Download Combining and Choosing Analytical Techniques

Combining and
Choosing Analytical
Techniques
Chapter 8
Mass Spectrometry
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One of the most common and useful analytical
tools used in combination with other techniques is
mass spectrometry.
Mass Spec can be used for:
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Quantitative analysis – a sophisticated and very sensitive
detector that can measure how much of a substance is
present
Qualitative – to provide a unique fingerprint of a
substance, this can be used to identify the substance from
an on-line database or to give information about the
structure of a new or unknown compound.
Instrumentation
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The key principal of
mass spec is that a
charged particle passing
through a magnetic field
is deflected along a
circular path of radius
proportional to the
mass: charge ratio, m/e
(can also be m/z).
Instrumentation
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The sample as a gas enters the evacuated tube.
Positive ions are formed in the ionisation chamber when an
electron beam dislodges electrons from the sample atoms
The positive ions are accelerated by an electric field
The ions enter a magnetic field perpendicular to their path.
This causes the ions to move in a curved path with a radius
that depends upon the m/z ratio of the ions.
Only ions moving in a curved path of a particular radius
corresponding to a fixed m/z ratio will reach the collector
The collector measures the current due to the ions reaching
the detector and the data is recorded as a mass spectrum.
How the spectrum is formed
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A molecular substance can give a range of
peaks in the spectrum.
Two factors cause the many peaks in the
spectrum:
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The fragmentation of the molecules into a large
number of different positive ions
The occurrence of different isotopes of the atoms
that make up the molecules
Fragmentation
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The high energy electron beam can knock just
one electron from the molecule to form a positive
ion:
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M + e- → M+ + 2eM+ is called the 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
Fragmentation
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Look at page 112.
What happens to ethanoic acid.
When an ion fragments into two smaller
parts, one ion will retain the electron to
become the uncharged free radical and the
other is positively charged.
Only the positive ions reach the detector
Isotope effects
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In the same spectrum additional peaks can be
formed due to the occurrence of different isotopes of
an element.
Chlorine for example has two isotopes.
 35Cl (76% abundance) and 37Cl (24% abundance)
Interpreting Mass Spectra
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The height of the peak gives the relative
concentration of the ions present.
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The highest peak is assigned an intensity of 100% and all
other peaks are measured relative to the base peak.
The relative intensities of the ions depend on:
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The energy of the bombarding electrons
The stability of the ion fragments formed
The ease with which ions can lose atoms
Your Turn
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Page 114
Questions 1-5
Combined Techniques
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A chemist presented with a sample will usually
have more than one technique to call upon.
For example acetylsalicylic acid, the active
ingredient in aspirin, can be analysed by
volumetric analysis, UV-visible spectroscopy,
HPLC, infrared spectroscopy and NMR
spectroscopy.
Each technique will give different information.
We can even combine some of the techniques.
Combined Techniques
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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 HPLC-mass
spectrometry (HPLC-MS)
Chromatography and Mass
Spectrometry
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The advantage of combining these two
techniques is that chromatography can separate a
complex sample into any number of components
and each one can be positively identified through
Mass spec.
These are used all the times in forensic analysis.
As individual components of the sample elute
leave the chromatography column they enter the
ionisation chamber of the mass spec.
Things to consider when choosing a
technique
Chemical Considerations
 Analyte
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Is the sample a metal or a non-metal?
Is it coloured or transparent?
What functional groups are present?
Is it volatile?
Is it soluble in water or other solvents
Chemical Considerations
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Sample
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How much is available?
The concentration of the analyte in the sample.
The physical state of the sample
What else is present in the sample can it
interfere
Economic Considerations
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Time
Cost of Equipment
Expertise
Quality of Data
Your Turn
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Page 122
Question 12 and 13
Worksheet