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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.html7/29/2019 Topic 12 GC and SFC
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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