Introduction heavy metal pollution

Analytical Techniques
Analysis of pollution load is an integral part of environmental management. In environmental samples heavy metals can exist in a range of physicochemical forms such as, hydrated metal ions and inorganic and organic complexes. There are many good analytical methods for analyzing the heavy metals in environment such as, atomic absorption spectrometry (AAS), inductively coupled plasma atomic emission spectrometry (ICP/AES), inductively coupled plasma mass spectrometry (ICP/MS), X-ray fluorescence (XRF) and ion chromatography (IC). Most of these techniques required sample digestion before quantification of metal. The aim of digestion is to achieve a selective or complete extraction of metals from the samples. Mostly, the digestion procedures are based on the addition of inorganic acids such as, aqua regia, HNO₃-HF, HFHNO₃-H₂SO₄-HClO₄, HNO₃-HClO₄ in a closed vessel, which may be heated on different sources (Jeneper & Hayao, 2005; Nieuwenhuize et al. 1991; Scancar et al. 2000; Hseu et al. 2002).

Atomic Absorption spectroscopy is based on absorption of radiation by atoms. Absorption results in the excitation of electrons of atoms which jump to the higher energy levels. The amount of energy absorbed in the form of photons by sample is measured by AAS. The energy required for an electron to leave an atom is known as ionization energy and is specific to each chemical element. Absorbance is directly proportional to the concentration of the analyte present in the sample (Garcia & Baez, 2012).

Inductively coupled plasma atomic emission spectrometry (ICP/AES) is based on principle that atoms emit light when excited by plasma. Plasma is ionized gas with very high temperature range from 7000 to 10000 ˚K. Excited atom emit characteristic spectra (Wang, et al. 2003). Inductively coupled plasma mass spectrometry (ICP-MS) is a very powerful, highly sensitive and specific technique for the analysis of trace (ppb-ppm) and ultra-trace (ppq-ppb) element and isotope. ICP-MS is composed of plasma (a high temperature i.e. 8000 ˚K ionization source), quadrupole mass spectrometer (MS) analyzer (sensitive rapid scan detector) and a distinctive interface. The detection of elements is done by their mass-to-charge ratio (m/z) and intensity of a specific peak in the mass spectrum is proportional to the amount of that isotope (element) in the original sample. ICP/AES and ICP/MS are the future techniques for heavy metal detection in environmental samples because of accuracy, rapid and multi element analysis (Tu et al. 2010).

X-ray fluorescence is a non-destructive method for analyzing samples. Fluorescence involves emission of an X-ray photon after ionization of atom by a primary X-ray beam. When primary X-ray beam strikes a sample, it interacts with electron and knocks it out of its inner shell forming voids. These voids present an unstable condition of atom, which stabilized when the void promptly filled by outer shell electron and give off X-ray with specific wavelength. This characteristic X-ray is the measure of elemental composition of a sample (Meirer et al. 2010).