Spectroscopy. Page 1 of 8 L.Pillay (2012)

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1 Spectroscopy Electromagnetic radiation is widely used in analytical chemistry. The identification and quantification of samples using electromagnetic radiation (light) is called spectroscopy. Light has a dual nature and its behaviour can be described in terms of a particle (or photon) and of a wave. We describe a wave in terms of wavelength and frequency: Where: λ = wavelength in cm c (the velocity of light) = cm sec -1 =frequency in s (the number of complete wave cycles that pass a particulate point per second) * We also use wavenumber ( ) in cm -1, which is the reciprocal of wavelength or. We measure the energy of the electromagnetic radiation, which is called a photon using Where h is Planck's constant, J s -1. Thus the electromagnetic spectrum can be described in terms of energy and wavelength. Shorter wavelengths have greater energy. The figure below shows the different regions in the electromagnetic spectrum. Page 1 of 8 L.Pillay (2012)

2 The interaction between light and matter can result in molecular or atomic transitions in energy. When energy is absorbed by an atom, a transition from lower to higher energy level will occur (remember that energy levels are quantised). Atoms can undergo electronic transitions, denoted E in the figure, (E 0 E 1, E 2 ), while molecules can undergo electronic, rotational (R) and vibrational (V) transitions due to the more complex nature of the bonds they form. electronic transition = Δ energy electrons vibrational transition = Δ vibration of bonds rotational transition = Δ rotation of molecules Spectroscopy is the quantification of the absorption, scattering, or emission of electromagnetic radiation by atoms or molecules. When the atom or molecule absorbs EM radiation, they may be converted to heat or re-emitted as light. Emission of EM radiation occurs when matter that has been excited thermally, chemically or by absorption loses energy. Sample exposure to radiation from different regions of the electromagnetic spectrum may provide information for determining a compounds structure (by identifying functional groups), and for the qualitative and quantitative determination of inorganic and organic compounds. Information about the analyte, both qualitative and quantitative can be obtained by measuring the amount of EM radiation absorbed or emitted by the sample. The radiation energy can be linked back to λ, a plot called a spectrum, can be obtained, providing useful information about the analyte. The figure above shows an absorption spectrum obtained when a sample is irradiated. Absorption will only occur if the energy of the photon must match the energy required for an electronic transition to occur. Atomic absorption spectra are simple and are generally represented by sharp bands, the number of which will depend on the number of valence electrons present. The atomic absorption spectrum shows at which wavelengths of light a particular atom will absorb photons therefore, each element has a characteristic absorption spectrum (electronic transitions occur in the UV-Vis region). In addition, because each energy transition has a different energy, absorption bands of different colour will be produced. Page 2 of 8 L.Pillay (2012)

3 Absorbance Absorption spectra of molecules are more complex than atomic spectra since in addition to electronic transitions, there are rotational and vibrational transitions. E = E electronic + E vibrational + E rotational The infra-red energy range is able to provide molecules with energy for vibrational transitions. Now instead of sharp bands, molecular spectra are characterised by broad absorption bands as shown in the spectrum of an indigo pigment below. This absorption spectrum has a maximum absorption in the UV/Vis region of approximately 620 nm. Wavelength in nm When white light passes through or is reflected by a coloured substance, a portion of the different wavelengths is absorbed. The remaining light will then assume the complementary colour to the wavelength absorbed which we see. So, the pigment above is absorbing UV radiation at approximately 620 nm. The complementary colour to this is a mixture of blue and violet resulting in an indigo colour. The absorption of UV/Vis light by a molecule is determined by the transitions that the valence electrons of that molecule undergo. The different types of electronic transitions that occur are shown below. Transitions involving p, s, and n electrons Transitions involving charge-transfer electrons ( Page 3 of 8 L.Pillay (2012)

4 The part of a molecule that is responsible for UV absorption is called a chromophore. Typical chromophores have atoms containing lone pairs of electrons or have double bonds (conjugation). Absorption occurs for chromophores with valence electrons that require low excitation energies. The bigger the energy gap, the lower the wavelength at which absorption occurs. Electronic transitions When light passes through a compound, the energy excites an electron from one of the bonding or non-bonding orbitals into one of the anti-bonding ones. The energy gaps between these levels determine the wavelength of the light absorbed, and those gaps will be different in different compounds. In order to absorb light in the region from nm the molecule must contain either pi bonds or atoms with non-bonding orbitals (lone pairs). The structure of the molecule determines the types of transition. * Transitions The energy required for this transition is large. For example, methane (which has only C-H bonds, and can only undergo * transitions) shows an absorbance maximum at 125 nm. Therefore absorption maxima due to these transitions are not seen in typical UV-Vis. spectra ( nm). n * Transitions n * transitions occur with saturated compounds with lone pairs. Although less energy is required for this transition, there are a limited number of organic functional groups with this transition in the UV region. This transition occurs in the wavelength range nm. n * and * Transitions Most absorption spectroscopy of organic compounds is based on transitions of n or electrons to the * excited state. This is because the absorption peaks for these transitions fall in the region of the spectrum nm. These transitions require electrons, therefore compounds must have unsaturated groups. Conjugation plays a role in absorption maxima. The energy required for these transitions is lowered due to delocalisation of electrons over all the atoms (due to conjugation). Compounds with high conjugation tend to be coloured e.g. carotene. Highly conjugated structures will also give UV spectra with many peaks. The table below gives examples of the wavelength range at which particular functional groups undergo electronic transitions. Transition (nm) (L mol -1 cm -1 ) examples * < alkanes, C-C and C-H n * OH, -NH 2, halogens n * C=O, -COOH, -nitro, * alkenes, alkynes, aromatics Page 4 of 8 L.Pillay (2012)

5 (L mol -1 cm -1 ) is the molar absorptivity of a compound. It is the measure of how strongly a species absorbs light at a particular wavelength. High molar absorptivities indicate that a compound is good at absorbing light at the given wavelength; this means that lower concentrations of this compound will be detected by UV spectroscopy. Instrumentation Molecular absorption spectroscopy is used to measure the absorption of light by chromophores for quantification. The measurement is carried out by a spectrophotometer. The instrument measures the intensity of light that reaches a detector after being passed through a sample. The components of a spectrophotometer are shown in the figure below. Two light sources are used for UV/Vis measurements, a deuterium lamp for the UV range ( nm) and a tungsten-halogen lamp for the visible region ( nm). Most instruments automatically change the light source as the wavelengths scan the visible and UV regions. The light is focused into the monochromator where it is dispersed by prism or diffraction grating into its component wavelengths. The entrance slit for light, light dispersion device, and exit slit are referred to as the monochromator. The angle of the light hitting the grating is changed resulting in the different wavelengths. Each wavelength of light is passed through a sample and a reference (blank) solution and the difference in light intensity between the sample and reference beams are detected and the light energy converted to electrons by the photomultiplier. Electron voltage is measured and converted to an absorbance reading. There are two different types of instruments commonly used; a single beam and a double beam instrument. A single beam instrument, light is passed through a reference solution followed by the sample solution. In the double beam instrument, light is split by a beam-splitter and one beam of light passes through the reference sample and the other through the sample solution at the same time. Page 5 of 8 L.Pillay (2012)

6 The power of the light source beam decreases when light is absorbed by a sample from P 0 to P. the fraction of light that is transmitted by the solution after absorption is called the transmittance. The transmittance (T) is commonly expressed as a %. The sample absorbance can be related to the transmittance. As the absorbance of the sample increases, the transmittance decreases. A transmittance of 100% represents a sample that has no absorbance. T %T 100% 10% 1% 0.1% A The absorbance of a sample is governed by the amount of absorbing molecules the sample has, and the path length over which the sample absorption occurs. This is the absorption law known as the Beer-Lambert Law. Where ε is the proportionality constant called the molar absorptivity measured in L mol -1 cm -1 ;l is the path length in cm, and c is the concentration of the absorbing species measured in mol L -1. The molar absorptivity is dependent on the wavelength at which the absorption measurements are carried out. In general, these measurements should be carried out at the wavelength at which maximum absorption occurs. At λ max, the absorbance readings are at maximum sensitivity, therefore allowing lower concentrations to be reliably measured. In addition, because the absorbance curve is at a peak at λ max, there is greater precision and accuracy as shown below. The path length is determined by the dimensions of the vessel which holds the reference or sample solution. These vessels are called cuvettes in UV analysis. They can be made from a number of different materials. Quartz and fused silica can be used in both the UV and visible range (the cut-off being 160 nm); however, these cells tend to be expensive. Normal glass can only be used for the visible range. Commonly used for the UV/Vis range are cheaper, disposable polycarbonate cuvettes. Material Wavelength range (nm) Glass Plastic Quartz below 380 Page 6 of 8 L.Pillay (2012)

7 Cuvettes may have different path lengths ranging from 1 10 cm, but the most common is 1 cm. Speciality cuvettes are used for volatile samples or when there are small sample volumes. These are shown in the figure below. typical cell stoppered cell for volatile solvents micro cell for small volumes flow through cell ultra micro cell 10 L cells come in many designs When using a cuvette, make sure that the sides of the cuvette are clean and free of scratches or fingerprints. If a double beam spectrometer is used, generally it is preferable if a matched set of cuvettes are used. The choice of solvent for UV/Vis analysis plays a critical role in the accuracy of your analysis. The solvent used as the reference (blank) and to dissolve the sample should not absorb in the same region as your sample. Different solvents have different regions in which they are transparent and the choice of solvent must take this into account. The solvent cut-off is the wavelength below which the solvent itself absorbs all of the light. Some commonly used solvents with their UV absorbance cut off wavelengths are shown in the table below. Solvent UV absorbance cutoff (nm) Acetone 329 Benzene 278 Dichloromethane 233 Ethanol 205 Toluene 285 Water 180 Limitations to Beers Law There are a number of factors that affect linearity (A α c) when using the Beer-Lambert Law. High sample concentrations (> 0.01 M) lead to electrostatic interactions between molecules may cause deviations in ε. changes to the refractive index and shifts in chemical equilibria. Scattering of light due to particulates in the sample. Fluorescence or phosphorescence of the sample. High signal to noise ratio (affects samples with low concentrations). Page 7 of 8 L.Pillay (2012)

8 Additional UV/Vis Spectroscopy Methods Species that are not UV active can still be measured if the species is converted to a chromophore or a chromophore is added. This type of analysis is called colorimetric analysis. Some examples of this type of analysis are: The addition of diphenylcarbazide to Cr(VI) to produce a red-violet complex which can then be analysed at 540 nm. The addition of 1,10-phenanthroline with Iron(II) ions to produce an orange-red complex which has a λ 510 nm. When two or more absorbing species are present in a mixture, the total absorbance as well as the component species absorbance can be measured provided some criteria are met. The total absorbance of the solution can be given by the sum of the individual absorbance at a particular wavelength. A total = A 1 + A A n where A 1 = ε 1 l c 1 ; A 2 = ε 2 l c 2 A n = ε n l c n M + N N M In the figure adjacent, there is no wavelength at which there is absorbance due to only one species. Here two wavelengths are chosen where the absorbance of the species differs considerably from the other (λ 1 for M and λ 2 for N). The molar absorptivities at each wavelength are determined using standards of M and N and constructing a calibration curve, ε being the slope at each point. A (@λ1) = ε 1 x l x c M + ε 2 x l x c N A (@λ2) = ε 1 x l x c M + ε 2 x l x c N Since ε and l are known, the simultaneous equations are used to solve for each concentration. (Practical 2 involves this concept) To determine the concentration of an absorbing species in a sample solution, Obtain an absorption spectrum of the absorbing species to be studied and determine λ max. Determine if other species in the sample solution absorb at the chosen wavelength, Prepare a calibration curve and plot the absorbance of each standard as a function of the concentrations. The concentrations of the standard solutions are chosen so that the %T of these solutions lies in the range of 70% to 10% transmittance. ( to an absorbance of ~ ) This should give a relative error <2%. Measure the absorbance of the sample solution at the chosen wavelength. The concentration of the sample solution may be obtained from the calibration curve. Remember to take account of any dilutions that have been carried out. Page 8 of 8 L.Pillay (2012)