chapter 4 1 atomic spectrocopy

7/28/2019 Chapter 4 1 atomic spectrocopy

1/34

Chapter 4

Atomic Spectroscopy


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


3/34

Optical spectroscopic methods

Atomic Absorption Spectroscopy

Atomic Emission Spectroscopy

Atomic Fluorescence Spectroscopy


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


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


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


7/34

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


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


9/34

Effect of temperature-boltzmann

distribution


10/34


11/34

Source of radiation

Hollow cathode lamps or electrode lessdischarge lamps


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


13/34

Monochromator

Isolation of the absorption line from

background light and from molecular

emissions originating in the flame (tuned to a

specific wavelength)


14/34

Detection


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


16/34

Detector

A photomultiplier measures the intensity ofthe incident light and generate an electrical

signal proportional to the intensity


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


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


19/34


20/34

Atomic Emission Spectroscopy


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.


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


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.)


24/34


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


25/34



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


27/34


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


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


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


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 /


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


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


34/34

chapter 4 1 atomic spectrocopy

Documents