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However, this molecule also absorbs at nm. Exercise 4. Explain your answer.

UV-Vis Spectroscopy

Protecting yourself from sunburn. Human skin can be damaged by exposure to ultraviolet light from the sun. We naturally produce a pigment, called melanin, which protects the skin by absorbing much of the ultraviolet radiation. Melanin is a complex polymer, two of the most common monomers units of which are shown below.

What is UV Vis Spectroscopy?

Overexposure to the sun is still dangerous, because there is a limit to how much radiation our melanin can absorb. PABA para -aminobenzoic acid was used in sunscreens in the past, but its relatively high polarity meant that it was not very soluble in oily lotions, and it tended to rinse away when swimming. Many sunscreens today contain, among other active ingredients, a more hydrophobic derivative of PABA called Padimate O.

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We have been talking in general terms about how molecules absorb UV and visible light — now let's look at some actual examples of data from a UV-vis absorbance spectrophotometer. The basic setup is the same as for IR spectroscopy: radiation with a range of wavelengths is directed through a sample of interest, and a detector records which wavelengths were absorbed and to what extent the absorption occurred. Schematic for a UV-Vis spectrophotometer.

UV-VIS Spectroscopy and Its Applications - Heinz-Helmut Perkampus - Google книги

Image from Wikipedia Commons. This compound absorbs light in the UV range due to the presence of conjugated pi-bonding systems. Notice also that the convention in UV-vis spectroscopy is to show the baseline at the bottom of the graph with the peaks pointing up.

Wavelength values on the x-axis are generally measured in nanometers nm rather than in cm -1 as is the convention in IR spectroscopy. Peaks in UV spectra tend to be quite broad, often spanning well over 20 nm at half-maximal height. Typically, there are two things that we look for and record from a UV-Vis spectrum. Here we use a unitless number called absorbance , abbreviated 'A'.

This contains the same information as the 'percent transmittance' number used in IR spectroscopy, just expressed in slightly different terms.

UV/Vis Applications Library

To calculate absorbance at a given wavelength, the computer in the spectrophotometer simply takes the intensity of light at that wavelength before it passes through the sample I 0 , divides this value by the intensity of the same wavelength after it passes through the sample I , then takes the log 10 of that number:. You can see that the absorbance value at nm A is about 1. Here, we see that the extended system of conjugated pi bonds causes the molecule to absorb light in the visible range.

Now, take a look at the spectrum of another food coloring, Blue Here, maximum absorbance is at nm, in the orange range of the visible spectrum, and the compound appears blue. Your email address will not be published. Qualitative and how much is this Quantitative? In this seminar, the focus was on the followings: 1.

Ultraviolet/Visible/Near Infrared Spectroscopy (UV/VIS/NIR)

Properties of electromagnetic radiations 2. Employment of these radiations in various analytical instruments. How UV-Vis. Spectrophotometers work.

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