Advanced Gas Chromatography
5
This chapter will concentrate on some relatively recent developments in the use of gas chromatography: 
capillary columns 
headspace analysis 
solid phase microextraction 5.1 Capillary (open tubular) columns Theoretical research into how separation occurs in gas chromatography led to the formulation of the Van Deemter equation (Eqn 5.1). B
H  A 3    C Eqn 5.1 
where H is the height of a theoretical plate,  is the mobile phase flow rate and A, B and C are constants related to particular aspects of the movement of compounds through a specific column/mobile phase combination. CLASS EXERCISE 5.1 (a) What is the meaning of the term “theoretical plate”? (b) How it is related to the term “N – the number of theoretical plates”? (c) How do H and N relate to column performance? In itself the Van Deemter equation isn’t terribly useful for other than pure research, but its application in optimising column performance is. Firstly, it tells us that there is an optimum flow rate for a column, as shown in Figure 5.1. The other important use of the Van Deemter equation was in the interpretation of the meaning of the three terms in the equation: the A, B and C terms. The causes of their contribution to the value of H are summarised in Table 5.1. 5. Advanced gas chromatography H
Flow rate
FIGURE 5.1 Effect on mobile phase flow rate on H TABLE 5.1 Significance of Van Deemter constants System Property Improvement in H caused by A Consistency of flow of analyte molecules in gas stream More uniform and smaller packing B Variation in speeds of molecules of the same Increased flow rate compound C Rate of transfer of solute between phases Large surface area but very thin films of stationary phase CLASS EXERCISE 5.2 (a) Describe the physical construction of a capillary column. (b) How do these physical characteristics match with the improvements suggested by the van Deemter equation in Table 5.1? AIT 5.2 5. Advanced gas chromatography Historical development of capillary (open tubular) columns The first capillary columns were the extreme in applying the van Deemter improvements . Table 5.2 summarises their design and the resulting problems. TABLE 5.2 Characteristics of the first capillary columns Design characteristic Problem very narrow bore, with very thin stationary phase films sample capacity was too low to be practical the stationary phase was coated directly 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 It took another 20‐30 years of development before capillary columns became commercially useful. The improvements involved: 
physical size (internal diameter, length) – sample capacity is related to internal diameter, so larger bore columns (up to 0.75 mm internal diameter) have been introduced – these are known as megabore columns; longer columns also became available 
stationary phase thickness – the limited sample capacity of early columns was also related to their very thin and inconsistent films; methods of manufacture have allowed a range of film thicknesses with great uniformity 
column wall material – fused silica is used in fibre optics, and is much stronger than glass or metal; it can be made much thinner, and when coated in a heat‐stable plastic, becomes very flexible 
method of binding the stationary phase to the column – two approaches are used to prevent column bleed: 
bonded: where the stationary phase is covalently bonded to the silica wall material; this is by far the best method, because it has less bleed during use, can be used to higher temperatures, and, when necessary, can be rinsed with solvents to remove accumulated non‐volatile materials, 
non‐bonded: certain stationary phases cannot be made as bonded phase; to improve the adsorption, some type of coating is applied to the wall which the stationary adheres to more effectively; these are variously known as porous layer (PLOT) or support coated (SCOT) Table 5.3 summarises the current options available for capillary columns. TABLE 5.3 Modern capillary column characteristics AIT Property Current technology 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 5.3 5. Advanced gas chromatography Capillary columns have improved the performance of GC separations substantially. This is not, however, because the value of H has been decreased significantly by the physical changes to the column. The value of H for modern capillary columns is not that much better than for the best packed column. However, the length of a capillary column means that the total number of plates (N) is much greater: typically 100, 000, compared to 5, 000 for a packed column. This is where the improvement in performance comes from. CLASS EXERCISE 5.3 Why can’t packed columns be made as long as capillary columns? The advantages of capillary columns include: 
improved efficiency (see Figure 5.3) 
greater column inertness (do not permanently adsorb certain species) 
greater reproducibility between runs 
fewer stationary phases needed 
more sensitive 
lower column bleed (a) (b) FIGURE 5.3 Comparison of performance of (a) capillary and (b) packed column for same sample Packed columns will, however, continue to be used. The disadvantages of capillary columns include: 
higher initial cost 
cannot be repaired 
non‐volatile species can block the column 
lower sample capacity AIT 5.4 5. Advanced gas chromatography Effect of column characteristics on performance Capillary columns can be purchased with a wide variety of not just stationary phases, but also variations in physical dimensions: 
internal diameter (i.d.) 
stationary phase film thickness 
length The dimensional properties affect certain performance characteristics, as shown in Figure 5.4 and Tables 5.4‐5.6 (in each case, one variable is changed, the others kept constant). Internal Diameter Sample Capacity Stat. Phase Thickness Mobile Phase Flow Rate Column Length Column Efficiency FIGURE 5.4 Relationship between column properties and performance TABLE 5.4 Effect on performance of internal diameter Internal diameter (mm) Efficiency (plates/m) Capacity (ng/analyte) 0.53 1,300 1000‐2000 0.32 2,300 400‐500 0.25 2,925 50‐100 0.20 3,650 <50 0.18 4,050 <50 0.10 7,300 <10 TABLE 5.5 Effect on performance of film thickness AIT Film thickness (um) Efficiency (plates/m) Capacity (ng/analyte) 0.1 4000 30 0.25 3500 75 1.0 3200 300 5.0 1300 1500 5.5 5. Advanced gas chromatography TABLE 5.6 Effect on performance of column length Length (m) Pressure (psi) Peak RT (min) Peak resolution (R) Efficiency (plates/m) 15 5.9 8.33 0.8 43,875 30 12.0 16.68 1.2 87,750 60 24.9 33.37 1.7 175,500 CLASS EXERCISE 5.4 Complete the table below, by predicting the effect (increase/decrease) on the following properties of changes in column characteristics. Change in characteristic Effect on Flow rate Sample capacity Efficiency Increase i.d. Increase s.p. thickness Increase length Given that manufacturers can supply numerous combinations of lengthy, i.d. and film thickness for the one stationary phase, the choice becomes difficult (see Table 5.7). And then of course there are at least four different column manufacturers! TABLE 5.7 Range of columns offered with same stationary phase (Supelco, AT1) AIT Length (m) Number of columns ID (mm) Film thickness (um) 5 2 0.05 0.05‐0.2 10 13 0.05‐0.53 0.05‐5.0 15 13 0.25‐0.53 0.1‐5.0 25 3 0.25‐0.53 0.2‐1.2 30 15 0.25‐0.53 0.1‐5.0 50 3 0.25‐0.53 0.2‐1.2 60 13 0.25‐0.53 0.1‐5.0 100 1 0.25 0.25 5.6 5. Advanced gas chromatography Some general comments about 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 Film thickness 
thinner films (< 0.25 μm) should be used for analytes with high (>300°C) boiling points 
thicker films give better capacity and work better with low b.p. compounds, but increase retention times and peak broadening Length 
a 30 m column provides the best balance of resolution, analysis time, and required column head pressure (for desired flow rate) 
a 30 m column with a thicker film may be as useful as a 60 m column Injection splitting One of the limitations of capillary columns is their much smaller sample capacity, compared with a packed column. In general, lower sample capacities mean that 1 L volumes are too much and cause flooding. Because injection volumes of less than this are not precise (even with an internal standard), a split injection system must be employed. This retains the heated chamber below the septum, but diverts some fraction of the mobile phase/sample mixture away from the column to waste. Essentially this is no more than a tap, which can be opened or closed to regulate the split ratio – how much of the sample goes to the column and how much is “lost”. Figure 5.5 shows a schematic representation of an injection splitter. mobile
split ratio
phase
control
septum
column
heated chamber
FIGURE 5.5 Schematic diagram of a split injection system The two basic limitations of split injections are: 
the possibility of a non‐representative split due to sample volatility differences between components 
loss of sensitivity for trace level components AIT 5.7 5. Ad
dvanced gas ch
hromatographyy 5.2 SSampling me
ethods for gaas chromato
ography Theree are some other optio
ons than th
he normal “stick the syyringe in the
e liquid sam
mple bottle”” appro
oach to gettting a samplle for injection. Each iss aimed at improving th
he analysis of o “difficult”” samp
ples: with highly vvolatile comp

ponents, eg fragrances 
with a large non‐volatilee content, egg soil Head
dspace analyysis This aanalyses the vapour above the samp
ple, rather th
he liquid/solid bulk. In so
ome cases, the vapour iss the m
major interesst in the sam
mple, eg fragrrances, the ssmell of food
dstuffs such as wine. In other cases,, the sample may have compo
onents in thee liquid or so
olid bulk thaat will cause problems in
n the GC, egg sugarr, salt, minerrals. In headspace anaalysis, the sam
mple (solid o
or liquid) is p
placed in a phial with a seeptum and aa sealed cap.. brated to obttain a reprod
ducible vapour. Most he
eadspace GC systems are
e It is then temperaature‐equilib
mated, and tthe vapour in the phial on the multti‐sample carrousel is evaacuated into
o the sample
e autom
loop for injection
n onto the column. c
Thee sample injeection could
d be done manually m
with
h a gas‐tightt syringge, but the reproducibilitty and sensittivity suffers. In heeadspace anaalysis, the saample is alreeady in the vvapour phasee, so the injeection port n
need not be
e very hot, which can be an advvantage for h
heat‐unstable compound
ds. Solid d phase micro
o‐extraction (SPME) opment, whiich uses a sp
pecial sampling device, w
which is inserrted into the
e This iis a relativelyy new develo
samp
ple or its vapour. It avoid
ds slow largee‐scale solven
nt extraction
ns. A pollymer‐coated
d silica fibre is exposed to the samp
ple for a period around 3
30 minutes, and organicc comp
pounds are aabsorbed on
nto the surfaace of the fib
bre. The fib
bre is then w
withdrawn back into the
e SPMEE unit, and ttaken to the GC. It is placed througgh the septum of the GC
C injection port, and the
e fibre pushed outt again. Th
he compounds desorb from f
the fib
bre and are separated in i the usuall ner. The pro
ocess is illustrrated in Figu
ure 5.6. mann
Adsorptiion from sam
mple Desorp
ption into GC
C FIGUR
RE 5.6 The SP
PME process AIT 5.8 5. Advanced gas chromatography There are a number of factors that affect the accuracy and precision of the technique: 
polarity of the fibre coating – the standard “like dissolves like” issue as with column choice 
thickness of the fibre coating – as with capillary columns, the coating thickness is related to sample capacity; a thick coating will retain volatiles better, but may not completely desorb the less volatile compounds, contaminating the next run 
sample agitation – stirring increases extraction efficiency but can make it less reproducible 
ionic strength – adding salt to liquid samples improves the extraction of organics onto the fibre 
sample volume – extraction efficiency is not constant for different sample volumes, so 1‐5 mL is recommended SPME units can be used in the field, and sealed with a special holder for transport back to the laboratory. They are also used with HPLC instruments. What You Need To Be Able To Do 
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explain the significance of the van Deemter equation outline the development of capillary columns compare capillary and packed columns describe the effect of column characteristics on performance describe injection systems for capillary columns describe the principles of headspace GC describe the principles of solid phase microextraction for GC analysis Revision Questions 1. Give TWO conclusions made from the Van Deemter equation. 2. (a) What were the problems encountered with the first capillary columns? (b) How were these problems solved in later capillary columns? 3. Why can’t packed columns be made as long as capillary ones? 4. Give THREE advantages of capillary columns. 5. Explain why the analysis of seawater for pesticides could not be done by direct injection of the sample onto a capillary column. 6. For a given stationary phase, what are the other properties that can be altered to change the column performance? 7. How do these properties affect the (a) sample capacity and (b) column efficiency? 8. Why is injection splitting necessary for capillary columns? 9. Outline an application of headspace analysis. 10. Outline an application of solid phase micro‐extraction. Answer guide on next page AIT 5.9 5. Advanced gas chromatography Answer guide 1. Improvement column of Table 5.1 2. (a) Low sample capacity, column bleed (b) Larger dimensions, chemically bonded st. phase 3. Impossible to push gas through the powder beyond about 3 m 5. Page 5.4 5. The high concentration of salt (non‐volatile) would block the column. 6. Int. diameter, length, st. phase thickness 7. Tables 5.4 & 5.5 8. Page 5.7 9/10. Section 5.2 AIT 5.10