Download - Chapter 4 1 atomic spectrocopy
-
7/28/2019 Chapter 4 1 atomic spectrocopy
1/34
Chapter 4
Atomic Spectroscopy
-
7/28/2019 Chapter 4 1 atomic spectrocopy
2/34
Optical spectroscopic methods
In Optical spectrometry, elements in thesample are converted to gaseous atoms orelementary ions by a process calledatomization. The absorption, emission, or
fluorescence of the atomic species in thevapor is then measured.
Determination of analytical sample
concentration Linear relationship between absorbance and
concentration
-
7/28/2019 Chapter 4 1 atomic spectrocopy
3/34
Optical spectroscopic methods
Atomic Absorption Spectroscopy
Atomic Emission Spectroscopy
Atomic Fluorescence Spectroscopy
-
7/28/2019 Chapter 4 1 atomic spectrocopy
4/34
Atomic Absorption Spectroscopy
Light Source Monochromator Detector Amplifier
E.g. Hollow
cathode lamp
Analyte solution
Atomiser Fuel (e.g. acetylene)Air
I0 It
Nebuliser, spray
chamber, and burner
-
7/28/2019 Chapter 4 1 atomic spectrocopy
5/34
How does it work?
The solution is aspirated into the flame as a fine spray
The solvent evaporates, leaving the dehydrated salt
The salt is dissociated into free gaseous atoms in the ground state
A certain fraction of these atoms absorbed energy from the flame
(some of them are collided each other) and be raised to the excited
electronic state
The excited state have a short lifetime and returned to the ground
state by emitting photon with characteristic wavelength,
The intensity of emission/absorption is directly proportional to theconc. of analyte in the solution, therefore a calibration curve of
emission/absorption intensity vs conc. is prepared
However, side reactions in the flame may decrease the population
of free atom and hence reduced the emission/absorption signal
hcE
-
7/28/2019 Chapter 4 1 atomic spectrocopy
6/34
Atomization
Desolvation and vaporization of ions or atoms
in a sample: high-temperature source such as
flame or graphite furnace
Flame atomic absorption spectroscopy
Graphite furnace atomic absorption spectroscopy
-
7/28/2019 Chapter 4 1 atomic spectrocopy
7/34
Flame atomic absorption spectroscopy
A flame provides a high-temperature source for desolvating and
vaporizing a sample to obtain free atoms for spectroscopic
analysis. In atomic absorption spectroscopy ground state atoms
are desired. For atomic emission spectroscopy the flame must
also excite the atoms to higher energy levels. The table lists
temperatures that can be achieved in some commonly usedflames.
Temperatures of some common flames
Fuel Oxidant Temperature (K)H2 Air 2000-2100
C2H2 Air 2100-2400
H2 O2 2600-2700
C2H2 N2O 2600-2800
-
7/28/2019 Chapter 4 1 atomic spectrocopy
8/34
Processes
In
Flame
M*excited
salt vapourised
*ATOMISED
*
Solution MXLiquid aerosol droplets
Salt mist of MXMolecules of MX
M
M+
ionised
MX
compound
formed
nebulisation
solvent evaporation
Dissociation
Thermal and chemical
-
7/28/2019 Chapter 4 1 atomic spectrocopy
9/34
Effect of temperature-boltzmann
distribution
-
7/28/2019 Chapter 4 1 atomic spectrocopy
10/34
-
7/28/2019 Chapter 4 1 atomic spectrocopy
11/34
Source of radiation
Hollow cathode lamps or electrode lessdischarge lamps
-
7/28/2019 Chapter 4 1 atomic spectrocopy
12/34
Hollow cathode lamp
Sources: hollow-cathode lamp (HCL), used in atomic absorbtion
Tube filled with inert gas (Ne or Ar)
Hollow cathode (negative) made with metal we want to detect
Run a high voltage between anode and cathode
This makes Ne or Ar ionize
Ne+ or Ar+ attracted to hollow metal cathode
As these ions hit the metal, atoms of metal are ejected into the gas
As metal atom interact with energetic electrons atoms are excited,
so generate light at that metals wavelength
since excitation is not by flame the linewidth is extra sharp
-
7/28/2019 Chapter 4 1 atomic spectrocopy
13/34
Monochromator
Isolation of the absorption line from
background light and from molecular
emissions originating in the flame (tuned to a
specific wavelength)
-
7/28/2019 Chapter 4 1 atomic spectrocopy
14/34
Detection
-
7/28/2019 Chapter 4 1 atomic spectrocopy
15/34
Calibration curve
Standards containing known concentrations of the analyte areintroduced into the instrument
Response is recorded
Response is corrected for instrument output obtained with a
blank Blank contains all of the components of the original sample except for
the analyte
Resulting data are then plotted to give a graph of correctedinstrument response vs. analyte concentration
An equation is developed for the calibration curve by a least-squares technique so that sample concentrations can becomputed directly
-
7/28/2019 Chapter 4 1 atomic spectrocopy
16/34
Detector
A photomultiplier measures the intensity ofthe incident light and generate an electrical
signal proportional to the intensity
-
7/28/2019 Chapter 4 1 atomic spectrocopy
17/34
Interference
Lowers the signal Chemical interference: formation of stable or
refractory compounds (not atomized at certain T)
Use higher temperature
Releasing agents, EDTA
Chelating agents, LaCl3
Ionization interference
Alkali metals easily ionized (low ionization energy) Energy level of ion lower than parent
Suppressed by adding elements that are easily
analyzed, CsCl when using Na or K
-
7/28/2019 Chapter 4 1 atomic spectrocopy
18/34
Disadvantages:
only solutions can be analysed
relatively large sample quantities required (1 2 mL)
problems with refractory elements
Advantages:
inexpensive (equipment, day-to-day running)
high sample throughput easy to use
high precision
-
7/28/2019 Chapter 4 1 atomic spectrocopy
19/34
-
7/28/2019 Chapter 4 1 atomic spectrocopy
20/34
Atomic Emission Spectroscopy
-
7/28/2019 Chapter 4 1 atomic spectrocopy
21/34
How it works?
A plasma source is used to dissociate the
sample into its constituent atoms or ions,
exciting them to a higher energy level.
They return to their ground state by emitting
photons of a characteristic wavelength
depending on the element present.
This light is recorded by an optical
spectrometer.
-
7/28/2019 Chapter 4 1 atomic spectrocopy
22/34
Plasma: atomization and excitation source A plasma is an electrically neutral,
highly ionized gas that consists of ions,
electrons, and atoms
A high-frequency current of 27.120MHz
is sent through a high-frequency coil,
generating a magnetic field. Plasma is then
formed from the gas (argon) flowing through
the coil. In the plasma, there are the samenumber of Ar+ and electrons, maintaining an
electrical balance.
Temperature reaching 10000K
Advantages over flame:
Lower inter-element interference (higher temperature With a single set of conditions signals for dozens of elements can be
recorded simultaneously
Monochromator
Detector
-
7/28/2019 Chapter 4 1 atomic spectrocopy
23/34
Interferences
two or more elements in the matrix emitting radiationat the same wavelength (e.g., Cu at 515.323 nm and Arat 515.139 nm). These spectral interferences can beminimized by using a high resolution system by using
several analytical lines for the detection of a singleelement
interference involving the formation of undesiredspecies (e.g., ions). It is important to note that an atom
of a specific element (e.g., Fe) has a different emissionspectra than one of its ions (e.g., Fe+, Fe+2, etc.)
-
7/28/2019 Chapter 4 1 atomic spectrocopy
24/34
Atomic Fluorescence Spectroscopy
Determination of single elements in analyticalsample
Determination of analytical sample concentration
Monitor the fluorescence emission from theexcited state
Atomic fluorescence spectra:
Resonance fluorescence
Radiationless transition + transition to ground state
Transition to lower state + radiationless transition
-
7/28/2019 Chapter 4 1 atomic spectrocopy
25/34
Atomic Fluorescence Spectroscopy
-
7/28/2019 Chapter 4 1 atomic spectrocopy
26/34
How it works?
the gaseous atoms obtained by flame orelectrothermal atomisation are excited to higherenergy levels by absorption of the
electromagnetic radiation and the fluorescenceemission from these excited atoms is measured.
This technique incorporates aspects of bothabsorption and emission.
Measure the fluorescence emission resultingfrom the relaxation of the excited atoms.
However, measurement is made 90o from source
-
7/28/2019 Chapter 4 1 atomic spectrocopy
27/34
-
7/28/2019 Chapter 4 1 atomic spectrocopy
28/34
Advantages and application
The main advantage of fluorescence detectioncompared to absorption measurements is the greatersensitivity achievable because the fluorescence signalhas a much lower background as compared to the one
observed in atomic absorption method Except for some favorable metals and metalloids (like
Pb, Cd, Tl, Se, Te, As, Sb, etc.) there is no specialadvantage over more established AAS. Thus, not
widespread use due to overwhelming success of AAS Applications: analysis of metals in lubricating oil,
seawater, biological substance, agriculture samples
-
7/28/2019 Chapter 4 1 atomic spectrocopy
29/34
Sample Preparation
The sample must be in the diluted form and
filtered for particulates Type of sample: blood, urine, tissues,
cerebral spinal fluid and other biologicalfluids by direct aspiration of the sample,
usually dilution with water is required toprevent clogging of the burner
In the preparation of standards, the matrix
of the analyte must always be matched, Eg.Analysing Zn in waste water, standardsolution is made up from ZnCl2
d d f h l ( b
-
7/28/2019 Chapter 4 1 atomic spectrocopy
30/34
Case study: determination of heavy metals (Pb, Cr,
As and K) in Selom plant
Raw Materials : Selom plant
Chemicals: nitric acid and hydrochloric acid with ratio of 3:1and distilled water
Procedure: dry the fresh Selom in the oven before ashingprocess to avoid unnecessary explosion
Burn the dry selom in the furnace at 500C for one hour
About 5 g of Selom ash is requiredbefore acid digestionprocess
The solution is then filtered and diluted to the requiredconcentration for AAS and/or ICP-OES analysis
Prepared the standard solution for each metal and obtained
their calibration curve From the regresion equation the concentration of each
element can be calculated
-
7/28/2019 Chapter 4 1 atomic spectrocopy
31/34
Ashing method
placingthe sample in an open inert vesseland destroying the combustible (organic)
portion of the sample by thermal
decomposition using a muffle furnace.Typical ashing temperatures are 450 to 550
C. Magnesium nitrate is commonly used as
an ashing aid. Charring the sample prior to
muffling is preferred. Charring is
accomplished using an open flame.
A id di ti /
-
7/28/2019 Chapter 4 1 atomic spectrocopy
32/34
Acid digestion/pressure
digestion/microwave digestion
Acid digestion process are employed for thedetermination of elements in solidsubsequent to sampling and mechanical
sample preparation in order to completelytransfer the analytes into solution so that itcan be introduced to analysis instrument- ICP-OES, AAS and ICP-MS
Common acid used, mineral acids (HCl, HNO3,HF, H2SO4 etc)
Representatives Detectin Limit by AAS and Flame
-
7/28/2019 Chapter 4 1 atomic spectrocopy
33/34
Representatives Detectin Limit by AAS and Flame
Emission Spectroscopy, FES/ICP
Element Wavelength (nm)Detection Limit (ppm)
AAS FES (ICP)
Ag 328.1 0.001(A) 0.01
Al 309.3 0.1 (N)
Au396.2 0.08
242.8 0.03 (N)
Ca267.7 3
422.7 0.003 (A) 0.0003
Cu 324.8 0.006 (A) 0.01
Eu 459.4 0.06 (N) 0.0008Hg 253.6 0.8 (A) 15
K 766.5 0.004 (A) 0.00008
Mg 285.2 0.004 (A) 0.1
Na 589 0.001 (A) 0.0008
Tl276.8 0.03 (A)
535 0.03
Zn 213.9 0.001(A) 15
Detection Limit (ppm): The conc required to give a signal equal to three times the
standard deviation of base line (Blank)
< 300 nm: AAS shows superior detectability because high thermal energy required
to excite the atom for emission at these wavelength
300 < < 400 nm: either method exhibit comparable detectability
-
7/28/2019 Chapter 4 1 atomic spectrocopy
34/34