Transcript Atomic-absorption (AA) spectroscopy

Atomic-Absorption (AA)
Spectroscopy
Overview

Excitation and Decay: Interplay of electron state
and photons
Emission
 Absorption


Instrument Components
Double beam configuration
 Light source
 Production of free atoms




Graphite furnace
Flame
Detector
Excitation and Decay: Interplay
of electron state and photons
– decay from high energy state
 Absorption – excitation to high energy state
 Two fundamentally different ways of
quantifying the concentration of an element!
 Emission
 Emission
– flame provides energy to excite
atoms
 Absorption – Gas phase atoms absorb photons
from light source
 Narrow
band! (unlike molecular absorption)
Instrument Schematic
Reference Beam
Flame
Lamp
Chopper
V ref
V
V sample
t
Light Source: Hollow Cathode
Lamp
Power Supply
+
-
Cup made of
metal of interest
window
anode
light
cathode
 The electric potential ionizes rare gas atoms and
accelerates them into the cathode where they
sputter metal atoms into the gas phase
 Collisions with gas atoms or electrons excite the
metal atoms
 On decay the metal atoms emit light
Atomization
Atomic absorption and emission requires gasphase atoms
 Liquid and solid samples must be vaporized

Flame
 graphite furnace

The gas-phase atoms absorb ultraviolet or visible
light and make transitions to higher electronic
energy levels.
 What about the inevitable subsequent emission of
photons on decay?

Detector
 Photo
multiplier tube
 Extremely sensitive
 Can detect single photons
 Produce a small current that is proportional
to the number of photons
 Op-amps produce a voltage that is then
digitized for analysis.
Flame
 Different
temperatures required for different
elements
 Air-Acetylene flame
 Preferred
flame for 35 elements
 Temperature of 2300 C
 Nitrous
Oxide-Acetylene flame
 Temperature
 Optimal
of 2900 C
temperature produces free gas
phase atoms but doesn’t ________
ionize them
Calibration
 ________
Beer’s
Law
 Non linearity (according to Perkin Elmer)
 Stray
light
 Non homogeneities of temperature and space in
the absorbing cell
 Line broadening
 Absorption at nearby lines
Getting a Concentration
 Flame
 Measures
something proportional to the
______________
concentration of the sample
 Analysis in the mg/L range (ppm)
 Graphite
Furnace
 Measures
a _______
mass
 Analysis in the mg/L range (ppb)