Spectroscopy is the study of interaction of electromagnetic radiation with matter based on the Bohr-Einstein frequency relationship . Excitation of electrons is due to the absorption of light in the ultraviolet and visible regions and this will only happen if the energy of incident light is exactly the energy required for the excitation. Basic principle of spectroscopy is the Beer-Lambert’s law, which states that absorption of incident radiation is directly proportional to the concentration and the thickness of the material. In UV-VIS Spectroscopy, a continuum range of wavelengths from 200nm to 1100nm is used. The visible region is obtained by a halogen lamp. The spectral energy of halogen lamp rapidly decreases below 400nm. Therefore a gas discharge lamp is used in ultraviolet region, for which hydrogen or deuterium lamp are the best. I have operated the automated UV-VIS Spectrometer (PerkinElmer Lambda 35), a double beam spectrometer for samples (dyes) rhodamine, thymol blue, phenol red and methylene blue. Now I am practicing visible spectroscopy by aligning a manual setup using an LED source, monochromator and a detector for same samples. Here the LED source, which has a spectral range of 420nm to 720nm is directed towards monochromator. The monochromator (Digikrom) has an entrance slit, collimating mirrors, diffraction grating, focusing mirror and an exit slit. The light entering through the slit is collimated and directed to the diffraction grating which is tilted to select the desired wavelength. The diffracted light is focused via a focusing mirror. Later it is passed through the exit slit towards the photo-detector. Photo detector is a device which counts the intensity of the transmitted radiation instantaneously. It consists of a simple p-n junction which can be made by defusing a p-type impurity into an n-type bulk silicon wafer or vice verse. The defused area is called active photodiode area which is coated by an anti reflecting thin film for maximum detection. Later I will be doing the preparation of the organic dyes and their uv-visible spectroscopic analysis.
Keywords: Ultraviolet-Visible-near Infrared (UV-VIS-NIR), Ultraviolet-Visible (UV-VIS) , Light emitting diode (LED).
Spectroscopy is the branch of science which deals with interaction of electromagnetic radiation with materials. In other words it is an analytical method for qualitative and quantitative analysis by use of light. This technique begins from Issac Newton’s experiment (1666-1672), which he defined the term ‘spectrum’ to describe the consecutive colours derived from the dispersion of white light through a prism. Later it was explained as any interaction of electomagnetic waves with matter. In early 19thcentury, Joseph von Fraunhofer made experimental advances with dispersive spectrometers that enabled spectroscopy to become a more precise, quantitative and scientific technique. The Bouguer-Lambert Beer law in 1852 made the basis for the quantitative evaluation of absorption measurements in the early period. This led firstly to colorimetry, then to photometry and finally to spectrophotometry. This evolution was along with the development of detectors for measuring light intensities, i.e. silicon photo-diode detector, which allow simultaneous measurement of the complete spectrum.
Recently, the definition has been expanded to include the study of the interactions between particles such as electrons, protons, and ions, as well as their interaction with other particles as a function of their collision energy. Spectroscopy is widely used as an exploratory tool in the fields of physics, chemistry, and astronomy, for determination of composition, physical structure and electronic structure of matter at atomic scale, molecular scale, macro scale, and over astronomical distances. Spectroscopic analysis has a key role in the development of the most fundamental theories in physics, including quantum mechanics, the special and general theories of relativity, and quantum electrodynamics. Spectroscopy has made a key role in development of scientific understanding.
Spectroscopy have became an invaluable aid towards structural identification in modern organic chemistry. Consequently, every student of science should be aware of the range of information available from spectroscopic techniques, and be given a practical introduction to the basic spectroscopic methods from the beginning of his training itself. Isolated experiments in spectroscopy have also been described for the beginner. With the development of quantum chemistry, increasing attention was paid to the correlation between light absorption and the structure of matter with the result that in recent decades a number of excellent discussions of the theory of electronic spectroscopy have been published.
Spectroscopic techniques have been applied virtually in all technical fields of science and technology. Radio-frequency spectroscopy of nuclei in a magnetic field has been employed in a medical technique called magnetic resonance imaging (MRI) to visualize the internal soft tissue of the body with unprecedented resolution. Microwave spectroscopy was under to discover the so-called three-degree blackbody radiation, the remnant of the big bang. The internal structure of the proton and neutron and the state of the early universe up to the first thousandth of a second of its existence are being unravelled with spectroscopic techniques using high-energy particle accelerators. The constituents of distant stars, intergalactic molecules, and even the primordial abundance of the elements before the formation of the first stars can be determined by optical, radio, and X-ray spectroscopy.
In this chapter, we talk about uv-visible spectroscopy. In which the spectroscopy is done corresponding to wavelengths varying from 200nm to 1100nm with material.
Spectroscopy is the study of interaction of electromagnetic radiation with matter based on the Bohr-Einstein frequency relationship , here h is the proportionality constant called Planck’s constant (6.626 x 10-34 J s) and v is frequancy. This relationship relates the discrete atomic or molecular energy with the frequency. When an Electromagnetic radiation is incident on a matter, phenomena like reflection, transmission, absorption, etc. are occurring. The measurement of intensity as a function of wavelength or frequency is defined as a spectrum. When the energy of incident photon is sufficient to excite the electron in the matter, the electron absorbs energy and get excited from ground state to a higher energy state.
Measurement of radiation intensity as a function of wavelength is described by spectroscopy. Spectrometers, spectrophotometers, spectrographs or spectral analyzers are referred to as spectral measurement devices. Spectroscopic techniques are extremely sensitive. Single atoms and even different isotopes of the same atom can be detected among 1020 or more atoms of a different species. Trace amounts of pollutants or contaminants are often detected most effectively by spectroscopic techniques. Certain types of microwave, optical, and gamma-ray spectroscopy are capable of measuring infinitesimal frequency shifts in narrow spectroscopic lines. Frequency shifts of one part in 1015 of the frequency can be observed with ultrahigh resolution laser techniques. Because of this sensitivity, the most accurate physical measurements have been frequency measurements. Spectroscopy now covers a sizable fraction of the electromagnetic spectrum. Spectroscopic techniques are not confined to electromagnetic radiation, however. Because the energy E of a photon is related to its frequency ν by the relation , spectroscopy is actually the measure of the interaction of photons with matter as a function of the photon energy. In instances where the probe particle is not a photon, spectroscopy refers to the measurement of how the particle interacts with the test particle or material as a function of the energy of the probe particle.
The measurement of radiation intensity against wavelength is defined as a spectrum. Spectrum can be differentiated as absorption and emission spectrum. In which an absorption spectrum gives the information about the measure of absorbance against wavelengths whereas, emission spectrum gives measure of emission due to photo-luminescence. Absorption spectrum is derived or calculated from the transmission spectrum which we are actually measuring. While emission spectrum itself is a transmission spectrum. Each of these spectrums explains the characteristics of that sample. Uv –visible spectroscopy consists of an absorption spectrum. An absorption spectrum gives information about the molar absorption coefficient, concentration of the sample, optical band gap etc.Example for spectra of a developed standard LED (red line) and a typical white LED (blue dotted line) is given below.
PRINCIPLE AND SETUP
Basic principle of spectroscopy is the Beer-Lambert’s law (also known as beer’s law) that relates the attenuation of light to the properties of the material through which the light is travelling. The law was discovered by Pierre Bouguer before 1729. It is often attributed to Johann Heinrich Lambert, who cited Bouguer's Essai d'optique sur la gradation de la lumière (Claude Jombert, Paris, 1729) and even quoted from it in his Photometria in 1760. Lambert's law stated that absorbance of a material is directly proportional to its thickness (path length). Much later, August Beer discovered another attenuation relation in 1852. Beer's law stated that absorbance is proportional to the concentrations of the material sample. The modern derivation of the Beer–Lambert law combines the two laws and correlates the absorbance to both the concentrations and the thickness of the material. Absorption spectra of chemical samples are generated when a beam of electromagnetic radiation is passed through a sample, and the chemical sample absorbs a portion of the photons of electromagnetic energy passing through the sample.
Spectroscopy can be done for a material by having a light source, a monochromator and a photo detector, which counts the number of photons. The light source is illuminated and passed through a monochromator which separates the white light into its consecutive colours, and is passed through the material. Intensity is measured against each wavelength. As the light source is passed through the setup, measurements are recorded for incident and transmitted radiations. These measurements are used to calculate the transmission and absorption spectra of the material. While many modern instruments perform Beer's law calculations by simply comparing a test sample with a reference sample which have a negligible absorbance. The graphing method assumes a straight-line relationship between absorbance and concentration, which is valid only for dilute solutions.
Transmittance, T = I/I0 Absorbance, A = 2-log(%T)
I –Transmitted radiation intensity I0 – Incident radiation intensity
When the light beams are passed through a dilute sample, the absorption will be less since there is only less number of absorbing particles presented. The light beam was passed through a concentrated sample. The intensity of the transmitted beam was considerably low, which leads to violation of Beer Lambert’s law.
The law thus states that for a dilute solution, A = Kcl
- A – absorbance
- K – molar absorbance coefficient
- c– molar concentration
- l - Path length
Ultraviolet -Visible Spectroscopy is one of the oldest and most widely used method in molecular spectroscopy. Within the whole electromagnetic spectrum, only ultraviolet and visible range which occupies only a very narrow frequencyregion corresponds to the discrete atomic or molecular energy levels. So called ‘electronic spectroscopy’. Excitation of electrons in both atoms and molecules are due to the absorption of light in the ultraviolet and visible range. Since the energy levels of matter are quantized, excitation will only happen if the energy of incident light is exactly the energy required of the excitation. Larger energy gap between the energy levels requires wavelengths of higher energy, resulting in absorption of shorter wavelength light. On every possible excitation, electrons are excited from a low energy ground state which is a completely filled orbital to a higher energy excited state empty anti-binding orbital (Fig. 3).
All molecules will undergo electronic excitation on following absorption of light. Absorption of light in the uv-visible range will only result in the following transitions(Fig. 4).
Therefore in order to absorb light in the region from 200 - 800 nm (where spectra are measured), the molecule must contain either a sigma bonds or atoms with non-bonding orbitals. There are some limits at either the side of electromagnetic spectrum, which are not fixed. Shorter wavelengths are restricted to measure by apparatus or it is not easier with our instruments. Longer wavelength doesn’t makes any sense when passed through a material since it exhibits considerably less energy. So most of the compounds exhibit no traceable absorption by the electronic excitation in this region.
Major applications of uv-vis spectroscopy are:
- Quantitative and Qualitative analysis.
- Determination of molecular weight.
- Determination of molar absorbance coefficient.
- Determination of unknown compound.
- Detection of functional group.
- Detection of isomers and geometrical isomers.
- Detection of impurities.
Major advantages of uv-vis spectroscopy are:
- High sensitivity.
- Require only small volume of sample.
- Linearity over wide range of concentration.
- Can be used with gradient elution.
Major disadvantages of uv-vis spectroscopy are:
- Not linear for high concentration.
- Does not work with compounds that do not absorb light at this wavelength region.
- Requirement of high voltage for initiation.
- Generates significant heat and requires external cooling.
In this chapter, uv-visible spectroscopy of the following dyes which were initially prepared at a concentration of 1 milli molar and later diluted to 25 micro molar are performing(Fig. 6):
- Thymol blue.
- Phenol red.
- Methylene blue.
In UV-VIS Spectroscopy, a continuum range of wavelengths from 200nm to 1100nm are used.The visible region is obtained by a halogen lamb also know as tungsten halogen, quartz-halogen or quartz iodine. It is an incandescent lamp and it consists of a compactly sealed tungsten filament in a transparent glass. It is a black-body source whose spectral energy distribution is described by Planck’s radiation formula. As the wavelength of emitted radiation moves to shorter region, temperature goes very high which in turns evaporates the tungsten coil, which results in a shorter lamp life. The evaporated tungsten coil condenses on the glass which reduces the spectral energy. In the modern century, the sealed glass is also filled with a mixture of inert gas and a small amount of halogen. This creates a halogen cycle that the evaporated tungsten decomposes back on the tungsten coil, the evaporation is reduced even in prolonged use resulting a higher spectral energy in the visible region. Thus keeping a longer life and maintaining a clear glass. However, the bulb should keep a temperature greater than 250 degree Celsius to keep the halogen cycle active.
The spectral energy of halogen lamp rapidly decreases below 400nm. Therefore a gas discharge lamp is used in ultraviolet region, for which hydrogen or deuterium lamp are the best. Generally we use deuterium lamp which is a low pressure gas-discharge light source in a spectroscopy. Basically a deuterium lamp uses a tungsten filament and anode is placed on opposite sides of a nickel box structure designed for the best output. However, the filament doesn’t produce the light. Instead of that, an arc is created in between the filament and anode. This setup is to heat the filament and is turned off after the discharge begins.
Various UV radiation sources are:
- Deuteriun lamp.
- Hydrogen lamp.
- Tungsten lamp.
- Xenon discharge lamp.
- Mercury arc lamp.
Various Visibe radiation sources are:
- Tungsten lamp.
- Mercury vapour lamp.
- Carbonone lamp.
The purpose of a monochromator is to produce a single spectral line from a broadband (multi-wavelength) source. In spectrometers, this can be used to collect light from an atomic emission source, like the atomic emission detector, and allow only a specific line to exit. It can also be used to isolate a single line from a light source such as a hollow cathode lamp. The simple monochromator shown here is called a Czerny-Turner monochromator.
The elements in the monochromator are:
- Entrance slit.
- Collimating mirror.
- Diffraction grating.
- Focusing mirror.
- Exit slit.
This is the basic expression governing diffraction gratings is mλ=d (sin i + sinθ)
Where, 'd' is the groove spacing, 'i' is the angle of incidence, 'θ'is the diffraction angle, 'λ' is the wavelength, and m is the order. This means that when d, m, and i are fixed, light of wavelength λ is diffracted in direction θ. The expression indicates the presence of higher-order light. If d, i, and λ are fixed, a different value of m results in a different value of θ. This indicates that light of wavelength λ diffracts in multiple angles θ, as shown in figure below. These light directions are named using a combination of the m value and the + or - sign, such as +1st-order light or -1st-order light. Incidentally, the light when m=0 is known as zero-order light, for which the diffraction angle θ is equal to the angle of incidence i. This is reflected as white light, equivalent to normal specular reflection. The various light orders of a diffraction grating result in dispersion of the energy and a reduction in light utilization efficiency. However, the diffracted light energy from a diffraction grating with a fine sawtooth profile is concentrated in the direction of the specular reflection, as shown in Figure. This wavelength is known as the "blaze wavelength." The diffraction grating in spectrophotometer is normally used near the blaze wavelength.
A photo detector is a semiconductor device which converts light energy to electrical energy. It consists of a simple P-N junction diode and is designed to work in reverse biased condition. The photons approaching the diode are absorbed by the photodiode and current is generated. It can be made by defusing a p-type impurity into a n-type bulk silicon wafer or vice verse. The defused area is called active photodiode area which is coated by an anti reflecting thin film for maximum detection and is covered by an illumination window. Non active area is deposited by thick layer of silicon oxide. Some photodiodes are manufactured with built-in filters and lenses having different surface areas. Response time of the photodiode is inversely proportional to the surface area. Solar cell is one of the best examples for photodiode. To increase the speed of response, a PIN junction is used instead of P-N junction.
Photo detectors may be classified by their mechanism for detection:
- PN Photodiode.
- Schottky Photo Diode.
- PIN Photodiode.
- Avalanche Photodiode.
Different photo detectors are used in different configurations. Single sensors are used to detect the overall light levels. For the detection of distributions along a line, a 1-D array of photo detectors are used similar to spectrophotometer and for image sensing, a 2-D array of photo detectors are used. Photo detectors can be identified by analysing its characteristics such as:
- Spectral response.
- Dark current.
- Response time.
- Quantum efficiency.
- Gain .
When a photon of high energy hits the diode, an electron-hole pair is generated. This mechanism electron-hole pair creates the current proportional to the intensity of the incoming photon and this mechanism is also called inner photoelectric effect.
An optical spectrometer basically measures the intensity of the light as a function of wavelength or frequency. Optical spectrometers are of two kinds. They are double beam spectrometer and single beam spectrometer. A single beam spectrometer have only one sample holder, and measure only one transmitted light beam while a double beam spectrometer have 2 sample holders, one for the sample which is to be analyzed and one for the reference sample to remove the errors while measurement. Double beam spectrometer measures the intensity of two transmitted light beams (sample and reference) simultaneously. Given below is a typical schematic representation of single and double beam spectrometers.
Image of the manually aligned spectrometer is given below, which consists of an LED source, monochromator (Digikrom) with hand control keyboard, focussing lenses, cuvette, photo detector and multi-meter to note down the intensity of transmitted light.
RESULTS AND DISCUSSION
Absorption spectrum of the dyes (25 micro molar concentration) using manually aligned setup are given below:
Absorption spectrum of the dyes (25 micro molar concentration) using advanced spectrometer are given below:
|Rhodamine||Thymol blue||Phenol red||Methylene blue|
|Using advanced spectrometer||86400||5164||24628||79932|
|Using manual setup||88800||8387||10088||120000|
Band gap determined using manually aligned spectrometer:
Band gap determined using advanced spectrometer:
|Rhodamine||Thymol blue||Phenol red||Methylene blue|
From the above observations, it has been noticed that there occurs an error of 0.53 % to 5.52 % in measurement of wavelength of maximum absorbance, 2.77 % to 62.35 % of error in determination of molar absorption coefficient and upto 25% of error in measurement of optical band gap using the manually aligned setup.
SOURCES OF ERRORS
- Lack of accuracy in manual alignment of optics.
- Lack of accuracy in monochromator.
- Low intensity of light coming from the monochromator.
- Lack of accuracy in focusing and collimating the light beam.
- Lack of accuracy in dye preparation.
- Lack of accuracy in readings taken for the intensity of transmitted light.
- Lack of accuracy in calculations and conversions.
In this chapter, I have done a comparison of accuracy in measurement of absorption spectrum, wavelength of maximum absorption, molar absorption coefficient (extinction coefficient) and optical band gap using uv-visible spectroscopy done by manually aligned spectrometer with the advanced spectrometer to determine the possible percentage of errors and also the sources of these errors have been discussed.
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First of all, I am thankful to my guide, Dr. Parag Sharma, Senior scientist, Physico Mechanical Metrology Division, CSIR-National Physical Laboratory(NPL), New Delhi, who has guided me throughout the period and made this project better. His special attention and motivation towards me and my project were the important factors that made this project done.
I would like to thank Dr. Ranjana Mehrotra (chief scientist), Mr. V.K. Jaiswal (senior scientist) and Dr. Shibu Saha (scientist) for the support to make this project better.
I would like to thank Mr. Prince Sharma for co-guiding me. Beyond his works, he had spend a lot of valuable time for guiding and motivating me for the project and I am grateful for his every contribution. I am also thankful to all the seniors in the department, especially, Bhumika Ray, Kaweri Gambhir, Rajeev Dwivedi, Vijeta, Swati Gangwar, Manjari Srivastava, Krishna Rathore, Narender Signh Bisht and Avina Reshel Dsouza.
I am also thankful to Indian Academy of science for giving me a wonderful opportunity to have an experience in the one of the best laboratory available across the country.
And finally, I am thankful to my beloved parents, teachers and my friends who have supported me throughout the period with their valuable suggestions and guidance.
Extinction coefficient also called Molar absorption coefficient, is a term which defines how much a chemical sample absorbs electromagnetic radiation at particular wavelength which is an intrincic property of the sample that is dependent on the chemical composition and structure of the sample. This coefficient relates the absorbance with the concentration of the sample and the path length of the light as A = Kcl, where "K" is the extinction coefficient.
Extinction coefficient can be determined as follows,
K= A/cl .
Optical band gap of a sample is a range of energy for which there occurs no electronic transitions due due to the radiations of lower energy.
Optical band gap can be calculated by the following method,
- Plot the absorption spectrum.
- From the absorption spectrum, calculate extinction coefficient for each wavelength.
- Calculate the energy corresponding to each wavelength.
- Calculate the square of the product of the energy and extinction coefficient corresponding to each wavelength.
- Plot the graph for energy v/s square of the product of the energy and extinction coefficient with energy on X-axis.
- Drwa a tangent for the obtained graph at the region of peak with constant positive slope and the X intercept is the optical band gap in eV.