W7 Atomic Spectroscopy Flashcards

1
Q

components of absorption atomic spectroscopy (AAS)

A

hollow cathode lamp to provide source of radiation that the atoms absorb

flame to break down sample into atomic state

monochromator

detector

amplifier

readout device

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2
Q

features of hollow cathode lamp

A

lamp must have lines narrower than 10^-3 to 10^-2 nm

high voltage across electrodes causes ionisation of Ar to Ar+ and e-

Ar+ stress the cathode and dislodges metal atoms (sputtering)

sputtered metal atoms collide with high energy electrons and are excited

excited atoms emit photons that have the same frequency as photons absorbed by analyte

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3
Q

first step of AAS

A

atomise sample to give an atomic gas

nebuliser introduces a sample in the form of fine spray of droplets called aerosol

sample spray mixes with the oxidant and fuel

flow spoilers or baffles block large droplets of liquid

only 5% of the initial sample reaches the flame/burner

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4
Q

what happens after the sample reaches the flame

A

desolvation occurs in primary combustion zone

particles vaporised and converted to gaseous atoms, elementary ions and molecular species in the internal region

oxides and hydroxides are formed at the outer cone

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5
Q

what gases are usually used for the fuel in the flame for readily excited heavy metal species

A

oxygen and N2O

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6
Q

difference in initial excitation between AAS and AES

A

AAS: use hollow cathode lamp

AES: occurs via thermal excitation, where temperature of a plasma source is used to excite the atoms

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7
Q

what is the source of flame for AES

A

inductively coupled plasma (ICP)

plasma is a hot, partially ionised gas containing relatively high concentrations of ions and electrons

audio-frequency power sources give ICP

temperature of 6000 to 10000 K is maintained in the plasma

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8
Q

advantages of ICP in AES

A

high temperature ensures that desolation and vaporisation are essentially complete and high atomisation is attained

atmosphere of Ar is chemically inert

temperature cross section of the flame is relatively uniform

optical path length is thin

can measure a large number of elements simultaneously

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9
Q

how does temperature affect atomic spectroscopy

A

varying the temperature by relatively small amounts does not affect the ground state population but affects the excited state population

atomic absorption is not as sensitive to temperature as atomic emission

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10
Q

what is natural broadening

A

governed by Heisenberg uncertainty principle

states that the shorter the lifetime of the excited state, the more uncertain is its energy

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11
Q

what is collisional broadening

A

occurs when atoms/molecules collide in a gas > energy levels of atoms get distributed > emitted/ absorbed light spread out over range of wavelength

more collisions > shorter lifetime in excited state

broadening increases with increasing temperature bc more collisions

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12
Q

what is doppler broadening

A

atoms move towards detector > detector detects radiation of higher frequency (shorter wavelength)

atoms move away from detector > detector detects radiation of lower frequency (longer wavelength)

atoms/molecules have distribution of speeds > emitted/absorbed light covers range of wavelength > broadening effect

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