Chemistry Instrumental Analysis Lecture 19 Chapter 12. Chem 4631

Transcription

1 Chemistry 4631 Instrumental Analysis Lecture 19 Chapter 12

2 There are three major techniques used for elemental analysis: Optical spectrometry Mass spectrometry X-ray spectrometry X-ray Techniques include: Diffraction Single-crystal Powder Spectrometry Absorbance Fluorescence

3 Fundamental Principles X-rays are part of the electromagnetic spectrum produced by transitions of electrons in the inner orbitals of atoms. Wavelength range from 10-5 to 100 Å.

4 Theory A source X-ray (or electron) strikes an inner shell electron. If at high enough energy (above absorption edge of element), it is ejected from the atom. Higher energy electrons cascade to fill vacancy, giving off characteristic X-rays. For elemental analysis of Na - U. (Lighter elements are possible with some instruments)

5 Fundamental Principles X-rays are produced by: bombardment of a metal target with a beam of energy electrons exposure of a substance to a primary beam of x-rays to produce a secondary beam of x- ray fluorescence use of a radioactive source with an x-ray decay process from a synchrotron radiation source

6 Fundamental Principles X-ray spectrum composed of a line spectrum of intense single wavelengths superimposed on a continuous background.

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8 Fundamental Principles Continuous X-ray spectrum Plot of the x-ray intensity at several wavelengths. The wavelength limits and intensity distribution is dependent on applied voltage but independent of the target material.

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10 Fundamental Principles Line Spectra Result from electronic transitions that involve the innermost atomic orbitals. A few intense emission lines are produced that correspond to K and L level transitions.

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12 Electron transitions in an atom, which produce the Kα, Kβ and Lα characteristic x-rays.

13 Energy-level diagram showing all the allowed electron transitions in a molybdenum atom.

14 Fundamental Principles Line Spectra Bragg discovered the characteristic lines. H. G. Moseley systemized the lines Moseley's law - the wavelength of any particular line decreases as the atomic number of the emitter increases. Linear relationship between the square root of the line frequency, v, and the atomic number, Z. v 1/2 = C(Z - s) C and s are constants

15 Moseley's relation between ν and Ζ for two characteristic lines

16 Diffraction When x-rays are scattered by a crystal, constructive and destructive interference takes place causing diffraction.

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18 Diffraction This diffraction can be described by Bragg s Law: nl = 2dsinq n integer d interplanar distance in the crystal l wavelength of x-ray radiation q angle of reflection

19 Source l selector Sample holder Transducer Signal processor

20 Sources Tubes Radioisotopes Secondary fluorescent sources

21 X-ray Tubes (Filament tubes) Invented by Coolidge in Hot-Cathode tube - tube is evacuated (in a vacuum). Electrons are supplied by a heated filament (W), called the cathode. Filament requires i = 1.5 to 5 A and 4 to 12 V to incandescence. Filament temperature ranges from 1800 to 2600 o C. The cathode is held at a high negative potential.

22 X-ray Tubes The electrons produced at the cathode are accelerated toward the anode, which is held at ground. Electrons strike the target (anode) at a high velocity. X-rays are produced and radiate in all directions. (~1% of the electron beam is converted to x-rays).

23 X-ray Tubes

24 Optics Includes a variety of slits, filters, monochromaters Purpose: to reduce stray radiation, produce x- ray spectra which display diffraction from a single wavelength. (each unique d-spacing will diffract different wavelengths at different angles).

25 Optics For Cu radiation, the basic emission contains the a 1, a 2 doublet and the b 1, b 3 transition. Usually the b radiation can be reduced to a few % of the a radiation by using filters, monochromater, or energy resolving detector. Most diffraction work uses the Cu Ka 1, Ka 2 doublet, which can be inconvenient at some angles.

26 Filters b - Filter A bandpass device used mainly to improve the ratio of Cu Ka to Cu Kb. If a polychromatic beam of radiation is passed through a filter, then preferential transmission of certain l's will occur. So need to find materials that has an absorption edge between the Ka doublet and the Kb doublet, to increase the a/b transmission ratio.

27 b - Filter For copper radiation, nickel is used since the nickel absorption edge (1.488 Å) lies between the Cu Ka (1.542 Å) and Cu Kb (1.392 Å) radiation.

28 Optics Various Slits The x-ray radiation passes through a series of slits on both the source side and the detector side. These slit width can be varied depending on the sample and x- ray scan parameters.

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30 Optics Monochromators Remove unwanted radiation. The crystals used for monochromators need to be mechanically strong, not affected by exposure to x-rays, and stable in air. Plane crystal monochromators are used in camera work, while curve (bent) crystal monochromators are used in powder diffractometers and fluorescence.

31 Monochromators Curved crystal monochromaters provide monochromatic radiation with low background, and furnish high intensity (compared to plane) and high resolving power. Materials used as crystals are: mica, gypsum, quartz, graphite Diffracted beam monochromator is made up of: a receiving slit, with a single crystal behind that, the detector is set at an angle to collect the l of interest diffracted by the crystal. The surface of the crystal, receiving slit, and detector slit all lie on the focusing circle of the monochromator.

32 Monochromators

33 Goniometer Goniometer circle - centered at the sample, with the x-ray source and detector on the circumference of the circle.

34 Detectors Majority of detectors depend on x-rays to ionize atoms (either as a gas or on a solid) Types of detectors: Proportional Geiger Scintillation Solid State (Semiconductor)

35 Proportional Detector Common detector A metal tube (cathode) filled with a gas (i.e. Ar, Xe, or Kr) and contains a thin metal wire (anode) running down the center. There is a constant potential difference between the cathode and anode.

36 Proportional Detector

37 Proportional Detector X-rays enter the tube through a transparent window and are absorbed by a gas The gas ejects a photoelectron and becomes ionize (an ion/electron pair is produced) - Ionized gas (+) moves toward the cathode (-) Electrons (-) move toward the anode (+) A small current is measured and related to the x-ray intensity. The ionization energy of the noble gas is ~30eV For one Cu x-ray photon, the energy is 8.04 KeV So ~270 electron-ion pairs are produced with CuKa

38 Scintillation Detector Incident x-ray hits a crystal causing it to fluoresce.

39 Scintillation Detector The crystal is NaI doped with 1%Tl (NaI/Tl). X-rays are absorbed by the crystal and raises electrons from the valence band to the conduction band in NaI. These electrons transfer energy to the Tl + ion. The excited Tl + returns to ground state and emits light (fluoresce at l = 420nm). A flash of light (scintillation) purple in color is produced in the crystal and is passed into a photomultiplier tube.

40 Scintillation Detector The photomultiplier tube is made up of a series (dynodes) of photocathodes.

41 Scintillation Detector The photocathodes are a photosensitive material made up of cesium-antimony intermetallic compound. Light strikes the 1st photocathode and electrons are ejected. These electrons are accelerated toward the next dynode by a potential difference (DV) Each dynode is 100V more positive than the proceeding one. As electrons hit the next dynode, more electrons are produced (multiplication). Last dynode is connected to a circuit.

42 Scintillation Detector This whole process takes less than a msec. So detector can handle rates of 10 5 counts/sec without loss. Advantage - efficient detector ~100% Disadvantage - energy resolution is not as good as the proportional detector or a solid state detector.

43 Solid-State Detector (Semiconductor detector) Made up of a single crystal consisting of a sandwich of intrinsic (pure) Si between a p-type layer (holes are carriers) and n-type layer (electrons are carriers). Forms a p-i-n diode.

44 Solid-State Detector The solid-state detector is made by taking Si (3-5 mm thick and 5-15 mm in diameter) that is lightly doped with boron (p-type). Li is applied to one face of the silicon and allowed to diffuse into the crystal at an elevated temperature. A gradient occurs, with one side higher in Li+ concentration than the other.

45 Solid-State Detector A bias is applied to create the p-i-n diode. When an x-ray photon hits the detector, electron-hole pairs are produced in the Si. These pairs are created when an energy of 3.8 ev (indirect gap of silicon) is exceeded. Number of electron-hole pairs, n equals: n = energy of the photon/energy required to create one pair For a CuKa photon, n = 8040/3.8 = 2116 pairs

46 Solid-State Detector The electron-hole pairs are swept to opposite poles by a bias, and the current is directed into a counting circuit. Advantage- excellent energy resolution, can resolve Ka and Kb Disadvantage must use bulky dewar to keep detector cool, long dead time, easy to overwhelm the detector.

47 Assignment HW Chapter 18 Due Today HW Chapter 8 and 9 Due 3/19/18 Read Chapter 12 Homework Chapter 12: 1-3, 9, 11 Homework Chapter 12 Due 03/21/18 Test III Friday Covers Lectures 14-19

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