Modern Exam 2 - AAS/AFS (Ch. 9)

Flame and electrothermal atomizers

The two most common atomizers for AAS/AFS.

Inductively coupled plasma (ICP)

The most common atomizer for AES.

Flame atomizers

Nebulization, desolvation, volatilization, dissociation

Nebulization

Droplet formation from collisions with gas oxidant and liquid.

Desolvation

Solvent evaporation to produce a finely divided solid molecular aerosol.

Volatilization

A flame produces gaseous molecules.

Dissociation, ionization, and excitation

Depends on the fuel and oxidant.

Interzonal region

The most widely used region of a flame atomizer because free atoms of the analyte are prevalent.

Monochromators can sample the radiation from a relatively small region of the flame so optimization occurs by adjusting the position of the flame with respect to the entrance slit.

Flame Temperature and Flame Absorption Profiles

laminar flow burner

Instrument for flame atomization

Most reproducible of all liquid-sample introduction methods developed for AAS and AFS. Has lower sensitivity and sample efficiency.

Properties of the Flame

Electrothermal Atomizer Furnace Steps

Drying (usually just above 110 deg. C.), Ashing (up to 1000 deg. C), Atomization (Up to 2000-3000 deg. C), and Cleanout (quick ramp up to 3500 deg. C or so).

Small sample size (1 μL or 10−10 g to 10−13 g) and a parts per trillion (ppt) concentration limit of detection.

Advantages of Electrothermal Atomizers

Irreproducibility (relative standard deviation is 5-10%), takes minutes per sample, and the analytical range is relatively narrow (<2 orders of magnitude).

Limitations of Electrothermal Atomizers

Plasma

An electrically conducting gaseous mixture containing a significant concentration of cations and electrons (net charge is zero).

2-3 times hotter than the hottest flame, more complete atomization, fewer chemical interferences, chemically inert environment, and a uniform temperature cross section.

Advantages of ICP

Requires a narrow source bandwidth relative to the width of an absorption line or band. Using ordinary spectrometers result in a nonlinear response, poor sensitivity, and small signals.

Beer's Law Limitation in AAS

Line Sources

Are used to mitigate the limitations of Beer's Law in AAS, but a separate source lamp is needed for each element.

Hallow-Cathode Lamps (HCL)

The most common line sources for AAS, utilizes sputtering and redeposition

Sputtering

Gaseous cations (Ne+ or Ar+) are generated to produce sufficient kinetic energy to dislodge the cathode surface creating an atomic cloud.

Redeposition

Atoms in the atomic cloud diffuse back to the cathode surfaces or to the glass walls.

Higher voltages lead to greater intensities but also an increase in Doppler broadening of the emission lines. Higher currents can lead to self-absorption.

HCL Tradeoff Considerations

The intensity of the source fluctuates at a constant frequency to eliminate residual effects and interferences from flame emission.

Source Modulation

Single-Beam Instruments

Modulate via the power source and can account for some effects of the flame on the signal.

Double-Beam Instruments

Modulate via an optical chopper and do not account for intensity loss and/scattering due to the flame.

Spectral interference

Arises when absorption or emission of interfering species overlaps with the analyte absorption or emission that resolution by the monochromator becomes impossible.

Chemical interferences

Result from reactions that occur during atomization that alter the absorption characteristics of the analyte.

Continuum-Source Correction

An interference correction method where absorbance is measured with a broadband source and an HCL, and the difference is taken.

Zeeman Effect Correction

An interference correction method that uses polarized light and a magnetic field; it is useful for electrothermal atomizers.

Source Self-Absorption/Self-Reversal Correction

An interference correction method based on the self-absorption behavior of radiation from an HCL at high and low currents.

Releasing agents

Cations that react preferentially with interfering anions to eliminate chemical interferences.

Protective agents

Prevent chemical interference by forming stable but volatile species with the analyte.

Flame AA Detection Limits

1-20 ppb, more reproducible with ~1% RSD.

Electrothermal AA Detection Limits

1-20 ppt ((10⁻¹⁰ to 10⁻¹³ g), more sensitive, but slower and with a higher RSD of 5-10%.

Atomic Fluorescence Spectroscopy (AFS)

Has few advantages when compared to AAS/AES and is expensive and specialized.

Electroless discharge lamp (EDL)

The source for AFS, which is about twice as intense as an HCL.

Nondispersive instruments (AFS)

Are lower cost, adaptable, high throughput, and allow for simultaneous collection