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Instrumental Analysis Noa > CH08 - X Ray Spectroscopy > Flashcards

CH08 - X Ray Spectroscopy Flashcards

XRF, XRA, XRE

1
Q

Every day life

A

Security, Medicine, Astronomy, Chemistry

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2
Q

The emission maxima depends on…

A

the element (exclusively)

Emission spectrum results from transitions of internal electrons. Therefore it does not depend on ionization state (unlike AES) or bonding.

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3
Q

Incoming radiation hits which electron?

A

the lowest energy electron (K)

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4
Q

what happens to this electron?

A

it gets kicked out

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5
Q

formula for change in E

A

change in E= E-E0

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6
Q

E0 is

A

K orbital (lowest energy)

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7
Q

E in E-E0 is

A

the incoming radiation

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8
Q

What happens to E0 when it is kicked out?

A

Another electron from a higher energy will take its place

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9
Q

formula for K alpha line energy

A

change in E= E1-E0 = K alpha line energy

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10
Q

The first xray emission line is from

A

Relaxation of whichever electron is taking the place of the electron being kicked out

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11
Q

The second E in E-E0 is…

A

Whichever orbital the electron being kicked off is located

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12
Q

Remembering E formula

A

Where it’s currently at - where its going ,= letter orbital where its going

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13
Q

Moseley’s law

A

The square root of the frequency of the x-ray emitted by an atom is proportional to its atomic number

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14
Q

Xray Absorption formula

A

I=I0e ^ -umpx

“Beer’s law” for XRA

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15
Q

Meaning of I in absorption formula?

A

transmitted power

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16
Q

Meaning of I0 in absorption intensity formula?

A

incident power

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17
Q

um=

A

mass absorption coefficient (units: (cm2)/gr)
- Characteristic value depends on element and absorbed wavelength.
- Does not depend on chemical or physical state of element.

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18
Q

p=
(It’s actually “rho”)

A

sample density (units: gr/cm)

“rho”

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19
Q

x=

A

sample thickness (units:cm)

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20
Q

Fluorescence

A

sample placed in a beam of Xrays. Some absorbed.

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21
Q

qualitative analysis with XRF?

A

xrays of characteristic wavelengths emiited

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22
Q

quantiative analysis with XRF?

A

Emission intensity proportional to element concentration

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23
Q

Bragg Equation

A

nλ=2dsinθ

n - differaction order
lamba - x ray wavelength
d - interplanar spacing
theta - x-ray angle of incidence

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24
Q

unit cell

A

smallest volume element thats completely represenative of the whole crystal

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25
Q

Miller Indices

A

integers of any family of parallel planes. the integers are recipricals of the coordinates in which the planes intersect x, y and z axes.

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26
Q

Given the set of Miller indices (4,4,0), (3,3,0), (2,2,0) for parallel crystal planes, which is the closest to the origin of
the set of planes?

A

(4,4,0), because indices are reciprocals of actual intersection points on axes.

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27
Q

essential components

A

source of monochromatic xrays (Cu anode),sample holder (spins),detector,data recording & processing

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28
Q

goniometer

A

heart of diffractometer\XRF spectrometer. keeps source, sample, & detector in correct relative positions & permits scattering angle to be controlled

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29
Q

xray tubes must have

A

Source of electrons,high voltage source to accelerate electrons ,metal target. components are encased in a vacuum tube.

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30
Q

filament tubes cathode filament

A

tungsten

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31
Q

filament tubes anode

A

Cu

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32
Q

filament tubes - what emits xrays

A

focal spot on anode

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33
Q

how do filament tubes work

A
  1. The cathode (filament) is heated and emits electrons by thermionic emission.
  2. Electrons are accelerated toward cathode by a sufficiently high voltage between the cathode and anode (2-100kV)
  3. Upon striking it, the electrons
    give up their energy at the metallic surface of the anode. If the electrons have been accelerated
    to a high enough velocity, energy
    is released as X-rays (0.1–100 Å)
  4. xray escape from windows made of material of low coefficient of absorption (Be, Al, mica)

2-100kV is the voltage range that can emit electrons. tube voltage is usually 4-50kV.

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34
Q

filament tubes disadvs

A

expensive, limited life time

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35
Q

filament tubes operate at approx

A

4-50kv

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36
Q

Kb & Ka

A

numerical subscripts relate to relative intensities ,Kb never resolved, only Ka noticeable ,Kb removed by filter

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37
Q

what b filter used for Cu tube

A

Ni b filter

38
Q

what is used as a filter

A

the element preceding the target element in the periodic table

39
Q

beta filter may be inserted in

A

either primary beam or secondary beam

40
Q

backscattered electrons - the higher the atomic number of the atoms,

A

the more backscattered electrons are bounced back out

41
Q

xray spectrum shows distribution of xray intensity as a function of either…

A

wavelength - wavelength dispersive spectrum WDS,or quantum energy - energy dispersive spectrum EDS

42
Q

wavelength dispersive spectrometers

A

radiation source -> sample -> wavelength selector -> detector

43
Q

energy dispersive spectroscopy

A

radiation source -> sample -> detector

44
Q

EDS

A

quick & simple analysis & data collection ,complete spectrum of energies obtained simultaneously

45
Q

WDS

A

spectrum acquired sequentially as full wavelength range is scanned ,takes longer

46
Q

ED detectors advs

A

fast qualitative analysis ,analyses all elements at once ,less expensive, simpler

47
Q

ED detectors disadvs

A

low count rates,poor resolution ,limited detection limits,poor light element detection

48
Q

WD detectors advs

A

high count rates,good resolution,good detection limits ,excellent quantitative analysis & light element detection

49
Q

WD detectors disadvs

A

slow qualitative analysis ,analyses 1/few elements at a time ,more expensive & complex

50
Q

WDS uses a crystal of

A

known d spacing to disperse xrays according to their wavelengths (analyzing crystal). Serves as a monochromator for X Rays.

51
Q

WDS crystal examples

A

LiF,PET,quartz

52
Q

WDS detector examples

A

scintillation counter ,sealed or gas flow proportional counter (the latter are gas filled detectors)

53
Q

EDS uses charge generated by

A

xray photons within a semiconductor wafer (Si(Li) or Ge type)

54
Q

what does EDS require

A

pulse processor ,multi channel analyser MCA

55
Q

EDS - some detectors need to be

A

cooled with liquid Ni or He to function correctly

56
Q

line spectrum

A

pattern of variably intense emissions at definite wavelengths

57
Q

characteristic xrays

A

xrays forming part of a line spectrum of an element

Manifests as sharp lines on brehmsstrahlung.

58
Q

real xray spectra combine

A

bremsstrahlung with characteristic xrays

59
Q

characteristic xrays result when

A

beam electrons eject inner shell electrons of specimen atoms

60
Q

continuum (bremsstrahlung) xrays result when

A

beam electrons interact with nucleus of specimen atoms.

Each electron generally
undergoes a series of collisions.

Each collision resulting in a photon of slightly different energy, creating emissions on a continuum of wavelengths.

Each electron

61
Q

EDS plot - duane-hunt limit

A

high energy value where continuum goes to 0

Continuum radiaiton intensity vs wavelength (or energy) plot, like figure 8.11 (a)

62
Q

bremsstrahlung

A

braking radiation, loss in energy as electron brakes emitted as photon

63
Q

Differences between XRF and XRE

A

XRF = X Ray Fluorescence -Electrons are ejected using radiation (X-ray of higher energy).
XRE = X Ray Emission
-Electrons are ejected by incident particles such as high energy electrons and ions (do they have to be charged?)

64
Q

K Line

A

X-ray emission lines from electron transitions terminating in the K shell.

Similar definition exists for L shell

65
Q

Auger Electron

A

The energy released when a core electron vacancy is filled knocks an electron out of the M shell. This electron is called an Auger Electron.

66
Q

Origin of absorption edges?

A
  1. As the wavelength of the X-ray decreases, its energy increases, its penetrating power increases, and the percent absorption decreases (downward slope).
  2. As the wavelength
    decreases further, the X-ray eventually has sufficient energy to displace electrons from the K shell => abrupt increase in absorption => K absorption edge is observed.
67
Q

Goniometer setup for WDXRF\XRD\XRA?

A

Analyzing crystal angle: θ
Detector angle: 2θ

68
Q

The wavelengths of the absorption edges and of the corresponding emission lines do not coincide. Why? Which one is longer?

A
  • energy required to dislodge an electron from an atom (absorption edge energy) is greater than the energy released when the dislodged electron is replaced by anouter shell electron (emitted X-ray energy).
  • (absorption edge wavelength) < (emmited X-ray wavelength)
69
Q

What is a primary X-ray beam?

A

In XRF, the beam of exciting X-rays is called the primary beam

70
Q

What is a secondary X-ray beam?

A

In XRF, the X-rays emitted from the sample (as a result of excitation by a primary x-ray beam) are called secondary X-rays

71
Q

The primary X-ray beam must have a λmin that is ____
than the absorption edge of the element to be excited.

A

Shorter - so it is energetic enough to eject an electron (be absorbed).

72
Q

XRD (X Ray Differaction)

A

Determines crystal structure from diffraction pattern of X-Rays from a crystal.

73
Q

Name 3 X-Ray Sources

A
  1. X-ray tube (filament tube): Most common source in XRD, XRF. Beam of high-energy electrons. Produces a broadband continuum of x-radiation (brehmsstrahlung from impact with a metal target) and element-specific X-ray lines
    (from ejecting inner core electrons from target metal)
  2. “XRF source”: a primary X-ray beam to eject inner core electrons to produce a secondary X-ray beam.
  3. Gamma rays from a radioactive isotope: the gamma rays are high energy X-rays.
74
Q

Mosley’s law describes the relationship between ______ number and wavelengths of ______, while Duane-Hunt law describes minimum wavelength of _____ as a function of _____.

A
  • Atomic Number
  • Characteristic Lines
  • All emmited X rays (including background brehmsstrahlung)
  • Accelerating voltage
75
Q

In XRF analysis, the target element of the primary source should have a _____ atomic number than the elements being examined in the sample

A

Greater
(Sample atoms are excited by characteristic lines, because those are more intense. Wavelengths of characteristic lines depend on Z, according to Moseley’s law)

76
Q

In XRF, sample atoms are excited by _____, because those are more intense.

A

characteristic lines

77
Q

In Moseley’s law, Why is the wavelength inversely dependent of Z?

A

Greater Z => more protons in nucleus => electrons are pulled down by a stronger force toward the nucleus => K_alpha transition emits more energy (shorter photon wavelength)

78
Q

How does a gas filled X ray detector work?

A
  1. X Rays fall on He gas in the tube, causing the following interaction:
    hν+He→He+ + e- + hν
  2. The ejected photoelectron has a very high kinetic energy (which is proportional to the X Ray wavelength).
  3. The photoelectron loses this kinetic energy by colliding with and ionizing many additional gas molecules as it moves to
    the center wire (by applied potential).
  4. The number of electrons created in 4 is proportional to the electron’s kinetic energy and therefore to the photon’s energy.
    - The gas filled tube is operated within the “proportional counter” voltage range, where the current pulse is proportional to the energy of the incoming X-ray photon (in the “geiger counter zone” it is no longer proportional).
79
Q

What are escape peaks (in a gas filled proportional counter)?

A
  1. An X ray photon ejects an inner shell electron of the filler gas and loses some of its energy (instead of transferring it as kinetic energy to photoelectrons).
  2. As a result, the photon creates an artefact peak at a wavelength corresponding to the energy:
    E’=E(incoming X ray)-E(filler gas characteristic X Ray)
    This peak is called an Escape Peak
80
Q

How does a scintillation detector work?

A

(1) formation of a photoelectron in the NaI(Tl) crystal after an X-ray photon hits the crystal
(2) emission of visible light photons from an excited state in the crystal, (3) production of photoelectrons from the cathode in the photomultiplier
(4) electron multiplication.

81
Q

Why is a PMT detector not enough for X rays?

A

PMT’s do not react to wavelengths in X ray (only UV Vis). The X rays must be first converted to UV Vis by exciting crystal electrons (NaITl crystal), as in a scintillation detector.

82
Q

What are sum peaks in EXRF and what causes them?

A

A single peak is registered at an energy that is the sum of the two peaks.

Cause: two high-intensity photons arrive too close in time, signal processing electronics cannot separate them.

83
Q

What causes escape peaks in EDXRF?

A

X rays from fluorescing analyte ejects an electron from K level of detector element (e.g. Si), “loses” part of its energy and creates an absorption line at lambda(analyte element)-lambda(detector element)

84
Q

In XRA, X-rays emitted from a particular element will be absorbed by elements with ______ atomic number.

A

A lower

85
Q

In XRD, it is possible to draw an
infinite number of crystal planes in three dimensions, but only those planes with ____ on
them reflect X-rays.

A

electron density
(reflections occur as a result of interactions between incident EM radiation and electrons on reflecting surface)

86
Q

_____ can be used to measure d, the distance between planes of electron density
in crystals and is the basis of X-ray crystallography.

A

Bragg equation nλ=2dsinθ

87
Q

Can you use XRD to characterize structure of liquid, gases and amorphous polymers?

A

No, because these lack an ordered structure.

88
Q

In XRF, Energy of the emitted X-ray is independent of _____ of the element; therefore, XRF is
generally considered to be an ______.

A
  • The chemical state
  • Elemental analysis method

In general, X rays wavelengths are indepnendent of the chemical state of the atom..

89
Q

Low-Z elements (3< Z < 11) such as Be cannot be analyzed in ____.

A

Air (Low Z=>long fluorescence lambda=> X-rays absorbed by air)

90
Q

XRF allows _____ analysis.

A

Multielement

91
Q

In multielement analysis, why is it important to have a high acceleration voltage (as high as 50kV) in X ray tube?

A

High acceleration voltage => electrons which excite target atoms are higher in energy => target atoms emit X rays in broader emission spectra

Not all of the electrons’ energies go to the target atoms so it’s better to set them with a high energy to begin with…

Instrumental Analysis Noa (7 decks)

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