Chapter 20 Atomic Spectroscopy

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.

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

How Temperature Affects Atomic Spectroscopy 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”

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 SPECTROSCOPY Elements 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 Spectroscopy Atomic “Absorption” Spectroscopy Radiation @ frequency is passed through flame. Intensity of transmitted radiation is measured.

Atomization: Flames, Furnaces, and Plasmas Use a premix burner Sample, oxidant, and fuel are mixed before going into flame

Atomization: Flames, Furnaces, and Plasmas Graphite furnace Provide greater sensitivity Requires less sample to analyze Sample injected into L’vov platform. Contact with heated platform atomizes

Atomization: Flames, Furnaces, and Plasmas 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

How Temperature Affects Atomic Spectroscopy 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?

Physical separation of a mixture into its individual components. 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 Column Theoretical plate

Chromatographic principle Stationary phase Mobile phase Sample mixture Equilibrium establishes at each point (ideally) 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

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

Basics of Chromatography Average linear flow rate Average linear flow of mobile phase

Types of Chromatography Liquid Chromatography Used to identify unknown plant pigments & other compounds. Paper Chromatography Can be used to separate the components of inks, dyes, plant compounds (chlorophyll), make-up, and many other substances Thin-Layer Chromatography Uses thin plastic or glass trays to identify the composition of pigments, chemicals, and other unknown substances. Gas Chromatography Used to determine the chemical composition of unknown substances, such as the different compounds in gasoline shown by each separate peak in the graph below.

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