Atomic Spectrometry Techniques

• Techniques that determine the elemental composition of an analyte are part of atomic spectrometry.
• This is based on the substance's electromagnetic or mass spectrum.

Atomic Spectrometry Methods

• Optical spectrometry and mass spectrometry are the two major categories.
• Atomic Emission Spectrometry (AES) analyzes light emission from excited atoms reverting to lower energy states.
• Atomic Absorption Spectrometry (AAS) measures light absorbed by gaseous atoms.
• Flame emission spectrometry (Flame AES/OES) is a type of AES.
• Inductively coupled plasma emission spectrometry (ICP-AES/ICP-MS) involves plasma emission.
• Atomic Fluorescence Spectrometry (AFS) is another method.

Comparing Atomic Spectroscopic Methods

• Flame-AAS (absorption) uses Air-Ac or N2O-Ac as an atomizer. Sensitivity is in ppm with short residence time, and is used for 1.5mL of single element
• Graphite Furnace-AAS uses an electrothermally heated graphite tube as an atomizer. Sensitivity is in ppb with long residence time, used for uL of 40 elements analysis
• Hydride generation-AAS uses Metal hydride with heating as an atomizer. Sensitivityis in ppb with long residence time, and is used for mL of single element analysis
• Cold Vapor uses Cold Vapor as an atomizer. Sensitivity is in ppb with long residence time, for mL of single element Mercury analysis
• ICP-OES (emission) uses Plasma as an atomizer. Sensitivity is in ppm with axial/radial application, used for mL of multielement analysis
• ICP-MS uses Plasma as an atomizer. Sensitivity is in ppb, ppt with axial/radial application, used for mL of multielement analysis

Common Principles of Atomic Spectroscopy

• Sample introduction involves nebulization and evaporation.
• Atomization (excitation or ionization) occurs via flame, furnace, or plasma.
• Spectrometer components include a light source, optical or mass spectrometer system, and a detector.
• Atomic Absorption requires an external light source
• Common components are a light source, monochromator, sample area, detector and readout system.

Criteria for Choosing a Method

• Multi-element analysis is more expensive but covers almost all elements, including hydrides and Hg. It is a fast, multi-element analysis
• Single-element analysis has mid-level prices and is a fast sequential multi-element analysis.
• Flame AAS (FAAS) is cheap and fast but only does single-element analysis.
• Graphite Furnace AAS (GFAAS) is more expensive and slow, but has better detection limits.
• ICP-MS covers almost all elements including hydrides, and enables fast, multi-element analysis.

Atomic Absorption Spectroscopy (AAS)

• AAS measures the absorption of radiation by atoms in the gaseous state.
• Applications include measuring metals and toxic substances
• AAS components include a flame (or graphite furnace or hydride), monochromator.

Flame AAS

• The flame is used for atomization.
• Key components include a flame, exhaust hood, inert gas, readout, autosampler, hollow cathode lamp (HCL), and electrodeless Discharge Lamps (EDL)..
• Inside the instrument are a monochromator, photomultiplier tube, detector.

FAAS traits

• The level of detection is in ppm.
• It is relatively insensitive due to incomplete volatilization.
• Reproducible with precision less than 1%.
• Has very low cost per analysis and is rapid.

Hollow Cathode Lamp

• It serves as a light source with a cathode composed of the element being measured.
• Each element analyzed requires a different lamp.
• The AA spectrometer stores the lamps in a compartment.
• A specific lamp is rotated to the position needed for a metal analysis.
• Can be single or multi-elemental.

Electrodeless Discharge Lamp (EDL)

• It contains a metal or salt of interest sealed in a quartz tube filled with Ne or Ar at low pressure.
• It is energized by a radio frequency or microwave radiation field.
• It is used for ionization of Ar to give a high frequency accelerated component.
• They can use light-generating substances that react with electrodes in typical lamps.
• EDL has extended bulb life
• EDL is more intense and sensitive than HCL with better precision and a lower detection limit.
• EDL is available for volatile elements like Sb, As, Bi, Cd, Ge, Pb, Hg, P, K, Rb, Se, Te, Th, Sn, Zn.

Light Chopper

• Placed between Hollow cathode lamp and the flame
• Light is chopped with a rotating half-mirror beam-splitter device
• The detector sees alternating light intensities.
• At one moment, the light emitted by the flame is read.
• At the next moment, it measures the light from both the flame emission and the intensity of the lamp.

Sample Introduction in AAS

• Samples dissolved in water.
• A flame is created, usually using ethyne & oxygen fuel.
• The aspirator sucks in liquid through the small tube from the container.
• Fine aerosol is created and mixed with fuel and oxidant for introduction to flame.
• Liquid is transferred to the flame for atomization.
• The metal atoms then absorb light from the source.

Burner-Nebulizer System

• The nebulizer sucks up liquid sample at a controlled rate and mixes it with fuel and oxidant.
• The combustion flames that come out will atomize and introduce the sample into the light path.
• The aerosol mist is swept through the spray chamber where 95% is drained out, then into the burner head producing vapor

Atomization

• Atomization converts the sample (solid or solution) into vapor particles, molecules, and gaseous atoms.
• Conversion of solution to aerosol form (nebulize or aspirate).
• Solvent removal (desolvation): MA(solution) → MA(solid).
• Sample vaporization: MA(solid) → MA(gas).
• Atomization into individual gaseous atoms: MA(gas) → M(gas) + A(gas).
• Following excitation: M•→ M*.
• Emission: M* → M• (for AES).
• Ionization: M → M+ (undesired).

Burner Head

• Composed of titanium for heat and corrosion resistance.
• A 10 cm single slot burner is optimal for air-acetylene flames. It produces a small, blue flame and has optimal length and sensitivity
• A 5 cm burner head is optimal for nitrous oxide acetylene flame applications. It produces a large flame which can be rotated for providing reduced sensitivity.

Common Oxidant Fuels

• Air-Acetylene ranges from 2100°C - 2400°C. It is sufficient to break down molecules of most elements to ground state atoms.
• Nitrous Oxide-Acetylene ranges in temperature from 2600 - 2800 °C, and can be used with hotter flames are required. It’s suitable for high-melting point refractory elements

Graphite Furnace AAS (GF-AAS)

• Includes an graphite furnace.
• GF-AAS components: stacked flame/furnace design, autosampler, HCL, EDL, inert gas, readout.
• Instead of a flame, the heater is an electrically heated graphite tube.
• Atomization efficiency is greater and detection limits are lower

Graphite Furnace (GF) Program

• Drying: 10 - 100 s at 50 – 200 °C to remove any solvent.
• Pyrolysis (Char): 10 - 100 s at 200 – 800°C to decompose the matrix.
• Atomization: 1 s at 2000 - 3000 °C for atomization of analytes.
• Graphite furnace AAS use a platform, mainly heated by radiation from tube wall, delaying the increase in temperature of the sample. Atomization occurs after the wall and the gaseous phase has reached a temperature plateau.

Graphite furnace vs Flame AAS

• Graphite furnace uses a smaller sample size (0.05μl-100μl) vs Flame AAS which requires larger sample size (3-5ml)
• Graphite furnace can analyize samples in pbb range vs Flame AAS which analyizes in ppm
• Graphite furnace has longer analysis time of 5 mins vs Flame AAS which is less than 10 seconds Graphite furnace a fixed vol of sample at a time vs Flame which is aspirated consistently
• Graphite furnace there is minimal sample preparation vs For Flame require prep like digestion
• Graphite furnace can handle solid and liquid samples vs Flame is only solution

Key features of Graphite Furnace

• Argon(Ar) gas is used in 2 stages to avoid oxidation. It’s stopped during atomization to extend residence time of the atoms
• The furnace needs to be cleaned with high current and cooled to ensure high efficiency for detection.

Hydride Generation AAS (HGAAS)

• HGAAS improves measurement a select group of metalloids that have matrix interferences.
• Volatile covalent hydrides are separated from the matrix.
• Use acidified aqueous solution to 1% aqueous solution of sodium borohydride to form As, Bi, Ge, Pb, Sb, Se, Sn, Te.
• Volatile hydride gas gets heated in a tube furnace or flame, generating the atoms of the analyte
• The detection limits well below the mg/L range are achievable.
• The extremely low detection limits results from a much higher sampling efficiency
• Limitation of HGAAS is that the gas formation is prevented, resulting in the technique undergoing interference.

Cold Vapor AAS

• Cold Vapor AAS is mainly for Mercury detection in samples
• The method does not require heating.
• Because Mercury has appreciable vapor pressure at room temperature, once reduced no atonization is required
• It's limited to Hg because no other elements offers the possibility of chemical reduction to mercury.
• Mercury is usually reduced by reaction with tin(II) chloride. Mercury vapour is then carried and drained out.

Flow Injection Mercury System

• FIMS (Flow Injection Mercury System) includes several key components.
• An optical unit with a detector is present.
• A series of pumps that carry the reagents and analyte to the reactor.
• Consists of a power switch and multiple functional elements such as a gas outlet, carrier gas line, and mixing coil.

Cold Vapor AAS Traits

• AAS has to be modified to measure Hg because AA requires atomization via high heat. With cold vapour AAS does not have a heat source
• Improved sensitivity is achieved through a 100% sampling efficiency with large sample volumes.
• All of the Hg in the sample solution are placed in the reaction flask.
• Then, it is chemically atomized & transported to the sample cell for measurement.

Czerny-Turner Grating Monochromator

• The Monochromator is a key component, and uses collimating mirros to focus on the diffraction grating, resulting in separation

Photomultiplier

• Has a photocathode and series of dynodes in an evacuated glass enclosure. Photons strike cathode and electrons are emitted
• Additional electrons are generated at each dynode
• Electrons accelerate to a series of dynodes by increasing voltages
• Amplified signal is collected and measured at the anode.