Accelerator Mass Spectroscopy

Accelerator Mass Spectroscopy (AMS) is a highly sensitive technique that is useful in isotopic analysis of specific elements in small samples (1mg or less of sample containing 10⁶ atoms or less of the isotope of interest).^[1]

Accelerator Mass Spectroscopy

AMS requires a particle accelerator, originally used in nuclear physics research, which limits its widespread use due to high costs and technical complexity.  Fortunately, UC Davis researchers have access to the Lawrence Livermore National Laboratory Center for Accelerator Mass Spectrometry (CAMS LLNL), one of over 180 AMS research facilities in the world.  AMS is distinct from conventional Mass Spectrometry (MS) because it accelerates ions to extremely high energies (millions of electron volts) compared to the thousands of electron volts in MS (1keV=1.6×10^-16 J).  This allows AMS to resolve ambiguities that arise in MS due to atomic and molecular ions of the same mass.  AMS is most widely used for isotope studies of ¹⁴C, which has applications in a variety of fields such as radiocarbon dating, climate studies, and biomedical analysis.^[2]  Some of the most fascinating applications of AMS range from exposure dating of surface rocks, ¹⁴C labeled drug tracer studies, and even radiocarbon dating of artifacts such as the Shroud of Turin and the Dead Sea Scrolls.^[3]

Theory

In conventional atomic mass spectrometry, samples are atomized and ionized, separated by their mass-to-charge ratio, then measured and/or counted by a detector.  Rare isotopes such as ¹⁴C present a challenge to conventional MS due to their low natural abundance and high background levels.  Researchers were challenged by isobaric interference (interference from equal mass isotopes of different elements exemplified by ¹⁴N in ¹⁴C analysis), isotopic interference (interference from equal mass to charge isotopes of different elements), and molecular interference (interference from equal mass to charge molecules, such as ¹²CH₂^-, ¹²CD, or ¹³CH^- in ¹⁴C analysis).   Most AMS systems employ an electrostatic tandem accelerator that has a direct improvement in background rejection, resulting in a 10⁸ time increase in the sensitivity of isotope ratio measurements.  As the natural abundance of ¹⁴C in modern carbon is 10^-12 (isotopic ratio of ¹⁴C:¹²C), a sensitivity of 10^-15 is a prerequisite for ¹⁴C analysis.

Figure 1.  A schematic of the AMS system at Lawrence Livermore National Laboratory Center for Accelerator Mass Spectrometry.

Figure 1, above, starts with a negative ion sputter source, which commonly consists of a stream of Cesium ions (Cs⁺) with energies of 2-3 keV focused on the surface of a solid sample in order to transfer enough energy to the target material to produce free atoms and ions of the sample material. This process, called sputtering, separates neutral, as well as positive and negative ions from the sample surface. The sample is held at a negative potential, and negatively charged ions are accelerated away from the sample, resulting in a beam of negative ions (Figure 2, below).  Cs⁺ is particularly useful in ¹⁴C studies because it does not form a negative ion from ¹⁴N, thereby eliminating isobar interference.^[4]  It is important to have a beam of negative ions entering the accelerator because the negative ions are attacted to the high -voltage terminal which results in their net acceleration.

Figure 2.  Cs sputter ion source.

The low energy (~5-10 keV) diverging beam that leaves the ion source is accelerated, focused and transported to the accelerator by the injector system.^[2]  CAMS LLNL employs a low-energy mass spectrometer that selects for the desired atomic mass^[5] that separates ions by their mass to charge ratio (¹²C, ¹³C, and ¹⁴C ions pass through separately).  Most AMS systems use sequential injection, a process that switches between stable and rare isotopes via the application of varying voltages to the electrically insulated vacuum chamber of the analyzer magnet.  In sequential injection, typical injection repetition rates are 10 sec^-1 to minimize variations in the electrical load.^[2]  This process allows the development of more versatile systems, allowing for analysis of a wide range of  isotopes.^[1]  The alternative to sequential injection is simultaneous injection, a process adopted in accelerators dedicated to ¹⁴C analysis.  A recombinator is used following sequential injection, which is a sequence of magnetic analyzers and quadrupole lenses that focus the stable and rare isotopes so they recombine and enter the accelerator together.

The traditional accelerator was first developed in the early 1930s for nuclear physics research.  In 1939, UC Berkeley scientists Luis Alvarez and Robert Cornog were the first to use AMS in the detection of ³He in nature using the 88-inch Berkeley cylclotron.^[5]  Now, over 70 years later, cyclotrons have been replaced by an accelerator type with greater energy stability: the tandem electrostatic accelerator.  An electrostatic accelerator works by accellerating particles though a magnetic field generated by high voltages using a mechanic transport system that continuously transports charges from ground to the insulated high-voltage terminal.  All tandem accelerators with a maximum terminal voltage above 5 MV use such a mechanical system.^[2]   The negative ions that enter the accelerator are attracted to the high-voltage terminal, which is what accellerates theCAMS LLNL employs a tandem Van de Graaff accelerator, in which a second acceleration of millions of volts is applied.  In all tandem accelerators, atoms are stripped at the high-voltage terminal using either a thin Carbon foil or Argon gas.  Stripping is the process in which two or more electrons are removed.  The Van de Graaff accelerator removes at least four electrons.  It is preferrable to remove at least three electrons because by this process that molecular isobars of ¹⁴C (such as ¹²CH₂^-, ¹²CD, or ¹³CH^-) are destroyed due to the high instability of their positively charged forms, and atomic C⁺ ions such as ¹²C⁺, ¹³C⁺, and ¹⁴C⁺ are separated due to their different mass to charge ratios.^[4]  The negative ions are changed to positively charged ions and are thus accelerated back to the ground potential in the high-energy part of the accelerator.  Transmission through a foil changes with time due to radiation damage and foil thickening, thus gas strippers are used in all modern analyzers due to their increased transmission stability.^[2]

Magnetic lenses focus the high energy particles leaving the accelerator into a magnetic dipole, (the high energy analyzing magnet).  Stable isotopes can be collected at off-axis beam stops where secondary focusing lenses and additional analyzing equipment remove unwanted ions and molecular fragments to eliminate background.  At CAMS LLNL, a magnetic quadrupole lens focuses the desired isotope and charge state to a high-energy mass spectrometer which passes ¹²C⁺ and ¹³C⁺ into Faraday cups and further focuses and stabilizes ¹⁴C in a quadrupole/electrostatic cylindrical analyzer that leads to a gas ionization detector.^[5]  The magnetic quadrupole and electrostatic selectors coupled together ensure high selectivity and sensitivity, respectively.  Other detectors commonly found in AMS systems include surface barrier, time-of-flight, gas filled magnets, and x-ray detectors.

Interpretation

Rare isotopes analyzed by AMS are always measured as a ratio of a stable, more abundant (but not too abundant) isotope.  For example, the ratio in ¹⁴C studies is generally shown as ¹⁴C/¹³C.  Less abundant isotopes are preferable in AMS because the decreased flux of ions reduces background and wear on the instrument, which is of particular concern due to the quick deterioration of particle detectors (performance deteriorates at rates higher than a few thousand particles per second^[1]).

Applications

Common radioisotope elements measured with AMS and their applications are shown in Table 1^[4], below.  Because ¹⁴C analysis is by far the most popular application of AMS, the methods discussed below are all techniques used involving ¹⁴C.

Table 1.  Radioisotope elements generally measured with AMS and their applications.

Element (Common Isotope)	Radioisotope with AMS	Natural abundance	Half-life (yr)	Study application
Hydrogen (1H)	3H	trace	12.33	Biological/biomedical Nutritional trace
Beryllium (9Be)	10Be	trace	1,510,000	Geochronology Hydrogeological study Exposure dating
Carbon (12C)	14C	1 x10-10%	5730	Biological/biomedical Nutritional trace
Aluminum (27Al)	26Al	trace, synthetic	720,000	Biological/biomedical Exposure dating
Chlorine (35Cl)	36Cl	7x10−11%	301,000	Earth Science Hydrogeological study Exposure dating Migration of nuclear  waste
Calcium (40Ca)	41Ca	trace, synthetic	116,000	Biological/biomedical Nuclear weapon testing
Nickel (58Ni)	59Ni	trace, synthetic	112,000	Nutritional trace
Iodine (127I)	129I	trace, synthetic	15,700,000	Biological/biomedical Migration of nuclear  waste Environmental study

Radiocarbon dating is an analytical method based on the rate of decay of ¹⁴C, a radioactive carbon isotope formed in the atmosphere by the reaction between neutrons from cosmic rays and ¹⁴N (neutron + ¹⁴N = ¹⁴C + proton).^[2]  Resultant ¹⁴C atoms are taken up by plants in the form of ¹⁴CO₂, then transferred to animals though the food chain.  When animals and plants die, they cease to uptake ¹⁴C, and a steady decay of ¹⁴C continues in their tissues over time.  ¹⁴C atoms decay via electron emission (β radiation) to form ¹⁴N, a process which has a half life of 5,730 years.^[5]  Radiocarbon levels in the atmosphere change according to complex patterns which are affected by a variety of fluctuations ranging from the sun’s solar activity and the earth’s magnetic field, to ocean ventilation rate and climate.  ¹⁴C analysis of tree rings, corals, lake sediments, ice cores, and other sources has led to a detailed record of ¹⁴C variations through time, allowing researchers to establish an official radiocarbon calibration curve (also referred to as a radiocarbon clock) dating back 26,000 calendar years. In the 1960s, nuclear weapons testing released large amounts of neutrons into the atmosphere, nearly doubling ¹⁴C activity.^[2]  Samples taken after this time period can be radiocarbon dated using a ¹⁴C bomb curve like the peak shown below in Figure 3, can retrieve very precise dates (within 1 year at the steepest part of the curve).

Figure 3.  The New Zealand curve (red) is representative of atmospheric ¹⁴C in the Southern Hemisphere, and the Austrian curve is representative of the Northern Hemisphere.

¹⁴C analysis provides valuable information in the radiocarbon dating of the world’s most priceless artifacts.  One such example of the monumental impact of ¹⁴C AMS is the radiocarbon dating of the Dead Sea Scrolls to dates from 300 BC to AD 61 by labs in Zurich and Arizona.  AMS has also contributed greatly to environmental and atmospheric studies by providing information regarding particle composition and origin.  In the biochemical field, synthesized ¹⁴C labeled compounds can be administered as a tracer dose for in-vivo human metabolic and drug studies which require AMS analysis of graphitized biological samples. 

AMS is a highly sensitive method for isotopic analysis that has numerous key applications that are only growing with advances in technology.  High costs and technical complexities that arise with the use of a particle accelerator are the only limits to the widespread use of AMS.  Recent times have seen the emergence of commercially available compact accelerators that use as low as 200 kV for radiocarbon dating and biomedical applications, and as particle accelerators become more commonplace, modifications to the instrument have also broadened the number of isotopes the instrument can measure.

---

1. [1].   Tuniz, C., Bird, J. R., Fink, D. and G. F. Herzog, Accelerator Mass Spectrometry. CRC PRess: Boca Raton, 1998; p 371.

1. [2].   Hellborg, R.; Skog, G., Accelerator mass spectrometry. Mass Spectrometry Reviews 2008, 27 (5), 398-427.

1. [3].   Gove, H. E., From Heroshima to the Iceman. Institute of Physics Publishing: London, 1999; p 225.

1. [4].  Kim, S.. Graphitization for biological, biomedical, and environmental carbon-14-accelerator mass spectrometry applications: Optimization and characterization. Ph.D. dissertation, University of California, Davis, United States -- California. Retrieved February 28, 2011, from Dissertations & Theses @ University of California.(Publication No. AAT 3379582).

1. [5].   https://www.llnl.gov/str/Holloway.htmlc

Fragmentation Patterns in Mass Spectra

This page looks at how fragmentation patterns are formed when organic molecules are fed into a mass spectrometer, and how you can get information from the mass spectrum.

The origin of fragmentation patterns

The formation of molecular ions

When the vaporized organic sample passes into the ionization chamber of a mass spectrometer, it is bombarded by a stream of electrons. These electrons have a high enough energy to knock an electron off an organic molecule to form a positive ion. This ion is called the molecular ion - or sometimes the parent ion.

The molecular ion is often given the symbol M⁺ or - the dot in this second version represents the fact that somewhere in the ion there will be a single unpaired electron. That's one half of what was originally a pair of electrons - the other half is the electron which was removed in the ionisation process.

Fragmentation

The molecular ions are energetically unstable, and some of them will break up into smaller pieces. The simplest case is that a molecular ion breaks into two parts - one of which is another positive ion, and the other is an uncharged free radical.

The uncharged free radical won't produce a line on the mass spectrum. Only charged particles will be accelerated, deflected and detected by the mass spectrometer. These uncharged particles will simply get lost in the machine - eventually, they get removed by the vacuum pump.

The ion, X⁺, will travel through the mass spectrometer just like any other positive ion - and will produce a line on the stick diagram. All sorts of fragmentations of the original molecular ion are possible - and that means that you will get a whole host of lines in the mass spectrum. For example, the mass spectrum of pentane looks like this:

It's important to realize that the pattern of lines in the mass spectrum of an organic compound tells you something quite different from the pattern of lines in the mass spectrum of an element. With an element, each line represents a different isotope of that element. With a compound, each line represents a different fragment produced when the molecular ion breaks up.

The molecular ion peak and the base peak

In the stick diagram showing the mass spectrum of pentane, the line produced by the heaviest ion passing through the machine (at m/z = 72) is due to the molecular ion.

The tallest line in the stick diagram (in this case at m/z = 43) is called the base peak. This is usually given an arbitrary height of 100, and the height of everything else is measured relative to this. The base peak is the tallest peak because it represents the commonest fragment ion to be formed - either because there are several ways in which it could be produced during fragmentation of the parent ion, or because it is a particularly stable ion.

Using fragmentation patterns

This section will ignore the information you can get from the molecular ion (or ions). That is covered in three other pages which you can get at via the mass spectrometry menu. You will find a link at the bottom of the page.

Example 1: Mass Spectrum of Pentane

Let's have another look at the mass spectrum for pentane: What causes the line at m/z = 57? How many carbon atoms are there in this ion? There can't be 5 because 5 x 12 = 60. What about 4? 4 x 12 = 48. That leaves 9 to make up a total of 57. How about C4H9+ then? C4H9+ would be [CH3CH2CH2CH2]+, and this would be produced by the following fragmentation: The methyl radical produced will simply get lost in the machine. The line at m/z = 43 can be worked out similarly. If you play around with the numbers, you will find that this corresponds to a break producing a 3-carbon ion: The line at m/z = 29 is typical of an ethyl ion, [CH3CH2]+: The other lines in the mass spectrum are more difficult to explain. For example, lines with m/z values 1 or 2 less than one of the easy lines are often due to loss of one or more hydrogen atoms during the fragmentation process.

Example 2: Pentan-3-one

This time the base peak (the tallest peak - and so the commonest fragment ion) is at m/z = 57. But this isn't produced by the same ion as the same m/z value peak in pentane. If you remember, the m/z = 57 peak in pentane was produced by [CH3CH2CH2CH2]+. If you look at the structure of pentan-3-one, it's impossible to get that particular fragment from it. Work along the molecule mentally chopping bits off until you come up with something that adds up to 57. With a small amount of patience, you'll eventually find [CH3CH2CO]+ - which is produced by this fragmentation: You would get exactly the same products whichever side of the CO group you split the molecular ion. The m/z = 29 peak is produced by the ethyl ion - which once again could be formed by splitting the molecular ion either side of the CO group.

Peak heights and the stability of ions

The more stable an ion is, the more likely it is to form. The more of a particular sort of ion that's formed, the higher its peak height will be. We'll look at two common examples of this.

Examples involving carbocations (carbonium ions)

Summarizing the most important conclusion from the page on carbocations:

Order of stability of carbocations

primary < secondary < tertiary

Applying the logic of this to fragmentation patterns, it means that a split which produces a secondary carbocation is going to be more successful than one producing a primary one. A split producing a tertiary carbocation will be more successful still. Let's look at the mass spectrum of 2-methylbutane. 2-methylbutane is an isomer of pentane - isomers are molecules with the same molecular formula, but a different spatial arrangement of the atoms.

Look first at the very strong peak at m/z = 43. This is caused by a different ion than the corresponding peak in the pentane mass spectrum. This peak in 2-methylbutane is caused by:

The ion formed is a secondary carbocation - it has two alkyl groups attached to the carbon with the positive charge. As such, it is relatively stable. The peak at m/z = 57 is much taller than the corresponding line in pentane. Again a secondary carbocation is formed - this time, by:

You would get the same ion, of course, if the left-hand CH₃ group broke off instead of the bottom one as we've drawn it. In these two spectra, this is probably the most dramatic example of the extra stability of a secondary carbocation.

Examples involving acylium ions, [RCO]⁺

Ions with the positive charge on the carbon of a carbonyl group, C=O, are also relatively stable. This is fairly clearly seen in the mass spectra of ketones like pentan-3-one.

The base peak, at m/z=57, is due to the [CH₃CH₂CO]⁺ ion. We've already discussed the fragmentation that produces this.

Using mass spectra to distinguish between compounds

Suppose you had to suggest a way of distinguishing between pentan-2-one and pentan-3-one using their mass spectra.

pentan-2-one		CH3COCH2CH2CH3
pentan-3-one		CH3CH2COCH2CH3

Each of these is likely to split to produce ions with a positive charge on the CO group. In the pentan-2-one case, there are two different ions like this:

• [CH₃CO]⁺
• [COCH₂CH₂CH₃]⁺

That would give you strong lines at m/z = 43 and 71.

With pentan-3-one, you would only get one ion of this kind:

• [CH₃CH₂CO]⁺

In that case, you would get a strong line at 57. You don't need to worry about the other lines in the spectra - the 43, 57 and 71 lines give you plenty of difference between the two. The 43 and 71 lines are missing from the pentan-3-one spectrum, and the 57 line is missing from the pentan-2-one one.

The two mass spectra look like this:

Computer matching of mass spectra

As you've seen, the mass spectrum of even very similar organic compounds will be quite different because of the different fragmentations that can occur. Provided you have a computer data base of mass spectra, any unknown spectrum can be computer analysed and simply matched against the data base.

13 C NMR PRIMER

C-13 NMR (CMR)
                        1.  NMR experiments may be done with any atom which has a nucleus with a magnetic spin.
                        2.  Examples of various isotopes with nuclear magnetic spin states are listed below.
                                    a)  The chart shows the frequency and magnetic field strength for which the nuclei have their
                                                resonance. Note that for a given isotope, resonant frequency is proportional to applied field.

isotope à	1H				2H	13C		19F
Ho, gauss	10,000	14,100	42,276	51,480	10,000	10,000	42,276	10,000	42,276
Resonant Frequency MHz	42.6	60.0	180	220.0	6.5	10.7	45.3	40.0	169.2

                        3.  A significant difference in proton NMR vs CMR is that ¹H is the most abundant isotope of hydrogen.
                                    a)  In CMR the lesser abundant isotope is observed. About 1% of all carbon atoms are C-13.
                                    b)  The lower abundance means a larger sample must be used in the instrument. 

                                                 Proton NMR requires about .1 mg of sample. CMR requires about 1-5 mg of sample.
                        4.  The ¹H NMR spectrum ranges typically from 0-12 ppm; the CMR spectrum ranges from 0-210 ppm.
                                    a)  Even slight differences in a carbon's chemical environment results in a significant chemical
                                                shift so that the number of carbon atoms (of different chemical environment) in the compound
                                                can be determined simply by adding up the number of signals in the spectrum.
                        5.  A great advantage of CMR is that the NMR experiment can be run in two different modes.
                                    a)  The "off-resonance decoupling"  experiment results in the splitting of each signal for each carbon
                                                 atom in the molecule by the hydrogen atoms attached to it.
                                                i)  This means that the number of hydrogen atoms attached to the carbon atom can be quickly
                                                            deduced simply by counting the peaks of the multiplet and subtracting 1.
 

                                     b)  In the proton-decoupled mode each carbon nucleus appears as a singlet.
                                                i)  The protons are kept from coupling with the C-13 nuclei by irradiating the sample with
                                                            radio waves which flip the protons:  they do not spend enough time in either spin state to
                                                            couple with the C-13 nuclei. 

                                                 ii)  Adjacent carbon atoms do not split the signal in C-13 NMR. Why not?
                                                            Answer:  Since there are so few C-13 atoms (~1%), the chances of having two C-13                                                          atoms next to each other in a molecule are small.                                                                         

                        6.  In CMR as in NMR, electronegative atoms attached to the carbon atom results in a downfield shift.
                                    a)  In addition, replacement of a hydrogen atom on a carbon with an alkyl group results in a
                                                downfield shift.

Chemical Shift, ppm	10.2 ppm	21.8	30.9	69.0
Answer	C-4	C-1	C-3	C-2

 

                       7.  In the CMR spectrum of 2-butanol above, assign which carbons are responsible for each observed
                                    peak. 
                        8.  Coupling constants could be used in proton NMR to distinguish cis and trans isomers of alkenes.
                                    a)  The chemical shift of the allylic carbons in CMR are lower for the cis isomer than the trans
                                                isomer by about 5 ppm, so this is another way of deciding upon which isomer is being studied.
                                    b)  Notice the chemical shifts for the isomers cis- and trans-2-butene.
                                             cis: 11.4 ppm                       trans: 16.8 ppm

                        9.  Unlike proton NMR, CMR integrals do not yield ratios of nonequivalent carbons   since the peak
                                     intensities in CMR are subject to distortion.
                        10.  Below are shown some CMR chemical shift correlations (ppm).

             H.  Spectral Interpretation
                        1.  It is not necessary to memorize the various chemical shifts for the different types of chemical
                                    environments.
                                    a)  Tables can be consulted.
                                    b)  It is helpful to remember some generalities such as, thinking of the proton NMR spectrum as a                                                             football field:
                                                i)  Alkanes resonate around the 10-20 yard line on the right side.
                                                ii)  Hydrogens bonded to carbons in turn bonded to highly electronegative groups (OH, Cl, Br)                                                             resonate near mid-field.
                                                iii)  Inductive shielding effects  are cumulative, so the greater the number of Cl's, O's, etc
                                                            bonded to a carbon, the more the C-H resonance will move downfield (past mid-field to
                                                            the other end zone.)
                                    c)  In CMR remember that double-bonded carbons resonate far downfield, C-O and C-X
                                                (X = halogens) moderately far downfield, and C=O very far downfield.
                        2.  From CMR one can find the number of nonequivalent carbons and in the off-resonance mode the
                                    number of hydrogens on each carbon.
                        3.  From proton NMR probably the most useful aspect is the spin-spin splitting patterns. 
                        4.  The integrations in proton NMR yield proton ratios in each chemical environment and should be
                                    evaluated.
                        5.  The chemical shift of ~7 is usually indicative of the presence of the benzene ring.
                                    a)  A signal between 9.5 and 10 usually indicates the presence of an aldehyde
                                    b)  A signal between 10 and 12 usually indicates the presence of a carboxylic acid.
            I. Sample Problems
                        1.  3-hexanol is treated with boiling sulfuric acid forming a mixture of isomeric alkenes:

The mixture was then separated by gas chromatography and the CMR spectra taken of each fraction.  The following CMR information resulted (in ppm):
Isomer	d, ppm
A	12.3, 13.5, 23.0, 29.3, 123.7, 130.6
B	13.4, 17.5, 23.1, 35.1, 124.7, 131.5
C	14.3, 20.6, 131.0
D	13.9, 25.8, 131.2

                                           Identify each isomer. 

            Answer.  Isomers A and B have six signals and therefore must have six nonequivalent carbons (III and IV).  Isomer B has two chemical shifts about 5 ppm greater than Isomer A (17.5 vs 13.5, 35.1 vs 29.3), which indicates isomer B must be compound III (the trans isomer) and A must be compound IV.

            By similar analysis, Isomer C must be compound I (the cis isomer) and Isomer D must be compound II. 

                        2.  A ¹H NMR experiment is done on a compound of formula C₄H₇Cl₃.  Use the following proton NMR data to determine the structure of the compound.  (s =  singlet, d = doublet;  t = triplet;  m = multiplet). 

                                    d   (ppm):    0.9 (t);  1.7 (m); 4.3 (m);  5.8 (d) 
                           Integration ratio:       3  :      2      :       1    :      1 

                        Answer. The triplet at .9 (upfield) with an integration of 3 (3 H's) is a methyl group next to a methylene group.  CH₃CH₂-
                        The doublet at 5.8 is so downfield that it is most likely -CHCl₂.  Being a doublet there must be only one hydrogen on the adjacent carbon.
                        The multiplet at 4.3 with an integration of 1 could account for this adjacent carbon with 1              hydrogen atom;  the fact that it is downfield           can be explained by the presence of a Cl atom and being next to a carbon with two Cl atoms.

                        -CHCl- 

                        The structure of the compound is likely to be 1,1,3-trichlorobutane 

                        3.  A compound of formula C₃H₇NO­₂ (an -NO₂ group is present – thank IR for that) has the
                            following  proton NMR spectrum.  Determine the structure of this compound. 

                                     Answer:  CH₃CH₂CH₂NO₂

    Ha: 1.0 ppm (t);  Hb: 2.0 ppm (m);  Hc: 4.4 ppm (t)

                        4.  A compound has the formula C₄H₇O₂Br. The signal at 10.97 ppm was moved onto the chart since the
                            chart paper only runs from 0-10 ppm.  Determine the structure based on the proton NMR spectrum. 

                         Answer.           CH­₃CH₂CHBrCO₂H                                                             

   Ha: 1.1 ppm (t);  Hb: 2.0 ppm (m);  Hc: 4.2 ppm (t);  Hd: 10.97 ppm (s)