Chapter 12: Modern Analytical Chemistry Fast Facts

Worksheet 12.2

Chapter 12: Modern analytical chemistry – fast facts

A.1 Analytical techniques

Chemical analysts identify and characterize unknown substances, determine the composition of a mixture and identify impurities.

Their work can be divided into:

Qualitative analysis: the detection of the presence,but not the quantity, of a substance in a mixture.
Quantitative analysis: the measurement of the quantity of a particular substance in a mixture.
Structural analysis: a description of how the atoms are arranged in a molecule.

Modern analytical methods generally work by analyzing the effect of different forms of energy on the substance analyzed. They are generally faster and more precise than traditional methods, such as those based, for example, on volumetric analysis.

Infrared spectroscopy is used to identify the bonds in a molecule.
Mass spectrometry is 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 of the arrangements of atoms in a molecule.
Nuclear magnetic resonance spectroscopyis used to show the chemical environment of certain isotopes (usually) in a molecule.
Information from only one technique is usually insufficient to determine or confirm a structure, so a combination of methods is needed.

A.2 Principles of spectroscopy

The electromagnetic spectrum includes, in order of decreasing frequency/energy:, γ rays,
X-rays, ultraviolet radiation, visible light, IR radiation, microwaves and radio waves (see Table 3 of the IB Data booklet).
Frequency (f) and wavelength (λ) are related by: c (speed of light) = fλ.
The energy of a photon (Ephoton) is related to the frequency (f) of the radiation by Planck’s equation:
Ephoton = hf (The equation is given in Table 1of the IB Data booklet). h is Planck’s constant (Table 2 of the IB Data booklet).

An emission spectrum is produced when an atom/molecule moves from a higher to a lower level with the emission of a photon of energy:

∆E = Ephoton = hf.

An absorption spectrum shows the radiation absorbed as atoms/molecules move from a lower to a higher energy level.

Different regions give different types of information, as they interact with substances in different ways.

Radio wavescan be absorbed by certain nuclei (usually ) causing them to reverse their spin. They are used in NMR and can give information about the chemical environment of H atoms.
Microwaves cause molecules to increase their rotational energy. This can give information about bond lengths. You are required to know the details at this level.
Infrared radiation is absorbed by certain bonds causing them to stretch or bend. This gives information about the bonds in a molecule.
Visible and ultraviolet light can produce electronic transitions and give information about the electronic energy levels within the atom or molecule. In organic chemistry it gives information about the π bonds in a molecule. In inorganic chemistry it gives information about the concentration of metal ions.

A.3 Infrared (IR) spectroscopy

Many spectroscopic methods use a double-beam method in which one beam is passed through the sample under investigation and the other through a reference sample.
In the double-beam IR spectrometer, IR radiation from a heated filament is split into two parallel beams.
Radiation is absorbed by the sample when it has the same frequency as the natural bond frequencies in the sample molecule.
When the radiation is not absorbed by the sample, the transmittance is 100% but when radiation is absorbed the transmittance falls to lower values.

Each bond vibrates and bends at a natural frequency which depends on the bond strength and the masses of the atoms. Light atoms vibrate at higher frequencies than heavier atoms and multiple bonds vibrate at higher frequencies than single bonds.
The IR spectrum can be used to identify bonds present, but not always the functional groups present.
A molecule can only be excited by infrared light if a change in vibrational motion produces a change in the dipole moment of the molecule.
The three vibrational modes of H2O are IR active as they produce a change in dipole moment.
Only three of the vibrational modes of CO2 are IR active.
The symmetric stretch produces no change dipole and so is IR inactive.

The IB Data booklet contains a table of IR absorptions for some bonds in organic molecules.

A.4 Mass spectrometry

The stages of operation:

vaporization of sample
ionization to produce M + ions by electron bombardment
acceleration of positive ions by electric field
deflection of ions by magnetic field perpendicular to their path
detection of ions.

The degree of deflection depends on the charge/mass ratio: the smaller the mass or the greater the charge, the greater the deflection.
The peak with largest mass represents the molecular (parent) ion and its mass gives the relative molecular mass.

The fragmentation pattern can help determine molecular structure.

/ (Mr– 15)
loss of CH3 / (Mr– 17)
loss ofOH / (Mr– 29)
loss of C2H5
or CHO / (Mr– 31)
loss of CH3O / (Mr – 45)
loss of COOH
Examples of fragments

A.5 Nuclear magnetic resonance (NMR) spectroscopy

A spinning hydrogen nucleus can be thought of as a small bar magnet. The energy between its two spin states depends on the strength of the external magnetic field applied by an electromagnetand its chemical environment.
Electrons in an external magnetic field orbit in such a way as to set up a magnetic field that opposes the applied field and so the frequency at which the hydrogen nuclei absorb radiation depends on the electron density near to the nucleus and therefore on the chemical environment.
The frequency of the NMR signal relative to this standard is called the chemical shift (δ) of the proton.

δ = ( (f – f0)/f0)  106 ppm

Hydrogen nuclei in particular environments have characteristic chemical shifts. A list is given in Table 18 of the IB Data booklet.

The number ofpeaks in an NMR spectrum equals the number of different environments of hydrogen atoms.
The integrated traceindicates the relative number of hydrogen atoms in the different environments.
NMR spectroscopy is non-invasive as the sample is recovered unchanged.
In magnetic resonance imaging (MRI)the patient is placed in a strong magnetic field chamber and bombarded with pulses of radio waves. Different parts of the body have different water-lipid ratios in the tissue and therefore absorb radio waves at different frequencies. The signals produced are decoded by a computer to produce a two- or three-dimensional image of organs in the human body.
MRI is ideal for detecting brain tumours, infections in the brain, spine and joints, and in diagnosing strokes and multiple sclerosis.

A.6 Atomic absorption (AA) spectroscopy

Atomic absorption spectroscopy determines the concentration of atoms, irrespective of how they are combined together.

It is an extremely sensitive method, allowing concentrations as low as one part per billion (ppb) to be determined.
It is used to measure the concentration of metals in water, soils, food and blood.
The non-excited vaporized metal atoms absorb radiation at a characteristic wavelength from an external source and are excited to a higher energy level. The log of ratio of the intensity of the transmitted light (I) to that of the incident light energy (Io) is proportional to the concentration of the metal atoms present.
A calibration curve is obtained using standard solutions of known concentrations, from which unknown concentrations can be determined.
The graph is linear for low concentrations in agreement with the Beer–LambertLaw: The absorbance of a given wavelength of light by a dilute solution is proportional to the concentration.

A = log10 (Io/I) = εcl

ε is molar absorptivity, c is concentration and l is the length of the radiation through the substance.
The atomic absorption spectrometer:

Themonochromatic sourcehas a hollow cathode with the vapour of the element under investigation. Radiation with wavelength of the maximum absorbance (λmax) for the element under investigation is generally used.
In the atomizer a flame is used to separate the sample into gaseous metal atoms and the solvent is evaporated. Ethyne (C2H2), hydrogen (H2) and propane (C3H8) are used as fuels.
Modern machines use the double-beam detectionprinciple to find the amount of light absorbed by atoms in the flame.

A.7 Chromatography

Chromatography separates and identifies the components of a mixture.
It is used in qualitative analysis: the identification of components present; and quantitative analysis: the determination of the amount a component is present.

The components have different affinities for the stationaryandmobile phasesand so are separated as the mobile phase moves through the stationary phase.
A component which has a strong attraction for the mobile phase will move quickly, whereas a component with a strong attraction for the stationary phase will be held back.
Partition chromatography uses a non-volatile liquid stationary phase held on an inert solid surface. The components distribute themselves between the two phases according to their relative solubility. Examples of partition chromatography include paper and gas–liquid chromatography.
Adsorption chromatographyuses a solid stationary phase and a mobile liquid or gas phase. Some components of the mixture are attracted to the solid surface and other components, which are less strongly bonded, travel faster with the mobile phase. As the stationary phase is generally a polar solid, the more polar solutes are more readily adsorbed than the less polar solutes. Thin-layer chromatography is an example.
In paper chromatography, the stationary phase is water bonded to the cellulose in paper and the liquid moving up the paper is the mobile phase. A spot of the mixture is placed on absorbent paper, which is then dipped in the liquid, which acts as the mobile phase. As the solvent soaks through the paper by capillary action, it moves past the spot. The components that are more soluble in the liquid are carried along with it leaving components that are more soluble in the water stationary phase. When some of the solvent has almost reached the top of the paper, the level is marked as the solvent front. The paper is then removed from the solvent and dried. The resulting chromatogram is treated with a locating agent or ultraviolet light if the different components are not visible. The organic dye ninhydrin is used to identify the different amino acids.
The different components are identified by their Rf value (retention factor).

Rf = distance moved by component above the base line/distance moved by solvent front.

Inthin-layer chromatography (TLC) the stationary phase is a thin layer of silica (silicon dioxide, SiO2) or alumina (aluminium oxide, Al2O3) on a glass or plastic support. The separation of the components is based on the differences in how the components are adsorbed by the stationary phase. The separation depends on the relative polarity of the components.
TLC has four advantages over paper chromatography:

It is quicker than paper chromatography as the silica or alumina particles are finer than the pores in paper.
It is more efficient, works on very small samples and the separated components can be easily recovered in a pure form.
The results are more easily reproduced.
A range of mixtures can be separated by changing the mobile and stationary phases.

TLC gives better and faster separation than paper chromatography and works with fairly small samples. The silica or alumina particles are finer than the pores in paper.
Incolumn chromatography the stationary phase is silica or alumina and the separation depends on whether a component is strongly adsorbed onto the surface or dissolves in the mobile solvent phase. The oxide powder is packed into a column with the solvent and the mixture applied at the top. The solvent slowly drips out of the bottom of the column, controlled by a tap, and fresh solvent added at the top so that the packing never becomes dry. As the mixture moves down it separates out into its components.

A.8 Ultraviolet and visible (UV–vis) spectroscopy

The 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.
Electrons in d orbitals localized along the axes have higher energy than electrons occupying d orbitals between the axes.
Ligands have a lone pair of electrons and include NH3, H2O and Cl–.
The energy separation, ∆E, between the d orbitals in the sub-levels and hence the colour of the complex depends on:

the nuclear charge and hence the identity of the central metal ion;
the charge density of the ligand;

the geometry of the complex ion (the electric field created by the ligand’s lone pair of electrons depends on the geometry of the complex ion);
the number of d electrons present and hence the oxidation number of the central metal ion.

The part of an organic molecule responsible for absorbing UV and visible radiationis called a chromophore, and generally includes unsaturated groups such as C = C, C = O, N = N, NO2 and the benzene ring.
A compound is more likely to absorb visible light and so appear coloured when it contains a conjugated systemof alternate C – C and C = C bonds, with the π electrons delocalized over a larger area. Benzene rings and other double bonds can also form part of a conjugated system.
Chlorophyll has extensive conjugation and has a green colour as it absorbs blue and red light.

Carotene has a conjugated system of π electrons and is an orange colour because it absorbs blue and green light.
The conjugated π system of retinol (right) absorbs visible light. The energy gained allows retinol to change its geometry, which allows an electrical signal to be sent along the optic nerve to the brain.
In acidic solution, the benzene rings in phenolphthalein are isolated and the compound is colourless.
In alkaline solution, the π system is extended throughout the molecule and it appears purple.
The concentration of metal ions in solution can be determined by measuring how much light, at the characteristic wavelength of the metal, is absorbed by the sample.A = log10 (Io/I) = εcl

Io I

This is known as the Beer–LambertLaw, ε is molar absorptivity and c is concentration.

A.9 Further NMR spectroscopy

NMR frequencies are measured against the standard signal produced by the twelve hydrogen nuclei in tetramethylsilane (TMS).
TMS produces a strong signal which, as silicon has lower electronegativity than carbon, does not overlap with any signals from the sample under investigation.
TMS is chemically inert and soluble in most organic solvents. It can be easily removed from the sample as it has a low boiling point.
In high resolution 1H NMR spectroscopy the peaks split as the effective magnetic field, experienced by particular nuclei, is modified by the magnetic field produced by neighbouring protons. This effect is known as spin–spin coupling.

The splitting patterns produced from different numbers of neighbouring protons can be deduced from Pascal’s triangle.
If a proton has Nprotons as nearest neighbours, its NMR peak is split into a group of N + 1 peaks.
When analyzing high-resolution NMR spectra, the following additional points should be noted:

Protons bonded to the same atom do not interact with one another as they are equivalent and behave as a group.
Protons on non-adjacent carbon atoms do not generally interact with one another.
The O–H single peak in ethanol does not split unless the sample is pure, as rapid exchange of the protons between ethanol molecules averages out the different possible spins.

A.10 Further chromatography

GLC is used to separate and identify small samples of gases and volatile liquids.
The sample is injected through a self-sealing cap into an oven where it is vaporized. The vapour is carried by an unreactive gas, the mobile phase, over a non-volatile liquid stationary phase.
Nitrogen and helium are typical carrier gases; long chain alkanes of high boiling point supported on a silicon dioxide surface act as the stationary phase.
The components of the sample are partitioned between the stationary and mobile phases, depending on their relative boiling points and relative solubilities in the two phases. They pass through the column at different rates, and so leave after different retention times.
Volatile components with low solubility in the liquid stationary phase have shorter retention times.

Unknown compounds can be identified by comparing the retention time with that of known compounds measured under the same conditions.
The area under each peak is a measure of the amount of each substance present.
The temperature of the oven is carefully controlled as it affects the rate at which molecules move through the apparatus.
GLC cannot be used for non-volatile substances or for substances which decompose at temperatures near their boiling points.
High-performance liquid chromatography (HPLC) is used when the sample is non-volatile or decomposes near its boiling point. High pressure is used to force the mixture through a column tightly packed with very fine solid particles.
The method can be used to separate components which are very similar to each other.
The choice of chromatographic technique depends on whether a quantitative or qualitative method is needed and on the nature of the sample.
Column chromatography can be used with large amounts, but GLC is very sensitive and so it can be used with smaller samples.
HPLC is the preferred method when the sample is non-volatile or decomposes near its boiling point.
GLC is used in the analysis of urine samples from athletes for illegal drugs; blood samples for ethanol levels and underground mine gases.
HPLC is used in the analysis of oil pollutants; alcoholic beverages; and antioxidants, sugars and vitamins in foods. It is used in the pharmaceutical and polymer industries, for quality control of insecticides and herbicides, and in biochemical and biotechnology research.