Atomic Absorption Spectrometry (AAS)

Atomic Absorption Spectrometry (AAS)

• AAS is an analytical technique used to determine the concentration of metal elements in a sample
• AAS measures the absorption of light by free atoms, following the Beer-Lambert Law
• Developed in the 1950s by Alan Walsh
• AAS is widely used in environmental, clinical, and industrial applications
• AAS helps in detecting trace metals with high accuracy and sensitivity

Principles of AAS

• Ground-state atoms absorb light at specific wavelengths unique to each element
• When a sample is atomized into a gaseous state, it interacts with light from a source emitting the element's characteristic wavelength
• The measured amount of light absorbed is directly proportional to the concentration of the element in the sample, described by Beer-Lambert's law
• AAS heats a sample to 2,000-3,000°C to atomize it, breaking chemical bonds and converting elements into free atoms
• The sample is nebulized into a flame or electric oven, where atomization occurs
• The atomic vapor passes through the optical path of the instrument for analysis
• AAS is used for qualitative and quantitative elemental analysis

Essential Parts of AAS

• Light Source
• Chopper
• Sample Introduction System
• Nebulizer
• Atomizer
• Monochromator
• Detector
• Amplifier
• Readout Device

Light Source

• The cylindrical hollow cathode lamp is used as a light source
• The lamp contains a cathode coated with the element being analyzed
• Contains an anode made of tungsten
• The anode and cathode are contained within a glass tube having a quartz window
• The glass enclosure of the discharge tube is charged with inactive gases, like Argon and Neon, at a decreased atmospheric pressure of 1-5 torr

Chopper

• A rotary device is positioned within the cathode lamp and a flame
• This eliminates macroscopic droplets and facilitates the entry of droplets with homogenous dimensions into the combustion zone
• Modulates the lamp’s continuous light into a pulsating form

Sample Introduction System

• The system is used for transferring samples into the atomizer

Nebulizer

• A device that aspirates the liquid sample into the spray chamber
• Converts liquid samples into a fine mist
• The fine mist is introduced into the atomization source for analysis

Two Common Nebulizers

• Concentric Nebulizer
• Angular or Crossflow Nebulizer

Atomizer

• Converts an analyte to a free gaseous atom
• Strips away the solvent, volatilizes the analytes, and dissociates the analytes into free atoms

Two Types of Atomizers

• Flame Atomizer
• Electrothermal Atomizer

Monochromators

• An optical instrument transmits a narrow range of wavelengths of light or other forms of radiation from a broader spectrum of wavelengths
• Can be used to isolate a desired band of lines by transmitting a narrow band

Types of Monochromators

• Prism
• Diffraction Gratings

Detector

• Usually a photomultiplier tube
• Converts light from a monochromator into a simplified electrical signal
• It can be adjusted to react to a particular wavelength or frequency

Amplifier

• The output produced by the detector is transmitted here
• This increases the current's magnitude by multiple orders of magnitude

Readout Devices

• Various types of readout systems are employed in atomic absorption spectroscopy.
• These include meters, chart recorders, and digital display meters
• Hard copies can be produced through the use of chart recorders, printers, or plotters

Operation of AAS

• Sample Preparation
• Instrument Setup
• Sample Analysis
• Instrument Shutdown and Cleaning

Sample Preparation

• If the sample is solid, it is dissolved in acid (e.g., nitric acid, hydrochloric acid) to convert it into a liquid
• The analyte is diluted to a concentration that falls within the instrument's detection range
• A series of standard solutions with known concentrations are prepared to create a calibration curve for measurement

Instrument Setup

• The hollow cathode lamp (HCL) specific to the target element is selected as the light source
• The monochromator is set to isolate the element's characteristic wavelength
• The appropriate atomizer is chosen
• For Flame AAS (FAAS), the sample is introduced into a flame for atomization and gas flow (fuel & oxidant) maintains a stable flame
• For Graphite Furnace AAS (GFAAS), the sample is heated in a graphite tube for precise atomization
• Using the standard solutions, the instrument is calibrated to establish a reference for concentration measurement

Sample Analysis

• The sample is aspirated into the instrument
• In FAAS, the nebulizer converts the liquid sample into a fine mist that enters the flame
• In GFAAS, the sample is injected into the graphite furnace, where it undergoes drying, ashing, and atomization
• Atomization occurs, breaking molecules into free atoms
• Light from the hollow cathode lamp (HCL) passes through the flame or furnace, and atoms absorb specific wavelengths of light

Instrument Shutdown and Cleaning

• The flame (FAAS) or graphite furnace (GFAAS) is turned off
• Gas supplies (fuel & oxidant) are shut down to prevent hazards
• The nebulizer and burner (FAAS) or graphite tube (GFAAS) are cleaned to prevent contamination

Data Analysis and Interpretation

• AAS works by measuring how much light a sample absorbs
• Different metals absorb different wavelengths, so picking the correct one is important
• Background correction helps remove extra signals so accurate readings of the metal are obtained
• Sometimes other substances in the sample also absorb light, which can alter results

Types of Background Correction

• Deuterium Background Correction (Old but Common)
  • Uses a separate deuterium lamp to measure background absorption
  • Works well for flame AAS, but not for wavelengths above 320 nm
  • It is not very accurate for correcting fine background interference because it uses a separate light source
• Zeeman Background Correction (More Advanced & Accurate)
  • Uses a magnetic field to split the light absorption into different parts
  • Accurately measures and removes the background using the same light source instead of a separate one
  • It works best with graphite furnace AAS but requires more powerful equipment

Data Interpretation

• Calibration Curve
  • The instrument is first calibrated using solutions with known concentrations to find an unknown concentration
  • The absorbance of these solutions is measured, and a graph (calibration curve) is created
  • When an unknown sample is tested, its absorbance is matched to the curve to find its concentration
• Spectral Readout
  • Each element absorbs light at a specific wavelength
  • When analyzing a sample, light of a specific wavelength is passed through the sample in a flame
  • The sample absorbs some of the light, and this absorption is measured

Reflectance Photometry

• A type of photometry that measures the color and reflectance properties of surfaces
• Analyzes how different surfaces reflect various amounts and wavelengths of light depending on their physical and chemical properties

Principles of Reflectance Photometry

• Measures the intensity of colored products on reagent strips
• Uses diffuse reflectance, where some light is absorbed, and the rest is scattered
• Detects only specific reflected wavelengths
• Calibrated with reflectance standards like magnesium carbonate or barium sulfate
• This process requires wavelength selection for accurate measurement
• It uses polychromatic light with filters or monochromatic light sources (e.g., LEDs)
• Expresses results as percent reflectance (%R) based on calibration
• Requires a microprocessor to convert non-linear data into a readable format

Essential Parts of the Macherey-Nagel QUANTOFIX Relax

• Touch-Screen which controls equipment functions
• Test Strip Slide which is a test strip retainer for autonomous start of analysis
• Printer Flap which opens for paper replacement,
• USB A -Interface which connects to a keyboard or bar code scanner
• USB B - Interface which connects to a computer (cable length¹ max. 3 m)
• LAN - Interface functions are currently not supported
• Serial Interface which connects to a computer (cable length¹ max. 3 m)
• On/Off Switch (I/O) which turns the equipment on and off
• Mains Connection which contacts to the provided power pack

Operation with MACHEREY-NAGEL QUANTOFIX Relax

• Power On, using the Power Switch (4) to turn on the instrument; the LCD Display/Key (3) should light up, indicating it's ready
• Load in a test strip carefully into the Strip Loading Plate (2) and make sure it's properly aligned so the instrument can move it into the correct measuring position
• Select necessary Test Type and Start Measurement, using the LCD Display/Key (3) to select the appropriate test type from the menu
• Follow the prompts on-screen to initiate the measurement. The strip loading plate will move the strip into the measuring area
• View Results on the LCD Display/Key (3.
• Optionally, Print Results; open the Printer Cover (1), insert paper, and use the appropriate function key on the LCD Display/Key (3) to print the results
• Remove the Test Strip, once the measurement is complete, take out the used test strip from the Strip Loading Plate
• Power Off, using the Power Switch (4) to turn off the instrument when finished

General Instrument Operation

• Prepare the Instrument
  • Place the Sample Plate: Position the sample plate (or test strip holder) in the center of the measurement chamber and that it is flat and aligned with the optical sensor
  • Power On, by plug in the power cord into the instrument and a grounded outlet, and press the Power button and wait for the startup screen to load
• System Check
  • Instrument will perform a self-test (lights, sensors, and motors activate); the plate/strip holder will adjust to the default measurement position
• Calibration
  • Insert Calibration Standard, use a white reference standard (e.g., Spectralon or a calibration strip) to set 100% reflectance, and follow the prompts: Select Calibrate > Start
  • Confirm Calibration: Wait until the display shows “Calibration Complete” or a green checkmark, and remove the reference tile/strip
• Load Samples, select measurement mode based on application (e.g., single measurement or batch processing)
• Start Measurement
  • Initiate Reading by pressing Start; the instrument will emit light and analyze reflectance
  • Wait for Completion until a progress bar or timer will display and do not open the chamber until display says “Measurement Done”

General Mode

• Place up to 10 test strips/samples in the holder's grooves
• Press End after loading the last sample

Single Mode

• Insert the first strip and press Start; repeat for each sample
• Press End after the final strip

Quick Mode

• Load all strips at once, then press End immediately
• Review Results
• View Data which will show reflectance values (e.g., %R) or calculated metrics (e.g., absorbance) on-screen and use arrow keys to scroll through results
• Export or Print, by pressing Print for a hard copy or Save to export data via USB/SD card
• Shut Down
• Remove Samples by discarding used test strips or store reusable plates
• Turn Off, by pressing and holding the Power button for 3 seconds and disconnect the power cord if not in use for extended periods

Data Analysis and Interpretation

• A light source illuminates the sample surface
• A detector captures the reflected light at specific angles and wavelengths
• The system is calibrated using reference materials with known reflectance values to ensure accuracy
• Adjustments are made to account for background signals or noise, ensuring that measurements reflect only the sample's properties
• Raw data is processed to remove noise and enhance signal quality, often involving statistical methods
• Reflectance spectra are analyzed to identify specific features corresponding to the sample's composition or characteristics
• Processed data is compared against known standards or databases to interpret the sample's properties accurately