Atomic Spectroscopy (chptr 20)

CHEM 310: Foundations of Analytical Chemistry

atomic spectroscopy

determination of elemental composition through evaluation of the characteristic electromagnetic spectrum of atoms

atomic emission spectroscopy (AES)

emission from a thermally populated state

atomic absorption spectroscopy (AAS)

absorption of sharp liens from a hollow cathode lamp

atomic fluorescence spectroscopy (AFS)

fluorescence following absorption of laser radiation

atomization

the process of converting an analyte in solid, liquid or solution form to a free gaseous atom; converting solution into vapor-phase free atoms

5 steps for atomization

1. nebulization

2. desolvation

3. liquefaction

4. vaporization

5. atomization

AAS: By measuring the amount of light absorbed, …

a quantitative determination of the amount of analyte element present can be made.

AAS: Absorption spectra of atoms are…

simpler and appear as sharp absorption lines because electronic states do no have vibrational or rotational levels.

AAS: Use of special light sources and careful selection of wavelength allows

the specific quantitative determination of individual elements in the presence of others.

AAS: Ease and speed to get precise and accurate measurements makes it ideal for…?

metal analysis!

AAS light source: the hollow cathode lamp

• inert gas atmosphere at a low pressure

• atoms of the gas are ionized in the electric discharge and accelerated to the cathode to dislodge atoms of the cathode metal (sputtering)

  • collisions with high energy electrons excites sputtered metal atoms

  • they relax to emit a line spectrum characteristic of the cathode metal

• typically limited to one metal

atomization by flame: pre-mixed burners

• mixes the fuel, oxidant, and sample before introducing the mixture to the flame

• sample is drawn into the nebulizer by the rapid oxidant (usually air) flow past the tip of the sample capillary

• spray is directed into a glass bead where droplets of sample is further broken into smaller particles

  • i.e. nebulization, forming an aerosol

• fuel also feeds into the chamber, sample is swept and introduced into the mixed fuel and oxidant gases in the burner

atomization by furnace: graphite furnace

• more sensitive than flame and uses less sample

• a few uL of sample is placed on graphite rod or within the depression of a tiny graphite crucible

  • solvent is evaporated by passing a current, temperature is increased very rapidly to 2000-3000C within a few seconds

• confines atomized sample in the optical path for several seconds (usually <1s)

  • leads to higher sensitivity

atomization by plasma: inductively coupled plasma (ICP)

• twice as hot as a combustion flame

• the high temperature, stable, and inert gas environment eliminate much of the interference found in the flame

• costs more to purchase and operate than a flame

AES operation

sample is subjected to a high thermal energy environment in order to produce excited state atoms, capable of emitting light

ideal AES source

• complete atomization of all elements

• controllable excitation energy

• sufficient excitation energy to excite all elements

• inert chemical environment

• no background

• accepts solutions, gases, and solids

• tolerant to various solution conditions and solvents

• simultaneous multi-element analysis

• reproducible atomization and excitation conditions

• accurate and precise analytical results

• inexpensive to maintain

• ease of operation

3 potential AES sources

1. flames

2. electric discharge (like arcs or sparks)

3. plasmas

AES source: flame

• background signals due to flame fuel and oxidants

• continuum emission from recombination reactions

  • like bands of atomic origin, these molecular bands are fairly broad:

    • H + OH —> H₂O + hv

    • CO + O —> CO₂ + hv

• flames used in AES nowadays are only used for a few elements

  • it’s cheap, but there are limitations

  • flame AES is often replaced by flame AAS

AES source: electric discharge, arcs and sparks

• arc - an electrical discharge between two or more conducting electrodes

• spark - an intermittent high-voltage discharge (few microseconds)

• limited to qualitative and semi-quantitative use

• particularly useful for solid samples (which get pressed into the electrode)

AES source: inductively coupled plasma

• long residence time results in high desolvation and volatilization rate

• high electron density suppresses ionization interference effects

• simultaneous multi-element analysis possible

• spectral interference is more likely for plasma than for flame due to the larger population of energetically higher states

  • background: Ar atomic lines

• Costs more than $50k

• operating costs are relatively high due to the Ar cost (10-15 mL/min) and training needed

comparison of atomic analysis methods

4 analytical interferences

1. spectral

2. ionization

3. chemical

4. matrix

spectral interference

= the overlap of analyte signal with signals due to other elements in the sample or signals due to the flame or furnace

• unwanted signals overlapping analyte signal

• solutions:

  • choose another wavelength for analysis

  • use high resolution spectrometers that are able to resolve closely spaced spectral lines

ionization interference

= desired signal is decreased due to ionization of analyte

ionization suppressants

• add an excess of an ionization suppressant to standards and samples

• there must be another Group I element which is more readily ionized, which will produce high concentrations of electrons that’ll suppress the ionization of your analyte

chemical interference

= caused by interaction with sample component that decreases atomization of analyte

• analyte metal forms a bond with an anion which won’t dissociate in the flame

  • so the ground state is never reached and absorbance doesn’t occur

  • usually the Group II elements (like Ca²⁺ and Mg²⁺) with phosphate

releasing agents

• add a releasing agent to standards and samples

  • it’ll bind the phosphate more strongly than Group II’s

• fuel rich flames can reduce certain oxidized analyte species that will be difficult to atomize

• higher flame temperatures eliminate many kinds of chemical interferences

matrix interference

= solvent effects; progressive enhancement or decrease in signal based on the nature of the solvent

• use method of standard addition

  • extrapolate backwards to the analyte concentration to automatically match matrix