In our discussion in “Introduction to the Electromagnetic Spectrum and Spectroscopy” we have discussed the different wavelengths for ultraviolet and visible lights which range from 10 nm to 400nm and 400nm to 780 nm respectively. The following chapter discusses to a greater extent the principles involved in the utility of ultraviolet-visible spectroscopy (UV-Vis) and the Beer-Lambert law which is useful in quantitative analysis of samples.

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Download full-text PDF Read full-text. Optical spect roscopy in the ultraviol et and visible light range (UV/VIS). Measurement principle in UV/VIS spectroscopy. Ultraviolet and visible (UV-Vis) absorption spectroscopy is the measurement of the. Attenuation of a beam of light after it passes through a sample or after reflection from. A sample surface.

Electromagnetic spectrum. We see only a very small portion of the electromagnetic spectrum. In spectrochemical methods, we measure the absorption of UV to far IR radiation. UV = 200-380 nm VIS = 280-780 nm IR = 0.78 mm-300 mm ©Gary Christian, Analytical Chemistry, 6th Ed. UV-Vis Spectroscopy is an analytical method used to measure the absorbance of ultra-violet or visible radiation through an analayte. The molecular absorption of the analayte corresponds to both excitation of valence electrons and excitation of electrons in different atomic orbitals. UV-Vis Spectroscopy is an effective technique for both.

As was seen in the chapter for the “Introduction to the Electromagnetic Spectrum and Spectroscopy”, the energy of the radiation can be calculated by the equation:
E = h . ν
Thus the energy of the radiation in the visible range is generally: 36 to 72 kcal/mole while that in the ultraviolet range goes as high as 143 kcal/mole. This energy irradiated on the molecules can result in changes in the electronic nature of the molecule i.e. changes between ground state and excited states of electrons within the system. As a result, UV-visible spectroscopy is also known as electronic spectroscopy.

Every time a molecule has a bond, the atoms in a bond have their atomic orbitals merged to form molecular orbitals which can be occupied by electrons of different energy levels. Ground state molecular orbitals can be excited to anti-bonding molecular orbitals.

The electrons in a molecule can be of one of three types: namely σ (single bond), π (multiple-bond), or non-bonding (n- caused by lone pairs). These electrons when imparted with energy in the form of light radiation get excited from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) and the resulting species is known as the excited state or anti-bonding state.

1. σ-bond electrons have the lowest energy level and are the most stable electrons. These would require a lot of energy to be displaced to higher energy levels. As a result these electrons generally absorb light in the lower wavelengths of the ultraviolet light and these transitions are rare.
2. π-bond electrons have much higher energy levels for the ground state. These electrons are therefore relatively unstable and can be excited more easily and would require lesser energy for excitation. These electrons would therefore absorb energy in the ultraviolet and visible light radiations.
3. n-electrons or non-bonding electrons are generally electrons belonging to lone pairs of atoms. These are of higher energy levels than π-electrons and can be excited by ultraviolet and visible light as well.

Most of the absorption in the ultraviolet-visible spectroscopy occurs due to π-electron transitions or n-electron transitions. Each electronic state is well defined for a particular system i.e. a double bond in 2-butene would have a particular energy level for the π-electons which when absorbs a specific (or quantized) amount of energy would get excited to the π* energy level for the electrons.

The figure below shows the different transitions between the bonding and anti-bonding electronic states.

Different transitions between the bonding and anti-bonding electronic states when light energy is absorbed in UV-Visible Spectroscopy.

When a sample is exposed to light energy that matches the energy difference between a possible electronic transition within the molecule, a fraction of the light energy would be absorbed by the molecule and the electrons would be promoted to the higher energy state orbital. A spectrometer records the degree of absorption by a sample at different wavelengths and the resulting plot of absorbance (A) versus wavelength (λ) is known as a spectrum. The wavelength at which the sample absorbs the maximum amount of light is known as λ_max. For example, shown below is the spectrum of isoprene. Isoprene is colorless as it does not absorb light in the visible spectrum, and has a λ_max of 222nm.

UV-visible spectrum of isoprene showing maximum absorption at 222 nm.

Chromophore

Certain chemical groups or entitities are susceptible to absorb light due to the electronic configuration of the electrons in the functional group. These groups are known as chromophores. For example, the table below lists commonly found chromophores and their estimated absorbances.

Chromophore	Example	Excitation	λmax (nm)	Solvent
C = C	Ethene	π → π*	171	Hexanes
C = O	Ethanal	π → π* n → π*	180 290	Hexane
N = O	Nitromethane	π → π* n → π*	200 275	Hexane

Effect of Conjugation

Conjugation of π-electrons affects the energy levels of the π-electrons. When two double bonds are conjugated, the electrons in them create four molecular orbitals (i.e. two bonding and two anti-bonding). See the figure below. As a result of this the highest occupied molecular orbital (HOMO) is at a higher energy state and the lowest unoccupied molecular orbital (LUMO) is of at a lower energy state. In order to excite this system, the energy that would be required to excite the electrons from the HOMO to the LUMO would therefore be reduced. As a result of this reduction in energy levels, the wavelength for absorption of conjugated molecules increases.

Diagram shows how a non-conjugated system would require more energy for absorption as compared to conjugated system.

Terminology for Absorption Shifts

Nature of the Shift	Descriptive Term
To Greater Absorbance	Hyperchromic
To Lesser Absorbance	Hypochromic
To Longer Wavelength	Bathchromic or Red Shift
To Shorter Wavelength	Hypsochromic or Blue Shift

Why Ultraviolet-Visible Spectra are not Sharp?

Between the different electronic energy levels are the vibrational energy levels caused due to vibrational changes within the system. It must be remembered that UV-visible light can excite molecular vibrational levels as well. As a result of this phenomenon, there is not one sharp peak obtained in the UV-Visible spectra, but rather a smooth curve shaped peak for absorption as will be seen in several examples.

Vibrational energy levels cause ultraviolet-visible spectra to be smooth and not sharp peaks.

Books on Analytical Chemistry and Spectroscopy

Check out these good books for analytical chemistry and spectroscopy

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1. Analytical Chemistry: An Introduction (Saunders Golden Sunburst Series) 7th Ed., by Douglas A. Skoog, Donald M. West, F. James Holler. 1999.
2. Fundamentals of Analytical Chemistry 8th Ed., by Douglas A. Skoog, Donald M. West, F. James Holler, Stanley R. Crouch. 2003.

What is UV-visible spectroscopy PDF?

Ultraviolet- Visible Spectroscopy. Ultraviolet and visible (UV-Vis) absorption spectroscopy is the measurement of the. attenuation of a beam of light after it passes through a sample or after reflection from. a sample surface. The visible spectrum ranges from 400 nm to about 800 nm.

What is the basic principle of UV-Visible Spectroscopy?

The Principle of UV-Visible Spectroscopy is based on the absorption of ultraviolet light or visible light by chemical compounds, which results in the production of distinct spectra. Spectroscopy is based on the interaction between light and matter.

What is the basic difference between UV and visible spectroscopy?

100 – 900 nm range is known as far ultraviolet and 190 – 400 nm is known as near ultraviolet. Visible range extends from 400 – 800 nm. Most of the commercial instruments cover 180 – 800 nm region. The technique is known as molecular absorption spectrophotometry or spectrophotometry or simply colorimetry.

What is meant by UV-Visible Spectroscopy?

Ultraviolet–visible spectroscopy or ultraviolet–visible spectrophotometry (UV–Vis or UV/Vis) refers to absorption spectroscopy or reflectance spectroscopy in part of the ultraviolet and the full, adjacent visible regions of the electromagnetic spectrum. This means it uses light in the visible and adjacent ranges.

What is the purpose of UV Visible Spectroscopy?

Uv Vis Spectroscopy Principle Pdf

UV/VIS/NIR spectroscopy is generally used to determine analyte concentrations or the chemical conversion of a component in solution. The technique measures the absorption of light across the desired optical range.

What is the purpose of UV spectroscopy?

UV-Vis Spectroscopy (or Spectrophotometry) is a quantitative technique used to measure how much a chemical substance absorbs light.

Which detector is used in UV spectroscopy?

photomultiplier tube

What are the detectors used in HPLC?

HPLC Detectors

• UV-Vis Detectors. The SPD-20A and SPD-20AV are general-purpose UV-Vis detectors offering an exceptional level of sensitivity and stability.
• Refractive Index Detector.
• Fluorescence Detectors.
• Evaporative Light Scattering Detector.
• Conductivity Detector.

What are the main parts of the spectrometer?

The spectrometer is an optical instrument used to study the spectra of different sources of light and to measure the refractive indices of materials (Fig. ). It consists of basically three parts. They are collimator, prism table and Telescope.১ এপ্রিল, ২০১৬

Which region of light wavelength is used in UV spectroscopy?

In UV/Vis/NIR spectroscopy the ultraviolet (170 nm to 380 nm), visible (380 nm to 780 nm), and near infrared (780 nm to 3300 nm) are used.

What is maximum absorbance?

(a) wavelength of maximum absorbance (λmax) The extent to which a sample absorbs light depends upon the wavelength of light. The wavelength at which a substance shows maximum absorbance is called absorption maximum or λmax.৩১ মার্চ, ২০১৬

Is benzene UV active?

Benzene exhibits very strong light absorption near 180 nm (ε > 65,000) , weaker absorption at 200 nm (ε = 8,000) and a group of much weaker bands at 254 nm (ε = 240).

Does benzene absorb UV light?

Note that both benzene and naphthacene absorb light in the near ultraviolet but that the latter does so much more intensely. A solution of naphthacene will absorb almost 100 times as much light at 250 nm. as a solution of benzene of the same molar concentration.

Which compound increases UV absorption?

Ag

How do you know if a compound is UV active?

Ones that do are said to be “UV-active” and ones that do not are “UV- inactive.” To be UV-active, compounds must possess a certain degree of conjugation, which occurs most commonly in aromatic compounds. One can then outline the spots with a pencil, while under the UV light, to mark their location.

What absorbs UV radiation?

As sunlight passes through the atmosphere, all UVC and most UVB is absorbed by ozone, water vapour, oxygen and carbon dioxide. Ozone is a particularly effective absorber of UV radiation. As the ozone layer gets thinner, the protective filter activity of the atmosphere is progressively reduced.৯ মার্চ, ২০১৬

Uv-visible Spectroscopy Principle And Instrumentation Pdf

How is UV generated?

UV radiation is produced either by heating a body to an incandescent temperature, as is the case with solar UV, or by passing an electric current through a gas, usually vaporized mercury. The latter process is the mechanism whereby UV radiation is produced artificially.

What is the major disadvantage of UV light as a disinfectant sterilant?

Disadvantages of UV disinfection? UV light needs the right amount of energy to be effective. UV light is effective for microorganisms not for chemicals. Photochemical damage caused by UV may be repaired by some organisms.২ জানু, ২০২০

How long does it take UV light to sterilize something?

30 minutes

Do hospitals use UV light?

The use of ultraviolet light systems is becoming more widely used in healthcare facilities for disinfecting patient and operating rooms. UV-C is also used in night time cleaning of laboratories and meat packing facilities. Another term used for UV-C disinfection is Ultraviolet Germicidal Irradiation (UVGI).

Uv-visible Spectroscopy Principle In Pdf Files

Does UV sanitizer really work?

That’s why UV light sanitizers can be a great alternative to products that might be out of stock. With any new technology, there will understandably be doubts about effectiveness, but research proves that in most cases UV sanitizers are effective in killing 99% of germs.১৪ নভেম্বর, ২০২০

What is the difference between ozone and UV?

There is sometimes confusion as to the difference between ozone and UV systems. Ozone is dissolved in water to kill microorganisms, destroy organics, and break down chloramines by oxidation. In comparison, UV light inactivates microorganisms and breaks down chloramines with light energy.২৫ ফেব, ২০১৯

Is it safe to breathe ozone?

When inhaled, ozone can damage the lungs. Relatively low amounts can cause chest pain, coughing, shortness of breath and throat irritation. Ozone may also worsen chronic respiratory diseases such as asthma and compromise the ability of the body to fight respiratory infections.

Does UV light destroy ozone?

UV light will create ozone from atmospheric oxygen at short wavelengths of less than 240 nanometers (nm). UV light will also destroy ozone and break ozone back down into atomic oxygen (O) and diatomic oxygen (O2) at wavelengths from about 200 nm to 315 nm.

Does a UV light create ozone?

A UV lamp “tuned” to 185nm can create ozone from oxygen (O2) by disrupting the O2 molecule and splitting it into two oxygen atoms. These two oxygen atoms attempt to attach to other oxygen molecule (O2). It is the attachment of this third oxygen atom that creates ozone (O3).১ জুন, ২০২০