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Volume 2
J Chem Tech App 2018, Volume 2
GLC IN DRUG ANALYSIS
Anil Batta
Govt.medical college, India
T
he purpose of the gas chromatograph is to separate mixtures into individual components that can be detected and measured
one at a time. A plot of the detector output is called a chromatogram, which charts the detector’s response as a function of time,
showing the separate components. The separation occurs based on differences in affinities for the two phases. As shown in the
figure, the sample is introduced into the GC column by way of a heated injector, which volatilizes all three components and intro-
duces them into the gas flowing over the stationary phase. He sample is introduced into the GC column by way of a heated injector,
which volatilizes all three components and introduces them into the gas flowing over the stationary phase. In this example, the
compound represented by the arrowhead has the least affinity for the stationary phase. As a result, it moves ahead of the other two
components and will reach the detector first. The compound symbolized by the diamond has the greatest affinity for the stationary
phase and spends the most time associated with it. As a result, this compound will be the last to reach the detector. Separation has
been achieved based on the different affinities of the three types of molecules found in the sample. In reality, complex mixtures
cannot always be completely separated, with some compounds emerging from the column simultaneously. In most forensic ap-
plications of GC, a sample is prepared by dissolving it in a solvent, and the solution is injected into the instrument using a syringe.
For example, to analyze a white powder suspected of being cocaine, a small portion is weighed out and dissolved in a solvent such
as methylene chloride, methanol, or chloroform. A tiny portion of the sample is then drawn up into a syringe and injected into the
heated injector port of the instrument. A tiny portion of the sample is then drawn up into a syringe and injected into the heated
injector port of the instrument. The mobile phase gas (called the carrier gas) also enters the injector port, picking up the volatilized
sample and introducing it into the column where the separation process occurs. If the sample contains cocaine, it will emerge from
the column at a given time (known as the retention time) that can be compared to the retention time of a known standard sample of
cocaine. The retention time in conjunction with information obtained from the detector is used to positively identify the compound
as cocaine if indeed it is present. Another method of sample introduction for GC is called pyrolysis, in which a solid sample such as
a fiber or paint chip is heated in a special sample holder to extreme temperatures, causing the sample to decompose into gaseous
components that can then be introduced into the GC. Pyrolysis is used when the sample is not readily soluble in common GC sol-
vents. A number of different detectors are available for use in gas chromatography. In forensic applications, the most commonly
used are mass spectrometry (often abbreviated as MSD for mass selective detector), flame ionization (FID), and nitrogen-phospho-
rus (NPD). The MSD is the most common of the three, principally because it can provide definitive identification of compounds (in
almost all cases) along with quantitative information.