Medical brain notes 14

  INTRODUCTION One of the most difficult and frustrating problems ever encountered in the domain of pharmaceutical analysis is that of the simultaneous separation, identification and above all the quantitation of more than one compound from a complex mixture in a pharmaceutical product.   A good of sophisticated chromatographic techniques of separation have been put forward since early fifties that may be categorized into the following  four  groups, namely :   ( a ) Gas-Solid adsorption Chromatography (GSC),   ( b ) Gas-Liquid partition Chromatography, (GLC),   ( c ) Liquid-Solid adsorption Chromatography (LSC), and   ( d ) Liquid-Liquid partition Chromatography (LLC).   The first two groups have been collectively termed as  ‘Gas Chromatography’.  Its phenomenal growth at almost logarithmic pace may be attributed to its unparalleled potential in resolving components of a complex mixture.  Gas chromatography  fundamentally...

  THEORY There are, in fact,  three  theories that have gained virtually wide recognition and acceptance in describ-ing a gas chromatographic separation, namely :   ( a ) Plate theory,   ( b ) Rate theory, and   ( c ) Random walk and nonequilibrium theory.   These different theories will be discussed briefly in the sections that follows :   1. PLATE THEORY   Martin and Synge* first proposed the ‘ plate theory ’ in 1941, whereby they merely compared the GC separation to fractional distillation. Thus, the ‘theoretical’ plate is the portion of the column wherein the solute is in complete equilibrium with the mobile and the stationary phase. This equilibrium is represented by the following expression : where,  K D  = Distribution coefficient.   Thus, the distribution of a solute after ‘ n ’ equilibrium (plates) may be defined by the expansion of the binomial in Eq. ( a ) below :               .......

  INSTRUMENTATION A gas chromatograph essentially comprises of  six  vital components, namely :   ( a ) Carrier Gas Regulator and Flow Meter,   ( b ) Sample Injection System,   ( c ) Separation Column,   ( d ) Thermal Compartment,   ( e ) Detectors,   ( f ) Recording of Signal Current, and   ( g ) Integrator.   These components shall be discussed briefly in the sections that follow : Figure 29.2, gives the sche-matic diagram of a standard GLC equipment showing the various parts : The sample is introduced into the vaporizer and enters the column along with the carrier gas at a constant flow through the detector oven. The reference sample also passes through the detector oven into the column which is maintained by column-oven heat control device. The detector picks up the signals of the sample as well as the reference substance one after the other which is duly amplified and the signal current recorded on a strip-chart recording devic...