Spectrophotometry. Theory

Transcription

1 is any technique that uses light to measure chemical concentrations. Theory When a sample absorbs electromagnetic radiation it undergoes a change in energy. The interaction happens between the sample and the particles in a beam of electromagnetic radiation (=photons). Spectroscopy is possible only if the photon s interaction with the sample leads to a change in characteristic properties of electromagnetic radiation e.g.: energy, velocity, amplitude, frequency, phase angle, polarization, and direction of propagation. When a photon is absorbed by a sample, it is destroyed, and its energy acquired by the sample. The energy of a photon, in joules, is related to its frequency, wavelength, or wavenumber by the following equations where h is Planck s constant, which has a value of x J s, c is the speed of light (3x10 8 m/s in vacuum), λ is wavelength, v is frequency. The frequency and wavelength of electromagnetic radiation vary over many orders of magnitude. Electromagnetic radiation is divided into different regions based on the type of atomic or molecular transition that gives rise to the absorption or emission of photons. The boundaries describing the electromagnetic spectrum can overlap between spectral regions. In absorption spectroscopy the energy carried by a photon is absorbed by the analyte, promoting the analyte from a lower-energy state to a higher-energy, or excited state. The source of the energetic state depends on the photon s energy. The electromagnetic spectrum in Figure shows that absorbing a photon of visible light causes a valence electron in the analyte to move to a higher-energy level. When an analyte absorbs infrared radiation, on the other hand, one of its chemical bonds experiences a change in vibrational energy. The intensity of photons passing through a sample containing the analyte is attenuated because of absorption. The measurement of this attenuation, which we call absorbance, serves as our signal. Absorption only occurs when the photon s energy matches the 1

2 difference in energy ( E) between two energy levels of the target molecule. A plot of absorbance as a function of the photon s energy is called an absorbance spectrum. Emission of a photon occurs when an analyte in a higher-energy state returns to a lowerenergy state. The higher-energy state can be achieved in several ways, including thermal energy, radiant energy from a photon, or by a chemical reaction. Emission following the absorption of a photon is also called photoluminescence, and that following a chemical reaction is called chemiluminescence. Sources of energy All forms of spectroscopy require a source of energy. In absorption spectroscopy this energy is supplied by photons. Emission and luminescence spectroscopy use thermal, radiant (photon), or chemical energy to promote the analyte to a less stable, higher energy state. A source of electromagnetic radiation must provide an output that is both intense and stable in the desired region of the electromagnetic spectrum. Sources of electromagnetic radiation are classified as either continuum or line sources. A continuum source emits radiation over a wide range of wavelengths, with a relatively smooth variation in intensity as a function of wavelength. Line sources, on the other hand, emit radiation at a few selected, narrow wavelength ranges. A continuum radiation source is often a tungsten filament ( nm), a deuterium arc lamp, which is continuous over the ultraviolet region ( nm), xenon arc lamps ( nm), or more recently, light emitting diodes (LED) for the visible wavelengths. Wavelength selection We usually try to select a single wavelength where the analyte is the only absorbing species. Unfortunately, we cannot isolate a single wavelength of radiation from a continuum source. Instead, a wavelength selector passes a narrow band of radiation to the sample. The simplest method for isolating a narrow band of radiation is to use an absorption or interference filter. Absorption filters work by selectively absorbing radiation from a narrow region of the electromagnetic spectrum. Interference filters use constructive and destructive interference to isolate a narrow range of wavelengths. One limitation of a filter is that they do not allow for a continuous selection of wavelength. If measurements need to be made at two wavelengths, monochromator provides for a continuous variation of wavelength selection. The construction of a typical monochromator with a grating is shown in Figure. Radiation from the source enters the monochromator through an entrance slit. The radiation is collected by a collimating mirror, which reflects a parallel beam of radiation to a diffraction grating. The diffraction grating is an optically reflecting surface with a large number of parallel grooves. Diffraction by the grating disperses the radiation in space, where a second mirror focuses the radiation onto a planar surface containing an exit slit. In some monochromators a prism is used in place of the diffraction grating. (see Figure) Radiation exits the monochromator and passes Entrance slit to the detector. As shown in Figure, a polychromatic source of radiation at the entrance slit is converted at the exit slit to a monochromatic source of finite bandwidth. The choice of which wavelength exits the Optical grating monochromator is determined by rotating the Mirrors diffraction grating. A narrower exit slit provides a smaller bandwidth and better resolution, but allows a smaller throughput of radiation. Monochromators are classified as either fixedwavelength or scanning. In a fixed-wavelength Exit slit monochromator, the wavelength is selected by manually rotating the grating. 2

3 Collimator lens Entrance slit Prism Focusing lens Exit slit A scanning monochromator includes a drive mechanism that continuously rotates the grating, allowing successive wavelengths to exit from the monochromator. Detectors The first detector for optical spectroscopy was the human eye, which, of course, is limited both by its accuracy and its limited sensitivity to electromagnetic radiation. Modern detectors use a sensitive transducer to convert a signal consisting of photons into an easily measured electrical signal. Phototubes and photomultipliers contain a photosensitive surface that absorbs radiation in the ultraviolet, visible, and near infrared (IR), producing an electric current proportional to the number of photons reaching the transducer. Other class of photon detectors uses a semiconductor as the photosensitive surface. Transmittance and absorbance The attenuation of electromagnetic radiation as it passes through a sample is described quantitatively by two separate, but related terms: transmittance and absorbance. Transmittance is defined as the ratio of the electromagnetic radiation s power exiting the sample, P T, to that incident on the sample from the source, P 0. Multiplying the transmittance by 100 gives the percent transmittance (%T), which varies between 100% (no absorption) and 0% (complete absorption). Attenuation of radiation as it passes through the sample leads to a transmittance of less than 1, without distinguish between the different ways in which the attenuation occurs e.g. reflection and absorption by the sample container, absorption by components of the sample matrix other than the analyte, and the scattering of radiation. To compensate for this loss we use a method blank. The radiation s power exiting from the blank is taken to be P 0. (a) Figure is showing the attenuation of radiation passing through a sample; P 0 is the radiant power from the source and P T is the radiant power transmitted by the sample. (b) Figure is showing that P 0 is redefined as the radiant power transmitted by the blank, correcting the transmittance in (a) for any loss of radiation. An alternative method for expressing the attenuation of electromagnetic radiation is absorbance, A, which is defined as Absorbance is the more common unit for expressing the attenuation of radiation because it is a linear function of the analyte s concentration. 3

4 Equations establish the linear relationship between absorbance and concentration, are known as the Beer Lambert law. When concentration is expressed using molarity, c (unit in mol L -1 ), effective light path length, l (cm) and the molar absorptivity, ε (with units of cm 1 M 1 ) is used A = ε l c The molar absorptivity gives the probability that the analyte will absorb a photon of given energy. As a result, values for ε depend on the wavelength of electromagnetic radiation. Calibration curves based on Beer Lambert law are used routinely in quantitative analysis. Limitations to Beer Lambert law deviations in absorptivity coefficients at high concentrations (>0.01M) due to electrostatic interactions between molecules in close proximity scattering of light due to particulates in the sample fluoresecence or phosphorescence of the sample shifts in chemical equilibria as a function of concentration (change of analyte conc.) non-monochromatic radiation, deviations can be minimized by using a relatively flat part of the absorption spectrum such as the maximum of an absorption band (see Figure ) stray light change of the solvent Ultraviolet-Visible Instrumentation Instruments using monochromators for wavelength selection are called spectrometers. In absorbance spectroscopy, where the transmittance is a ratio of two radiant powers, the instrument is called a spectrophotometer. The simplest spectrophotometer is a single-beam instrument equipped with a fixed wavelength monochromator, the block diagram for which is shown in Figure. The limitations of fixed-wavelength, single-beam spectrophotometers are minimized by using the double-beam spectrophotometer. A chopper controls the radiation s path, alternating it between the sample, the blank, and a shutter. The signal processor uses the chopper s 4

5 known speed of rotation to resolve the signal reaching the detector into that due to the transmission of the blank (P 0 ) and the sample (P T ). By including an opaque surface as a shutter it is possible to continuously adjust the 0% T response of the detector. A scanning monochromator allows for the automated recording of spectra. Double-beam instruments are more versatile than single-beam instruments, being useful for both quantitative and qualitative analyses. Practice Procedure 1. - Determination of the concentration of NiSO 4 solution by standard addition method The standard addition is one of the calibration methods. The standard solution (solution of known concentration of analyte) is added to the unknown solution so any impurities in the unknown are accounted for in the calibration. The operator does not know how much was in the solution initially but does know how much standard solution was added, and knows how the readings changed before and after adding the standard solution. Thus, the operator can extrapolate and determine the concentration initially in the unknown solution. Materials: M NiSO 4 solution Equipements: - 6 pcs. volumetric flask (100,00 ml) - pipettes Step 1. Take one of the unknown concentration of NiSO 4 solution in ml volumetric flask and fill to the mark with distilled water. Step 2. Number 6 pcs ml volumetric flasks from 1-6, and measure ml from solution prepared in Step 1. into them. Add different volume of NiSO 4 standard according to the table, and fill all to mark. 5

6 Step 3. Measure the absorbance at 390 nm. Calculate the concentration of NiSO 4 in each flasks, excluding the unknown concentration. Flask No ml unknown sol. fill to 10 ml unknown sol. + 3 ml 0.25 M NiSO 4 sol. fill to 10 ml unknown sol. + 6 ml 0.25 M NiSO 4 sol. fill to 10 ml unknown sol. + 9 ml 0.25 M NiSO 4 sol. fill to 10 ml unknown sol ml 0.25 M NiSO 4 sol. fill to 10 ml unknown sol ml 0.25 M NiSO 4 sol. fill to Absorbance at 390 nm Data analysis 1. Step 1. Draw the concentration as a function of absorbance Step 2. Extrapolate the regression line into the negative concentration region; obtain the concentration of the unknown solution as the intercept of the regression line with the X-axis. Step 3. Use the least square method to calculate linear regression. From y=ax+b you can get the concentration, as 0=ax+b, where x will be the conc. value. Step 4. Calculate the NiSO 4 solution s concentration in mg/100 ml units! (M NiSO4 = 155 g/mol) Procedure 2. - Determination of methylene blue concentration Solution: - 0,001 m/m% methylene blue solution Equipment: - test tubes - pipettes (10 ml) Step 1. At first determine the absorption spectra of the 0.001% methylene blue solution. Fill a cuvette with the solution and measure its absorbance between 530 and 700 nm with 5 nm steps. Step 2. Calibration solution preparation. Take 6 test tubes and number them from 1 to 6. Fill 4 ml 0.001% methylene blue solution into the 1 st, and fill to 10 ml (=add 6 ml water). Fill 5-5 ml water into all the other tubes. Add 5 ml solution from 1 to 2 and mix well. Take 5 ml solution from 2 to 3 and mix. Continue with all the tubes. 6

7 Step 3. Search for the highest absorbance value measured in Step 1. that will be the absorption maxima. Measure the absorbance of all samples from the calibration and the unknown at the wavelength of the absorption maxima. Data analysis 2. Step 1. Draw the absorption spectra (absorbance as a function of wavelength). Step 2. Draw the calibration line from absorbance data obtained from calibration solutions. Step 3. Calculate the regression line (least square method). Using the regression equation obtained the unknown methylene blue concentration (in m/m%) from its absorbance data. Questions 1. What a spectrum is? 2. Write the steps for a calibration with standard addition method 3. Which limitations have the Lambert-Beer law? 4. Show the functions of the parts of a single/double beam spectrophotometer! 5. Give a rough description about the light sources/monochromators/detectors power (P) The flux of energy per unit time. intensity (I) The flux of energy per unit time per area. photon A particle of light carrying an amount of energy equal to hν. transmittance (T) The ratio of the radiant power passing through a sample to that from the radiation s source. absorbance (A) The attenuation of photons as they pass through a sample absorbance spectrum A graph of a sample s absorbance of electromagnetic radiation versus wavelength (or frequency or wavenumber). emission The release of a photon when an analyte returns to a lower-energy state from a higher-energy state. continuum source A source that emits radiation over a wide range of wavelengths. monochromator A wavelength selector that uses a diffraction grating or prism, and that allows for a continuous variation of the nominal wavelength. monochromatic Electromagnetic radiation of a single wavelength. spectrophotometer An instrument for measuring absorbance that uses a monochromator to select the wavelength. Bibliography: Harris Quantitative Chemical Analysis, 7 th ed. Harvey Modern analytical chemistry, 1 st ed