Download Analysis of Chemicals

GCE
Chemistry
Context study
Edexcel Advanced Subsidiary GCE in Chemistry (8CH01)
Edexcel Advanced GCE in Chemistry (9CH01)
Analysis of Chemicals for AS
October 2007
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Contents
Introduction
1
Analysis of chemicals
3
Unit 1 topic 1.5: Atomic structure and the periodic table
3
The age of the Earth
3
Determining the composition of the solar wind
4
The pharmaceutical industry
4
Drugs testing for athletes
5
Accurate relative atomic masses
5
Unit 2 topic 2.12: Mass spectra and IR
7
Analysis of organic molecules
References
7
9
Introduction
This document is designed to help teachers to understand the contemporary context of
analysis of chemicals. It should give teachers information on this context and on how to
research it further if they wish. This document could also be given to students as introductory
material.
Context study (Analysis of Chemicals for AS) — Edexcel AS/A GCE in Chemistry (8CH01/9CH01)
— Issue 1 — October 2007 © Edexcel Limited 2007
1
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Context study (Analysis of Chemicals for AS) — Edexcel AS/A GCE in Chemistry (8CH01/9CH01)
— Issue 1 — October 2007 © Edexcel Limited 2007
Analysis of chemicals
Mass spectrometry (MS) is an extremely valuable analytical tool used over a broad range of
scientific disciplines. It is often used in conjunction with other techniques such as gas
chromatography (GC), high performance liquid chromatography (HPLC).
Unit 1 topic 1.5: Atomic structure and the periodic table
The age of the Earth
Geologists can date rocks or even obtain the age of the Earth. Here is an example using the
rubidium/strontium dating method.
Natural rubidium consists of two isotopes, 85Rb and 87Rb. The latter is radioactive with a halflife of nearly fifty billion years. It emits a β particle and turns into the non-radioactive isotope
87
Sr. Natural strontium also contains 87Sr. Most minerals that contain rubidium also contain
some strontium, so the age calculation must take into account the presence of the 87Sr at the
beginning of the time interval.
As molten rock material cools, first one mineral and then another solidifies. Rubidium is
crystallised with potassium compounds fairly late in the solidification process together with
some strontium compounds. Different rocks of the same age will have different Rb:Sr ratios
although the initial 87Sr:86Sr ratio will be the same in all samples.
If there is a non-radiogenic isotope of the daughter element present in the mineral, it can be
used as a reference, and the ratios of the parent and daughter elements plotted as ratios with
that reference isotope. The slope of the curve then gives the time interval. (Non-radiogenic
means that it is not produced by radioactive decay.)
The rubidium/strontium dating method uses the non-radioactive isotope strontium-86 as a
comparison standard. The relative amounts of 87Sr to 86Sr and 87Rb to 86Sr in different rock
samples believed to be the same age are determined with great precision using a highresolution mass spectrometer. A plot of these points, shown in figure 1, fits a straight line.
The slope of this graph can be used to calculate the time since the solid rock formed from the
molten material, ie the age of the Earth.
The 4.5 billion year age for the Earth is consistent with the results of the potassium/argon and
uranium/lead estimations. Similar results are also obtained from the study of spontaneous
fission events from uranium-238 and plutonium-244.
Context study (Analysis of Chemicals for AS) — Edexcel AS/A GCE in Chemistry (8CH01/9CH01)
— Issue 1 — October 2007 © Edexcel Limited 2007
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Figure 1 — Plot of 87Sr v 87Rb standardised against 86Sr (Source: Kane 1987)
For figure 1, the half-life, t1/2, for 87Rb = 4.88 x 1010 years, so the age of the rock =
4.53 x 109 years.
Determining the composition of the solar wind
An exciting example of the use of mass spectrometers in astronomy is to determine the
composition of the solar wind. The Solar and Heliospheric Observatory (SOHO) satellite has a
group of instruments aboard known collectively as the Charge, Element and Isotope Analysis
System (CELIAS). Up-to-date results indicate that hydrogen and helium make up 99.9 per cent
of the wind. The sensitive mass spectrometer has made measurements of the trace
constituents, which include isotopes of silicon, sulphur, calcium, chromium, iron and nickel.
Isotopes of neon and argon have been discovered in the wind, which were undetected in
earlier satellite missions. The isotopes provide clues about where the solar wind originates in
the dynamic structure of the sun.
The pharmaceutical industry
Mass spectrometry provides four essential requirements for the pharmaceutical industry,
namely:
•
sensitivity — the accurate measurement of small samples
•
selectivity — can differentiate similar chemical environments even in stereoisomers
•
speed — fast turnaround times for analysis
•
high throughput — dealing with large volumes of samples.
It is a valuable tool in each stage of drug development and has become the preferred
analytical method for trace-mixture analysis. It is used in conjunction with sample
preparation, and chromatographic separation using GC or HPLC.
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Context study (Analysis of Chemicals for AS) — Edexcel AS/A GCE in Chemistry (8CH01/9CH01)
— Issue 1 — October 2007 © Edexcel Limited 2007
Modern methods of surface chemical characterization play an important role in the study and
development of pharmaceutical products. Time-of-flight secondary ion mass spectrometry
(ToF-SIMS) is one of the most important surface analysis techniques. (Source: CSMA
laboratories)
The energy of an ion produced by electron bombardment is proportional to the square of its
velocity, so the heavier the molecule the more slowly it passes through the spectrometer. In
ToF-SIMS, short electrical pulses are applied to the material being studied. The time taken for
the ions released to travel to the detector is measured. This time depends on the m/z value of
the molecular ion. No magnetic deflection or focusing is required with this method.
The two interesting case studies below concern analysis of contamination of pharmaceutical
products. These typically arise post production and show the versatility of mass spectrometry.
1
Tablets from a commercial manufacturer were turning yellow (from white) on exposure to
light. Mass spectrometry revealed that light was converting a CH2COOH group to an
aldehyde group leading to a new structure with a molecule of molar mass 30 less than the
original.
2
A certain liquid drug showed signs of micro-crystallization, in the bulk. Careful extraction
of a crystalline region onto filter paper followed by ToF-SIMS revealed that a polymer
additive in the bottle stopper had leached out and reacted with the drug molecule.
Controlled release drug-delivery systems for heart or diabetic patients allow improved quality
of life with fewer side effects and increased life expectancy. One approach is to encapsulate a
drug bead within a multi-layer polymer coating where the properties of the latter control the
rate of drug release. Since the production of this type of multi-component system is difficult,
analysis of the final product is vital. Mass spectrometry (ToF-SIMS) has proved ideally suited
for this task and can provide an assessment of the thickness and uniformity of the coating
layers as well as the distribution of the drug and other excipients (released substances) within
the bead.
Drugs testing for athletes
Unfortunately a small minority of athletes who want to enhance their performance use drugs
such as anabolic steroids sometimes during competitions. The same techniques that chemists
use to analyse new products can of course be applied to drug testing.
Accurate relative atomic masses
A traditional example of the uses of mass spectrometry is where chemists and physicists need
to know accurate relative atomic masses (RAM).
The relative abundances of the isotopes of an element may be obtained with a mass
spectrometer. For example, the relative abundances of krypton isotopes is shown in figure 2.
Context study (Analysis of Chemicals for AS) — Edexcel AS/A GCE in Chemistry (8CH01/9CH01)
— Issue 1 — October 2007 © Edexcel Limited 2007
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Figure 2 — Experimental mass spectrum of krypton (Source: Krane 1987)
Krypton isotope mass
relative
abundance
78
77.92
0.0012
80
79.92
0.0200
82
81.91
0.12
83
82.91
0.12
84
83.91
0.57
86
85.91
0.17
Kr
Kr
Kr
Kr
Kr
Kr
Table 1 — Calculation to obtain the relative atomic mass (RAM)
(Source: www.apsidium.com/elements/014.htm)
A weighted average of the isotopes in table 1 above gives a value of 83.90, the accepted
atomic mass of krypton which appears in the periodic table. Other isotopes of krypton are
known, but they do not appear in natural samples because they are unstable (radioactive).
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Context study (Analysis of Chemicals for AS) — Edexcel AS/A GCE in Chemistry (8CH01/9CH01)
— Issue 1 — October 2007 © Edexcel Limited 2007
Unit 2 topic 2.12: Mass spectra and IR
Analysis of organic molecules
In the analysis of organic molecules the technique of mass spectrometry, infrared
spectroscopy and nmr spectroscopy are all used in the ‘detective work’ of elucidating a
structure. As a common example, a simple molecule ethyl ethanoate will be analysed here.
Bear in mind that a microanalysis giving the empirical formula will invariably be available to
the chemist. In this example the empirical formula is C2H4O.
Figure 3 — The mass spectrum of ethylethanoate
The mass spectrum is given in figure 3. The final peak at 88 is the molecular ion M + and is
extremely useful information. This confirms C4H8O2 as the molecular formula, but there are
four structural isomers within this formula, ethylethanoate, CH3COOC2H5, methylpropanoate,
CH3CH2COOCH3, 1-propylmethanoate, HCOOCH2CH2CH3 and 2-propylmethanoate,
HCOOCH(CH3)2. (A very small peak at M+1 may be seen in some spectra. This is due to the
presence of one atom of the C13 isotope in a molecule, giving a mass of one more than the
molecular ion.)
Under the electron bombardment in a mass spectrometer, esters break at the C-O bond
producing a stable fragment of formula RCO+. This is usually the most intense (longest) line in
the spectrum.
Ethylethanoate will therefore have a major line at m/z = 43 due to the CH3CO+ ion,
methylpropanaoate will have a major line at m/z = 57 due to the CH3CH2CO+ ion and the two
methanoate esters will have a major line at m/z = 29 caused by the HCO+ ion, as will the
others, but this line is then caused by the CH3CH2+ ion.
Context study (Analysis of Chemicals for AS) — Edexcel AS/A GCE in Chemistry (8CH01/9CH01)
— Issue 1 — October 2007 © Edexcel Limited 2007
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Further fragmentation of the main fragments is likely and a common peak at 15 is due to the
methyl ion, CH3+, which again is almost invariably seen. The mass spectrum does not give the
order of the groups but does tell which of the main groups are present. A trial guess at the
spectrum — say ethyl ethanoate — can explain the fragmentation pattern, whereas another
guess — say ethanal — could be eliminated.
The infrared spectrum is presented in figure 4.
Figure 4 — The IR spectrum of ethylethanoate (ethyl acetate)
For convenience the wave numbers of the absorptions are shown. The key peak to focus on
occurs at 1743 cm-1. This is a very large absorption and is characteristic not only of the
carbonyl group but more specifically the carbonyl component of the ester group range
(1750-1730 cm-1). C-H stretching at around 2900-3000 cm-1 will be almost invariably be present
but in this case is indicative of the CH bonds in alkanes. Sometimes this absorption is seen as a
‘shoulder’ on an O-H peak. Often it is useful to note missing peaks such as a broad peak
around 3600-3300, which eliminates O-H or COOH. Whilst C-O stretching (1200-1150 cm-1)
might be a useful peak to look for. The remaining peaks in the ‘fingerprint region’ (ie less
than 1400 cm-1) may be used to confirm the exact compound being analysed by comparing the
spectrum with a database such as SDBS.
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Context study (Analysis of Chemicals for AS) — Edexcel AS/A GCE in Chemistry (8CH01/9CH01)
— Issue 1 — October 2007 © Edexcel Limited 2007
References
Lee M S — Spectroscopic methods of analysis — Mass spectrometry Vol 1 issue 1, pp 2535-2551
(available online www.dekker.com/sdek/linking~db=enc~content=a713491567, 2002)
Morrison R T and Boyd R N — Organic Chemistry, 5th edition (Allyn and Bacon, Inc, 1987)
Structure spectroscopy examples for GCE
Chapman B — Transition Metal, Quantitative Kinetics and Applied Organic Chemistry
(Nelson, 2001)
Houghton J — Global warming: The complete briefing, 3rd edition (Cambridge University
Press, 2004)
Extra reading
A detailed and in-depth overview of current MS technologies and applications can be obtained
from the recent proceedings of the American Society for Mass Spectrometry Conference on
Mass Spectrometry and Allied Topics (www.asms.org) and the Association of Biomolecular
Resource Facilities (www.abrf.org).
Websites
Dr Rod Beavon’s Chemistry
pages
http://home.clara.net/rod.beavon/chemistry_contents.htm
Extended periodic table
www.apsidium.com/ext_pt/expertab.htm
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Context study (Analysis of Chemicals for AS) — Edexcel AS/A GCE in Chemistry (8CH01/9CH01)
— Issue 1 — October 2007 © Edexcel Limited 2007
9
October 2007
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