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