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What Is Spectroscopy?

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What Is Spectroscopy? What is Spectroscopy? • Spectroscopy: rely on the interactions of between electromagnetic radiation and matter. Ch. 18 Fundamentals of • Spectrometry and spectrometric methods: Spectrophotometry -a large group of analytical methods that are based on atomic and molecular spectroscopy -the measurement of the intensity of radiation with a photoelectric transducer or other type of electronic device. • Colorimetry: based on absorption of visible light. Wavelengths and Color Absorption Spectrum • Light shinning on a sample causes electrons to be excited from the ground state to an excited state • wavelengths of that energy are removed from transmitted spectra To Do a Spectroscopic Analysis Single-beam Absorbance Experiment • You need: – A continuous light source Irradiance – A wavelength selector Irradiance out – A sample cell in – A detector Lamp or laser grating, prism or filter Pathlength • The sample cell is called cuvet and can be made of many substances Irradiance (P): the energy per unit area in the light beam (W/m2). – Glass (good for visible) Transmittance (T): the fraction of original light not absorbed by the – Quartz (UV-vis) sample. T = P/P0 Monochromatic light: consists of “one color” (one wavelength). Chromophore: the part of a molecule responsible for light absorption 1 Absorbance and Beer’s Law Relation between Transmittance and • Absorbance (A): the amount light absorbed by the sample is related to transmittance (T): Absorbance P ’ 0 P ’ A = log −= logT A = log 0 −= logT P P • Beer’s law relates the absorbance of a chemical to its concentration: P/Po %T A = ε λλ bcA 1 100 0 b : the pathlength, typically in cm, and c is the 0.1 10 1 concentration of the chemical species in M 0.01 1 2 ε : the molar absorptivity, the unit that tells how much light is absorbed for a given wavelength. ε has units of M-1 cm-1 Absorption Methods - Beer’s Law Absorption Methods - Beer’s Law A = abc = εbc • The light being shined on A = abc = εεεbc the sample must be monochromatic (one color where a => absorptivity or wavelength) b => path length c => concentration A • The analyte must not be ε => molar absorptivity participate in a concentration dependent c equilibrium Limitations to Beer’s Law Spectroscopic Procedures • Real Limitations – high • You may have a single-beam or double beam -1 1.0 1 - 10 mg l NO -N 3 concentrated solutions, – Single-beam instrument has one sample holder, you r = 0.8791 concentrated electrolyte 0.8 solutions (proximity must swap blank and sample 0.6 alters molecular – Double-beam instrument splits light output between Abs 0.4 absorption). two holders so you can measure blank and sample 0.2 – A baseline spectrum is a spectrum of a reference 0.0 • Chemical Limitations – 0 2 4 6 8 10 solution (solvent or reagent blank) -1 absorbing species [NO 3-N] (mg l ) participate in association – try to do an analysis at the max if we can or dissociation reactions, e.g. weak acids in HNO + H O →← H O + NO − 2 2 3 2 concentrated solutions, complexation. 2 Spectroscopic Procedures Example: Find the absorbance and transmittance of a • Keep the absorbance reading of your sample below 1. 0.0220 M solution with a molar absorptivity of 15.5 M-1 cm-1 in a 2.00 cm pathlength cell – % transmittance is related logarithmically with concentration – for 1-99% transmittance you can detect ~ 2 orders of -1 -1 magnitude in analyte concentration; any orders of magnitude Aλ=ε λ bc = (15.5 M cm )(2.00 cm)(0.0220 M)= 0.68 greater than that will be detected in the range of 0-1% T logT = −A • Dilute the solution if you have to so that the −0.68 transmittance reading is not maxed out in that region. T =10 = 0.21 21% of the irradiated light emerges from the analyte sample Example: Consider a 250 mL solution of 28.8 mg benzene in hexane. If the absorbance of your 1 cm cell is 0.266 at λ=200nm, what is the ε value in M-1 cm-1 for benzene (FW: 78.11)? 28.8mg C6H6 1g 1000ml 1mol C6H6 c = 250 ml sol’n 1000mg 1L 78.11 g C6H6 -3 =A /cb =A/c c=[C6H6]=1.321x10 M ε ε=0.266/1.321x10-3=201.5 M-1 cm-1 3.
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