File Name: uv vis spectroscopy and its applications .zip
UV spectroscopy involves the transitions of electrons within molecule or ion from a lower to a higher electronic energy level or vice-versa by the absorption or emission of radiation's falling in the UV-visible range of electromagnetic spectrum. This means it uses light in the visible and adjacent ranges.
The absorption or reflectance in the visible range directly affects the perceived color of the chemicals involved. In this region of the electromagnetic spectrum , atoms and molecules undergo electronic transitions.
Absorption spectroscopy is complementary to fluorescence spectroscopy , in that fluorescence deals with transitions from the excited state to the ground state , while absorption measures transitions from the ground state to the excited state.
Molecules containing bonding and non-bonding electrons n-electrons can absorb energy in the form of ultraviolet or visible light to excite these electrons to higher anti-bonding molecular orbitals. Spectroscopic analysis is commonly carried out in solutions but solids and gases may also be studied.
The Beer—Lambert law states that the absorbance of a solution is directly proportional to the concentration of the absorbing species in the solution and the path length. It is necessary to know how quickly the absorbance changes with concentration. This can be taken from references tables of molar extinction coefficients , or more accurately, determined from a calibration curve. The presence of an analyte gives a response assumed to be proportional to the concentration. For accurate results, the instrument's response to the analyte in the unknown should be compared with the response to a standard; this is very similar to the use of calibration curves.
The response e. The wavelengths of absorption peaks can be correlated with the types of bonds in a given molecule and are valuable in determining the functional groups within a molecule. The spectrum alone is not, however, a specific test for any given sample. The nature of the solvent, the pH of the solution, temperature, high electrolyte concentrations, and the presence of interfering substances can influence the absorption spectrum.
Experimental variations such as the slit width effective bandwidth of the spectrophotometer will also alter the spectrum. The method is most often used in a quantitative way to determine concentrations of an absorbing species in solution, using the Beer—Lambert law :. The Beer—Lambert Law is useful for characterizing many compounds but does not hold as a universal relationship for the concentration and absorption of all substances.
A 2nd order polynomial relationship between absorption and concentration is sometimes encountered for very large, complex molecules such as organic dyes Xylenol Orange or Neutral Red , for example.
UV—Vis spectroscopy is also used in the semiconductor industry to measure the thickness and optical properties of thin films on a wafer. UV—Vis spectrometers are used to measure the reflectance of light, and can be analyzed via the Forouhi—Bloomer dispersion equations to determine the Index of Refraction n and the Extinction Coefficient k of a given film across the measured spectral range.
The Beer—Lambert law has implicit assumptions that must be met experimentally for it to apply; otherwise there is a possibility of deviations from the law. The chemical and physical conditions of a test sample therefore must match reference measurements for conclusions to be valid.
It is important to have a monochromatic source of radiation for the light incident on the sample cell. A given spectrometer has a spectral bandwidth that characterizes how monochromatic the incident light is. In reference measurements, the instrument bandwidth bandwidth of the incident light is kept below the width of the spectral lines.
When a test material is being measured, the bandwidth of the incident light should also be sufficiently narrow. Reducing the spectral bandwidth reduces the energy passed to the detector and will, therefore, require a longer measurement time to achieve the same signal to noise ratio. In liquids, the extinction coefficient usually changes slowly with wavelength. A peak of the absorbance curve a wavelength where the absorbance reaches a maximum is where the rate of change in absorbance with wavelength is smallest.
Another important factor is the purity of the light used. The most important factor affecting this is the stray light level of the monochromator.
The detector used is broadband; it responds to all the light that reaches it. If a significant amount of the light passed through the sample contains wavelengths that have much lower extinction coefficients than the nominal one, the instrument will report an incorrectly low absorbance. Any instrument will reach a point where an increase in sample concentration will not result in an increase in the reported absorbance, because the detector is simply responding to the stray light.
In practice the concentration of the sample or the optical path length must be adjusted to place the unknown absorbance within a range that is valid for the instrument. Sometimes an empirical calibration function is developed, using known concentrations of the sample, to allow measurements into the region where the instrument is becoming non-linear.
As a rough guide, an instrument with a single monochromator would typically have a stray light level corresponding to about 3 Absorbance Units AU , which would make measurements above about 2 AU problematic.
A more complex instrument with a double monochromator would have a stray light level corresponding to about 6 AU, which would therefore allow measuring a much wider absorbance range. At sufficiently high concentrations, the absorption bands will saturate and show absorption flattening. The concentration at which this occurs depends on the particular compound being measured. One test that can be used to test for this effect is to vary the path length of the measurement. In the Beer—Lambert law, varying concentration and path length has an equivalent effect—diluting a solution by a factor of 10 has the same effect as shortening the path length by a factor of If cells of different path lengths are available, testing if this relationship holds true is one way to judge if absorption flattening is occurring.
Solutions that are not homogeneous can show deviations from the Beer—Lambert law because of the phenomenon of absorption flattening. This can happen, for instance, where the absorbing substance is located within suspended particles. The last reference describes a way to correct for this deviation. Some solutions, like copper II chloride in water, change visually at a certain concentration because of changed conditions around the coloured ion the divalent copper ion. For copper II chloride it means a shift from blue to green,  which would mean that monochromatic measurements would deviate from the Beer—Lambert law.
These include spectral interferences caused by absorption band overlap, fading of the color of the absorbing species caused by decomposition or reaction and possible composition mismatch between the sample and the calibration solution.
The UV—visible spectrophotometer can also be configured to measure reflectance. The basic parts of a spectrophotometer are a light source, a holder for the sample, a diffraction grating in a monochromator or a prism to separate the different wavelengths of light, and a detector. The detector is typically a photomultiplier tube , a photodiode , a photodiode array or a charge-coupled device CCD. Single photodiode detectors and photomultiplier tubes are used with scanning monochromators, which filter the light so that only light of a single wavelength reaches the detector at one time.
The scanning monochromator moves the diffraction grating to "step-through" each wavelength so that its intensity may be measured as a function of wavelength.
Fixed monochromators are used with CCDs and photodiode arrays. As both of these devices consist of many detectors grouped into one or two dimensional arrays, they are able to collect light of different wavelengths on different pixels or groups of pixels simultaneously. A spectrophotometer can be either single beam or double beam. In a single beam instrument such as the Spectronic 20 , all of the light passes through the sample cell.
This was the earliest design and is still in common use in both teaching and industrial labs. In a double-beam instrument, the light is split into two beams before it reaches the sample. One beam is used as the reference; the other beam passes through the sample. Some double-beam instruments have two detectors photodiodes , and the sample and reference beam are measured at the same time. In other instruments, the two beams pass through a beam chopper , which blocks one beam at a time.
The detector alternates between measuring the sample beam and the reference beam in synchronism with the chopper. There may also be one or more dark intervals in the chopper cycle. In this case, the measured beam intensities may be corrected by subtracting the intensity measured in the dark interval before the ratio is taken. In a single-beam instrument, the cuvette containing only a solvent has to be measured first.
The light source consists of a Xenon flash lamp for the ultraviolet UV as well as for the visible VIS and near-infrared wavelength regions covering a spectral range from up to nm. The lamp flashes are focused on a glass fiber which drives the beam of light onto a cuvette containing the sample solution.
The beam passes through the sample and specific wavelengths are absorbed by the sample components. The remaining light is collected after the cuvette by a glass fiber and driven into a spectrograph.
The spectrograph consists of a diffraction grating that separates the light into the different wavelengths, and a CCD sensor to record the data, respectively. The whole spectrum is thus simultaneously measured, allowing for fast recording. Samples are typically placed in a transparent cell, known as a cuvette.
Test tubes can also be used as cuvettes in some instruments. The type of sample container used must allow radiation to pass over the spectral region of interest.
The most widely applicable cuvettes are made of high quality fused silica or quartz glass because these are transparent throughout the UV, visible and near infrared regions. Glass and plastic cuvettes are also common, although glass and most plastics absorb in the UV, which limits their usefulness to visible wavelengths. Specialized instruments have also been made. These include attaching spectrophotometers to telescopes to measure the spectra of astronomical features.
UV—visible microspectrophotometers consist of a UV—visible microscope integrated with a UV—visible spectrophotometer. A complete spectrum of the absorption at all wavelengths of interest can often be produced directly by a more sophisticated spectrophotometer.
In simpler instruments the absorption is determined one wavelength at a time and then compiled into a spectrum by the operator.
UV—visible spectroscopy of microscopic samples is done by integrating an optical microscope with UV—visible optics, white light sources, a monochromator , and a sensitive detector such as a charge-coupled device CCD or photomultiplier tube PMT.
As only a single optical path is available, these are single beam instruments. Modern instruments are capable of measuring UV—visible spectra in both reflectance and transmission of micron-scale sampling areas. The advantages of using such instruments is that they are able to measure microscopic samples but are also able to measure the spectra of larger samples with high spatial resolution.
As such, they are used in the forensic laboratory to analyze the dyes and pigments in individual textile fibers,  microscopic paint chips  and the color of glass fragments. They are also used in materials science and biological research and for determining the energy content of coal and petroleum source rock by measuring the vitrinite reflectance.
Microspectrophotometers are used in the semiconductor and micro-optics industries for monitoring the thickness of thin films after they have been deposited. In the semiconductor industry, they are used because the critical dimensions of circuitry is microscopic. A typical test of a semiconductor wafer would entail the acquisition of spectra from many points on a patterned or unpatterned wafer. The thickness of the deposited films may be calculated from the interference pattern of the spectra.
In addition, ultraviolet—visible spectrophotometry can be used to determine the thickness, along with the refractive index and extinction coefficient of thin films as described in Refractive index and extinction coefficient of thin film materials. A map of the film thickness across the entire wafer can then be generated and used for quality control purposes. Using optical fibers as a transmission element of spectrum of burning gases it is possible to determine a chemical composition of a fuel, temperature of gases, and air-fuel ratio.
From these measurements, the concentration of the two species can be calculated.
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Spectroscopy is the investigation and measurement of spectra produced by matter interacting with or emitting electromagnetic radiation. Originally, spectroscopy was defined as the study of the interaction between radiation and matter as a function of wavelength. Now, spectroscopy is defined as any measurement of a quantity as a function of wavelength or frequency.
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UV-VIS spectroscopy is one of the oldest methods in molecular spectroscopy. The definitive UV-VIS Spectroscopy and Its Applications Download book PDF.
Vivekkumar K. Priyanka R. Patel 1. Divya Y.
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