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Chapter 1

Chapter 1. Introduction. Instrumental Methods of Analysis. Goal – detection of analyte concentration and/or structure Method – design an instrument that furnishes this information based upon some particular response of the analyte. Stimuli for Various Instrumental Methods. Light. Optics.

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Chapter 1

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  1. Chapter 1 Introduction

  2. Instrumental Methods of Analysis Goal – detection of analyte concentration and/or structure Method – design an instrument that furnishes this information based upon some particular response of the analyte.

  3. Stimuli for Various Instrumental Methods Light Optics Electricity Fragmentation Equilibrium

  4. Common Instrumental Methods • Optical Spectroscopic Techniques • Ultraviolet and Visible (UV-Vis) Spectroscopy • Fluorescence and Phosphorescence • Atomic Spectroscopy - both emission (ICP-AES) and absorption (AAS) • Fourier Transform Infrared (FTIR) and Raman Spectroscopy • Xray Spectroscopy • Nuclear Magnetic Resonance (NMR) Spectroscopy • Chromatographic Techniques • Gas Chromatography (GC) • High Performance Liquid Chromatography (HPLC) • Capillary Electrophoresis • Electrochemical Techniques • Voltammetry • Polarography • Coulometry • Miscellaneous Techniques • Mass Spectrometry

  5. Basic Components of Instruments(from “Instrumental Methods of Analysis”, 7th ed. Willard et. al.) Input Transducers (Detector) Signal Transformation Modules Signal Generators Output Transducers • the analyte itself,e.g. [H+] pH electrode • stimulated analyte, e.g. Xrays, flames, lasers, etc • thermocouple • thermistor • photodiode • photomultiplier tube • CCD • electrode • amplification • differentiation • integration • Fourier Transformation • printers • computers • chart recorders • analog and digital meters • oscilloscopes

  6. Basic Statistics Review Gaussian Distribution: Frequency 

  7. Mean – measure of the central tendency or average of the data (accuracy) N   Infinite population Finite population Standard Deviation – measure of the spread of the data (reproducibility) Infinite population Finite population

  8. Confidence Intervals

  9. Calibration Curves The signal from the instrument must be calibrated with external standards of known concentration. Unknown concentrations are obtained via extrapolation. • assume – • linear relationship (least squares fit = trendline in Microsoft Excel) • concentration of external standards exactlyknown signal (S)  concentration (C)  “blank”

  10. S C Sensitivity A measure of the ability of an instrument to discriminate between small differences in analyte concentration, i.e. generates a large change in signal for a small change in concentration. Depends on two factors, (1) the slope of the calibration curve, and (2) the precision of each measured point in the calibration curve (error bars = ) slope = The top calibration curve with the larger slope has higher calibration sensitivity – a larger signal change results from the same change in concentration – calibration sensitivity = m The analytical sensitivity()depends on the precision of each measured point as well as the slope –  = m/ S C signal (S)  concentration (C) 

  11. Limit of Detection (LOD) What is the minimum detectable signal level? SLOD = <S>blank + 3 blank generally agreed upon definition

  12. An equation to calculate the LOD can be derived from the calibration curve -

  13. The LOD can be graphically calculated also -

  14. Dynamic Range Range of concentrations where calibration curves remain linear.

  15. Example 1-2 A least squares analysis of calibration data for the determination of lead based on its flame emission spectrum yielded the equation: S = 1.12 CPb + 0.312 where CPb is the lead concentration in ppm and S is a measure of the relative intensity of the lead emission line. The following replicate data were then obtained: Calculate (a) the calibration sensitivity, (b) the analytical sensitivity, (c) the LOD.

  16. Chapter 6 An Introduction to Spectroscopic Methods

  17. Electromagnetic Radiation Mutually perpendicular electric and magnetic fields traveling at a constant speed c (in vacuum) c = 2.998 x 108 m/s

  18. Wave Characteristics c =  where  = wavelength,  = frequency e.g. what is the wavelength of a radio station broadcasting at 105.1 MHz?

  19. Light as a Particle - Photons Ephoton = h Compare the energies of “blue” (420nm) vs. “red” (650nm) photons:

  20. Electromagnetic Spectrum

  21. Superposition of Waves & Constructive - Destructive Interference

  22. Diffraction of Light http://cache.eb.com/eb/image?id=44791&rendTypeId=4

  23. Diffraction of Light The process in which a parallel beam of light is bent as it passes by a sharp edge or through a narrow opening. from “Optics” by Hecht and Zajac

  24. Fraunhofer Diffraction from a Single Slit A parallel beam of light diffracts from a rectangular aperture, and the diffracted light is collected and focused by a lens into the “far field”. from “Optics” by Hecht and Zajac

  25. from “Optics” by Hecht and Zajac

  26. Fraunhofer Diffraction from a Single Slit Monochromatic Source red = 700 nm, slit width = 5 m 0th order 1storder 1storder 3rdorder 2ndorder 3rdorder 2ndorder

  27. Fraunhofer Diffraction from a Single Slit Polychromatic (“White”) Source blue = 400 nm, red = 700 nm, NIR = 1000 nm slit width = 5 m white 1st order 1st order

  28. 3-D View of Fraunhofer Diffraction from a Single Slit Occurs off the entrance and exit slits of Monochromators

  29. Refraction of Light modified from http://www.sorouche.com/journal/uploads/entries/DarkSideoftheMoon.jpg

  30. Refractive Index Consider what happens to light as it travels from one medium to another - Light slows down as it enters a denser medium. The frequency is source-dependent and therefore does not change. Since the velocity has decreased, and since u = , then the wavelength decreased. The refractive index is a measure of how much the velocity has changed: n = c/u note: n  1.00 where n = refractive index c = speed of light in vacuum u = speed in medium

  31. Dispersion of Light The refractive index varies inversely with wavelength – u =  = c n 2 generalized dispersion curve for glass n Snells Law: angle of refraction increases with n nblue > nred so larger dispersion angle in the blue. “blue”   “red”

  32. blue refracted more width of blue bands greater than red bands red diffracted more equal width of color bands Comparison of Diffraction and Refraction

  33. Interactions of Radiation and Matter Electrons can absorb and emit photons if the photon energy matches the energy gap between the final and initial states. n = 4 n = 3 n = 2 photon of light (h) h h n = 1 absorption emission Ephoton = E = Ef - Ei

  34. Interactions of Radiation and Matter - Absorption

  35. Interactions of Radiation and Matter - Emission

  36. Interactions of Radiation and Matter - Luminescence

  37. Interactions of Radiation and Matter - Scattering

  38. Atomic vs. Molecular Transitions

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