Spectrochemical methods

Transcription

1 Spectrochemical methods G. Galbács The interactions of radiations and matter are the subject of spectroscopy py or spectrochemical methods (also called spectrometry). Spectrochemical methods usually measure the electromagnetic radiation produced (emitted) or absorbed by molecular or atomic species of interest. It has to be added though that spectroscopy nowadays includes some methods that do not involve EM radiation, such as acoustic and mass spectroscopy. A spectrum always ay refers to a graph gap which shows the intensity of radiation or the count of particles as a function of energy. Spectroscopic methods are among the most selective and sensitive analytical methods, and therefore are the second most widely used methods (after chromatography). Spectroscopy also played an important role in the development of atomic theory.

2 Properties of electromagnetic radiation EM radiation (e.g. light) is a form of energy that is transported through space at very high velocities. EM radiation has a double nature. On one hand, it can be described as a wave with properties of wavelength, frequency, velocity and amplitude. It also shows wave phenomena such as refraction, reflection, interference, diffraction. But in contrast to e.g. acoustic waves, EM waves require no supporting medium for transmission (it propagates also in vacuum). Properties of electromagnetic radiation The frequency of EM radiation is determined by its source and remains constant regardless of the medium traversed. In constrast, the velocity of propagation depends both on the wavelength and the medium. The governing equation is v= ν λ wherevisthevelocity.in air, v is very close to c (speed of light in vacuum). Wavenumber (k, ν ) is defined as the reciprocate of lambda in cm.

3 Properties of electromagnetic radiation On the other hand, EM radiation can also be described as consisting of photons or quanta (particles). It is neccessary, as the wave model fails to account for the absorption and emission processes. For these processes, EM radiation is best treated as discrete packets of energy. The energy of a photon can be related to its wavelength, frequency or wavenumber h v E = h ν = = h c ν λ where h is the Planck constant ( J s). Radiant power (P, sometimes also called intensity) in watts is the energy of a beam that reaches a given area per unit time. The power of radiation is directly proportional to the number of photons per second. Some calculational examples part 1. Calculate the wavenumber of a beam of EM radiation that has: a) a wavelength of 5 µm b) a frequency of 100 GHz The correct answers are: a) 2000 cm -1 b) 3.33 cm -1

4 Some calculational examples part 2. Calculate the energy in joules for one photon of the radiation that has: a) a wavelength of 1 µm b) a frequency of 2 THz Remember that the value of the Planck constant is J s. The correct answers are: a) J b) J Some calculational examples part 3. Calculate the power in watts for a laser light pulse that has a duration of 10 ns and a pulse energy of 110 mj. The correct answer is : W Calculate how many photons the above laser light pulse contains, if it has a wavelength of 1 µm. The correct answer is :

5 The electromagnetic spectrum Classification of spectrochemical methods can given in terms of the species they deal with (atomic or molecular) or the radiation processes involved (e.g. absorption, emission). But most often it is done in terms of the energy. See the electromagnetic spectrum: The electromagnetic spectrum Strictly speaking, light only refers to EM radiation in the visible range (ca. 380 to 800 nm), but it is often used in the UV (ultra violet) and IR (infrared) range too (see the visible spectrum below). Light is associated with electronic transitions in atoms and molecules. Note that spectrochemical methods working in the UV, visible and IR range are often called optical methods.

6 Spectroscopic measurement modes In spectroscopy, the sample is stimulated in some way by applying energy in the form of heat, electrical energy, light, particles or chemical reaction. Pi Prior to the stimulus, the analyte is predominantly in its lowest energy state or ground state. The stimulus then causes some of the analyte to undergo a transition to a higher energy state, or excited state. In most cases, we acquire information about the analyte either by measuring the EM radiation emitted as the analyte returns to ground state or by measuring the amount of EM radiation absorbed by the analyte during excitation. In fact, four different measurement modes can be identified: emission spectroscopy absorption spectroscopy photoluminescence spectroscopy mass spectrometry Emission spectroscopy In emission spectroscopy, the analyte is stimulated by electrical, thermal or chemical energy. We record the emission spectrum, gy p, which is the intensity (power) of emitted radiation as a function of wavelength or frequency.

7 Emission spectroscopy schematic of the instrument In emission spectroscopy, the analyte is stimulated by electrical, thermal or chemical energy. We record the emission spectrum, which is the intensity (power) of emitted radiation as a function of wavelength or frequency. Absorption spectroscopy In absorption spectroscopy, the analyte is stimulated by EM radiation from an external source and some of this incident radiation is absorbed be the analyte. We record the absorption spectrum, which is the amount of absorbed intensity (power) of incident radiation as a function of wavelength or frequency.

8 Absorption spectroscopy schematic of the instrument In absorption spectroscopy, the analyte is stimulated by EM radiation from an external source and some of this incident radiation is absorbed be the analyte. We record the absorption spectrum, which is the amount of absorbed intensity (power) of incident radiation as a function of wavelength or frequency. Photoluminescence spectroscopy In photoluminescence spectroscopy, the analyte is also stimulated by EM radiation from an external source Some of this incident by EM radiation from an external source. Some of this incident radiation is absorbed by the analyte and we record the emission spectrum. In other words, we do emission spectroscopy after excitation by EM radiation. Two subclasses of this spectroscopy are the fluorescence and phosphorescence spectroscopies.

9 Photoluminescence spectroscopy schematic In photoluminescence spectroscopy, the analyte is also stimulated by EM radiation from an external source. Some of this incident radiation is absorbed by the analyte and we record the emission spectrum. In other words, we do emission spectroscopy after excitation by EM radiation. Two subclasses of this spectroscopy are the fluorescence and phosphorescence spectroscopies. Mass spectrometry In mass spectrometry, e the sample components are ionized using chemical, thermal or electric energy and then the ions are be separated according to their m/z (mass over charge) ratio by a mass analyzer. The number of charged particles for each m/z ratio reaching the detector is counted. This principle of operation needs no EM radiation to be applied or measured, hence its instrumentation is unique. The example here shows the mass spectrum of air. N 2 + N + O 2 + Ar + O + H 2 O + CO 2 +

10 Instrumentation - optical materials Sample holders, windows, lenses and wavelength selecting elements all must transmit radiation in the wavelength region of interest. This limits the choice of optical materials. For example, simple glass is fine for the VIS range, but fused silica or quartz is needed in the UV. In the IR range, halide salts are often used. Instrumentation sample holders in UV/Vis range cuvettes for liquids (and gases)

11 Instrumentation radiation sources Absorption and luminescence spectroscopies need a stable radiation source that emits a radiation of high intensity. In terms of wavelength, spectroscopic radiation sources fall into two categories: continuum sources and line sources. Continuum sources emit radiation in a broad spectral range (intensity changes slowly as a function of wavelength). Line sources emit radiation only at specific wavelengths. Sources can also be classified as continuous or pulsed, according to their operation as a function of time. continuum spectrum line spectrum Continuum radiation sources in the UV/Vis range As continuum radiation sources in the UV, typically deuterium or hydrogen lamps (on the left) and in the VIS range, tungsten lamps (on the right) are used. Tungsten lamps are ordinary filament lamps. Deuterium (or hydrogen) lamps are filled with low pressure H 2 gas, thatt is excited by electrical energy. This excited species then dissociates to two H atoms and a UV photon. The energy of the photon can vary within a range of energies.

12 Continuum radiation sources in the IR range IR continuum radiation sources are normally inert heated solids. Examples include SiC rods (Globar lamp) and cylinders of a composite made of 85% ZrO 2 and 15% YO 2 (Nenrst glower). Line sources in the UV/Vis range hollow cathode lamps Hollow cathodes are made of the metal of interest (or its compound). The high voltage (several hundreds of volts) connected to between The high voltage (several hundreds of volts) connected to between the cathode and anode will produce electrons that collide with and ionize inert gas atoms, which in turn will sputter the cathode (-) material. The sputtered atoms will be excitated by further collisions.

13 Line sources in the UV/Vis range - lasers Laser operation is based on the stimulated emission of radiation initiated by population inversion in an active medium by electrical/optical l/ i l pumping. Laser beams are: highly collimated possessing very high intensity highly monochromatic pulsed or continuous coherent (interference capable) Instrumentation monochromators Czerny-Turner Bunsen

14 Instrumentation photon detectors (ca nm) Phototube Photoelectron multiplier (PMT) Alkali li metal or metal oxide (Cs-Sb) 90 V or more Every next dynode is held on a ca. 100 V more positive potential Dynodes produce secondary electrons PMTs are very sensitive and fast detectors with very wide linear dynamic range (e.g. amplification is 10 9, dynamic range is 9-10 orders, etc.) Instrumentation semiconductor photon detectors Doped Si semiconductors Photodiode Without light, conductance is very low (na-µa) under reverse bias. When photons strike the p-n junction, they create hole- electron pairs, thus extra carriers. The created current will be proportional to the radiant power. For IR ranges, an InGaAs semiconductor is needed

15 Instrumentation array/multichannel detectors Photodiode arrays (PDA) Linear PDA or CCD array Charge coupled detectors (CCD) Instrumentation thermal detectors (IR range) Thermal detectors, that sense the heating effect of photons, are needed in the IR range. The four common classes are the following: Bolometers are very thin, black metallic conductor layers (e.g. Pt soot, Sb, etc.) that have very little reflectivity. Operation is based on the fact that the resistance of metals depend on T. Thermopiles contain thermocouples (two metals or alloys connected to each other) that respond to temperature. Semiconductor detectors, such as InGaAs. Golay cell is a pneumatic cell, in which Xe gas is expanding/contracting in response to the IR radiation and this causes an internal light beam to be deflected from a thin mirror (T).

16 Radiation absorption the Lambert-Beer law If a parallel beam of monochromatic radiation passes through an absorbing sample, then the intensity of the beam will be attenuated. Transmittance (T) and absorbance (A) can be defined as: T = I I 0 I0 A = lg = lg T = I 1 lg T The Lambert-Beer law declares that A is linearly proportional to the thickness of the sample and its concentration. A = ε c l where ε is the molar absorptivity, c is the molar concentration and l is the thickness in cm. Note, that A refers to absorbance at a given wavelength, and ε is also a function of wavelength. Radiation absorption the absorption spectrum The absorption spectrum shows the absorbance as the function of wavelength. As it is recorded in one setting, with the other measurement conditions fixed (c and l is constant), it practically depicts ε(λ). Visible absorption spectra of KMnO 4 solutions of various concentrations

17 Radiation absorption multicomponent samples Absorption at a wavelength is an additive feature. This means that if there are more than one species in the sample, the absorbance will be the sum of absorbances for each compounds at the given wavelength. A total = ΣA i = Σ( ε i c i l) = l Σ( ε i c ) i Colored curves are individual spectra of components; dotted curve is the spectrum of the mixture absorbance nm Some calculational examples part 4. Calculate the transmittance for a solution whose absorbance is The correct answer is: T= Explain, how the following quantities change when we double the cuvette length (for the measurement of the same solution): a) absorbance b) transmittance The correct answers are: a) A 2 =2 A 1 b) T 2 =T 1 T 1

18 Some calculational examples part 5. Calculate the molar absorptivity for a 0.008M solution in a 15 mm cuvette, whose absorbance is The correct answer is: ε= dm 3 /mol cm -1 Calculate the absorbance of a sample prepared by mixing 10 ml of a 0.15M solution and 20 ml of a 0.35M solution of the same compound (ε= 4.5dm 3 /mol cm -1, cuvette length is 1 cm). The correct answer is: 1.26 Derivative spectroscopy Similarly to other analytical methods, it is also beneficial in spectroscopy to plot the first derivative of the spectrum curve as p py p p it can reveal very fine details. See the examples below.

19 Radiation absorption deviations from Beer s law There are deviations from the linear law. Some of these are real and some are due to chemical changes, or instrumental imperfections. Concentration limitation. Validity is limited to diluted solutions (concentrations well below 0.01 M) due to interactions between the analyte species. Chemical deviations. When the absorbing species in the sample undergo association, dissociation or reaction with the solvent, the resulting species will absorb differently. This example shows the case of an unbuffered solution of HA. At higher concentrations, dissociation diminishes. A - HA Radiation absorption deviations from Beer s law Instrumental deviations. The law is only valid if the radiation is monochromatic and no stray light reaches the detector. Real spectrometers always have some flaws/limited bandpass, etc. Effect of polychromatic radiation Effect of stray light

20 Radiation absorption deviations from Beer s law Non-specific radiation losses (e.g. scattering, reflection) and differences between sample holders (cuvettes) can also result in deviations from the law. Losses due to reflection at cell interfaces can amount to as much as 8-10%, which is comparable to the true light attenuation, therefore needs to be corrected for... Blank correction This can be done by blank correction. First, a blank (only solvent containing) solution is measured in an identical (or the same) cuvette and the resulting signal is used for the correction. The attenuation of light in the blank solution is due to the losses associated with the cuvette and the absorption of the solvent. I A = lg I I I I I I blank sample losses sample true = I = I 0 true 0 = I = I 0 = I I true I true sample losses I blank I losses 0 + I 0 + I I blank blank

21 Single/double beam spectrophotometers The blank correction has to be done manually (sequentially) with a single beam spectrophotometer. With a double beam spectrophotometer, it is easier because it has two cuvette holders: I sample and I blank are measured in parallel. Of course, the cuvettes need to be matched (very similar) in this case. See the schematics below. single beam spectrophotometer double beam spectrophotometer Molecular (UV/Vis) absorption spectrometry

22 UV/Vis molecular absorption spectroscopy Analytical information in the UV/Vis range Light absorption of molecules in the UV and Vis range is mainly due to electronic transitions between molecular orbitals, and d-d orbitals inmetal ions (valence shell electronic transitions ii that is). As in molecules usually many electrons are residing at very similar energy levels, therefore UV/Vis absorption features in the spectrum appear as bands, or wide peaks ( nm breadth). Useful spectral range of UV/Vis spectrophotometry is the ca. 190 to 800 nm range, limited mainly by the optical materials used to make sample holders (cuvettes) and the absorption of common solvents. UV/Vis molecular absorption spectroscopy Absorption by organic molecules In the case of organic molecules, light absorption in this range comes mainly from n π * and π π * transitions. Other transitions (such as σ σ * and n σ * ) usually are not excited in this range, so single bonds and isolated (non conjugated) double bonds do not typically give rise to UV/Vis absorption. wavelengths of these transitions are too low, so we don t see them in UV/Vis spectra

23 UV/Vis molecular absorption spectroscopy Absorption by organic molecules Chromophores are the parts of molecules that make them colorful, that is absorb light in the UV/Vis range. For organic molecules, these are parts with ihconjugated double bonds, triple bonds, or bonds with ih heteroatoms (in the latter case, for the nonbonding electrons). UV/Vis molecular absorption spectroscopy Absorption by organic molecules It should be noted however, that absorption wavelengths (peak maxima) for the various bonds or atomic environments are not sharply fixed, but dependsd on structural variations i and the solvent. Solvent effects arise from the interaction of either the ground or excited state of the molecule, thus changing the wavelength or absorption by ca nm (blue or red shift) Also a polar solvent or red shift). Also, a polar solvent for a polar solute means more interaction, thereby smearing over absorption, leading to the loss of fine absorption features in he spectra. See the example of phenol on the right (polar solute case).

24 UV/Vis molecular absorption spectroscopy Absorption by inorganic species The ions and complexes of many transition elements are colored due to absorption involving transitions from filled to unfilled d-orbitals. (for rare earth ions the transitions are for 4f and 5f electrons). The energy differences between these orbitals depend on oxidation state and the ligand the ion is bonded to. UV/Vis molecular absorption spectroscopy Quantitative applications Quantitative applications involve the use of Lambert-Beer law to the determination of the concentration of an analyte. For increased sensitivity, ii i long pathlength h cells/cuvettes can be used. Direct applications, that is when a molecule strongly absorb UV/Vis light (e.g. KMnO 4 ) are quite straightforward, but relatively rare. It is more common that a non-absorbing (e.g. colorless) analyte species is quantitatively converted into another chemical form that absorbs much strongly (chemical conversion). Here, the general concept is that we either attach a chromophore to the analyte species or oxidise/reduce it to produce a chromofore. Possibilities are wide, as this approach can be used not only on molecules but also on metal ions (e.g. complex formation) and inorganic ions. An important advantage of this approach is that the chemical reaction can provide extra selectivity to the determination.

25 UV/Vis molecular absorption spectroscopy Quantitative applications iodate determination conversion of metal ions into colorful complexes: 2 reagent: diethyl-dithiocarbamate 2 reagent: diphenyl-tiocarbazone UV/Vis molecular absorption spectroscopy Selected quantitative applications - environmental

26 UV/Vis molecular absorption spectroscopy Selected quantitative applications clinical samples o-toluidine Green color, absorption at 630 nm UV/Vis molecular absorption spectroscopy Qualitative applications In solution, the qualitative applications of UV/Vis spectrophotometry is limited. It is due to the fact that absorption bands are very wide and not too characteristic i (h (their wavelength is influenced by many factors, e.g. association/dissociation, ph). Chemical conversion is usually needed to provide selectivity. Visible absorption spectrum of benzene in various phases/media

27 UV/Vis molecular absorption spectroscopy Qualitative applications effect of ph (isosbestos point) The protonated and deprotonated form of many molecules have different light absorption, hence the ph has a strong effect on the spectrum. The presenceof an isosbestos point in the spectrum isan evidence that only two principal species are present. phenol red benzeneazodiphenylamine UV/Vis molecular absorption spectroscopy Characterization applications equilibrium constant (K) If we know the characteristic wavelengths (and ε) for some PX and X compounds, that are in equilibrium with each other according to P + X PX K = [PX] [P][X] then the equilibrium constant for the reaction can be determined spectrophotometrically (Scatchard plot). Consider a series of solutions, in which increments of X are added to a constant amount of P 0. Substituting [P]= P 0 -[PX] into K s expression, we get [PX] = K (P [X] 0 [PX]) meaning that a plot of [PX]/[X] yields a straight line with a slope of - K. [PX] and [X] are determined photometrically.

28 UV/Vis molecular absorption spectroscopy Characterization applications photometric titrations Spectrophotometry can also be used as an end-point detection method for titrations, assuming that the titration reaction consumes or generates a chromophore at a given monitoring wavelength. Examples include acid-base titrations in the presence of an indicator dye or complexometric titrations. Precipitation titrations can obviously not be generally followed spectrophotometrically. Example: determination of a bismuth/copper mixture. At 745 nm, Cu-EDTA absorbs strongly, but neither does EDTA nor Bi-EDTA