Study Notes on Atomic Spectroscopy Based on Flame Atomization

ATOMIC SPECTROSCOPY BASED ON FLAME ATOMIZATION (CHAPTER 3)

Lesson Outcomes

  • Explain the principles of Flame Atomic Spectroscopy (absorption and emission)

  • Draw and label the schematic diagram of AAS and AES

  • Explain the functions of each component in AAS and AES

  • Discuss the differences in terms of parts and functions of AAS and AES

3.1 Fundamental Principle

Atomic Spectroscopy

  • A class of spectroscopic methods where the species examined in the spectrometer are in the form of ATOMS (not molecules or ions).

  • Spectral transitions involve electronic energy levels in the visible, UV, and X-ray regions.

  • The wavelengths of the observed absorption/emission lines are characteristic of a specific ELEMENT.

  • The intensity of the spectral line is proportional to the number of atoms undergoing the corresponding transition.

3.2 Flame AAS

AAS Principle

  • AAS is based on principles similar to the flame test used in qualitative analysis.

  • When an alkali metal salt (e.g., sodium, lithium) or a calcium, strontium, or barium salt is heated in a Bunsen flame, it produces characteristic flame colors:

    • Na → yellow

    • Li → crimson

    • Ca → brick red

    • Sr → crimson

    • Ba → green

  • In the flame, ions are reduced to gaseous metal atoms, and the high temperature excites a valence electron to a higher-energy orbital.

  • The atom emits energy as visible light when the electron falls back to the lower energy orbital (ground state).

  • The ground state atom absorbs light of the same characteristic wavelengths that it emits upon returning from the excited state.

  • The intensity of the absorbed light is proportional to the concentration of the element in the flame, useful for quantitative analysis.

  • Absorbance or emission of atomic vapor is measured. Oxidation states (e.g., Fe²⁺, Fe³⁺) cannot be distinguished by this method.

3.2.1 Atomization Process (Sample Introduction)

  • Atomization is critical for atomic spectroscopy, converting a sample into an atomic vapor.

  • The process involves:

    1. Nebulization: Conversion of the liquid sample to a fine spray.

    2. Desolvation: Solid atoms are mixed with the gaseous fuel.

    3. Volatilization: Solid atoms are converted to vapor in the flame (can include molecules/ions).

  • The atomizer is the device used for converting samples to atomic vapor.

3.2.2 Flame Atomization Process

  • The specific processes during atomization:

    1. Nebulization: The liquid sample is converted into a fine aerosol for introduction into the flame.

    2. Desolvation: Solid atoms are mixed with gaseous fuel and oxidant.

    3. Volatilization: Solid atoms are converted to vapor in the flame.

3.2.3 Properties of Flame Fuel

  • The following oxidants and fuels can be used, with their respective temperature ranges and maximum burning velocities:

    • Natural gas + Air: 1700-1900 °C, 39-43 cm/s

    • Natural gas + Oxygen: 2700-2800 °C, 370-390 cm/s

    • Hydrogen + Air: 2000-2100 °C, 300-400 cm/s

    • Hydrogen + Oxygen: 2550-2700 °C, 900-1400 cm/s

    • Acetylene + Air: 2100-2400 °C, 158-266 cm/s

    • Acetylene + Oxygen: 3050-3150 °C, 1100-2480 cm/s

    • Acetylene + Nitrous oxide: 2600-2800 °C, 285 cm/s

  • The selection of flame type depends on the volatilization temperature of the atom of interest, with Acetylene/Air being the most common.

  • Types of flames involved can significantly impact the sensitivity and results of the measurements.

Flame Structure

  • The primary combustion zone contains non-atomized species and fuel species emitting in the blue region of the electromagnetic spectrum.

  • The interzonal region is the hottest part of the flame where atomic absorption is optimal.

  • In the secondary combustion zone, oxidation of the atoms occurs, leading to the formation of molecular oxides that disperse into the surroundings.

Nebulizer

  • The nebulizer is a crucial component that:

    • Sucks up the liquid sample (aspiration).

    • Produces a fine aerosol for introduction into the flame.

    • Thoroughly mixes the aerosol with fuel and oxidant, creating a heterogeneous mixture.

    • Smaller droplets lead to higher sensitivity for the element.

Methods of Sample Introduction in AAS

  • Pneumatic Nebulization: For solutions or slurries.

  • Ultrasonic Nebulization: For solutions.

  • Electrothermal Vaporization: Suitable for solid, liquid, or solution samples.

  • Hydride Generation: For specific soluble elements.

  • Direct Insertion: For solid powder samples.

  • Laser Ablation: For solid metal samples.

  • Spark or Arc Ablation: For conducting solids.

  • Glow-Discharge Sputtering: For conducting solids.

Burners

  • The burner, where chemical reactions occur, consists of:

    • Inlet tube,

    • Fuel and air inlets,

    • Nebulizer,

    • Mixing cell,

    • Reaction/sample cell (the flame).

  • Two main types of burners in flame spectroscopy are:

    1. Turbulent Flow Burner: Combines nebulizer and burner, allows large sample introduction, but has shorter path length and noise issues.

    2. Laminar Flow Burner: Nebulizes sample via oxidant flow, longer path length enhances sensitivity but has a lower sample introduction rate.

3.3 Instrumentation for Flame AAS

  • Single Beam AAS Instrument involves components like:

    • Modulated power source,

    • Readout,

    • Shutter,

    • Amplifier,

    • Ebert grating monochromator,

    • Flame.

  • Double Beam AAS Instrument includes:

    • Lamp (chopper),

    • Czerny-Turner monochromator,

    • Flame,

    • Detector (photomultiplier tube), and readout components.

3.3.1 Line Sources

  • Radiation Source Types:

    • Hollow Cathode Lamp (HCL): Provides precise wavelengths needed for analysis; contains the element of interest.

    • Electrodeless Discharge Lamp (EDL): Contains metal or salt sealed in a quartz tube, ionizes noble gas to excite elements, greater intensities than HCL.

3.3.2 Source Modulation

  • Employed to eliminate interference caused by flame emissions and background gas radiation.

  • Light chopper (a rotating disk) facilitates measurement by alternating between signals from the source and the flame, allowing for absorbance determination by comparing readings.

3.3.3 Interferences in AAS

  • Interference Types:

    • Spectral Interference: Physical processes affecting light intensity at the analytical wavelength due to combustion products or sample matrix.

    • Chemical Interference: Alterations in analyte absorbance due to chemical processes that form thermally stable compounds, affecting dissociation.

    • Ionization Interference: Creation of ions in hot flames can reduce ground state atom availability for light absorption, most critical for elements like Mg, Ca, Na, etc.

3.3.4 Sample Preparation & Quantitative Analysis

  • Sample preparation methods vary; some samples require no preparation, while others involve:

    • Wet Digestion: Converts solids into solutions using acids under heat and pressure.

    • Dry Ashing: Burns samples at high temperatures to remove organic matter, suitable for non-volatile elements.

  • Quantitative analysis follows Beer’s Lambert Law: A = εbc where the concentration of analytes is determined using calibration curves and standard addition methods to overcome matrix effects.

  • Samples are measured for absorbance and related to known standards for concentration calculations.

Example: Standard Addition Method

  • Used to counteract matrix effects by adding known increments of standard solutions to aliquots of the sample, measuring absorbances, and plotting for concentrations.

Detection Limits & Accuracy

  • Detection limits for FAAS vary, with some elements measurable in ppb ranges and others undetectable.

  • AAS generally boasts high sensitivity with ppm level concentrations.

3.4 Flame Emission Spectroscopy

Atomic Emission

  • Emission occurs when an excited atom relaxes, emitting electromagnetic radiation (photons). The intensity of emitted light relates to concentration.

  • The relationship between intensity and concentration is generally linear, adhering to the expression I = kC where I is intensity, k is a constant, and C is concentration.

Principles Common to AAS & AES

  • Both methods involve atomization for measuring concentrations:

    • AAS measures absorbed radiation.

    • AES measures emitted radiation from excited atoms, which varies based on the excitation source and the reaction processes involved.

Excitation Sources:

  • Sources include flame (typically for light elements) and plasma (commonly for heavier metals) due to their higher temperatures.

  • Plasma sources include Inductively Coupled Plasma (ICP), Direct Current Plasma (DCP), and Microwave-Induced Plasma (MIP).

  • ICP typically serves as the preferred method due to efficiency and low chemical interference.

Conclusion

  • Comparing FAAS and FAES:

    • FAAS measures absorbed light; FAES measures emitted light.

    • FAAS requires a light source, while FAES does not.

    • Each method serves unique applications depending on the analytical requirements and the elemental concentrations being assessed.