Monday, September 23, 2019

Ion chromatography Essay Example | Topics and Well Written Essays - 1500 words

Ion chromatography - Essay Example After the discovery of X-rays and the establishment of association of absorption characteristics and atomic number of an element, X-rays have been widely used for analytical and diagnostic purposes. X-ray fluorescence spectrometry (XRF) developed in to two modes of analysis; wavelength dispersive spectrometry (WDXRF) or isolation of narrow wavelength bands by diffraction through a crystal and energy dispersive spectrometry (EDXRF) using proportional detectors for isolation of narrow bands (figure 1). XRF is widely used for quantitative analysis of almost all elements of periodic table with accuracies up to tenth of a percent and at concentrations as low as few ppms (Jenkins, 2000). Figure 1: Energy Dispersive & Wavelength Dispersive X-ray Fluorescent Spectroscopy PROPERTIES OF X-RAYS X-rays form a part of the electromagnetic spectrum between the wavelength ranges of 0.01-10nm. X-rays are produced when an accelerated electron collides with a target element thereby losing energy; the l ost energy forming the X-rays. Less than 1% of the lost energy is used for X-ray production, rest being lost as heat. Ek = eV = 1/2mv2 Where, Ek – kinetic energy, e – Charge of electron (1.6 ? 10-19 C), V – Applied voltage, m – Mass of electron (9.11 ? 10-31 kg), v – Velocity of electron (m/s) Figure 2: X-ray spectrum of Mo at different voltages (Menke) PRINCIPLE Deceleration of an incident high energy electron beam by atomic electrons of the sample leads to emission of a band of radiations of broad wavelength termed continuum or white or polychromatic radiation or ‘bremsstrahlung’ (German for breaking radiation) (figure 2). For a sample comprising of multiple elements, white radiation leads to excitation of characteristic lines enabling identification of the constituent elements (Jenkins, 1999). X-ray beam with energy (E) incident on an element with binding energy (?) of the atomic electrons, such that E > ? might lead to emission of electrons from its orbital position. This phenomenon is known as photoelectric effect. Kinetic energy of the emitted electron = E-? The photoelectric effect results in formation of a characteristic peak when the hole in the inner shell is filled by a higher energy electron from the outer shell (figure 3). Figure 3: Schematic Representation of an X-ray Fluorescence Process (K, n=1; L, n=2; M, n=3) (Menke) However, each incident X-ray beam does not lead to single transition, but since atoms comprise of multiple orbitals, multiple transitions are possible. Each of these transitions result in a number of XRF peaks in the spectrum and are characteristic of the sample element (figure 4). Contrary to this some holes are filled by an internal rearrangement process (Auger effect) and therefore, do not result in characteristic spectrum. Fluorescent Yield = Number of holes resulting in Characteristic photon emission/ Total Number of holes For elements with low atomic number the fluorescent yie ld is very small. Moreover, fluorescent yield for L transitions is lesser than that for K transitions and that for M is even lesser. Selection rules for normal lines in spectral diagram require the principal quantum number (n) to change at least

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