5. Advances in Gas Chromatography
Topics covered
• capillary columns
• headspace analysis
• solid phase micro-extraction
Capillary columns - research
• Van Deemter derived an equation to explain chromatography in general
H  A3 
B

 C
• H is the height of a theoretical plate
•  is the mobile phase flow rate
• A, B and C are constants related to particular aspects of the movement of
compounds through a specific column/mobile phase combination
Exercise 5.1
a)
•
What is the meaning of the term “theoretical plate”?
the smallest volume of st. phase where sep’n occurs
b)
•
How is H related to the term “N – the number of theoretical plates”?
length ÷ N
c)
•
How do H and N relate to column performance?
the smaller the value of H, the more N, the better
Van Deemter equation importance
• there is an optimum flow rate for maximising the separation
H
Flow rate
• the meaning of the A, B and C terms
Table 5.1
System property
Improvement in H caused by
A
Consistency of flow of analyte
molecules in gas stream
More uniform and smaller packing
B
Travel of particles in a straight line Increased flow rate
C
Rate of transfer of solute
between phases
Large surface area but very thin
films of stationary phase
Exercise 5.2
a)
•
Describe the physical construction of a capillary column.
long, narrow, hollow, s.p. bound to walls
b)
How do these physical characteristics match with the improvements
suggested?
A - no packing means as uniform as possible and molecular size
B – hollow tube increases flow rates
C - length and thin films allow rapid phase transfer
•
•
•
The first capillary columns
• first columns applied van Deemter conclusions to the max
very narrow bore, with very thin
stationary phase films
sample capacity was too low to
be practical
the stationary phase was
adsorbed onto the inner walls of
the column
major “bleed” problems (where
the stationary phase isn’t
stationary!)
constructed from stainless steel,
copper or glass
not flexible, and could not be
coiled readily without breaking,
limiting the lengths available
Improvements
• physical size (internal diameter, length)
• sample capacity is related to internal diameter
• larger bore columns (up to 0.75 mm internal diameter) (known as
megabore columns)
• longer columns also became available (up to 100 m)
Improvements
• stationary phase thickness
• limited sample capacity of early columns also related to their very thin
and inconsistent films
• improved methods of manufacture
• allow a range of film thicknesses with great uniformity
Improvements
• method of binding the stationary phase to the column
• to prevent column bleed:
• bonded
• covalently bonded to the silica wall material
• less bleed during use
• can be used to higher temperatures
• rinsable
• porous layer (PLOT) or support coated (SCOT)
• a layer of porous material is bonded to the inner walls of the column
• the stationary phase adsorbs into this (like packed column particles)
Improvements
• column wall material
• fused silica is used in fibre optics
• much stronger than glass or metal
• can be made much thinner
• when coated in a heat-stable plastic, becomes very flexible (these are
known as FSOT) columns)
Current status
Wall material
Fused silica
Stationary phase bonding
Bonded phases
Length
10 – 100 m
Internal diameter
0.1 – 0.75 mm
Film thickness
0.1 – 5 m
Benefits
•
•
•
•
improved efficiency substantially over packed columns
H is not much improved because of “improvements” in useability
capillary columns are much longer
N is much greater 100,000 vs 5,000
Exercise 5.3
Why can’t packed columns be made as long as capillary columns?
• gas can’t be forced through more than 3m of packing
Advantages
•
•
•
•
•
•
improved efficiency
greater column inertnesspacked
(do not permanently adsorb certain species)
greater reproducibility between runs
fewer stationary phases needed
more sensitive
lower column bleed
same sample
capillary
Disadvantages
•
•
•
•
higher initial cost
cannot be repaired
non-volatile species can block the column
lower sample capacity
Column characteristics & performance
• look at a GC column catalogue and you are bombarded with choice
• for each different stationary phase, there are up to 50 options based on:
• internal diameter (i.d.)
• stationary phase film thickness
• length
• each affects performance
• capacity
• flow rate
• efficiency
Column characteristics & performance
Internal
Diameter
Sample
Capacity
Stat. Phase
Thickness
Mobile Phase
Flow Rate
Column
Length
Column
Efficiency
Exercise 5.4
Sample capacity
Flow rate
Efficiency
Increase i.d.
Increase
Increase
Decrease
Increase s.p.
thickness
Increase
No effect
Decrease
Increase length
No effect
Decrease
Increase
Column choice
Stationary phase
• choose the least polar column that will do the job (columns indicated with
the number 5, eg BP5, will separate about 90% of mixtures
Internal diameter
• 0.25 mm i.d. columns have the best compromise between efficiency and
capacity
• narrow bore columns may require special fittings
Column choice
Film thickness
• thinner films – analytes with high boiling points
• thicker films
• better capacity, low b.p. compounds
• increase retention times and peak broadening
Length
• 30 m – best balance of resolution, analysis time, and required column head
pressure
• a 30 m column with a thicker film may be as useful as a 60 m column
Injection splitting
•
•
•
•
•
•
even megabore columns have smaller capacity than packed columns
less than 1 uL of concentrated sample
less than this not precise (even with an internal standard)
use of a split injection system
two pathways after injection port – one to waste, one to column
tap regulates proportion
mobile
phase
split ratio
control
septum
column
heated chamber
Headspace analysis
•
•
•
•
•
for samples with highly volatile components, eg fragrances
where the vapour is of more interest
sample temperature-equilibrated to obtain a reproducible vapour
larger injection volumes (100 uL) used because of low concs in gases
injection port doesn’t need to be heated
Solid phase micro-extraction
• alternative to solvent extraction
• pen-like cartridge with extendable coated fibre which adsorbs
volatiles (by polarity attraction)
• becomes the “syringe”
• various factors influence efficiency