4. 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 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
CB
AH 3
Exercise 4.1
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
• the meaning of the A, B and C terms
H
Flow rate
Table 4.1
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 4.2
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 4.3Why 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 inertness (do not permanently adsorb certain species)• greater reproducibility between runs• fewer stationary phases needed• more sensitive• lower column bleed
same sample
capillary
packed
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
InternalDiameter
Stat. PhaseThickness
ColumnLength
SampleCapacity
Mobile PhaseFlow Rate
ColumnEfficiency
Exercise 4.4
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
columnseptum
mobile phase
heated chamber
split ratio control
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