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bio lab 3 - Biology 05LA Winter Quarter 2008 Lab 3 page 1...

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Biology 05LA – Winter Quarter 2008 Lab 3 – page 1 LAB 3 : SPECTROPHOTOMETRY AND QUANTITATIVE DATA ANALYSIS This exercise will introduce the use of a spectrophotometer (or colormeter) to measure a particular chemical property of many biological (and non-biological) materials. We will also learn to quantify spectrophotometric data, learn several "wet lab" techniques, and learn how to properly prepare scientific graphs. Be advised that you will be using all of these newly acquired skills in the labs of the weeks to come, so come to lab prepared to master these skills – your potential success in these future labs will be greatly enhanced if you do so. Absorption Spectra All solutions that are colored to the human eye absorb electromagnetic radiation in the visible portion of the electromagnetic spectrum (Fig. 1). That is why they are colored. The color that we perceive represents those wavelengths of light that are not absorbed by the substance, or, conversely, represents those wavelengths that are transmitted . For example, if a solution is green, it absorbs in the blue and red portions of the spectrum. If it is red, we would expect it to absorb in the green and blue portions of the spectrum. Another way of saying this is that a solution transmits light of a color complementary to that which it absorbs (Fig. 1). Figure 1: The E lectrom agnetic S pectrum. The range of wavelengths of the EMS is shown. The visible spectrum represents a narrow band of the EMS. Spectrophotometry can also utilize part of the infra red and ultra violet bands. The approximate wavelengths of the colors in the visible spectrum are given in the color wheel. Colors that are complementary fall on opposite sides of the wheel. 10 6 10 4 10 2 10 0 10 -2 10 -4 10 -6 10 -8 10 -10 10 -12 10 -14 Comparable Lengths: Wavelength (in cm) UCR to Rubidoux Size of blue whale. McDonald’s Hamburger. Resolving power of human eye. Red blood cell. Hertzian waves; radio, TV, and microwaves. Infra red. Visible spectrum . Ultra violet. Common Name X-ray. Gamma ray. Cosmic ray. AIDS virus H atom. Electron Red (650nm) Orange (600nm) Yellow (550nm) Green (500nm) Blue (450nm) Violet (400nm)
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Biology 05LA – Winter Quarter 2008 Lab 3 – page 2 The absorption of certain wavelengths of electromagnetic radiation is as characteristic of a compound as is its molecular weight, solubility properties, melting point or any other intrinsic property. In fact, the absorption spectrum of a compound may be used to identify it. The spectrum may be thought of as a “fingerprint” of a compound. This description of the properties of a solution, applies equally well to a solid substance. The difference is that light that is not absorbed by a substance is reflected from its surface. The ability to absorb and reflect electromagnetic radiation of different wavelengths is an intrinsic property of substances as well as solutions.
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