Text: Principles of Instrumental Analysis, 5th Ed., Skoog, Holler, Nieman, Harcourt Brace, 1998

1. Text: Principles of Instrumental Analysis, 5th Ed., Skoog, Holler, Nieman, Harcourt Brace, 1998

17. Standard Addition Method • Usually involves adding one or more increments of a standard solution to sample aliquots of the same size (spiking)

18. Lab 1: Spectrophotometric Analysis of a Mixture of Absorbing Substances • Purpose is to determine the individual concentrations of a mixture of absorbing substances • Gain experience working with a UV-Vis Spectrophotometer • Practice several analytical techniques • Understand absorbance and application of the Beer-Lambert Law

19. Background: Absorption of Radiation • Absorption – A process in which electromagnetic energy is transferred to the atoms, ions, or molecules composing a sample • Promotes particles from their normal room temperature state (ground state) to one or more higher-energy states. • Atoms, molecules or ions have a limited number of discrete energy levels • For absorption to occur, the energy of the exciting photon must exactly match the energy difference between the ground state and an excited state of the absorbing species

20. b P0 P Absorbing solution of concentration, c Absorption Methods • Absorbance A of a medium is defined A = -log10T = log10P0/P • Beer-Lambert Law is defined A = Єbc

21. Lab Report Write-up • Introduction to spectroscopy, instrument basics, absorption principles and Beer-Lambert Law • Experimental section • Specific instrumention (www.oceanoptics.com) • Experimental procedures • Results • Abs vs. wavelength spectra • Plots of concentration vs. absorbance, including equations of lines and R2 • Red at λ1 and at λ2 • Yellow at λ1 and at λ2 • Tables • Dilutions • Red absorbance by concentration • Yellow absorbance by concentration • Є values • Equations and unknown concentrations • Conclusions • References

24. Signals and Noise • Analytical measurements consist of 2 components • Signal • Noise • Signal to noise ratio • S/N = x/s = mean / standard deviation

25. Sources of Noise in Instrumental Analysis • Chemical Noise • Instrumental Noise • Thermal noise • Shot noise • Flicker noise • Environmental noise

26. Signal to Noise Enhancement • Hardware • Software • Ensemble Averaging • Boxcar Averaging • Digital Averaging • Fourier transformation

27. An Introduction to Spectrometric Methods • Spectroscopy • Interactions of various types of radiation with matter • Electromagnetic radiation (light, X-Rays) • Ions and electrons

28. Properties of EMR • Described by means of sine wave • Wavelength, frequency, velocity, amplitude • Particle model of radiation is necessary • Represented as electric and magnetic fields that undergo sinusoidal oscillations at right angles to each other and the direction of propogation

29. vi = nli • Frequency is determined by source and remains invariant • Velocity depends on medium • Velocity (air or vacuum) = c = 3.00 x 108 m/s = ln

30. Transmission of Radiation • Refractive index • A measure of the interaction of radiation with the medium it travels through hi = c/vi

31. Scattering of Radiation • Small fraction of radiation is scattered as it passes through a medium • Rayleigh Scattering (elastic) • Scattering by molecules with wavelengths smaller than wavelength of radiation • Its intensity is proportional to 1/l4 • Raman Scattering (inelastic)

32. Diffraction of Radiation • All types of EMR exhibit diffraction • Is a consequence of interference • A parallel beam of radiation is bent as it passes a barrier or slit • nl = BC sin q (Bragg Equation)

33. The Photoelectric Effect • Experiments revealed that a spark jumped more readily between 2 charged spheres when their surfaces were illuminated with light • EMR is a form of energy that releases electrons from metallic surfaces • Below a certain frequency, no additional sparks (electrons) are observed

34. E = hn (Einstein) • eV0 = hn - w • E = hn = eV0 + w

35. Emission of Radiation • EMR is produced when excited particles (atoms, ions, or molecules) relax to lower energy levels by giving up their excess energy as photons • Excitation can be brought about by • Bombardment with electrons • Irradiation with a beam of EMR

36. Radiation from an excited source is characterized by an emission spectrum • Plot of relative power of emitted radiation vs wavelength or frequency • Types of spectra • Line • Band • Continuum

37. Absorption of Radiation • In absorption, EM energy is transferred to atoms, molecules comprising the sample • Absorption promotes these particles from RT state to a higher-energy excited state • For absorption to occur, the energy of the exciting photon must exactly match the energy difference between the ground state and one of the excited states of the absorbing species

38. Atomic Absorption • Passage of radiation through a medium that consists of monoatomic particles results in absorption of a few frequencies • Simplicity is due to small number of possible energy states for the absorbing particles

39. Molecular Absorption • More complex because the number of energy states is large compared to isolated atoms • The energy, E, associated with the molecular bands: E = Eelectronic + Evibrational + Erotational

40. Components of Optical Instruments • Stable source of radiant energy • Transparent sample container • Device that isolates a restricted region of the spectrum • Radiation detector • Signal processor and readout

41. Sources of Radiation • Source must generate a beam of radiation with sufficient power • Output must be stable for reasonable periods • Radiant power of a source varies exponentially with the voltage of its power supply • Continuum (tungsten) • Line (lasers)

42. Wavelength Selectors • Narrow bandwidth is required • Filters • Monochromators, consisting of • Entrance slit • Collimating lens (or mirror) • Grating (or prism, historical) • Focusing element • Exit slit

43. Radiation Transducers • Convert radiant energy into an electrical signal • Photon transducers • Photomultiplier tube (PMT) • Contain a photoemissive surface • Emit a cascade of electrons when struck by electrons • Useful for measurement of low radiant power

44. Component Configuration for Optical Absorption Spectroscopy Source Lamp Sample Holder Wavelength Selector Signal Processor and Readout Photoelectric Transducer

45. Atomic Absorption Spectrometry • Most widely used method for determination of single elements in analytical chemistry • Quantification of energy absorbed from an incident radiation source from the promotion of elemental electrons from the ground state • Technique relies on a source of free elemental atoms electronically excited by monochromatic light

46. Sample Introduction in AAS • Flame • Method of supplying atom source • Utilizes a nebulizer in conjunction with air/acetylene flame • Solvent evaporates • Metal salt vaporizes and is reduced to complete the atomization process • Radiation source is a hollow cathode lamp

47. Graphite Furnace AAS • Samples are atomized by electrothermal atomization • Provide an increase in sensitivity and improved safety compared to Flame AAS instruments • Applications

48. Mass Spectrometry • Relies on separating gaseous charged ions according to their mass-to-charge ratio (m/z) • Widely used in conjunction with other analytical techniques

49. Operating Principles • Sample inlet • Sample ionization • Ion acceleration by an electric field • Ion dispersion according to m/z • Identification of ion mass

50. Mass to Charge Ratio • Obtained by dividing the atomic or molecular mass of an ion, m, by the number of charges, z, of the ion • Most ions are singly charged