Tuesday 2 August 2016

Gas Chromatography
Introduction:
A Gas Chromatograph is used to detect the components based on the selective affinity of components towards the adsorbent materials. The sample is introduced in the liquid/gas form with the help of GC syringe into the injection port, it gets vaporized at injection port then passes through column with the help of continuously flowing carrier stream (mobile phase), mainly H2 (for TCD), and gets separated/detected at the detection port with suitable temperature programming. We visualize this on computer in the form of peaks. Carrier medium can be liquid (e.g. HPLC) or gas (e.g. GC) for the ease of separation/detection, if it is gas then called gas chromatography otherwise called liquid chromatography. Different chemical constituents of the sample travel through the column at different rates .

                       

It depends upon
1. Physical properties
 2. Chemical properties
 3. Interaction with a specific column filling (stationary phase).
As the chemicals exit the end of the column, they are detected and identified electronically. The function of the stationary phase in the column is to separate different components, causing each one to exit the column at a different time (retention time). Other parameters that can be used to alter the order or time of retention are the carrier gas flow rate, and the temperature. Physical Components involve inlet port, Adsorption column, detector port, flow controller (to control the flow of carrier gas), etc.
Types of columns:
Packed Column: Packed columns are 1.5 - 10 m in length and have an internal diameter of 2 – 4 mm. The tubing is usually made of stainless steel or glass and contains a packing of finely divided, inert, solid support material (eg. diatomaceous earth) that is coated with a liquid or solid stationary phase. The nature of the coating material determines what type of materials will be most strongly adsorbed.
Capillary Column: Capillary columns have a very small internal diameter, on the order of a few tenths of millimeters, and lengths between 25-60 meters are common. The inner column walls are coated with the active materials.
Some columns are quasi solid filled with many parallel micro pores.Most capillary columns are made of fused silica with a polyimide outer coating. These columns are flexible, so a very long column can be wound into a small coil.
Effect of temperature:
Temperature dependence of molecular adsorption and of the rate of progression along the column necessitates a careful control of the column temperature to within a few tenths of a degree for precise work. Reducing the temperature produces the greatest level of separation, but can result in very long elution times.
Carrier gas:
The choice of carrier gas or mobile phase is important, with hydrogen being the most efficient and providing the best separation. However, helium has a larger range of flow rates that are comparable to hydrogen in efficiency, with the added advantage that helium is non-flammable, and works with a greater number of detectors. Therefore, helium is the most common carrier gas used.
Injection port:
The sample to be analyzed is loaded at the injection port via a hypodermic syringe.  The injection port is heated in order to volatilize the sample.  Once in the gas phase, the sample is carried onto the column by the carrier gas, typically helium. 
                       


The carrier gas is also called the mobile phase.  Gas chromatographs are very sensitive instruments.  Typically samples of one microliter or less are injected on the column.  These volumes can be further reduced by using what is called a split injection system in which a controlled fraction of the injected sample is carried away by a gas stream before entering the column.
 Column:
The column is where the components of the sample are separated.  The column contains the stationary phase.  Gas chromatography columns are of two types—packed and capillary.  Capillary columns are those in which the stationary phase is coated on the interior walls of a tubular column with a small inner diameter.  We will use a capillary column in this experiment. 
The stationary phase in our column is a polysiloxane material.  The basic structure of the polymeric molecules is shown below, where n indicates a variable number of repeating units and R indicates an organic functional group.  In our  columns, 5% of the “R’s” are methyl groups (-CH3) and 95% of the “R’s” are phenyl groups (-C6H5)
                       

 This polymeric liquid has a high boiling point that prevents it from evaporating off the column during the experiment.
The components in the sample get separated on the column because they take different amounts of time to travel through the column depending on how strongly they interact with the stationary phase.  As the components move into the column from the injection port they dissolve in the stationary phase and are retained.  Upon re-vaporization into the mobile phase they are carried further down the column.  This process is repeated many times as the components migrate through the column.  Components that interact more strongly with the stationary phase spend proportionally less time in the mobile phase and therefore move through the column more slowly.  Normally the column is chosen such that it’s polarity matches that of the sample.  When this is the case, the interaction and elution times can be rationalized according to Raoult’s law and the relationship between vapor pressure and enthalpy of vaporization.  The rule of thumb is that retention times correlate with boiling points.  (Do not expect an exact quantitative correlation, i.e. one with an R-value close to one, for this simple model.  You will be using a non-polar column and the interaction between an alcohol molecule and the stationary phase will be subject by weak van der Waals forces.)
 The rate at which compounds move through the column depends on the nature of the interaction between the compound and the stationary phase.  Other variables that affect this rate are column temperature and carrier gas flow rate.  Not only do you waste valuable resources (your time and chart paper) but broadening of the peaks and loss of resolution will become marked when the elution times are too long.  This broadening is an expected consequence of diffusion.  The theory of diffusion shows that the width of a peak is roughly proportional to the square root of elution time.  Thus the optimum conditions are those that result in complete separation of the peaks in the shortest possible time.
Detectors:  
A number of detectors are used in gas chromatography. The most common are the Flame ionization detector (FID) and the thermal conductivity detector (TCD). While TCDs are essentially universal and can be used to detect any component other than the carrier gas (as long as their thermal conductivities are different than that of the carrier gas, at detector temperature), FIDs are sensitive primarily to hydrocarbons, and are more sensitive to them than TCD.
                                   



Both detectors are also quite robust. Since TCD is nondestructive, it can be operated in-series before an FID (destructive), thus providing complementary detection of the same eluents.
Applications:
Gas Chromatography is widely used in pharmaceutical industries for analytical research and development, quality control, quality assurance, production, pilot plants departments for active pharmaceutical ingredients (API), bulk drugs and formulations. It is used for process and method development, identification of impurities in API. It is an integral part of research associated with medicinal chemistry (synthesis and characterization of compounds), pharmaceutical analysis (stability testing, impurity profiling), pharmacognosy, pharmaceutical process control (Figure 7), pharmaceutical biotechnology etc
                                                                                                                                                         Alok Sharma


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