6
Gas Chromatography
6.1 Revision
The following questions cover the important concepts that you should have understood in the first
year introduction to chromatography.
1.
How does a gas chromatograph separate a mixture?
2.
What limitation on analytes does gas chromatography have?
3.
Draw a block diagram showing the essential components of a typical gas chromatograph.
4.
What carrier gases are commonly used in gas chromatography? What component of the
chromatography system is the carrier gas?
5.
What is the effect on elution of increasing the temperature?
6.
How is the stationary phase chosen for a given analysis, e.g. ethanol in alcoholic beverages?
7.
Why is the injection port heated?
8.
What are the two common methods for determining the identity of compounds by GC?
6. Gas Chromatography
6.2 Columns
Columns in gas chromatography are of two basic types: packed and capillary. Packed columns were
the only type for many years, capillary columns being a development of the last 30 years. Their basic
properties are compared in Table 6.1. In this chapter, we will be focussing in packed columns. Capillary
columns will be dealt with in the Advanced Instrumental Techniques subject.
TABLE 6.1 Comparison of GC column types
Type
Length (m)
Diameter (mm)
Stationary phase condition
Packed
1-3
3-5
adsorbed into powder, filling column
Capillary
10-50
0.1-0.7
adsorbed onto walls of hollow column
A packed column is in reality three different components:
•
the tubing itself,
•
the powder or packing, and
•
the liquid stationary phase
Tubing
Tubing for packed columns is most commonly made from stainless steel, though some are glass. Their
dimensions are described in Table 6.1. Stainless steel columns can be uncoiled and emptied of
contaminated material, repacked and recoiled. Glass columns obviously can’t be uncoiled, and the
packing must be blown out and in with compressed air, a less convenient option.
Packing material
If you look at a glass packed column, what you see is a fine white powder. The powder itself is not the
stationary phase, but an inert support material onto which the stationary phase is coated. The most
common support materials are firebrick (also known as Chromosorb), and Kieselguhr (fossilised single
cell organisms, also called diatomaceous earth). The essential properties of the support material
include small and uniform particle size and large surface area.
Stationary phases
The stationary phase in gas-liquid chromatography (GLC, the more common form) is a liquid coated in
thin layers on the support in packed columns or the walls in capillary columns. Compounds suitable
for use as stationary phases must fulfil various requirements:
•
low volatility - each column has a defined maximum operating temperature, which is
approximately 200°C below the boiling point of the liquid; if this is exceeded, column bleed evaporation of the stationary phase - will occur,
•
thermal stability - prolonged heating should not cause significant levels of decomposition, and
•
chemically inert - the analytes cannot be chemically altered by the stationary phase
The two main chemical types of compounds, which are obviously of high FW (to give high b.p.s) are:
•
polysiloxanes (PS) – repeating Si-O-Si units with two other groups bonded to each two silicon
•
polyethylene glycols (PEGs) – repeating units of O-CH2CH2-O
The siloxanes form the basis for non-polar columns, and the PEGs the polar ones. The substituent
groups on the siloxanes affect their polarity. For example, the most non-polar of all is 100% methyl
(Me) substituted, while others have some percentage of phenyl (Ph, benzene ring), trifluoropropyl (FP)
and cyanopropyl (CP) groups instead, producing slightly polar phases.
Adv. Chrom.
6.2
6. Gas Chromatography
Table 6.2 shows a range of commonly used stationary phases in packed columns, and their important
characteristics. Most are used in both types of columns. Different companies give different model
numbers/names to the same stationary phase.
TABLE 6.2 Characteristics of common GLC stationary phases
Composition
Names
Polarity
Max. Temp.
Applications
100% Me PS
OV1, SE30, DB1,
HP1
Non-polar
325
No selectivity, separates by
b.p.
5% Ph PS
DB-5, HP-5, BP-5
Non-polar
325
alkaloids, organochlorine
pesticides
50% Ph PS
OV-17
Non-polar
325
Steroids, aromatics
7% CP, 7% Ph PS
AT-1701, DB-1701
Medium
polar
280
Pharmaceuticals
PEG
BP20, Carbowax
20M, HPInnowax
Polar
260
Alkanols, essential oils,
non-chlorinated pesticides
CLASS EXERCISE 6.1
What would be a suitable column for the separation of:
(a) petrol components
(b)
soil extract containing DDT and other pesticides
Column preparation and conditioning
In preparing a packed column, the support is coated with stationary phase by making a solution of the
liquid in a volatile solvent, producing a slurry with the appropriate amount of support, and then
evaporating the solvent off. The proportion of stationary phase to support is important, and is called
the loading. Typically loadings range from 1 to 10% by weight.
A new column of either type must be conditioned, by running at maximum temperature for an
extended period with the carrier gas flowing so that any remaining solvent or other chemicals involved
in manufacture are removed. The carrier gas must be flowing whenever the column is heated,
otherwise significant oxidation of the stationary phase will occur.
Are packed columns a dying breed?
Capillary columns separate mixtures more rapidly and with better resolution than packed columns. So
why bother with packed columns? Because they can cope with high levels of non-volatiles (eg salts,
sugars) which block capillary columns, and because they can cope with higher sample volumes. They
can be repacked at little cost, whereas once a capillary column stops working, it is terminal.
Adv. Chrom.
6.3
6. Gas Chromatography
6.3 Detectors
Modern GLC instruments put severe demands on the detectors employed, requiring them to respond
to nanogram or less quantities of analyte passing through the detector within a few seconds. Desirable
properties for a detector include:
•
wide linear response – to maximise the operating range for the instrument,
•
good stability with time – to avoid continual recalibration, and
•
response to all compounds – to reduce the need for separate analysis of different components,
though in certain cases, the complete insensitivity of a detector to a particular species or group
of species may in fact be an advantage.
There are many types of detectors available for gas chromatographs, but we will only discuss the two
most common in detail.
Thermal conductivity (TCD)
Also known as a katharometer, the TCD compares the thermal conductivity of the eluant gas with the
carrier gas. It does this by sensing the change in resistance of a current-carrying filament as a result of
changes in the composition of the gas flowing over it. The presence of an eluting compound changes
the thermal conductivity of the gas, and thus the conduction properties to the filament.
Most TCDs use a pair of filaments, one placed before the injection port as a reference and the
second at the end of the column for the eluant gas. Use of the reference allows for detection of
variations only in gas conductivity, and also for variations in flow rate.
To column
Filaments
Reference stream
From column
Eluant stream
Waste
From cylinder
FIGURE 6.1 Schematic diagram of a dual-filament TCD
To maximise sensitivity, the carrier gas should have a thermal conductivity significantly different to the
organic species eluting in it. This is achieved with helium, which has the second highest thermal
conductivity known for gases (hydrogen is highest, but is too unsafe). Thus, when a compound elutes,
the thermal conductivity of the gas stream decreases, and the filament cools, causing a decrease in its
resistance, and an increase in current.
TABLE 6.3 Advantages & disadvantages of thermal conductivity detectors
Advantages
•
simplicity
•
cheapness (around $2,000)
•
response to all species
•
non-destructive (essential if the eluants are
to be analysed further after separation)
Adv. Chrom.
Disadvantages
•
comparatively poor sensitivity (5-10 ug/mL
of eluant gas)
6.4
6. Gas Chromatography
Flame ionisation (FID)
This measures the electrical conductivity of the gas stream from the column after passing it through a
hydrogen/air flame, relying on the fact that combustible species produce ions as intermediates in the
burning process. These ions are collected by a measuring electrode placed over the flame. Figure 6.2
shows a typical construction of such a detector.
FIDs are most sensitive to organic compounds with long carbon chains, since these produce
more ions. Oxidised carbons, such as those found in carbonyl compounds and alkanols produce a
smaller response. Hence, the detector response is not at all uniform for different species, even those
of the same functional group. For example, 1-octanol will produce many more ions than methanol,
due to the long carbon chain. FIDs are completely insensitive to non-combustible species, such as
water, carbon dioxide and nitrogen (which becomes the ideal carrier gas, since it is far cheaper than
helium) and oxygen. This can be an advantage if these species are present in a sample, but are not of
analytical interest, e.g. when analysing ethanol in alcoholic beverages, the water present does not
register and separation is not particularly critical.
FIGURE 6.2 Schematic diagram of a flame ionisation detector
(from http://acpcommunity.acp.edu/Facultystaff/hass/oc1/exp/nabh4/nabh4des.html )
TABLE 6.4 Advantages & disadvantages of flame ionisation detectors
Advantages
Disadvantages
•
very sensitive to hydrocarbons (1 ng/mL)
•
require extra gas supplies (hydrogen, air)
•
insensitive to water
•
can be hard to get lit
•
relatively cheap ($5-10,000)
Other commonly employed detectors for GC are:
• electron capture (ECD) – use a radioactive source of electrons, which are strongly absorbed by
certain analytes, such as those containing halogens or P; ideal for pesticides
• mass spectrometer (MSD) – not only generates a chromatogram but also a mass spectrum for
every peak, allowing identification; very expensive ($50,000) but very powerful; used for all drug
testing
Adv. Chrom.
6.5
6. Gas Chromatography
6.4 Qualitative analysis
Identification by retention time comparison and spiking have been discussed in the first year
introductory subject. The former suffers if there are temperature or gas flow variations, and is
particularly difficult if retention times of different species are very similar. Spiking overcomes these
problems, but is time consuming and may not be possible if little sample is available.
For a laboratory undertaking analyses of similar mixtures in a regular basis, a table of the
retention times of common analytes could be drawn up for comparison purposes. While not providing
a definite match (even if the RTs were the same) it would at least make spiking less of a hit and miss
exercise.
However, because small variations in temperature and gas pressure will affect the rate of
progress in the column, a standard (or two) would have to be run to “calibrate” the instrument on the
day against the table. For example, if, under the standard conditions of the table, hexane had a
retention time of 110 seconds, but on the day it came in at 120 seconds, all sample peak times would
be slightly longer than the table would suggest. Taking this into account would only require a simple
bit of scaling up or down.
6.5 Quantitative analysis
This involves consideration of two related but different things: firstly, how to measure the instrument
response and secondly, how to calibrate that response against standards.
I. Measuring instrument response
Depending on the type of instrumental output available, the detector response may be a peak on a
computer screen, on a roll of plain paper or on graph paper. The truest measure of the amount of
compound producing that peak is the area under the peak.
How that response is measured varies greatly, again dependent on the sophistication of the
instrument. Most modern instruments will have some form of electronic integration which produces
a numerical value, and is the most accurate method by far. The numerical value has no meaning in
itself, and is only useful compared to other numbers generated by that integration system.
Older/simpler instruments may produce the output directly to a chart recorder, and
measurement of the response must then be done by the technician. A variety of methods have been
used for manual measurements, including cutting the peak out and weighing it (!!!) but the only one
that is really of any value is peak height.
CLASS EXERCISE 6.2
If peak area is the true measure of the amount of compound, why is peak height recommended for
measurements on chart paper?
Each different compound produces a different response from each different detector, and that
response will vary slightly from day to day. Therefore, the response must be calibrated for accurate
use by the use of standards, as described below.
Adv. Chrom.
6.6
6. Gas Chromatography
II. Calibrating against standards
Calibration standards
This is based on the area or height of the analyte peak in the sample by comparison with standards
injected under the same conditions. Matrix interference is not normally a problem because the matrix
components are separated from the analyte. However, the volume injected determines the peak size,
and it is very difficult to ensure reproducible injections, given the difficulties of the hot injection port,
and the very small volumes. Therefore, any accurate GC analysis requires an improvement on normal
calibration standards. That improvement is the internal standard method, described in the previous
chapter in the flame emission section, and repeated below.
Internal standards
The basic principles of the internal standard method are covered in Chapter 8. Suffice to say here that
its use in gas chromatography is to deal with the major error in quantitative chromatography - injection
volume variations.
If the syringe is only partly filled accidentally, then both the analyte and internal standard peaks
will decrease by the same proportion. In GLC, the internal standard is chosen to give a peak near, but
resolved from the analyte peak, and as always, it cannot be already part of the sample.
This means there will be quite a number of possible candidates for the job, and if they all meet
the usual criteria, then in principle, they are all equally suitable. There will be one further measure of
suitability which can only be determined by actually running a sample with the possible internal
standards added: where does the internal standard elute relative to the analyte(s)?
1.
2.
The chosen compound should not overlap the analyte(s) in the chromatogram.
The chosen compound should not extend the run time if possible.
The reasons for these rules should be obvious, and each can be bent somewhat. A slight overlap of
peaks can be dealt with by adjusting operating temperatures, while it may not be possible to find a
compound that elutes between the solvent and analytes, so the best choice will be the one that
extends the run least.
EXAMPLE 6.1
The conversion of ethanol to ethanoic acid for vinegar manufacture has to be tested to make sure that
no ethanol remains, and also for the yield of ethanoic acid. The following compounds are suggested as
internal standards (retention times in brackets):
methanol (2)
(a)
ethanol (2.5)
1-propanol (4)
ethanoic acid (4.5)
propanoic acid (5.5)
Identify any compounds which would definitely be unsuitable as the internal standard?
The two species that are or may be present – ethanoic acid and ethanol – are unsuitable.
(b)
Of the compounds that remain as suitable, choose the one you believe is best, and explain your
answer.
Methanol is the quickest, but is quite different to the main analyte, ethanoic acid.
1-propanol fits nicely between the two analytes.
Propanoic acid extends the run time.
1-propanol would be best.
Adv. Chrom.
6.7
6. Gas Chromatography
EXERCISE 6.3
You need to analyse the products of the conversion of cyclohexanol to cyclohexanone. The mixture is
extracted into hexane for analysis. The following compounds have been suggested as possible internal
standards.
Compound
RT (min)
cyclohexanone
6.5
cyclohexanol
8.6
2-hexanone
6.8
2-hexanol
8.9
cyclopentanone
5.2
hexane
1.8
(a)
(b)
Suitable (Y/N)?
Reason if No
Identify any compounds which would definitely be unsuitable as the internal standard? Explain
why.
Of the compounds that remain as suitable, choose the one you believe is best, and explain your
answer.
EXERCISE 6.4
The products of the nitration of toluene can be determined by GLC. The main products are 2nitrotoluene and 4-nitrotoluene with some 2,4-dinitrotoluene, and some toluene would remain
unreacted. Suggest a suitable internal standard and explain your choice.
Adv. Chrom.
6.8
6. Gas Chromatography
6.6 Optimisation of GC separations
It is a basic observation in chromatography of any type that the longer a species is retained within the
system, the more its peak will spread out: this is known not surprisingly (or imaginatively) as peak
broadening. This is clearly shown in Figure 6.3. The reason for this is simple: molecules of the same
compound do not move at exactly the same speed, and the longer they travel, the farther behind, the
slower ones will fall.
0
200
400
600
800
1000
1200
1400
FIGURE 6.3 Chromatogram showing the effect on peak shape of retention
EXERCISE 6.5
Why is peak broadening a problem?
There are four factors that influence the rate at which a compound travels through a GC column:
• the type of column (packed or capillary)
• the polarity of the stationary phase
• the temperature of the oven
• the flow rate of the carrier gas
On the assumption that an appropriate stationary phase and type of column has been chosen, then
the options for speeding up the elution of a slow compound are limited to increasing the flow rate
and/or increasing the temperature. For each column, there is an optimum flow rate for efficiency of
separation, so increasing the rate beyond this would seem to be self-defeating. Therefore, it is
temperature which provides the main control over retention times.
Adv. Chrom.
6.9
6. Gas Chromatography
500
450
400
Ad. RT (s)
350
300
250
200
150
100
50
0
50
70
90
110
130
150
170
190
Temp. (deg. C)
FIGURE 6.4 Effect on retention time of temperature
The effect of temperature on retention is obviously not linear, as seen in Figure 6.4. It should be clear
that for a slow compound (high RT), a relatively small change in temperature will make a substantial
difference in retention time.
A successful chromatogram has two basic requirements:
• the analyte peak(s) are well-resolved from other peaks
• all components are eluted in as short a time as possible
Nowhere in the rulebook does it say that all peaks are perfectly separated, but it does say that all peaks
must be out before the next injection.
EXERCISE 6.6
Explain those two observations:
• all peaks do not have to be perfectly separated
•
all peaks must be out before the next injection
Adv. Chrom.
6.10
6. Gas Chromatography
EXERCISE 6.7
Comment on the following chromatograms, in terms of whether they are satisfactory or need improving
based on the specified analysis requirements. In each case, it is the same sample containing four
compounds (A-D).
(a) Analytes are A & B.
(b) All four compounds are analytes
(c)
Analyte is C.
Adv. Chrom.
6.11
6. Gas Chromatography
It is also not in the GC rulebook that the temperature has to stay the same through the run. It is
obviously simpler if a temperature can be found that gives good separation and gets all species out
quickly. A chromatogram run under constant temperature is known as isothermal.
However, when a sample has components (analyte or otherwise) of widely varying boiling points
and polarities, it may not be possible to achieve an efficient separation using isothermal conditions.
FIGURE 6.5 Isothermal and programmed chromatograms (a) 45°C, (b) 145°C and (c) programmed rise from 30°C to 180°C
(from Skoog, Principles of Instrumental Analysis, Saunders.)
In Figure 6.5, chromatogram (a), run at low temperature, shows the early peaks as well resolved, but
the more strongly retained compounds elute very slowly with drastic broadening. Regardless if which
species are the analytes, this is not successful.
In chromatogram (b) at a higher temperature, the slow peaks are speeded up and sharpened,
but at the cost of resolution of the early peaks. If these early peaks were of no interest, then this would
be a satisfactory run. However, if all compounds or some of the early ones are of interest, then it too
is unsuccessful.
In cases such as this, the use of temperature programming is required. This is where the GC
oven is programmed to increase (it is always increase) in temperature during the run. In
chromatogram (c), a lower initial temperature is used to allow separation of the initial peaks, and then
the temperature is increased to hurry up the “slowcoaches”.
Adv. Chrom.
6.12
6. Gas Chromatography
EXERCISE 6.8
There is something wrong with chromatogram 6.5(c). What is it?
The shape of the temperature program is determined by the actual separation problem. Well-spaced
peaks across the chromatogram may only require the simple program in (a), whereas close early peaks
would be better suited by (b), and close later peaks by program (c).
Temp.
(a)
(b)
(c)
Time
FIGURE 6.6 Typical temperature program shapes
Working out the ideal program is a process of trial and error, with educational guessing thrown in. A
series of different temperature isothermal runs is suggested, followed by some thought as to the best
option. Good luck!
6.7 Measuring performance
Resolution in chromatography is defined as the separation of adjacent peaks. Achieving satisfactory
resolution in the shortest possible time is the aim when setting up a method in GC and HPLC.
The standard measure of column efficiency is known as the number of theoretical plates (N).
The term theoretical plate reflects the origins of chromatography theory in fractional distillation
processes, where a plate was one of the pieces of glass etc in the fractionating column. Basically, the
more theoretical plates, the more efficient the column will be at separating two substances. N can be
estimated from the chromatogram, as shown in Equation 6.1.
t
N = 16 r
 tw



2
Eqn 6.1
where tr is the retention time and tw the peak width measured at the base of the peak in the same
units as retention time. This could both be in mm, using a ruler, or in seconds, best measured by the
instrument.
Column performance is very commonly quoted in plates/metre: well-packed columns will have
1,000-2,000 plates/m, while capillary columns are somewhat higher (2,000-3,000/m). Capillary
columns gain their greater resolving power by extra length.
Adv. Chrom.
6.13
6. Gas Chromatography
EXERCISE 6.9
Calculate and comment on the value of N for in the following situations:
Retention time (min)
4.35
2.15
(a)
(b)
Peak width (s)
26.3
2.4
Column type
packed
capillary
Column length (m)
3
25
Analyte peaks need to be resolved for accurate measurement of peak area (or height, though it is less
affected). However, close together peaks do not have to have flat baseline between them. They can
overlap slightly (joined at the “ankles”) without causing significant error. There is a simple measure
using retention times and peak widths to check the resolution of overlapping peaks, as shown in
Equation 6.2.
 t − t r1 

R = 2  r 2
t
+
t
w2 
 w1
Eqn 6.2
where tr and tw are retention times and peak widths for peaks 1 (faster) & 2 (slower), measured in the
same units. In practice, satisfactory resolution is indicated by an R value of greater than 1.
EXERCISE 6.10
Check the resolution of peaks giving the following measures:
Compound
1
2
Adv. Chrom.
RT (min)
3.25
3.55
Peak width (s)
8.6
9.6
6.14
6. Gas Chromatography
What You Need To Be Able To Do
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
define important terminology
distinguish between packed and capillary columns
list common stationary phases
select a suitable stationary phase for a given separation
explain how packed columns are repacked
describe the operation of TC & FI detectors
compare relative advantages & disadvantages of common detectors
compare methods of peak identification
compare methods of measuring detector response
explain why an internal standard is necessary for accurate GC analysis
explain how a suitable internal standard is chosen
calculate sample concentrations using the internal standard method
explain the requirements for a successful chromatogram
describe how a chromatogram can be improved by temperature variations
calculate performance parameters
Revision questions
1.
Describe the process of repacking a GC column.
2.
What physical differences are there between packed and capillary columns?
3.
Explain why column bleed occurs.
4.
5.
6.
7.
8.
9.
10.
11.
12.
A new technician unused to GC instruments installs a new column, and conditions it at 250°C
without turning the carrier gas on. When the instrument is used next, no sensible results can
be obtained. Explain why.
Give two reasons why packed columns will continue to be used when capillary columns are much
more efficient.
(a) Briefly describe how a thermal conductivity detector works.
(b) What advantages do TCDs have over other GC detectors?
Justify your choice of a suitable stationary phase and detector for the following analyses:
(a) ethanol in alcoholic beverages
(b) composition of a complex mixture of species in lavender oil
(c) trace levels of DDT and PCBs in water
(d) hydrocarbon contamination of soils
If the instrument’s electronic integration system wasn’t working, give two means by which you
could measure the analyte response. Which is better? Why?
Why is it necessary to use an internal standard for accurate GC quantitative analysis?
What physical/chemical properties are most important in choosing an internal standard?
Why is necessary that the internal standard is not present in the sample?
Xylene mixtures contain all three dimethyl benzene isomers. The analysis involves dissolving the
sample in cyclohexane before injection.
Which of the following compounds would be the best internal standard: 1,4-dimethylbenzene,
methylbenzene, benzene, cyclohexane. Explain your answer:
Adv. Chrom.
6.15
6. Gas Chromatography
13.
Given the following data, calculate the resolution value for the two separations, and the number
of theoretical plates in each column. Comment on the results, given that column 1 is 3 m packed
column, and column 2 is a 10 m capillary column.
Peak
Column 1
tr (min)
tw (min)
5.78
0.55
6.30
0.60
Compound 1
Compound 2
14.
Column 2
tr (min)
tw (min)
4.05
0.14
4.99
0.18
The chromatogram below shows the elution of ethanol, 1-butanol and 1-octanol from a polar
column at a temperature of 60°C. The column has a maximum operating temperature of 175°C.
B
A
C
0
2
4
6
8
10
Retention time (minutes)
12
(a)
(b)
(c)
(d)
Comment on the appearance of the chromatogram. Is it satisfactory?
Why is peak C such a different shape to peaks A & B?
Which compound do you expect belongs to each peak? How would you make sure?
What difference to the chromatogram would occur if the following changes were made?
(i) the temperature is increased to 80°C
(ii) the temperature is increased to 160°C
(iii) the temperature is increased to 200°C
(iv) the cylinder of carrier gas begins to run out
(v) a non-polar column is used
(vi) the initial temperature remained at 60°C and was increased after three
minutes to 150°C in the next two minutes
(vii) 1-pentanol is added to the mixture
(e) As the chromatogram appears above, is peak height be a suitable measure for the relative
quantities of each compound?
(f) The sample was a dilute aqueous solution directly injected. What detector must have been
used? Why?
Answers to these questions on following page.
Answers to class exercises can be found in the Powerpoint file provided on the website.
Adv. Chrom.
6.16
6. Gas Chromatography
Revision Questions
1.
see page 3
2.
capillary columns are longer, narrow and are hollow with the SP bonded to the inner walls
3.
If the column is heated beyond its maximum temperature, the Stat. phase becomes volatile.
4.
Heating the column with no gas flowing causes it to be oxidised and destroyed.
5.
see page 3
6.
(a) see pages 3/4 (b) see Table 5.3
7.
(a)
(b)
(c)
(d)
8.
Peak height is the best alternative, others include estimating area by triangle shape, counting
squares and weighing the paper
9.
To compensate for variations in injection volume.
10.
Similar boiling point and functional groups.
11.
The IS method relies on the response from the IS only changing if the injection volume changes.
If the compound is already in the sample, this will give an extra response, which would be
assumed to be due to an injection change and cause an error.
12.
Methylbenzene & benzene would both be OK, but methylbenzene is closest to the analytes, and
therefore the better choice. the other two are present in the sample.
13.
Avge N: Column 1 - 1765, Column 2 – 12800; 1 is packed, 2 is capillary
Resolution: Column 1 - 0.9, not good; Column 2 – 5.9, fine.
14.
(a) No, C takes too long to come out, wasting time and making measurement of its response
inaccurate.
(b) peak broadening - the longer a compound is retained, the more it spreads out
(c) A - ethanol, B - 1-butanol, C - 1-octanol. Spiking.
(d) (i) all compounds would have shorter retention times, A & B might lose resolution
(ii) A & B would elute at the same time, C would be much quicker and sharper
(iii) column bleed, evident by sloping baseline
(iv) retention times would increase
(v) loss of separation of ethanol & butanol since they would have little or no
attraction to the column; octanol would and would still be slow
(vi) A & B would still be resolved, and C would be much quicker and sharper
(vii) a peak between B & C
(e) it is for A & B, but not for C because of its width and low height
(f) FID, since there would have been a large water peak otherwise
BP20 (polar), FID (ignores water), TCD would be OK since high sensitivity is not needed
BP20 (good for essential oils), MS (identification by spectrum)
DB5 & ECD (ideal for organochlorine pesticides)
OV1 (non-polar), FID (for sensitivity)
Adv. Chrom.
6.17