GC Flashcards
GC Instrument
- Injector port/oven
- column and column oven
- detector
- Computer
GC Columns
- Packed
- WCOT - wall coated open tubular
- SCOT - solid-support coated tubular
- PLOT - polymer layer open tubular
Open tubular benefits
- much lower flow resistance
- can be much longer
- can be much narrower
- very high plate counts
- high efficiencies
- better separtion
Increasing Resolution
Smaller internal diameter
longer column
Selectivity
How strongly the analyte interacts with the stationary phase
Choosing Column
- choose stationary phase that compliments polarity of analyte
- retention depends on structure of analyte and stationary phase
- low retention leads to poor separation
GC phases
mobile phase is a carrier gas
stationary phase is a non-volatile liquid bound to column or solid particles/polymer
Partition Coefficient factors
activity constant (intermolecular force strength)
vapour pressure
selectivity calculation
ratio between two analytes distribution coefficients (or activity coefficients)
determined by interaction between analytes and stationary phase
Non polar SP
low selectivities
separates by boiling point/distillation
e.g. dimethyl polysiloxane
polar SP
separates by dipole differences additionally
e.g cyanopropyl phenyl
or carbowax
or phenyl groups
Efficiency calculation
Column length divided by height equivalent of a theoretical plate
Golay Equation
Height of separation
no eddy diffusion
only longitudal diffusion and resistance to mass transit
C term for both liquid and gas phases
(for thin films liquid term is 0)
factors of liquid C term
- stationary film thickness
- diffusion coefficient in stationary phase
- retention
- flow rate
factors of B term
- diffusion of analyte in mobile phase
- obstruction factor
- flow rate
factors of gas C term
- diffusion coefficient in gas
- column diameter
- retention
- flow rate
Overloading
Compound mass exceeds capacity of WCOT
peak fronting
lower efficiency and resolution
poor reproducibility
peak distortion
Film Thickness Advantages
Increased Interaction
Increased Retention
better resolution of early peaks
reduced tailing (silanol shielding)
better for volatilve
Film Thickness cons
Increased column bleed (deteriation)/ noise
poorer resolution (longer retention)
Volatile vs non volatile compounds column
volatiles should be analysed on thick film columns to increase retention
high molecular weight compounds should be analysed thinner film columns to decrease run time and minimize bleed from higher temps
Increasing length advantages
Increased interation
increased retention
higher efficiency
resolution increases by square of length
though longer run times
Column Temperature
Alters distribution constants/partitioning towards mobile phase
increases vapour pressure
decreases retention
Gradient temperature advantages
same resolution
sharper late eluting peaks
faster run time
Carrier gases
H2, He, N2
Optimal flow rates of carrier gases
N < He < H
Anayltes Diffusion carrier gases
H2 > He > N2
NItrogen gas
Best efficiency
but smallest range of optimal flows (high C term)
gives lower detection limit with ionisation
Hydrogen Carrier Gas
highest optimal flow rates
highest diffusion
explosive
Helium carrier gas
in between N2 and H2
good efficiency and good flow rates
most common
but expensive
Injection Types
Split
Splitless
On Column
Split injection
Only a fraction of sample goes to column rest to waste
Split injection advantages
simple
small sample sizes less overloading
high flow rate
high resolution
inject without dilution
inject dirty samples
higher temps
Split injection disadvantages
no trace analysis
split may not represent sample
Splitless injection
Entire sample introduced to column
Splitless Advantages
high sensitivity trace analysis
Splitless Disadvantages
Prone to overloading
Time Consuming
colder temps
temperature program to vaporise entire
dilution required
optimisation required
not well suited to volatile compounds (must be 30˚C below solvent)
On column
Used for samples that deompose above thier boiling points
no hot injector
injected straight on to column
solvent trapping
column temp vaporises sovent after injection
solvent trapping
analyte and solvent condense
solute molecules become trapped in solvent band
leads to sharper peaks
Cold Trapping
low temperature
high BP anlytes condense at head of column
low BP analytes and solvent elute early separated
then column heated to separate high BP
Kovat’s Retention Index
for comparing retention of hydrocarbons
I = 100n
higher index means higher retention and eluted later
Detectors
Thermal Conductivity Detector
Flame Ionisation Detector
Electron Capture Detector
Thermal Conductivity Detector
- Anlayte changes thermal conductivity
- change measured by electrodes relative to carrier gas
- requires significant difference (e.g. H2)
- higher flow rates decrease sensitivity
- least sensitive
- but universl
Flame Ionisation Detector
- analyte ionised by flame
- hydrocarbons selectively oxidised creating current
- current proportional to carbon atoms
- selective for organics
Electron Capture Detector
Carrier gas ionised by beta-radiation ejecting electrons
electronegative analytes absorb free electron
combine with cations decreasing current
selective for inorganics
Detector sensitivity
ECD > FID > TCD
Increasing film thickness
Reduced analyte wall interactions
increased sample capacity
broader peaks
increased column bleed
good for analytes with low BPs
good for high conc samples
decreasing film thickness
sharper peaks (retention)
decreased bleed
increased signal to noise
decreased sample capacity
shorter retention
good for high BP analytes
increased wall interactions
Phase ratio
Ranks retention ability of columns
smaller ratio means more retention (low BP volatiles)
higher ration means less retention (heavier analytes)
Separation mechanisms
separations are effected by partitioning
the partitioning depends on the analyte and the column
non-polar columns are mainly van der waals and higher boiling points have longer retention
More polar columns have dipole-dipole interactions which affect retention (not just BP)
Sample Preparations Methods
Derivatisation
pyrolysis
Solid phase extraction (SPE)
purge and trap
Derivatisation
chemically react analyte functional groups to improve:
- sample volatility (decrease polarity)
- selectivity
- detectability
- stability
Derivatisation Reactions
- silylation (less surface adsorption as well)
- acylation
- alkylation
- esterification (FAME)
e. g. acids, hydroxyls, O, S, N, P etc
Pyrolysis
Thermal decomposition of sample
creates more volatile analytes
characteristic of original structure
though may not work
e.g DNA proteins, micro-organisms
Solid Phase MicroExtraction
SPME
solid phase micro extraction
put fibre in liquid sample/head space to collect volatile analytes by adsorption
then put fibre in injector and thermally desorb
Purge and trap
purge liquid sample with gas
trap volatile analytes in absorbent tube
layers of increasing absorbent strength
aim to collect 100% of volatiles from sample
thermally desorb in injector and cold trap
Non polar interactions/retention
dimethyl polysiloxane
- dispersion forces or van der Waals
- increase with size
- elute in order of BP
Intermediate Polar Columns Interactions/Retention
methylphenylpolysiloxan
Dipole dipole
π-π bonding
hydrogen bonding induced dipole
basic interactions
combination of BP and force strength
Polar Column Interactions/Retention
carbowax
strong dipole dipole
strong induced dipole
hydrogen bonding
basic interactions
separation determined by differences in strength of analyte interactions/polarity
Smaller internal diameter
decreased phase ratio
higher retention
higher efficiency N=L/dc
smaller capacity
shorter columns with same efficiency
Increasing Column Length
increased efficiency N=L/dc
smaller than proportional increase in resolution
run time increases faster
increased back pressure
Total Ion Chromatogram
Sums all mass peaks of scan over time
Extracted Ion Chromatogram
Single mass monitored over time
Selective Ion Monitoring
Monitor a few m/z
more sensitive
target analysis
Selected Reaction Monitoring
Delivers specific fragment to detector
sensitive and quantitive
Time of Flight
Mass of ion assigned based on time to reach detector
- larger m/z slower than small m/z
- sensitive and full scan
Spectral Matching
Identify unknown compound by fragmentation
