Medical brain notes 14

  INTRODUCTION Allan Walsh, in 1955*, was the pioneer for the introduction of  atomic absorption spectroscopy  (AAS), which eventually proved to be one of the best-known-instrumental-techniques in the analytical armamentarium, that has since been exploited both intensively and extensively in carrying out the quantitative determination of trace metals in liquids of completely diversified nature, for instance : blood serum-for Ca 2+ , Mg 2+ , Na +  and K +  ; edible oils-Ni 2+  ; beer samples-Cu +  ; gasoline (petrol)-Pb 2+  ; urine-Se 4+  ; tap-water-Mg 2+  ; Ca 2+  ; lubricating oil-Vanadium (V).   The atomic absorption spectroscopy (AAS) may be accomplished either by using a  flame -whereby the sample solution is aspirated directly into a flame or by using an  electrothermal  device-whereby the sample solution is first evaporated and then ignited on a hot surface. It has been noticed that gaseous metal atoms in ...

  THEORY The underlying principle of atomic absorption spectroscopy (AAS) is the absorption of energy exclu-sively by ground state atoms while they are in the gaseous form.   It may be further expatiated as follows below :   A solution consisting of certain metallic species when aspirated into a flame, it will give rise to the corresponding vapours of metallic species. As it has already been discussed under flame emission spectroscopy (FES) : Some metal atoms would be raised directly to an energy level to such an extent as to emit the particular radiation of the metal. At this critical point, a sufficiently large quantum of the metal atoms of a particular element would still remain in the non-emitting ground-state, which in turn shall be receptive of light radiation having their own specific wavelength. Consequently, when a light of this wavelength is passed through a flame ; along the atoms of the metallic species, a portion of the same would be absorbed ; and the result...

  INSTRUMENTATION The atomic absorption spectrophotometers are essentially of  two  types, namely :   ( a ) Single-beam Atomic Absorption Spectrophotometer, and   ( b ) Double-beam Atomic Absorption Spectrophotometer.   These two instruments shall be discussed briefly here along with their vital components.   1.  SINGLE-BEAM ATOMIC ABSORPTION SPECTROPHOTOMETER   The schematic diagram of a single-beam atomic spectrophotometer in illustrated in Figure 26.1.   A = Anode (Tungsten),   B= Glass-shield,   C= Neon or Argon at 1-5 torr,   D= Quartz or Pyrex Window,   E= Inlet for Acetylene,   F = Inlet for air,   G = Liquid sample sucked in by an atomizer,   HCL = Hollow cathode Lamp,   I = Chopper (a rotating shutter),   J = Flame,   K 1  and K 2  = Slits,   L = Prism or Grating,   M = Photocell,   O = Drain outlet to maintain a constant pressure head in the mixing chamb...

  IMPORTANT ASPECTS OF ATOMIC ABSORPTION SPECTROSCOPY The following  three  important aspects of atomic absorption spectroscopy shall be discussed here briefly, namely :   ( i ) Analytical Techniques,   ( ii ) Detection Limit and Sensitivity, and   ( iii ) Interferences.   1. ANALYTICAL TECHNIQUES   In atomic absorption spectroscopy (AAS) the technique using  calibration curves  and the  standard   addition method  are both equally suitable for the quantitative determinations of elements.   1.1. Calibration Curves   Theoretically, the absorbance must be proportional to concentrations, however, deviations from lin-earity usually take place. Therefore, it is necessary to prepare an  empirical calibration curve  (ECC). For this, the standard solutions of the element(s) to be determined are employed to plot the ECC from which the contents in the ‘ test solutions’  may be measured conveniently.   F...