Chapter 20

Atomic Spectroscopy

• By exposing these atoms to temperature they are able to “jump” to high energy levels and in return, emit light.

• Atomic “absorption” is the absorption of light by free atoms.

• An atomic “absorption” spectrophotometer is an instrument that uses this principle to analyze the concentration of metals in solution.

• Atomic “Emission” Spectroscopy– No lamp required.– Light emitted by hot atoms where electrons promoted

excited states in the flame.

ATOMIC SPECTROSCOPY

Absorption promotes electrons from their ground state to higher-energy excited state.

• Boltzmann Distribution (Math!)– Consider molecule with two energy levels

– Atoms/molecules can have more than one state at a given energy.• This is called Degeneracy, labeled as “g”

How Temperature Affects Atomic Spectroscopy

ATOMIC SPECTROSCOPY

• Flame Emission and Atomic Absorption Spectroscopy (3 main types)– Atomic Emission (with thermal excitation), AES– Atomic Absorption, (with optical photon unit) AAS– Atomic Florescence, AFS

• Three types of high-temperature plasmas – The inductively coupled plasma (ICP).– The direct current plasma (DCP).– The microwave induced plasma (MIP).

ATOMIC SPECTROSCOPY

• Advantages of AAS– Determination of 68 metals– Ability to make ppb determinations on major

components of a sample– Precision of measurements by flame are better

than 1% rsd.– Sample preparation is simple (often involving only

dissolution in an acid)– Instrument easy to tune and operate

ATOMIC SPECTROSCOPYElements that are highlighted in

pink are detectable by AAS

Atomic Spectroscopy

• The light that is focused into the flame is produced by a hollow cathode lamp. Inside the lamp is a cylindrical metal cathode containing the metal for excitation, and an anode. When a high voltage is applied across the anode and cathode, the metal atoms in the cathode are excited into producing light with a certain emission spectrum.

Atomic Spectroscopy

• The type of hollow cathode tube depends on the metal being analyzed. For analyzing the concentration of copper in an ore, a copper cathode tube would be used, and likewise for any other metal being analyzed. The electrons of the atoms in the flame can be promoted to higher orbitals for an instant by absorbing a set quantity of energy (a quantum).

Atomic Spectroscopy

• This amount of energy is specific to a particular electron transition in a particular element. As the quantity of energy put into the flame is known, and the quantity remaining at the other side (at the detector) can be measured, it is possible to calculate how many of these transitions took place, and thus get a signal that is proportional to the concentration of the element being measured.

• Atomic “Absorption” Spectroscopy– Radiation @ frequency is passed through flame.

Intensity of transmitted radiation is measured.

Atomic Spectroscopy

• Flame– Use a premix burner• Sample, oxidant, and fuel are mixed before going into flame

Atomization: Flames, Furnaces, and Plasmas

• Furnace– Graphite furnace• Provide greater sensitivity • Requires less sample to analyze

– Sample injected into L’vovplatform. Contact with heated platform atomizes

Atomization: Flames, Furnaces, and Plasmas

• Plasma– Ar gas + analyte fed into

torch– Tesla coil ionizes Ar gas– Free electrons

accelerated by radio freq. superheat gas to 6 – 10K Kelvin by colliding with atoms.

Atomization: Flames, Furnaces, and Plasmas

• Nebulizer/Atomizer– Makes analyte into a fine mist used for testing

Atomization: Flames, Furnaces, and Plasmas

• Examples!The lowest excited state of a sodium atom lies 3.371E-19 J/atom above the ground state.Degeneracy of the excited state is 2.Ground state degeneracy is 1.

Calculate the fraction of Na atoms in the excited state in an oxyacetylene flame at 2600 K:

What if 2610 K?

How Temperature Affects Atomic Spectroscopy

Chromatography

Physical separation of a mixture into its individual components.

Chromatography

From Wikipedia ...

• Chromatography (from Greek word for chromos for colour) is the collective term for a family of laboratory techniques for the separation of mixtures. It involves passing a mixture which contains the analyte through a stationary phase, which separates it from other molecules in the mixture and allows it to be isolated.

How Does Chromatography Work?• In all chromatographic separations, the sample is transported in a

mobile phase. The mobile phase can be a gas, a liquid, or a supercritical fluid.

• The mobile phase is then forced through a stationary phase held in a column or on a solid surface. The stationary phase needs to be something that does not react with the mobile phase or the sample.

• The sample then has the opportunity to interact with the stationary phase as it moves past it. Samples that interact greatly, then appear to move more slowly. Samples that interact weakly, then appear to move more quickly. Because of this difference in rates, the samples can then be separated into their components.

How Does Chromatography Work?

How Does Chromatography Work??

• Greater separation occurs with: –greater number of theoretical plates (N) –as plate height (H or HETP) becomes smaller

L = N H or H = L / N where L is length of column, N is number of plates, and H is height of plates or height equivalent to theoretical plate (HETP)

Plate Theory - Martin & Synge 1954 Nobel Laureates

• View column as divided into a number (N) of adjacent imaginary segments called theoretical plates –Within each theoretical plate analyte(s) completely

equilibrate between stationary phase and mobile phase

ColumnTheoretical plate

Chromatographic principle

The molecules of the mixture interact with the molecules of

the Mobile and Stationary Phase

Retardation of rate of movement of

molecules

Each molecule interacts differently with MP and

SP

Different distribution coefficients and different net

rates of migration

Stationary phase

Mobile phaseSamplemixture Equilibrium

establishes at each point (ideally)

Plate Theory – Practical Considerations

• It is not unusual for a chromatography column to have millions of theoretical plates• Greater separation occurs with:

–greater number of theoretical plates (N) –as plate height (H or HETP) becomes smaller

L = N H or H = L / N where L is length of column, N is number of plates, and H is height of plates or height equivalent to theoretical plate (HETP)

Plate Theory – Practical Considerations

• Greater separation occurs with: –greater number of theoretical plates (N) –as plate height (H or HETP) becomes smaller

L = N H or H = L / N where L is length of column, N is number of plates, and H is height of plates or height equivalent to theoretical plate (HETP)

Basics of Chromatography

• Separation of Analytes• Type of Equilibrium

M

S

CCK

Basics of Chromatography

• Average linear flow rate• Average linear flow of mobile phase

RtL

MtL

Types of Chromatography

Gas ChromatographyUsed to determine the chemical composition of unknown substances, such as the different compounds in gasoline shown by each separate peak in the graph below.

Paper ChromatographyCan be used to separate the components of inks, dyes, plant compounds (chlorophyll), make-up, and many other substances

Liquid ChromatographyUsed to identify unknown plant pigments & other compounds.

Thin-Layer ChromatographyUses thin plastic or glass trays to identify the composition of pigments, chemicals, and other unknown substances.

Principle and Instrumentation of GC

Principle and Instrumentation of GC

• The volatile sample in the liquid matter is injected to the column.

• The temperature of column is setted in which sample can be vaporized.

• The gas is pumped and then it carries the sample (gas) through the column.

• The sample which has higher boiling point/melting point will be retained by column, in contrast the lower one will be carried through the column faster.

• Means the higher b.p/m.p wil be lower the Rt and vice versa