CHAPTER 4: ATOMIC SPECTROSCOPY

General3 major type of spectrometric method

of identifying elements in the samplesi. Optical spectrometryii. Mass spectrometryiii. X-ray spectrometry

In Optical spectrometry, elements in the sample are converted to gaseous atoms or elementry ions by a process called atomization. The absorption, emission, or fluorescence of the atomic species in the vapour is then measured.

Optical spectrophotometryCommoni. Atomic absorption spectroscopy , Eg. AAS instrumentii. Atomic emission spectroscopy, ICP-OESiii. Atomic fluorescence spectroscopy, AFS

Note: Atomization is usually achieved either by flames, electrically heating or by plasmas.

Process Occurring in Flame (Gary D. Christian 6th ed, pg 522)

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,

hcE

Continue….

Eg. characteristic wavelength of elements:

The intensity of emission/absorption is directly proportional to the conc. 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

Elements (nm)Na 589Ca 422Zn 213

Continue….

There are three fundamental processes that can occur in the atomic level.

Eo

Ej

Atomic emission Atomic absorption(from ground state)

Ej

Ei

Ej

Ei

Atomic emission(thermal excitiation)

Atomic flourescence(light excitiation)

ProcessesIn Flame

M*excited

salt vapourised

*ATOMISED *

Solution MX

Liquid aerosol droplets

Salt mist of MX

Molecules of MX

M

M+ionised

MXcompoundformed

nebulisation

solvent evaporation

DissociationThermal and chemical

Characteristic wavelength, transition elements

Distribution between ground and excited states- Most atoms are in the ground state

The relatives populations of ground-states N0 and excited-state Ne at a given flame temperature via the Maxwell-Boltzmann Equation

kT

E

o

e

o

e egg

NN

ge and g0 are the statistical weights of the excited and ground stateE is the difference in energy between the excited and lower or ground statek the Boltzmanns Constant 1.38062 x 10-23J K-1 T is the temperature in K

Therefore, the more atoms in the excited state the higher the intensity of the emission.

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) 0.01Al 309.3 0.1 (N)

Au 396.2 0.08242.8 0.03 (N)

Ca 267.7 3422.7 0.003 (A) 0.0003

Cu 324.8 0.006 (A) 0.01Eu 459.4 0.06 (N) 0.0008Hg 253.6 0.8 (A) 15K 766.5 0.004 (A) 0.00008

Mg 285.2 0.004 (A) 0.1Na 589 0.001 (A) 0.0008

Tl 276.8 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 wavelength300 < < 400 nm: either method exhibit comparable detectability

Energy level diagram illustrating energy changes associated with absorption of electromagnetic radiation

A = pure rotational changer (far IR)B = rotational + vibrational changes (near IR)C = rotational + vibrational + electronic transition (Vis + UV)Eo = electronic ground stateE1 = first electronic exited state

Calibration CurveStandards containing known concentrations

of the analyte are introduced into the instrument

Response is recordedResponse is corrected for instrument output

obtained with a blank◦ Blank contains all of the components of the original

sample except for the analyteResulting data are then plotted to give a

graph of corrected instrument response vs. analyte concentration

An equation is developed for the calibration curve by a least-squares technique so that sample concentrations can be computed directly

Atomic Absorption Spectrophotometer- Schematic Diagram For a Double Beam Instrument

MONOCHROMATOR PHOTODETECTOR

AMPLIFIER ANDREADOUT

LIGHTSOURCE

flame

SampleChoppe

r• Double beam instrument that measures the ratio P0/P (Beer Lambert Law)•The source beam is alternately sent through the flame and around the flame by the chopper• The detector measures these alternately and the logarithm of the ratio is displayed• The detector amplifier is tuned to receive only radiation modulated at the frequency of the chopper and the radiation emitted by the flame is recorded (characteristic of individual element)

Major component:

Atomic Absorption Spectrophotometer- Major Componet

A. Sources: hollow-cathode lamp (HCL), used in atomic absorbtionTube filled with inert gas (Ne or Ar)Hollow cathode (negative) made with metal we want to detectRun a high voltage between anode and cathode

This makes Ne or Ar ionizeNe+ or Ar+ attracted to hollow metal cathodeAs 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 wavelengthsince excitation is not by flame the linewidth is extra sharp

Note: Flame used for atomization only not to excited the electron of the free atom

Atomic Absorption Spectrophotometer- Major Componet

B. Burner: pre-mix burner containing of nebulizer, chamber and burner, desolvation/drying process

C. Flame: 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 used flames.

Temperatures of some common flamesFuel Oxidant Temperature (K)H2 Air 2000-2100C2H2 Air 2100-2400H2 O2 2600-2700C2H2 N2O 2600-2800

Sample PreparationThe sample must be in the diluted form

and filtered for particulatesType of sample: blood, urine, tissues,

cerebral spinal fluid and other biological fluids by direct aspiration of the sample, usually dilution with water is required to prevent clogging of the burner

In the preparation of standards, the matrix of the analyte must always be matched, Eg. Analysing Zn in waste water, standard solution is made up from ZnCl2

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 3:1 and distilled water Procedure: dry the fresh Selom in the oven before

ashing process to avoid unnecessary explosion Burn the dry selom in the furnace at 500C for one

hour About 5 g of Selom ash is required before acid

digestion process The solution is then filtered and diluted to the

required concentration 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

Ashing  methodplacingthe sample in an open inert vessel and 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.

Acid digestion/pressure digestion/microwave digestion

Acid digestion process are employed for the determination of elements in solid subsequent to sampling and mechanical sample preparation in order to completely transfer the analytes into solution so that it can be introduced to analysis instrument- ICP-OES, AAS and ICP-MS

Common acid used, mineral acids (HCl, HNO3, HF, H2SO4 etc)