W7 Atomic Spectroscopy Flashcards
types of atomic spectroscopy
absorption of sharp lines from hollow cathode lamp
emission from a thermally populated excited state
emission from an excited state due to initial absorption of radiation
components of absorption atomic spectroscopy (AAS)
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
features of hollow cathode lamp
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
first step of AAS
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
what happens after the sample reaches the flame
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
what gases are usually used for the fuel in the flame for readily excited heavy metal species
oxygen and N2O
difference in initial excitation between AAS and AES
AAS: use hollow cathode lamp
AES: occurs via thermal excitation, where temperature of a plasma source is used to excite the atoms
what is the source of flame for AES
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
advantages of ICP in AES
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
how does temperature affect atomic spectroscopy
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
what is natural broadening
governed by Heisenberg uncertainty principle
states that the shorter the lifetime of the excited state, the more uncertain is its energy
