If someone told you that fi nding new ways to solve some of society’s greatest threats to human health involved using technology that has existed since the beginning of the 20th century, you would probably give them a strange look. One of the recent challenges of mass spectrometry has been its application to medical research. Traditional mass spectrometry is useful for analysing small to medium-sized molecules;
but biological molecules, particularly macromolecules such as proteins, that are large, fragile to ionize and sensitive to temperature are not compatible with the high temperatures and high-energy electrons found in the average mass spectrometer.
Mass spectrometry has been part of the chemist’s toolkit for many years, but being able to make accurate measurements of the molecular masses of large
proteins was a dream. With many considering it impossible, two chemists using a lot of imagination and creativity caused a revolutionary breakthrough in our understanding of protein structure. In 1988 John Fenn from the USA published a paper on an electrospray mass spectrometry method that involved a simple modifi cation to a typical mass spectrometer.
Instead of the sample being vaporized and then ionized, the process was reversed, and the sample was fi rst ionized in solution and then vaporized. At the same time, in Japan Koichi Tanaka, using a soft laser
desorption method, found a way to use an intense laser pulse to ionize macromolecules. Since then these new technologies have become the standard methods for the structural analysis of biological molecules. Society has benefi ted from these advances in knowledge. In the fi eld of drug development many more molecules can now be analysed, new mechanisms for the spread of malaria have been discovered, the early diagnosis of some cancers is now possible, and hazardous molecules formed during food preparation have been identifi ed.
This story shows how, using technology, chemists can bring information, insights and analytical skills to bear on matters of public concern. It also highlights how long periods of continuity and major shifts in thinking are also persistent features of the way in which new knowledge is acquired in science. Like the advances in mass spectrometry, these changes often involve looking at existing knowledge in completely different ways.
• How do beliefs about what is valued in society infl uence the pursuit of which analytical technology tools will be developed and how they are used? Consider the social, political and economic forces.
• Compare the ways in which a technological invention such as the mass spectrometer might be comparable to imagining and creating a work of art.
1 Describe how the molecules become positive ions in the mass spectrometer.
2 Complete the table below stating the masses of the following fragments.
Fragment ion m/z ratio CH3+
C2H5+ OH+ CH2Cl+ CH3CO+ CH3O+
3 Including the molecular ion, state the molecular formula and the m/z ratio of three fragments you would expect to fi nd in the mass spectrum of methanol.
Section 1.3 Exercises
4 The mass spectrum of 3-heptanone is shown below.
relative abundance
m/z
20 40
27 29
41
57
72 85
114
60 80
10 20 30 40 50 60 70 100 90 80
100 0
120 O
CH3CH2—C—CH2CH2CH2CH3
Suggest the probable formula for the ions producing peaks at mass number:
a 114 b 85 c 57 d 29
5 The mass spectrum of compound B contains major peaks at m/z = 88, 73, 45, 43 and 29. Confi rm that this fragmentation pattern is consistent with compound B being ethyl ethanoate.
6 A student believes that the mass spectrum of compound C (below) is that of ethene. Explain why the student is mistaken, using at least two peaks on the spectrum to support your answer.
100
80
60
relative abundance
40
m/z 0
10 20
20 15 17
31
45
46
30 40 50
7 Referring again to the spectrum of compound C in question 6, deduce the identity of compound C, giving reasons for your deduction.
CHAPTER 1 MODERN ANALYTICAL CHEMISTRY 8 The mass spectrum of compound D is shown below. Deduce the identity of
compound D and explain why you made that deduction.
100
relative abundance
50
m/z 27 29
41 42
43
57 72
Nuclear magnetic resonance (NMR) spectroscopy makes use of the interaction between atoms and both a magnetic fi eld and electromagnetic radiation. NMR spectroscopy is one of the most powerful tools for investigating the structure of organic molecules and it has important medical applications. The technique is based on the fact that the nuclei of some atoms, when placed in a strong magnetic fi eld, absorb radiation in the radio wave region of the electromagnetic spectrum.
As early as 1930, it was found that certain atomic nuclei have a property called spin, and that, in spinning, these nuclei create a magnetic fi eld and so behave as if they were tiny bar magnets. Nuclei that exhibit this property include
1H, 13C, 19N and 31P. They all have an odd number of nucleons (protons and neutrons). Of these, the most commonly used for analysis in NMR is the 1H nucleus because it is so common in organic matter. When placed in a strong magnetic fi eld, the 1H nucleus has two possible orientations of its magnetic fi eld: in the same direction as the magnetic fi eld with spin = +12, or in the opposite direction with spin = −12. In fi gure 1.4.1 it can be seen that if radiation of the correct wavelength (and so of the correct energy) is applied to the nucleus, the energy is absorbed and the spinning nuclear magnet fl ips and becomes aligned at the higher
energy state (spin = −12). The appropriate wavelength that causes this behaviour is in the radio wave region of the electromagnetic spectrum.
When the fl ip occurs, the nucleus is said to be in resonance with the applied radiation, hence the name nuclear magnetic resonance.
A schematic diagram of the NMR spectrometer is shown in fi gure 1.4.2. A sample is dissolved in a suitable solvent and placed in a thin glass tube.
The tube is placed between the
1.4 NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY
Figure 1.4.1 (a) A spinning nucleus creates a magnetic field and so acts like a tiny bar magnet. (b) Spinning nuclei have two energy levels (E1 and E2) in an applied magnetic field. Absorption of energy causes transitions between these energy levels.
axis of spin
a b
E2
E1
E2
E1
N
S
direction of
spin applied magnetic field applied magnetic field
absorption of energy radio wave
energy
poles of a magnet. A strong magnetic fi eld is applied and the sample is irradiated with radio waves from an antenna coil (green coil in fi gure 1.4.2).
The sample absorbs energy and hydrogen nuclei are excited. The energy is then emitted and is picked up by the radio receiver coil (blue in the fi gure 1.4.2) and the signal is passed on to the computer.
Figure 1.4.2 Schematic diagram of the NMR spectrometer.
north south
magnet magnet
sample
I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I
I I I I I I I I I I I
detector radio-frequency
generator
recorder
Depending on the applied magnetic fi eld, a different frequency of radio waves will be needed to excite the nuclei. As the strength of the applied magnetic fi eld increases, the frequency (and hence energy) of the radio waves needed to excite the nuclei from spin = +12 to spin = −12 will increase.
Figure 1.4.3 As the strength of the applied magnetic field increases, the frequency (and hence energy) of the radio waves needed to excite the nuclei will increase.
?
1 2
2.34 4.73 7.0 11.75 Applied magnetic
field, Bo (tesla) ν
(MHz)
100 200 300 500
1 2
In practice, the wavelength is kept constant and the nuclei are exposed to a range of applied magnetic fi eld strengths. As the fi eld strength increases, 1H nuclei absorb and produce an NMR signal. The environment of the 1H nuclei infl uences
CHEM COMPLEMENT
Spinning nuclei
The basic research that culminated in NMR spectroscopy dates to the 1930s, when two physicists, Otto Stern and Isidor Rabi, determined that certain nuclei have spin.
For their research into this phenomenon, Stern and Rabi were awarded the Nobel Prize in Physics in 1943 and 1944 respectively. In the 1940s, Edward Purcell and Felix Bloch developed the technique of NMR to work out complex molecular structures.
Purcell and Bloch shared the Nobel Prize in Physics in 1952 for this research. In the 1960s, scientists used the principles of NMR to develop the diagnostic tool of magnetic resonance imaging (MRI). The initial basic research into nuclear spin ultimately led to the development of this important medical tool.
In 2003 the Nobel Prize for Physiology and Medicine was awarded to Paul C. Lauterbur and Sir Peter Mansfield for their work on magnetic resonance imaging.
Figure 1.4.4 A view of the inside of an MRI
CHAPTER 1 MODERN ANALYTICAL CHEMISTRY the way in which the hydrogen nucleus responds to a given magnetic fi eld.
Electrons around each nucleus are spinning and so have an associated magnetic fi eld that shields the nucleus from the applied magnetic fi eld. In response, a greater amount of energy is needed for the nucleus to be excited.
An actual NMR spectrometer is a very expensive instrument that is highly dependent on modern computer technology. In the past 20 years this analytical tool has advanced signifi cantly due to advances in superconducting magnets and computers. The magnetic fi elds that are used are very great indeed, making the NMR measurements more sensitive than with smaller magnetic fi elds. If we compare the magnetic fi eld used in an NMR spectrometer to the approximate value of the Earth’s magnetic fi eld, we fi nd that the fi eld in the NMR spectrometer is approximately 10 000 times that of the Earth’s magnetic fi eld. However, even with such high magnetic fi elds, the change in energy between the two spin states of the 1H nuclei is very small: 10–5 times that of the energy transitions undergone in infrared spectroscopy and 10–7 times that of electronic transitions in UV–visible spectroscopy.
The exact point of absorption of energy by the 1H nucleus also depends on its immediate electronic environment. Electrons close to the 1H nucleus shield it slightly from the magnetic fi eld and so alter the position at which energy is absorbed. Thus the energy absorbed by a 1H nucleus bonded to an oxygen atom (–OH) will be different from that absorbed by a 1H nucleus bonded to a carbon atom (for example, in a CH3 group of an organic compound). Chemically equivalent nuclei (those in the same environment) all absorb at the same position on the NMR spectrum, while those in different environments absorb at different positions. The position of the peak along the horizontal axis of an NMR spectrum is known as the chemical shift. Hydrogen atoms that are bonded to more electronegative atoms have a greater shift than those in more electron-dense environments. Chemical shift is discussed in more detail in the higher level part of this option.
Worked example 1
The structural formula of butan-1-ol is given in fi gure 1.4.5. In this molecule the hydrogen atoms exist in four different environments.
1 The three green hydrogen atoms are in the CH3 group at the end of the chain.
2 The two pink hydrogen atoms are in a CH2 group adjacent to the CH3 and CH2OH groups.
3 The two blue hydrogen atoms are in a CH2 group that is bonded to a carbon atom that also has an –OH bonded to it.
4 The one red hydrogen atom is bonded to the oxygen atom.
When an NMR spectrum for this compound is examined, we fi nd four peaks (from lowest chemical shift to highest) with a peak area ratio 3 : 2 : 2 : 1.
The intensity of a signal on an NMR spectrum gives an indication of the number of nuclei causing the signal. An NMR spectrum can therefore provide information on the ‘types’ of 1H nuclei in molecules, as well as (from the relative intensities of the signal) the number of nuclei of each type.
The area under each signal peak is measured electronically (integrated) and the results presented in a ‘stair-step’ fashion. The height of each step is Deducing structures using 1H NMR spectra
A.5.1
Deduce the structure of a compound given information from its 1H NMR spectrum.
© IBO 2007
C C
C H
H H
H H
O
H H
H
Figure 1.4.5 The structural formula of propan-1-ol showing the four different environments of the hydrogen atoms.
proportional to the number of nuclei causing the peak. Therefore, to compare peak areas we simply need to measure the step heights in the integrated spectrum. WORKThese integrated spectra are extremely useful in interpreting
1H NMR spectra.
Figure 1.4.6 An integrated NMR spectrum allows determination of the relative numbers of each type of nucleus.
increasing signal intensity
integrated 1H spectrum of ethanol CH3CH2OH
The NMR spectra and structural formulas of propanone and of dimethoxymethane are given in fi gure 1.4.7.
8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0
Figure 1.4.7 The NMR spectra and structural formulas of (a) propanone and (b) dimethoxymethane. The NMR spectra of these compounds are very straightforward due to the carbonyl and ether functional groups.
a Propanone (CH3COCH3): There is only one environment for the hydrogen atoms in this molecule. The six hydrogen atoms are identically placed in two –CH3 groups bonded to a carbonyl group. As a result there is only one peak in the NMR spectrum. (Note that the peak marked TMS is a reference peak and not due to the propanone sample.)
WORKSHEET 1.2