads to large ion currents for

phosphorous and nitrogen a selective detector that is

500x as sensitive as FID

Flame photometric detector: specialized form of UV

emission from flame products

Photoionization detector: UV irradiation used to ionize

analytes, detected by an ion current.

And, of course, the mass spectrometer (MS)


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Examples of GC Detection: Petroleum Analysis

An example of atomic

spectroscopy, usingmicrowave-induced

plasma (MIP), to

selectively detect lead

(Pb) containing

compounds in gasoline

See pg 710 of Skoog for

an example of oxygen

(O) and carbon (C)

detection for separating

hydrocarbons


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Examples of ECD Detection: Pesticide Analysis

Data from Agilent, https://reader009.{domain}/reader009/html5/0420/5ad96ca3ec7e9/5ad96cb58b793.jpg


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Interpretation of GC Data

Common use: develop a method to separate compounds of

interest by spiking, and use retention times to determine

whether a compound is present or not in unknowns Watch out for compounds with the same retention time!

GC can function as a negative teste.g. rule out the presence of

ethyl acetate in my sample.

Relative retention time:

QuantitativeKovats retention index (I) based on normal alkanes

the retention index of these compounds is independent of

temperature and packing I= 100z(zis the number of carbons in a compound)

Relative retention index:

stdRARttr )/()(

zRzR

zRBR

tt

ttzI

)log()log(

)log()log(100100

1


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Purge and Trap GC for Volatile Organic Compounds

Invented 30 years ago by T. A. Bellar at the US EPA

Principle: Inert gas is bubbled through an aqueous sample

Gas carries analytes to headspace above sample, through to a

sorbent trap

After a collection period, the sorbent trap is heated to desorb the

analytes The desorbed analytes are injected into a GC

Results:

ppb detection of VOCs like benzene, decane, halomethanes,

etc in water samples

Commercialized by Teledyne Tekmar (e.g. the Velocity XPT)

and used worldwide

Legally-mandated for water analysis in many areas


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Chemical Derivatization for GC Analysis GC is only applicable to lower molecular weight compounds with

significant (> ~60 torr) volatility

Polar functional groups reduce volatility For other compounds, another separations approach can be used

(LC, etc) orderivatization can be explored

Derivatization: chemical reaction(s) that modify an analyte so that it is

easier to separate or detect

Advantages:

Can lower LOD (increase sensitivity)

Can stabilize heat-sensitive compounds

Can avoid tailing in GC caused by on-column reactions (carbonyl,

amino, imino)

Can improve the separation of closely-related molecules

Disadvantage:

Requires running a reaction, with all its complexities


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Chemical Derivatization for GC Analysis

A typical derivitization reactions silylation of an alcohol:

Common derivatives that reduce polarity:

Groups Derivative

Alcohol (OH) Alkyl ester, alkyl ether, silyl ether

Carboxylic acid (COOH) Alkyl ester, silyl ester

Amino (-NH2) Acyl derivative, silyl derivative

Imino (=NH) Silyl derivative

Aldehyde (COH) Dimethyl acetal

Thiol (SH) Thioether, silylthioether

OH + Si

CH3

CH3

Cl + HClCH3 Si

CH3

CH3

CH3O

Other derivatives contain halogens for ECD detection


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Applications of Derivatization and GC in Doping

Example: derivatization of androgens (like testosterone)

for GC-MS analysis. Detection limits can be as low as 0.2ng/mL

In one procedure, derivitization with TMS is used in

conjunction with a series of pretreatment and extraction

steps, followed by GC-MS:

O

OH

H

H

H

testosterone

K. Shimada , K. Mitamura, T. Higashi, J. Chrom. A., 935, 2001, 141

172.

O

O

H

H

H

Si


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Hyphenation of GC and MS

The first useful hyphenated method?

Continuous monitoring of the column effluent by a massspectrometer or MSD

Very easy to interfacecapillary GC columns have low enough

flow rates, and modern MS systems have high enough

pumping rates, that GC effluent can be fed directly into theionization chamber of the MS (for EI or CI, etc)

Larger columns require a jet separator

Most common systems use quadrupole or ion trap mass

analyzers (MSD)


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Supercritical Fluids

Phase diagrams

show regionswhere a substance

exists in a certain

physical state

Beyond the critical

point, a gas cannot

be converted into

the liquid state, no

matter how muchpressure is applied!


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Supercritical Fluids

Photos of CO2 as it goes from a gas/liquid to a supercritical fluid

Images from http://www.chem.leeds.ac.uk/People/CMR/criticalpics.html

1

2

3

4

Meniscus

Increasing

temp
http://www.chem.leeds.ac.uk/People/CMR/criticalpics.htmlhttp://www.chem.leeds.ac.uk/People/CMR/criticalpics.html


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Extractions with Supercritical Fluids

Why use supercritical fluid extraction (SFE)?

Supercritical fluids can solvate just as well as organic

solvents, but they have these advantages:

Higher diffusivities

Lower viscosities Lower surface tensions

Inexpensive

Pure

Easy to dispose of.

Basic utility many of the same features apply to SFC, so

we introduce them here with SFE.


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Extractions with Supercritical Fluids

Pure CO2 is able to extract a wide range of non-polar and

moderately polar analytes.

Modifiers (such as methanol) at v/v% of 1-10% can be

used to help solubilize polar compounds.

Other supercritical fluids can be used (note that NH3 is

reactive and corrosive, while N2O and pentane are

flammable)


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Some Uses of SFE

Environmental analysis:

total petroleum hydrocarbons

polyaromatic hydrocarbons

organochloropesticides in soils

Food industry:

Extraction of fats

Extraction of caffeine

Density-stepping SFEused as a form of mini-chromatography


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Supercritical Fluid Chromatography (SFC)

SFC is the next logical step from SFE

A supercritical fluid is used as the mobile phase

hardware is otherwise similar to GC.


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Control of Pressure in SFC

Pressure affects the

retention (capacity) factork

Why? The density of the SF

mobile phase increases with

more pressure

More dense mobile phase

means more solvatingpower (more molecules)

More solvating power

means faster elution times

Changing the pressure inSFC is somewhat

analogous to changing the

solvent gradient in LC


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Detectors for SFC

Detectors are generally similar to those used in GC and

LC

Major advantage of SFC over HPLC: SFC can use the

universal FID as a detector

SFC can also use UV, IR, and fluorescence detectors

SFC is compatible with MS hyphenation


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Applications of SFC

Why use SFC over other techniques? Consider speed

and capability as well as expense

Topic 12 GC and SFC

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