Técnicas e instrumentos de recolección de datos

Most common methods of analysis of metal ions in samples are; atomic absorption spectroscopy (AAS), (Omonayi et al., 2011), X- ray fluorescence (XRF) (Beckholf et al., 2006), Induced coupled plasma with atomic emission spectroscopy (ICP-AES) (Wanjau et al., 2002) and mass spectrometry (MS) (Becker et al., 2008). This study used EDX spectroscopy since it is a multi-elemental analysis and FAAS because it is easy to use, precise and has high sensitivity in addition it was available.

2.7.1.1 Working principles of FAAS

The FAAS works on the principle of absorption of radiation by atoms at discrete wavelengths characteristics of absorbing species (Skoog et al., 1998). The radiation beam is attenuated by the amount that is proportional to the concentration of the element under consideration in the atomizer (American chemical society, 2006). The concentration of an element is measured by the absorption of radiation of characteristic frequency by free atoms of an element. The strength of this method is that atoms absorb only a very narrow range of wavelength as compared to the molecular species. A hollow cathode lamp produces light of certain wavelength with cathode made of element of interest (Van Loon, 1981). Atomization of the element can be achieved by introducing a fine spray of test solution through the nebulizer into air/ acetylene or nitrous oxide, acetylene flame.

When solution enters into the flame, it is dispersed into a mist of very small droplets, which gets evaporated to give a dry salt. Part of the salt is dissociated into atoms of the element of interest, which then absorbs radiation from an external source. The

unabsorbed radiation from the flame is allowed to pass through a monochromator, which isolates the existing spectral lines of the light source. The resulting radiation is then quantified by the detector and the absorbed quantity determined by calculating the difference from the incident radiation.

The working of the atomic absorption spectroscopy involves absorption of light that is associated with process of transition from one steady state to another. For instance the case of study state 1 and 2 where > , the 1-2 transition results in the absorption of light with frequency as shown by Maxwell- Boltzmann equations 2.1 and 2.2;

⁄ ……….equation 2.1

Where h is the planks constant (Skoog et al., 1998)

The Maxwell -Boltzmann law gives relative number of atoms in the excited state to the number of atoms in ground state as follows;

⁄ ⁄ ( ⁄ ) ………equation 2.2

Where K is the Boltzmann constant t is the temperature in degrees kelvin and E2 is the

energy difference between the excited state and ground state. The quantities P2 and P1

are statistical factors that are determined by the number of states having equal energy at each quantum level. The relative fraction in the excited state is dependent on temperature (Skoog et al., 1998).

The components of an atomic absorption spectrometer include; (a) Hollow cathode source

It is a source of radiation used in most AAS instruments. The cathode is made of metal of interest. The lamp consists of tungsten anode and a cylindrical cathode sealed in a glass tube that is filled with neon or argon at a pressure of 1 to 5 torr. Figure 2.3 shows the scheme of FAAS.

Figure 2.3: Scheme of FAAS (b) Atomizer

There are two types of commonly used atomizer; flame and electro thermal. In the flame the final temperature is determined by flow rate and the ratio of oxidant to fuel. When the disolvation occurs in the flame, it is evaporated to produce a finely divided solid molecule aerosol. Dissociation of these molecules then leads to an atomic gas. Some of the atoms then ionize to give cations and electrons.

(c) Monochromators

Monochromator are filters, prisms or gratings that disperse radiations so that selected wavelengths corresponding to a particular energy within the sample is transmitted to a detector.

(d) Detectors

There are several kinds of detectors including phototubes, photomultipliers tubes photo diode arrays detectors. These detectors convert radiant energy into electrical signal.

(e) Readout system

Read out system is digital and coupled with microprocessors that allow the programing of various aspects bringing simplicity to separate procedures such as calibration and calculation of concentration.

2.7.1.2 Working principles of energy dispersive x- ray spectroscopy

An energy x- ray spectrophotometer equipped with solid state detector was employed for the analysis of the elemental oxides. Energy-dispersive spectrometers employ pulse height analysis. Detector which gives output pulses proportional in height to the x-ray photon energy is used in conjunction with a pulse height analyzer (in this case a multichannel type) (Russ, 1984). A solid state detector is used because of its better energy resolution. Incident x-ray photons cause ionization in the detector, producing an electrical charge, which is amplified by a sensitive preamplifier located close to the detector. Both detector and preamplifier are cooled with liquid nitrogen to minimize electronic noise. Si (Li) or Si drift detectors are commonly in use. Since the electron probe analyses only to a shallow depth, specimens should be well polished so that surface roughness does not affect the results. Sample preparation was essentially as for reflected light microscopy, with the provision that only vacuum compatible materials must be used. Opaque samples may be embedded in epoxy resin blocks. For transmitted light viewing, polished thin sections on glass slides are prepared. In

principle, specimens of any size and shape (within reasonable limits) can be analyzed. Holders are commonly provided for 25 mm diameter round specimens and for rectangular glass slides. Standards are either mounted individually in small mounts or in batches in normal sized mounts.

The relative intensity of an X-ray line is approximately proportional to the mass concentration of the element concerned (Castaing, 1951). This relationship is due to the fact that the mass of the sample penetrated by the incident electrons is approximately constant regardless of composition. (The electrons are decelerated by interactions with bound electrons and the number of these per atom is equal to the atomic number, which, in turn, is approximately proportional to the atomic weight). Given this approximation, an 'apparent concentration (C') can be derived using equation 2.3.

[

⁄ ] ………..equation 2.3

Where and are the intensities measured for specimen and standard respectively, and is the concentration of the element concerned in the standard. The energy dispersive x-ray spectrometer is especially useful for qualitative analysis because a complete spectrum can be obtained very quickly. Aids to identification are provided, such as facilities for superimposing the positions of the lines of a given element for comparison with the recorded spectrum. In performing qualitative x-ray analysis, we have to identify the specific energy of the characteristic x-ray peaks for each element. This information is available in the form of tabulations, graphs or as computer

database. The energy-dispersive x-ray spectrometer is an attractive tool for qualitative x-ray microanalysis.