Satish Pradhan Dnyanasadhana College, Thane-400604 Satish Pradhan Dnyanasadhana College, Thane-400604. Department of Chemistry M.Sc. Analytical Chemistry Sem-II 1.1 General introduction to spectroscopic methods

Syllabus 1.1 General introduction to spectroscopic methods:, Recapitulation Of Basic Concepts, general instrumentation: sources, wavelength selectors, sample containers, radiation transducers, signal processors and read out system, fiber optics, types of optical instruments, Fourier transform optical instruments.[5L] 1.2 Molecular ultra violet and visible spectroscopy: factors affecting molecular absorption, types of transitions, pH, temperature, solvent, effect of substituents, Derivative and dual wavelength spectroscopy, applications including simultaneous determinations.

Recapitulation Of Basic Concepts

? What is Spectroscopy?

What is Electromagnetic Radiation Electromagnetic radiation, or light, is a form of energy whose behavior is described by the properties of both waves and particles. The optical properties of electromagnetic radiation, such as diffraction, are explained best by describing light as a wave

Energy Increases Wavelength Decreases Electromagnetic Spectrum

W A V E L N G T h I C R S Energy Increases

Properties of electromagnetic radiation Frequency: The number of oscillations of an electromagnetic wave per second (n). Wavelength: The distance between any two consecutive maxima or minima of an electromagnetic wave (l). Wave number:The reciprocal of wavelength (–n).

GENERAL INSTRUMENTATION Radiation Source Wavelength selector Sample Cell Detector Amplifier Read Out Device

In this energy is supplied by photons. Sources of Energy In this energy is supplied by photons. Absorption Spectroscopy Scattering Techniques

Thermal or chemical energy source . luminescence spectroscopy Sources of Energy Thermal or chemical energy source . Emission Spectroscopy luminescence spectroscopy

Type of Source 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

The most common sources of thermal energy flames Flame sources use the combustion of a fuel and an oxidant such as acetylene and air, to achieve temperatures of 2000–3400 K. plasmas Plasmas, which are hot, ionized gases, provide temperatures of 6000–10,000 K.

Exothermic reactions also may serve as a source of energy Chemical Sources of Energy Exothermic reactions also may serve as a source of energy In chemiluminescence the analyte is raised to a higher-energy state by means of a chemical reaction, emitting characteristic radiation when it returns to a lower-energy state When the chemical reaction results from a biological or enzymatic reaction, the emission of radiation is called bioluminescence. Commercially available “light sticks” and the flash of light from a firefly are examples of chemiluminescence and bioluminescence, respectively.

Electromagnetic spectrum Radiation Source H2 and D2 lamp Wavelength region continuum source from 160–380 nm Useful for UV molecular absorption UV Spectroscopy

Electromagnetic spectrum Radiation Source tungsten lamp Wavelength region 400-800 nm Useful for Vis molecular absorption Visible Spectroscopy

Electromagnetic spectrum Radiation Source Xe arc lamp Wavelength region continuum source from 200–1000 nm Useful for Vis molecular absorption molecular fluorescence Spectroscopy

Electromagnetic spectrum Radiation Source Nernst glower Wavelength region continuum source from 0.4–20 um IR Useful for Vis molecular absorption molecular absorption Spectroscopy

Useful for Sr.No. Radiation Source Wavelength region 1 H2 and D2 lamp continuum source from 160–380 nm UV molecular absorption 2 tungsten lamp continuum source from 320–400 nm Vis molecular absorption 3 Xe arc lamp continuum source from 200–1000 nm molecular fluorescence 4 Nernst glower continuum source from 0.4–20 um IR molecular absorption 5 Globar source from 1–40 um 6 Nichrome wire continuum source from 0.75–20 um 7 hollow cathode lamp line source in UV/Vis 200-800 nm UV/Vis atomic absorption 8 Hg vapor lamp 9 laser line source UV/Vis 200-800 nm atomic and molecular absorption, fluorescence and scattering

Wavelength selectors Absorption filters work by selectively absorbing radiation from a narrow region of the electromagnetic spectrum. Commercially available absorption filters provide effective bandwidths from 30–250 nm. The maximum throughput for the smallest effective band passes, however, may be only 10% of the source’s emission intensity over that range of wavelengths. Interference filters uses constructive and destructive interference to isolate a narrow range of wavelengths. A simple example of an absorption filter is a piece of colored glass. A purple filter, for example, removes the complementary color green from 500–560 nm. Interference filters are more expensive than absorption filters, but have narrower effective bandwidths, typically 10–20 nm, with maximum throughputs of at least 40%.

Wavelength Selection Using Monochromators One limitation of an absorption or interference filter is that they do not allow for a continuous selection of wavelength. If measurements need to be made at two wavelengths, then the filter must be changed in between measurements. A further limitation is that filters are available for only selected nominal ranges of wavelengths. An alternative approach to wavelength selection, which provides for a continuous variation of wavelength, is the monochromator. The construction of a typical monochromator is shown in Figure 10.12. 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

WAVELENGTH SELECTORS LIGHT DISPERSING DEVICES Filters Absorption Filters Interference Filters MONOCHROMATORS Prism Monochromator Grating Interferometers

1.Filters 24 MCL PCL

2. PRISM MONOCHROMATOR FOCUSING LENS COLLIMATING LENS PRISM LAMBDA-1 LAMBDA-2 PRISM ENTRANCE SLIT EXIT SLIT PRISM MONOCHROMATOR

PRISM MONOCHROMATOR

GRATING MONOCHROMATOR

GRATING MONOCHROMATOR MAGINIFIED VIEW ENLARGED VIEW GRATING MONOCHROMATOR

Interferometers as wavelength selector In FTIR Spectroscopy Interferometer: A device that allows all wavelengths of light to be measured simultaneously, eliminating the need for a wavelength selector interferometer simultaneously allows source radiation of all wavelengths to reach the detector. Radiation from the source is focused on a beam splitter that transmits half of the radiation to a fixed mirror, while reflecting the other half to a movable mirror. The radiation recombines at the beam splitter, where constructive and destructive interference determines, for each wavelength, the intensity of light reaching the detector. As the moving mirror changes position, the wavelengths of light experiencing maximum constructive interference and maximum destructive interference also changes. The signal at the detector shows intensity as a function of the moving mirror’s position, expressed in units of distance or time. The result is called an interferogram, or a time domain spectrum. The time domain spectrum is converted mathematically, by a process called a Fourier transform, to the normal spectrum (also called a frequency domain spectrum) of intensity as a function of the radiation’s energy.

Interferometers as wavelength selector In FTIR Spectroscopy Source Beam splitter Moving Mirror Sample Detector Fixed mirror

Sample cell

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. Ideally the detector’s signal, S, should be a linear function of the electromagnetic radiation’s power, P, S = kP + D where k is the detector’s sensitivity, and D is the detector’s dark current, or the background electric current when all radiation from the source is blocked from the detector.

Transducer A device that converts a chemical or physical property, such as pH or photon intensity, to an easily measured electrical signal, such as a voltage or current.

Transducer Photon Transducer Thermal Transducer

PHOTOCELL DETECTOR (--) D D G C E- G B A +

PHOTO TUBE DETECTOR Photo Cathode Collector Anode - AMPLIFIER RECORDER

Thermal Transducers Infrared radiation generally does not have sufficient energy to produce a measurable current when using a photon transducer. A thermal transducer, therefore, is used for infrared spectroscopy. The absorption of infrared photons by a thermal transducer increases its temperature, changing one or more of its characteristic properties.

The pneumatic transducer, for example, consists of a small tube filled with xenon gas equipped with an IR-transparent. window at one end, and a flexible membrane at the other end. A blackened surface in the tube absorbs photons, increasing the temperature and, therefore, the pressure of the gas. The greater pressure in the tube causes the flexible membrane to move in and out, and this displacement is monitored to produce an electrical signal.

Signal processor : A device, such as a meter or computer, that displays the signal from the transducer in a form that is easily interpreted by the analyst. .

COMPUTERS EQUIPPED WITH DIGITAL ACQUISITION BOARDS SIGNAL PROCESSOR ANALOG METERS OR DIGITAL METERS RECORDERS COMPUTERS EQUIPPED WITH DIGITAL ACQUISITION BOARDS

What is function of Signal processer ? The electrical signal generated by the transducer is sent to a signal processor where it is displayed in a more convenient form for the analyst. The signal processor also may be used to calibrate the detector’s response, to amplify the signal from the detector, to remove noise by filtering, or to mathematically transform the signal.

membrane displacement Detector Class Wavelength Range Output Signal PHOTOTUBE PHOTON 200–1000 nm current PHOTO MULTIPLIER 110–1000 nm SI PHOTODIODE 250–1100 nm PHOTOCONDUCTOR 750–6000 nm change in resistance PHOTOVOLTAIC CELL 400–5000 nm current or voltage THERMOCOUPLE THERMAL 0.8–40 mm voltage THERMISTOR PNEUMATIC 0.8–1000 mm membrane displacement PYROELECTRIC 0.3–1000 mm

Types Of Optical Instruments

1. Instrument: Single Beam Colorimeter Monochromatic light Photocell /PMT detector Read Out Device U.V.Light & visible light Filter or Monochromator

2. Atomic Absorption Spectrophotometer Rotating Chopper Hollow Cathode Lamp P.M.T.Detector Flame Amplifier Read Out Grating Power Supply Sample Solution

3. Flame Photometer Slit Collimating Mirror P.M.T.Detector Amplifier Read Out Fuel Oxidant Prism Monochromator Sample Solution

2. Instrument: Single Beam Fuorimeter Monochromatic light U.V.Light & visible light Primary filter Secondary filter Photocell /PMT detector Read Out Device

Turbidimeter 4 visible light Filter Photocell Detector Read Out Device Sample Cell visible light Filter Technique is used when concentration of suspended particles are high In this intensity of transmitted light is measured

Nephelometer 5 visible light Light Trap Photocell Detector Sample Cell Graduated Disc Collimating Lens Light Trap visible light Photocell Detector Technique is used when concentration of suspended particles are less In this intensity of scattered light is measured Sample Cell Read Out Device

Fourier transform optical instruments

Fourier Transform- Infrared Spectroscopy (FT-IR) The mid-IR region covers the range from 4000 to 400 cm-1. The basic principle of the IR technique is that various organic functional groups absorb infrared light at specific wavelengths. Thus, since every organic molecule has a unique chemical composition, it also has a unique infrared spectrum. Biological samples are composed of proteins, carbohydrates, lipids and nucleic acids. Since these molecules contain functional organic groups, the IR spectrum consists of bands from all these components. The infrared spectrum is very complex, and it contains a large amount of information. To evaluate the data, it is necessary to use multivariate statistical analysis.

Energy for Atom and Molecule Electronic Energy Electronic Energy Atom Atom Rotational Energy Vibrational Energy

CH3----------------- CH3 Rotational Energy CH3----------------- CH3 Motion of molecule from the centre of joining the nuclei

Bond length 154 pm, Stretching Vibrational Energy Bond length 154 pm, Stretching 10 pm. For a C-C bond with a bond length of 154 pm, the variation is about 10 pm.

C C C 4o Bending 10 pm Vibrational Energy For C-C-C bond angle a change of 4o is typical. This moves a carbon atom about 10 pm.

Regions of Absorption U.V. 200-400 nm Visible 400-800 nm