Atomic Absorption and Graphite Furnace Instruments

Question 1

What is pneumatic nebulizer?

Answer 1

Where sample solution is drawn by rapid flow of oxidant (usually air) past the tip of the sample capillary

Question 2

Name eight techniques of sample introduction for atomic spectroscopy, if possible, indicate whether the sample is meant to be solid or solution

Answer 2

Pneumatic nebulizers
Ultrasonic nebulizers
Electrothermal vaporizeres (liquid and solid)
Hydride generation technique
Direct sample insertation (solid)
Arc and spark ablation (solid)
Laser ablation
Glow discharge techniques

Question 3

Describe ultrasonic nebulizers

Answer 3

Similar aerosol effect to pneumatic, however, quartz crystal used to produce a dense, homogenous mist.

Question 4

Describe electrothermal vaporizers (liquid and solid)

Answer 4

Sample is vaporized by rapid resistive heating of a graphite or tantalum material. Material is entrained in a flow of inert gas. A transient peak is observed. Peak height or peak area used to quantify.

Question 5

Describe Hydride generation techniques

Answer 5

Volatile hydrides are generated by a chemical reaction (see below). Higher transfer efficiency of specific analyte types; arsenic, antimony, tin, selenium, bismuth and lead. Results in an increase in sensitivity of 10 to 100.

Question 6

Describe direct sample insertion (solid)

Answer 6

Sample is physical placed into atomizer by a probe or similar and transient signal is produced

Question 7

Describe arc and spark ablation (solid)

Answer 7

Electrical discharges involving the surface of a sample can lead to the ablation of surface material to form a plume of particulate and vapors which is then swept into atomizer by inert gas.

Question 8

Describe laser ablation

Answer 8

Material ablated from surface by an intense laser beam. Versatile because the material can be of almost any type

Question 9

Describe glow discharge techniques

Answer 9

A glow discharge takes place in a low-pressure atmosphere (1 to 10 torr) of argon gas between a pair of electrodes maintained at a dc potential of 250 to 1000 V. The argon gas to break down into positively charge argon ions and electrons. The electric field accelerates the argon ions to the cathode surface that contains the sample. Neutral sample atoms are sputtered from surface.

Question 10

Name and describe five sample atomization techniques

Answer 10

Flame atomization: the sample is nebulized into a gaseous oxidant flow, mixed with a fuel and then transported into a flame region for atomization.
Eletrothermal atomization: a small aliquot of sample (microliter) injected into a graphite furnace where it is atomized via resistively heating to ~3000°C
Glow discharge atomization: small amounts of solid conductive samples are sputtered form a surface acting as an electrical cathode. An electrically accelerated stream of ionized argon is provides the energy for the sputtering and atomization process
Hydride atomization: volatile metal hydrides are produced from a chemical reaction and then atomized by heating in a quartz tube.
Cold-vapor atomization: mercury metal has a sufficient volatility to be atomized by vaporization and thus the atomic absorption (@253.7 nm) can be obtained cold. Can produce low ppb detection limits.

Question 11

Label and describe the components of a flame structure.

Answer 11

• *Primary combustion zone**: in hydrocarbon, blue luminescence zone (C₂, CH etc. )Thermal equilibrium not yet reached.
• *Interzonal region**: common zone to use form spectroscopy. Rich in free atoms and the hottest part of the flame

Primary combustion zone: in hydrocarbon, blue luminescence zone (C₂, CH etc. ). Thermal equilibrium not yet reached.
Interzonal region: common zone to use form spectroscopy. Rich in free atoms and the hottest part of the flame
Secondary combustion zone: atoms and other reagents converted to more stable species, such as oxides.

atoms and other reagents converted to more stable species, such as oxides.

Question 12

What is the function of electrothermal atomizers (graphite furnace) and its advantages/disadvantages?

Answer 12

Small aliquot (microliter) of sample (or entire sample) is introduced into a graphite furnace that is heated electrically to 2000°C to 3000°C.

Advantages: sample size of 0.5 microliter to 10 microliter, with detection limits of 10-10 to 10 -13 g of analyte.
Disadvantages: reproducibility (RSD) 5% to 10% whereas in flame 1% or better. Method are slow, dynamic range is low.

Normally graphite furnace is normally only used when detection limits are an issue.

Question 13

Name and describe the types of interference in atomic absorption.

Answer 13

Spectral interference: These results from the absorption or emission of radiation by an interfering species that overlaps or cannot be resolve form the absorption of the analyte species.
Chemical interference: these result from various chemical processes that are occurring during atomization that alter the absorption characteristics of the analyte

Question 14

How to correct spectral interferences in graphite furnace?

Hint: there are three ways.

Answer 14

Using hollow cathode can result in very narrow and well characterized emission lines.
Use background correction for fuel and oxidant interferences
Two line correction: an additional spectral line from the source, close in frequency to the analyte wavelength can be employed. (special case and rare)
Continuum source correction: signal from a continuous (deuterium lamp) is alternately passed through the analyte zone. Most common
Zeeman effect correction (see image): in a strong magnetic field, the magnetic field generated by the spinning electron alters the “energy” or wavelength of transitions. For Singlet transitions, 3 lines –sigma, pi, +sigma result. Pi lines absorb radiation polarized parallel to magnetic field. Sigma perpendicular to field.

Question 15

How to treat chemical interference in atomic absorption?

Answer 15

• Typically, treat flame as a stable solvent and apply equilibrium concepts
  
  • Low volatility compound: anion presence leads to non-volatile ionic species ie. Calcium absorbance drops off linearly with sulfate or phosphate contamination.
  • The presence of other metals can also reduce signal possibly due to complex, heat stable metal oxides being formed.

Dissociation Equilibria: in a flame complex equilibrium are established such as alkaline-earth oxides are stable, lead to broad molecular bands. Other reactions also reduce such as Na + Cl ↔ NaCl

So, reduce or eliminate by higher temperature

• Add releasing agents to preferably bind with interference
• Add protective agents to form stable volatile complexes such as EDTA

Question 16

What is plasma?

Answer 16

An electrically conducting gaseous mixture containing significant concentrations of cations and electrons.