Option A
Analytical Chemistry
NOTES – Option A: Analytical Chemistry
PART 1: Mass Spectrometry & 1H-NMR
Reasons for using analytical techniques:
 Structure determination
 Analysis of composition of substances
 To determine purity of a substance
Examples of analytical techniques
 Atomic absorption spectroscopy
 Infrared spectroscopy
 Mass spectrometry
 1H nuclear magnetic resonance (NMR)
 Chromatography
Mass spectrometry
Used to determine relative atomic and molecular masses. The fragmentation pattern can be used as a fingerprint
technique to identify unknown substances or for evidence for the arrangements of atoms in a molecule.
How it works (5 basic steps):
1) ____________________________: if the sample is not already as gas, the sample is heated to this point.
2) ____________________________: sample is bombarded with a stream of high energy electons. In practice, the
instrument is set so that only ions with a single positive charge are formed (M+).
3) ____________________________: resulting unipositive ions pass through slits in parallel plates under the
influence of an electric field.
4) ____________________________: ions are then passed over an external magnetic field. The magnetic field
causes the ions to be deflected, and the amount of deflections is proportional to the charge/mass ratio. Ions
with smaller masses are deflected more than heavier ions. Ions with higher charges are deflected more as they
interact more effectively with the magnetic field.
5) ____________________________: positive ions of a particular mass/charge ratio are detected and a signal is
sent to a recorder. The strength of the signal is a measure of the number of ions with that charge/mass ratio
that are detected.
Example: Find the relative atomic mass (Ar) of naturally occurring lead from the data below. Record your answer to the
nearest tenth.
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Option A
Analytical Chemistry
Determining the molecular mass of a compound
 Can also use to determine relative molecular mass of a compound (Mr)
 If empirical formula is known, can be used to determine molecular formula.
Fragmentation Patterns
 Ionization process involves an e- from an electron gun hitting the incident species and removing an electron:
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This collision can be so energetic that it causes the molecule to break up into different fragments.
The largest mass peak corresponds to a parent ion passing through the instrument unscathed, but other ions
produced as a result of this break up are also detected.
 The fragmentation pattern can provide useful evidence for the structure of the compound.
 A chemist pieces together the fragments to form a picture of the complete molecule, just as the archaeologist
finds clues about the past from pieces of artifacts discovered on the ground.
Example: ethanol
 The highest mass fragment represents the Mr of the compound.
 Fragments provide clues about structure because certain numbers correspond to particular groups.
Fragments you will be expected to recognize:
Nuclear magnetic resonance (NMR) spectroscopy
Used to show the chemical environment of certain isotopes (hydrogen, carbon, phosphorous and fluorine) in a molecule
and so gives vital structural information.
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Option A
Analytical Chemistry
Nuclear magnetic resonance (NMR) spectroscopy
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Nuclei of atoms with an odd number of protons spin and behave like tiny bar magnets.
If placed in external magnetic field, some of these nuclei will line up with applied field and, if they have sufficient
energy, some will line up against it.
This arrangement leads to two nuclear energy levels; the energy needed for the nuclei to reverse their spin and
flip over can be provided by radio waves (RF).
Resonance: when the radio waves have the exact frequency needed to make the nuclei flip over and spin in the opposite
direction
The chemical shift
 As electrons shield the nucleus from the full effects of the external magnetic field, differences in electron
distribution produce different energy separations between the two spin energy levels.
 Nuclei in different _________________________________________ produce different signals in the spectrum.
 Proton or 1H NMR is particularly useful.
 Hydrogen nuclei are in all organic molecules.
 Act as spies and give information about their position in a molecule.
The TMS standard
 If we are to measure shift, we need a standard, or point of reference from which to measure.
 Tetramethylsilane (TMS) is the perfect standard.
 All 12 H’s are in identical chemical environments, so one signal is recorded.
 Because Si is less electronegative than C, TMS absorbs radio waves in a different region from that absorbed by H
attached only to C.
 This ensures that the signal does not overlap with any signals under investigation
 TMS is also inert.
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Option A
 TMS is soluble in most organic solvents.
 TMS can be easily removed from the sample because it has a low boiling point.
The chemical shift
 Position of NMR signal relative to the standard.
 Mathematically, the chemical shift () of a proton in a molecule is defined as:
Analytical Chemistry
Where f and f0 are the frequencies of the radio waves absorbed by the protons in the sample and TMS respectively.
Why make things so complicated???
 While absolute frequency of signal depends on the strength of the magnetic field, the chemical shift relative to
the standard is constant.
Interpreting 1H NMR spectra
High resolution 1H NMR spectra
Refer to pp. 88-89 of Option A chapter online
NMR is used in body scanners
 Protons (1H) in water molecules within human cells can be detected by magnetic resonance imaging (MRI),
giving a three-dimensional view of organs in the human body.
 31P is useful in determining extent of damage from a heart attack and monitoring the control of diabetes.
 Standard MRI Machine:
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Option A
Analytical Chemistry
PART 2: Spectroscopy (IR and UV-vis)
Spectroscopy is the main method we have of probing into the atom and the molecule.
Energy of electromagnetic radiation is carried in discrete packets of energy called photons or quanta.
• E = energy of a single photon of radiation
• h = 6.63 x 10-34 Js (Plank’s constant)
• f = frequency of the radiation
Example: Calculate the energy of a photon of visible light with a frequency of 3.0 x 1014 s-1. Express in kJ mol-1.
Note: f x  = c = 3.0 x 108 ms-1. Thus, f = c/ 
In IR spectroscopy, the frequency of radiation is often measured as number of waves per centimeter (cm-1), also called
wavenumber.
Example: Calculate the wavenumber in cm-1 for an IR wave with a frequency of 3 x 1013 s-1.
Info from different em regions (you need to be able to analyze data of underlined regions)
• Radio waves can be absorbed by certain nuclei causing them to reverse their spin. Used in NMR and can give
info about the environment of certain atoms.
• Microwaves cause molecules to increase their rotational energy. Can give info about bond lengths.
• Infrared (IR) absorbed by certain bodies causing them to stretch or bend. Gives info about bonds in a molecule.
• Visible and UV can produce electronic transitions and give info about energy levels within the atom or molecule.
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Option A
Analytical Chemistry
• X-rays are produced when electrons make transitions between inner energy levels. They have wavelengths of
the same order of magnitude as the inter-atomic distances in crystals and produce diffraction patterns which
provide direct evidence of atomic structure.
• Gamma rays cause changes in the energy of atomic nuclei. Not helpful to the analytical chemist.
Absorption and emission spectra
• Absorption: black lines where light is absorbed by sample.
• Emission: bands of colored lines where light is transmitted from excited sample.
Infrared (IR) Spectroscopy
Natural frequency of a chemical bond
• Chemical bonds are like springs.
• Bonds vibrate and bend at a natural frequency which depends on the bond strength and the masses of the
atoms.
– Light atoms vibrate at higher freq. than heavier atoms
– Multiple bonds vibrate at higher freq. than single bonds.
Using IR to excite molecules
• Energy needed to excite the bonds in a molecule to make them vibrate with greater amplitude occurs in the IR
region.
• IR radiation can cause a bond to stretch or bend.
• Stretching and bending
a) bending (symmetrical)
b) stretching (asymmetrical)
c) stretching (symmetrical)
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•
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Not all vibrations absorb IR.
For absorption, there must be a change in bond polarity (dipole moment) as the vibration occurs.
Thus, diatomic gas molecules such as H2, Cl2 and O2 do not absorb IR.
TABLE 1: Vibrations of H2O, SO2 & CO2
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Option A
Double-beam IR spectrometer
• One beam passes through sample
• Second beam passes through reference
– Purpose of reference: to eliminate absopions
caused by CO2 and H2O vapor in air, or absorptions
from the bonds in the solvent used.
• Baseline = 100% transmittance
Matching wavenumbers with bonds
• <1500 cm-1 = “fingerprint region” (lots of overlap)
• C=O (1700-1750) – very strong
• O-H (2500-3600) – broad and strong
• N-H (3300-3500) - usually sharper than O-H
IR spectrum of ethanol, CH3CH2OH
Analytical Chemistry
IR spectrum of ethyl ethanoate, CH3COOCH2CH3
UV-vis Spectroscopy
•
Similar to IR spectroscopy, but uses UV and visible light instead to analyze solutions of complex metal ions and
organic compounds.
• Horizontal axis: wavelength (not wavenumber as in IR)
• Vertical axis: absorbance / intensitry of the absorption (not %transmittance as in IR)
• UV-vis light have sufficient energy to excite electrons in higher energy levels in complex ions and molecules.
• When a full range of wavelengths of UV-vis radiation is passed through sample, an absorption spectrum is
obtained.
• Each characteristic absorption corresponds to an electronic transition.
• Substance that appear colored absorb certain wavelengths of light in the visible region and transmit the
remaining wavelengths.
• When the energy needed to excite an electron is in the UV region of the spectrum and all visible light is
transmitted, the substance appears white.
Transition metal complexes
Recall from unit 3 that d orbitals in a transition metal complex are split into two levels by the
electric field created by the lone pair of electrons of the surrounding ligands. For example,
when light passes through a sol’n of [Cu(H2O)6]2+, one 3d e- is excited from the lower to the
higher energy sub-level. A photon of orange light is absorbed and the light of the
complementary color, blue is transmitted.
Recall from unit 3…
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Option A
Analytical Chemistry
Colored complexes
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In the free ion the five d-orbitals are degenerate (of equal energy). However, in complexes the d orbitals are split into two distinct energy
levels.
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The energy difference between the levels corresponds to a specific frequency and wavelength in the visible region of the electromagnetic
spectrum.
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When the complex is exposed to light, energy of a specific wavelength is absorbed and electrons are excited from the lower level to the
higher level.
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Cu2+(aq) appears blue because it is the complementary color to the wavelengths that have been absorbed.
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The energy separation between the orbitals and hence the color of the complex depends on the following factors:
o Nuclear charge (based on identity of the central metal ion)
o Charge density of the ligand
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Ex: NH3 has a higher charge density than H2O and so produces a larger split in the d sublevel.
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[Cu(H2O)6]2+ absorbs red-orange light and appears pale blue
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[Cu(NH3)4(H2O)2]2+ absorbs the higher energy yellow light and appears deep blue
o Number of d electrons present (and hence the oxidation # of the central ion)
o Shape of the complex ion
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Electric field created by the ligand’s lone pair of electrons depends on the geometry of the complex ion
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If the d sublevel is completely empty, as in Sc3+, or completely full, as in Cu+ or Zn2+, no transitions within the d sublevel can take place
and the complexes are colorless.
o NOTE: it is important to distinguish between the words “clear” and “colorless.” Neither AP, nor IB, will give credit for use of the
word clear (which means translucent) when colorless should have been used. Think about it, something can be pink and clear…
colorless means something else.
The Spectrochemical Series - when ligands are arranged in order of their ability to split the d orbitals:
I- < Br- < Cl- < OH- < H2O < NH3 < CNIodine causes the smallest splitting; Cyanide ions cause the largest splitting
Organic Molecules
Compounds containing unsaturated groups, such as C=C, C=O, -N=N-, -NO2 and the benzene ring can absorb in the UV or
visible part of the spectrum. A compound is more likely to absorb visible light (and so appear colored) when it contains a
conjugated system of alternate C=C and C-C bonds, with the  electrons delocalized over a larger area.
• Example: -carotene - absorbs in the blue region; transmits orange)
• Example: phenolphthalein - absorbs UV light in acidic sol’n, so appears colorless (not conj). In basic solution, the
pi bonds all overlap, allowing electrons to be delocalized across the entire molecule; now it absorbs green light
and appears purple.
Using UV-vis spectroscopy and Beer-Lambert Law to determine concentration:
Concentration of metal ions in sol’n can be determined by measuring how much light, at the characteristic  of the
metal, is absorbed by the sample.
• Ex: Cobalt (II) compounds are pink in sol’n (octahedral complex absorbs green light).
– Monochromator selects green light of appropriate ; pass light through sample; photocell detector
measures intensity of light transmitted.
• Intensity of light transmitted through a sol’n falls exponentially as the path length (l) ↑.
• The Beer-Lambert Law is generally expressed in logarithms to base 10, with the ratio log10(I0/I) defined as the
absorbance (A).
•
Degree of absorption depends on concentration.
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Option A
Analytical Chemistry
• As the Beer-Lambert Law only applies strictly to dilute sol’ns, it is not generally used directly.
• Instead, the absorbance of standard solutions with a range of concentrations is measured and plotted on a
graph to produce a calibration curve.
• This allows the concentration of a metal in a sample to be read of once the absorbance is known.
PART 3: Atomic Absorption Spectroscopy (AA) and Chromatography
Atomic Absorption (AA)
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Measures energy absorbed as electrons are promoted to higher energy level.
Extremely sensitive – measures concentrations as low as 1 ppb
Used to measure concentrations of metals in water, blood, soils, etc.
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Recall that Beer-Lambert’s law only applies strictly to dilute sol’ns, so it is not generally used directly.
In practice, known concentrations are used to produce calibration curves.
Absorbance of unknown is then measured and concentration can be read by interpolation of the calibration
curve.
Chromatography
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A technique for separating and identifying components in a mixture.
Components have different affinities for two phases: a stationary phase and a mobile phase
A component which has a strong attraction for the mobile phase will move quickly, while a component with a
strong attraction for the stationary phase will be held back.
Stationary and mobile phases are carefully selected so that the different components of the mixture will move
at different speeds and thus be separated effectively.
For example, if the stationary phase is nonpolar and a polar mobile phase is employed, components will
separate out based on their relative polarities (most polar will travel fastest).
Partition chromatography
• Non-volatile liquid stationary phase
• Inert solid surface
• Components distribute themselves between the two phases according to their relative solubility.
• Most soluble = fastest
• Examples: Paper, GLC and TLC
Adsorption chromatography
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Option A
Analytical Chemistry
• Solid stationary phase
• Mobile liquid or gas phase
• Components distribute themselves between the two phases according to their relative polarity.
• Most polar = fastest (if stationary phase is a polar solid)
• Example: LC, HPLC and at times TLC
Main types of chromatography
• Paper chromatography
• Thin layer chromatography (TLC)
• Column chromatography
• Gas-liquid chromatography (GLC)
• Gas chromatography – mass spectrometry (GCMS)
• High Performance Liquid Chromatography (HPLC)
Paper chromatography
• Paper composed mostly of cellulose.
• Polar due to hydroxyl groups
• “Dry” paper is actually ~10% water (H-bonded to –OH) groups
o Polar water bonded to paper = stationary phase
o Added solvent (eluent) = mobile phase
• Small amt. mixture spotted onto paper
• Starting point marked
• Paper suspended in small quantity of solvent (eluent)
• Closed container (prevents solvent evaporation & speeds process)
• As solvent rises, components partition between two phases bases on relative solubility.
• Solvent front is marked at end of separation
• Colored components can be seen/measured
• Other components may be made visible with stain (i.e. iodine or ninhydrin) or UV light
• Each solute will have a constant retention factor (Rf) for a given eluent.
• If two substances have similar Rf values in one solvent, the paper can be turned 90 and eluted with a different
solvent for further separation.
o Known as two-way chromatography
o Often used to separate amino acids
Calculating solute retention factors:
Rf 
distancemoved by solute
distancemoved by solvent (eluent)
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Option A
• Example: Calculate the Rf values for each of the components in the mixture.
Analytical Chemistry
Thin layer chromatography (TLC)
• Similar to paper chromatography
• Thin layer of a solid is painted onto an inert support, such as glass
o Common solids:
 Alumina (Al2O3)
 Silica (SiO2)
• When absolutely dry works by adsorption
• However, like paper, silica and alumina are very hygroscopic (water-loving) and therefore separation occurs
more by partition, with water as the stationary phase.
• Separated components can be recovered by scraping off the section containing the component and dissolving it
in a suitable solvent.
• Pregnancy tests may use TLC to detect pregnanediol in urine.
• Advantages of TLC over paper chromatography:
o Approx. three times faster
o More efficient – small samples can be separated and components can easily be recovered in pure form
o Results are more easily reproduced
o Range of mixtures can be separated by changing mobile and stationary phases
Column Chromatography
• Essentially large-scale TLC
• Usually used to separate mixture components for further use rather than simple ID.
• Stationary phase often alumina or silical gel (SiO2xH2O)
How it works:
o Mixture dissolved in minimum volume of solvent
o Added to top of column
o Sample is eluted (fresh solvent added to the top to wash sample down the
stationary phase)
o Components separated as they travel at different rates – collected at
bottom as different fractions.
o Solvent can also be changed to elute more tightly held components.
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Option A
Analytical Chemistry
Gas-liquid chromatography (GLC)
• Used to separate and ID gases and volatile liquids
How it works:
o Sample injected into oven where it is vaporized
o Vapor carried by inert gas (mobile phase) over a non-volatile liquid (stationary phase).
 N2 and He are typical carrier gases
 Long-chain alkanes w/ high b.p.’s supported on a SiO2 (glass) surface typically act as stationary
phase
• Components generally exit to a flame ionization detector connected to a chart recorder.
• The less volatile (lower b.p.) the component, the greater the retention time.
• Used to identify components that can vaporize without decomposition
• Examples:
o Illegal steroids and stimulants in urine
o Blood-alcohol levels
o Underground mine gases
Gas chromatography – mass spectrometry (GCMS)
• Like GLC, but employs mass spectrometry instead of flame ionization detector at exit point.
• Gas chromatography – mass spectrometry (GCMS)
• Computer matches the spectra and gives a printout of all components and their concentrations.
• Very powerful tool and used extensively in forensic science, medicine, etc.
• Mass spectrometer is connected to a computer which contains a library of the spectra of all known compounds.
High Performance Liquid Chromatography (HPLC)
• Like LC (column chromatography) but employs pressure instead of just gravity to force mobile phase through
column. (Ex: lab we did separating grape Kool-Aid components by pushing solvents through syringe)
• Stationary phase is typically silica with long-chain alkanes adsorbed to surface,
• Efficient separation, so long columns not needed.
• Separated components often detected via IR spectroscopy.
• Like GLC, results recorded onto chart showing different retention times.
• Can also be used to separate enantiomers (chiral separation) using columns containing optically active material.
• Other uses include analysis of oil pollutants, alcoholic beverages, food (antioxidants, sugars and vitamins),
pharmaceuticals, polymers, quality control of insecticides and herbicides.
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