Further Reading
6.2 Instrumentation
6.2.3 Atomizers
6.2.3.2 Furnace Atomizers
Furnace atomization is defined using several names such as Graphite Furnace Atomic Absorption Spectrometry (GFAAS) or ETAAS. In contrast to the flame atomizers in which the analyte atoms are in a dynamic equilibrium, in furnace atomizers the signal is caused by only the number of atoms derived from the sample placed in the furnace.
Ionized atomic vapour, M+(g) + e- M+*(g) + e- Ionization
Atomic vapour, M(g) M*(g) Atomization Molecular vapour, MA(g) MA(g)
Vaporization Solid aerosol
Desolvation Liquid aerosol
Nebulization
Nebulizer and spray chamber
Suction by vacuum Sample Solution
Flame
*
Figure 6.4 The stages of analyte species in a flame atomizer
The furnace atomizer mainly consists of a graphite cuvette that is typically 3–5 cm long and 5–9 mm in diameter, as shown in Figure 6.5. The graphite cuvette is kept in an Ar or N2atmosphere to prevent burning of carbon. Ar is a better inert gas since in the presence of carbon N2will form CN radicals whose spectrum will cause spectral interferences. Graphite sublimes at around 3000 °C and thus can be heated up to this temperature in an inert environment. The cuvette is resistively heated by electricity.
The protective jacket around the cuvette sustains the inert atmosphere; cooling of the system is provided by water circulation. 5–50 µL of sample solution is introduced through the sampling port either manually by a plastic-tip pipette or an automatic sam- pling device. The position of the drop in cuvette is important; the drop position should be reproducible. Recently, a system for observing this position has been developed using an optical monitoring device whose image can be displayed on the screen of a personal computer, as shown in Figure 6.6.
The heating of graphite cuvette can be programmed regarding the time and the temperature, allowing custom design of the steps required for a particular analysis.
The following steps are commonly employed:
Drying. The sample is usually in a solution form. A temperature below the boil- ing point of solution is applied. The drying should be smooth; boiling should not take place. Otherwise, the contents may be scattered to different locations of the atomizer in each determination, and thus the precision and accuracy may are adversely affected. The drying regime may consist of a linear heating stage, a ramp or their mixture. The inert gas flow is maintained to remove the solvent vapour.
Ashing. This is very similar to the ashing step that is commonly performed in many analytical procedures requiring the destruction of matrix. The purpose of ash- ing is to oxidize and remove the organic content in the sample. The ashing temper- ature should be as high as possible to remove the maximum matrix content, and yet
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Figure 6.5 Schematic representation of a graphite cuvette and signal formation
low enough not to cause any analyte atomization. The ashing period commonly includes ramp heating so that the organic compounds are sequentially and smoothly removed without causing any explosions in the sample mass. Typically, temperatures ranging in 200–2000 °C with inert gas flow are employed for ashing. The heating regime may be designed by the performer as needed. An incomplete ashing will leave organic materials in the cuvette; this will possibly cause interference during the atomization stage. The presence of the organic molecules and decomposition prod- ucts in cuvette during atomization cause a high extraneous background signal; this can be separately observed in most AA spectrometers.
Atomization. After the sample is cleaned from its organic content as much as pos- sible, temperature of the system is suddenly increased to cause analyte atomization.
Depending on the analyte, applied temperatures vary between 1200 and 3000 °C.
Excessively high temperature adversely affects the cuvette life; in addition, the sen- sitivity may be lowered due to the rapid expansion of the inert gas transporting the analyte species out and minimizing the residence time. The atomization of analyte will include several steps from its solid compound to analyte atoms. During these intermediate stages, volatile oxides and/or halides of analyte may form. Owing to the increase in temperature, the inert gas will rapidly expand and will tend to diffuse out Figure 6.6 Monitoring of the sample drop position in furnace atomizer using an optical device and personal computer; A, correct injection depth; B, capillary too low; C, capil- lary too high (Reproduced with permission from Thermo Electron Corporation, UK)
of the cuvette together with the analyte species, leaving the volume in which the ana- lytical signal is to be formed. Although the inert gas flow in the cuvette is usually interrupted during the atomization cycle, the rapid expansion of the contents may transport some of the analyte in unatomized form, out of the cuvette; this will cause a reduction in the analytical sensitivity. Therefore, if the rate of increase in temper- ature is faster than the diffusion rate of the volatile intermediate analyte species, an efficient atomization will take place; the intermediate species will still form, but without being able to diffuse out of the atomizer they will be decomposed to provide the analyte atoms. In modern graphite atomizers, a maximum power is applied for a rapid heating, as soon as the temperature of the cuvette reaches to the desired level, this is optically detected by graphite furnace emission and the power is leveled off.
As the analyte atoms are formed and then diffuse out of the cuvette, a transient sig- nal is formed and recorded, as shown in Figure 6.5. During atomization, inert gas flow may be continued or a reduced or mini-flow may be applied at the expense of sensitivity. The analytical signal may be the height of the peak in units of A, absorbance, or the peak area may be utilized using the units of A⋅s, absorbance times seconds.
Cleaning. The atomization temperature should be only as high to effect the ana- lyte atomization, as mentioned above. However, the sample often includes other species that remains unvolatilized after the atomization stage. Therefore, in order to clean the cuvette prior to the next atomization, maximum temperature available for the system is applied for about 5 s accompanied by the inert gas flow. In order to have an efficient and reproducible atomization, the temperature programme for a furnace atomizer should be carefully optimized. A typical temperature programme is shown in the Figure 6.7.
It should be remembered that the instrumentation available today allows using as high as about 20 steps for a temperature programme; especially the drying and ash- ing steps may contain several linear and/or ramp heating regimes as required for the optimum conditions. When a sample with a new matrix is to be analyzed using ETAAS, it is customary to optimize the maximum ashing and the atomization tem- peratures as an important part of the temperature programme. This is often realized by using an ash-atomize plot, as shown by the Figure 6.8. Iterations may be neces- sary using this approach. Using a predetermined atomization temperature, the sam- ple is ashed at varying temperatures prior to atomization. The recorded atomization signal starts to decrease if the ashing temperature employed becomes high enough to effect analyte atomization, leaving less analyte to the atomization step. For the other part of this plot, a predetermined ashing temperature is used and the atomiza- tion temperature is varied. Usually the minimum atomization temperature that will effect full atomization is selected for the reasons mentioned above. The reactions and the behaviours of analyte and interferants are complex in an atomizer; therefore, the comments to be made here should be taken only as general guidelines.
In most furnace designs, the cuvette is in contact with the graphite electrodes at both ends providing a longitudinal heating. In this case, similar to any other resistor, the graphite cuvette will have the highest temperature in the centre and will be grad- ually cooler towards the ends. Having a thermal gradient in the atomizer causes problems; for example, atoms formed in the centre may be condensed at the cooler
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Figure 6.7 A typical temperature programme for a furnace atomizer. a: drying; b: ashing; c:
atomization; d: cleaning; AA: atomic absorption (corrected); BA: background absorption
Figure 6.8 Ash-atomize plots in a graphite furnace atomization
ends. Recently, novel designs were made available where the cuvettes are heated not from the ends, but its sides providing a transverse heating. The principle of longitu- dinal and transverse heating with the temperature profiles along the cuvette is shown in Figure 6.9. Almost ideal isothermal conditionsare obtained using transverse heat- ing with a more uniform temperature distribution as compared to the longitudinally heated cuvette.
There are many other methods and approaches to improve the quality of a deter- mination using ETAAS; these will be handled after the interferences and background correction methods are discussed.