Olaf Schulz, Product Manager for ICP-OES, talks to AZoM about why users are switching from FAA techniques to ICP-OES techniques.
Could you give us a brief overview of Flame AAS and ICP-OES technology?
FAA technique relies on ground state atoms and the absorption of light that passes through the flame containing the atoms. Especially for trace and minor concentrations and where only few elements need to be analyzed, FAA is the technique of choice and likely will be in the future.
ICP-OES requires different, much higher temperatures for emission to take place for most of the elements. Because of the high temperatures present in an inductively coupled argon plasma not only atomic, but also ionic emission takes place. This is an advantage since many metals have sensitive ionic emission lines. One reason why ICP-OES, compared to the AAS technique, provides higher detection sensitivity.
What are the main reasons why someone would switch from using Flame AAS to ICP-OES spectrometers?
Today’s ICP-OES systems are predominantly simultaneous measuring systems and so the analyses times are independent from the number of elements to be analyzed – a particular advantage if many elements need to be analyzed. FAA is purely sequential (element by element analysis). But FAAs are fast, require only about 3 sec for one replicate measurement versus 30 sec ICPs typically use. This means that when more than 10 elements need to be analyzed an ICP is faster. In praxis the parity point is lower since an FAA requires lamp changes, stabilization, another pre-flush etc.
The other reasons are freedom from chemical interferences and a generally higher sensitivity for many elements. Also for wide concentration ranges and in the case of samples with higher amounts of totally dissolved solids, the ICP-OES technique offers advantages.
What are the key strengths and limitations of each technology’s analytical capabilities for specific element ranges including technical aspects such as spectral interferences as well as chemical and ionization interferences?
Because chemical flames are used, the maximum achievable temperatures are limited. This has advantages for the determination of the group I alkali elements. While those elements can even be determined in emission mode, FAA provides by far better sensitivities compared to the ICP-OES technique. However, this is also a disadvantage. Elements with, for example, a higher affinity to oxygen may easily form oxides, which will not dissociate since the temperatures in the flame are not high enough. These so called chemical interferences are quite common and need to be addressed by using additional chemicals or different flame parameters for different elements.
In contrast, an inductively coupled argon plasma with temperatures up to 10,000K is more powerful, extremely robust and is thus less influenced by the sample introduced. It is also chemically inert and does not influence the constituents of the sample. Chemical interferences are therefore completely eliminated; existing ionization interferences are easily compensated by only viewing a cross section of the plasma from the side. Inductively coupled plasmas are also insensitive to high sample loads and ideally suited for the direct introduction of organic matrices.
ICP-OES provides generally a higher sensitivity for metals and also enables the measurements of non-metals. Because of the high temperatures in the plasma the sensitive ionic emission lines of those elements are usable.
Drawbacks are spectral interferences. Emission spectra, particularly with metals, can be line rich. This fact needs to be addressed with an appropriate line selection, mathematical correction and optical design.
How does operational cost differ between the two?
ICP in general does not have the best reputation in terms of gas consumption, since it is known to consume a lot of argon. While there is a drastic difference when comparing per minute flow rates and for less than 10 elements and the use of acetylene-air only, the FAA technique has a cost advantage. An ICP-OES advantage is achieved when 17 elements or more need to be analyzed. In cases where nitrous oxide needs to be used as an oxidant the parity point is even reached much earlier. ICP, in principle, enjoys a cost advantage the higher the number of elements that need to be analyzed, since the cost remain the same, independent from the number of elements to be analyzed.
Obviously a much higher number of samples can be processed per day and, additionally, the ICP can be easily automated and since no flammable gases are involved, the instrument can be left unattended.
Are there any significant differences in throughput?
Measuring element by element and replicate by replicate, much more time is spent compared to the fully simultaneous capture of the spectrum. While an ICP-OES, independent from the number of elements to be analyzed, is easily able to process up to 320 samples in 8 h, at 17 elements flame atomic absorption only manages up to 140.
Would switching incur significant cost?
ICP-OES has become much less expensive in recent years and so several FAA applications are now well within the scope of the ICP-OES method. Switching does not incur huge costs. The ICP-OES typically fits the same space as the FAA instrument and only requires an exhaust, argon and electric power for operation.
How do the detection limits differ between AAS and ICP-OES?
Particularly entry level ICP-OES compare well to the flame atomic absorption technique. While a number of elements show a comparable performance, flame atomic absorption provides a higher sensitivity for the group I alkali elements, while the refractory elements, the metals, can be analyzed with higher sensitivity using ICP-OES. Since there is seldom a need for trace level analysis of alkali elements, an entry level ICP should therefore be able to fully replace flame atomic absorption in almost any case.
How have SPECTRO developed systems like the GENESIS to offer better limits of detection?
Most important is minimized light loss so that as much of the light emitted in the plasma as possible is available for evaluation. This can be achieved by using few optical components and by avoiding transmission optical components.
This is exactly the concept of an optical system in Paschen-Runge mount. With this technique, all components of the optical system are arranged on a circle. A further developed design of this concept, called ORCA, is used in all SPECTRO ICPs. The one installed in our entry level ICP, the SPECTRO GENESIS, used for comparison with the FAA technique, provides fully simultaneous detection in a wavelength range between 175 and 770 nm. There are no transmission optical components included and so it also provides high transmission in the UV as well as a uniform resolution over a wide spectral range.
How do you plan on developing systems like the GENESIS further?
I believe operational costs will be an even bigger subject in the years to come and so, a further reduction will definitely be on the agenda for the future.
Where can our readers learn more?
A good book for further reading is definitely “Analytical Atomic Spectrometry with Flames and Plasmas by" José A.C. Broekaert.
About Olaf Schulz
Olaf Schulz is the Product Manager for ICP-OES and is responsible for the strategic development of the complete product line including application development and product support. Olaf’s experience in optical emission spectroscopy for materials analysis and testing spans more than 30 years. With an engineering degree in physics, he started his career in the auto industry, initially at Robert Bosch GmbH and then at Dr. Ing. h.c. F. Porsche AG. Olaf moved to SPECTRO Analytical Instruments GmbH in 1989 with assignments in application development, sales, and marketing.
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