Further Reading
6.5 A General Evaluation and Capabilities of AAS Systems
There have been no exciting improvements in instrumental design characteristics of AA spectrometers in the last decade. Most of the instruments in the market are well designed, excellent spectrometers. Although very powerful techniques such as ICP- OES and ICP-MS are now available with multi-element capabilities and thus high sample throughput figures, AAS still survives in many laboratories, as it is compar- atively economical and simple instrument.
Among the many designs offered in the market, the choices are to be made for the single- and double-beam spectrometers, Zeeman and D2background correction sys- tems, instruments with different levels of automation and unattended operation, fairly different accessories for HGAAS and CVAAS, now mostly flow systems. The deci- sion will depend on the specific needs and the workload of a laboratory. While a mostly academic research laboratory requires a flexible system, on the other extreme is a laboratory with a well-defined routine and heavy workload requiring a well- automated spectrometer to produce results accurate and fast. For the best economical decisions, one should always choose the least sensitive system if the problem could be solved this way. Therefore, if a simple flame atomizer is sufficient, the purchase of an ETAAS system is not needed until the new problems arise justifying its presence.
Feedback systems for most of the single-beam instruments are so perfect that the need for a double-beam design is not really justified; besides the latter has a poorer S/Nvalue. Regarding which background system to prefer, although very often the Zeeman systems are presented as the best, the very same manufacturers having this claim continue to produce spectrometers with D2source background correction sys- tems. The contemporary D2 designs are now much improved with their high modu- lation frequencies; so that sudden changes in background signals can be successfully corrected. Instrument manufacturers have documentation illustrating the capabilities of background correction systems, the weakness and better performances of each background system for specific analysis problems. The customer with well-defined analytical problems should discuss these properties and decide accordingly.
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Figure 6.18 Schematic illustration for AA-6200 spectrometer (Adapted with permission from Shimadzu Corporation, Japan)
Figure 6.19 Schematic illustration of contrAA continuum source, multi-element AAS system (Adapted by permission from U. Heitmann, ISAS, Department Berlin, Germany) S: Xe source; AC: atom cell, flame or furnace; EG: echelle grating; P: prism;
D: detector, CCD; HRM: high-resolution monochromator
Flow systems for flame, HGAAS and CVAAS are very commonly available.
Home-made batch systems can be easily and economically prepared by the labora- tories that may find these sufficient. Commercial flow systems provide higher sam- ple throughput, well suited to heavy routine work.
A double beam AA spectrometer, Model AA-6200, manufactured by Shimadzu Corporation is shown in Figure 6.18. In such a typical double-beam instrument, D2 background correction is sufficient to solve most of the background absorption
Table 6.1. Detection limits using AAS techniques,µg/La(Adapted from Reference 40)
Analyte FAAS HGAASb ETAAS Analyte FAAS HGAASb ETAAS
Ag 1.5 0.005 Na 0.3 0.005
Al 45 0.1 Nb 1500
As 150 0.03 0.05 Nd 1500
Au 9 0.15 Ni 6 0.07
B 1000 20 P 75000 130
Ba 15 0.35 Pb 15 0.05
Be 1.5 0.008 Pd 30 0.09
Bi 30 0.03 0.05 Pr 7500
Ca 1.5 0.01 Pt 60 2.0
Cd 0.8 0.002 Rb 3 0.03
Co 9 0.15 Re 750
Cr 3 0.004 Rh 6
Cs 15 Ru 100 1.0
Cu 1.5 0.014 Sb 45 0.15 0.05
Dy 50 Sc 30
Er 60 Se 100 0.03 0.05
Eu 30 Si 90 1.0
Fe 5 0.06 Sm 3000
Ga 75 Sn 150 0.1
Gd 1800 Sr 3 0.025
Ge 300 Ta 1500
Hf 300 Tb 900
Hg 300 0.009c 0.6 Te 30 0.03 0.1
Ho 60 Ti 75 0.35
In 30 Tl 15 0.1
Ir 900 3.0 Tm 15
K 3 0.005 U 15000
La 3000 V 60 0.1
Li 0.8 0.06 W 1500
Lu 1000 Y 75
Mg 0.15 0.004 Yb 8
Mn 1.5 0.005 Zn 1.5 0.02
Mo 45 0.03 Zr 450
aAll the values were found by using standard aqueous solutions and are based on 3s (98% confidence level). System 2 electrodeless discharge lamps were used whenever available with a Model AAnalyst 800 spectrometer. ETAAS detection limits were found by using 50 µL sample volumes, an integrated platform and full STPF conditions using AAnalyst 800 spectrometer.
bHGAAS detection limits were determined using MHS-15 Mercury/Hydride system.
cHg detection limits by CVAAS technique were found by using a FIAS-100 or FIAS-400 flow injection system with amalgamation accessory.
problems. The flame atomizer can be replaced by a heated quartz atomizer for HGAAS or cold absorption tube for CVAAS. As almost all other AAS instruments in the market, Shimadzu AA-6200 is controlled and operated by a personal com- puter; a few adjustments can be done manually.
Recently, a continuum source AAS instrument has been manufactured by Analytik Jena A. G. The system employs a single continuum Xe source to cover a wavelength range of 189–900 nm, an Echelle monochromator and a CCD array detector system.
This instrument is named as contrAAand is capable of multi-element determination in a sequential mode. A schematic illustration of contrAA is given in Figure 6.19.
A list of detection limits is given in Table 6.1; the values are provided by Perkin Elmer Instruments.40The values given in Table 6.1 are not necessarily comprehen- sive. Some of these elements can be determined by HGAAS technique although the table contains no such data; among these are Ge, Pb and Sn. On the other hand, although detection limits are given for some elements, their determination by AAS is not a part of common practice. Some of the rare earth elements are in this group, such as Gd, Lu, Nd, Pr, Sm and Tb. ICP-OES and ICP-MS techniques offer better detection limits for most of these elements. Table 6.1, however, contains the data available from the laboratory of a well-known manufacturer, and is very useful to have a general idea about the capabilities of AAS. Detection limits are available from literature and also from other manufacturers; the figures for an element may vary as much as 10-fold depending on the source. The user should remember that most of the commercial figures are for an easy water background. The detection limit for real samples and complex matrices may be about 10 times higher. It should also be remembered that LOQ values, determining the lower limit of actual applications, are about 5–10 times higher than detection limits.
References
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Further Reading
1. B.V. L’vov,Atomic Absorption Spectrochemical Analysis, (Translated by J.H.
Dixon), American Elsevier Publishing Company, Inc., New York, 1970.
2. B. Welz and M. Sperling, Atomic Absorption Spectrometry, 3rd edn, Wiley- VCH, New York, 1999.
3. L.H.C. Lajunen,Spectrochemical Analysis by Atomic Absorption and Emission, The Royal Society of Chemistry, Cambridge, 1992.
4. J.R. Dean,Atomic Absorption and Plasma Spectroscopy, 2nd edn, Analytical Chemistry by Open Learning, Wiley, Chichester, 1997.