Featured Article
Please see our Atomic Absorption Spectrophotometer section to find manufacturers that sell these products
Atomic absorption spectrometers (AAS) measure the concentration of atoms in a sample in the parts-per-billion range based on their ability to absorb light at a specific wavelength. Most samples analyzed by AAS are liquid samples, and solid samples can also be analyzed after liquefaction.
There are several atomic absorption spectrometers to choose from depending on the specific application.
AAS methods
Atomic absorption spectrometers require a light source, an atomizer, a monochromator, and a detector. AAS systems are either flame atomic absorption spectrometers (F-AAS) or graphite atomic absorption spectrometers (G-FAAS). In F-AAS, the atomizer is a flame furnace, and in G-FAAS, the atomizer is a graphite furnace.
The sample is introduced into the atomizer. In F-AAS, the heat of the flame produces the free atoms. In G-FAAS, the electrical energy in the graphite furnace produces the free atoms.
Generally, the source for atomic absorption is a hollow cathode lamp, which provides emission lines that correspond to the analyte’s absorption spectrum.
A light beam from the light source passes through the atomizer and into the monochromator. The free atoms absorb the light at a characteristic wavelength. The light directed into the detector produces an electrical signal proportional to the light intensity, and the intensity of the light absorbed is an indication of the concentration of the atoms.
Hydride vaporization method
Certain elements form volatile hydrides, and an inert gas carries the volatile hydrides to the atomizer. This method is an add-on to the flame and graphite atomization methods.
Mercury vaporization method
Mercury is reduced to elemental mercury, and an inert gas carries the elemental mercury to the atomizer. This method is used singly with specialized AAS for the determination of mercury or as an add-on to the flame and graphite atomization methods.
Correction for background absorption
Several methods are available for correcting the background absorption. Subtracting the background absorption from the total absorption gives a measure of the analyte absorption.
In the deuterium (D2) method, the light of a hollow cathode lamp provides a measure of the total absorption, and the light of a deuterium lamp provides a measure of the background absorption. In the highspeed self-reversal (SR) method, the self-reversal of the power of the hollow cathode lamp provides a measure of the background absorption. In the Zeeman method, the applied magnetic field gives a measure of the background absorption.
AAS purchasing considerations
In choosing an AAS, many factors should be considered.
Application
An important consideration in selecting an AAS is its area of application.
GBC Scientific Equipment offers the SensAA and the XplorAA AAS, which are compact systems suitable for use in various areas, such as pharmaceutical, clinical, and raw material analyses.
Agilent Technologies offers the 55BAA for corrosive environments. Other issues to consider are the cost of the instruments and the availability of technical support.
“The 210VGP is not only suitable for use in various applications such as mining, metal plating, silicone chip makers, environmental labs, chemistry labs, pharmacology, biophysics, and toxicology research, as well as an aid to understand principles of spectroscopy, it is also economical,” stated Eric Anderson, General Manager at Buck Scientific Equipment.
According to Dan Davies, TOC/AA/ICP/X-Ray Product Manager at Shimadzu Scientific Instruments, “The AA-6200 and AA-7000 are popular as they come with free phone technical support for the life of the instruments.”
Atomizer
The choice of the atomizer and the vaporization method determines sample throughput and the detection limit achievable for individual elements. A flame atomizer provides high sample throughput and a detection limit in the range of 1–100 ppb. A graphite atomizer offers lower sample throughput and a detection limit in the range of 1–0.01 ppb. Incorporating the hydride vaporization and the mercury vaporization methods with both F-AAS and G-AAS enhances the detection limits by 10–100 fold.
The AAnalyst™ 200 and AAnalyst 400 from PerkinElmer Inc. utilize a flame atomizer. The PinAAcle™ 900 series provide mercury and hydride vaporization capabilities.
Buck Scientific offers the 201VGP, which utilizes a flame furnace and a hydride atomizer.
In graphite and flame atomic absorption spectrometers (GF-AAS), both flame and graphite atomizers are present but share the light source. Once the light source is selected, depending on the application, it is pointed toward one or the other atomizer.
The TRACE AI1200 from Aurora Biomed comes with an automatic switch between the F-AAS and G-AAS atomizers.
Beam configuration
Another important consideration is beam configuration. The choice of beam configuration should be made in the context of selecting the atomizer.
AAS use either a single-beam or double-beam configuration. In the single-beam configuration, the light beam is modulated to differentiate between the sample and reference beam. In the double-beam configuration, the light beam is split into the sample and reference beam.
Generally, a single-beam configuration does not provide for correction of light fluctuation compensation, while a double-beam configuration does.
The 205AAS and Accusys 211 from Buck Scientific employ a “stable” single-beam configuration.
The AA-6200 from Shimadzu utilizes a doublebeam configuration and the AA-700 an optical double-beam configuration.
Lamp type
An important consideration in selecting an AAS is the choice of lamp.
Generally, a different lamp is used for each element, and a few elements may be combined in a multielement lamp. Consideration must be given to the wavelength that provides the best sensitivity and to the intensity of the emission line.
The 205AAS from Buck Scientific provides two hollow cathode lamps.
The AA-6200 from Shimadzu has a six-lamp turret with hollow cathode and deuterium lamps.
The 240ZAA and the 280Z AA from Agilent Technologies have a single light source with eight fixed lamp positions.
Background absorption
An important factor in selecting an AAS is the method used for correcting background absorption. This correction affects the detection limits of the AAS.
The AA-6200 from Shimadzu offers the D2 method, and the AA-7000 the SR and D2 methods. The iCE™ 3400 from Thermo Fisher Scientific and the PinAAcle from PerkinElmer offer the D2 and Zeeman methods.
Customization
An important factor in selecting an AAS is correcting for background absorption, which affects the detection limits of the AAS as well. The 240 AA from Agilent Technologies comes with the optional vapor generation accessories VGA 77 and ETC 60 for the analysis of mercury and hydrides.
The AA-7000F from Shimadzu is supplied with the optional AAC-7000 Auto Atomizer Changer, which creates capability for both flame and furnace analysis without changing the atomizer.
Table 1 – Providers of atomic absorption spectrometers
Representative AAS
Agilent Technologies: 240 AA, 240 FS AA, 240Z AA, 280FS AA, 280Z AA, 55B AA, AA Duo
Analytik Jena: contrAA® series (300, 600, 700), novAA® series (350, 400P), ZEEnit series (650 P, 700 P), solid AA®
Angstrom Advanced Inc.: AAS320N
Aurora Biomed Inc.:TRACE AI1200, TRACE 1300, TRACE 1800
Buck Scientific: 205AAS, 210VGP, Accusys 211
GBC Scientific Equipment: SavantAA Zeeman, SavantAA AAS, SavantAA Σ AAS, SensAA, XplorAA
Labnics Equipment: LAAS-210, LAAS-320, LAAS-810, LAAS-990, LAAS A Series (110A, 120A, 130A), LAAS B Series (110B, 120B, 130B)
PerkinElmer Inc.: AAnalyst™ 200, AAnalyst 400, PinAAcle™ 900F, PinAAcle 900Z, PinAAcle 900H, PinAAcle 900T
PG Instruments Ltd.: AA500G, AA500F, AA500FG, AA990F
Qualitest International Inc.: WFX-110A/120A/130A, WFX- 110/120/130, WFX-210, WFX-320
SAFAS S.A.: AA 191, AA 181, AA 170
Shimadzu: AA-6200, AA-7000
Spectrolab Systems: S-990F, S-990G, S-990FG
Thermo Fisher Scientific: ICE™ 3300, ICE 3400, ICE 3500
U-Term International (H.K.) Ltd.: ST-AAS-01, ST-AAS-02, ST-AAS-03, ST-AAS-7002
Going forward
Customization options provide important features to improve on detection limits and provide for a wider spectrum of elements analyzed.
Providers
A list of providers of AAS is given in Table 1.
Lina Genovesi, Ph.D., JD, is a Technical, Healthcare, and Business Writer based in Princeton, NJ; e-mail: [email protected]; www.linagenovesi.com
Please see our Atomic Absorption Spectrophotometer section to find manufacturers that sell these products