This preview shows page 1. Sign up to view the full content.
Unformatted text preview: CHAPTER 12 MODERN ANALYTICAL CHEMISTRY (IB OPTION A) SUMMARY
Analytical chemistry involves qualitative and quantitative analysis of a sample to determine its chemical composition and structure and to determine purity. Analytical methods are usually faster, more precise and easier to automate than ‘wet’ methods, and a combination of analytical techniques is often used to obtain complete structural information. Spectroscopy Many analytical techniques involve spectroscopy - the way in which the absorption or emission of electromagnetic radiation by substances varies with frequency involving radiation such as UV, IR and microwave. The energy of a quantum of radiation is directly proportional to its frequency and the energy is inversely proportional to its wavelength. Thus high frequency (and hence short wavelength) radiation carry a great deal of energy and those of low frequency radiation carry much less. A particle (atom, molecule or ion) can absorb a quantum of light and this will affect its state depending on the amount of energy that the quantum carries: γ-rays can bring about changes in the nucleus. X-rays cannot cause changes in the nucleus, but have enough energy to remove electrons in inner filled shells of atoms. Ultraviolet and visible light have enough energy to affect the valence electrons. UV radiation causes sunburn with too much exposure to sunlight. Microwaves affect the rotational state of molecules. Radio waves can alter the spin state of some nuclei when they are exposed to magnetic fields and are used in NMR spectroscopy. In emission spectroscopy the frequency of the radiation emitted by excited particles dropping to a lower energy state is studied. In absorption spectroscopy radiation of a wide range of frequencies is passed through the sample and the way in which the absorption of radiation varies with its frequency is studied. Energy of particular frequencies is absorbed and used to enable a particle to move from a lower to a higher energy state. A double-beam spectrometer allows radiation from a source that is split into two equal beams, one passed through the sample and the other through a reference containing the same solvent but without the substance being studied. The two beams are then recombined at the detector. The signals from the sample and reference beams are then compared electronically to see if the sample absorbs radiation of any frequency. As the spectrum of the sample is scanned, the frequency of the radiation is varied and a graph of absorption against frequency, wavelength or wavenumber is drawn. Comparison of the spectrum of the unknown compound with a data bank enables its identification. IR: A quantum of infrared radiation has sufficient energy to excite a molecule to a higher vibrational level. In order to absorb infrared light a vibrational motion must result in a change in the dipole moment of the molecule. The information from an IR spectrum can be used to identify bonds present, but not always the functional groups present. In a mass spectrometer, fragmentation of a molecule occurs at the weakest bonds and where branches occur. For a molecule, a plot of relative intensities of signals of various m/z values from the fragmentation pattern is called a mass spectrum and is highly characteristic of a particular compound, like its fingerprint. Mass spectra are used to prove the identity of a compound and to help establish its structure. Some simple fragments detected in a mass spectrometer include: (Mr – 15)+ loss of CH3, (Mr – 17)+ loss of OH, (Mr – 18)+ loss of H2O, (Mr – 29)+ loss of C2H5 or CHO, (Mr – 31)+ loss of CH3O, (Mr –45)+ loss of COOH. NMR is an extremely diagnostic tool for the determination of structure as an NMR spectrum provides information of the chemical environment of the protons in a molecule. Electrons in a magnetic field orbit in such a way as to set up a magnetic field that opposes the applied field. Thus the magnetic field experienced by the nucleus, and hence the precise frequency at which it absorbs radiation, depends on the electron density near to the nucleus and therefore on the chemical environment of the nucleus.
© IBID Press 2007 1 CHAPTER 12 MODERN ANALYTICAL CHEMISTRY (IB OPTION A) SUMMARY
The body scanner operates on the principle of NMR spectroscopy. The main constituents of the body that contain hydrogen atoms, and hence produce signals, are water and lipids. Different parts of the body have different water-lipid ratios in the tissue and therefore absorb radio frequency radiation in different ways. The patient is placed in a strong magnetic field and as the scanner is moved around the body, data about the absorption at various angles can be accumulated to allow a three-dimensional image of the various organs in the body to be built up. The advantage of this technique called Magnetic Resonance Imaging (MRI) in body scanning is obvious as radio waves are harmless. MRI is used to diagnose cancer, Multiple Sclerosis (MS) and other conditions. Tetramethylsilane (TMS), (CH3)4Si is used as a reference in NMR spectroscopy and all absorptions measured against it. TMS has the advantage of being chemically inert, producing a single strong signal, as it has 12 hydrogens in identical chemical environments, it absorbs radiation at lower frequency and different from most organic compounds, so it does not interfere with other absorption signals. Also, it has a low boiling point and the pure compound can be obtained by easy removal of TMS. Interpretation of an NMR spectrum involves: • Number of peaks: Since each peak represents hydrogen atoms in the same chemical environment, then the number of peaks gives the number of different chemical environments hydrogen atoms are present in. • chemical shift: TMS is used as a reference at δ = 0 ppm. Different chemical environments produce different chemical shifts that can be identified using a data of chemical shifts (such as the IB Data Booklet). • Area under each peak is proportional to the number of hydrogen atoms present in that particular chemical environment. • Spin-spin splitting takes place since the spins of adjacent protons interact with each other; the splitting pattern depends on the number of protons it interacts with. Splitting patterns in high resolution spectra provide (n+1) peaks if there are n number of adjacent hydrogen atoms in the same chemical environment. Atomic spectroscopy is mainly used for the determination of trace metals in organic and inorganic samples such as aluminum in blood serum, calcium in blood serum, plants, soil samples, in water (for water hardness), mercury and lead in food samples amongst others. The non-excited vaporized metal atoms absorb its characteristic radiation from an external source and become excited, that is, it causes transition from the ground state to the excited state. Each metal absorbs light of characteristic wavelength or frequency. The ratio of the intensity of the transmitted light to that of the incident light energy is proportional to the concentration of the metal atoms present and excited. Thus, a calibration curve can be obtained by using standard solutions of known concentrations. The concentration of the unknown can then be determined from the calibration curve The components of AA spectrophotometer consist of a monochromatic light source of the same wavelength as that absorbed by the metal being detected. The application of high voltage across the electrodes excites the atoms on the cathode producing a constant, intense light source characteristic of the metal. The light source is focused into the flame in an atomizer unit. A flame or graphite furnace is used to atomize the sample into gaseous metal atoms. This involves three steps: desolvation (removing the solvent), vaporization of the solid sample to a gas and dissociation of the compound to free atoms (atomization). The choice of fuels includes the gases ethyne (acetylene, C2H2), hydrogen (H2) and propane (C3H8) all of which are explosive. The monochromatic light detector is set to the specific wavelength characteristic of the metal atoms being detected. Once the sample is introduced in the flame, it absorbs some of the monochromatic light from the hollow cathode lamp. There is a decrease in the intensity due to the atomic absorption which depends on the concentration of the metal atoms present. The monochromatic detector signal appears as the absorption in a computer read-out.
© IBID Press 2007 2 CHAPTER 12 MODERN ANALYTICAL CHEMISTRY (IB OPTION A) SUMMARY
Chromatography Chromatography is a method of analysis that is used for the separation of components of a mixture (for example the separation of chlorophylls in plant extracts), as well as qualitative analysis, that is identification of components present (such as the detection of amino acids in a mixture) and quantitative analysis, that is how much of a component is present (such as detection of levels of alcohol in blood). Chromatography operates on the principle of partition or adsorption. Partition involves the way in which components of a mixture distribute themselves between two immiscible liquid phases, depending on their solubility in each phase. • Adsorption involves the way a substance bonds to the surface of a solid stationary phase, made of polar solid. Thus, solubility of a component in the liquid phase and how strongly it is adsorbed to the solid stationary phase determines the rate at which it elutes. Depending on the polarity of the components, different strength of interaction will take place between each component in the mixture and the polar solid stationary phase. Thus, the more polar a component, stronger the interaction to the solid phase, slower the rate of elution. The greater the solubility in the mobile phase, the faster the rate of elution. The difference lies only in the nature of the two phases and hence the type of bonding that operates between the components of the mixture and these phases. Adsorption is the case in Column chromatography, High Performance Liquid Chromatography (HPLC) and sometimes TLC. In paper chromatography, water bonded to the cellulose in paper is the stationary phase and the organic liquid moving past it is the mobile phase. A spot of the mixture is applied to absorbent paper, rather like filter paper. The end of this is then dipped in the solvent used to ‘develop’ the chromatogram. The solvent soaks through the paper by capillary action, moving past the spot where the mixture was applied and onwards. The components that bond strongly to the solvent will be carried along in the direction that the solvent is moving, whereas those that do not bond to it will remain almost stationary. Rf is the ratio of the distance traveled by the component to the distance traveled by the solvent. Thin layer chromatography (TLC) is very similar to paper chromatography and the physical arrangement being almost identical. The difference is that the stationary phase is a thin layer, usually of silica (silicon dioxide, SiO2) or alumina (aluminum oxide, Al2O3) on a glass or plastic support. When dry, the separation of the components depends on the extent to which the components bond to the surface, i.e. are adsorbed by the stationary layer of silica or alumina, which in turn mainly depends on the polarity of the substance. SiO2 and Al2O3 both contain highly polar oxygen atom and thus have a high attraction for polar water. Thus in the presence of water, it becomes the stationary phase adsorbed to the silica or alumina and separation takes place more by partition. Because the particles in TLC are much finer than the pores in paper it usually gives better and faster separation and works with fairly small samples. The principle of column chromatography is very similar to adsorption in thin layer chromatography, as the stationary phase is usually silica or alumina and separation depends on whether a component is strongly adsorbed onto the surface of this or remains dissolved in the mobile solvent phase used to elute the column. The oxide powder is packed into a column with the solvent and the mixture applied at the top of the packing. The solvent is allowed to slowly drip 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. If the components are colorless, then separate fractions of the solution leaving the column in liquid chromatography must be collected and tested for the presence of the components of the mixture.
3 • • • © IBID Press 2007 CHAPTER 12 MODERN ANALYTICAL CHEMISTRY (IB OPTION A) SUMMARY
Where the mobile phase is an inert gas, the rate at which the components move through the liquid phase is dependent on its volatility, for example in Gas Liquid Chromatography (GLC). Thus, when components are being partitioned between two phases, the more volatile a component in the mobile phase, the faster it will be carried. In GLC the mobile phase is a gas and the stationary phase is packed into a very long (often a number of metres) thin column, that is coiled into a helix packed with an oxide (usually SiO2 or Al2O3), or more frequently (in HRGC, high resolution gas chromatography) a very thin column is coated on the inside with an oxide layer in which separation occurs because molecules of the mixture are adsorbed onto the surface of the oxide. Sometimes, the oxide acts as a support for a high boiling point oil or wax. In this case separation depends on the partition of the components between the gas phase and solution in the oil. In GLC, the mixture is injected into a steady gas flow at the start of the column and it vaporizes at the temperature used. The column is housed in an oven which controls the rate at which the sample passes through the column. The components of the mixture are detected as they reach the end of the column, either by the effect they have on the thermal conductivity of the gas, or by the current that results from the ions formed when they are burnt in a flame. The results are shown as a graph of the detector signal against the time, the retention time, since the mixture was injected into the gas flow and the area under the peak is proportional to the amount of the component in the mixture. GLC is used to identify components that can vaporize without decomposition such as analysis of vegetable oil mixtures, analysis of gas mixtures from underground mines or from petrochemicals, analysis of components of fruit odors, detection of drugs in urine samples from athletes, detection of steroids, and blood alcohol levels. Gas Chromatography-Mass Spectrometry, GC-MS is a powerful technique that involves coupling the output of the gas chromatography column to the input of the mass spectrometer that identifies each component as it elutes by means of its mass spectrum. This is particularly useful in food and drug testing as well as in forensic science. The principle of High Performance Liquid Chromatography (HPLC) is similar to gas chromatography except that the mobile phase is a liquid, forced under high pressure (up to 107 Pa) through a rather shorter column (usually 10 − 30 centimetres long), rather than a gas. Its advantage over gas chromatography lies in the fact that it can be used for non-volatile and ionic substances. One of the major weaknesses of this technique is that the detector systems are less sensitive than those usually used in gas chromatography. For a volatile sample, generally gas chromatography on a suitable column will offer the best solution. For a non-volatile sample HPLC will often provide the solution. Column chromatography is most suitable for preparative purposes, whilst paper and thin layer techniques involve the minimum amount of apparatus, if all that is required is a simple qualitative check. The energy carried by a quantum of light in the UV and visible regions of the spectrum corresponds to the difference in energy between filled and unfilled electron orbitals in many ions and molecules. In solution, the ions of many metals are colorless. In some such ions (for example, sodium ions) the difference in energy between the highest filled orbital and the lowest unfilled orbital is quite large and so they only absorb very short wavelength UV light. In the transition metals, the difference in energy between filled and unfilled split d-orbitals is much smaller so that these ions in solution absorb energy in the far UV and visible regions, the latter being responsible for the fact that many of these ions are colored, and are of different colors. The color of these ions is the complementary color of the colors absorbed. UV-visible spectroscopy is used in assaying of metal ions, organic structure determination and detection of drug metabolites. Orbitals in a sub-level are all at the same energy. However, in the presence of ligands, Lewis bases with lone electron pair(s), the d orbitals are split into two sets at different energy levels.
© IBID Press 2007 4 CHAPTER 12 MODERN ANALYTICAL CHEMISTRY (IB OPTION A) SUMMARY
The difference in energy, ΔE corresponds to wavelength in the visible light. The transitions between split d orbitals are only possible if there are partially filled d orbitals containing 1 to 9 electrons. The number of d electrons present, the oxidation number of the metal, the different ligands and the transition metal present produce different degrees of splitting, giving rise to different colors. In the same way that atoms and ions have atomic orbitals, molecules have molecular orbitals, some of which are filled, others are unfilled. In molecules, such as water, and in simple ions, the difference in energy between the highest filled orbital and the lowest unfilled orbital is quite large, they only absorb very short wavelength UV light and the solution appears colorless. Carotene which has extensive conjugation and is an orange color because it absorbs blue and green light. Completely saturated compounds show no absorption in the visible and the far UV region. A double-bonded compound absorbs in the far UV region. Presence of two or more chromophores, unsaturated organic groups that absorb radiation mostly in the UV and visible regions specially if conjugated, lower the energy difference between orbitals and increases the wavelength of radiation absorbed. When there is an extended system of delocalised bonds and conjugated double bonds the absorption moves into the visible region to produce a colored compound such as carotene which is an orange because it absorbs blue and green light. The amount of light of a particular frequency which a solution absorbs depends on the nature of the compound which determines the molar extinction (or absorption) coefficient, ε; (units of ε: dm3 mol−1 cm−1.), its concentration (c in mol dm−3) and the distance the light passes through the solution (l, in cm). A = εcl. This is known as the Beer-Lambert law, and A (which is unit-less) is the absorbance of the solution, the reading given directly by most UV/visible spectrophotometers. Iron content in blood serum and the copper content of bronze (by converting Cu to Cu2+) can be determined using a standardization or calibration curve. (Shaded areas indicate AHL material) © IBID Press 2007 5 ...
View Full Document
This note was uploaded on 09/23/2010 for the course CS 001 taught by Professor Jix during the Spring '10 term at Riverside Community College.
- Spring '10