Year 12 Chemistry :~ Chapter 8 Combining and choosing analytical techniques
A chemist presented with a sample will usually have more then one technique to call upon.
Each technique will give different information. One technique many be better suited to
determine the concentration of the substance, while another gives information about
the structure. In this chapter the following techniques are discussed.
 Mass spectrometry
 Combined techniques, such as gas chromatography – mass spectrometry (GC-MS)
 The synchrotron
 Factors chemists consider when choosing an analytical technique.
8.1 Mass Spectrometry
One of the most common and useful analytical tools used in combination with other
techniques.
 Quantitative analysis: how much of the substance
 Qualitative analysis: provide a unique fingerprint of a substance.
Instrumentation
The key principle of mass spectrometry is that a charged particle passing through a
magnetic field is deflected along a circular path or radius proportional to the
mass : charge ratio, m/e.
 The sample is added as a gas,
 Positive ions are formed in the ionisation chamber
 The positive ions are accelerated by an electric field,
 The ions enter a magnetic field perpendicular to their path, deflection depends
upon the mass to charge ration (m/e) of the ions.
 Only ions of a particular radius will reach the collector,
 Voltage or the strength of the magnetic field can be varied,
 A mass spectrum is formed.
How a spectrum is formed
Peaks in the spectrum:
 The fragmentation of the moles into a large number of different positive ions.
 Different isotopes of the atoms that make up the molecule.
Fragmentation
The high energy electron beam can knock just one electron from the molecule, M, to
form a positive ion, M+:
M + e-  M+ + 2ebombarding
electron
M+ is called the molecular ion or parent molecular ion.
The parent molecular ion is a radical, with one unpaired electron. It is chemically
unstable and so will often break into smaller fragments consisting of more positive ions
and uncharged free radicals.
For example, ethanoic acid, CH3COOH, will ionise as follows:
CH3COOH + e-  CH3COOH+ + 2eParent molecular
Ion: m/e = 60
CH3COOH+ will fragment:
CH3COOH+  CH3+ +
.COOH
Frag ion
uncharged
m/e = 15
free radical
CH3+ may fragment further:
CH3+  CH2+ + .H
Frag ion
m/e = 14
Isotopes effects
In the same spectrum additional peaks can be formed due to the occurrence of
different isotopes of an element.
The height of the peak in the mass spectrum gives the relative concentration of the ions
present.
The relative intensities of he ions depend on:
 the energy of the bombarding electrons
 the stability of the ion fragments formed
 the ease with which ions can lose atoms.
The mass spectrum is unique to the compound and so can be used to identify a
substance.
Applications include the analysis of pharmaceutical samples, medical screening,
environmental samples such as contaminants in water, and geological samples such as oil
and gas.
8.2 Combined techniques
Many instruments combine two techniques to provide more detailed and rapid
information about a sample. The most commonly used combined techniques are gas
chromatography – mass spectrometry (GC-MS) and high performance liquid
chromatography – mass spectrometry (HPLC-MS).
Chromatography and mass spectrometry
The advantage of the techniques of GC-MS and HPLC-MS is that the chromatography
can separate a complex sample into any number of components and each one can be
positively identified through mass spectrometry.
As the individual components of the sample elute leave the chromatography column, they
enter the ionisation chamber of the mass spectrometer where they are bombarded with
a stream of electrons. The positive ions formed are accelerated into a quadropole mass
spectrometer (see figure 8.6). The MS cycles through the range of mass-to-charge
ratios to form a scan for each component.
Synchrotron – the ultimate combined technique
A synchrotron is a giant collection of spectrometers linked via circular channels and
covering an area equal to half a football field. Australia’s first synchrotron, costing
$206 million, has been built adjacent to Monash University, Clayton, Victoria and opened
for business in 2007.
A synchrotron accelerates and circulates electrons to close to the speed of light.
Synchrotrons have a number of averages over conventional spectrometers:
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This technique has been used to
 help develop the anti-influenza drug Relenza
 investigate the optimum conditions for heating, cooling and stirring by chocolate
manufacturers.
 To test ceramic and metal surfaces used in jet aircraft engines
Combining analyses to learn more
In many cases chemists faced with an unknown sample will employ a number of analytical
techniques to get more information about the substance
Analysis of ethanol
 Redox reaction, resulting in a colour change (early breathalyser kits)
 Volumetric analysis (ethanol content in wine)
 IR spectrophotometer (breathalyser or blood sample)
 Fuel cell (breathalyser)
 GC-MS
 NMR
8.3 Assessing analytical techniques
What factors do chemists consider when choosing the analytical technique or techniques
to use? Factors to consider fall into two main types: chemical considerations and
economic factors.
*Ensure you have a copy of table 8.1 pg 120 in your summary notes.
Chemical considerations
Analyte
Is the analyte a metal or non-metal? Is it coloured or transparent? What functional
groups are present? Is the analyte volatile? Is it soluble in water or other solvents?
Sample
Consider the amount of sample available, the concentration of the analyte in the sample,
the physical state of the sample and possible interference from other compounds in the
sample.
Economic considerations
Time
eg, single analysis, continuos monitoring, multi-element determination and preparation
Cost of Equipment
eg, some NMR machines use expensive liquid helium as a coolant.
Expertise
Quality data
Costs are generally greater for high quality data.
Questions: : 2, 4, 5, 11, 6, 7, 8, 12, 9, 10, 14, 17, 19 & 20
HOLIDAY WORK
 Complete Chapter 8
 Complete any unfinished questions from Chapters 1 – 8. Use your timeline.
 Complete Summary Notes for all Chapters 1 – 8 in preparation with mid year exam.
 Complete A+ TEST 4!
 Worksheet 11: Decision making.
 Revise for NEAP TEST on Chromatography and Spectrometry
o pg 124 – 128 will help