Atomic spectroscopy methods

Atomic spectroscopy methods are based on light absorption and emission of atoms in the gas phase. The goal is elemental analysis - identity and concentration

of a specific element in the sample; chemical and structural information are lost. The sample is destroyed.

Design of instrumentation to probe a material

Signal Generation-sample excitation

Input transducer-detection of analytical signal

Signal modifier-separation of signals or amplification

Output transducer-translation & interpretation

Characterization of Properties

chemical state

structure

orientation

interactions

general properties

Molecular Methods

macro Vs micro

pure samples Vs mixtures

qualitative Vs quantitative

surface Vs bulk

large molecules (polymers, biomolecules)

Elemental Analysis

bulk, micro, contamination (matrix)

matrix effects

qualitative Vs quantitative

complete or specific element

chemical state

Techniques for reducing matrix effects include:

1. Matrix substitution - dissolving sample into liquid or gas solution, grinding sample with KBr powder.

2. Separation - using chromatography, solvent extraction, etc. to isolate analyte from complex matrix.

3. Preconcentration - collecting the analyte from sample into a much smaller volume to raise its concentration.

4. Derivatization - chemically modifying the analyte to improve volatility, light absorption, complex formation, etc., so that the instrument can more easily measure concentration.

5. Masking - modifying interferences so that they are no longer detected by the instrument.

Extreme trace elemental analysis

Direct instrumental determination - multi-element - direct excitation---should be least expensive

These are relative physical methods requiring appropriate standards & systematic errors like spectral interferences occur

NAA, XRF, sputtered neutral MS

Extreme trace elemental analysis

Multi-stage procedures --- sample separation and preparation before quantitation

Standards are less of a problem

Time consuming & subject to losses or contamination

Chromatography coupled with analysis

Molecular Spectroscopy
IR, UV-Vis, MS, NMR

What are interactions with radiation

Means of excitation (light sources)

Separation of signals (dispersion)

Detection (heat, excitation, ionization)

Interpretation (qualitative easier than quantitative)

Techniques

spectroscopy (UV, IR, AA)

NMR

mass spectrometry

chromatography (GC, HPLC)

measure radioactivity, crystallography, PCR, gas phase analysis

Reason to understand how an instrument works

What results can be obtained

What kind of materials can be characterized

Where can errors arise

Atomic spectroscopy

Outer shell electrons excited to higher energy levels

Many lines per atom (50 for small metals over 5000 for larger metals)

Lines very sharp (inherent linewidth of 0.00001 nm)

Collisional and Doppler broadening (0.003 nm)

Strong characteristic transitions

Atomic spectroscopy for analysis

Flame emission - heated atoms emit characteristic light

Electrical or discharge emission - higher energy sources with more lines

Atomic absorption - light absorbed by neutral atoms

Atomic fluorescence - light used to excite atom then similar to FES

Slide 14

General issues with flames

Turbulence / stability / reproducibility

Fuel rich mixtures more reducing to prevent refractory formation

High temperature reduces oxide interferences but decreases ground state population of neutrals (fluctuations are critical)

Inductively Coupled Plasma

AA Instrument Schematic

Atomic Absorption

AA instrumentation

Radiation source (hollow cathode lamps)

Optics (get light through ground state atoms and into monochromator)

Ground state reservoir (flame or electrothermal)

Monochromator

Detector , signal manipulation and readout device

Hollow Cathode Lamp

Emission is from elements in cathode that have been sputtered off into gas phase

Light Source

Hollow Cathode Lamp - seldom used, expensive, low intensity

Electrodeless Discharge Lamp - most used source, but hard to produce, so its use has declined

Xenon Arc Lamp - used in multielement analysis

Lasers - high intensity, narrow spectral bandwidth, less scatter, can excite down to 220 nm wavelengths, but expensive

Atomizers

Flame Atomizers - rate at which sample is introduced into flame and where the sample is introduced are important

AA - Flame atomization

Use liquids and nebulizer

Slot burners to get large optical path

Flame temperatures varied by gas composition

Molecular emission background (correction devices )

Sources of error

solvent viscosity

temperature and solvent evaporation

formation of refractory compounds

chemical (ionization, vaporization)

salts scatter light

molecular absorption

spectral lines overlap

background emission

Atomizers

Flame Atomizers - rate at which sample is introduced into flame and where the sample is introduced is important

Graphite Furnace Atomizers - used if sample is too small for atomization, provides reducing environment for oxidizing agents - small volume of sample is evaporated at low temperature and then ashed at higher temperature in an electrically heated graphite cup. After ashing, the current is increased and the sample is atomized

Electrothermal atomization

Graphite furnace (rod or tube)

Small volumes measured, solvent evaporated, ash, sample flash volatilized into flowing gas

Pyrolitic graphite to reduce memory effect

Hydride generator

Graphite Furnace

Graphite Furnace AA

Closeup of graphite furnace

Controls for graphite furnace

Detector

Photomultiplier Tube

has an active surface which is capable of absorbing radiation

absorbed energy causes emission of electrons and development of a photocurrent

encased in glass which absorbs light

Charge Coupled Device

made up of semiconductor capacitors on a silicon chip, expensive

Background corrections

Two lines (for flame)

Deuterium lamp

Smith-Hieftje (increase current to broaden line)

Zeeman effect (splitting of lines in a strong magnetic field)

Atomic Absorption

Assumptions: (i) Beer's law holds for the atoms in the flame or graphite furnace, and (ii) the concentration

of atoms in the flame or furnace is proportional to the concentration of analyte in the sample.

Calculations: The usual calibration curves or standard addition problems.

Beer’s Law

A = e bC (Beer’s Law)

where e = molar absorptivity (units M^-1cm^-1 ); b = sample thickness (cell pathlength) in cm; and C = conc. in M (mol/L). , is a property of the analyte and of wavelength; identification of the analyte

(qualitative analysis) is possible from the spectrum (e vs 8). Note that the sensitivity m is equal to e b.

Problems with Technique

Precision and accuracy are highly dependent on the atomization step

Light source

molecules, atoms, and ions are all in heated medium thus producing three different atomic emission spectra

Problems continued

Line broadening occurs due to the uncertainty principle

limit to measurement of exact lifetime and frequency, or exact position and momentum

Temperature

increases the efficiency and the total number of atoms in the vapor

but also increases line broadening since the atomic particles move faster.

increases the total amount of ions in the gas and thus changes the concentration of the unionized atom

Interferences

If the matrix emission overlaps or lies too close to the emission of the sample, problems occur (decrease in resolution)

This type of matrix effect is rare in hollow cathode sources since the intensity is so low

Oxides exhibit broad band absorptions and can scatter radiation thus interfering with signal detection

If the sample contains organic solvents, scattering occurs due to the carbonaceous particles left from the organic matrix


	Atomic spectroscopy methods are based on light absorption and emission of atoms in the gas phase. The goal is elemental analysis - identity and concentration
	of a specific element in the sample; chemical and structural information are lost. The sample is destroyed.


	Signal Generation-sample excitation
	Input transducer-detection of analytical signal
	Signal modifier-separation of signals or amplification
	Output transducer-translation & interpretation


	chemical state
	structure
	orientation
	interactions
	general properties


	macro Vs micro
	pure samples Vs mixtures
	qualitative Vs quantitative
	surface Vs bulk
	large molecules (polymers, biomolecules)


	bulk, micro, contamination (matrix)
	matrix effects
	qualitative Vs quantitative
	complete or specific element
	chemical state


	1. Matrix substitution - dissolving sample into liquid or gas solution, grinding sample with KBr powder.
	2. Separation - using chromatography, solvent extraction, etc. to isolate analyte from complex matrix.
	3. Preconcentration - collecting the analyte from sample into a much smaller volume to raise its concentration.
	4. Derivatization - chemically modifying the analyte to improve volatility, light absorption, complex formation, etc., so that the instrument can more easily measure concentration.
	5. Masking - modifying interferences so that they are no longer detected by the instrument.


	Direct instrumental determination - multi-element - direct excitation---should be least expensive
	These are relative physical methods requiring appropriate standards & systematic errors like spectral interferences occur
	NAA, XRF, sputtered neutral MS


	Multi-stage procedures --- sample separation and preparation before quantitation
	Standards are less of a problem
	Time consuming & subject to losses or contamination
	Chromatography coupled with analysis


	What are interactions with radiation
	Means of excitation (light sources)
	Separation of signals (dispersion)
	Detection (heat, excitation, ionization)
	Interpretation (qualitative easier than quantitative)


	spectroscopy (UV, IR, AA)
	NMR
	mass spectrometry
	chromatography (GC, HPLC)
	measure radioactivity, crystallography, PCR, gas phase analysis


	What results can be obtained
	What kind of materials can be characterized
	Where can errors arise


	Outer shell electrons excited to higher energy levels
	Many lines per atom (50 for small metals over 5000 for larger metals)
	Lines very sharp (inherent linewidth of 0.00001 nm)
	Collisional and Doppler broadening (0.003 nm)
	Strong characteristic transitions


	Flame emission - heated atoms emit characteristic light
	Electrical or discharge emission - higher energy sources with more lines
	Atomic absorption - light absorbed by neutral atoms
	Atomic fluorescence - light used to excite atom then similar to FES


	Turbulence / stability / reproducibility
	Fuel rich mixtures more reducing to prevent refractory formation
	High temperature reduces oxide interferences but decreases ground state population of neutrals (fluctuations are critical)


	Radiation source (hollow cathode lamps)
	Optics (get light through ground state atoms and into monochromator)
	Ground state reservoir (flame or electrothermal)
	Monochromator
	Detector , signal manipulation and readout device


	Hollow Cathode Lamp - seldom used, expensive, low intensity
	Electrodeless Discharge Lamp - most used source, but hard to produce, so its use has declined
	Xenon Arc Lamp - used in multielement analysis
	Lasers - high intensity, narrow spectral bandwidth, less scatter, can excite down to 220 nm wavelengths, but expensive


	Flame Atomizers - rate at which sample is introduced into flame and where the sample is introduced are important


	Use liquids and nebulizer
	Slot burners to get large optical path
	Flame temperatures varied by gas composition
	Molecular emission background (correction devices )


	solvent viscosity
	temperature and solvent evaporation
	formation of refractory compounds
	chemical (ionization, vaporization)
	salts scatter light
	molecular absorption
	spectral lines overlap
	background emission


	Flame Atomizers - rate at which sample is introduced into flame and where the sample is introduced is important
	Graphite Furnace Atomizers - used if sample is too small for atomization, provides reducing environment for oxidizing agents - small volume of sample is evaporated at low temperature and then ashed at higher temperature in an electrically heated graphite cup. After ashing, the current is increased and the sample is atomized


	Graphite furnace (rod or tube)
	Small volumes measured, solvent evaporated, ash, sample flash volatilized into flowing gas
	Pyrolitic graphite to reduce memory effect
	Hydride generator


	Photomultiplier Tube
		has an active surface which is capable of absorbing radiation
		absorbed energy causes emission of electrons and development of a photocurrent
		encased in glass which absorbs light
	Charge Coupled Device
		made up of semiconductor capacitors on a silicon chip, expensive


	Two lines (for flame)
	Deuterium lamp
	Smith-Hieftje (increase current to broaden line)
	Zeeman effect (splitting of lines in a strong magnetic field)


	Assumptions: (i) Beer's law holds for the atoms in the flame or graphite furnace, and (ii) the concentration
	of atoms in the flame or furnace is proportional to the concentration of analyte in the sample.
	Calculations: The usual calibration curves or standard addition problems.


	A = e bC (Beer’s Law)
	where e = molar absorptivity (units M^-1cm^-1 ); b = sample thickness (cell pathlength) in cm; and C = conc. in M (mol/L). , is a property of the analyte and of wavelength; identification of the analyte
	(qualitative analysis) is possible from the spectrum (e vs 8). Note that the sensitivity m is equal to e b.


	Precision and accuracy are highly dependent on the atomization step
	Light source
	molecules, atoms, and ions are all in heated medium thus producing three different atomic emission spectra


	Line broadening occurs due to the uncertainty principle
		limit to measurement of exact lifetime and frequency, or exact position and momentum
	Temperature
		increases the efficiency and the total number of atoms in the vapor
		but also increases line broadening since the atomic particles move faster.
		increases the total amount of ions in the gas and thus changes the concentration of the unionized atom


	If the matrix emission overlaps or lies too close to the emission of the sample, problems occur (decrease in resolution)
	This type of matrix effect is rare in hollow cathode sources since the intensity is so low
	Oxides exhibit broad band absorptions and can scatter radiation thus interfering with signal detection
	If the sample contains organic solvents, scattering occurs due to the carbonaceous particles left from the organic matrix