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«Dissertation zur Erlangung des Grades “Doktor der Naturwissenschaften” Im Promotionsfach Geowissenschaften Am Fachbereich Chemie, Pharmazie und ...»

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Atomic absorption is one of the key processes in determining analyte concentration in atomic absorption spectroscopy. The process involves in producing free ground-state atoms of the element of interest and the production of free atoms occurs in atomizers and is called atomization. The laminar flow premix system burner is usually applied in modern day atomic absorption spectroscopy. It contains a nebulizer, a premix chamber and a burner head. The premix chamber is designed to mix fuel, oxidant and sample with an additional purpose of filtering off large droplets as these droplets could enhance light-scattering effects in the flame.

Method

Liquid sample is usually introduced into a burner through the nebulizer by the venture action of the nebulizer oxidant with sample at first being aspirated and passing through the nebulizer the liquid stream is then subsequently broken into a droplet spray. The sample and standard solutions viscosity should stay similar in order to avoid the possibility of physical interference problems and particulate matter is not allowed to be present in samples due to severe damage of nebulizers by clogging. Burner heads for premix burners usually present with a single slot 5-10 cm in length. It allows the radiation beam from the source to traverse a lengthy path of atoms and the magnitude of the absorbance is related to the path length of the radiation beam in the flame.

Fig. (4) Zones of a flame burning on a premix burner (Section 8.1 Ref. 78)

Flame is the most ideal atomizer for atomic absorption spectroscopy. Despite extensive research flame is still a rather least well understood component in AAS. However, flames can be broadly defined as primary combustion, interconal, and secondary combustion zones figure(4) with not uniform in composition, length, or cross section. The primary combustion zone is rarely used for absorption work due to reducing and noise conditions and the secondary combustion zone is also not applicable because of the relative low temperature and oxidizing condition. It is therefore only the interconal zone which operates in thermodynamic equilibrium and elongation of the zone can be achieved by shielding a flame.

Method

The atomization process (figure 5) in flames can further be described in follows. Liquid mist produced by the nebulizer and further processed to extract the finest droplets in the premix chamber evaporates on entering the flame. The produced analyte compounds move upward into hotter regions of the flame and agglomerated into small solid particles followed by dissociation into atoms.

Fig. (5) Atomization processes resulting when liquid mist is introduced into a flame (Section 8.1 Ref. 78) Air-acetylene flame is most frequently used in atomic absorption. Choice of flame type is governed by the temperature required. High temperature flame is always preferable in order to minimize chemical and nonspecific background interferences. However, hotter flames (e.g.

nitrous oxide-acetylene, above 3000 oC) can cause severe ionization problem with degradation in measurement precision and inferior detection limits. In the region below 200 nm (e.g. arsenic

193.7 nm), flame gas absorption of the source radiation can be a very serious problem and the argon-entrained air-hydrogen flame should be used to produce best flame transparency conditions. Flame temperature can be adjusted by the proportions of oxidant (air, nitrous oxide) and fuel (hydrogen, acetylene). It is widely acceptable that fuel-rich flames are cooler than oxidant rich flames.

Method (3.4) Hydride generation atomic absorption spectroscopy (HG-AAS) Metals and metalloids can be analyzed by method like atomic absorption spectroscopy (AAS) but for certain metalloids (e.g. Arsenic) such method is not applicable due to the interferences, poor reproducibility and poor detection limits for the method to several metalloids.

Therefore, an improved method so called hydride generation atomic absorption spectroscopy (HGAAS) method has been introduced to rectify the AAS problem with an additional hydride generation module. Many main parts of the HGAAS system are identical to the AAS such as a hollow cathode lamp, air/acetylene flame and optical system. The HGAAS system contains a further complex hydride generation system without using nebulizer and the system is under continuous flow instead of batch flow. The purposes of different parts of the system are briefly summarized in the follows table (12).

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The hollow cathode lamp (HCL) is made up of the element of interest with a low internal pressure of an inert gas. With apply electrical current that the metal can be excited and characteristic spectral lines of particular element can be emitted. The emitted light is directly through the lamp’s glass transparent window in UV and visible wavelengths. In the hydride generation system metalloid oxyanions react with sodium borohydride and HCl to produce volatile hydrides (e.g. AsH3 and SeH2). Care must be taken to produce the specific metalloid oxidation state before the sample is introduced into the hydride generation system and in addition to ensure the time from reagent mixing and the volatile hydride is separated from the liquid and sent to the optical cell has to be accurate. These can be achieved by using a peristaltic pump and tubing of specific lengths. The liquid mixture flows through a unit length of tube and eventually reaches into a gas/liquid separator where the hydride and some gaseous hydrogen bubble out and being purged into the optical cell via a gas transfer line. The following figure (6) demonstrated the systematic set-up for the hydride generator.





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The optical cell is made of fused silica glass tube in which the hollow cathode lamp beam can pass on the way to the monochromator and PMT. The fused silica glass tube locates on top of the normal AAS air/acetylene flame. The gaseous, metalloidal hydride flows into the optical cell from the hydride generation module pushes by an inert purge gas. In the optical cell it decomposes into the elemental form in which can absorb the hallow cathode light beam. The monochromator can isolate the analytical line emitted from the hollow cathode lamp with a specific wavelength and slit width. The role of the monochromator between HGAAS and AAS are the same in which to allow the light not absorbed by the analyte atoms in the optical cell to reach the PMT. Before an analyte is aspirated, a measured signal is generated by the PMT as light from the HCL passes through the optical cell. When analyte atoms are present in the cell from hydride decomposition and in the mean time the sample is aspirated and some of that light is absorbed by those atoms (with only volatile hydride gets to the optical cell and then only decomposed hydride produces the elemental form). This causes a decrease in PMT signal that is proportional to the amount of analyte. This last is true inside the linear range for that element using that slit and that analytical line. The signal is therefore a decrease in measure light. The following figure (7) revealed the arrangement of the optical cell and the flame in the hydride generation atomic absorption spectroscopy (HGAAS) system.

Fig. (7) Figure shows arrangement of optical cell and the flame in HG-AAS (Section 8.1 Ref. 85)

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With double beam instrument, a hollow cathode lamp is divided into two paths under a rotating mirror. One pathway passes through the optical cell and another is around the cell. Both beams are recombined in space so they both hit the PMT but separated in time. The beams alternate quickly back and forth along the two paths. One instant the PMT beam is split by the rotating mirror and the sample beam passes through the cell and hits the PMT. In next instance, the HCL beam passes through a hole in the mirror and passes directly to the PMT without passing through the optical cell. The difference between these beams is the amount of light absorbed by atoms in the optical cell. All of the above application is to compensate for drift of the output of the hollow cathode lamp or PMT.

Operation of HG-AAS instrument can further be described in follows. Lighting up AAS flame process involves setting up the optical cell in right position (above the burner) and connecting hydride gas transfer line. Fuel (Appendix C) and oxidant (Appendix C) are turned on and the flame is lit with the instrument’s auto ignition system. Few minutes later the flame is stable, acidic blank solution is flowed through the sample inlet tube for 10 minutes and deionized water is aspirated between samples in order to stabilize the system. Fuel/air mixture flow control is crucial due to elemental response is based on successful decomposition of the volatile hydride in the heated optical cell. The flame for decomposing volatile hydride in the heated optical cell should maintain at 800oC throughout the measurement. The instrumental performance can be maximized by ensuring the analyte concentrations in the middle linear response range of the instrument and moreover the fuel/oxidant mix flow should be adjusted until the optimized light absorbance is achieved. Furthermore, the position of burner’s head, optical cell and sample uptake rate should be corrected for greatest light absorbance can be obtained. After the measurement, all the inlet tubes (Sodium borohydride, concentrated HCl and sample inlet) should be flowed with deionized water and the fuel supply should be stopped with ignition also to be turned off at the same time. After all the plastic tubing that is stretched around the peristaltic pump head is loosened to length its’ lifetime and the purge gas should also be switched off.

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(3.5) Nonlinearity in analytical calibration graphs Fig. (8) Calibration graph showing departures from ideality (Section 8.1 Ref. 78) An element in which can be determined by AAS is generally containing 4 to 5 orders of magnitude concentration working range. At the upper end of the concentration range, it is common to have the graph bend towards the concentration axis (figure 8) (absorbance against concentration). This could be due to the failure of the monochromator and slit system to prevent multiple, close-spaced lines from reaching the detector. Several lines of differing absorption coefficient (e.g. unresolved multiplets or nonaborbing lines) fall on the detector would lead to nonlinear relationship between absorbance and analyte concentration. It is also important to ensure that the emission source linewidth must be narrower than the absorption linewidth due to variation of absorption coefficient at different frequency.

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(3.6) X-ray fluorescence analysis XRF is a noninvasive analytical methods which are now commonly in use. Bulk specimens can be analyzed under this technique with samples prepared as compressed powder pellets or fused glass discs. X-ray then applies to the samples by an x-ray tube in a potential between 10 to 100 kV. Atoms of the sample interact with this primary radiation and leads to ionization of discrete orbital electrons and electronic rearrangement subsequently occurs with electrons de-excites back to the ground state. As a result, the intensity of the characteristic radiation emission can be measured with a wavelength dispersive x-ray spectrometer and different emission lines in wavelength are obtained and compared with a standard sample.

X-ray is a high-frequency electromagnetic radiation of energy intermediate between the far ultraviolet and gamma-ray regions of the spectrum. Moseley (1913, 1914) formulated the wavelength of an x-ray emission (λ) with the atomic number of an element (Z) by a formula.

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Fig. (9) Example shows schematic representation of the mechanism of x-ray fluorescence of an iron atom leading to the emission of an Fe Kα x-ray (Section 8.1 Ref. 60) The underlying theory of Moseley is related to transitions of electrons between orbitals of discrete energy in an atom. The following simplified Bohr model (figure 9) can be used as an aid of explaining x-ray spectra.

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Fig. (10) Diagram of the electron orbital structure of an iron atom. The number of electrons occupying each shell is indicated, together with the corresponding ionization energy (in keV). Interactions between orbital electrons cause individual subshells to split in energy. In these circumstances, it is energetically more favourable for the last two electrons to occupy the N-shell rather than the M-shell, as might otherwise be expected (Section 8.1 Ref. 60) The energy of an electron orbital is defined as the energy required removing that electron from the influence of the atom and so opposing the electrostatic attraction between negatively charged electron and positively charged nucleus. This energy is measured by the ionization potential of the orbital electron and thus electrons occupying orbital’s of the highest ionization represent states that are energetically the most stable. In figure(10), K-shell possess largest ionization potential with L-, M- and N-shells are progressively more weakly bound to the nucleus. A high-energy x-ray photon can interact with an orbital electron and the electrons in any of the occupied shells can be removed from the electron orbitals by ionization. A vacancy left in K-shell (figure (VII)) can leave the atom in a highly unstable state and to restore the stability, an electron from the L- or M- shell falls down the potential energy gradient to refill this vacancy. The surplus energy which must be lost for this transition to take place is emitted as an x-ray having an energy characteristic of the difference between the two orbital levels.

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(3.7) Inductively coupled plasma mass spectrometry (ICP-MS) ICP-MS is a robust trace multi-element analysis technique which encompassed four main components shown at following figure (11).



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