Atomic Absorption Spectroscopy Vocabulary

Flashcards of key vocabulary terms related to atomic absorption spectroscopy

Atomic Spectroscopy

Atomic spectroscopy techniques include flame-AAS, ET-AAS, FIAS-AAS for absorption, and flame-AES and ICP-AES for emission, used for quantitative or qualitative chemical information.

Principle of AAS

The interaction between an atom and radiation in the visible region, where the atom absorbs radiation and undergoes an electronic energy level transition.

Resonance Line in AAS

A spectral line of radiation from an atom at a wavelength coinciding with the light absorbed by the atom in its ground state, having the strongest intensity and important for quantitative analysis.

Use of AAS

AAS measures the amount of visible radiation absorbed by an atom, and concentration is obtained from a linear relationship of measured absorbances with known concentrations.

Types of AAS

Types of AAS include flame-AAS and electrothermal-AAS (GFAAS), or based on sample introduction using hydride generation.

Components in flame-AAS

Radiation, sample introduction, atomizer, wavelength selector, detector, signal processor, and amplifier.

Radiation Source in AAS

Hollow cathode lamp (HCL) and electrodeless discharge lamp; function to provide radiation at discrete wavelengths to the atomizer.

Radiation Source Requirements

Must provide stable, intense radiation at a specific wavelength to the atomizer with a narrow linewidth.

Why Narrow Line Widths?

Requires a narrow source bandwidth relative to the absorption line width to maintain linearity between absorbance and analyte concentration; emitted radiation band/line width must be less than the absorption bandwidth in the atomizer.

Dynamic Range

Linear region of calibration curve where the lower limit is ten times the standard deviation of the blank.

Hollow Cathode Lamp (HCL)

Consists of a tungsten anode and cylindrical cathode constructed of the target element, sealed in a glass tube filled with Ne or Ar at low pressure.

Hollow Cathode Lamp Function

Produces line source specific to the element used to construct the cathode; the cathode must be capable of conducting a current.

Ionization of Ar in HCL

Applying voltage creates a current, where the anode pulls electrons off fill gas (Ar) atoms, creating argon cations (Ar+).

Sputtering

If the voltage is high, fill gas cations (Ar+) dislodge metal atoms from the cathode surface, producing ground state metal atomic vapor.

Emission in HCL

Excited atom returns to ground state, emitting a characteristic narrow beam (~0.5 nm) high-intensity radiation at which the analyte absorbs.

Electrodeless Discharge Lamp (EDL)

Sealed quartz tube containing low-pressure inert gas (Ar) and a small quantity of the metal or its salt; contains no electrode; RF coil generates RF or microwave radiation to ionize argon gas.

Function of Atomizer

To convert the target analyte present in the sample into gaseous atoms; precision and accuracy depend on the atomization step and sample introduction.

Types of Atomizers

Continuous atomizers introduce samples in a steady manner (plasmas and flames), while discrete atomizers introduce samples discontinuously (electrothermal atomizer).

Processes Occurring During Atomization

Nebulization, desolvation, volatilization, and dissociation/atomization.

Processes Leading to Atoms, Molecules, and Ions

Nebulizer converts solution sample into fine droplets; high temperature evaporates solvent leaving dry aerosol particles; further heating volatizes particles producing atomic, molecular, and ionic species.

Band and Continuum Spectra

When atomic spectra are generated, band and continuum spectra are also produced, potentially causing interference, which should be minimized by proper wavelength choice, background correction, and atomization conditions.

Types of Flames

Air is the oxidant (1700-2400°C) for easily decomposed samples; O2 or nitrous oxide is the oxidant (2500-3100°C) for refractory samples.

Flame Burning Velocity

Flame rises above the burner where flow velocity = burning velocity; flow velocity < burning velocity causes flashback.

Regions in a Flame

Primary combustion zone, interzonal zone, and secondary combustion zone.

Electrothermal Atomizers (ETA)

Samples are introduced discontinuously; solids or liquids are placed in an electrically heated graphite tube or cup; involves drying, ashing, and atomizing steps.

Methods of Sample Introduction

Pneumatic nebulization, ultrasonic nebulization, electrothermal vaporization, hydride generation, direct insertion, laser ablation, spark or arc ablation, glow-discharge sputtering

Criteria for Sample Introduction

Reproducible measurement, high atomization efficiency, no spectral or chemical interferences, direct sample introduction.

Hydride Generation

Used to determine As, Se, and Hg to improve signal detection, especially at trace levels; installed in FIAS-AAS system.

Flow Injection Analaysis System

Is used to mix the sample solution(containing the target element) with the reducing agent

Types of Interferences in Atomic Spectroscopy

Involves spectral and chemical interferences.

Spectral Interferences

Absorption or emission of an interfering species that overlaps or lies close to the analyte absorption or emission.

Chemical Interferences

Various chemical processes occurring during atomization alter the absorption or emission characteristics of the analyte.

Flame Emission Effects

Emission of radiation by excited atoms in the flame; emitted radiation of the same wavelength as the monochromator setting cannot be eliminated; minimized by source modulation and choppers.

To Minimize Spectral Interferences

Matrix products, variation in analytical variables, radiation buffer, and standard addition method.

Source Modulation

Light source is modulated to create an AC signal at the detector; detector receives AC signal of radiation source and DC signal from emission process.

Chemical Interferences

Formation of low volatility compounds, dissociation equilibria, and ionization equilibria.

Formation of Low Volatility Compounds

Analytes react with anions or cations to form low volatility compounds, reducing the population of ground state atomic vapor.

To Minimize the Effects of Low Volatility Compounds

Higher flame temperature, releasing agents, and protecting agents.

Releasing Agents

Cations that react preferentially with the interferant, preventing the analyte from reacting with the interferant.

Protecting Agents

Prevent interference by forming stable but volatile species with the analyte; common reagents include EDTA and 8-hydroxiquinoline.

Dissociation Equilibria

In the atomizer, there are dissociation and association reactions of oxides and hydroxides, influencing the metallic constituents in the elemental state.

Ionization in Flames

Elements ionize to some degree in flame, with an equilibrium established between atoms, ions, and electrons.

Ionization Suppressor

An easily ionizable species provides a high concentration of electrons in the flame, suppressing ionization of the analyte.

Effects of Flame Temperature

Flame temperatures determine the efficiency of atomization and the relative number of excited and unexcited atoms.

Matrix in Atomic Spectroscopy

Medium that contains everything in the sample except the analyte; standard addition method is used to account for severe matrix effects.